Crystal structure and electrochemical properties of LiFe1-xVxPO4F1-δOδ cathode materials for lithium-ion batteries

Tavorite-structured polyanion compounds, such as LiVPO4F, LiFePO4F and LiVPO4O, are promising cathode candidates of Li-ion batteries due to their structural stability as a result of strong covalent bonds. A few publications related to LiVPO4FLiFePO4F and LiVPO4FLiVPO4O solid solutions have been reported to date. In this work, LiFePO4FLiVPO4O solid solution, i.e. LiFe1-xVxPO4F1-δOδ (0 ≤ x ≤ 1; 0 ≤ δ ≤ 0.36) is first prepared by a two-step solid-state route using the pre-synthesized FePO4 and VPO4O powders. Sloping discharge profiles shown in solid-solution cells indicate a single-phase behavior which is different from the two-phase reaction of end members. The structure determination, refinement and electrochemical properties are studied. The meachanisms are concluded involved in redox energies of cations. Redox energies are tuned within a wide range 1.5-4.5 V in polyanion-type cathodes, through the inductive effect introduced by cation (V for Fe) and anion (O for F) substitution.