Phase stability and its impact on the electrochemical performance of VOPO ₄ and LiVOPO ₄

Vanadyl phosphate (VOPO4) is an attractive candidate as a Li-ion battery cathode with potential to reach higher energy density than that of olivine LiFePO4. However, the limited knowledge of the complex polymorphism of VOPO4 and LiVOPO4 prevents any systematic optimization. Here we present a comprehensive first-principles study on the phase stability of VOPO4 and LiVOPO4 and its impact on the electrochemical properties. βVOPO4 and αLiVOPO4 are predicted to be the most stable phases of VOPO4 and LiVOPO4, respectively. The crucial factor that determines the phase stability is the connection between cation polyhedra including the tilting of VO6 octahedra and the distance between cations, but not the bonding between cations and anions. The calculated data well explain the experimentally observed transition from the αII- to the αI-phase and from the ε- to the α-phase induced by lithiation. The transition from the β- to the α-phase is also possible but is associated with much larger structural deformation. The voltages calculated using possible phase transitions taken into account are in excellent agreement with experimental measurements. The diffusivity in βLiVOPO4 and αLiVOPO4 differs by five orders of magnitude at 300 K. These results elucidate the importance of the phase stability of VOPO4 and LiVOPO4 in the application of Li-ion battery cathodes and pave the way for their future improvements.