Float-charging protocol in rechargeable Zn–MnO2 batteries: Unraveling the key role of Mn2+ additives in preventing spontaneous pH changes

Aqueous rechargeable Zn–MnO2 batteries have been attracting increasing interest in recent years due to their relatively high energy density, non-toxicity, non-flammability, and low-cost. However, the battery chemistry still faces several challenges limiting the performance of some key performance indicators and preventing its commercial deployment. Among these, cycle stability and lifespan are important, especially for stationary energy applications, and require improvement. In this work, a spontaneous reaction between Zn metal and electrolytes evolving hydrogen and Zn ions is shown to drive the electrolyte pH towards alkaline conditions, which in turn hinders the reversible electrochemical reactions at both positive and negative electrodes. Proton release during the oxidation of a Mn2+ additive and promotion of the oxygen evolution reaction (OER) at the positive electrode by implementing a float-charging protocol (constant voltage at the end of the charging process) is proposed as a simple strategy to restore the initial pH. Using a pH indicator dissolved in the electrolyte and assembling the electrochemical cell in a spectrophotometer cuvette, the effect of floating voltage is evaluated in operando, allowing optimization of the floating voltage to enable the electrolyte pH to remain stable.