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 var epsilon-phases. β- and var epsilon-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.