Rechargeability of aqueous sulfate Zn/MnO2 batteries enhanced by accessible Mn2+ ions

Abstract The Zn/MnO 2 battery is safe, low cost and comes with a high energy density comparable to Li-ion batteries. However, irreversible spinel phases formed at the MnO 2 electrode limits its cyclability. A viable solution to overcome this inactive phase is to use an aqueous ZnSO 4 -based electrolyte, where pH is mildly acidic leading to a different reaction mechanism. Most importantly, the addition of MnSO 4 achieves excellent cyclability. How accessible Mn 2+ ions in the electrolyte enhances the reversibility is presented. With added Mn 2+ , the capacity retention is significantly improved over 100 cycles. Zn 2+ insertion plays an important role on the reversibility and a hydrated layered Zn-buserite structure formed during charge is reported. Furthermore, Zn 4 SO 4 (OH) 6 · 5H 2 O precipitates during discharge but is not involved in the electrochemical reaction. This precipitate both buffers the pH and partly insulates the surface. Described in operando study show how the phase transformations and the failure mechanisms depend on the presence of Mn 2+ -ions in the electrolyte. These results give insight necessary to improve this battery further to make it a worthy contender to the Li-ion battery in large scale energy storage solutions.