MXenes represent a burgeoning family of two-dimensional (2D) functional materials with a variety of applications that highly rely on termination-mediated surface functionalization, but the understanding of termination is limited. Here, we take Ti3C2T2 (T = O, F, OH, and H) as an example of MXenes, to demonstrate how termination stabilizes the Ti3C2 monolayer matrix by saturating the nonbonding valence electrons of the surface Ti atom by the low-energy orbitals of the termination. This is achieved by orbitally resolved density of states analysis by simply yet efficiently manipulating the internal coordination of the octahedral crystal field to match exactly the Cartesian coordination. Highly degenerate 3d orbitals of surface Ti split in such a way that it exhibits pseudogaps whose widths predict a stability order Ti3C2O2 > Ti3C2F2 > Ti3C2(OH)2 > Ti3C2H2 > Ti3C2, consistent with Bader charge analysis, thermodynamic calculations, and experimental results. This new criterion could have implications in the general context of ubiquitous termination phenomena of MXenes and other relevant termination-functionalized 2D materials.

Nb-based ‘312’ MAX phase has not been recognized so far, raising a hypothesis that Nb doping would destabilize the isostructural Ti3AlC2. Here we report that (Ti1−xNbx)3AlC2 could persist with a doping limitation up to x = 0.15. As demonstrated by HAADF-STEM analysis, Nb dopants homogeneously distribute among polycrystalline grains at the microscale and randomly occupy the Ti sites at the atomic level. Beyond the limitation, Nb-doped ‘312’ phase Ti3AlC2 decomposes into (Ti,Nb)C, Nb-doped ‘211’ phase Ti2AlC, and Nb-based ‘413’ phase. Compared to pristine Ti3AlC2, the compressive strength of (Ti0.9Nb0.1)3AlC2 at 1200 °C increases by 130%, whereas doping at this level impairs the oxidation resistance. Improving high-temperature strength without deteriorating oxidation resistance can be achieved by 5% Nb doping.

Porous LiMn2O4 hollow microspheres were facilely prepared by incorporation of Li and Mn elements into a spherical polymeric precursor through copolymerization of lithium and manganese acetates with resorcinol and hexamethylenetetramine and then burning off the organic matrix at appropriate temperatures in air. The LiMn2O4 inherited the spherical morphology of the polymeric precursor but showed hollow porous structure assembled by nanocrystals of about 50–100 nm in size. When tested as cathode of Li-ion batteries, the LiMn2O4 hollow spheres exhibited excellent rate capability and cycle stability. A discharge capacity of above 90 mAh g−1 was maintained at 10 C (1 C = 120 mA g−1), and the cells can still deliver a discharge capacity over 100 mAh g−1 after another 115 cycles at 0.5 C. With such excellent electrochemical properties, the prepared LiMn2O4 hollow microspheres could be promising cathode of Li-ion batteries for long term and high power applications.

Carbon vacancies are commonly present in two-dimensional (2D) MXenes that hold promise in a variety of applications whereas their behavior remains unknown. Here we report on the influence of carbon vacancies on the structural stability, electronic properties and stiffness of MXenes by taking Ti2CT2 (T = O, F, and OH) as an example. According to the first-principles calculations, the formation energies of carbon vacancies in MXenes are lower than those in other typical 2D materials including graphene and MoS2, in combination with high migration energies. These two features mean that carbon-vacant MXenes are thermodynamically and dynamically stable as further evidenced by the absence of structural reconstruction both in the ground state and at ambient temperature. Interestingly, carbon vacancies that are usually considered as defects substantially offer a new opportunity on at least two aspects: enhanced electronic conduction and reduced stiffness corresponding to improved flexibility. The localized states in the vicinity of the Fermi level introduced by carbon vacancies account for the prominent metallic characteristics in carbon-vacant Ti2CT2 MXenes.

[100] is believed to be a tough diffusion direction for Li+ in LiFePO4, leading to the belief that the rate performance of [100]-oriented LiFePO4 is poor. Here we report the fabrication of 12 nm-thick [100]-oriented LiFePO4 nanoflakes by a simple one-pot solvothermal method. The nanoflakes exhibit unexpectedly excellent electrochemical performance, in stark contrast to what was previously believed. Such an exceptional result is attributed to a decreased thermodynamic transformation barrier height (Δμb) associated with increased active population.

Success in the exfoliation of the stacked T-functionalized titanium carbide MXenes Tin+1CnT2 (T = OH, O, and F) would potentially extend their application scope, which requires an understanding of the nature of interlayer coupling. Here, we report for the first time the intrinsic interlayer coupling in pristine MXenes on the basis of first-principles calculations by taking long-range interaction into account. It is demonstrated that the functional terminations (OH, O, and F) weaken the interlayer coupling as compared with the bare counterparts, whereas the coupling is significantly stronger than van der Waals bonding as specified by the fact that the binding energies of stacked Tin+1CnT2 are 2–6 times those of well-known graphite and MoS2 with weak interlayer coupling. With binding energies in the range of 1–3.3 J m−2, the successful exfoliation of stacked Tin+1CnT2 into monolayers invariably requires further weakening of the interlayer coupling.

MXenes represent an emerging family of conductive two-dimensional materials. Their representative, Ti3C2Tx, has been recognized as an outstanding member in the field of electrochemical energy storage. However, an in-depth understanding of fundamental processes responsible for the superior capacitance of Ti3C2Tx MXene in acidic electrolytes is lacking. Here, to understand the mechanism of capacitance in Ti3C2Tx MXene, we studied electrochemically the charge/discharge processes of Ti3C2Tx electrodes in sulfate ion-containing aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the H2SO4 electrolyte bonds with the terminal O in the negative electrode upon discharging while debonding occurs upon charging. Correspondingly, the reversible bonding/debonding changes the valence state of Ti element in the MXene, giving rise to the pseudocapacitance in the acidic electrolyte. In stark contrast, only electric double layer capacitance is recognized in the other electrolytes of (NH4)2SO4 or MgSO4. The charge storage ways also differ: ion exchange dominates in H2SO4, while counterion adsorption in the rest. Hydronium that is characterized by smaller hydration radius and less charge is the most mobile among the three cations, facilitating it more kinetically accommodated on the deep adsorption sites between the MXene layers. The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance, and ion exchange-featured charge storage, simultaneously contribute to the superior capacitance of Ti3C2Tx MXene in acidic electrolytes.Keywords: charging mechanism; electrochemical capacitor; in situ Raman spectroscopy; MXene; pseudocapacitance; supercapacitor; two-dimensional materials;

Binder-free MXene films comprising Ti3C2Tx nanoflakes in a face-to-face fashion, achieved by a simple dropping-mild baking approach, exhibit high gravimetric capacitances up to 499 F g−1 with excellent cyclability and rate performances. The entire electrode system including Ni-foam and MXene film shows volumetric capacitances in the range of 84–226 F cm−3 depending on the loadings of MXene as active material.

We present a comparative study on the static and dynamical properties of bare Ti3C2 and T-terminated Ti3C2T2 (T = O, F, OH) monosheets using density functional theory calculations. First, the crystal structures are optimized to be of trigonal configurations (Pm1), which are thermodynamically and dynamically stable. It is demonstrated that the terminations modulate the crystal structures through valence electron density redistribution of the atoms, particularly surface Ti (Ti2) in the monosheets. Second, lattice dynamical properties including phonon dispersion and partial density of states (PDOS) are investigated. Phonon PDOS analysis shows a clear collaborative feature in the vibrations, reflecting the covalent nature of corresponding bonds in the monosheets. In the bare Ti3C2 monosheet, there is a phonon band gap between 400 and 500 cm−1, while it disappears in Ti3C2O2 and Ti3C2(OH)2 as the vibrations associated with the terminal atoms (O and OH) bridge the gap. Third, both Raman (Eg and A1g) and infrared-active (Eu and A2u) vibrational modes are predicted and conclusively assigned. A comparative study indicates that the terminal atoms remarkably influence the vibrational frequencies. Generally, the terminal atoms weaken the vibrations in which surface Ti atoms are involved while strengthening the out-of-plane vibration of C atoms. Temperature-dependent micro Raman measurements agree with the theoretical prediction if the complexity in the experimentally obtained lamellae for the Raman study is taken into account.

Nanolaminated Ti3AlC2 honeycomb monolith with parallel and uniform holes has been prepared through a facile extrusion route by using Ti3AlC2 powder as the main raw material. The fabricated honeycomb monolith has high compressive strength of 133 ± 11 and 59 ± 9 MPa, along and perpendicular to the extrusion direction, respectively. It also has good electrical conductivity, and excellent match of thermal expansion coefficient with the washcoat material of γ-Al2O3. These combined properties endow the honeycomb monolith a promising candidate as catalysis substrate for cleaning vehicle exhaust.

Structure and vibrational dynamics of T-terminated titanium carbide monosheets Ti2CT2 (T = O, F, OH) are studied by means of first-principles calculations to understand their inherent relation. Terminations modulate the crystal structures through the redistribution of valence electron density among the atoms in the monosheets, particularly Ti atoms. Phonon partial density of states analysis shows a clear feature of collaborative vibration, which reflects the covalent nature of bonds in the monosheets. Two metrics of covalency and cophonicity proposed very recently are adopted to quantitatively correlate the vibrational properties with the electrostructural characteristics of the system. A remarkable positive correlation between the covalency and vibrational dynamics specified as Raman shifts and IR wavenumbers is found. The bond-specific covalency metrics depend on not only the identity of terminations but also the thickness of the two-dimensional titanium carbides. For example, in the case of Ti3C2T2 with increased thickness, red shift in Raman shifts and IR wavenumbers occurs as a result of the decrease in covalency.

We design four-group experiments to understand the morphological and orientational diversity of LiFePO4 crystallites in hydrothermal and/or solvothermal syntheses. In the solvothermal synthesis, in which water is highly deficient, the starting Li3PO4 nanoparticle likely directly evolves into LiFePO4 through an in situ transition mechanism. In contrast, under the other three conditions, i.e., hydrothermal synthesis with stoichiometric H3PO4, hydrothermal and solvothermal syntheses in the presence of excess H3PO4, the starting Li3PO4 nanoparticle follows three diverse paths, generating different precursors and/or intermediates whose compositions and dissolution properties are remarkably divergent. Such divergences in reaction paths dramatically influence the colloidal stability of the small, primary nanosheets participating in oriented-attachment aggregation growth, resulting in the diversity of the resultant LiFePO4 in crystallite size from nanometer to micrometer shape (rod-like platelet, slab, and flake), and orientation ([010], [100] and [211]) as well as point defect concentration. Electrochemical performances of the diverse LiFePO4 crystallites synthesized in this study correlate well with the morphological and orientational diversity, shedding light on the tailored synthesis of LiFePO4 crystallites for high-performance lithium-ion batteries.

Wear performance of a near equi-volume TiC–Ni2AlTi cermet with minor NiAl was evaluated by reciprocal sliding against Si3N4 balls. Both coefficient of friction (COF) and specific wear rate (SWR) decrease with the applied load in the range from 5 to 20 N, reaching minimums of 0.34 and 2.2 × 10−6 mm3/Nm, respectively, at 20 N. To understand the novel wear resistance, interfacial microstructure was investigated. As indicated by high resolution transmission electron microscopy observations, the interfaces are either coherent (TiC/NiAl and Ni2AlTi/NiAl) or semi-coherent (TiC/Ni2AlTi). Depending on the grain size of Ni2AlTi, two types of TiC/Ni2AlTi interface were observed. For the micrometer or sub-micrometer sized Ni2AlTi grains, the orientation relationship (OR) is (111) TiC ∥ (220) Ni2AlTi, [1\(\bar{1}\)0] TiC ∥ [1\(\bar{1}\)0] Ni2AlTi, while for the Ni2AlTi grains in tens of nanometers, the OR is (020) TiC ∥ (002) Ni2AlTi, [101] TiC ∥ [010] Ni2AlTi. The strongly bonded coherent and semi-coherent interfaces impede the failure of the heterophase boundaries, which accounts for the excellent wear resistance of the newly prepared cermet.

Improving electrochemical properties of hydrothermally synthesized LiFePO4 powders is of immense technological significance and has been a subject of much scientific inquiry for many years. As reported previously, reversing the feeding sequence of starting materials and/or introducing ethylene glycol (EG) could significantly improve the electrochemical performance of hydrothermally synthesized LiFePO4. However, the mechanism remains unclear. Here, we report a systematic study to understand the mechanism from viewpoints of crystal growth and defect concentration control. Combining the results of experimental and theoretical investigations, the improvement in electrochemical performance is attributed to simultaneous suppression of crystal growth along the [010] direction and reduced defect concentration of the antisite. The reduction in antisite defects is readily monitored by significant red shift of the infrared (IR) absorption band around 1000 cm−1 which is assigned to the symmetric stretching P–O vibration of the PO4 tetrahedron, as indicated by theoretical calculation. With this knowledge in mind, an output as high as 450 g L−1 (autoclave volume), and an enhanced specific discharge capacity of 165 A h kg−1 (close to the theoretical unity of 170 A h kg−1) at 0.1 C are achieved.

Due to the large volume fraction of electrochemically inactive materials involved in conventional thin calendered electrodes, it is challenging to approach high volumetric capacity. Here, we report a proof-of-concept study on LiFePO4-based bulk electrodes, as opposed to conventional thin calendered electrodes. The bulk electrodes are hierarchically porous, electro-conductive, metal current collector- and polymeric binder-free. In combination with 3D pore topologies and electronic conduction, even with a small quantity of conductive agents (5.6 wt%), the as-prepared bulk electrodes exhibit an ultrahigh volumetric capacity and a stable cycling life.

The low-temperature hydrothermal synthesis method has been drawing ever-growing attention due to the fact that it has many advantages over conventional methods for preparing promising cathode material LiFePO4. However, the mechanism for hydrothermal synthesis of LiFePO4 remains unclear. Here, the hydrothermal reaction mechanism of LiFePO4 is systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and specific surface analysis. As evidenced by apparent precursor dissolution, fast hydrothermal formation, and significant decrease in particle size with adding alcohols and/or carbon black in the reaction system, a dissolution−precipitation mechanism accounts for the hydrothermal synthesis of LiFePO4. Moreover, we identified tetraphosphate in the LiFePO4 precursor. This compound undergoes hydrolysis upon heating during the hydrothermal process, resulting in a remarkable decline of pH value.

Improving electrochemical properties of hydrothermally synthesized LiFePO4 powders is of immense technological significance and has been a subject of much scientific inquiry for many years. As reported previously, reversing the feeding sequence of starting materials and/or introducing ethylene glycol (EG) could significantly improve the electrochemical performance of hydrothermally synthesized LiFePO4. However, the mechanism remains unclear. Here, we report a systematic study to understand the mechanism from viewpoints of crystal growth and defect concentration control. Combining the results of experimental and theoretical investigations, the improvement in electrochemical performance is attributed to simultaneous suppression of crystal growth along the [010] direction and reduced defect concentration of the antisite. The reduction in antisite defects is readily monitored by significant red shift of the infrared (IR) absorption band around 1000 cm−1 which is assigned to the symmetric stretching P–O vibration of the PO4 tetrahedron, as indicated by theoretical calculation. With this knowledge in mind, an output as high as 450 g L−1 (autoclave volume), and an enhanced specific discharge capacity of 165 A h kg−1 (close to the theoretical unity of 170 A h kg−1) at 0.1 C are achieved.

We present a comparative study on the static and dynamical properties of bare Ti3C2 and T-terminated Ti3C2T2 (T = O, F, OH) monosheets using density functional theory calculations. First, the crystal structures are optimized to be of trigonal configurations (Pm1), which are thermodynamically and dynamically stable. It is demonstrated that the terminations modulate the crystal structures through valence electron density redistribution of the atoms, particularly surface Ti (Ti2) in the monosheets. Second, lattice dynamical properties including phonon dispersion and partial density of states (PDOS) are investigated. Phonon PDOS analysis shows a clear collaborative feature in the vibrations, reflecting the covalent nature of corresponding bonds in the monosheets. In the bare Ti3C2 monosheet, there is a phonon band gap between 400 and 500 cm−1, while it disappears in Ti3C2O2 and Ti3C2(OH)2 as the vibrations associated with the terminal atoms (O and OH) bridge the gap. Third, both Raman (Eg and A1g) and infrared-active (Eu and A2u) vibrational modes are predicted and conclusively assigned. A comparative study indicates that the terminal atoms remarkably influence the vibrational frequencies. Generally, the terminal atoms weaken the vibrations in which surface Ti atoms are involved while strengthening the out-of-plane vibration of C atoms. Temperature-dependent micro Raman measurements agree with the theoretical prediction if the complexity in the experimentally obtained lamellae for the Raman study is taken into account.

Success in the exfoliation of the stacked T-functionalized titanium carbide MXenes Tin+1CnT2 (T = OH, O, and F) would potentially extend their application scope, which requires an understanding of the nature of interlayer coupling. Here, we report for the first time the intrinsic interlayer coupling in pristine MXenes on the basis of first-principles calculations by taking long-range interaction into account. It is demonstrated that the functional terminations (OH, O, and F) weaken the interlayer coupling as compared with the bare counterparts, whereas the coupling is significantly stronger than van der Waals bonding as specified by the fact that the binding energies of stacked Tin+1CnT2 are 2–6 times those of well-known graphite and MoS2 with weak interlayer coupling. With binding energies in the range of 1–3.3 J m−2, the successful exfoliation of stacked Tin+1CnT2 into monolayers invariably requires further weakening of the interlayer coupling.

Binder-free MXene films comprising Ti3C2Tx nanoflakes in a face-to-face fashion, achieved by a simple dropping-mild baking approach, exhibit high gravimetric capacitances up to 499 F g−1 with excellent cyclability and rate performances. The entire electrode system including Ni-foam and MXene film shows volumetric capacitances in the range of 84–226 F cm−3 depending on the loadings of MXene as active material.

Due to the large volume fraction of electrochemically inactive materials involved in conventional thin calendered electrodes, it is challenging to approach high volumetric capacity. Here, we report a proof-of-concept study on LiFePO4-based bulk electrodes, as opposed to conventional thin calendered electrodes. The bulk electrodes are hierarchically porous, electro-conductive, metal current collector- and polymeric binder-free. In combination with 3D pore topologies and electronic conduction, even with a small quantity of conductive agents (5.6 wt%), the as-prepared bulk electrodes exhibit an ultrahigh volumetric capacity and a stable cycling life.