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

Abstract Tavorite-structured polyanion compounds, such as LiVPO 4 F, LiFePO 4 F and LiVPO 4 O, 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 LiVPO 4 F LiFePO 4 F and LiVPO 4 F LiVPO 4 O solid solutions have been reported to date. In this work, LiFePO 4 F LiVPO 4 O solid solution, i.e. LiFe 1- x V x PO 4 F 1- δ O δ (0 ≤  x  ≤ 1; 0 ≤  δ  ≤ 0.36) is first prepared by a two-step solid-state route using the pre-synthesized FePO 4 and VPO 4 O 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.