In Situ Electrochemical Transformation Reaction of Ammonium-Anchored Heptavanadate Cathode for Long-Life Aqueous Zinc-Ion Batteries.

Rechargeable aqueous zinc-ion batteries (ZIBs) are promising portable and large-scale grid energy storage devices, as they are safe and economical. However, developing suitable ZIB cathode materials with excellent cycling performance characteristics remains a challenging task. Here, ammonium heptavanadate (NH4)2V7O16·3.2H2O (NHVO) nanosquares with mixed-valence V5+/V4+ as a cathode are developed for high-performance ZIBs. The layered NHVO shows a capacity of 362 mA h g-1 at 0.05 A g-1, with a high energy density of 263.5 W h kg-1. It exhibits an initial specific capacity of 250.7 mA h g-1 at a current density of 4 A g-1 and retains 255 mA h g-1 capacity after 1000 charge/discharge cycles. The V7O16-based cathode was demonstrated with a phase transition to the V2O5-based cathode upon initial cycling. Moreover, the in situ generated V2O5-based cathodes show excellent electrochemical properties, which provide a different perspective on the electrochemical reaction of cathode materials for aqueous ZIBs.