Yurui Gao

Improved electron/Li-ion transport and oxygen stability of Mo-doped Li2MnO3

Li2MnO3 is an important building block for stabilizing the structure as well as ensuring the high specific lithium storage capacity of xLi2MnO3·(1 − x)LiMO2 (M = Ni, Co, Mn, etc.) cathode materials for lithium-ion batteries. However, the drawbacks of Li2MnO3 such as its low conductivity and oxygen evolution during delithiation make cathode materials less attractive in terms of safety, rate and cycling performances than traditional cathode materials. This work aims to improve the properties of Li2MnO3-related cathode materials by doping molybdenum (Mo) into Li2MnO3 (C2/c). First-principles calculations within the PBE + U scheme show that Mo doping is beneficial for improving both the dynamic and thermodynamic properties of Li2MnO3 by reducing the band gap and increasing the number of electronic states near the Fermi level. This promotes Li-ion diffusion between the lithium layer and transition-metal layer and charge transference from Mo to O, lowering the delithiation potential, adding Mo as another charge compensation donator upon Li removal, and enhancing the stability of oxygen according to the reaction enthalpy. Therefore, Mo doping is expected to be an effective way to improve the structural stability and rate performance of Li2MnO3 and xLi2MnO3·(1 − x)LiMO2 cathode materials.

Molybdenum Substitution for Improving the Charge Compensation and Activity of Li2MnO3

Lithium-rich layer-structured oxides xLi2 MnO3 ⋅ (1-x)LiMO2 (0<x<1, M=Mn, Ni, Co, etc.) are interesting and potential cathode materials for high energy-density lithium ion batteries. However, the characteristic charge compensation contributed by O(2-) in Li2 MnO3 leads to the evolution of oxygen during the initial Li(+) ion extraction at high voltage and voltage fading in subsequent cycling, resulting in a safety hazard and poor cycling performance of the battery. Molybdenum substitution was performed in this work to provide another electron donor and to enhance the electrochemical activity of Li2 MnO3 -based cathode materials. X-ray diffraction and adsorption studies indicated that Mo(5+) substitution expands the unit cell in the crystal lattice and weakens the LiO and MnO bonds, as well as enhancing the activity of Li2 MnO3 by lowering its delithiation potential and suppressing the release of oxygen. In addition, the chemical environment of O(2-) ions in molybdenum-substituted Li2 MnO3 is more reversible than in the unsubstituted sample during cycling. Therefore molybdenum substitution is expected to improve the performances of the Li2 MnO3 -based lithium-rich cathode materials.

Anti-P2 structured Na0.5NbO2 and its negative strain effect

Layer-structured oxides are studied for their essential roles in various applications (e.g. high-energy batteries and superconductors) due to their distinctive physical structures and chemical properties. Most of the layered AxMO2 (A = alkali ions, M = transition metals) are composed of MO6 octahedra and various A coordination polyhedra such as octahedra (O), tetrahedra (T) or trigonal prisms (P). Herein, we report a new layered oxide material, anti-P2 Na0.5NbO2, which is composed of NbO6 trigonal prisms and NaO6 octahedra. Its lattice shrinks as sodium (Na) ions are intercalated in it and expands when the ions are deintercalated (a negative volume or strain effect). Analysis by X-ray absorption spectroscopy and density functional theory (DFT) calculations indicates that the negative volume effect is mainly a result of the enhanced interlayer (Na–O) interaction and the weakened Nb–Nb and Nb–O bonding in the O–Nb–O slab upon Na intercalation. Moreover, Na0.5NbO2 exhibits high structural stability, a long cycle life and prominent rate performance for Na-ion batteries. These distinctive features make Na0.5NbO2 an ideal “volume buffer” to compensate for positive-strain electrode materials. These findings will arouse great interest in anti-P2 layered oxides for materials science and applications, and enrich the understanding of novel negative-strain materials for energy storage either as excellent independent active electrode materials or as volume buffers for constructing long-life composite electrodes made of positive-strain materials.

Workfunction, a new viewpoint to understand the electrolyte/electrode interface reaction

Severe gassing during cycling hinders the application of spinel Li4Ti5O12 as a zero-strain anode material for constructing high power-density and long-lifespan Li-ion batteries. The gassing issue is caused by the interface reaction between Li4Ti5O12 and the electrolyte. This article is aimed towards understanding the Li4Ti5O12/electrolyte interface reaction and the chemical stability of Li4Ti5O12 from a new viewpoint, surface workfunction, the energy required to take away one electron from the Fermi level. Density functional theory (DFT) calculations indicate that the workfunction decreases due to the presence of the Li-rich surface (Li+-occupied 16c sites) of Li3+xTi6−xO12. Meanwhile, the chemical potential increases and even reaches the LUMO of the carbonate electrolyte, easily inducing interface reactions. This means that the electrochemically lithiated phase Li7Ti5O12 is responsible for the electrolyte decomposition on the surface. The Li-rich surfaces of Li4Ti5O12 generated during material preparation or chemical lithiation can also trigger the interface reactions. From the combination of the experimental and calculation results, we believe that the interface reaction involves losses of electrons and Li+ ions from the Li-rich surface of Li4Ti5O12 or Li7Ti5O12, and the reduction of the electrolyte. In addition, O vacancies on the surface decrease the workfunction and further promote the reaction.

Li2C2, a High‐Capacity Cathode Material for Lithium Ion Batteries

As a typical alkaline earth metal carbide, lithium carbide (Li2C2) has the highest theoretical specific capacity (1400 mA h g(-1)) among all the reported lithium-containing cathode materials for lithium ion batteries. Herein, the feasibility of using Li2C2 as a cathode material was studied. The results show that at least half of the lithium can be extracted from Li2C2 and the reversible specific capacity reaches 700 mA h g(-1). The C≡C bond tends to rotate to form C4 (C≡C⋅⋅⋅C≡C) chains during lithium extraction, as indicated with the first-principles molecular dynamics (FPMD) simulation. The low electronic and ionic conductivity are believed to be responsible for the potential gap between charge and discharge, as is supported with density functional theory (DFT) calculations and Arrhenius fitting results. These findings illustrate the feasibility to use the alkali and alkaline earth metal carbides as high-capacity electrode materials for secondary batteries.