Protocol in Evaluating Capacity of Zn-Mn Aqueous Batteries: A Clue of pH

In literature, Zn-Mn aqueous batteries (ZMABs) confront abnormal capacity behavior, such as capacity fluctuation and diverse “unprecedented performances.” Because of the electrolyte additive-induced complexes, various charge/discharge behaviors associated with different mechanisms are being reported. However, the current performance assessment remains unregulated, and only the electrode or electrolyte is considered. The lack of a comprehensive and impartial performance evaluation protocol for ZMABs hinders forward research and commercialization. In this chapter, we first propose a pH clue (proton-coupled reaction) to understand different mechanisms and normalize the capacity contribution. Then, a series of performance metrics, including rated capacity (C_r) and electrolyte contribution ratio from Mn²⁺ (Cf M), are systematically discussed based on diverse energy storage mechanisms. The relationship between Mn (II) ↔ Mn (III) ↔ Mn (IV) conversion chemistry and proton consumption/production is subsequently established. Finally, we propose concrete design concepts of a tunable H⁺/Zn²⁺/Mn²⁺ storage system for customized application scenarios, opening the door for next-generation highsafety and reliable energy storage systems.