Vanadyl phosphates of VOPO₄ as a cathode of Li-ion rechargeable batteries

Seven phases of VOPO₄ were synthesized and their Li intercalation properties were investigated. Li can be reversibly intercalated into all phases of VOPO₄ and potential plateaus for Li intercalation and de-intercalation exist at 3.7 and 3.9 V, respectively, except for the β- and ε-phases. β- and ε-phases of VOPO₄ exhibit slightly higher flat potentials of 3.9 and 4.1 V. The capacity for Li intercalation is strongly affected by the crystal structure of VOPO₄ and the Li intercalation capacity increases with increasing average valence of V. Among the VOPO₄ crystal phases, it was found that the δ-phase exhibits the largest reversible capacity of ca. 130 mAh/g. The capacity for Li de-intercalation in δ-phase VOPO₄ hardly decreased over 35 charge and discharge cycles. On the other hand, the Li intercalation capacity in δ-phase VOPO₄ decreased with increasing current density for charge and discharge and temperature. However, even at a current density of 0.4 mA/cm², corresponding to C/5, fairly large capacity of ca. 60 mAh/g was sustained, and at a operating temperature or 323 K, a capacity of 110 mAh/g was exhibited at 0.04 mA/cm².