Cation-regulated MnO2 reduction reaction enabling long-term stable zinc–manganese flow batteries with high energy density†

Abstract

Aqueous Zn–Mn flow batteries (Zn–Mn FBs) are a potential candidate for large-scale energy storage due to their high voltage, low cost, and environmental friendliness. However, the unsatisfactory performance due to the sluggish MnO₂ reduction reaction (MnRR) kinetics leads to low discharge voltage (typically <1.7 V) and poor stability (typically <1000 cycles), which hinders their practical application. Here, we successfully achieve a reversible Mn²⁺/MnO₂ reaction by a cation-regulated MnO₂ formation/decomposition process. The dual role of Mg²⁺ addition in locking free water and forming Mg-doped MnO₂ compounds with enlarged atomic spacing was revealed, leading to excellent electrolyte stability and highly reversible MnRR. The Zn–Mn FBs with Mg²⁺ exhibit a high discharge voltage of 1.91 V at 20 mA cm⁻² and superior long-term stability for over 2600 cycles, thus realizing a considerably high energy density (38.2 mW h cm⁻² per cycle and 23.75 W h cm⁻² cumulatively). This work underscores the importance of electrolyte engineering to the reversibility of Mn-based reactions and its potential for high power and energy density applications.