Dual characteristics of Mn2+ dissolution from manganese oxide cathode materials in aqueous zinc batteries: trade-offs between active material loss and double capacity contribution

Abstract

Rechargeable aqueous Zn–MnO₂ batteries are promising for large-scale energy storage systems. Complicated reaction processes have been revealed at the cathode in this mildly acidic system, with Mn²⁺ dissolution occurring as a universal phenomenon across different mechanisms. However, the understanding of the roles of Mn²⁺ dissolution has evolved over the past decade. With the Zn²⁺/H⁺ de/intercalation mechanism based on the Mn(IV)/Mn(III) redox couple, Mn²⁺ dissolution results in active material loss and should be inhibited. With the dissolution/deposition reaction with the Mn(IV)/Mn(II) redox couple, on the other hand, Mn²⁺ is the discharged product and the dissolution should be promoted. Currently, the underlying evaluation criterion to understand and regulate the Mn²⁺ dissolution process for specific energy storage mechanisms remains unclear, which hinders the further exploitation of aqueous Zn–MnO₂ batteries. Herein, rather than simply compiling recent progress, we present a dialectical discussion on the roles of Mn²⁺ dissolution. Accordingly, we propose performance metrics to quantitatively identify dominant mechanisms of energy storage and capacity loss in MnO₂ cathodes. They function as the Mn²⁺ dissolution indicator to understand the trade-offs between its dual characteristics of active material loss vs. double capacity contribution. These analyses establish a protocol which would allow the suitable utilization of Mn²⁺ dissolution for desired mechanisms and optimize corresponding solutions for the different capacity fading behaviors. Finally, future strategies are proposed to boost the two-electron Mn dissolution/deposition process and revive this high capacity reaction path of the MnO₂ cathode family, which would open up an effective avenue towards high-performance aqueous energy storage systems.