A new sodium ion preintercalated and oxygen vacancy-enriched vanadyl phosphate cathode for aqueous zinc-ion batteries

At present, layered vanadium-oxygen structures have attracted wide attention for multivalent metal ion storage, especially in aqueous zinc-ion batteries (AZIBs), due to the attractive layered structure and large specific capacity based on V⁵⁺/V³⁺ double electron transfer. However, in addition to a large specific capacity, a high output voltage is necessary to achieve a high specific energy density. Vanadium oxide and vanadate usually feature low working voltages, serious structural degradation and limited practical. To alleviate these problems, some cathode modification strategies have been proposed that improve the operating voltage, structural stability and diffusion kinetics of multivalent metal ions. In this paper, vanadyl phosphate (Na_y(VO_1-x)₃(PO₄)₂) nanosheets preintercalated with sodium ions and modified with oxygen vacancies were prepared via a facile one-step liquid phase treatment. The Na_y(VO_1-x)₃(PO₄)₂ nanosheet cathode for AZIBs delivered a high specific capacity of 75.3 mAh g^-1 at 0.1 A g^-1 and retained 27.5 mAh g^-1 after 4000 cycles at 2 A g^-1. Subsequently, the as-prepared Na_y(VO_1-x)₃(PO₄)₂ nanosheets were physically and electrochemically characterized, and a possible mechanism of Zn²⁺ insertion/extraction and structural decomposition was proposed based on ex situ XRD and XPS characterizations. Our work provides a simple method for simultaneously introducing sodium ion preintercalation and oxygen vacancies into vanadyl phosphate structures, and provides some insights into the zinc storage mechanism.