Co-reporter:Yue Deng;Christopher Eames;Benoit Fleutot;Rénald David;Jean-Noël Chotard;Emmanuelle Suard;Christian Masquelier
ACS Applied Materials & Interfaces March 1, 2017 Volume 9(Issue 8) pp:7050-7058
Publication Date(Web):January 27, 2017
DOI:10.1021/acsami.6b14402
Lithium superionic conductor (LISICON)-related compositions Li4±xSi1–xXxO4 (X = P, Al, or Ge) are important materials that have been identified as potential solid electrolytes for all solid state batteries. Here, we show that the room temperature lithium ion conductivity can be improved by several orders of magnitude through substitution on Si sites. We apply a combined computer simulation and experimental approach to a wide range of compositions (Li4SiO4, Li3.75Si0.75P0.25O4, Li4.25Si0.75Al0.25O4, Li4Al0.33Si0.33P0.33O4, and Li4Al1/3Si1/6Ge1/6P1/3O4) which include new doped materials. Depending on the temperature, three different Li+ ion diffusion mechanisms are observed. The polyanion mixing introduced by substitution lowers the temperature at which the transition to a superionic state with high Li+ ion conductivity occurs. These insights help to rationalize the mechanism of the lithium ion conductivity enhancement and provide strategies for designing materials with promising transport properties.Keywords: diffusion mechanism; energy storage; LISICON; mixed polyanion effect; solid electrolyte;
Co-reporter:Juliette Billaud;Christopher Eames;Nuria Tapia-Ruiz;Matthew R. Roberts;Andrew J. Naylor;A. Robert Armstrong;Peter G. Bruce
Advanced Energy Materials 2017 Volume 7(Issue 11) pp:
Publication Date(Web):2017/06/01
DOI:10.1002/aenm.201601043
The silicate compounds Li2MSiO4 (where M = Mn, Fe, Co) have received significant attention recently as Li intercalation electrodes. Overwhelmingly they exhibit relatively poor kinetics of ion intercalation. By synthesizing Li-rich solid solutions of the form Li2+2xFe1−xSiO4 (with 0 ≤ x ≤ 0.3), the structural requirements for fast ion transport and hence relatively fast intercalation have been identified. Specifically the presence of additional Li+ in interstitial sites, not normally occupied in the stoichiometric Li2FeSiO4 compound, enhances ion transport by more than two orders of magnitude. The results, obtained by combining electrochemical measurements, with powder X-ray and neutron diffraction and atomistic modeling of the ion dynamics, provide valuable guidance in designing future intercalation electrodes with high Li-ion transport and, hence, fast electrode kinetics.
Co-reporter:Bethan Charles;Jessica Dillon;Oliver J. Weber;Mark T. Weller
Journal of Materials Chemistry A 2017 vol. 5(Issue 43) pp:22495-22499
Publication Date(Web):2017/11/07
DOI:10.1039/C7TA08617B
The routes and kinetics of the degradation of thin films of methylammonium (MA)/formamidinium (FA) lead iodide perovskites (MA1−xFAxPbI3, 0 ≤ x ≤ 1) under dry atmospheric conditions have been investigated. MA-rich phases decompose to the precursor iodide salts and PbI2, while FA-rich phases convert mainly to the yellow hexagonal phase. The reactivity is strongly inhibited for mixed cation phases of MA1−xFAxPbI3, for x = 0.4 to 0.6, where the decomposition routes available to end member phases become less favourable. It is shown that for pristine films with x = 0.6, PbI2 formation can be completely suppressed for up to 10 days. Kinetic analysis reveals that the rate of PbI2 formation decays exponentially with increasing FA content until x = 0.7, beyond which the FA containing perovskite transforms rapidly to the hexagonal phase. Ab initio simulations of the decomposition reaction energies fully support the increased kinetic stability found experimentally for the mixed A-cation perovskites.
Co-reporter:Nick Aristidou;Christopher Eames;Saif A. Haque
Journal of Materials Chemistry A 2017 vol. 5(Issue 48) pp:25469-25475
Publication Date(Web):2017/12/12
DOI:10.1039/C7TA06841G
Halide perovskites offer low cost and high efficiency solar cell materials but serious issues related to air and moisture stability remain. In this study we show, using UV-vis, fluorescence and time of flight secondary ion mass spectrometry (ToF-SIMS) techniques, that the degradation of methylammonium lead iodide solar cells is significantly accelerated when both air and moisture are present in comparison to when just air or moisture is present alone. Using ab initio computational techniques we identify the thermodynamic driving force for the enhanced reactivity and highlight the regions of the photoexcited material that are the most likely reaction centres. We suggest that water catalyses the reaction by stabilising the reactive superoxide species, enabling them to react with the methylammonium cation.
Co-reporter:Jennifer Heath;Hungru Chen
Journal of Materials Chemistry A 2017 vol. 5(Issue 25) pp:13161-13167
Publication Date(Web):2017/06/27
DOI:10.1039/C7TA03201C
Developing rechargeable magnesium batteries has become an area of growing interest as an alternative to lithium-ion batteries largely due to their potential to offer increased energy density from the divalent charge of the Mg ion. Unlike the lithium silicates for Li-ion batteries, MgFeSiO4 can adopt the olivine structure as observed for LiFePO4. Here we combine advanced modelling techniques based on energy minimization, molecular dynamics (MD) and density functional theory to explore the Mg-ion conduction, doping and voltage behaviour of MgFeSiO4. The Mg-ion migration activation energy is relatively low for a Mg-based cathode, and MD simulations predict a diffusion coefficient (DMg) of 10−9 cm2 s−1, which suggest favourable electrode kinetics. Partial substitution of Fe by Co or Mn could increase the cell voltage from 2.3 V vs. Mg/Mg2+ to 2.8–3.0 V. The new fundamental insights presented here should stimulate further work on low-cost silicate cathodes for Mg batteries.
Co-reporter:Hungru Chen and M. Saiful Islam
Chemistry of Materials 2016 Volume 28(Issue 18) pp:6656
Publication Date(Web):September 6, 2016
DOI:10.1021/acs.chemmater.6b02870
Lithium-rich oxide electrodes with layered structures have attracted considerable interest because they can deliver high energy densities for lithium-ion batteries. However, there is significant debate regarding their redox chemistry. It is apparent that the mechanism of lithium extraction from lithium-rich Li2MnO3 is not fully understood, especially in relation to the observed O2 evolution and structural transformation. Here, delithiation and kinetic processes in Li2MnO3 are investigated using ab initio simulation techniques employing high level hybrid functionals as they reproduce accurately the electronic structure of oxygen hole states. We show that Li extraction is charge-compensated by oxidation of the oxide anion, so that the overall delithiation reaction involves lattice oxygen loss. Localized holes on oxygen (O–) are formed as the first step but are not stable leading to oxygen dimerization (with O–O ∼ 1.3 Å) and eventually to the formation of molecular O2. Oxygen dimerization facilitates Mn migration onto octahedral sites in the vacated lithium layers. The results suggest that reversible oxygen redox without major structural changes is only possible if the localized oxygen holes are stabilized and oxygen dimerization suppressed. Such an understanding is important for the future optimization of new lithium-rich cathode materials for high energy density batteries.
Co-reporter:Yifei Yuan, Stephen M. Wood, Kun He, Wentao Yao, David Tompsett, Jun Lu, Anmin Nie, M. Saiful Islam, and Reza Shahbazian-Yassar
ACS Nano 2016 Volume 10(Issue 1) pp:539
Publication Date(Web):December 9, 2015
DOI:10.1021/acsnano.5b05535
Controlled synthesis of nanomaterials is one of the grand challenges facing materials scientists. In particular, how tunnel-based nanomaterials aggregate during synthesis while maintaining their well-aligned tunneled structure is not fully understood. Here, we describe the atomistic mechanism of oriented attachment (OA) during solution synthesis of tunneled α-MnO2 nanowires based on a combination of in situ liquid cell transmission electron microscopy (TEM), aberration-corrected scanning TEM with subangstrom spatial resolution, and first-principles calculations. It is found that primary tunnels (1 × 1 and 2 × 2) attach along their common {110} lateral surfaces to form interfaces corresponding to 2 × 3 tunnels that facilitate their short-range ordering. The OA growth of α-MnO2 nanowires is driven by the stability gained from elimination of {110} surfaces and saturation of Mn atoms at {110}-edges. During this process, extra [MnOx] radicals in solution link the two adjacent {110} surfaces and bond with the unsaturated Mn atoms from both surface edges to produce stable nanowire interfaces. Our results provide insights into the controlled synthesis and design of nanomaterials in which tunneled structures can be tailored for use in catalysis, ion exchange, and energy storage applications.Keywords: interface; nanowire; oriented attachment; surface structure; tunnel;
Co-reporter:Yue Deng; Christopher Eames; Jean-Noël Chotard; Fabien Lalère; Vincent Seznec; Steffen Emge; Oliver Pecher; Clare P. Grey; Christian Masquelier
Journal of the American Chemical Society 2015 Volume 137(Issue 28) pp:9136-9145
Publication Date(Web):June 29, 2015
DOI:10.1021/jacs.5b04444
Solid electrolytes that are chemically stable and have a high ionic conductivity would dramatically enhance the safety and operating lifespan of rechargeable lithium batteries. Here, we apply a multi-technique approach to the Li-ion conducting system (1–z)Li4SiO4–(z)Li3PO4 with the aim of developing a solid electrolyte with enhanced ionic conductivity. Previously unidentified superstructure and immiscibility features in high-purity samples are characterized by X-ray and neutron diffraction across a range of compositions (z = 0.0–1.0). Ionic conductivities from AC impedance measurements and large-scale molecular dynamics (MD) simulations are in good agreement, showing very low values in the parent phases (Li4SiO4 and Li3PO4) but orders of magnitude higher conductivities (10–3 S/cm at 573 K) in the mixed compositions. The MD simulations reveal new mechanistic insights into the mixed Si/P compositions in which Li-ion conduction occurs through 3D pathways and a cooperative interstitial mechanism; such correlated motion is a key factor in promoting high ionic conductivity. Solid-state 6Li, 7Li, and 31P NMR experiments reveal enhanced local Li-ion dynamics and atomic disorder in the solid solutions, which are correlated to the ionic diffusivity. These unique insights will be valuable in developing strategies to optimize the ionic conductivity in this system and to identify next-generation solid electrolytes.
Co-reporter:Julian Roos, Christopher Eames, Stephen M. Wood, Alexander Whiteside and M. Saiful Islam
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 34) pp:22259-22265
Publication Date(Web):29 Jul 2015
DOI:10.1039/C5CP02711J
The recently discovered lithium-rich cathode material Li7Mn(BO3)3 has a high theoretical capacity and an unusual tetrahedral Mn2+ coordination. Atomistic simulation and density functional theory (DFT) techniques are employed to provide insights into the defect and redox chemistry, the structural changes upon lithium extraction and the mechanisms of lithium ion diffusion. The most favourable intrinsic defects are Li/Mn anti-site pairs, where Li and Mn ions occupy interchanged positions, and Li Frenkel defects. DFT calculations reproduce the experimental cell voltage and confirm the presence of the unusually high MnV redox state, which corresponds to a theoretical capacity of nearly 288 mA h g−1. The ability to reach the high manganese oxidation state is related to both the initial tetrahedral coordination of Mn and the observed distortion/tilting of the BO3 units to accommodate the contraction of the Mn–O bonds upon oxidation. Molecular dynamics (MD) simulations indicate fast three-dimensional lithium diffusion in line with the good rate performance observed.
Co-reporter:Stephen M. Wood
The Journal of Physical Chemistry C 2015 Volume 119(Issue 28) pp:15935-15941
Publication Date(Web):June 17, 2015
DOI:10.1021/acs.jpcc.5b04648
Polyanionic phosphates have the potential to act as low-cost cathodes and stable framework materials for Na ion batteries. The mixed phosphates Na4M3(PO4)2P2O7 (M = Fe, Mn, Co, Ni) are a fascinating new class of materials recently reported to be attractive Na ion cathodes which display low-volume changes upon cycling, indicative of long-lifetime operation. Key issues surrounding intrinsic defects, Na ion migration mechanisms, and voltage trends have been investigated through a combination of atomistic energy minimization, molecular dynamics (MD), and density functional theory simulations. For all compositions, the most energetically favorable defect is calculated to be the Na/M antisite pair. MD simulations suggest Na+ diffusion extends across a 3D network of migration pathways with an activation barrier of 0.20–0.24 eV, and diffusion coefficients (DNa) of 10–10–10–11 cm2 s–1 at 325 K, suggesting good rate capability. The voltage trends indicate that doping the Fe-based cathode with Ni can significantly increase the voltage, and hence the energy density.
Co-reporter:M. Saiful Islam and Craig A. J. Fisher
Chemical Society Reviews 2014 vol. 43(Issue 1) pp:185-204
Publication Date(Web):07 Nov 2013
DOI:10.1039/C3CS60199D
Energy storage technologies are critical in addressing the global challenge of clean sustainable energy. Major advances in rechargeable batteries for portable electronics, electric vehicles and large-scale grid storage will depend on the discovery and exploitation of new high performance materials, which requires a greater fundamental understanding of their properties on the atomic and nanoscopic scales. This review describes some of the exciting progress being made in this area through use of computer simulation techniques, focusing primarily on positive electrode (cathode) materials for lithium-ion batteries, but also including a timely overview of the growing area of new cathode materials for sodium-ion batteries. In general, two main types of technique have been employed, namely electronic structure methods based on density functional theory, and atomistic potentials-based methods. A major theme of much computational work has been the significant synergy with experimental studies. The scope of contemporary work is highlighted by studies of a broad range of topical materials encompassing layered, spinel and polyanionic framework compounds such as LiCoO2, LiMn2O4 and LiFePO4 respectively. Fundamental features important to cathode performance are examined, including voltage trends, ion diffusion paths and dimensionalities, intrinsic defect chemistry, and surface properties of nanostructures.
Co-reporter:David A. Tompsett ; Stephen C. Parker
Journal of the American Chemical Society 2014 Volume 136(Issue 4) pp:1418-1426
Publication Date(Web):January 6, 2014
DOI:10.1021/ja4092962
MnO2 is a technologically important material for energy storage and catalysis. Recent investigations have demonstrated the success of nanostructuring for improving the performance of rutile MnO2 in Li-ion batteries and supercapacitors and as a catalyst. Motivated by this we have investigated the stability and electronic structure of rutile (β-)MnO2 surfaces using density functional theory. A Wulff construction from relaxed surface energies indicates a rod-like equilibrium morphology that is elongated along the c-axis, and is consistent with the large number of nanowire-type structures that are obtainable experimentally. The (110) surface dominates the crystallite surface area. Moreover, higher index surfaces than considered in previous work, for instance the (211) and (311) surfaces, are also expressed to cap the rod-like morphology. Broken coordinations at the surface result in enhanced magnetic moments at Mn sites that may play a role in catalytic activity. The calculated formation energies of oxygen vacancy defects and Mn reduction at key surfaces indicate facile formation at surfaces expressed in the equilibrium morphology. The formation energies are considerably lower than for comparable structures such as rutile TiO2 and are likely to be important to the high catalytic activity of rutile MnO2.
Co-reporter:Yuri G. Andreev ; Pooja M. Panchmatia ; Zheng Liu ; Stephen C. Parker ; M. Saiful Islam ;Peter G. Bruce
Journal of the American Chemical Society 2014 Volume 136(Issue 17) pp:6306-6312
Publication Date(Web):April 8, 2014
DOI:10.1021/ja412387c
The shape of nanoparticles can be important in defining their properties. Establishing the exact shape of particles is a challenging task when the particles tend to agglomerate and their size is just a few nanometers. Here we report a structure refinement procedure for establishing the shape of nanoparticles using powder diffraction data. The method utilizes the fundamental formula of Debye coupled with a Monte Carlo-based optimization and has been successfully applied to TiO2-B nanoparticles. Atomistic modeling and molecular dynamics simulations of ensembles of all the ions in the nanoparticle reveal surface hydroxylation as the underlying reason for the established shape and structural features.
Co-reporter:Christopher Eames
Journal of the American Chemical Society 2014 Volume 136(Issue 46) pp:16270-16276
Publication Date(Web):October 13, 2014
DOI:10.1021/ja508154e
Two-dimensional transition metal carbides (termed MXenes) are a new family of compounds generating considerable interest due to their unique properties and potential applications. Intercalation of ions into MXenes has recently been demonstrated with good electrochemical performance, making them viable electrode materials for rechargeable batteries. Here we have performed global screening of the capacity and voltage for a variety of intercalation ions (Li+, Na+, K+, and Mg2+) into a large number of M2C-based compounds (M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta) with F-, H-, O-, and OH-functionalized surfaces using density functional theory methods. In terms of gravimetric capacity a greater amount of Li+ or Mg2+ can be intercalated into an MXene than Na+ or K+, which is related to the size of the intercalating ion. Variation of the surface functional group and transition metal species can significantly affect the voltage and capacity of an MXene, with oxygen termination leading to the highest capacity. The most promising group of M2C materials in terms of anode voltage and gravimetric capacity (>400 mAh/g) are compounds containing light transition metals (e.g., Sc, Ti, V, and Cr) with nonfunctionalized or O-terminated surfaces. The results presented here provide valuable insights into exploring a rich variety of high-capacity MXenes for potential battery applications.
Co-reporter:Christopher Eames, John M. Clark, Gwenaelle Rousse, Jean-Marie Tarascon, and M. Saiful Islam
Chemistry of Materials 2014 Volume 26(Issue 12) pp:3672
Publication Date(Web):May 20, 2014
DOI:10.1021/cm5008203
Layered LiFeSO4OH has recently attracted interest as a sustainable cathode material for rechargeable lithium batteries that offers favorable synthesis and processing routes. Here, the defect chemistry, lithium-ion transport pathways, and cell voltages of layered LiFeSO4OH are investigated by atomistic modeling and density functional theory (DFT) methods and compared with the tavorite polymorph. The results indicate that the layered phase exhibits two-dimensional (2D) lithium-ion diffusion with low activation energies of ∼0.2 eV for long-range transport within the bc-plane, which is important for good rate capability. The tavorite phase also shows 2D lithium-ion diffusion but with higher activation energies of ∼0.7 eV. Using DFT+U techniques the experimental voltage and structural parameters are accurately reproduced for the tavorite polymorph. For the layered structure, similar accuracy in both cell voltage and structure can only be obtained if a van der Waals functional is included in the DFT methodology to account for the interlayer binding.
Co-reporter:John M. Clark, Christopher Eames, Marine Reynaud, Gwenaëlle Rousse, Jean-Noël Chotard, Jean-Marie Tarascon and M. Saiful Islam
Journal of Materials Chemistry A 2014 vol. 2(Issue 20) pp:7446-7453
Publication Date(Web):27 Mar 2014
DOI:10.1039/C3TA15064J
The search for high voltage cathodes for lithium-ion batteries has led to recent interest in the monoclinic Li2Fe(SO4)2 material which has a voltage of 3.83 V vs. lithium, the highest recorded for a fluorine-free iron-based compound. Here we investigate the defect, surface and lithium migration properties of the Li2M(SO4)2 (M = Fe, Mn, Co) materials using combined atomistic modelling and density functional theory (DFT) techniques. All intrinsic defect types including Li/M antisite disorder are found to be of high energy, suggesting insignificant concentrations. Low activation energies are found for lithium migration along the a-axis channels giving rise to long-range 1D diffusion, which are supported by molecular dynamics (MD) simulations. For the crystal morphology a significant surface area is exposed to these 1D diffusion channels, which would allow facile Li insertion and extraction. Using DFT simulations we reproduce the high voltage of the Li2Fe(SO4)2 material in accord with electrochemical data and also examine local structural distortions on lithium extraction.
Co-reporter:David A. Tompsett, Stephen C. Parker and M. Saiful Islam
Journal of Materials Chemistry A 2014 vol. 2(Issue 37) pp:15509-15518
Publication Date(Web):29 Jul 2014
DOI:10.1039/C4TA00952E
Hollandite (α-)MnO2 gives superior performance compared to other MnO2 polymorphs in surface sensitive applications in supercapacitors and catalysis. However, a thorough understanding of its atomic-scale surface properties is lacking, which we address here using density functional theory (DFT). A Wulff construction based upon relaxed surface energies demonstrates that the equilibrium morphology expresses the low index (100), (110) and (111) surfaces as well as the high index (211) and (112) surfaces. The predicted morphology exhibits clear elongation along the c-axis which is consistent with the large number of nanorod type structures that are obtainable experimentally. The surface structures expressed in the morphology are discussed in detail and it is found that α-MnO2 gives rise to larger surface relaxations than are observed for the less open rutile structured MnO2. Enhanced magnetic moments at surface sites are rationalised by a crystal field argument. Experimental studies consistently find that α-MnO2 has higher catalytic activity than other polymorphs of MnO2. In this work, calculated formation energies for oxygen vacancy defects at the expressed surfaces are demonstrably lower, by ∼1 eV, than for rutile MnO2 surfaces [Tompsett et al., JACS, 2014, 136, 1418]. The lowest vacancy formation energy occurs at the (112) surface, which despite its relative high Miller index constitutes 17% of the surface area of the calculated morphology. This may play a key role in the favourable catalytic performance observed for α-MnO2 in a broad range of applications.
Co-reporter:John M. Clark, Prabeer Barpanda, Atsuo Yamada and M. Saiful Islam
Journal of Materials Chemistry A 2014 vol. 2(Issue 30) pp:11807-11812
Publication Date(Web):05 Jun 2014
DOI:10.1039/C4TA02383H
Na-ion batteries are currently the focus of significant research activity due to the relative abundance of sodium and its consequent cost advantages. Recently, the pyrophosphate family of cathodes has attracted considerable attention, particularly Li2FeP2O7 related to its high operating voltage and enhanced safety properties; in addition the sodium-based pyrophosphates Na2FeP2O7 and Na2MnP2O7 are also generating interest. Herein, we present defect chemistry and ion migration results, determined via atomistic simulation techniques, for Na2MP2O7 (where M = Fe, Mn) as well as findings for Li2FeP2O7 for direct comparison. Within the pyrophosphate framework the most favourable intrinsic defect type is found to be the antisite defect, in which alkali-cations (Na/Li) and M ions exchange positions. Low activation energies are found for long-range diffusion in all crystallographic directions in Na2MP2O7 suggesting three-dimensional (3D) Na-ion diffusion. In contrast Li2FeP2O7 supports 2D Li-ion diffusion. The 2D or 3D nature of the alkali-ion migration pathways within these pyrophosphate materials means that antisite defects are much less likely to impede their transport properties, and hence important for high rate performance.
Co-reporter:Alexander Whiteside, Craig A. J. Fisher, Stephen C. Parker and M. Saiful Islam
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 39) pp:21788-21794
Publication Date(Web):01 Sep 2014
DOI:10.1039/C4CP02356K
The expansion of batteries into electric vehicle and grid storage applications has driven the development of new battery materials and chemistries, such as olivine phosphate cathodes and sodium-ion batteries. Here we present atomistic simulations of the surfaces of olivine-structured NaFePO4 as a sodium-ion battery cathode, and discuss differences in its morphology compared to the lithium analogue LiFePO4. The calculated equilibrium morphology is mostly isometric in appearance, with (010), (201) and (011) faces dominant. Exposure of the (010) surface is vital because it is normal to the one-dimensional ion-conduction pathway. Platelet and cube-like shapes observed by previous microscopy studies are reproduced by adjusting surface energies. The results indicate that a variety of (nano)particle morphologies can be achieved by tuning surface stabilities, which depend on synthesis methods and solvent conditions, and will be important in optimising electrochemical performance.
Co-reporter:Pooja M. Panchmatia, A. Robert Armstrong, Peter G. Bruce and M. Saiful Islam
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 39) pp:21114-21118
Publication Date(Web):03 Jul 2014
DOI:10.1039/C4CP01640H
Layered Li1+xV1−xO2 has attracted recent interest as a potential low voltage and high energy density anode material for lithium-ion batteries. A greater understanding of the lithium-ion transport mechanisms is important in optimising such oxide anodes. Here, stoichiometric LiVO2 and Li-rich Li1.07V0.93O2 are investigated using atomistic modelling techniques. Lithium-ion migration is not found in LiVO2, which has also previously shown to be resistant to lithium intercalation. Molecular dynamics simulations of lithiated non-stoichiometric Li1.07+yV0.93O2 suggest cooperative interstitial Li+ diffusion with favourable migration barriers and diffusion coefficients (DLi), which are facilitated by the presence of lithium in the transition metal layers; such transport behaviour is important for high rate performance as a battery anode.
Co-reporter:David A. Tompsett
The Journal of Physical Chemistry C 2014 Volume 118(Issue 43) pp:25009-25015
Publication Date(Web):September 30, 2014
DOI:10.1021/jp507189n
The nanostructuring of rutile MnO2 has been demonstrated to improve its performance for electrochemical storage and catalysis. Despite the progress of recent experimental works in exploiting this to enhance the material’s performance in important technological systems such as Li-ion batteries the detailed atomic-scale mechanisms still require explanation. The ability of surfaces and interfaces to produce intriguing phenomena including superconductivity and magnetism has been firmly established by intensive research in recent years. In this work we use density functional theory calculations to demonstrate that key surfaces of rutile MnO2 possess electronically conducting surface states, in contrast to the insulating bulk material. The surface band structure demonstrates that the conducting states are associated with both surface manganese and oxygen sites. Furthermore, the metallic conductivity is found to be anisotropic for the (001) surface, which may be exploited in device applications. The implications for the energy storage capacity and catalytic activity of rutile MnO2 are discussed in light of the need for good electron transport in Li-ion batteries, supercapacitors, and Li–O2 batteries.
Co-reporter:David A. Tompsett, Steve C. Parker, Peter G. Bruce, and M. Saiful Islam
Chemistry of Materials 2013 Volume 25(Issue 4) pp:536
Publication Date(Web):January 22, 2013
DOI:10.1021/cm303295f
Manganese oxide materials are attracting considerable interest for clean energy storage applications such as rechargeable Li ion and Li–air batteries and electrochemical capacitors. The electrochemical behavior of nanostructured mesoporous β-MnO2 is in sharp constrast to the bulk crystalline system, which can intercalate little or no lithium; this is not fully understood on the atomic scale. Here, the electrochemical properties of β-MnO2 are investigated using density functional theory with Hubbard U corrections (DFT+U). We find good agreement between the measured experimental voltage, 3.0 V, and our calculated value of 3.2 V. We consider the pathways for lithium migration and find a small barrier of 0.17 eV for bulk β-MnO2, which is likely to contribute to its good performance as a lithium intercalation cathode in the mesoporous form. However, by explicit calculation of surface to bulk ion migration, we find a higher barrier of >0.6 eV for lithium insertion at the (101) surface that dominates the equilibrium morphology. This is likely to limit the practical use of bulk samples, and demonstrates the quantitative importance of surface to bulk ion migration in Li ion cathodes and supercapacitors. On the basis of the calculation of the electrostatic potential near the surface, we propose an efficient method to screen systems for the importance of surface migration effects. Such insight is valuable for the future optimization of manganese oxide nanomaterials for energy storage devices.Keywords: cathode; DFT; lithium battery; manganese oxides; supercapacitor; surface;
Co-reporter:David A. Tompsett and M. Saiful Islam
Chemistry of Materials 2013 Volume 25(Issue 12) pp:2515
Publication Date(Web):May 22, 2013
DOI:10.1021/cm400864n
MnO2 is attracting considerable interest in the context of rechargeable batteries, supercapacitors, and Li–O2 battery applications. This work investigates the electrochemical properties of hollandite α-MnO2 using density functional theory with Hubbard U corrections (DFT+U). The favorable insertion sites for Li-ion and Na-ion insertion are determined, and we find good agreement with measured experimental voltages. By explicit calculation of the phonons we suggest multiple insertion sites are accessible in the dilute limit. Significant structural changes in α-(Li,Na)xMnO2 during ion insertion are demonstrated by determining the low energy structures. The significant distortions to the unit cell and Mn coordination are likely to be active in causing the observed degradation of α-MnO2 with cycling. The presence of Li2O in the structure reduces these distortions significantly and is the probable cause for the good experimental cycling stability of α-[0.143Li2O]-MnO2. However, the presence of Na2O is less effective in reducing the distortion of the Na-ion intercalated form. We also find a distinct change in the favored Li-ion insertion site, not identified in previous studies, for lithiation of α-LixMnO2 at x > 0.5. The migration barriers for both Li-ions and Na-ions increase from <0.3 eV in the dilute limit to >0.48 eV for α-(Li,Na)0.75MnO2. Finally, the electronic density of states in α-MnO2 with the incorporation of Li2O has the character of a full metal, not a half metal as was suggested in previous work. This may be key to its good performance as a catalyst in Li–O2 batteries.Keywords: Li–air; cathode; DFT; Li2O; lithium battery; manganese oxides; sodium battery;
Co-reporter:C. Eames, A. R. Armstrong, P. G. Bruce, and M. S. Islam
Chemistry of Materials 2012 Volume 24(Issue 11) pp:2155
Publication Date(Web):May 23, 2012
DOI:10.1021/cm300749w
The search for new low cost, safe, and high capacity cathodes for lithium batteries has focused attention recently on Li2FeSiO4. The material presents a challenge because it exhibits complex polymorphism, and when it is electrochemically cycled there is a significant drop in the cell voltage related to a structural change. Systematic studies based on density functional theory techniques have been carried out to examine the change in cell voltages and structures for the full range of Li2FeSiO4 polymorphs (βII, γs, and γII) including the newly elucidated cycled structure (termed inverse-βII). We find that the cycled structure has a 0.18–0.30 V lower voltage than the directly synthesized polymorphs in accord with experimental observations. The trends in cell voltage have been correlated to the change in energy upon delithiation from Li2FeSiO4 to LiFeSiO4 in which the cation–cation electrostatic repulsion competes with distortion of the tetrahedral framework.Keywords: cathode; computer simulation; electronic structure; lithium battery; silicates;
Co-reporter:Satyajit Phadke, Juan C. Nino and M. Saiful Islam
Journal of Materials Chemistry A 2012 vol. 22(Issue 48) pp:25388-25394
Publication Date(Web):29 Oct 2012
DOI:10.1039/C2JM32940A
Atomistic simulation techniques are used to perform a comparative study of intrinsic defects, dopant incorporation and protonic groups in two lanthanum phosphate compounds, namely, the orthophosphate (LaPO4) and the ultraphosphate (LaP5O14). The suitability of dopant incorporation predicted from the dopant solution energies (with Ca and Sr the most favorable) is in excellent agreement with trends in ionic conductivity from recent experimental investigations. The defect chemistry of the phosphates related to protonic defects and oxygen vacancies created from extrinsic doping is investigated. The results indicate favorable orientations for the protonic defect within the structures. The binding energies for proton–dopant interactions indicate that defect association may occur. In LaPO4 it was observed that the relaxed local atomic structure around an oxygen vacancy is analogous to the formation of a P2O7 pyrophosphate anion.
Co-reporter:Jesse T. R. Dufton, Aron Walsh, Pooja M. Panchmatia, Laurie M. Peter, Diego Colombara and M. Saiful Islam
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 20) pp:7229-7233
Publication Date(Web):18 Apr 2012
DOI:10.1039/C2CP40916J
As the demand for photovoltaics rapidly increases, there is a pressing need for the identification of new visible light absorbing materials for thin-film solar cells that offer similar performance to the current technologies based on CdTe and Cu(In,Ga)Se2. Metal sulphides are the ideal candidate materials, but their band gaps are usually too large to absorb significant fractions of visible light. However, by combining Cu+ (low binding energy d10 band) and Sb3+/Bi3+ (low binding energy s2 band), the ternary sulphides CuSbS2 and CuBiS2 are formed, which have been gathering recent interest for solar cell applications. Using a hybrid density functional theory approach, we calculate the structural and electronic properties of these two materials. Our results highlight the stereochemical activity of the Sb and Bi lone pair electrons, and predict that the formation of hole carriers will occur in the Cu d10 band and hence will involve oxidation of Cu(I).
Co-reporter:John M. Clark;Dr. Shin-ichi Nishimura; Atsuo Yamada; M. Saiful Islam
Angewandte Chemie 2012 Volume 124( Issue 52) pp:13326-13330
Publication Date(Web):
DOI:10.1002/ange.201205997
Co-reporter:John M. Clark;Dr. Shin-ichi Nishimura; Atsuo Yamada; M. Saiful Islam
Angewandte Chemie International Edition 2012 Volume 51( Issue 52) pp:13149-13153
Publication Date(Web):
DOI:10.1002/anie.201205997
Co-reporter:A. Robert Armstrong ; Navaratnarajah Kuganathan ; M. Saiful Islam ;Peter G. Bruce
Journal of the American Chemical Society 2011 Volume 133(Issue 33) pp:13031-13035
Publication Date(Web):July 8, 2011
DOI:10.1021/ja2018543
The importance of exploring new low-cost and safe cathodes for large-scale lithium batteries has led to increasing interest in Li2FeSiO4. The structure of Li2FeSiO4 undergoes significant change on cycling, from the as-prepared γs form to an inverse βII polymorph; therefore it is important to establish the structure of the cycled material. In γs half the LiO4, FeO4, and SiO4 tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse βII polymorph on cycling involves inversion of half the SiO4, FeO4, and LiO4 tetrahedra, such that they all now point in the same direction, eliminating edge sharing between cation sites and flattening the oxygen layers. As a result of the structural changes, Li+ transport paths and corresponding Li–Li separations in the cycled structure are quite different from the as-prepared material, as revealed here by computer modeling, and involve distinct zigzag paths between both Li sites and through intervening unoccupied octahedral sites that share faces with the LiO4 tetrahedra.
Co-reporter:Rajesh Tripathi, Grahame R. Gardiner, M. Saiful Islam, and Linda F. Nazar
Chemistry of Materials 2011 Volume 23(Issue 8) pp:2278
Publication Date(Web):March 30, 2011
DOI:10.1021/cm200683n
A new family of fluorosulfates has attracted considerable attention as alternative positive electrode materials for rechargeable lithium batteries. However, an atomic-scale understanding of the ion conduction paths in these systems is still lacking, and this is important for developing strategies for optimization of the electrochemical properties. Here, the alkali-ion transport behavior of both LiFeSO4F and NaFeSO4F are investigated by atomistic modeling methods. Activation energies for numerous ion migration paths through the complex structures are calculated. The results indicate that LiFeSO4F is effectively a three-dimensional (3D) lithium-ion conductor with an activation energy of ∼0.4 eV for long-range diffusion, which involve a combination of zigzag paths through [100], [010], and [111] tunnels in the open tavorite lattice. In contrast, for the related NaFeSO4F, only one direction ([101]) is found to have a relatively low activation energy (0.6 eV). This leads to a diffusion coefficient that is more than 6 orders of magnitude lower than any other direction, suggesting that NaFeSO4F is a one-dimensional (1D) Na-ion conductor.Keywords: atomistic modeling; ion transport; Li-ion battery; lithium ion conductor; lithium iron fluorosulfate; sodium ion conductor; sodium iron fluorosulfate;
Co-reporter:M. Saiful Islam, Robert Dominko, Christian Masquelier, Chutchamon Sirisopanaporn, A. Robert Armstrong and Peter G. Bruce
Journal of Materials Chemistry A 2011 vol. 21(Issue 27) pp:9811-9818
Publication Date(Web):29 Mar 2011
DOI:10.1039/C1JM10312A
Polyoxyanion compounds, particularly the olivine-phosphate LiFePO4, are receiving considerable attention as alternative cathodes for rechargeable lithium batteries. More recently, an entirely new class of polyoxyanion cathodes based on the orthosilicates, Li2MSiO4 (where M = Mn, Fe, and Co), has been attracting growing interest. In the case of Li2FeSiO4, iron and silicon are among the most abundant and lowest cost elements, and hence offer the tantalising prospect of preparing cheap and safe cathodes from rust and sand! This Highlight presents an overview of recent developments and future challenges of silicate cathode materials focusing on their structural polymorphs, electrochemical behaviour and nanomaterials chemistry.
Co-reporter:Dr. Pooja M. Panchmatia;Dr. Alodia Orera;Gregory J. Rees; Mark E. Smith;Dr. John V. Hanna;Dr. Peter R. Slater; M. Saiful Islam
Angewandte Chemie International Edition 2011 Volume 50( Issue 40) pp:9328-9333
Publication Date(Web):
DOI:10.1002/anie.201102064
Co-reporter:Lorenzo Malavasi, Craig A. J. Fisher and M. Saiful Islam
Chemical Society Reviews 2010 vol. 39(Issue 11) pp:4370-4387
Publication Date(Web):17 Sep 2010
DOI:10.1039/B915141A
This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).
Co-reporter:Cristina Tealdi;Piercarlo Mustarelli
Advanced Functional Materials 2010 Volume 20( Issue 22) pp:
Publication Date(Web):
DOI:10.1002/adfm.201090100
Abstract
Novel melilite-type gallium-oxides are attracting attention as promising new oxide-ion conductors with potential use in clean energy devices such as solid oxide fuel cells. Here, an atomic-scale investigation of the LaSrGa3O7-based system using advanced simulation techniques provides valuable insights into the defect chemistry and oxide ion conduction mechanisms, and includes comparison with the available experimental data. The simulation model reproduces the observed complex structure composed of layers of corner-sharing GaO4 tetrahedra. A major finding is the first indication that oxide-ion conduction in La1.54Sr0.46Ga3O7.27 occurs through an interstitialcy or cooperative-type mechanism involving the concerted knock-on motion of interstitial and lattice oxide ions. A key feature for the transport mechanism and high ionic conductivity is the intrinsic flexibility of the structure, which allows considerable local relaxation and changes in Ga coordination.
Co-reporter:Cristina Tealdi;Piercarlo Mustarelli
Advanced Functional Materials 2010 Volume 20( Issue 22) pp:3874-3880
Publication Date(Web):
DOI:10.1002/adfm.201001137
Abstract
Novel melilite-type gallium-oxides are attracting attention as promising new oxide-ion conductors with potential use in clean energy devices such as solid oxide fuel cells. Here, an atomic-scale investigation of the LaSrGa3O7-based system using advanced simulation techniques provides valuable insights into the defect chemistry and oxide ion conduction mechanisms, and includes comparison with the available experimental data. The simulation model reproduces the observed complex structure composed of layers of corner-sharing GaO4 tetrahedra. A major finding is the first indication that oxide-ion conduction in La1.54Sr0.46Ga3O7.27 occurs through an interstitialcy or cooperative-type mechanism involving the concerted knock-on motion of interstitial and lattice oxide ions. A key feature for the transport mechanism and high ionic conductivity is the intrinsic flexibility of the structure, which allows considerable local relaxation and changes in Ga coordination.
Co-reporter:Grahame R. Gardiner and M. Saiful Islam
Chemistry of Materials 2010 Volume 22(Issue 3) pp:1242
Publication Date(Web):December 22, 2009
DOI:10.1021/cm902720z
Olivine-type phosphates have attracted considerable attention as cathode materials for rechargeable lithium batteries. Here, the defect and ion transport properties of the mixed-metal material LiFe0.5Mn0.5PO4 are investigated by atomistic modeling methods. The intrinsic defect type with the lowest energy is the cation antisite defect, in which Li and Fe/Mn ions exchange positions. As found in the LiFePO4 material, lithium ion diffusion in the mixed-metal system occurs down the b-axis channels following a curved path. Migration energies for Fe and Mn antisite cations on Li sites suggest that Mn defects would impede bulk Li mobility in LiFe0.5Mn0.5PO4 to a greater extent than Fe antisite defects in LiFePO4. Association or binding energies for various defect clusters comprised of lithium vacancies and/or antisite cations are examined.
Co-reporter:Stephen J. Stokes and M. Saiful Islam
Journal of Materials Chemistry A 2010 vol. 20(Issue 30) pp:6258-6264
Publication Date(Web):22 Jun 2010
DOI:10.1039/C0JM00328J
Defect reactions, water incorporation and proton-dopant association in the BaZrO3 and BaPrO3 perovskite materials are investigated using well-established atomistic simulation techniques. The interatomic potential models reproduce the experimental cubic BaZrO3 and orthorhombic BaPrO3 structures. The high defect energies suggest that significant intrinsic disorder (either Frenkel, Schottky or reduction) in BaZrO3 is unlikely, which is consistent with the relative chemical stability of this system. In contrast, favourable redox processes are found for intrinsic reduction of BaPrO3, and oxidation of acceptor-doped BaPrO3, the latter leading to p-type conduction properties as observed experimentally. Binding energies for dopant-OH pairs in BaZrO3 indicate the weakest association for Gd and Y dopants, and the strongest association for Sc. The high binding energies for all the dopant-OH pair clusters in BaPrO3 suggest strong proton trapping effects, which would be detrimental to proton conductivity. The water incorporation or hydration energy is found to be less exothermic for BaZrO3 than for BaPrO3, the higher exothermic value for the latter suggesting that water incorporation extends to higher temperatures in accord with the available thermodynamic data. The energies and pathways for oxide ion migration in both materials are also investigated.
Co-reporter:P. M. Panchmatia, A. Orera, E. Kendrick, J. V. Hanna, M. E. Smith, P. R. Slater and M. S. Islam
Journal of Materials Chemistry A 2010 vol. 20(Issue 14) pp:2766-2772
Publication Date(Web):18 Feb 2010
DOI:10.1039/B924220A
Apatite-type oxide-ion conductors have attracted considerable interest as potential fuel cell electrolytes. Atomistic modelling techniques have been used to investigate oxygen interstitial sites, protonic defects and water incorporation in three silicate and three germanate-based apatite-systems, namely La8Ba2(SiO4)6O2, La9.33(SiO4)6O2, La9.67(SiO4)6O2.5, La8Ba2(GeO4)6O2, La9.33(GeO4)6O2, and La9.67(GeO4)6O2.5. The simulation models reproduce the complex experimental structures for all of these systems. The interstitial defect simulations have examined the lowest energy configuration and confirm this site to be near the Si/GeO4 tetrahedra. The water incorporation calculations identify the O–H protonic site to be along the O4 oxygen channel as seen in naturally occurring hydroxy-apatites. The results also show more favourable and exothermic water incorporation energies for the germanate-based apatites. This is consistent with recent experimental work, which shows that Ge-apatites take up water more readily than the silicate analogues.
Co-reporter:Corinne Arrouvel, Stephen C. Parker and M. Saiful Islam
Chemistry of Materials 2009 Volume 21(Issue 20) pp:4778
Publication Date(Web):October 2, 2009
DOI:10.1021/cm900373u
TiO2−B is a highly promising anode material for rechargeable lithium batteries. Computational studies based on density functional theory (DFT) have been carried out on this material focusing on key issues relating to lithium insertion sites and lithium diffusion paths. Our simulation model shows good reproduction of the observed crystal structure of TiO2−B. Electronic structure calculations suggest that the lowest energy lithium site is a slightly off-center position in the b-axis channel for low lithium concentration (x < 0.125 for LixTiO2−B). Our calculated cell voltages are compatible with values from electrochemical measurements. Low Li migration energies are found for pathways along the b-axis channel and the [001] c-axis direction, suggesting significant Li ion mobility in this anode material.
Co-reporter:N. Kuganathan and M. S. Islam
Chemistry of Materials 2009 Volume 21(Issue 21) pp:5196
Publication Date(Web):October 9, 2009
DOI:10.1021/cm902163k
A new family of silicate materials has attracted interest for potential use in rechargeable lithium batteries. The defect chemistry, doping behavior, and lithium diffusion paths in the Li2MnSiO4 cathode material are investigated by advanced modeling techniques. Our simulations show good reproduction of both monoclinic and orthorhombic structures of Li2MnSiO4. The most favorable intrinsic defect type is found to be the cation anti-site defect, in which Li and Mn ions exchange positions. The migration energies suggest differences in intrinsic Li mobility between the monoclinic and orthorhombic polymorphs, which would affect their rate capability as rechargeable electrodes. The results indicate curved Li diffusion paths and confirm the anisotropic nature of Li transport, which is probably general for the Li2MSiO4 (M = Mn, Fe, Co) family of compounds. Subvalent doping by Al on the Si site is energetically favorable and could be a synthesis strategy to increase the Li content.
Co-reporter:Alison Jones, Peter R. Slater and M. Saiful Islam
Chemistry of Materials 2008 Volume 20(Issue 15) pp:5055
Publication Date(Web):July 16, 2008
DOI:10.1021/cm801101j
Apatite-type oxides of general formula La9.33+x(SiO4)6O2 + 3x/2 have been attracting considerable interest recently because of their observed high oxide-ion conductivity and potential use in solid oxide fuel cells (SOFCs), oxygen sensors, and ceramic membranes. In this paper, computer modeling techniques are used to investigate, at the atomic level, the energetics of defect formation, oxide-ion migration, and cation migration in the oxygen-excess apatite silicate, La9.67(SiO4)6O2.5. Recent research has suggested that oxide-ion conduction in these apatite systems proceeds by an interstitial mechanism. Our results support this view and have revealed how the flexibility of the SiO4 substructure plays a crucial role in facilitating oxide-ion migration: the presence of interstitial oxide ions creates pseudo-“SiO5” units, which can effectively pass along the c direction by oxygen transfer. La vacancy migration is also examined and, as expected, found to have a high energy barrier.
Co-reporter:Craig A. J. Fisher, Veluz M. Hart Prieto and M. Saiful Islam
Chemistry of Materials 2008 Volume 20(Issue 18) pp:5907
Publication Date(Web):August 26, 2008
DOI:10.1021/cm801262x
The defect chemistry, doping behavior, and ion migration in olivine-type materials LiMPO4 (M = Mn, Fe, Co, and N) are investigated by atomistic simulation techniques. The most favorable intrinsic defect type is found to be the cation antisite defect, in which Li and M ions exchange positions. Li migration is found to occur preferentially down [010] channels, following a curved trajectory. Defect association or binding energies for pair clusters composed of combinations of lithium vacancies, antisite cations, and small polaron species are investigated. Migration energies for divalent antisite cations on Li sites suggest that such defects would impede Li diffusion in LiMPO4 to varying degrees. Calculation of dopant substitution energies for cations with charges +1 to +5 indicate that supervalent doping (e.g., Ga3+, Ti4+, Nb5+) on either Li or M sites is energetically unfavorable and does not result in a large increase in electronic (small polaron) species.
Co-reporter:Craig A. J. Fisher and M. Saiful Islam
Journal of Materials Chemistry A 2008 vol. 18(Issue 11) pp:1209-1215
Publication Date(Web):31 Jan 2008
DOI:10.1039/B715935H
Advanced simulation techniques are used to provide atomic-scale insight into the surface structures and crystal morphologies of the lithium battery cathode material LiFePO4. Relaxed surface structures and energies are reported for 19 low index planes. The calculated equilibrium morphology takes on a rounded, isometric appearance, with {010}, {201}, {011}, and {100} faces prominent. Almost all of the low energy surfaces are lithium-deficient relative to the bulk lattice, requiring Li vacancies at the surface. The calculated growth morphology exhibits the {010}, {100} and {101} faces, with an elongated hexagonal prism-like shape; this morphology is more consistent with experimentally observed LiFePO4 particles. The exposure of the (010) surface in our calculated equilibrium and growth morphologies is significant since it is normal to the most facile pathway for lithium ion conduction (along the [010] channel), and hence important for the reversible insertion/de-insertion of lithium ions. SEM images of plate-like crystallites from hydrothermal synthesis are also simulated by our methods, and exhibit large (010) faces.
Co-reporter:J. R. Tolchard;P. R. Slater;M. S. Islam
Advanced Functional Materials 2007 Volume 17(Issue 14) pp:
Publication Date(Web):21 AUG 2007
DOI:10.1002/adfm.200600789
Novel apatite-type silicates are attracting considerable interest as a new family of oxide-ion conductors with potential use in fuel cells and ceramic membranes. Combined computer modeling and X-ray absorption (EXAFS) techniques have been used to gain fresh insight, at the atomic level, into the site selectivity and local structures of a wide range of dopants in these apatite materials. The results indicate that an unusually broad range of dopant ions (in terms of size and charge state) can substitute for La in the La9.33Si6O26 apatite, in accord with current experimental data. The range is much wider than that observed for doping on a single cation site in most other oxide-ion conductors, such as the perovskite LaGaO3. In addition, our local structural investigation demonstrates that this dopant behavior is related to the flexibility of the silicate substructure, which allows relatively large local distortion and alteration of the site volumes. This could be a key factor in the high oxide-ion conductivity exhibited by these apatite silicates. Indeed, the breadth of possible doping regimes in these novel materials provides new opportunities to design and optimize the conduction properties for fuel cell electrolytes.
Co-reporter:John M. Clark, Christopher Eames, Marine Reynaud, Gwenaëlle Rousse, Jean-Noël Chotard, Jean-Marie Tarascon and M. Saiful Islam
Journal of Materials Chemistry A 2014 - vol. 2(Issue 20) pp:NaN7453-7453
Publication Date(Web):2014/03/27
DOI:10.1039/C3TA15064J
The search for high voltage cathodes for lithium-ion batteries has led to recent interest in the monoclinic Li2Fe(SO4)2 material which has a voltage of 3.83 V vs. lithium, the highest recorded for a fluorine-free iron-based compound. Here we investigate the defect, surface and lithium migration properties of the Li2M(SO4)2 (M = Fe, Mn, Co) materials using combined atomistic modelling and density functional theory (DFT) techniques. All intrinsic defect types including Li/M antisite disorder are found to be of high energy, suggesting insignificant concentrations. Low activation energies are found for lithium migration along the a-axis channels giving rise to long-range 1D diffusion, which are supported by molecular dynamics (MD) simulations. For the crystal morphology a significant surface area is exposed to these 1D diffusion channels, which would allow facile Li insertion and extraction. Using DFT simulations we reproduce the high voltage of the Li2Fe(SO4)2 material in accord with electrochemical data and also examine local structural distortions on lithium extraction.
Co-reporter:M. Saiful Islam, Robert Dominko, Christian Masquelier, Chutchamon Sirisopanaporn, A. Robert Armstrong and Peter G. Bruce
Journal of Materials Chemistry A 2011 - vol. 21(Issue 27) pp:NaN9818-9818
Publication Date(Web):2011/03/29
DOI:10.1039/C1JM10312A
Polyoxyanion compounds, particularly the olivine-phosphate LiFePO4, are receiving considerable attention as alternative cathodes for rechargeable lithium batteries. More recently, an entirely new class of polyoxyanion cathodes based on the orthosilicates, Li2MSiO4 (where M = Mn, Fe, and Co), has been attracting growing interest. In the case of Li2FeSiO4, iron and silicon are among the most abundant and lowest cost elements, and hence offer the tantalising prospect of preparing cheap and safe cathodes from rust and sand! This Highlight presents an overview of recent developments and future challenges of silicate cathode materials focusing on their structural polymorphs, electrochemical behaviour and nanomaterials chemistry.
Co-reporter:P. M. Panchmatia, A. Orera, E. Kendrick, J. V. Hanna, M. E. Smith, P. R. Slater and M. S. Islam
Journal of Materials Chemistry A 2010 - vol. 20(Issue 14) pp:NaN2772-2772
Publication Date(Web):2010/02/18
DOI:10.1039/B924220A
Apatite-type oxide-ion conductors have attracted considerable interest as potential fuel cell electrolytes. Atomistic modelling techniques have been used to investigate oxygen interstitial sites, protonic defects and water incorporation in three silicate and three germanate-based apatite-systems, namely La8Ba2(SiO4)6O2, La9.33(SiO4)6O2, La9.67(SiO4)6O2.5, La8Ba2(GeO4)6O2, La9.33(GeO4)6O2, and La9.67(GeO4)6O2.5. The simulation models reproduce the complex experimental structures for all of these systems. The interstitial defect simulations have examined the lowest energy configuration and confirm this site to be near the Si/GeO4 tetrahedra. The water incorporation calculations identify the O–H protonic site to be along the O4 oxygen channel as seen in naturally occurring hydroxy-apatites. The results also show more favourable and exothermic water incorporation energies for the germanate-based apatites. This is consistent with recent experimental work, which shows that Ge-apatites take up water more readily than the silicate analogues.
Co-reporter:Jesse T. R. Dufton, Aron Walsh, Pooja M. Panchmatia, Laurie M. Peter, Diego Colombara and M. Saiful Islam
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 20) pp:NaN7233-7233
Publication Date(Web):2012/04/18
DOI:10.1039/C2CP40916J
As the demand for photovoltaics rapidly increases, there is a pressing need for the identification of new visible light absorbing materials for thin-film solar cells that offer similar performance to the current technologies based on CdTe and Cu(In,Ga)Se2. Metal sulphides are the ideal candidate materials, but their band gaps are usually too large to absorb significant fractions of visible light. However, by combining Cu+ (low binding energy d10 band) and Sb3+/Bi3+ (low binding energy s2 band), the ternary sulphides CuSbS2 and CuBiS2 are formed, which have been gathering recent interest for solar cell applications. Using a hybrid density functional theory approach, we calculate the structural and electronic properties of these two materials. Our results highlight the stereochemical activity of the Sb and Bi lone pair electrons, and predict that the formation of hole carriers will occur in the Cu d10 band and hence will involve oxidation of Cu(I).
Co-reporter:David A. Tompsett, Stephen C. Parker and M. Saiful Islam
Journal of Materials Chemistry A 2014 - vol. 2(Issue 37) pp:NaN15518-15518
Publication Date(Web):2014/07/29
DOI:10.1039/C4TA00952E
Hollandite (α-)MnO2 gives superior performance compared to other MnO2 polymorphs in surface sensitive applications in supercapacitors and catalysis. However, a thorough understanding of its atomic-scale surface properties is lacking, which we address here using density functional theory (DFT). A Wulff construction based upon relaxed surface energies demonstrates that the equilibrium morphology expresses the low index (100), (110) and (111) surfaces as well as the high index (211) and (112) surfaces. The predicted morphology exhibits clear elongation along the c-axis which is consistent with the large number of nanorod type structures that are obtainable experimentally. The surface structures expressed in the morphology are discussed in detail and it is found that α-MnO2 gives rise to larger surface relaxations than are observed for the less open rutile structured MnO2. Enhanced magnetic moments at surface sites are rationalised by a crystal field argument. Experimental studies consistently find that α-MnO2 has higher catalytic activity than other polymorphs of MnO2. In this work, calculated formation energies for oxygen vacancy defects at the expressed surfaces are demonstrably lower, by ∼1 eV, than for rutile MnO2 surfaces [Tompsett et al., JACS, 2014, 136, 1418]. The lowest vacancy formation energy occurs at the (112) surface, which despite its relative high Miller index constitutes 17% of the surface area of the calculated morphology. This may play a key role in the favourable catalytic performance observed for α-MnO2 in a broad range of applications.
Co-reporter:Craig A. J. Fisher and M. Saiful Islam
Journal of Materials Chemistry A 2008 - vol. 18(Issue 11) pp:NaN1215-1215
Publication Date(Web):2008/01/31
DOI:10.1039/B715935H
Advanced simulation techniques are used to provide atomic-scale insight into the surface structures and crystal morphologies of the lithium battery cathode material LiFePO4. Relaxed surface structures and energies are reported for 19 low index planes. The calculated equilibrium morphology takes on a rounded, isometric appearance, with {010}, {201}, {011}, and {100} faces prominent. Almost all of the low energy surfaces are lithium-deficient relative to the bulk lattice, requiring Li vacancies at the surface. The calculated growth morphology exhibits the {010}, {100} and {101} faces, with an elongated hexagonal prism-like shape; this morphology is more consistent with experimentally observed LiFePO4 particles. The exposure of the (010) surface in our calculated equilibrium and growth morphologies is significant since it is normal to the most facile pathway for lithium ion conduction (along the [010] channel), and hence important for the reversible insertion/de-insertion of lithium ions. SEM images of plate-like crystallites from hydrothermal synthesis are also simulated by our methods, and exhibit large (010) faces.
Co-reporter:Satyajit Phadke, Juan C. Nino and M. Saiful Islam
Journal of Materials Chemistry A 2012 - vol. 22(Issue 48) pp:NaN25394-25394
Publication Date(Web):2012/10/29
DOI:10.1039/C2JM32940A
Atomistic simulation techniques are used to perform a comparative study of intrinsic defects, dopant incorporation and protonic groups in two lanthanum phosphate compounds, namely, the orthophosphate (LaPO4) and the ultraphosphate (LaP5O14). The suitability of dopant incorporation predicted from the dopant solution energies (with Ca and Sr the most favorable) is in excellent agreement with trends in ionic conductivity from recent experimental investigations. The defect chemistry of the phosphates related to protonic defects and oxygen vacancies created from extrinsic doping is investigated. The results indicate favorable orientations for the protonic defect within the structures. The binding energies for proton–dopant interactions indicate that defect association may occur. In LaPO4 it was observed that the relaxed local atomic structure around an oxygen vacancy is analogous to the formation of a P2O7 pyrophosphate anion.
Co-reporter:Alexander Whiteside, Craig A. J. Fisher, Stephen C. Parker and M. Saiful Islam
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 39) pp:NaN21794-21794
Publication Date(Web):2014/09/01
DOI:10.1039/C4CP02356K
The expansion of batteries into electric vehicle and grid storage applications has driven the development of new battery materials and chemistries, such as olivine phosphate cathodes and sodium-ion batteries. Here we present atomistic simulations of the surfaces of olivine-structured NaFePO4 as a sodium-ion battery cathode, and discuss differences in its morphology compared to the lithium analogue LiFePO4. The calculated equilibrium morphology is mostly isometric in appearance, with (010), (201) and (011) faces dominant. Exposure of the (010) surface is vital because it is normal to the one-dimensional ion-conduction pathway. Platelet and cube-like shapes observed by previous microscopy studies are reproduced by adjusting surface energies. The results indicate that a variety of (nano)particle morphologies can be achieved by tuning surface stabilities, which depend on synthesis methods and solvent conditions, and will be important in optimising electrochemical performance.
Co-reporter:Stephen J. Stokes and M. Saiful Islam
Journal of Materials Chemistry A 2010 - vol. 20(Issue 30) pp:NaN6264-6264
Publication Date(Web):2010/06/22
DOI:10.1039/C0JM00328J
Defect reactions, water incorporation and proton-dopant association in the BaZrO3 and BaPrO3 perovskite materials are investigated using well-established atomistic simulation techniques. The interatomic potential models reproduce the experimental cubic BaZrO3 and orthorhombic BaPrO3 structures. The high defect energies suggest that significant intrinsic disorder (either Frenkel, Schottky or reduction) in BaZrO3 is unlikely, which is consistent with the relative chemical stability of this system. In contrast, favourable redox processes are found for intrinsic reduction of BaPrO3, and oxidation of acceptor-doped BaPrO3, the latter leading to p-type conduction properties as observed experimentally. Binding energies for dopant-OH pairs in BaZrO3 indicate the weakest association for Gd and Y dopants, and the strongest association for Sc. The high binding energies for all the dopant-OH pair clusters in BaPrO3 suggest strong proton trapping effects, which would be detrimental to proton conductivity. The water incorporation or hydration energy is found to be less exothermic for BaZrO3 than for BaPrO3, the higher exothermic value for the latter suggesting that water incorporation extends to higher temperatures in accord with the available thermodynamic data. The energies and pathways for oxide ion migration in both materials are also investigated.
Co-reporter:Julian Roos, Christopher Eames, Stephen M. Wood, Alexander Whiteside and M. Saiful Islam
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 34) pp:NaN22265-22265
Publication Date(Web):2015/07/29
DOI:10.1039/C5CP02711J
The recently discovered lithium-rich cathode material Li7Mn(BO3)3 has a high theoretical capacity and an unusual tetrahedral Mn2+ coordination. Atomistic simulation and density functional theory (DFT) techniques are employed to provide insights into the defect and redox chemistry, the structural changes upon lithium extraction and the mechanisms of lithium ion diffusion. The most favourable intrinsic defects are Li/Mn anti-site pairs, where Li and Mn ions occupy interchanged positions, and Li Frenkel defects. DFT calculations reproduce the experimental cell voltage and confirm the presence of the unusually high MnV redox state, which corresponds to a theoretical capacity of nearly 288 mA h g−1. The ability to reach the high manganese oxidation state is related to both the initial tetrahedral coordination of Mn and the observed distortion/tilting of the BO3 units to accommodate the contraction of the Mn–O bonds upon oxidation. Molecular dynamics (MD) simulations indicate fast three-dimensional lithium diffusion in line with the good rate performance observed.
Co-reporter:John M. Clark, Prabeer Barpanda, Atsuo Yamada and M. Saiful Islam
Journal of Materials Chemistry A 2014 - vol. 2(Issue 30) pp:NaN11812-11812
Publication Date(Web):2014/06/05
DOI:10.1039/C4TA02383H
Na-ion batteries are currently the focus of significant research activity due to the relative abundance of sodium and its consequent cost advantages. Recently, the pyrophosphate family of cathodes has attracted considerable attention, particularly Li2FeP2O7 related to its high operating voltage and enhanced safety properties; in addition the sodium-based pyrophosphates Na2FeP2O7 and Na2MnP2O7 are also generating interest. Herein, we present defect chemistry and ion migration results, determined via atomistic simulation techniques, for Na2MP2O7 (where M = Fe, Mn) as well as findings for Li2FeP2O7 for direct comparison. Within the pyrophosphate framework the most favourable intrinsic defect type is found to be the antisite defect, in which alkali-cations (Na/Li) and M ions exchange positions. Low activation energies are found for long-range diffusion in all crystallographic directions in Na2MP2O7 suggesting three-dimensional (3D) Na-ion diffusion. In contrast Li2FeP2O7 supports 2D Li-ion diffusion. The 2D or 3D nature of the alkali-ion migration pathways within these pyrophosphate materials means that antisite defects are much less likely to impede their transport properties, and hence important for high rate performance.
Co-reporter:M. Saiful Islam and Craig A. J. Fisher
Chemical Society Reviews 2014 - vol. 43(Issue 1) pp:NaN204-204
Publication Date(Web):2013/11/07
DOI:10.1039/C3CS60199D
Energy storage technologies are critical in addressing the global challenge of clean sustainable energy. Major advances in rechargeable batteries for portable electronics, electric vehicles and large-scale grid storage will depend on the discovery and exploitation of new high performance materials, which requires a greater fundamental understanding of their properties on the atomic and nanoscopic scales. This review describes some of the exciting progress being made in this area through use of computer simulation techniques, focusing primarily on positive electrode (cathode) materials for lithium-ion batteries, but also including a timely overview of the growing area of new cathode materials for sodium-ion batteries. In general, two main types of technique have been employed, namely electronic structure methods based on density functional theory, and atomistic potentials-based methods. A major theme of much computational work has been the significant synergy with experimental studies. The scope of contemporary work is highlighted by studies of a broad range of topical materials encompassing layered, spinel and polyanionic framework compounds such as LiCoO2, LiMn2O4 and LiFePO4 respectively. Fundamental features important to cathode performance are examined, including voltage trends, ion diffusion paths and dimensionalities, intrinsic defect chemistry, and surface properties of nanostructures.
Co-reporter:Pooja M. Panchmatia, A. Robert Armstrong, Peter G. Bruce and M. Saiful Islam
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 39) pp:NaN21118-21118
Publication Date(Web):2014/07/03
DOI:10.1039/C4CP01640H
Layered Li1+xV1−xO2 has attracted recent interest as a potential low voltage and high energy density anode material for lithium-ion batteries. A greater understanding of the lithium-ion transport mechanisms is important in optimising such oxide anodes. Here, stoichiometric LiVO2 and Li-rich Li1.07V0.93O2 are investigated using atomistic modelling techniques. Lithium-ion migration is not found in LiVO2, which has also previously shown to be resistant to lithium intercalation. Molecular dynamics simulations of lithiated non-stoichiometric Li1.07+yV0.93O2 suggest cooperative interstitial Li+ diffusion with favourable migration barriers and diffusion coefficients (DLi), which are facilitated by the presence of lithium in the transition metal layers; such transport behaviour is important for high rate performance as a battery anode.
Co-reporter:Lorenzo Malavasi, Craig A. J. Fisher and M. Saiful Islam
Chemical Society Reviews 2010 - vol. 39(Issue 11) pp:NaN4387-4387
Publication Date(Web):2010/09/17
DOI:10.1039/B915141A
This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).
Co-reporter:Joshua C. Treacher, Stephen M. Wood, M. Saiful Islam and Emma Kendrick
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 48) pp:NaN32752-32752
Publication Date(Web):2016/11/16
DOI:10.1039/C6CP06777H
The importance of developing new low-cost and safe cathodes for large-scale sodium batteries has led to recent interest in silicate compounds. A novel cobalt orthosilicate, Na2CoSiO4, shows promise as a high voltage (3.3 V vs. Na/Na+) cathode material for sodium-ion batteries. Here, the synthesis and room temperature electrochemical performance of Na2CoSiO4 have been investigated with the compound found to yield a reversible capacity greater than 100 mA h g−1 at a rate of 5 mA g−1. Insights into the crystal structures of Na2CoSiO4 were obtained through refinement of structural models for its two polymorphs, Pn and Pbca. Atomistic modelling results indicate that intrinsic defect levels are not significant and that Na+ diffusion follows 3D pathways with low activation barriers, which suggest favourable electrode kinetics. The new findings presented here provide a platform on which future optimisation of Na2CoSiO4 as a cathode for Na-ion batteries can be based.
Co-reporter:Jennifer Heath, Hungru Chen and M. Saiful Islam
Journal of Materials Chemistry A 2017 - vol. 5(Issue 25) pp:NaN13167-13167
Publication Date(Web):2017/05/31
DOI:10.1039/C7TA03201C
Developing rechargeable magnesium batteries has become an area of growing interest as an alternative to lithium-ion batteries largely due to their potential to offer increased energy density from the divalent charge of the Mg ion. Unlike the lithium silicates for Li-ion batteries, MgFeSiO4 can adopt the olivine structure as observed for LiFePO4. Here we combine advanced modelling techniques based on energy minimization, molecular dynamics (MD) and density functional theory to explore the Mg-ion conduction, doping and voltage behaviour of MgFeSiO4. The Mg-ion migration activation energy is relatively low for a Mg-based cathode, and MD simulations predict a diffusion coefficient (DMg) of 10−9 cm2 s−1, which suggest favourable electrode kinetics. Partial substitution of Fe by Co or Mn could increase the cell voltage from 2.3 V vs. Mg/Mg2+ to 2.8–3.0 V. The new fundamental insights presented here should stimulate further work on low-cost silicate cathodes for Mg batteries.
