Phase stability and its impact on the electrochemical performance of VOPO4 and LiVOPO4

Abstract

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 α_II- to the α_I-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.