Fundamental understanding of the proton and zinc storage in vanadium oxide for aqueous zinc-ion batteries

Vanadium oxide (VOx) materials have gained considerable interest for rechargeable aqueous zinc ion batteries because of their structural and electrochemical diversities. However, the charge storage mechanism of the VOx cathode remains a topic of discussion. Herein we demonstrate a high-performance Zn/VOx cell where the binder-free VOx cathode with a layered structure is electrodeposited on a graphite substrate. This cathode with a high mass loading of 6 mg cm−2 exhibits high specific capacity of 402 mAh g−1 at the current density of 0.26 A g−1 in 6 M ZnCl2 aqueous electrolyte. Good cycling stability of ~89% capacity retention can be achieved for 10,000 cycles at the fast discharge rate of 7.8 A g−1. Electrochemical and spectroscopy analysis indicates that the VOx cathode experiences an interactive dual-ion storage mechanism, including sequential H+ and Zn2+ insertion, as well as H+/Zn2+ co-insertion processes during discharging. The H+ and Zn2+ insertion kinetics is also studied. Results identify that excessive H+ storage in the initial discharge region will block the subsequent Zn2+ insertion and thus decrease the discharge capacity. The interfacial charge transfer and the ion diffusion processes of the dual-ion storage in VOx are crucial to achieving good electrochemical performance.