First-principles study of VS<sub>2</sub> ascathode material for Li-ion batteries

Abstract

With the increase of performance requirements for lithium-ion batteries (LIBs), it is particularly important to study and develop new electrodes for lithium-ion batteries. In this work, a 3 × 3 × 1 supercell of VS<sub>2</sub> is constructed, and the possibility of using it as an anode material for lithium-ion batteries is study by the first-principles method based on density functional theory. Through the analysis of the energy band diagram, it is found that VS<sub>2</sub> has metallic properties. Combining the density of states diagram, the analysis shows that the energy band near the Fermi level of VS<sub>2</sub> is contributed by the 3d state of V and the 3p state electrons of S, which means that the conductive properties of VS<sub>2</sub> are largely affected by the 3d state of V and the 3p state electrons of S. Of the vacancies, bridge sites, and top sites of lithium adsorbing vanadium (V), the top site has the lowest adsorption energy, indicating that lithium will preferentially adsorb the top site of vanadium (V). Through first-principles molecular dynamics simulations of the top position of adsorbed vanadium (V), it is found that at a temperature of 300k, the total energy of the system and the magnitude of the total temperature fluctuation can reach a steady state, indicating that lithium can exist at the top position of stably adsorbed vanadium (V). Moreover, the interlayer spacing of the double-layer VS<sub>2</sub> reaches 3.67 Å, which is larger than the interlayer spacing of graphene. From the top position to the vacancy, its diffusion barrier is only 0.20eV. Its interlayer spacing is larger than the double-layer graphene’s, and its diffusion barrier is lower than graphene’s, indicating that lithium has very good diffusivity on the VS<sub>2</sub> surface, and lithium can migrate quickly on the VS<sub>2</sub> surface, which is conducive to the rapid charge-discharge process of LIB. In addition to excellent electrical conductivity, VS<sub>2</sub> has good mechanical properties. The calculated Young's modulus is 96.82N/m, and the Young's modulus and Poisson's ratio do not decrease after adsorbing lithium, indicating that the rigidity of VS<sub>2</sub> will not be reduced in the diffusion and migration process of lithium. On the other hand, it has excellent deformation resistance. In order to be more accurate and closer to the actual situation, a double-layer VS<sub>2</sub> model is constructed, with a maximum number of lithium atoms adsorbed between layers being 18. The calculated theoretical capacity of VS<sub>2</sub> (466 mAh/g) is higher than the theoretical capacity of graphene (372 mAh/g). Our results indicate that VS<sub>2</sub> has excellent electrical conductivity and mechanical stiffness, making it a promising cathode material for lithium-ion batteries.