A bi-cation electrolyte for a 1.7 V aqueous Zn ion battery.

Rechargeable aqueous zinc-ion batteries (ZIBs) are receiving increased attention due to their high safety and low cost. However, their practical application is plagued by their low energy density as a result of the low output voltage and the narrow voltage window of aqueous electrolytes. Here, we explored a ZIB with wider potential window by using bi-cation (1 M Al(CF3SO3)3/1 M Zn(CF3SO3)2) as the electrolyte and α-MnO2 as the cathode, obtaining a discharge voltage of 1.7 V, ~0.3 V higher than the value reported earlier. The resultant cell delivers a record high energy density of 448 Wh kg-1 (based on MnO2 mass) and retains 100% capacity over 1000 cycles. The ion storage mechanism and the role of Al3+ in enlarging the output voltage were elucidated. This research indicates the important role of using bi-cations to improve the electrochemical performance of aqueous ZIBs, opening a new way to increase the energy density of aqueous energy storage devices.