Co-reporter:Xiao Hua, Zheng Liu, Michael G. Fischer, Olaf Borkiewicz, Peter J. Chupas, Karena W. Chapman, Ullrich Steiner, Peter G. Bruce, and Clare P. Grey
Journal of the American Chemical Society September 27, 2017 Volume 139(Issue 38) pp:13330-13330
Publication Date(Web):August 5, 2017
DOI:10.1021/jacs.7b05228
TiO2 (B) has attracted considerable attention in recent years because it exhibits the largest capacity among all studied titania polymorphs, with high rate performance for Li intercalation being achieved when this material is nanostructured. However, due to the complex nature of its lithiation mechanism and practical challenges in probing Li structure in nanostructured materials, a definitive understanding of the lithiation thermodynamics has yet to be established. A comprehensive mechanistic investigation of the TiO2 (B) nanoparticles is therefore presented using a combination of in situ/operando X-ray pair distribution function (PDF) and electrochemical techniques. The discharge begins with surface reactions in parallel with Li insertion into the subsurface of the nanoparticles. The Li bulk insertion starts with a single-phase reaction into the A2 site, a position adjacent to the b-channel. A change of the Li diffusion pathway from that along this open channel to that along the c-direction is likely to occur at the composition of Li0.25TiO2 until Li0.5TiO2 is attained, leading to a two-step A2-site incorporation with one step kinetically distinct from the other. Subsequent Li insertion involves the C′ site, a position situated inside the channel, and follows a rapid two-phase reaction to form Li0.75TiO2. Due to the high diffusion barrier associated with the further lithiation, Li insertion into the A1 site, another position adjacent to the channel neighboring the A2 sites, is kinetically restricted. This study not only explores the lithiation reaction thermodynamics and mechanisms of nanoparticulate TiO2 (B) but also serves as a strong reference for future studies of the bulk phase, and for future calculations to study the Li transport properties of TiO2 (B).
Co-reporter:Matthew T. Dunstan, Hannah Laeverenz Schlogelhofer, John M. Griffin, Matthew S. Dyer, Michael W. Gaultois, Cindy Y. Lau, Stuart A. Scott, and Clare P. Grey
The Journal of Physical Chemistry C October 12, 2017 Volume 121(Issue 40) pp:21877-21877
Publication Date(Web):September 14, 2017
DOI:10.1021/acs.jpcc.7b05888
Among the many different processes proposed for large-scale carbon capture and storage (CCS), high-temperature CO2 looping has emerged as a favorable candidate due to the low theoretical energy penalties that can be achieved. Many different materials have been proposed for use in such a process, the process requiring fast CO2 absorption reaction kinetics as well as being able to cycle the material for multiple cycles without loss of capacity. Lithium ternary oxide materials, and in particular Li2ZrO3, have displayed promising performance, but further modifications are needed to improve their rate of reaction with CO2. Previous studies have linked rates of lithium ionic conduction with CO2 absorption in similar materials, and in this work we present work aimed at exploring the effect of aliovalent doping on the efficacy of Li2ZrO3 as a CO2 sorbent. Using a combination of X-ray powder diffraction, theoretical calculations, and solid-state nuclear magnetic resonance, we studied the impact of Nb, Ta, and Y doping on the structure, Li ionic motion, and CO2 absorption properties of Li2ZrO3. These methods allowed us to characterize the theoretical and experimental doping limit into the pure material, suggesting that vacancies formed upon doping are not fully disordered but instead are correlated to the dopant atom positions, limiting the solubility range. Characterization of the lithium motion using variable-temperature solid-state nuclear magnetic resonance confirms that interstitial doping with Y retards the movement of Li ions in the structure, whereas vacancy doping with Nb or Ta results in a similar activation energy as observed for nominally pure Li2ZrO3. However, a marked reduction in the CO2 absorption of the Nb- and Ta-doped samples suggests that doping also leads to a change in the carbonation equilibrium of Li2ZrO3, disfavoring the CO2 absorption at the reaction temperature. This study shows that a complex mixture of structural, kinetic, and dynamic factors can influence the performance of Li-based materials for CCS and underscores the importance of balancing these different factors in order to optimize the process.
Co-reporter:Joshua M. Stratford, Martin Mayo, Phoebe K. Allan, Oliver Pecher, Olaf J. Borkiewicz, Kamila M. Wiaderek, Karena W. Chapman, Chris J. Pickard, Andrew J. Morris, and Clare P. Grey
Journal of the American Chemical Society May 31, 2017 Volume 139(Issue 21) pp:7273-7273
Publication Date(Web):May 4, 2017
DOI:10.1021/jacs.7b01398
The alloying mechanism of high-capacity tin anodes for sodium-ion batteries is investigated using a combined theoretical and experimental approach. Ab initio random structure searching (AIRSS) and high-throughput screening using a species-swap method provide insights into a range of possible sodium–tin structures. These structures are linked to experiments using both average and local structure probes in the form of operando pair distribution function analysis, X-ray diffraction, and 23Na solid-state nuclear magnetic resonance (ssNMR), along with ex situ 119Sn ssNMR. Through this approach, we propose structures for the previously unidentified crystalline and amorphous intermediates. The first electrochemical process of sodium insertion into tin results in the conversion of crystalline tin into a layered structure consisting of mixed Na/Sn occupancy sites intercalated between planar hexagonal layers of Sn atoms (approximate stoichiometry NaSn3). Following this, NaSn2, which is predicted to be thermodynamically stable by AIRSS, forms; this contains hexagonal layers closely related to NaSn3, but has no tin atoms between the layers. NaSn2 is broken down into an amorphous phase of approximate composition Na1.2Sn. Reverse Monte Carlo refinements of an ab initio molecular dynamics model of this phase show that the predominant tin connectivity is chains. Further reaction with sodium results in the formation of structures containing Sn–Sn dumbbells, which interconvert through a solid-solution mechanism. These structures are based upon Na5–xSn2, with increasing occupancy of one of its sodium sites commensurate with the amount of sodium added. ssNMR results indicate that the final product, Na15Sn4, can store additional sodium atoms as an off-stoichiometry compound (Na15+xSn4) in a manner similar to Li15Si4.
Co-reporter:Hao Liu, Min-Ju Choe, Raul A. Enrique, Bernardo Orvañanos, Lina Zhou, Tao Liu, Katsuyo Thornton, and Clare P. Grey
The Journal of Physical Chemistry C June 8, 2017 Volume 121(Issue 22) pp:12025-12025
Publication Date(Web):May 16, 2017
DOI:10.1021/acs.jpcc.7b02819
Li–Fe antisite defects are commonly found in LiFePO4 particles and can impede or block Li diffusion in the single-file Li diffusion channels. However, due to their low concentration (∼1%), the effect of antisite defects on Li diffusion has only been systematically investigated by theoretical approaches. In this work, the exchange between Li in solid LiFePO4 (92.5% enriched with 6Li) and Li in the liquid Li electrolyte solution (containing natural abundance Li, 7.6% 6Li and 92.4% 7Li) was measured as a function of time by both ex situ and in situ solid-state nuclear magnetic resonance experiments. The experimental data reveal that the time dependence of the isotope exchange cannot be modeled by a simple single-file diffusion process and that defects must play a role in the mobility of ions in the LiFePO4 particles. By performing kinetic Monte Carlo simulations that explicitly consider antisite defects, which allow Li to cross over between adjacent channels, we show that the observed tracer exchange behavior can be explained by the presence of channels with paired Li–Fe antisite defects. The simulations suggest that Li diffusion across the antisite is slow (10–16 cm2 s–1) and that the presence of antisite defects is widespread in the LiFePO4 particles we examined, where ∼80% channels are affected by such defects.
Co-reporter:Wei Meng, Roberta Pigliapochi, Paul M. Bayley, Oliver Pecher, Michael W. Gaultois, Ieuan D. Seymour, Han-Pu Liang, Wenqian Xu, Kamila M. Wiaderek, Karena W. Chapman, and Clare P. Grey
Chemistry of Materials July 11, 2017 Volume 29(Issue 13) pp:5513-5513
Publication Date(Web):June 5, 2017
DOI:10.1021/acs.chemmater.7b00428
V6O13 is a promising Li-ion battery cathode material for use in the high temperature oil field environment. The material exhibits a high capacity, and the voltage profile contains several plateaus associated with a series of complex structural transformations, which are not fully understood. The underlying mechanisms are central to understanding and improving the performance of V6O13-based rechargeable batteries. In this study, we present in situ X-ray diffraction data that highlight an asymmetric six-step discharge and five-step charge process, due to a phase that is only formed on discharge. The LixV6O13 unit cell expands sequentially in c, b, and a directions during discharge and reversibly contracts back during charge. The process is associated with change of Li ion positions as well as charge ordering in LixV6O13. Density functional theory calculations give further insight into the electronic structures and preferred Li positions in the different structures formed upon cycling, particularly at high lithium contents, where no prior structural data are available. The results shed light into the high specific capacity of V6O13 and are likely to aid in the development of this material for use as a cathode for secondary lithium batteries.
Co-reporter:Oliver Pecher;Javier Carretero-González;Kent J. Griffith
Chemistry of Materials January 10, 2017 Volume 29(Issue 1) pp:213-242
Publication Date(Web):October 27, 2016
DOI:10.1021/acs.chemmater.6b03183
Improving electrochemical energy storage is one of the major issues of our time. The search for new battery materials together with the drive to improve performance and lower cost of existing and new batteries is not without its challenges. Success in these matters is undoubtedly based on first understanding the underlying chemistries of the materials and the relations between the components involved. A combined application of experimental and theoretical techniques has proven to be a powerful strategy to gain insights into many of the questions that arise from the “how do batteries work and why do they fail” challenge. In this Review, we highlight the application of solid-state nuclear magnetic resonance (NMR) spectroscopy in battery research: a technique that can be extremely powerful in characterizing local structures in battery materials, even in highly disordered systems. An introduction on electrochemical energy storage illustrates the research aims and prospective approaches to reach these. We particularly address “NMR in battery research” by giving a brief introduction to electrochemical techniques and applications as well as background information on both in and ex situ solid-state NMR spectroscopy. We will try to answer the question “Is NMR suitable and how can it help me to solve my problem?” by shortly reviewing some of our recent research on electrodes, microstructure formation, electrolytes and interfaces, in which the application of NMR was helpful. Finally, we share hands-on experience directly from the lab bench to answer the fundamental question “Where and how should I start?” to help guide a researcher’s way through the manifold possible approaches.
Co-reporter:Clayton Cozzan;Geneva Laurita;Jerry G. Hu;Kent J. Griffith;Ram Seshadri
Inorganic Chemistry February 20, 2017 Volume 56(Issue 4) pp:2153-2158
Publication Date(Web):February 6, 2017
DOI:10.1021/acs.inorgchem.6b02780
SiAlON ceramics, solid solutions based on the Si3N4 structure, are important, lightweight structural materials with intrinsically high strength, high hardness, and high thermal and chemical stability. Described by the chemical formula β-Si6–zAlzOzN8–z, from a compositional viewpoint, these materials can be regarded as solid solutions between Si3N4 and Al3O3N. A key aspect of the structural evolution with increasing Al and O (z in the formula) is to understand how these elements are distributed on the β-Si3N4 framework. The average and local structural evolution of highly phase-pure samples of β-Si6–zAlzOzN8–z with z = 0.050, 0.075, and 0.125 are studied here, using a combination of X-ray diffraction, NMR studies, and density functional theory calculations. Synchrotron X-ray diffraction establishes sample purity and indicates subtle changes in the average structure with increasing Al content in these compounds. Solid-state magic-angle-spinning 27Al NMR experiments, coupled with detailed ab initio calculations of NMR spectra of Al in different AlOqN4–q tetrahedra (0 ≤ q ≤ 4), reveal a tendency of Al and O to cluster in these materials. Independently, the calculations suggest an energetic preference for Al–O bond formation, instead of a random distribution, in the β-SiAlON system.
Co-reporter:Megan M. Butala, Martin Mayo, Vicky V. T. Doan-Nguyen, Margaret A. Lumley, Claudia Göbel, Kamila M. Wiaderek, Olaf J. Borkiewicz, Karena W. Chapman, Peter J. Chupas, Mahalingam Balasubramanian, Geneva Laurita, Sylvia Britto, Andrew J. Morris, Clare P. Grey, and Ram Seshadri
Chemistry of Materials April 11, 2017 Volume 29(Issue 7) pp:3070-3070
Publication Date(Web):March 27, 2017
DOI:10.1021/acs.chemmater.7b00070
In the pursuit of high-capacity electrochemical energy storage, a promising domain of research involves conversion reaction schemes, wherein electrode materials are fully transformed during charge and discharge. There are, however, numerous difficulties in realizing theoretical capacity and high rate capability in many conversion schemes. Here we employ operando studies to understand the conversion material FeS2, focusing on the local structure evolution of this relatively reversible material. X-ray absorption spectroscopy, pair distribution function analysis, and first-principles calculations of intermediate structures shed light on the mechanism of charge storage in the Li–FeS2 system, with some general principles emerging for charge storage in chalcogenide materials. Focusing on second and later charge/discharge cycles, we find small, disordered domains that locally resemble Fe and Li2S at the end of the first discharge. Upon charge, this is converted to a Li–Fe–S composition whose local structure reveals tetrahedrally coordinated Fe. With continued charge, this ternary composition displays insertion–extraction behavior at higher potentials and lower Li content. The finding of hybrid modes of charge storage, rather than simple conversion, points to the important role of intermediates that appear to store charge by mechanisms that more closely resemble intercalation.
Co-reporter:Oliver Pecher, David M. Halat, Jeongjae Lee, Zigeng Liu, Kent J. Griffith, Marco Braun, Clare P. Grey
Journal of Magnetic Resonance 2017 Volume 275() pp:127-136
Publication Date(Web):February 2017
DOI:10.1016/j.jmr.2016.12.008
•New external automatic tuning/matching (eATM) robot system.•Enables “on-the-fly” recalibration of the resonance circuit during NMR experiments.•Enhanced efficiency of variable offset cumulative spectra (VOCS) and other broadband measurements.•Various applications to paramagnetic, quadrupolar, and/or multinuclear systems.•Automated acquisition during variable temperature, static, and/or MAS NMR experiments.We have developed and explored an external automatic tuning/matching (eATM) robot that can be attached to commercial and/or home-built magic angle spinning (MAS) or static nuclear magnetic resonance (NMR) probeheads. Complete synchronization and automation with Bruker and Tecmag spectrometers is ensured via transistor-transistor-logic (TTL) signals. The eATM robot enables an automated “on-the-fly” re-calibration of the radio frequency (rf) carrier frequency, which is beneficial whenever tuning/matching of the resonance circuit is required, e.g. variable temperature (VT) NMR, spin-echo mapping (variable offset cumulative spectroscopy, VOCS) and/or in situ NMR experiments of batteries. This allows a significant increase in efficiency for NMR experiments outside regular working hours (e.g. overnight) and, furthermore, enables measurements of quadrupolar nuclei which would not be possible in reasonable timeframes due to excessively large spectral widths. Additionally, different tuning/matching capacitor (and/or coil) settings for desired frequencies (e.g.7Li and 31P at 117 and 122 MHz, respectively, at 7.05 T) can be saved and made directly accessible before automatic tuning/matching, thus enabling automated measurements of multiple nuclei for one sample with no manual adjustment required by the user. We have applied this new eATM approach in static and MAS spin-echo mapping NMR experiments in different magnetic fields on four energy storage materials, namely: (1) paramagnetic 7Li and 31P MAS NMR (without manual recalibration) of the Li-ion battery cathode material LiFePO4; (2) paramagnetic 17O VT-NMR of the solid oxide fuel cell cathode material La2NiO4+δ; (3) broadband 93Nb static NMR of the Li-ion battery material BNb2O5; and (4) broadband static 127I NMR of a potential Li–air battery product LiIO3. In each case, insight into local atomic structure and dynamics arises primarily from the highly broadened (1–25 MHz) NMR lineshapes that the eATM robot is uniquely suited to collect. These new developments in automation of NMR experiments are likely to advance the application of in and ex situ NMR investigations to an ever-increasing range of energy storage materials and systems.Figure optionsDownload full-size imageDownload high-quality image (149 K)Download as PowerPoint slide
Co-reporter:Michal LeskesGunwoo Kim, Tao Liu, Alison L. Michan, Fabien Aussenac, Patrick Dorffer, Subhradip Paul, Clare P. Grey
The Journal of Physical Chemistry Letters 2017 Volume 8(Issue 5) pp:
Publication Date(Web):February 14, 2017
DOI:10.1021/acs.jpclett.6b02590
Forming a stable solid electrolyte interphase (SEI) is critical for rechargeable batteries’ performance and lifetime. Understanding its formation requires analytical techniques that provide molecular-level insight. Here, dynamic nuclear polarization (DNP) is utilized for the first time to enhance the sensitivity of solid-state NMR (ssNMR) spectroscopy to the SEI. The approach is demonstrated on reduced graphene oxide (rGO) cycled in Li-ion cells in natural abundance and 13C-enriched electrolyte solvents. Our results indicate that DNP enhances the signal of outer SEI layers, enabling detection of natural abundance 13C spectra from this component of the SEI on reasonable time frames. Furthermore, 13C-enriched electrolyte measurements at 100 K provide ample sensitivity without DNP due to the vast amount of SEI filling the rGO pores, thereby allowing differentiation of the inner and outer SEI layer composition. Developing this approach further will benefit the study of many electrode materials, equipping ssNMR with the necessary sensitivity to probe the SEI efficiently.
Co-reporter:Evan N. Keyzer;Peter D. Matthews;Zigeng Liu;Andrew D. Bond;Dominic S. Wright
Chemical Communications 2017 vol. 53(Issue 33) pp:4573-4576
Publication Date(Web):2017/04/20
DOI:10.1039/C7CC01938F
The development of rechargeable Ca-ion batteries as an alternative to Li systems has been limited by the availability of suitable electrolyte salts. We present the synthesis of complexes of Ca(PF6)2 (a key potential Ca battery electrolyte salt) via the treatment of Ca metal with NOPF6, and explore their conversion to species containing PO2F2− under the reaction conditions.
Co-reporter:Cindy Y. Lau;Matthew T. Dunstan;Wenting Hu;Stuart A. Scott
Energy & Environmental Science (2008-Present) 2017 vol. 10(Issue 3) pp:818-831
Publication Date(Web):2017/03/15
DOI:10.1039/C6EE02763F
Chemical looping combustion (CLC) has been proposed as an efficient carbon capture process for power generation. Oxygen stored within a solid metal oxide is used to combust the fuel, either by releasing the oxygen into the gas phase, or by direct contact with the fuel; this oxyfuel combustion produces flue gases which are not diluted by N2. These materials can also be used to perform air-separation to produce a stream of oxygen mixed with CO2, which can subsequently be used in the conventional oxyfuel combustion process to produce sequesterable CO2. The temperature and oxygen partial pressures under which various oxide materials will react in this way are controlled by their thermodynamic equilibria with respect to reduction and oxidation. While many materials have been proposed for use in chemical looping, many suffer from poor kinetics or irreversible capacity loss due to carbonation, and therefore applying large scale in silico screening methods to this process is a promising way to obtain new candidate materials. In this study we report the first such large scale screening of oxide materials for oxyfuel combustion, utilising the Materials Project database of theoretically determined structures and ground state energies. From this screening several promising candidates were selected due to their predicted thermodynamic properties and subjected to initial experimental thermodynamic testing, with SrFeO3−δ emerging as a promising material for use in CLC. SrFeO3−δ was further shown to have excellent cycling stability and resistance to carbonation over the temperatures of operation. This work further advances how in silico screening methods can be implemented as an efficient way to sample a large compositional space in order to find novel functional materials.
Co-reporter:Jeongjae Lee;Ieuan D. Seymour;Andrew J. Pell;Siân E. Dutton
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 1) pp:613-625
Publication Date(Web):2016/12/21
DOI:10.1039/C6CP06338A
Rechargeable battery systems based on Mg-ion chemistries are generating significant interest as potential alternatives to Li-ion batteries. Despite the wealth of local structural information that could potentially be gained from Nuclear Magnetic Resonance (NMR) experiments of Mg-ion battery materials, systematic 25Mg solid-state NMR studies have been scarce due to the low natural abundance, low gyromagnetic ratio, and significant quadrupole moment of 25Mg (I = 5/2). This work reports a combined experimental 25Mg NMR and first principles density functional theory (DFT) study of paramagnetic Mg transition metal oxide systems Mg6MnO8 and MgCr2O4 that serve as model systems for Mg-ion battery cathode materials. Magnetic parameters, hyperfine shifts and quadrupolar parameters were calculated ab initio using hybrid DFT and compared to the experimental values obtained from NMR and magnetic measurements. We show that the rotor assisted population transfer (RAPT) pulse sequence can be used to enhance the signal-to-noise ratio in paramagnetic 25Mg spectra without distortions in the spinning sideband manifold. In addition, the value of the predicted quadrupolar coupling constant of Mg6MnO8 was confirmed using the RAPT pulse sequence. We further apply the same methodology to study the NMR spectra of spinel compounds MgV2O4 and MgMn2O4, candidate cathode materials for Mg-ion batteries.
Co-reporter:R. J. Clément;J. Xu;D. S. Middlemiss;J. Alvarado;C. Ma;Y. S. Meng;C. P. Grey
Journal of Materials Chemistry A 2017 vol. 5(Issue 8) pp:4129-4143
Publication Date(Web):2017/02/21
DOI:10.1039/C6TA09601H
Structural processes occurring upon electrochemical cycling in P2-Nax[LiyNizMn1−y−z]O2 (x, y, z ≤ 1) cathode materials are investigated using 23Na and 7Li solid-state nuclear magnetic resonance (ssNMR). The interpretation of the complex paramagnetic NMR data obtained for various electrochemically-cycled NaxNi1/3Mn2/3O2 and NaxLi0.12Ni0.22Mn0.66O2 samples is assisted by state-of-the-art hybrid Hartree–Fock/density functional theory calculations. Two Na crystallographic environments are present in P2-Nax[LiyNizMn1−y−z]O2 compounds, yet a single 23Na NMR signal is observed with a shift in-between those computed for edge- and face-centered prismatic sites, indicating that Na-ion motion between sites in the P2 layers results in an average signal. This is the first time that experimental and theoretical evidence are provided for fast Na-ion motion (on the timescale of the NMR experiments) in the interlayer space in P2-type NaxTMO2 materials. A full assignment of the 7Li NMR data confirms that Li substitution delays the P2 to O2 phase transformation taking place in NaxNi1/3Mn2/3O2 over the range 1/3 ≥ xNa ≥ 0. 23Na ssNMR data demonstrate that NaxNi1/3Mn2/3O2 samples charged to ≥3.7 V are extremely moisture sensitive once they are removed from the cell, water molecules being readily intercalated within the P2 layers leading to an additional Na signal between 400 and 250 ppm. By contrast, the lithiated material NaxLi0.12Ni0.22Mn0.66O2 shows no sign of hydration until it is charged to ≥4.4 V. Since both TMO2 layer glides and water intercalation become increasingly favorable as more vacancies are present in the Na layers, the higher stability of the Li-doped P2 phase at high voltage can be accounted for by its higher Na content at all stages of cycling.
AbstractNon-aqueous Li–O2 batteries are promising for next-generation energy storage. New battery chemistries based on LiOH, rather than Li2O2, have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru-catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e− oxygen reduction reaction, the H in LiOH coming solely from added H2O and the O from both O2 and H2O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li2O2, LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long-lived battery. An optimized metal-catalyst–electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.
Co-reporter:Michael A. Hope;David M. Halat;Pieter C. M. M. Magusin;Subhradip Paul;Luming Peng
Chemical Communications 2017 vol. 53(Issue 13) pp:2142-2145
Publication Date(Web):2017/02/09
DOI:10.1039/C6CC10145C
Surface-selective direct 17O DNP has been demonstrated for the first time on CeO2 nanoparticles, for which the first three layers can be distinguished with high selectivity. Polarisation build-up curves show that the polarisation of the (sub-)surface sites builds up faster than the bulk, accounting for the remarkable surface selectivity.
Co-reporter:Zigeng Liu;Jeongjae Lee;Guolei Xiang;Hugh F. J. Glass;Evan N. Keyzer;Siân E. Dutton
Chemical Communications 2017 vol. 53(Issue 4) pp:743-746
Publication Date(Web):2017/01/05
DOI:10.1039/C6CC08430C
Bi nanowires as anode materials for Mg ion batteries exhibit excellent electrochemical behaviour, forming Mg3Bi2; this is in part ascribed to the rapid Mg mobility between the two Mg sites of Mg3Bi2, as revealed by the 25Mg NMR spectra of Mg3Bi2 formed electrochemically and via ball-milling. A mechanism involving hops into vacant Mg sites is proposed.
Co-reporter:Matthew T. Dunstan, Anubhav Jain, Wen Liu, Shyue Ping Ong, Tao Liu, Jeongjae Lee, Kristin A. Persson, Stuart A. Scott, John S. Dennis and Clare P. Grey
Energy & Environmental Science 2016 vol. 9(Issue 4) pp:1346-1360
Publication Date(Web):21 Jan 2016
DOI:10.1039/C5EE03253A
The implementation of large-scale carbon dioxide capture and storage (CCS) is dependent on finding materials that satisfy several different criteria, the most important being minimising the energy load imposed on the power plant to run the process. The most mature CCS technology, amine scrubbing, leads to a loss of 30% of the electrical work output of the power station without capture, which is far too high for widespread deployment. High-temperature CO2 absorption looping has emerged as a technology that has the potential to deliver much lower energy penalties, but further work is needed to find and develop an optimal material. We have developed a combined computational and experimental methodology to predict new materials that should have desirable properties for CCS looping, and then select promising candidates to experimentally validate these predictions. This work not only has discovered novel materials for use in high-temperature CCS looping, but analysis of the entirety of the screening enables greater insights into new design strategies for future development.
Co-reporter:Alison L. Michan; Giorgio Divitini; Andrew J. Pell; Michal Leskes; Caterina Ducati
Journal of the American Chemical Society 2016 Volume 138(Issue 25) pp:7918-7931
Publication Date(Web):May 27, 2016
DOI:10.1021/jacs.6b02882
The solid electrolyte interphase (SEI) of the high capacity anode material Si is monitored over multiple electrochemical cycles by 7Li, 19F, and 13C solid-state nuclear magnetic resonance spectroscopies, with the organics dominating the SEI. Homonuclear correlation experiments are used to identify the organic fragments −OCH2CH2O–, −OCH2CH2–, −OCH2CH3, and −CH2CH3 contained in both oligomeric species and lithium semicarbonates ROCO2Li, RCO2Li. The SEI growth is correlated with increasing electrode tortuosity by using focused ion beam and scanning electron microscopy. A two-stage model for lithiation capacity loss is developed: initially, the lithiation capacity steadily decreases, Li+ is irreversibly consumed at a steady rate, and pronounced SEI growth is seen. Later, below 50% of the initial lithiation capacity, less Si is (de)lithiated resulting in less volume expansion and contraction; the rate of Li+ being irreversibly consumed declines, and the Si SEI thickness stabilizes. The decreasing lithiation capacity is primarily attributed to kinetics, the increased electrode tortuousity severely limiting Li+ ion diffusion through the bulk of the electrode. The resulting changes in the lithiation processes seen in the electrochemical capacity curves are ascribed to non-uniform lithiation, the reaction commencing near the separator/on the surface of the particles.
Co-reporter:Kent J. Griffith; Alexander C. Forse; John M. Griffin
Journal of the American Chemical Society 2016 Volume 138(Issue 28) pp:8888-8899
Publication Date(Web):June 6, 2016
DOI:10.1021/jacs.6b04345
Nanostructuring and nanosizing have been widely employed to increase the rate capability in a variety of energy storage materials. While nanoprocessing is required for many materials, we show here that both the capacity and rate performance of low-temperature bronze-phase TT- and T-polymorphs of Nb2O5 are inherent properties of the bulk crystal structure. Their unique “room-and-pillar” NbO6/NbO7 framework structure provides a stable host for lithium intercalation; bond valence sum mapping exposes the degenerate diffusion pathways in the sites (rooms) surrounding the oxygen pillars of this complex structure. Electrochemical analysis of thick films of micrometer-sized, insulating niobia particles indicates that the capacity of the T-phase, measured over a fixed potential window, is limited only by the Ohmic drop up to at least 60C (12.1 A·g–1), while the higher temperature (Wadsley–Roth, crystallographic shear structure) H-phase shows high intercalation capacity (>200 mA·h·g–1) but only at moderate rates. High-resolution 6/7Li solid-state nuclear magnetic resonance (NMR) spectroscopy of T-Nb2O5 revealed two distinct spin reservoirs, a small initial rigid population and a majority-component mobile distribution of lithium. Variable-temperature NMR showed lithium dynamics for the majority lithium characterized by very low activation energies of 58(2)–98(1) meV. The fast rate, high density, good gravimetric capacity, excellent capacity retention, and safety features of bulk, insulating Nb2O5 synthesized in a single step at relatively low temperatures suggest that this material not only is structurally and electronically exceptional but merits consideration for a range of further applications. In addition, the realization of high rate performance without nanostructuring in a complex insulating oxide expands the field for battery material exploration beyond conventional strategies and structural motifs.
Co-reporter:Evan N. Keyzer; Hugh F. J. Glass; Zigeng Liu; Paul M. Bayley; Siân E. Dutton; Clare P. Grey;Dominic S. Wright
Journal of the American Chemical Society 2016 Volume 138(Issue 28) pp:8682-8685
Publication Date(Web):June 30, 2016
DOI:10.1021/jacs.6b04319
Mg(PF6)2-based electrolytes for Mg-ion batteries have not received the same attention as the analogous LiPF6-based electrolytes used in most Li-ion cells owing to the perception that the PF6– anion decomposes on and passivates Mg electrodes. No synthesis of the Mg(PF6)2 salt has been reported, nor have its solutions been studied electrochemically. Here, we report the synthesis of the complex Mg(PF6)2(CH3CN)6 and its solution-state electrochemistry. Solutions of Mg(PF6)2(CH3CN)6 in CH3CN and CH3CN/THF mixtures exhibit high conductivities (up to 28 mS·cm–1) and electrochemical stability up to at least 4 V vs Mg on Al electrodes. Contrary to established perceptions, Mg electrodes are observed to remain electrochemically active when cycled in the presence of these Mg(PF6)2-based electrolytes, with no fluoride (i.e., MgF2) formed on the Mg surface. Stainless steel electrodes are found to corrode when cycled in the presence of Mg(PF6)2 solutions, but Al electrodes are passivated. The electrolytes have been used in a prototype Mg battery with a Mg anode and Chevrel (Mo3S4)-phase cathode.
Co-reporter:Ieuan D. Seymour; Derek S. Middlemiss; David M. Halat; Nicole M. Trease; Andrew J. Pell
Journal of the American Chemical Society 2016 Volume 138(Issue 30) pp:9405-9408
Publication Date(Web):July 12, 2016
DOI:10.1021/jacs.6b05747
Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of 17O NMR in the same materials has not yet been reported. In this work, we present a combined 17O NMR and hybrid density functional theory study of the local O environments in Li2MnO3, a model compound for layered Li-ion batteries. After a simple 17O enrichment procedure, we observed five resonances with large 17O shifts ascribed to the Fermi contact interaction with directly bonded Mn4+ ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the 17O shifts of environments similar to the 4i and 8j sites in pristine Li2MnO3, respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li2MnO3 structure. From the ratio of the intensities of the different 17O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying 17O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides.
Journal of the American Chemical Society 2016 Volume 138(Issue 6) pp:1955-1961
Publication Date(Web):January 19, 2016
DOI:10.1021/jacs.5b12423
Sodium batteries have seen a resurgence of interest from researchers in recent years, owing to numerous favorable properties including cost and abundance. Here we examine the feasibility of studying this battery chemistry with in situ NMR, focusing on Na metal anodes. Quantification of the NMR signal indicates that Na metal deposits with a morphology associated with an extremely high surface area, the deposits continually accumulating, even in the case of galvanostatic cycling. Two regimes for the electrochemical cycling of Na metal are apparent that have implications for the use of Na anodes: at low currents, the Na deposits are partially removed on reversing the current, while at high currents, there is essentially no removal of the deposits in the initial stages. At longer times, high currents show a significantly greater accumulation of deposits during cycling, again indicating a much lower efficiency of removal of these structures when the current is reversed.
Co-reporter:Phoebe K. Allan; John M. Griffin; Ali Darwiche; Olaf J. Borkiewicz; Kamila M. Wiaderek; Karena W. Chapman; Andrew J. Morris; Peter J. Chupas; Laure Monconduit
Journal of the American Chemical Society 2016 Volume 138(Issue 7) pp:2352-2365
Publication Date(Web):January 29, 2016
DOI:10.1021/jacs.5b13273
Operando pair distribution function (PDF) analysis and ex situ 23Na magic-angle spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy are used to gain insight into the alloying mechanism of high-capacity antimony anodes for sodium-ion batteries. Subtraction of the PDF of crystalline NaxSb phases from the total PDF, an approach constrained by chemical phase information gained from 23Na ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electrochemically; a-Na3–xSb (x ≈ 0.4–0.5), a structure locally similar to crystalline Na3Sb (c-Na3Sb) but with significant numbers of sodium vacancies and a limited correlation length, and a-Na1.7Sb, a highly amorphous structure featuring some Sb–Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na3–xSb and, finally, crystalline Na3Sb. Desodiation results in the formation of an electrode formed of a composite of crystalline and amorphous antimony networks. We link the different reactivity of these networks to a series of sequential sodiation reactions manifesting as a cascade of processes observed in the electrochemical profile of subsequent cycles. The amorphous network reacts at higher voltages reforming a-Na1.7Sb, then a-Na3–xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na3–xSb without the formation of a-Na1.7Sb. a-Na3–xSb is converted to crystalline Na3Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature 23Na NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.
Co-reporter:Alison L. Michan, Michal Leskes, and Clare P. Grey
Chemistry of Materials 2016 Volume 28(Issue 1) pp:385
Publication Date(Web):November 20, 2015
DOI:10.1021/acs.chemmater.5b04408
The solid electrolyte interphase (SEI) passivating layer that grows on all battery electrodes during cycling is critical to the long-term capacity retention of lithium-ion batteries. Yet, it is inherently difficult to study because of its nanoscale thickness, amorphous composite structure, and air sensitivity. Here, we employ an experimental strategy using 1H, 7Li, 19F, and 13C solid-state nuclear magnetic resonance (ssNMR) to gain insight into the decomposition products in the SEI formed on silicon electrodes, the uncontrolled growth of the SEI representing a major failure mechanism that prevents the practical use of silicon in lithium-ion batteries. The voltage dependent formation of the SEI is confirmed, with the SEI growth correlating with irreversible capacity. By studying both conductive carbon and mixed Si/C composite electrodes separately, a correlation with increased capacity loss of the composite system and the low-voltage silicon plateau is demonstrated. Using selective 13C labeling, we detect decomposition products of the electrolyte solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) independently. EC decomposition products are present in higher concentrations and are dominated by oligomer species. Lithium semicarbonates, lithium fluoride, and lithium carbonate products are also seen. Ab initio calculations have been carried out to aid in the assignment of NMR shifts. ssNMR applied to both rinsed and unrinsed electrodes show that the organics are easily rinsed away, suggesting that they are located on the outer layer of the SEI.
Co-reporter:Fiona C. Strobridge, Hao Liu, Michal Leskes, Olaf J. Borkiewicz, Kamila M. Wiaderek, Peter J. Chupas, Karena W. Chapman, and Clare P. Grey
Chemistry of Materials 2016 Volume 28(Issue 11) pp:3676
Publication Date(Web):April 25, 2016
DOI:10.1021/acs.chemmater.6b00319
The delithiation mechanisms occurring within the olivine-type class of cathode materials for Li-ion batteries have received considerable attention because of the good capacity retention at high rates for LiFePO4. A comprehensive mechanistic study of the (de)lithiation reactions that occur when the substituted olivine-type cathode materials LiFexCo1–xPO4 (x = 0, 0.05, 0.125, 0.25, 0.5, 0.75, 0.875, 0.95, 1) are electrochemically cycled is reported here using in situ X-ray diffraction (XRD) data and supporting ex situ 31P NMR spectra. On the first charge, two intermediate phases are observed and identified: Li1–x(Fe3+)x(Co2+)1–xPO4 for 0 < x < 1 (i.e., after oxidation of Fe2+ to Fe3+) and Li2/3FexCo1–xPO4 for 0 ≤ x ≤ 0.5 (i.e., the Co-majority materials). For the Fe-rich materials, we study how nonequilibrium, single-phase mechanisms that occur discretely in single particles, as observed for LiFePO4 at high rates, are affected by Co substitution. In the Co-majority materials, a two-phase mechanism with a coherent interface is observed, as was seen in LiCoPO4, and we discuss how it is manifested in the XRD patterns. We then compare the nonequilibrium, single-phase mechanism with the bulk single-phase and coherent interface two-phase mechanisms. Despite the apparent differences between these mechanisms, we discuss how they are related and interconverted as a function of Fe/Co substitution and the potential implications for the electrochemistry of this system.
Co-reporter:Gosuke Oyama, Oliver Pecher, Kent J. Griffith, Shin-ichi Nishimura, Roberta Pigliapochi, Clare P. Grey, and Atsuo Yamada
Chemistry of Materials 2016 Volume 28(Issue 15) pp:5321
Publication Date(Web):July 5, 2016
DOI:10.1021/acs.chemmater.6b01091
Alluaudite sodium iron sulfate Na2+2xFe2–x(SO4)3 is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fe3+/Fe2+ redox reactions (3.8 V vs Na/Na+), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), 23Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fe3+/Fe2+ redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions.
Co-reporter:Nicole M. Trease, Ieuan D. Seymour, Maxwell D. Radin, Haodong Liu, Hao Liu, Sunny Hy, Natalya Chernova, Pritesh Parikh, Arun Devaraj, Kamila M. Wiaderek, Peter J. Chupas, Karena W. Chapman, M. Stanley Whittingham, Ying Shirley Meng, Anton Van der Van, and Clare P. Grey
Chemistry of Materials 2016 Volume 28(Issue 22) pp:8170
Publication Date(Web):October 7, 2016
DOI:10.1021/acs.chemmater.6b02797
The doping of Al into layered Li transition metal (TM) oxide cathode materials, LiTMO2, is known to improve the structural and thermal stability, although the origin of the enhanced properties is not well understood. The effect of aluminum doping on layer stabilization has been investigated using a combination of techniques to measure the aluminum distribution in layered LiNi0.8Co0.15Al0.05O2 (NCA) over multiple length scales with 27Al and 7Li MAS NMR, local electrode atom probe (APT) tomography, X-ray and neutron diffraction, DFT, and SQUID magnetic susceptibility measurements. APT ion maps show a homogeneous distribution of Ni, Co, Al, and O2 throughout the structure at the single particle level in agreement with the high-temperature phase diagram. 7Li and 27Al NMR indicates that the Ni3+ ions undergo a dynamic Jahn–Teller (JT) distortion. 27Al NMR spectra indicate that the Al reduces the strain associated with the JT distortion, by preferential electronic ordering of the JT lengthened bonds directed toward the Al3+ ion. The ability to understand the complex atomic and orbital ordering around Al3+ demonstrated in the current method will be useful for studying the local environment of Al3+ in a range of transition metal oxide battery materials.
Journal of Materials Chemistry A 2016 vol. 4(Issue 17) pp:6433-6446
Publication Date(Web):12 Apr 2016
DOI:10.1039/C6TA00673F
The nature of a phase transition plays an important role in controlling the kinetics of reaction of an electrode material in a lithium-ion battery. The actual phase transition path can be affected by particle size and cycling rate. In this study, we investigated the phase transition process during the electrochemical Li intercalation of anatase TiO2 as a function of particle size (25 nm and 100 nm), cycling rate (1C, 2C, 5C, 10C, 20C) and temperature (room temperature and 80 °C) by in situ synchrotron X-ray diffraction. The phase transition was found to be affected by the particle size: the 100 nm particles react simultaneously via a conventional nucleation and growth, i.e. two-phase, mechanism, while the 25 nm particles react sequentially via a two-phase mechanism. The Li miscibility gap decreases with increasing cycling rate, yet the phase separation was not suppressed even at a cycle rate of 20C. An increase in temperature from room temperature to 80 °C significantly improves the electrode's electrochemical performance despite undergoing a two-phase reaction. The failure to observe a continuous structural transition from tetragonal TiO2 to orthorhombic Li0.5TiO2 even at high rates and elevated temperature was attributed to the high energy barrier of a continuous phase transition path.
Co-reporter:John M. Griffin, Alexander C. Forse, Clare P. Grey
Solid State Nuclear Magnetic Resonance 2016 Volumes 74–75() pp:16-35
Publication Date(Web):April–May 2016
DOI:10.1016/j.ssnmr.2016.03.003
•Applications of solid-state NMR for the study of supercapacitors are reviewed.•An overview of the basic principles of supercapacitors is given.•Ring current effects provide the primary tool for the identification of charge-storing species.•Ex situ and in situ experimental methodologies are discussed.•NMR measurements reveal the charge storage process is much more complex than is commonly assumed.Electrochemical double-layer capacitors, or ‘supercapacitors’ are attracting increasing attention as high-power energy storage devices for a wide range of technological applications. These devices store charge through electrostatic interactions between liquid electrolyte ions and the surfaces of porous carbon electrodes. However, many aspects of the fundamental mechanism of supercapacitance are still not well understood, and there is a lack of experimental techniques which are capable of studying working devices. Recently, solid-state NMR has emerged as a powerful tool for studying the local environments and behaviour of electrolyte ions in supercapacitor electrodes. In this Trends article, we review these recent developments and applications. We first discuss the basic principles underlying the mechanism of supercapacitance, as well as the key NMR observables that are relevant to the study of supercapacitor electrodes. We then review some practical aspects of the study of working devices using ex situ and in situ methodologies and explain the key advances that these techniques have allowed on the study of supercapacitor charging mechanisms. NMR experiments have revealed that the pores of the carbon electrodes contain a significant number of electrolyte ions in the absence of any charging potential. This has important implications for the molecular mechanisms of supercapacitance, as charge can be stored by different ion adsorption/desorption processes. Crucially, we show how in situ NMR experiments can be used to quantitatively study and characterise the charging mechanism, with the experiments providing the most detailed picture of charge storage to date, offering the opportunity to design enhanced devices. Finally, an outlook for future directions for solid-state NMR in supercapacitor research is offered.
Co-reporter:Tao Liu;Javier Carretero-González;Elizabeth Castillo-Martínez;Gunwoo Kim;Paul M. Bayley;Zigeng Liu
Science 2016 Volume 352(Issue 6286) pp:
Publication Date(Web):
DOI:10.1126/science.aad8843
Abstract
Lithium-oxygen (Li-O2) batteries cycle reversibly with lithium iodide (LiI) additives in dimethoxyethane (DME) to form lithium hydroxide (LiOH). Viswanathan et al. argue that because the standard redox potential of the four-electron (e−) reaction, 4OH– ↔ 2H2O + O2 + 4e–, is at 3.34 V versus Li+/Li, LiOH cannot be removed by the triiodide ion (I3–). However, under nonaqueous conditions, this reaction will occur at a different potential. LiOH also reacts chemically with I3– to form IO3–, further studies being required to determine the relative rates of the two reactions on electrochemical charge.
We described a lithium-oxygen (Li-O2) battery comprising a graphene electrode, a dimethoxyethane-based electrolyte, and H2O and lithium iodide (LiI) additives, lithium hydroxide (LiOH) being the predominant discharge product. We demonstrate, in contrast to the work of Shen et al., that the chemical reactivity between LiOH and the triiodide ion (I3–) to form IO3– indicates that LiOH can be removed on charging; the electrodes do not clog, even after multiple cycles, confirming that solid products are reversibly removed.
Co-reporter:Ieuan D. Seymour, David J. Wales, and Clare P. Grey
The Journal of Physical Chemistry C 2016 Volume 120(Issue 35) pp:19521-19530
Publication Date(Web):August 5, 2016
DOI:10.1021/acs.jpcc.6b05307
Layered LiMnO2 is a potential Li ion cathode material that is known to undergo a layered to spinel transformation upon delithiation, as a result of Mn migration. A common strategy to improve the structural stability of LiMnO2 has been to replace Mn with a range of metal dopants, although the mechanism by which each dopant stabilizes the structure is not well understood. In this work we characterize ion-migration barriers using hybrid eigenvector-following (EF) and density functional theory to study how trivalent dopants (Al3+, Cr3+, Fe3+, Ga3+, Sc3+, and In3+) affect Mn migration during the initial stage of the layered to spinel transformation in Li0.5MnO2. We demonstrate that dopants with small ionic radii, such as Al3+ and Cr3+, can increase the barrier for migration, but only when they are located in the first cation coordination sphere of Mn. We also demonstrate how the hybrid EF approach can be used to study the migration barriers of dopant species within the structure of Li0.5MnO2 efficiently. The transition state searching methodology described in this work will be useful for studying the effects of dopants on structural transformation mechanisms in a wide range of technologically interesting energy materials.
Co-reporter:Tao Liu, Gunwoo Kim, Mike T. L. Casford, and Clare P. Grey
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 23) pp:4841-4846
Publication Date(Web):November 14, 2016
DOI:10.1021/acs.jpclett.6b02267
Superoxide-based nonaqueous metal–oxygen batteries have received considerable research attention as they exhibit high energy densities and round-trip efficiencies. The cycling performance, however, is still poor. Here we study the cycling characteristic of a Na–O2 battery using solid-state nuclear magnetic resonance, Raman spectroscopy, and scanning electron microscopy. We find that the poor cycling performance is primarily caused by the considerable side reactions stemming from the chemical aggressiveness of NaO2 as both a solid-phase and dissolved species in the electrolyte. The side reaction products cover electrode surfaces and hinder electron transfer across the electrode–electrolyte interface, being a major reason for cell failure. In addition, the available electrode surface and porosity change considerably during cell discharging and charging, affecting the diffusion of soluble species (superoxide and water) and resulting in inhomogeneous reactions across the electrode. This study provides insights into the challenges associated with achieving long-lived superoxide-based metal–O2 batteries.
Co-reporter:Sylvia Britto; Michal Leskes; Xiao Hua; Claire-Alice Hébert; Hyeon Suk Shin; Simon Clarke; Olaf Borkiewicz; Karena W. Chapman; Ram Seshadri; Jaephil Cho
Journal of the American Chemical Society 2015 Volume 137(Issue 26) pp:8499-8508
Publication Date(Web):June 8, 2015
DOI:10.1021/jacs.5b03395
Vanadium sulfide VS4 in the patronite mineral structure is a linear chain compound comprising vanadium atoms coordinated by disulfide anions [S2]2–. 51V NMR shows that the material, despite having V formally in the d1 configuration, is diamagnetic, suggesting potential dimerization through metal–metal bonding associated with a Peierls distortion of the linear chains. This is supported by density functional calculations, and is also consistent with the observed alternation in V–V distances of 2.8 and 3.2 Å along the chains. Partial lithiation results in reduction of the disulfide ions to sulfide S2–, via an internal redox process whereby an electron from V4+ is transferred to [S2]2– resulting in oxidation of V4+ to V5+ and reduction of the [S2]2– to S2– to form Li3VS4 containing tetrahedral [VS4]3– anions. On further lithiation this is followed by reduction of the V5+ in Li3VS4 to form Li3+xVS4 (x = 0.5–1), a mixed valent V4+/V5+ compound. Eventually reduction to Li2S plus elemental V occurs. Despite the complex redox processes involving both the cation and the anion occurring in this material, the system is found to be partially reversible between 0 and 3 V. The unusual redox processes in this system are elucidated using a suite of short-range characterization tools including 51V nuclear magnetic resonance spectroscopy (NMR), S K-edge X-ray absorption near edge spectroscopy (XANES), and pair distribution function (PDF) analysis of X-ray data.
Co-reporter:Gunwoo Kim; John M. Griffin; Frédéric Blanc; Sossina M. Haile
Journal of the American Chemical Society 2015 Volume 137(Issue 11) pp:3867-3876
Publication Date(Web):March 3, 2015
DOI:10.1021/jacs.5b00280
17O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ≥107 Hz. Variable-temperature 17O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P–O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P–O2, P–O3d, and P–O3a axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1–H1 bond. The activation energy of this process is slightly lower than that obtained from the 1H exchange process (0.70 ± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C 2013, 117, 6504−6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps.
Co-reporter:Alexander C. Forse; John M. Griffin; Céline Merlet; Paul M. Bayley; Hao Wang; Patrice Simon
Journal of the American Chemical Society 2015 Volume 137(Issue 22) pp:7231-7242
Publication Date(Web):May 14, 2015
DOI:10.1021/jacs.5b03958
Ionic liquids are emerging as promising new electrolytes for supercapacitors. While their higher operating voltages allow the storage of more energy than organic electrolytes, they cannot currently compete in terms of power performance. More fundamental studies of the mechanism and dynamics of charge storage are required to facilitate the development and application of these materials. Here we demonstrate the application of nuclear magnetic resonance spectroscopy to study the structure and dynamics of ionic liquids confined in porous carbon electrodes. The measurements reveal that ionic liquids spontaneously wet the carbon micropores in the absence of any applied potential and that on application of a potential supercapacitor charging takes place by adsorption of counterions and desorption of co-ions from the pores. We find that adsorption and desorption of anions surprisingly plays a more dominant role than that of the cations. Having elucidated the charging mechanism, we go on to study the factors that affect the rate of ionic diffusion in the carbon micropores in an effort to understand supercapacitor charging dynamics. We show that the line shape of the resonance arising from adsorbed ions is a sensitive probe of their effective diffusion rate, which is found to depend on the ionic liquid studied, as well as the presence of any solvent additives. Taken as whole, our NMR measurements allow us to rationalize the power performances of different electrolytes in supercapacitors.
Journal of the American Chemical Society 2015 Volume 137(Issue 42) pp:13612-13623
Publication Date(Web):September 30, 2015
DOI:10.1021/jacs.5b08434
The morphology of a nanomaterial (geometric shape and dimension) has a significant impact on its physical and chemical properties. It is, therefore, essential to determine the morphology of nanomaterials so as to link shape with performance in specific applications. In practice, structural features with different length scales are encoded in a specific angular range of the X-ray or neutron total scattering pattern of the material. By combining small- and wide-angle scattering (typically X-ray) experiments, the full angular range can be covered, allowing structure to be determined accurately at both the meso- and the nanoscale. In this Article, a comprehensive morphology analysis of lithium-ion battery anode material, TiO2 (B) nanoparticles (described in Ren, Y.; Liu, Z.; Pourpoint, F.; Armstrong, A. R.; Grey, C. P.; Bruce, P. G. Angew. Chem. Int. Ed. 2012, 51, 2164), incorporating structure modeling with small-angle X-ray scattering (SAXS), pair distribution function (PDF), and X-ray powder diffraction (XRPD) techniques, is presented. The particles are oblate-shaped, contracted along the [010] direction, this particular morphology providing a plausible rationale for the excellent electrochemical behavior of these TiO2(B) nanoparticles, while also provides a structural foundation to model the strain-driven distortion induced by lithiation. The work demonstrates the importance of analyzing various structure features at multiple length scales to determine the morphologies of nanomaterials.
Co-reporter:Hee Jung Chang; Andrew J. Ilott; Nicole M. Trease; Mohaddese Mohammadi; Alexej Jerschow
Journal of the American Chemical Society 2015 Volume 137(Issue 48) pp:15209-15216
Publication Date(Web):November 2, 2015
DOI:10.1021/jacs.5b09385
Lithium dendrite growth in lithium ion and lithium rechargeable batteries is associated with severe safety concerns. To overcome these problems, a fundamental understanding of the growth mechanism of dendrites under working conditions is needed. In this work, in situ 7Li magnetic resonance (MRI) is performed on both the electrolyte and lithium metal electrodes in symmetric lithium cells, allowing the behavior of the electrolyte concentration gradient to be studied and correlated with the type and rate of microstructure growth on the Li metal electrode. For this purpose, chemical shift (CS) imaging of the metal electrodes is a particularly sensitive diagnostic method, enabling a clear distinction to be made between different types of microstructural growth occurring at the electrode surface and the eventual dendrite growth between the electrodes. The CS imaging shows that mossy types of microstructure grow close to the surface of the anode from the beginning of charge in every cell studied, while dendritic growth is triggered much later. Simple metrics have been developed to interpret the MRI data sets and to compare results from a series of cells charged at different current densities. The results show that at high charge rates, there is a strong correlation between the onset time of dendrite growth and the local depletion of the electrolyte at the surface of the electrode observed both experimentally and predicted theoretical (via the Sand’s time model). A separate mechanism of dendrite growth is observed at low currents, which is not governed by salt depletion in the bulk liquid electrolyte. The MRI approach presented here allows the rate and nature of a process that occurs in the solid electrode to be correlated with the concentrations of components in the electrolyte.
Co-reporter:Hyeyoung Jung, Phoebe K. Allan, Yan-Yan Hu, Olaf J. Borkiewicz, Xiao-Liang Wang, Wei-Qiang Han, Lin-Shu Du, Chris J. Pickard, Peter J. Chupas, Karena W. Chapman, Andrew J. Morris, and Clare P. Grey
Chemistry of Materials 2015 Volume 27(Issue 3) pp:1031
Publication Date(Web):January 5, 2015
DOI:10.1021/cm504312x
Metallic germanium is a promising anode material in secondary lithium-ion batteries (LIBs) due to its high theoretical capacity (1623 mAh/g) and low operating voltage, coupled with the high lithium-ion diffusivity and electronic conductivity of lithiated Ge. Here, the lithiation mechanism of micron-sized Ge anodes has been investigated with X-ray diffraction (XRD), pair distribution function (PDF) analysis, and in-/ex-situ high-resolution 7Li solid-state nuclear magnetic resonance (NMR), utilizing the structural information and spectroscopic fingerprints obtained by characterizing a series of relevant LixGey model compounds. In contrast to previous work, which postulated the formation of Li9Ge4 upon initial lithiation, we show that crystalline Ge first reacts to form a mixture of amorphous and crystalline Li7Ge3 (space group P3212). Although Li7Ge3 was proposed to be stable in a recent theoretical study of the Li–Ge phase diagram (Morris, A. J.; Grey, C. P.; Pickard, C. J. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 054111), it had not been identified in prior experimental studies. Further lithiation results in the transformation of Li7Ge3, via a series of disordered phases with related structural motifs, to form a phase that locally resembles Li7Ge2, a process that involves the gradual breakage of the Ge–Ge bonds in the Ge–Ge dimers (dumbbells) on lithiation. Crystalline Li15Ge4 then grows, with an overlithiated phase, Li15+δGe4, being formed at the end of discharge. This study provides comprehensive experimental evidence, by using techniques that probe short-, medium-, and long-range order, for the structural transformations that occur on electrochemical lithiation of Ge; the results are consistent with corresponding theoretical studies regarding stable lithiated LixGey phases.
Co-reporter:Fiona C. Strobridge, Bernardo Orvananos, Mark Croft, Hui-Chia Yu, Rosa Robert, Hao Liu, Zhong Zhong, Thomas Connolley, Michael Drakopoulos, Katsuyo Thornton, and Clare P. Grey
Chemistry of Materials 2015 Volume 27(Issue 7) pp:2374
Publication Date(Web):February 27, 2015
DOI:10.1021/cm504317a
Nanosized, carbon-coated LiFePO4 (LFP) is a promising cathode for Li-ion batteries. However, nano-particles are problematic for electrode design, optimized electrodes requiring high tap densities, good electronic wiring, and a low tortuosity for efficient Li diffusion in the electrolyte in between the solid particles, conditions that are difficult to achieve simultaneously. Using in situ energy-dispersive X-ray diffraction, we map the evolution of the inhomogeneous electrochemical reaction in LFP-electrodes. On the first cycle, the dynamics are limited by Li diffusion in the electrolyte at a cycle rate of C/7. On the second cycle, there appear to be two rate-limiting processes: Li diffusion in the electrolyte and electronic conductivity through the electrode. Three-dimensional modeling based on porous electrode theory shows that this change in dynamics can be reproduced by reducing the electronic conductivity of the composite electrode by a factor of 8 compared to the first cycle. The poorer electronic wiring could result from the expansion and contraction of the particles upon cycling and/or the formation of a solid-electrolyte interphase layer. A lag was also observed perpendicular to the direction of the current: the LFP particles at the edges of the cathode reacted preferentially to those in the middle, owing to the closer proximity to the electrolyte source. Simulations show that, at low charge rates, the reaction becomes more uniformly distributed across the electrode as the porosity or the width of the particle-size distribution is increased. However, at higher rates, the reaction becomes less uniform and independent of the particle-size distribution.
Co-reporter:Jongsik Kim, Andrew J. Ilott, Derek S. Middlemiss, Natasha A. Chernova, Nathan Pinney, Dane Morgan, and Clare P. Grey
Chemistry of Materials 2015 Volume 27(Issue 11) pp:3966
Publication Date(Web):May 13, 2015
DOI:10.1021/acs.chemmater.5b00856
Although substitution of aluminum into iron oxides and oxyhydroxides has been extensively studied, it is difficult to obtain accurate incorporation levels. Assessing the distribution of dopants within these materials has proven especially challenging because bulk analytical techniques cannot typically determine whether dopants are substituted directly into the bulk iron oxide or oxyhydroxide phase or if they form separate, minor phase impurities. These differences have important implications for the chemistry of these iron-containing materials, which are ubiquitous in the environment. In this work, 27Al and 2H NMR experiments are performed on series of Al-substituted goethite, lepidocrocite, and 2-line ferrihydrite in order to develop an NMR method to track Al substitution. The extent of Al substitution into the structural frameworks of each compound is quantified by comparing quantitative 27Al MAS NMR results with those from elemental analysis. Magnetic measurements are performed for the goethite series to compare with NMR measurements. Static 27Al spin–echo mapping experiments are used to probe the local environments around the Al substituents, providing clear evidence that they are incorporated into the bulk iron phases. Predictions of the 2H and 27Al NMR hyperfine contact shifts in Al-doped goethite and lepidocrocite, obtained from a combined first-principles and empirical magnetic scaling approach, give further insight into the distribution of the dopants within these phases.
Co-reporter:Rıza Dervişoğlu, Derek S. Middlemiss, Frédéric Blanc, Yueh-Lin Lee, Dane Morgan, and Clare P. Grey
Chemistry of Materials 2015 Volume 27(Issue 11) pp:3861
Publication Date(Web):May 1, 2015
DOI:10.1021/acs.chemmater.5b00328
A structural characterization of the hydrated form of the brownmillerite-type phase Ba2In2O5, Ba2In2O4(OH)2, is reported using experimental multinuclear NMR spectroscopy and density functional theory (DFT) energy and GIPAW NMR calculations. When the oxygen ions from H2O fill the inherent O vacancies of the brownmillerite structure, one of the water protons remains in the same layer (O3) while the second proton is located in the neighboring layer (O2) in sites with partial occupancies, as previously demonstrated by Jayaraman et al. ( Solid State Ionics 2004, 170, 25−32) using X-ray and neutron studies. Calculations of possible proton arrangements within the partially occupied layer of Ba2In2O4(OH)2 yield a set of low energy structures; GIPAW NMR calculations on these configurations yield 1H and 17O chemical shifts and peak intensity ratios, which are then used to help assign the experimental MAS NMR spectra. Three distinct 1H resonances in a 2:1:1 ratio are obtained experimentally, the most intense resonance being assigned to the proton in the O3 layer. The two weaker signals are due to O2 layer protons, one set hydrogen bonding to the O3 layer and the other hydrogen bonding alternately toward the O3 and O1 layers. 1H magnetization exchange experiments reveal that all three resonances originate from protons in the same crystallographic phase, the protons exchanging with each other above approximately 150 °C. Three distinct types of oxygen atoms are evident from the DFT GIPAW calculations bare oxygens (O), oxygens directly bonded to a proton (H-donor O), and oxygen ions that are hydrogen bonded to a proton (H-acceptor O). The 17O calculated shifts and quadrupolar parameters are used to assign the experimental spectra, the assignments being confirmed by 1H–17O double resonance experiments.
Co-reporter:Shou-Hang Bo, Clare P. Grey, and Peter G. Khalifah
Chemistry of Materials 2015 Volume 27(Issue 13) pp:4630
Publication Date(Web):June 10, 2015
DOI:10.1021/acs.chemmater.5b01040
The reversible room temperature intercalation of Mg2+ ions is difficult to achieve but may offer substantial advantages in the design of next-generation batteries if this electrochemical process can be successfully realized. Two types of quadruple ribbon-type transition metal borates (MgxFe2–xB2O5 and MgVBO4) with high theoretical capacities (186 and 360 mAh/g) have been synthesized and structurally characterized through the combined Rietveld refinement of synchrotron and time-of-flight neutron diffraction data. Neither MgVBO4 nor MgxFe2–xB2O5 can be chemically oxidized at room temperature, though Mg can be dynamically removed from the latter phase at elevated temperatures (approximately 200–500 °C). It is found that Mg diffusion in the MgxFe2–xB2O5 structure is more facile for the inner two octahedral sites than for the two outer octahedral sites in the ribbons, a result supported by both the refined site occupancies after Mg removal and bond valence sum difference map calculations of diffusion paths in the pristine material. Mg diffusion in this pyroborate MgxFe2–xB2O5 framework is also found to be tolerant to the presence of Mg/Fe disorder since Mg ions can diffuse through interstitial channels which bypass Fe-containing sites.
Co-reporter:Ieuan D. Seymour, Sudip Chakraborty, Derek S. Middlemiss, David J. Wales, and Clare P. Grey
Chemistry of Materials 2015 Volume 27(Issue 16) pp:5550
Publication Date(Web):August 3, 2015
DOI:10.1021/acs.chemmater.5b01674
The migration mechanism associated with the initial layered-to-spinel transformation of partially delithiated layered LiMnO2 was studied using hybrid eigenvector-following coupled with density functional theory. The initial part of the transformation mechanism of Li0.5MnO2 involves the migration of Li into both octahedral and tetrahedral local minima within the layered structure. The next stage of the transformation process involves the migration of Mn and was found to occur through several local minima, including an intermediate square pyramidal MnO5 configuration and an independent Mn3+ to Mn2+ charge-transfer process. The migration pathways were found to be significantly affected by the size of the supercell used and the inclusion of a Hubbard U parameter in the DFT functional. The transition state searching methodology described should be useful for studying the structural rearrangements that can occur in electrode materials during battery cycling, and more generally, ionic and electronic transport phenomena in a wide range of energy materials.
Co-reporter:Alexander C. Forse, Céline Merlet, Phoebe K. Allan, Elizabeth K. Humphreys, John M. Griffin, Mesut Aslan, Marco Zeiger, Volker Presser, Yury Gogotsi, and Clare P. Grey
Chemistry of Materials 2015 Volume 27(Issue 19) pp:6848
Publication Date(Web):September 25, 2015
DOI:10.1021/acs.chemmater.5b03216
The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The ring currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use computer simulations to show that the carbon domain size increases with the temperature used in the carbon synthesis. The techniques developed here are applicable to a wide range of porous carbons and offer new insights into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated carbons.
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 34) pp:22311-22320
Publication Date(Web):29 Jul 2015
DOI:10.1039/C5CP02331A
The Carr–Purcell–Meiboom–Gill (CPMG) sequence is commonly used in high resolution NMR spectroscopy and in magnetic resonance imaging for the measurement of transverse relaxation in systems that are subject to diffusion in internal or external gradients and is superior to the Hahn echo measurement, which is more sensitive to diffusion effects. Similarly, it can potentially be used to study dynamic processes in electrode materials for lithium ion batteries. Here we compare the 7Li signal decay curves obtained with the CPMG and Hahn echo sequences under static conditions (i.e., in the absence of magic angle spinning) in paramagnetic materials with varying transition metal ion concentrations. Our results indicate that under CPMG pulse trains the lifetime of the 7Li signal is substantially extended and is correlated with the strength of the electron–nuclear interaction. Numerical simulations and analytical calculations using Floquet theory suggest that the combination of large interactions and a train of finite pulses, results in a spin locking effect which significantly slows the signal's decay. While these effects complicate the interpretation of CPMG-based investigations of diffusion and chemical exchange in paramagnetic materials, they may provide a useful approach to extend the signal's lifetime in these often fast relaxing systems, enabling the use of correlation experiments. Furthermore, these results highlight the importance of developing a deeper understanding of the effects of the large paramagnetic interactions during multiple pulse experiments in order to extend the experimental arsenal available for static and in situ NMR investigations of paramagnetic materials.
The Journal of Physical Chemistry C 2015 Volume 119(Issue 43) pp:24255-24264
Publication Date(Web):September 23, 2015
DOI:10.1021/acs.jpcc.5b06647
Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO2 looping, where the material reacts reversibly with CO2 to form Li2CO3, among other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li2CO3 has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li2CO3 over the temperature range of 293–973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy, and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li2CO3. Importantly, we have shown that these processes may be coupled in this material, with the initial carbonate ion rotations aiding the subsequent hopping of lithium ions within the structure. Additionally, this study shows that it is possible to measure dynamic processes in powder or crystalline materials indirectly through a combination of NMR spectroscopy and theoretical calculations.
The Journal of Physical Chemistry C 2015 Volume 119(Issue 29) pp:16443-16451
Publication Date(Web):May 20, 2015
DOI:10.1021/acs.jpcc.5b03396
The growth of lithium microstructures during battery cycling has, to date, prohibited the use of Li metal anodes and raises serious safety concerns even in conventional lithium-ion rechargeable batteries, particularly if they are charged at high rates. The electrochemical conditions under which these Li microstructures grow have, therefore, been investigated by in situ nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and susceptibility calculations. Lithium metal symmetric bag cells containing LiPF6 in EC/DMC electrolytes were used. Distinct 7Li NMR resonances were observed due to the Li metal bulk electrodes and microstructures, the changes in peak positions and intensities being monitored in situ during Li deposition. The changes in the NMR spectra, observed as a function of separator thickness and porosity (using Celgard and Whatmann glass microfiber membranes) and different applied pressures, were correlated with changes in the type of microstructure, by using SEM. Isotopically enriched 6Li metal electrodes were used against natural abundance predominantly 7Li metal counter electrodes to investigate radiofrequency (rf) field penetration into the Li anode and to confirm the assignment of the higher frequency peak to Li dendrites. The conclusions were supported by calculations performed to explore the effect of the different microstructures on peak position/broadening, the study showing that Li NMR spectroscopy can be used as a sensitive probe of both the amount and type of microstructure formation.
Co-reporter:Tao Liu;Michal Leskes;Wanjing Yu;Amy J. Moore;Lina Zhou;Paul M. Bayley;Gunwoo Kim
Science 2015 Volume 350(Issue 6260) pp:530-533
Publication Date(Web):30 Oct 2015
DOI:10.1126/science.aac7730
Solving the problems with Li-air batteries
Li-air batteries come as close as possible to the theoretical limits for energy density in a battery. By weight, this is roughly 10 times higher than conventional lithium-ion batteries and would be sufficient to power cars with a range comparable to those with gasoline engines. But engineering a Li-air battery has been a challenge. Liu et al. managed to overcome the remaining challenges: They were able to avoid electrode passivation, turn limited solvent stability into an advantage, eliminate the fatal problems caused by superoxides, achieve high power with negligible degradation, and even circumvent the problems of removing atmospheric water.
Nanostructured oxides find multiple uses in a diverse range of applications including catalysis, energy storage, and environmental management, their higher surface areas, and, in some cases, electronic properties resulting in different physical properties from their bulk counterparts. Developing structure-property relations for these materials requires a determination of surface and subsurface structure. Although microscopy plays a critical role owing to the fact that the volumes sampled by such techniques may not be representative of the whole sample, complementary characterization methods are urgently required. We develop a simple nuclear magnetic resonance (NMR) strategy to detect the first few layers of a nanomaterial, demonstrating the approach with technologically relevant ceria nanoparticles. We show that the 17O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency 17O chemical shifts being observed for the lower coordinated surface sites. H217O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. 17O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature. Density functional theory calculations confirm the assignments and reveal a strong dependence of chemical shift on the nature of the surface. These results open up new strategies for characterizing nanostructured oxides and their applications.
Co-reporter:Kimberly A. See ; Michal Leskes ; John M. Griffin ; Sylvia Britto ; Peter D. Matthews ; Alexandra Emly ; Anton Van der Ven ; Dominic S. Wright ; Andrew J. Morris ; Clare P. Grey ;Ram Seshadri
Journal of the American Chemical Society 2014 Volume 136(Issue 46) pp:16368-16377
Publication Date(Web):November 10, 2014
DOI:10.1021/ja508982p
The high theoretical gravimetric capacity of the Li–S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li–S phase diagram using computational techniques and complement this with an in situ 7Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li2S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li+-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li2S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li2S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.
Co-reporter:Fiona C. Strobridge, Raphaële J. Clément, Michal Leskes, Derek S. Middlemiss, Olaf J. Borkiewicz, Kamila M. Wiaderek, Karena W. Chapman, Peter J. Chupas, and Clare P. Grey
Chemistry of Materials 2014 Volume 26(Issue 21) pp:6193
Publication Date(Web):October 9, 2014
DOI:10.1021/cm502680w
In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO4 (space group Pnma) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li2/3(Co2+)2/3(Co3+)1/3PO4 phase. Two resonances are observed for Li2/3CoPO4 with intensity ratios of 2:1 and 1:1 in the 31P and 7Li NMR spectra, respectively. An ordering of Co2+/Co3+ oxidation states is proposed within a (a × 3b × c) supercell, and Li+/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co3+ ions and Li vacancies are found to order along the b-axis. Two other low energy Li+/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO4–CoPO4 convex hull and they may be readily interconverted by Li+ hops along the b-direction.
Co-reporter:Zigeng Liu, Yan-Yan Hu, Matthew T. Dunstan, Hua Huo, Xiaogang Hao, Huan Zou, Guiming Zhong, Yong Yang, and Clare P. Grey
Chemistry of Materials 2014 Volume 26(Issue 8) pp:2513
Publication Date(Web):March 24, 2014
DOI:10.1021/cm403728w
Na3V2(PO4)2F3 is a novel electrode material that can be used in both Li ion and Na ion batteries (LIBs and NIBs). The long- and short-range structural changes and ionic and electronic mobility of Na3V2(PO4)2F3 as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution 23Na and 31P solid-state nuclear magnetic resonance (NMR). The 23Na NMR spectra and XRD refinements show that the Na ions are removed nonselectively from the two distinct Na sites, the fully occupied Na1 site and the partially occupied Na2 site, at least at the beginning of charge. Anisotropic changes in lattice parameters of the cycled Na3V2(PO4)2F3 electrode upon charge have been observed, where a (= b) continues to increase and c decreases, indicative of solid-solution processes. A noticeable decrease in the cell volume between 0.6 Na and 1 Na is observed along with a discontinuity in the 23Na hyperfine shift between 0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement of unpaired electrons within the vanadium t2g orbitals. The Na ion mobility increases steadily on charging as more Na vacancies are formed, and coalescence of the resonances from the two Na sites is observed when 0.9 Na is removed, indicating a Na1–Na2 hopping (two-site exchange) rate of ≥4.6 kHz. This rapid Na motion must in part be responsible for the good rate performance of this electrode material. The 31P NMR spectra are complex, the shifts of the two crystallograpically distinct sites being sensitive to both local Na cation ordering on the Na2 site in the as-synthesized material, the presence of oxidized (V4+) defects in the structure, and the changes of cation and electronic mobility on Na extraction. This study shows how NMR spectroscopy complemented by XRD can be used to provide insight into the mechanism of Na extraction from Na3V2(PO4)2F3 when used in a NIB.
Co-reporter:Fiona C. Strobridge, Derek S. Middlemiss, Andrew J. Pell, Michal Leskes, Raphaële J. Clément, Frédérique Pourpoint, Zhouguang Lu, John V. Hanna, Guido Pintacuda, Lyndon Emsley, Ago Samoson and Clare P. Grey
Journal of Materials Chemistry A 2014 vol. 2(Issue 30) pp:11948-11957
Publication Date(Web):09 Jun 2014
DOI:10.1039/C4TA00934G
Olivine-type LiCoPO4 (LCP) is a high energy density lithium ion battery cathode material due to the high voltage of the Co2+/Co3+ redox reaction. However, it displays a significantly poorer electrochemical performance than its more widely investigated isostructural analogue LiFePO4 (LFP). The co-substituted LiFexCo1−xPO4 olivines combine many of the positive attributes of each end member compound and are promising next-generation cathode materials. Here, the fully lithiated x = 0, 0.25, 0.5, 0.75 and 1 samples are extensively studied using 31P solid-state nuclear magnetic resonance (NMR). Practical approaches to broadband excitation and for the resolution of the isotropic resonances are described. First principles hybrid density functional calculations are performed on the Fermi contact shift (FCS) contributions of individual M–O–P pathways in the end members LFP and LCP and compared with the fitted values extracted from the LiFexCo1−xPO4 experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFexCo1−xPO4 have been calculated and used to assign the NMR spectra. Due to the additional unpaired electron in d6 Fe2+ as compared with d7 Co2+ (both high spin), LFP is expected to have larger Fermi contact shifts than LCP. However, two of the Co–O–P pathways in LCP give rise to noticeably larger shifts and the unexpected appearance of peaks outside the range delimited by the pure LFP and LCP 31P shifts. This behaviour contrasts with that observed previously in LiFexMn1−xPO4, where all 31P shifts lay within the LiMnPO4–LFP range. Although there are 24 distinct local P environments in LiFexCo1−xPO4, these group into seven resonances in the NMR spectra, due to significant overlap of the isotropic shifts. The local environments that give rise to the largest contributions to the spectral intensity are identified and used to simplify the assignment. This provides a tool for future studies of the electrochemically-cycled samples, which would otherwise be challenging to interpret.
Co-reporter:Shou-Hang Bo, Kyung-Wan Nam, Olaf J. Borkiewicz, Yan-Yan Hu, Xiao-Qing Yang, Peter J. Chupas, Karena W. Chapman, Lijun Wu, Lihua Zhang, Feng Wang, Clare P. Grey, and Peter G. Khalifah
Lithium iron borate (LiFeBO3) has a high theoretical specific capacity (220 mAh/g), which is competitive with leading cathode candidates for next-generation lithium-ion batteries. However, a major factor making it difficult to fully access this capacity is a competing oxidative process that leads to degradation of the LiFeBO3 structure. The pristine, delithiated, and degraded phases of LiFeBO3 share a common framework with a cell volume that varies by less than 2%, making it difficult to resolve the nature of the delithiation and degradation mechanisms by conventional X-ray powder diffraction studies. A comprehensive study of the structural evolution of LiFeBO3 during (de)lithiation and degradation was therefore carried out using a wide array of bulk and local structural characterization techniques, both in situ and ex situ, with complementary electrochemical studies. Delithiation of LiFeBO3 starts with the production of LitFeBO3 (t ≈ 0.5) through a two-phase reaction, and the subsequent delithiation of this phase to form Lit–xFeBO3 (x < 0.5). However, the large overpotential needed to drive the initial two-phase delithiation reaction results in the simultaneous observation of further delithiated solid-solution products of Lit–xFeBO3 under normal conditions of electrochemical cycling. The degradation of LiFeBO3 also results in oxidation to produce a Li-deficient phase D-LidFeBO3 (d ≈ 0.5, based on the observed Fe valence of ∼2.5+). However, it is shown through synchrotron X-ray diffraction, neutron diffraction, and high-resolution transmission electron microscopy studies that the degradation process results in an irreversible disordering of Fe onto the Li site, resulting in the formation of a distinct degraded phase, which cannot be electrochemically converted back to LiFeBO3 at room temperature. The Li-containing degraded phase cannot be fully delithiated, but it can reversibly cycle Li (D-Lid+yFeBO3) at a thermodynamic potential of ∼1.8 V that is substantially reduced relative to the pristine phase (∼2.8 V).
Co-reporter:Rıza Dervişoğlu, Derek S. Middlemiss, Frédéric Blanc, Lesley A. Holmes, Yueh-Lin Lee, Dane Morgan and Clare P. Grey
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 6) pp:2597-2606
Publication Date(Web):03 Dec 2013
DOI:10.1039/C3CP53642D
Structural characterization of Brownmillerite Ba2In2O5 was achieved by an approach combining experimental solid-state NMR spectroscopy, density functional theory (DFT) energetics, and GIPAW NMR calculations. While in the previous study of Ba2In2O5 by Adler et al. (S. B. Adler, J. A. Reimer, J. Baltisberger and U. Werner, J. Am. Chem. Soc., 1994, 116, 675–681), three oxygen resonances were observed in the 17O NMR spectra and assigned to the three crystallographically unique O sites, the present high resolution 17O NMR measurements under magic angle spinning (MAS) find only two resonances. The resonances have been assigned using first principles 17O GIPAW NMR calculations to the combination of the O ions connecting the InO4 tetrahedra and the O ions in equatorial sites in octahedral InO6 coordination, and to the axial O ions linking the four- and six-fold coordinated In3+ ions. Possible structural disorder was investigated in two ways: firstly, by inclusion of the high-energy structure also previously studied by Mohn et al. (C. E. Mohn, N. L. Allan, C. L. Freeman, P. Ravindran and S. Stølen, J. Solid State Chem., 2005, 178, 346–355), where the structural O vacancies are stacked rather than staggered as in Brownmillerite and, secondly, by exploring structures derived from the ground-state structure but with randomly perturbed atomic positions. There is no noticeable NMR evidence for any substantial occupancy of the high-energy structure at room temperature.
Co-reporter:Xiao Hua ; Rosa Robert ; Lin-Shu Du ; Kamila M. Wiaderek ; Michal Leskes ; Karena W. Chapman ; Peter J. Chupas
The Journal of Physical Chemistry C 2014 Volume 118(Issue 28) pp:15169-15184
Publication Date(Web):June 11, 2014
DOI:10.1021/jp503902z
Conversion materials for lithium ion batteries have recently attracted considerable attention due to their exceptional specific capacities. Some metal fluorides, such as CuF2, are promising candidates for cathode materials owing to their high operating potential, which stems from the high electronegativity of fluorine. However, the high ionicity of the metal–fluorine bond leads to a large band gap that renders these materials poor electronic conductors. Nanosizing the active material and embedding it within a conductive matrix such as carbon can greatly improve its electrochemical performance. In contrast to other fluorides, such as FeF2 and NiF2, good capacity retention has not, however, been achieved for CuF2. The reaction mechanisms that occur in the first and subsequent cycles and the reasons for the poor charge performance of CuF2 are studied in this paper via a variety of characterization methods. In situ pair distribution function analysis clearly shows CuF2 conversion in the first discharge. However, few structural changes are seen in the following charge and subsequent cycles. Cyclic voltammetry results, in combination with in situ X-ray absorption near edge structure and ex situ nuclear magnetic resonance spectroscopy, indicate that Cu dissolution is associated with the consumption of the LiF phase, which occurs during the first charge via the formation of a Cu1+ intermediate. The dissolution process consequently prevents Cu and LiF from transforming back to CuF2. Such side reactions result in negligible capacity in subsequent cycles and make this material challenging to use in a rechargeable battery.
Co-reporter:Alexander C. Forse ; John M. Griffin ; Volker Presser ; Yury Gogotsi
The Journal of Physical Chemistry C 2014 Volume 118(Issue 14) pp:7508-7514
Publication Date(Web):March 14, 2014
DOI:10.1021/jp502387x
Nuclear magnetic resonance (NMR) spectroscopy is increasingly being used to study the adsorption of molecules in porous carbons, a process which underpins applications ranging from electrochemical energy storage to water purification. Here we present density functional theory (DFT) calculations of the nucleus-independent chemical shift (NICS) near various sp2-hybridized carbon fragments to explore the structural factors that may affect the resonance frequencies observed for adsorbed species. The domain size of the delocalized electron system affects the calculated NICSs, with larger domains giving rise to larger chemical shieldings. In slit pores, overlap of the ring current effects from the pore walls is shown to increase the chemical shielding. Finally, curvature in the carbon sheets is shown to have a significant effect on the NICS. The trends observed are consistent with existing NMR results as well as new spectra presented for an electrolyte adsorbed on carbide-derived carbons prepared at different temperatures.
Co-reporter:Hao Liu;Fiona C. Strobridge;Olaf J. Borkiewicz;Kamila M. Wiaderek;Karena W. Chapman;Peter J. Chupas
Science 2014 Volume 344(Issue 6191) pp:
Publication Date(Web):27 Jun 2014
DOI:10.1126/science.1252817
Structured Abstract
Introduction
The ability to achieve high cycling rates in a lithium-ion battery is limited by the Li transport within the electrolyte; the transport of Li ions and electrons within the electrodes; and, when a phase transformation is induced as a result of the Li compositional changes within an electrode, the nucleation and growth of the second phase. The absence of a phase transformation involving substantial structural rearrangements and large volume changes is generally considered to be key for achieving high rates. This assumption has been challenged by the discovery that some nanoparticulate electrode materials, most notably LiFePO4, can be cycled in a battery at very high rates, even though they cycle between two phases during battery operation. This apparent contradiction has been reconciled by the hypothesis that a nonequilibrium solid solution can be formed during reaction to bypass the nucleation step.
Phase transformation from LiFePO4 (blue) to FePO4 (red). The delithiation (indicated by yellow arrows) proceeds at high rates via the formation of a nonequilibrium solid solution phase LixFePO4 (intermediate purple color), avoiding a classical nucleation process (indicated by dashed arrows). When the reaction is interrupted, the particles relax into the equilibrium configuration (shaded region), where only single-phase particles of LiFePO4 and/or FePO4 are present.
Rationale
To test this proposal, in situ techniques with high temporal resolution must be used to capture the fast phase transformation processes. We performed in situ synchrotron x-ray diffraction (XRD), which readily detects the structural changes and allows for fast data collection, on a LiFePO4-Li battery at high cycling rates, conditions that are able to drive the system away from equilibrium. We used an electrode comprising ~190-nm LiFePO4 particles, carbon, and binder (30:60:10 weight %), along with an electrochemical cell designed to yield reproducible results over multiple cycles, even at high rates. The high carbon content ensures that the reaction at high rates is not limited by either the electronic conductivity or ionic diffusion within the electrode composite. We compared the experimental results with simulated XRD patterns, in which the effects of strain versus compositional variation were explored. We then adapted a whole-pattern fitting method to quantify the compositional variation in the electrode during cycling.
Results
The XRD patterns, collected during high-rate galvanostatic cycling, show the expected disappearance of LiFePO4 Bragg reflections on charge and the simultaneous formation of FePO4 reflections. In addition, the development of positive intensities between the LiFePO4 and FePO4 reflections indicates that particles with lattice parameters that deviate from the equilibrium values of LiFePO4 and FePO4 are formed. The phenomenon is more pronounced at high currents. Detailed simulations of the XRD patterns reveal that this lattice-parameter variation cannot be explained by a LiFePO4-FePO4 interface within the particles, unless the size of the interface is similar to or greater than the size of the entire particle. Instead, the results indicate compositional variation either within or between particles.
Conclusion
The results demonstrate the formation of a nonequilibrium solid solution phase, LixFePO4(0 < x < 1), during high-rate cycling, with compositions that span the entire composition between two thermodynamic phases, LiFePO4 and FePO4. This confirms the hypothesis that phase transformations in nanoparticulate LiFePO4 proceed, at least at high rates, via a continuous change in structure rather than a distinct moving phase boundary between LiFePO4 and FePO4. The ability of LiFePO4 to transform via a nonequilibrium single-phase solid solution, which avoids major structural rearrangement across a moving interface, helps to explain its high-rate performance despite a large Li miscibility gap at room temperature. The creation of a low-energy nonequilibrium path by, for example, particle size reduction or cation doping should enable the high-rate capabilities of other phase-transforming electrode materials.
Co-reporter:Frédéric Blanc, Michal Leskes, and Clare P. Grey
Accounts of Chemical Research 2013 Volume 46(Issue 9) pp:1952
Publication Date(Web):June 21, 2013
DOI:10.1021/ar400022u
Electrochemical cells, in the form of batteries (or supercapacitors) and fuel cells, are efficient devices for energy storage and conversion. These devices show considerable promise for use in portable and static devices to power electronics and various modes of transport and to produce and store electricity both locally and on the grid. For example, high power and energy density lithium-ion batteries are being developed for use in hybrid electric vehicles where they improve the efficiency of fuel use and help to reduce greenhouse gas emissions. To gain insight into the chemical reactions involving the multiple components (electrodes, electrolytes, interfaces) in the electrochemical cells and to determine how cells operate and how they fail, researchers ideally should employ techniques that allow real-time characterization of the behavior of the cells under operating conditions. This Account reviews the recent use of in situ solid-state NMR spectroscopy, a technique that probes local structure and dynamics, to study these devices.In situ NMR studies of lithium-ion batteries are performed on the entire battery, by using a coin cell design, a flat sealed plastic bag, or a cylindrical cell. The battery is placed inside the NMR coil, leads are connected to a potentiostat, and the NMR spectra are recorded as a function of state of charge. 7Li is used for many of these experiments because of its high sensitivity, straightforward spectral interpretation, and relevance to these devices. For example, 7Li spectroscopy was used to detect intermediates formed during electrochemical cycling such as LixC and LiySiz species in batteries with carbon and silicon anodes, respectively. It was also used to observe and quantify the formation and growth of metallic lithium microstructures, which can cause short circuits and battery failure. This approach can be utilized to identify conditions that promote dendrite formation and whether different electrolytes and additives can help prevent dendrite formation. The in situ method was also applied to monitor (by 11B NMR) electrochemical double-layer formation in supercapacitors in real time. Though this method is useful, it comes with challenges. The separation of the contributions from the different cell components in the NMR spectra is not trivial because of overlapping resonances. In addition, orientation-dependent NMR interactions, including the spatial- and orientation-dependent bulk magnetic susceptibility (BMS) effects, can lead to resonance broadening. Efforts to understand and mitigate these BMS effects are discussed in this Account.The in situ NMR investigation of fuel cells initially focused on the surface electrochemistry at the electrodes and the electrochemical oxidation of methanol and CO to CO2 on the Pt cathode. On the basis of the 13C and 195Pt NMR spectra of the adsorbates and electrodes, CO adsorbed on Pt and other reaction intermediates and complete oxidation products were detected and their mode of binding to the electrodes investigated. Appropriate design and engineering of the NMR hardware has allowed researchers to integrate intact direct methanol fuel cells into NMR probes. Chemical transformations of the circulating methanol could be followed and reaction intermediates could be detected in real time by either 2H or 13C NMR spectroscopy. By use of the in situ NMR approach, factors that control fuel cell performance, such as methanol cross over and catalyst performance, were identified.
Co-reporter:Hao Wang ; Alexander C. Forse ; John M. Griffin ; Nicole M. Trease ; Lorie Trognko ; Pierre-Louis Taberna ; Patrice Simon
Journal of the American Chemical Society 2013 Volume 135(Issue 50) pp:18968-18980
Publication Date(Web):November 25, 2013
DOI:10.1021/ja410287s
Electrochemical capacitors, commonly known as supercapacitors, are important energy storage devices with high power capabilities and long cycle lives. Here we report the development and application of in situ nuclear magnetic resonance (NMR) methodologies to study changes at the electrode–electrolyte interface in working devices as they charge and discharge. For a supercapacitor comprising activated carbon electrodes and an organic electrolyte, NMR experiments carried out at different charge states allow quantification of the number of charge storing species and show that there are at least two distinct charge storage regimes. At cell voltages below 0.75 V, electrolyte anions are increasingly desorbed from the carbon micropores at the negative electrode, while at the positive electrode there is little change in the number of anions that are adsorbed as the voltage is increased. However, above a cell voltage of 0.75 V, dramatic increases in the amount of adsorbed anions in the positive electrode are observed while anions continue to be desorbed at the negative electrode. NMR experiments with simultaneous cyclic voltammetry show that supercapacitor charging causes marked changes to the local environments of charge storing species, with periodic changes of their chemical shift observed. NMR calculations on a model carbon fragment show that the addition and removal of electrons from a delocalized system should lead to considerable increases in the nucleus-independent chemical shift of nearby species, in agreement with our experimental observations.
Co-reporter:Derek S. Middlemiss, Andrew J. Ilott, Raphaële J. Clément, Fiona C. Strobridge, and Clare P. Grey
Chemistry of Materials 2013 Volume 25(Issue 9) pp:1723-1734
Publication Date(Web):April 1, 2013
DOI:10.1021/cm400201t
Solid-state nuclear magnetic resonance (NMR) of paramagnetic samples has the potential to provide a detailed insight into the environments and processes occurring in a wide range of technologically-relevant phases, but the acquisition and interpretation of spectra is typically not straightforward. Structural complexity and/or the occurrence of charge or orbital ordering further compound such difficulties. In response to such challenges, the present article outlines how the total Fermi contact (FC) shifts of NMR observed centers (OCs) may be decomposed into sets of pairwise metal–OC bond pathway contributions via solid-state hybrid density functional theory calculations. A generally applicable “spin flipping” approach is outlined wherein bond pathway contributions are obtained by the reversal of spin moments at selected metal sites. The applications of such pathway contributions in interpreting the NMR spectra of structurally and electronically complex phases are demonstrated in a range of paramagnetic Li-ion battery positive electrodes comprising layered LiNiO2, LiNi0.125Co0.875O2, and LiCr0.125Co0.875O2 oxides; and olivine-type LiMPO4 and MPO4 (M = Mn, Fe, and Co) phosphates. The FC NMR shifts of all 6/7Li and 31P sites are decomposed, providing unambiguous NMR-based proof of the existence of local Ni3+-centered Jahn–Teller distortions in LiNiO2 and LiNi0.125Co0.875O2, and showing that the presence of M2+/M3+ solid solutions and/or M/M′ isovalent transition metal (TM) mixtures in the olivine-type electrodes should lead to broad and potentially interpretable NMR spectra. Clear evidence for the presence of a dynamic Jahn–Teller distortion is obtained for LiNixCo1–xO2. The results emphasize the utility of solid-state NMR in application to TM-containing battery materials and to paramagnetic samples in general.
Co-reporter:Matthew T. Dunstan, Wen Liu, Adriano F. Pavan, Justin A. Kimpton, Chris D. Ling, Stuart A. Scott, John S. Dennis, and Clare P. Grey
Chemistry of Materials 2013 Volume 25(Issue 24) pp:4881
Publication Date(Web):November 15, 2013
DOI:10.1021/cm402875v
A novel compound for carbon capture and storage (CCS) applications, the 6H perovskite Ba4Sb2O9, was found to be able to absorb CO2 through a chemical reaction at 873 K to form barium carbonate and BaSb2O6. This absorption was shown to be reversible through the regeneration of the original Ba4Sb2O9 material upon heating above 1223 K accompanied by the release of CO2. A combined synchrotron X-ray diffraction, thermogravimetric, and microscopy study was carried out to characterize first the physical absorption properties and then to analyze the structural evolution and formation of phases in situ. Importantly, through subsequent carbonation and regeneration of the material over 100 times, it was shown that the combined absorption and regeneration reactions proceed without any significant reduction in the CO2 absorption capacity of the material. After 100 cycles the capacity of Ba4Sb2O9 was ∼0.1 g (CO2)/g (sorbent), representing 73% of the total molar capacity. This is the first report of a perovskite-type material showing such good properties, opening the way for studies of new classes of inorganic oxide materials with stable and flexible chemical compositions and structures for applications in carbon capture.Keywords: carbon capture and storage; CO2 absorption; perovskite; thermogravimetric analysis; X-ray diffraction;
A series of layered oxides within the NaxNiix/2Mn1–x/2O2 (2/3 ≤ x ≤ 1) system were synthesized by classical solid-state methodologies. A study of their long and short-range structure was undertaken by combining X-ray diffraction and NMR spectroscopy. A transition from P2 to O3 stacking was observed at x > 0.8 when samples were made at 900 °C, which was accompanied by disordering of ions in the transition metal layer. The magnetic properties of the materials were consistent with this picture of ordering, with all samples showing antiferromagnetic character. At x = 2/3, competition between a P2 and a P3 structure, with different degrees of transition metal ordering, was found depending on the synthesis temperature. Na/Li exchange led to structures with octahedral or tetrahedral coordination of the alkali metal, and Li/Ni crystallographic exchange in the resulting O3 phases. The transition from alkali metal prismatic coordination to octahedral/tetrahedral coordination involves [TMO6]∞ layer shearing that induces some structural disorder through the formation of stacking faults.
Co-reporter:Alexander C. Forse, John M. Griffin, Hao Wang, Nicole M. Trease, Volker Presser, Yury Gogotsi, Patrice Simon and Clare P. Grey
Physical Chemistry Chemical Physics 2013 vol. 15(Issue 20) pp:7722-7730
Publication Date(Web):26 Mar 2013
DOI:10.1039/C3CP51210J
A detailed understanding of ion adsorption within porous carbon is key to the design and improvement of electric double-layer capacitors, more commonly known as supercapacitors. In this work nuclear magnetic resonance (NMR) spectroscopy is used to study ion adsorption in porous carbide-derived carbons. These predominantly microporous materials have a tuneable pore size which enables a systematic study of the effect of pore size on ion adsorption. Multinuclear NMR experiments performed on the electrolyte anions and cations reveal two main environments inside the carbon. In-pore ions (observed at low frequencies) are adsorbed inside the pores, whilst ex-pore ions (observed at higher frequencies) are not adsorbed and are in large reservoirs of electrolyte between carbon particles. All our experiments were carried out in the absence of an applied electrical potential in order to assess the mechanisms related to ion adsorption without the contribution of electrosorption. Our results indicate similar adsorption behaviour for anions and cations. Furthermore, we probe the effect of sample orientation, which is shown to have a marked effect on the NMR spectra. Finally, we show that a 13C → 1H cross polarisation experiment enables magnetisation transfer from the carbon architecture to the adsorbed species, allowing selective observation of the adsorbed ions and confirming our spectral assignments.
Co-reporter:Michal Leskes, Amy J. Moore, Gillian R. Goward, and Clare P. Grey
The Journal of Physical Chemistry C 2013 Volume 117(Issue 51) pp:26929-26939
Publication Date(Web):November 27, 2013
DOI:10.1021/jp410429k
A multi-nuclear solid-state NMR approach is employed to investigate the lithium–air battery, to monitor the evolution of the electrochemical products formed during cycling, and to gain insight into processes affecting capacity fading. While lithium peroxide is identified by 17O solid state NMR (ssNMR) as the predominant product in the first discharge in 1,2-dimethoxyethane (DME) based electrolytes, it reacts with the carbon cathode surface to form carbonate during the charging process. 13C ssNMR provides evidence for carbonate formation on the surface of the carbon cathode, the carbonate being removed at high charging voltages in the first cycle, but accumulating in later cycles. Small amounts of lithium hydroxide and formate are also detected in discharged cathodes and while the hydroxide formation is reversible, the formate persists and accumulates in the cathode upon further cycling. The results indicate that the rechargeability of the battery is limited by both the electrolyte and the carbon cathode stability. The utility of ssNMR spectroscopy in directly detecting product formation and decomposition within the battery is demonstrated, a necessary step in the assessment of new electrolytes, catalysts, and cathode materials for the development of a viable lithium–oxygen battery.
Co-reporter:Gunwoo Kim, Frédéric Blanc, Yan-Yan Hu, and Clare P. Grey
The Journal of Physical Chemistry C 2013 Volume 117(Issue 13) pp:6504-6515
Publication Date(Web):February 28, 2013
DOI:10.1021/jp312410t
Local dynamics and hydrogen bonding in CsH2PO4 have been investigated by 1H, 2H, and 31P solid-state NMR spectroscopy to help provide a detailed understanding of proton conduction in the paraelectric phase. Two distinct environments are observed by 1H and 2H NMR, and their chemical shifts (1H) and quadrupolar coupling constants (2H) are consistent with one strong and one slightly weaker H-bonding environment. Two different protonic motions are detected by variable-temperature 1H MAS NMR and T1 spin–lattice relaxation time measurements in the paraelectric phase, which we assign to librational and long-range translational motions. An activation energy of 0.70 ± 0.07 eV is extracted for the latter motion; that of the librational motion is much lower. 31P NMR line shapes are measured under MAS and static conditions, and spin–lattice relaxation time measurements have been performed as a function of temperature. Although the 31P line shape is sensitive to the protonic motion, the reorientation of the phosphate ions does not lead to a significant change in the 31P CSA tensor. Rapid protonic motion and rotation of the phosphate ions is seen in the superprotonic phase, as probed by the T1 measurements along with considerable line narrowing of both the 1H and the 31P NMR signals.
Co-reporter:Yuri Janssen ; Derek S. Middlemiss ; Shou-Hang Bo ; Clare P. Grey ;Peter G. Khalifah
Journal of the American Chemical Society 2012 Volume 134(Issue 30) pp:12516-12527
Publication Date(Web):June 18, 2012
DOI:10.1021/ja301881c
The crystal structure of the promising Li-ion battery cathode material LiFeBO3 has been redetermined based on the results of single crystal X-ray diffraction data. A commensurate modulation that doubles the periodicity of the lattice in the a-axis direction is observed. When the structure of LiFeBO3 is refined in the 4-dimensional superspace group C2/c(α0γ)00, with α = 1/2 and γ = 0 and with lattice parameters of a = 5.1681 Å, b = 8.8687 Å, c = 10.1656 Å, and β = 91.514°, all of the disorder present in the prior C2/c structural model is eliminated and a long-range ordering of 1D chains of corner-shared LiO4 is revealed to occur as a result of cooperative displacements of Li and O atoms in the c-axis direction. Solid-state hybrid density functional theory calculations find that the modulation stabilizes the LiFeBO3 structure by 1.2 kJ/mol (12 meV/f.u.), and that the modulation disappears after delithiation to form a structurally related FeBO3 phase. The band gaps of LiFeBO3 and FeBO3 are calculated to be 3.5 and 3.3 eV, respectively. Bond valence sum maps have been used to identify and characterize the important Li conduction pathways, and suggest that the activation energies for Li diffusion will be higher in the modulated structure of LiFeBO3 than in its unmodulated analogue.
Co-reporter:Lucienne Buannic ; Frédéric Blanc ; Derek S. Middlemiss
Journal of the American Chemical Society 2012 Volume 134(Issue 35) pp:14483-14498
Publication Date(Web):June 13, 2012
DOI:10.1021/ja304712v
Hydrated BaSn1–xYxO3–x/2 is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn1–xYxO3–x/2 (0.10 ≤ x ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The 119Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to x = 0.20, while at higher substitution levels, Y–O–Y linkages are avoided, leading, at x = 0.50, to strict Y–O–Sn alternation of B-site cations. These results are confirmed by the absence of a “Y–O–Y” 17O resonance and supported by the 17O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by 89Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by 119Sn and 89Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at x = 0.50, which is ascribed to the presence of residual Sn–O–Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature 119Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y–O–Sn cation ordering, which prevents proton trapping on the more basic Y–O–Y sites.
Journal of the American Chemical Society 2012 Volume 134(Issue 23) pp:9708-9720
Publication Date(Web):May 3, 2012
DOI:10.1021/ja301963e
17O–1H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross-polarization magic angle spinning (CPMAS) NMR, CP rotational-echo double resonance (CP-REDOR) NMR, and heteronuclear correlation (HETCOR) NMR spectroscopy to distinguish between Brønsted acid sites with different O–H distances. The accessibility of the various Brønsted acid sites was quantified by adsorbing the basic probe molecule trimethylphosphine in known amounts. On the basis of these experiments, locations of different Brønsted acid sites in H-Mordenite (H-MOR) were proposed. The use of 17O chemical shift correlations to help assign sites is discussed.
Co-reporter:Rosa Robert, Dongli Zeng, Antonio Lanzirotti, Paul Adamson, Simon J. Clarke, and Clare P. Grey
Chemistry of Materials 2012 Volume 24(Issue 14) pp:2684
Publication Date(Web):June 13, 2012
DOI:10.1021/cm3005375
Ex situ and in situ Synchrotron X-ray fluorescence imaging coupled with selective micro-X-ray absorption near-edge spectroscopy (μXANES) and micro-X-ray diffraction (μXRD) were used to investigate the electrochemical lithiation of the layered oxysulfide Sr2MnO2Cu3.5S3. Microfocused X-ray fluorescence (XRF) imaging was used to image the elemental components within the battery electrode while μXANES and μXRD provided information about the Cu oxidation state and phase distribution, respectively. Sr2MnO2Cu3.5S3 operates by a combined insertion/displacement mechanism. After 1 mol of Li intercalation, Cu metal extrusion is observed by μXRD, which also reveals the formation of the Sr2MnO2Cu3.5–xLixS3 phase. Ex situ μXRF images of the electrode after 3.75 mol of Li intercalation show segregated Cu metal and Sr2MnO2Cu3.5–xLixS3 particles, while in situ μXRF imaging experiments reveal that the Cu and Mn elemental distribution maps are highly correlated to the particle orientation giving different results when the particle is oriented either perpendicular or parallel to the incident beam. In situ electrochemical synchrotron XRF imaging has the advantage over the ex situ mode in that it allows the reaction mechanism of a single particle to be followed vs time. In situ μXRF imaging data suggest that the microstructure of the electrode, on a microscale level, is not affected by the Cu extrusion process.Keywords: cathode materials; insertion/displacement reactions; Li-ion batteries; oxysulfides; scanning X-ray fluorescence imaging;
Co-reporter:Frédérique Pourpoint, Xiao Hua, Derek S. Middlemiss, Paul Adamson, Da Wang, Peter G. Bruce, and Clare P. Grey
Chemistry of Materials 2012 Volume 24(Issue 15) pp:2880
Publication Date(Web):June 25, 2012
DOI:10.1021/cm300662m
Pair distribution function (PDF) analyses of synchrotron data obtained for the anode materials Li1+xV1–xO2 (0 ≤ x ≤ 0.1) have been performed to characterize the short to medium range structural ordering. The data show clear evidence for the magnetically-induced distortion of the V sublattice to form trimers, the distortion persisting at even the highest excess Li content considered of x = 0.1. At least three distinct local environments were observed for the stoichiometric material LiVO2 in 6Li nuclear magnetic resonance (NMR) spectroscopy, the environments becoming progressively more disordered as the Li content increases. A two-dimensional Li–Li correlation NMR experiment (POST-C7) was used to identify the resonances corresponding to Li within the same layers. NMR spectra were acquired as a function of the state of charge, a distinct environment for Li in Li2VO2 being observed. The results suggest that disorder within the Li layers (in addition to the presence of Li within the V layers as proposed by Armstrong et al. Nat. Mater.2011, 10, 223–229) may aid the insertion of Li into the Li1+xV1–xO2 phase. The previously little-studied Li2VO2 phase was also investigated by hybrid density functional theory (DFT) calculations, providing insights into magnetic interactions, spin–lattice coupling, and Li hyperfine parameters.Keywords: 6,7Li NMR; first principles calculations; LiVO2; PDF analyses; POST-C7; spin−lattice interaction; trimer;
Co-reporter:Shou-Hang Bo, Feng Wang, Yuri Janssen, Dongli Zeng, Kyung-Wan Nam, Wenqian Xu, Lin-Shu Du, Jason Graetz, Xiao-Qing Yang, Yimei Zhu, John B. Parise, Clare P. Grey and Peter G. Khalifah
Journal of Materials Chemistry A 2012 vol. 22(Issue 18) pp:8799-8809
Publication Date(Web):2012/03/26
DOI:10.1039/C2JM16436A
Lithium iron borate (LiFeBO3) is a particularly desirable cathode material for lithium-ion batteries due to its high theoretical capacity (220 mA h g−1) and its favorable chemical constituents, which are abundant, inexpensive and non-toxic. However, its electrochemical performance appears to be severely hindered by the degradation that results from air or moisture exposure. The degradation of LiFeBO3 was studied through a wide array of ex situ and in situ techniques (X-ray diffraction, nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy and spectroscopy) to better understand the possible degradation process and to develop methods for preventing degradation. It is demonstrated that degradation involves both Li loss from the framework of LiFeBO3 and partial oxidation of Fe(II), resulting in the creation of a stable lithium-deficient phase with a similar crystal structure to LiFeBO3. Considerable LiFeBO3 degradation occurs during electrode fabrication, which greatly reduces the accessible capacity of LiFeBO3 under all but the most stringently controlled conditions for electrode fabrication. Comparative studies on micron-sized LiFeBO3 and nanoscale LiFeBO3–carbon composite showed a very limited penetration depth (∼30 nm) of the degradation phase front into the LiFeBO3 core under near-ambient conditions. Two-phase reaction regions during delithiation and lithiation of LiFeBO3 were unambiguously identified through the galvanostatic intermittent titration technique (GITT), although it is still an open question as to whether the two-phase reaction persists across the whole range of possible Li contents. In addition to the main intercalation process with a thermodynamic potential of 2.8 V, there appears to be a second reversible electrochemical process with a potential of 1.8 V. The best electrochemical performance of LiFeBO3 was ultimately achieved by introducing carbon to minimize the crystallite size and strictly limiting air and moisture exposure to inhibit degradation.
Co-reporter:Kenneth J. Rosina, Meng Jiang, Dongli Zeng, Elodie Salager, Adam S. Best and Clare P. Grey
Journal of Materials Chemistry A 2012 vol. 22(Issue 38) pp:20602-20610
Publication Date(Web):31 Aug 2012
DOI:10.1039/C2JM34114J
The structural properties of layered Li[Li1/9Ni1/3Mn5/9]O2 positive electrodes nominally coated with aluminum fluoride are studied. Coatings were prepared by using aqueous solutions with various concentrations of aluminum and fluorine and are compared with samples treated under similar conditions but with aqueous HCl solutions. Samples were investigated following heat treatment at 120 °C and 400 °C with powder X-ray diffraction, transmission electron microscopy including energy dispersive X-ray spectroscopy (TEM/EDS), elemental analysis via inductively coupled plasma-optical emission spectroscopy (ICP-EA), and both 6Li and 27Al magic angle spinning NMR spectroscopy. The TEM/EDS and 27Al NMR data provide support for an aluminum-rich amorphous coating that, following drying at 120 °C, comprises six coordinated, partially hydrated aluminum environments. Heat treatment at 400 °C results in a phase that resembles partially fluorinated γ- or γ′-Al2O3, at least locally. An Al:F ratio of 2:1 is obtained in stark contrast to the ratio used in the original solution (1:3). No AlF3 is detected by PXRD and instead some evidence for a protonated phase (formed by ion exchanging protons for lithium) is detected along with Li[Li1/9Ni1/3Mn5/9]O2 after drying. This phase disappears on heating to 400 °C, suggesting some reorganization of bulk Li[Li1/9Ni1/3Mn5/9]O2 and possibly some incorporation of Al into the structure. This is in agreement with the 6Li NMR spectra, which indicate that the local environments that are found in the Ni-free end member of the series Li[Li(1/3−2x/3)NixMn(2/3−x/3)]O2 (i.e. Li2MnO3) are enhanced on sintering.
Co-reporter:Nicole M. Trease, Lina Zhou, Hee Jung Chang, Ben Yunxu Zhu, Clare P. Grey
Solid State Nuclear Magnetic Resonance 2012 Volume 42() pp:62-70
Publication Date(Web):April 2012
DOI:10.1016/j.ssnmr.2012.01.004
The application of in situ nuclear magnetic resonance (NMR) to investigate batteries in real time (i.e., as they are cycling) provides fruitful insight into the electrochemical structural changes that occur in the battery. A major challenge for in situ static NMR spectroscopy of a battery is, however, to separate the resonances from the different components. Many resonances overlap and are broadened since spectra are acquired, to date, in static mode. Spectral analysis is also complicated by bulk magnetic susceptibility (BMS) effects. Here we describe some of the BMS effects that arise in lithium ion battery (LIB) materials and provide an outline of some of the practical considerations associated with the application of in situ NMR spectroscopy to study structural changes in energy materials.Graphical AbstractHighlights► Monitor structural changes utilizing NMR in energy materials during electrochemical cycling. ► Outline experimental considerations for in situ static NMR of energy materials. ► Complicated spectra due to bulk magnetic susceptibility effect. ► Examples given for metallic, paramagnetic, and diamagnetic lithium species. ► Orientation of battery can reduce the BMS effects.
Co-reporter:Jongsik Kim, Wei Li, Brian L. Philips and Clare P. Grey
Energy & Environmental Science 2011 vol. 4(Issue 10) pp:4298-4305
Publication Date(Web):01 Sep 2011
DOI:10.1039/C1EE02093E
Phosphate adsorption on the surfaces of the iron oxyhydroxide polymorphs goethite, akaganeite, and lepidocrocite were studied by using 31P static spin-echo mapping NMR experiments to determine how this environmentally-important anion binds to common soil minerals. The large 31P hyperfine shifts confirm the formation of inner-sphere complexes between the phosphate anion and the iron oxyhydroxide surface, the large shifts indicating the presence of Fe3+–O–P covalent bonds. Binding was explored as a function of pH and phosphate concentrations, the phosphate ion binding via two oxygen ions to the oxyhydroxide surface under all conditions and for all the surfaces. To support our analysis of the NMR spectra, adsorption of dimethyl phosphinic acid (DPA) on iron oxyhydroxides was also investigated, since this ion can only bond via one Fe–O–P interaction to the surface. The 31P hyperfine shifts observed for this anion were 50% of those seen for the phosphate anions, confirming that the phosphate ions bind to the surface via two P–O–Fe linkages.
Co-reporter:Frédéric Blanc ; Derek S. Middlemiss ; Zhehong Gan
Journal of the American Chemical Society 2011 Volume 133(Issue 44) pp:17662-17672
Publication Date(Web):September 14, 2011
DOI:10.1021/ja2053557
Doped lanthanum gallate perovskites (LaGaO3) constitute some of the most promising electrolyte materials for solid oxide fuel cells operating in the intermediate temperature regime. Here, an approach combining experimental multinuclear NMR spectroscopy with density functional theory total energy and GIPAW NMR calculations yields a comprehensive understanding of the structural and defect chemistries of Sr- and Mg-doped LaGaO3 anionic conductors. The DFT energetics demonstrate that Ga–VO–Ga (VO = oxygen vacancy) environments are favored (vs Ga–VO–Mg, Mg–VO–Mg and Mg–O–Mg–VO–Ga) across a range y = 0.0625, 0.125, and 0.25 of fractional Mg contents in LaGa1–yMgyO3–y/2. The results are interpreted in terms of doping and mean phase formation energies (relative to binary oxides) and are compared with previous calculations and experimental calorimetry data. Experimental multinuclear NMR data reveal that while Mg sites remain six-fold coordinated across the range of phase stoichiometries, albeit with significant structural disorder, a stoichiometry–dependent minority of the Ga sites resonate at a shift consistent with GaV coordination, demonstrating that O vacancies preferentially locate in the first anion coordination shell of Ga. The strong Mg–VO binding inferred by previous studies is not observed here. The 17O NMR spectra reveal distinct resonances that can be assigned by using the GIPAW NMR calculations to anions occupying equatorial and axial positions with respect to the GaV–VO axis. The disparate shifts displayed by these sites are due to the nature and extent of the structural distortions caused by the O vacancies.
Co-reporter:Hao Wang ; Thomas K.-J. Köster ; Nicole M. Trease ; Julie Ségalini ; Pierre-Louis Taberna ; Patrice Simon ; Yury Gogotsi
Journal of the American Chemical Society 2011 Volume 133(Issue 48) pp:19270-19273
Publication Date(Web):November 1, 2011
DOI:10.1021/ja2072115
11B NMR spectroscopy has been used to investigate the sorption of BF4– anions on a highly porous, high surface area carbon, and different binding sites have been identified. By implementing in situ NMR approaches, the migration of ions between the electrodes of the supercapacitors and changes in the nature of ion binding to the surface have been observed in real time.
Co-reporter:Zhouguang Lu, Hailong Chen, Rosa Robert, Ben Y. X. Zhu, Jianqiu Deng, Lijun Wu, C. Y. Chung, and Clare P. Grey
Chemistry of Materials 2011 Volume 23(Issue 11) pp:2848
Publication Date(Web):May 12, 2011
DOI:10.1021/cm200205n
The effects of citric acid (CA) and ammonium (NH4+) ions on the structural and morphological transformations of olivine LiFePO4 upon hydrothermal treatment are systematically investigated, as a function of reaction time, by using a combination of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), magic-angle-spinning nuclear magnetic resonance (MAS NMR), and Fourier transform infrared absorption spectroscopy (FTIR). In the presence of both CA and NH4+ ions, the structures evolve from amorphous precursors to crystalline (NH4)FePO4·H2O and finally LiFePO4. The initial olivine particles adopt an egglike shape and appear to form from the fusing of (NH4)FePO4·H2O plates. This metastable morphology evolves to form a mixture of cubic and rhombic particles. These particles are then etched, resulting in hollow structures and then ultimately barrel-like particles, after over 120 h of hydrothermal reaction at 180 °C. The final morphology is close to the equilibrium structure proposed by Islam et al. [Fisher, C. A. J.; Islam, M. S. J. Mater. Chem.2008, 18, 1209]. The presence of NH4+ ions (as detected by FTIR) adsorbed on the surfaces of these particles, seems to slow growth along certain directions, resulting in cubic/rhombic-shaped particles. The formation of hollow particles is ascribed to the opposing effects of etching (from CA) and surface protection (from NH4+). The electrochemical performances vary significantly with particle shape. The hollow and roughened spindle-like particles (formed in the absence of NH4+ ions) exhibit superior electrochemical properties, compared to the other particles, because of their higher specific surface areas and shorter Li+ ion diffusion lengths. The facile synthesis of olivine LiFePO4 particles with very different morphologies provides an interesting platform for further fundamental investigation into the shape-dependent electrochemical performance and electrochemical lithium intercalation and deintercalation mechanisms of olivine LiFePO4.Keywords: hydrothermal; lithium ion batteries; lithium iron phosphates; shape controlled particles;
Co-reporter:Ulla Gro Nielsen, Ivo Heinmaa, Ago Samoson, Juraj Majzlan, and Clare P. Grey
Chemistry of Materials 2011 Volume 23(Issue 13) pp:3176
Publication Date(Web):June 9, 2011
DOI:10.1021/cm2003929
Detailed insight into the magnetic properties and mobility of the different deuteron species in jarosites (AFe3(SO4)2(OD)6, A = K, Na, D3O) is obtained from variable-temperature 2H MAS NMR spectroscopy performed from 40 to 300 K. Fast MAS results in high-resolution spectra above the Néel transition temperature (i.e., in the paramagnetic regime). The 2H NMR hyperfine shift (δ), measured as a function of temperature, is a very sensitive probe of the local magnetic environment. Two different magnetic environments are observed: (i) Fe2–OD groups and D3O+ ions in stoichiometric regions of the sample. Here, the δ(2H) values are proportional to the bulk susceptibility and follow a Curie–Weiss law above 150 K. (ii) Fe-OD2 groups and D2O molecules located near the Fe3+ vacancies in the structure. The Fe3+ ions near these vacancies show strong local antiferromagnetic couplings even high above the Néel temperature (of ca. 65 K). The D2O and D3O+ ions located on the jarosite A site can be distinguished in the 2H NMR spectra due to the different temperature dependence of their isotropic shifts. Motion of the D3O+ ions was followed by investigating the isostructural (diamagnetic) compound (D3O)Al3(SO4)2(OD)6 and an activation energy of 6.3(4) kJ/mol is determined for the D3O+ motion. Our NMR results support theories that ascribe the spin glass behavior that is observed for (H3O)Fe3(SO4)2(OD)6 but not for the other cation substituted jarosites, to the disorder of the D3O+ ions and/or a less distorted Fe coordination environment. No signs of proton transfer reactions from the D3O+ ion to the framework are observed.Keywords: 2H NMR; iron sulfate; jarosite; Kagomé; paramagnetism; proton dynamics; solid-state NMR; spin-glass;
Co-reporter:Jordi Cabana, Junichi Shirakawa, Masanobu Nakayama, Masataka Wakihara and Clare P. Grey
Journal of Materials Chemistry A 2011 vol. 21(Issue 27) pp:10012-10020
Publication Date(Web):11 Mar 2011
DOI:10.1039/C0JM04197A
The structure and lithium mobility have been investigated for A- and B-Li3Fe2(PO4)3, before and after mechanical milling and lithium insertion, by using Li NMR. The data indicate that the milling step induces a significant amount of defects in the structure, while it improves the ability of the material to take up lithium. The lithium mobility in the different samples was studied by collecting NMR spectra at different temperatures, extensive lithium mobility being observed for both polytypes at temperatures above 150 °C. This mobility was found to be enhanced after milling. The enhancement in the electrode material utilization is ascribed to both a reduction of the diffusion lengths (particle size) and an increase in the intrinsic mobility of lithium in the sample.
Co-reporter:Dr. Thomas K.-J. Köster;Dr. Elodie Salager;Dr. Andrew J. Morris;Dr. Baris Key;Dr. Vincent Seznec;Dr. Mathieu Morcrette; Chris J. Pickard; Clare P. Grey
Co-reporter:Dr. Thomas K.-J. Köster;Dr. Elodie Salager;Dr. Andrew J. Morris;Dr. Baris Key;Dr. Vincent Seznec;Dr. Mathieu Morcrette; Chris J. Pickard; Clare P. Grey
Angewandte Chemie International Edition 2011 Volume 50( Issue 52) pp:12591-12594
The Journal of Physical Chemistry C 2011 Volume 115(Issue 5) pp:2030-2037
Publication Date(Web):January 19, 2011
DOI:10.1021/jp109201t
Brønsted acid sites play a key role in controlling the catalytic performances of acidic catalysts. A determination of the structure of the acid site, in particular, the O−H bond length, is important for the understanding of acid strength. The apparent O−H distances in zeolite HY and HZSM-5 extracted from 17O−1H rotational echo double resonance (REDOR) NMR data acquired at room temperature are noticeably longer than those extracted from ab initio calculations, due to the presence of some restricted motions at room temperature, such as zeolite framework vibrations and O−H librational motion. Our 17O−1H REDOR NMR results for zeolite HY and HZSM-5 obtained at a lower temperature of 183 K, where some of these motions are frozen out, are presented here. By comparing the line shapes obtained by simulation with the SIMPSON package with the experimental data, an O−H distance of ∼0.97−0.98 Å was obtained for zeolite HY, which is consistent with the previous ab initio calculation results. The results demonstrate that low-temperature REDOR NMR can provide more accurate estimates of the O−H distance, which should prove useful in structure−function correlations and as a method to explore the effects of hydrogen bonding.
Co-reporter:Baris Key ; Mathieu Morcrette ; Jean-Marie Tarascon
Journal of the American Chemical Society 2010 Volume 133(Issue 3) pp:503-512
Publication Date(Web):December 20, 2010
DOI:10.1021/ja108085d
Lithium ion batteries (LIBs) containing silicon negative electrodes have been the subject of much recent investigation, because of the extremely large gravimetric and volumetric capacities of silicon. The crystalline-to-amorphous phase transition that occurs on electrochemical Li insertion into crystalline Si, during the first discharge, hinders attempts to link the structure in these systems with electrochemical performance. We apply a combination of local structure probes, ex situ 7Li nuclear magnetic resonance (NMR) studies, and pair distribution function (PDF) analysis of X-ray data to investigate the changes in short-range order that occur during the initial charge and discharge cycles. The distinct electrochemical profiles observed subsequent to the first discharge have been shown to be associated with the formation of distinct amorphous lithiated silicide structures. For example, the first process seen on the second discharge is associated with the lithiation of the amorphous Si, forming small clusters. These clusters are broken in the second process to form isolated silicon anions. The (de)lithiation model helps explain the hysteresis and the steps in the electrochemical profile observed during the lithiation and delithiation of silicon.
Journal of the American Chemical Society 2010 Volume 133(Issue 2) pp:262-270
Publication Date(Web):December 13, 2010
DOI:10.1021/ja104852q
Morphology control of functional materials is generally performed by controlling the growth rates on selected orientations or faces. Here, we control particle morphology by “crystal templating”: by choosing appropriate precursor crystals and reaction conditions, we demonstrate that a material with rhombohedral symmetry—namely the layered, positive electrode material, LiCoO2—can grow to form a quadruple-twinned crystal with overall cubic symmetry. The twinned crystals show an unusual, concaved-cuboctahedron morphology, with uniform particle sizes of 0.5−2 μm. On the basis of a range of synthetic and analytical experiments, including solid-state NMR, X-ray powder diffraction analysis and HRTEM, we propose that these twinned crystals form via selective dissolution and an ion-exchange reaction accompanied by oxidation of a parent crystal of CoO, a material with cubic symmetry. This template crystal serves to nucleate the growth of four LiCoO2 twin crystals and to convert a highly anisotropic, layered material into a pseudo-3-dimensional, isotropic material.
Co-reporter:Jongsik Kim ; Derek S. Middlemiss ; Natasha A. Chernova ; Ben Y. X. Zhu ; Christian Masquelier
Journal of the American Chemical Society 2010 Volume 132(Issue 47) pp:16825-16840
Publication Date(Web):November 5, 2010
DOI:10.1021/ja102678r
Iron phosphates (FePO4) are among the most promising candidate materials for advanced Li-ion battery cathodes. This work reports upon a combined nuclear magnetic resonance (NMR) experimental and periodic density functional theory (DFT) computational study of the environments and electronic structures occurring in a range of paramagnetic Fe(III) phosphates comprising FePO4 (heterosite), monoclinic Li3Fe2(PO4)3 (anti-NASICON A type), rhombohedral Li3Fe2(PO4)3 (NASICON B type), LiFeP2O7, orthorhombic FePO4·2H2O (strengite), monoclinic FePO4·2H2O (phosphosiderite), and the dehydrated forms of the latter two phases. Many of these materials serve as model compounds relevant to battery chemistry. The 31P spin−echo mapping and 7Li magic angle spinning NMR techniques yield the hyperfine shifts of the species of interest, complemented by periodic hybrid functional DFT calculations of the respective hyperfine and quadrupolar tensors. A Curie−Weiss-based magnetic model scaling the DFT-calculated hyperfine parameters from the ferromagnetic into the experimentally relevant paramagnetic state is derived and applied, providing quantitative finite temperature values for each phase. The sensitivity of the hyperfine parameters to the composition of the DFT exchange functional is characterized by the application of hybrid Hamiltonians containing admixtures 0%, 20%, and 35% of Fock exchange. Good agreement between experimental and calculated values is obtained, provided that the residual magnetic couplings persisting in the paramagnetic state are included. The potential applications of a similar combined experimental and theoretical NMR approach to a wider range of cathode materials are discussed.
Co-reporter:Alexander C. Forse; Céline Merlet; John M. Griffin
Journal of the American Chemical Society () pp:
Publication Date(Web):March 31, 2016
DOI:10.1021/jacs.6b02115
Supercapacitors (or electric double-layer capacitors) are high-power energy storage devices that store charge at the interface between porous carbon electrodes and an electrolyte solution. These devices are already employed in heavy electric vehicles and electronic devices, and can complement batteries in a more sustainable future. Their widespread application could be facilitated by the development of devices that can store more energy, without compromising their fast charging and discharging times. In situ characterization methods and computational modeling techniques have recently been developed to study the molecular mechanisms of charge storage, with the hope that better devices can be rationally designed. In this Perspective, we bring together recent findings from a range of experimental and computational studies to give a detailed picture of the charging mechanisms of supercapacitors. Nuclear magnetic resonance experiments and molecular dynamics simulations have revealed that the electrode pores contain a considerable number of ions in the absence of an applied charging potential. Experiments and computer simulations have shown that different charging mechanisms can then operate when a potential is applied, going beyond the traditional view of charging by counter-ion adsorption. It is shown that charging almost always involves ion exchange (swapping of co-ions for counter-ions), and rarely occurs by counter-ion adsorption alone. We introduce a charging mechanism parameter that quantifies the mechanism and allows comparisons between different systems. The mechanism is found to depend strongly on the polarization of the electrode, and the choice of the electrolyte and electrode materials. In light of these advances we identify new directions for supercapacitor research. Further experimental and computational work is needed to explain the factors that control supercapacitor charging mechanisms, and to establish the links between mechanisms and performance. Increased understanding and control of charging mechanisms should lead to new strategies for developing next-generation supercapacitors with improved performances.
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 34) pp:NaN22320-22320
Publication Date(Web):2015/07/29
DOI:10.1039/C5CP02331A
The Carr–Purcell–Meiboom–Gill (CPMG) sequence is commonly used in high resolution NMR spectroscopy and in magnetic resonance imaging for the measurement of transverse relaxation in systems that are subject to diffusion in internal or external gradients and is superior to the Hahn echo measurement, which is more sensitive to diffusion effects. Similarly, it can potentially be used to study dynamic processes in electrode materials for lithium ion batteries. Here we compare the 7Li signal decay curves obtained with the CPMG and Hahn echo sequences under static conditions (i.e., in the absence of magic angle spinning) in paramagnetic materials with varying transition metal ion concentrations. Our results indicate that under CPMG pulse trains the lifetime of the 7Li signal is substantially extended and is correlated with the strength of the electron–nuclear interaction. Numerical simulations and analytical calculations using Floquet theory suggest that the combination of large interactions and a train of finite pulses, results in a spin locking effect which significantly slows the signal's decay. While these effects complicate the interpretation of CPMG-based investigations of diffusion and chemical exchange in paramagnetic materials, they may provide a useful approach to extend the signal's lifetime in these often fast relaxing systems, enabling the use of correlation experiments. Furthermore, these results highlight the importance of developing a deeper understanding of the effects of the large paramagnetic interactions during multiple pulse experiments in order to extend the experimental arsenal available for static and in situ NMR investigations of paramagnetic materials.
Co-reporter:Jordi Cabana, Junichi Shirakawa, Masanobu Nakayama, Masataka Wakihara and Clare P. Grey
Journal of Materials Chemistry A 2011 - vol. 21(Issue 27) pp:NaN10020-10020
Publication Date(Web):2011/03/11
DOI:10.1039/C0JM04197A
The structure and lithium mobility have been investigated for A- and B-Li3Fe2(PO4)3, before and after mechanical milling and lithium insertion, by using Li NMR. The data indicate that the milling step induces a significant amount of defects in the structure, while it improves the ability of the material to take up lithium. The lithium mobility in the different samples was studied by collecting NMR spectra at different temperatures, extensive lithium mobility being observed for both polytypes at temperatures above 150 °C. This mobility was found to be enhanced after milling. The enhancement in the electrode material utilization is ascribed to both a reduction of the diffusion lengths (particle size) and an increase in the intrinsic mobility of lithium in the sample.
Co-reporter:Fiona C. Strobridge, Derek S. Middlemiss, Andrew J. Pell, Michal Leskes, Raphaële J. Clément, Frédérique Pourpoint, Zhouguang Lu, John V. Hanna, Guido Pintacuda, Lyndon Emsley, Ago Samoson and Clare P. Grey
Journal of Materials Chemistry A 2014 - vol. 2(Issue 30) pp:NaN11957-11957
Publication Date(Web):2014/06/09
DOI:10.1039/C4TA00934G
Olivine-type LiCoPO4 (LCP) is a high energy density lithium ion battery cathode material due to the high voltage of the Co2+/Co3+ redox reaction. However, it displays a significantly poorer electrochemical performance than its more widely investigated isostructural analogue LiFePO4 (LFP). The co-substituted LiFexCo1−xPO4 olivines combine many of the positive attributes of each end member compound and are promising next-generation cathode materials. Here, the fully lithiated x = 0, 0.25, 0.5, 0.75 and 1 samples are extensively studied using 31P solid-state nuclear magnetic resonance (NMR). Practical approaches to broadband excitation and for the resolution of the isotropic resonances are described. First principles hybrid density functional calculations are performed on the Fermi contact shift (FCS) contributions of individual M–O–P pathways in the end members LFP and LCP and compared with the fitted values extracted from the LiFexCo1−xPO4 experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFexCo1−xPO4 have been calculated and used to assign the NMR spectra. Due to the additional unpaired electron in d6 Fe2+ as compared with d7 Co2+ (both high spin), LFP is expected to have larger Fermi contact shifts than LCP. However, two of the Co–O–P pathways in LCP give rise to noticeably larger shifts and the unexpected appearance of peaks outside the range delimited by the pure LFP and LCP 31P shifts. This behaviour contrasts with that observed previously in LiFexMn1−xPO4, where all 31P shifts lay within the LiMnPO4–LFP range. Although there are 24 distinct local P environments in LiFexCo1−xPO4, these group into seven resonances in the NMR spectra, due to significant overlap of the isotropic shifts. The local environments that give rise to the largest contributions to the spectral intensity are identified and used to simplify the assignment. This provides a tool for future studies of the electrochemically-cycled samples, which would otherwise be challenging to interpret.
Co-reporter:Kenneth J. Rosina, Meng Jiang, Dongli Zeng, Elodie Salager, Adam S. Best and Clare P. Grey
Journal of Materials Chemistry A 2012 - vol. 22(Issue 38) pp:NaN20610-20610
Publication Date(Web):2012/08/31
DOI:10.1039/C2JM34114J
The structural properties of layered Li[Li1/9Ni1/3Mn5/9]O2 positive electrodes nominally coated with aluminum fluoride are studied. Coatings were prepared by using aqueous solutions with various concentrations of aluminum and fluorine and are compared with samples treated under similar conditions but with aqueous HCl solutions. Samples were investigated following heat treatment at 120 °C and 400 °C with powder X-ray diffraction, transmission electron microscopy including energy dispersive X-ray spectroscopy (TEM/EDS), elemental analysis via inductively coupled plasma-optical emission spectroscopy (ICP-EA), and both 6Li and 27Al magic angle spinning NMR spectroscopy. The TEM/EDS and 27Al NMR data provide support for an aluminum-rich amorphous coating that, following drying at 120 °C, comprises six coordinated, partially hydrated aluminum environments. Heat treatment at 400 °C results in a phase that resembles partially fluorinated γ- or γ′-Al2O3, at least locally. An Al:F ratio of 2:1 is obtained in stark contrast to the ratio used in the original solution (1:3). No AlF3 is detected by PXRD and instead some evidence for a protonated phase (formed by ion exchanging protons for lithium) is detected along with Li[Li1/9Ni1/3Mn5/9]O2 after drying. This phase disappears on heating to 400 °C, suggesting some reorganization of bulk Li[Li1/9Ni1/3Mn5/9]O2 and possibly some incorporation of Al into the structure. This is in agreement with the 6Li NMR spectra, which indicate that the local environments that are found in the Ni-free end member of the series Li[Li(1/3−2x/3)NixMn(2/3−x/3)]O2 (i.e. Li2MnO3) are enhanced on sintering.
Co-reporter:Michael A. Hope, David M. Halat, Pieter C. M. M. Magusin, Subhradip Paul, Luming Peng and Clare P. Grey
Chemical Communications 2017 - vol. 53(Issue 13) pp:NaN2145-2145
Publication Date(Web):2017/01/24
DOI:10.1039/C6CC10145C
Surface-selective direct 17O DNP has been demonstrated for the first time on CeO2 nanoparticles, for which the first three layers can be distinguished with high selectivity. Polarisation build-up curves show that the polarisation of the (sub-)surface sites builds up faster than the bulk, accounting for the remarkable surface selectivity.
Co-reporter:Joshua M. Stratford, Phoebe K. Allan, Oliver Pecher, Philip A. Chater and Clare P. Grey
Chemical Communications 2016 - vol. 52(Issue 84) pp:NaN12433-12433
Publication Date(Web):2016/09/22
DOI:10.1039/C6CC06990H
Operando
23Na solid-state NMR and pair distribution function analysis experiments provide insights into the structure of hard carbon anodes in sodium-ion batteries. Capacity results from “diamagnetic” sodium ions first adsorbing onto pore surfaces, defects and between expanded layers, before pooling into larger quasi-metallic clusters/expanded carbon sheets at lower voltages.
Co-reporter:Jeongjae Lee, Ieuan D. Seymour, Andrew J. Pell, Siân E. Dutton and Clare P. Grey
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 1) pp:NaN625-625
Publication Date(Web):2016/11/11
DOI:10.1039/C6CP06338A
Rechargeable battery systems based on Mg-ion chemistries are generating significant interest as potential alternatives to Li-ion batteries. Despite the wealth of local structural information that could potentially be gained from Nuclear Magnetic Resonance (NMR) experiments of Mg-ion battery materials, systematic 25Mg solid-state NMR studies have been scarce due to the low natural abundance, low gyromagnetic ratio, and significant quadrupole moment of 25Mg (I = 5/2). This work reports a combined experimental 25Mg NMR and first principles density functional theory (DFT) study of paramagnetic Mg transition metal oxide systems Mg6MnO8 and MgCr2O4 that serve as model systems for Mg-ion battery cathode materials. Magnetic parameters, hyperfine shifts and quadrupolar parameters were calculated ab initio using hybrid DFT and compared to the experimental values obtained from NMR and magnetic measurements. We show that the rotor assisted population transfer (RAPT) pulse sequence can be used to enhance the signal-to-noise ratio in paramagnetic 25Mg spectra without distortions in the spinning sideband manifold. In addition, the value of the predicted quadrupolar coupling constant of Mg6MnO8 was confirmed using the RAPT pulse sequence. We further apply the same methodology to study the NMR spectra of spinel compounds MgV2O4 and MgMn2O4, candidate cathode materials for Mg-ion batteries.
Co-reporter:Evan N. Keyzer, Peter D. Matthews, Zigeng Liu, Andrew D. Bond, Clare P. Grey and Dominic S. Wright
Chemical Communications 2017 - vol. 53(Issue 33) pp:NaN4576-4576
Publication Date(Web):2017/03/31
DOI:10.1039/C7CC01938F
The development of rechargeable Ca-ion batteries as an alternative to Li systems has been limited by the availability of suitable electrolyte salts. We present the synthesis of complexes of Ca(PF6)2 (a key potential Ca battery electrolyte salt) via the treatment of Ca metal with NOPF6, and explore their conversion to species containing PO2F2− under the reaction conditions.
Co-reporter:Zigeng Liu, Jeongjae Lee, Guolei Xiang, Hugh F. J. Glass, Evan N. Keyzer, Siân E. Dutton and Clare P. Grey
Chemical Communications 2017 - vol. 53(Issue 4) pp:NaN746-746
Publication Date(Web):2016/12/12
DOI:10.1039/C6CC08430C
Bi nanowires as anode materials for Mg ion batteries exhibit excellent electrochemical behaviour, forming Mg3Bi2; this is in part ascribed to the rapid Mg mobility between the two Mg sites of Mg3Bi2, as revealed by the 25Mg NMR spectra of Mg3Bi2 formed electrochemically and via ball-milling. A mechanism involving hops into vacant Mg sites is proposed.
Co-reporter:Alexander C. Forse, John M. Griffin, Hao Wang, Nicole M. Trease, Volker Presser, Yury Gogotsi, Patrice Simon and Clare P. Grey
Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 20) pp:NaN7730-7730
Publication Date(Web):2013/03/26
DOI:10.1039/C3CP51210J
A detailed understanding of ion adsorption within porous carbon is key to the design and improvement of electric double-layer capacitors, more commonly known as supercapacitors. In this work nuclear magnetic resonance (NMR) spectroscopy is used to study ion adsorption in porous carbide-derived carbons. These predominantly microporous materials have a tuneable pore size which enables a systematic study of the effect of pore size on ion adsorption. Multinuclear NMR experiments performed on the electrolyte anions and cations reveal two main environments inside the carbon. In-pore ions (observed at low frequencies) are adsorbed inside the pores, whilst ex-pore ions (observed at higher frequencies) are not adsorbed and are in large reservoirs of electrolyte between carbon particles. All our experiments were carried out in the absence of an applied electrical potential in order to assess the mechanisms related to ion adsorption without the contribution of electrosorption. Our results indicate similar adsorption behaviour for anions and cations. Furthermore, we probe the effect of sample orientation, which is shown to have a marked effect on the NMR spectra. Finally, we show that a 13C → 1H cross polarisation experiment enables magnetisation transfer from the carbon architecture to the adsorbed species, allowing selective observation of the adsorbed ions and confirming our spectral assignments.
Journal of Materials Chemistry A 2016 - vol. 4(Issue 17) pp:NaN6446-6446
Publication Date(Web):2016/04/12
DOI:10.1039/C6TA00673F
The nature of a phase transition plays an important role in controlling the kinetics of reaction of an electrode material in a lithium-ion battery. The actual phase transition path can be affected by particle size and cycling rate. In this study, we investigated the phase transition process during the electrochemical Li intercalation of anatase TiO2 as a function of particle size (25 nm and 100 nm), cycling rate (1C, 2C, 5C, 10C, 20C) and temperature (room temperature and 80 °C) by in situ synchrotron X-ray diffraction. The phase transition was found to be affected by the particle size: the 100 nm particles react simultaneously via a conventional nucleation and growth, i.e. two-phase, mechanism, while the 25 nm particles react sequentially via a two-phase mechanism. The Li miscibility gap decreases with increasing cycling rate, yet the phase separation was not suppressed even at a cycle rate of 20C. An increase in temperature from room temperature to 80 °C significantly improves the electrode's electrochemical performance despite undergoing a two-phase reaction. The failure to observe a continuous structural transition from tetragonal TiO2 to orthorhombic Li0.5TiO2 even at high rates and elevated temperature was attributed to the high energy barrier of a continuous phase transition path.
Co-reporter:R. J. Clément, J. Xu, D. S. Middlemiss, J. Alvarado, C. Ma, Y. S. Meng and C. P. Grey
Journal of Materials Chemistry A 2017 - vol. 5(Issue 8) pp:NaN4143-4143
Publication Date(Web):2017/01/19
DOI:10.1039/C6TA09601H
Structural processes occurring upon electrochemical cycling in P2-Nax[LiyNizMn1−y−z]O2 (x, y, z ≤ 1) cathode materials are investigated using 23Na and 7Li solid-state nuclear magnetic resonance (ssNMR). The interpretation of the complex paramagnetic NMR data obtained for various electrochemically-cycled NaxNi1/3Mn2/3O2 and NaxLi0.12Ni0.22Mn0.66O2 samples is assisted by state-of-the-art hybrid Hartree–Fock/density functional theory calculations. Two Na crystallographic environments are present in P2-Nax[LiyNizMn1−y−z]O2 compounds, yet a single 23Na NMR signal is observed with a shift in-between those computed for edge- and face-centered prismatic sites, indicating that Na-ion motion between sites in the P2 layers results in an average signal. This is the first time that experimental and theoretical evidence are provided for fast Na-ion motion (on the timescale of the NMR experiments) in the interlayer space in P2-type NaxTMO2 materials. A full assignment of the 7Li NMR data confirms that Li substitution delays the P2 to O2 phase transformation taking place in NaxNi1/3Mn2/3O2 over the range 1/3 ≥ xNa ≥ 0. 23Na ssNMR data demonstrate that NaxNi1/3Mn2/3O2 samples charged to ≥3.7 V are extremely moisture sensitive once they are removed from the cell, water molecules being readily intercalated within the P2 layers leading to an additional Na signal between 400 and 250 ppm. By contrast, the lithiated material NaxLi0.12Ni0.22Mn0.66O2 shows no sign of hydration until it is charged to ≥4.4 V. Since both TMO2 layer glides and water intercalation become increasingly favorable as more vacancies are present in the Na layers, the higher stability of the Li-doped P2 phase at high voltage can be accounted for by its higher Na content at all stages of cycling.
Co-reporter:Shou-Hang Bo, Feng Wang, Yuri Janssen, Dongli Zeng, Kyung-Wan Nam, Wenqian Xu, Lin-Shu Du, Jason Graetz, Xiao-Qing Yang, Yimei Zhu, John B. Parise, Clare P. Grey and Peter G. Khalifah
Journal of Materials Chemistry A 2012 - vol. 22(Issue 18) pp:NaN8809-8809
Publication Date(Web):2012/03/26
DOI:10.1039/C2JM16436A
Lithium iron borate (LiFeBO3) is a particularly desirable cathode material for lithium-ion batteries due to its high theoretical capacity (220 mA h g−1) and its favorable chemical constituents, which are abundant, inexpensive and non-toxic. However, its electrochemical performance appears to be severely hindered by the degradation that results from air or moisture exposure. The degradation of LiFeBO3 was studied through a wide array of ex situ and in situ techniques (X-ray diffraction, nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy and spectroscopy) to better understand the possible degradation process and to develop methods for preventing degradation. It is demonstrated that degradation involves both Li loss from the framework of LiFeBO3 and partial oxidation of Fe(II), resulting in the creation of a stable lithium-deficient phase with a similar crystal structure to LiFeBO3. Considerable LiFeBO3 degradation occurs during electrode fabrication, which greatly reduces the accessible capacity of LiFeBO3 under all but the most stringently controlled conditions for electrode fabrication. Comparative studies on micron-sized LiFeBO3 and nanoscale LiFeBO3–carbon composite showed a very limited penetration depth (∼30 nm) of the degradation phase front into the LiFeBO3 core under near-ambient conditions. Two-phase reaction regions during delithiation and lithiation of LiFeBO3 were unambiguously identified through the galvanostatic intermittent titration technique (GITT), although it is still an open question as to whether the two-phase reaction persists across the whole range of possible Li contents. In addition to the main intercalation process with a thermodynamic potential of 2.8 V, there appears to be a second reversible electrochemical process with a potential of 1.8 V. The best electrochemical performance of LiFeBO3 was ultimately achieved by introducing carbon to minimize the crystallite size and strictly limiting air and moisture exposure to inhibit degradation.
![dibenzo[ghi,mno]fluoranthene](http://img.cochemist.com/ccimg/5900/5821-51-2.png)
![dibenzo[ghi,mno]fluoranthene](http://img.cochemist.com/ccimg/5900/5821-51-2_b.png)
