δ-VOPO4 nanosheet with intrinsic V4+ defective as high-performance cathode for sodium-ion battery

Abstract Transition metal phosphates have become of great interest as cathode materials for rechargeable ion batteries because of their low cost and environmental friendliness, but suffer from low-intrinsic conductivity. Here, we report that a δ phase VOPO4 (δ-VPO) nanosheets with intrinsic V4+ defect, can deliver a specific capacity of 130 mA h/g at 0.2 C (1 C = 166 mA/g) with 75% capacity retention after 500 cycles as a cathode of sodium-ion batteries (SIBs), which is superior to previously reported α-VPO as well as sodiated VPO counterparts. Our results demonstrate that the high V4+ defect concentration of VPO is responsible for the better high-rate performance due to enhanced conductivity and reduced redox polarization, while δ-VPO enabling high cycling stability resulting from the limited expansion of the lateral size. This work offers a potentially alternative cathode material with controllable phases and defects towards high-performance SIBs.