A Pyrophosphate Bifunctional Cathode with Inductive Effect for High–Voltage and Self–Charging Zinc Ion Battery

With the growing demand for new energy storage devices, rechargeable aqueous zinc ion batteries (ZIBs) have attracted widespread attention due to their low cost and high safety. Among the cathode materials for ZIBs, polyanionic–based cathode materials with high voltage, high stability, and low cost have great potential. In this paper, tetragonal Na₂VOP₂O₇ was prepared using a simple sol–gel method. The discharge platform voltage amounted to 1.8 V and had good rate and cycle performance due to the inductive effect of pyrophosphate. Then, a protective layer of Zn–hydroxyapatite (ZnHAP) modification was applied to the cathode surface, which can inhibit the hydrolysis of vanadium ions. The capacity was enhanced by 19 % after modification and the capacity retention after 100 cycles was also higher. Interestingly, the Na₂VOP₂O₇ cathode also possesses a self–charging effect, recovering to 48 % of its initial capacity with an open–circuit voltage (OCV) of 1.1 V within a certain period, and light exposure can reduce the self–charging time by 83 %. These beneficial results indicate that the pyrophosphate bifunctional cathode with inductive effect has a great potential to construct high–voltage and multifunctional zinc ion battery.