Unraveling the high Energy efficiency for Zn||metal hexacyanoferrate batteries in a zinc-potassium hybrid configuration

Aqueous zinc batteries suffer from high overpotentials due to the sluggish diffusion of Zn2+ in the host and low potential of the cathode partially owing to the high desolvation energy for the hydrated Zn2+ cations, which lead to the unsatisfactory Energy efficiency although high Coulombic efficiency can be achieved in most cases. Here we achieved a high Energy efficiency up to 89% for the energy storage of Zn2+ ions through selecting cathode with stable frameworks and suitable tunnels. Furthermore, through screening different metal ions, K+ ion stands out due to its weak hydration structure and smallest charge/radius ratio. The overpotential for both Zn2+ and K+ ions were researched, and the dominated factor is the small diffusion barrier of charge carriers in the selected cathode structure. Based on the pre-built cathode and screened charge carrier of K+, a high discharge voltage plateau around 1.9 V was obtained, with a small overpotential (< 0.12 V), thus an extraordinary Energy efficiency around 93% that is almost irrelevant to the current density was obtained. We wish our research can arouse attention of community on Energy efficiency of aqueous electrolyte batteries.