Ion-Anchored Strategy for MnO₂/Mn²⁺ Chemistry without “Dead Mn” and Corrosion

Abstract

The utilization of MnO₂/Mn²⁺ chemistry in near-neutral pH acetate aqueous electrolytes provides an opportunity to achieve a higher energy density (theoretical capacity 616 mA h/g, discharge platform >1.5 V). However, this Zn-MnO₂ aqueous battery suffers from inevitable "dead Mn" and proton corrosion. In this study, we discover that the diffusion of the cathode reaction intermediate Mn³⁺ is intrinsic for the generation of "dead Mn", and the accumulation of "dead Mn" increases the H⁺ which shuttles to the anode, inducing serious corrosion. A pH-neutral hydrogel ion-anchored strategy is proposed here not only to restrict the diffusion of Mn³⁺ but also to suppress the proton transference. This hydrogel ion anchor is designed by deprotonating a series of monomers undergoing in situ free radical polymerization at the cathode interface. The anionic monomer with a moderate binding energy to manganese ions is screened to anchor Mn³⁺, which enhances the reversibility of the MnO₂/Mn²⁺ reaction. Simultaneously, a substantial amount of anionic groups and hydrophilic functional groups in the hydrogel effectively constrains the proton shuttle to corrode the anode. Consequently, the Zn/MnO₂ battery achieves exceptional cyclic stability of the MnO₂/Mn²⁺ reaction, sustaining 8500 cycles even at a relatively low current density and discharge current density of 1 mA/cm². Our findings highlight the importance of anchoring Mn³⁺ at the cathode interface and offer valuable insights for advancing practical applications of MnO₂/Mn²⁺ reactions.