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

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 LiVPO4F1-delta O delta (delta < 1). The computational results indicate that (1) the thermodynamically metastable 'triclinic' polymorph of VPO4F1-delta O delta, 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) (Li0.667VPO4F versus Li0.667VPO4F0.917O0.083), inferring that the presence of O-F(-) defects can induce the asymmetric redox pathway of LiVPO4F1-delta O delta. 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.