Towards high-areal-capacity aqueous zinc–manganese batteries: promoting MnO2 dissolution by redox mediators

Abstract

Aqueous manganese (Mn) batteries based on the deposition–dissolution reaction of Mn²⁺/MnO_2(s) have attracted great attention due to their low cost, high voltage, and high safety. However, the incomplete dissolution of MnO₂ and exfoliated MnO₂ from mechanical cracks of thick MnO₂ layers (lost capacity) prevent long-term stable operation at high areal capacity (>2.0 mA h cm⁻²). Here, we propose a mediator strategy to facilitate MnO₂ dissolution and recover ‘lost’ capacity from exfoliated MnO₂, which improves the cycling stability at high areal capacity. UV-visible spectroscopy was used to verify the working principle of the mediator strategy. The iodide (I⁻) mediator chemically reduces solid MnO₂ to form Mn²⁺ and oxidizes to tri-iodide (I₃⁻), which then can get reduced at the electrode returning to I⁻, completing one mediation cycle. The zinc–manganese (Zn–Mn) battery with the iodide mediator shows improved cycling stability at 2.5 mA h cm⁻² (400 vs. 100 cycles, static mode) and 15 mA h cm⁻² (225 vs. 60 cycles, flow mode). We further increased the areal capacity and demonstrated 50 mA h cm⁻² for more than 50 cycles, which is the highest areal capacity achieved for reported Zn–Mn batteries. This mediator strategy is introduced into aqueous Mn-based batteries for the first time, which could also shed light on other deposition-based batteries to achieve stable operation at high areal capacity.