Electrochemistry of rechargeable aqueous zinc/zinc-sulphate/manganese-oxide batteries and methods for preparation of high-performance cathodes

Abstract

Rechargeable aqueous Zn/ZnSO₄/MnO₂ batteries, due to their high performance and safety, have attracted much attention as promising candidates for grid-scale energy storage. Despite intensive efforts in previous studies, the cathode electrochemistry remains largely unidentified. We observed new insights into the cathode electrochemistry in Zn/ZnSO₄/MnO₂ batteries and found that, regardless of the physicochemical states of manganese in the pristine, as-prepared cathode, birnessite is the fated compound. The birnessite consisted of both Mn(III) and Mn(IV) with Zn²⁺ as the foreign cation. Birnessite stores energy via chemical redox reactions. Mn(III) has the highest priority for discharge. Mn(IV) discharges stepwise, first to MnO–OH, and then MnO–OH continuously discharges to Mn²⁺. During recharge, birnessite is produced stepwise via Mn²⁺ → MnO–OH and MnO–OH → birnessite. Zinc sulphate hydroxide plays essential roles in driving discharge and recharge via its formation with hydroxyl ions (by-products of discharge) and via neutralization of proton ions (by-products of recharge). We inserted birnessite into three-dimensional networks of single-walled carbon nanotubes (3D-SWCNT-networks) via electrochemical deposition and the superior conductive properties of the birnessite/3D-SWCNT-networks have extended the cycling numbers of the rechargeable aqueous Zn/ZnSO₄/MnO₂ batteries over 11 000 with a capacity retention > 82.5% at a high discharge/recharge rate of 1.5 A g⁻¹. The rechargeable aqueous Zn/ZnSO₄/MnO₂ batteries with birnessite/3D-SWCNT-networks as cathodes are the promising devices for applications in long-term energy storage.