Understanding the effects of oxygen defects on the redox reaction pathways in LiVPO4F by combining ab initio calculations with experiments

Abstract

Tavorite LiVPO₄F has great potential as a cathode material due to its high energy density, superior thermal stability and fast kinetics. It has been controversial whether the redox reaction pathway of LiVPO₄F is symmetric or not, but the origin of such symmetric/asymmetric redox reaction pathways has not been clearly understood. By combining ab initio calculations with experiments, we found that oxygen defects on fluorine sites, O_F⁻, can be a key factor that affects the symmetry of the redox pathway of LiVPO₄F_1−δO_δ (δ < 1). The computational results indicate that (1) the thermodynamically metastable ‘triclinic’ polymorph of VPO₄F_1−δO_δ, which maintains the structural symmetry of LiVPO₄F, can be kinetically stabilized considering its marginally higher energy than the stable ‘monoclinic’ phase; and (2) the O_F⁻ defects slightly destabilize the intermediate phase (x = 0.667) (Li_0.667VPO₄F versus Li_0.667VPO₄F_0.917O_0.083), inferring that the presence of O_F⁻ defects can induce the asymmetric redox pathway of LiVPO₄F_1−δO_δ. We also experimentally found that the concentration of O_F⁻ defects is critically affected by the synthesis process of LiVPO₄F, and VPO₄F with lower O_F⁻ concentration can support the possible presence of the metastable (triclinic) phase, leading to the redox pathway being prone to being symmetric. Therefore, controlling oxygen defects by a synthesis processes can affect electrochemical performance via different redox reaction pathways of LiVPO₄F. This understanding provides further possibilities for improving the electrochemical performance.