Inner-Sphere Electron Transfer Enabling Highly Reversible Mn²⁺/MnO₂ Conversion toward Energy-Dense Electrolytic Zinc–Manganese Batteries

Abstract

High-voltage electrolytic Zn//MnO₂ batteries show great potential for large-scale energy storage due to their affordability, eco-friendliness and high safety. However, their practical application is hindered by capacity losses due to incomplete MnO₂ dissolution. Herein, we propose the strategy by coupling a 1,4-benzoquinone (1,4-BQ)/hydroquinone (HQ) redox mediator pair with <i>in situ</i> modulation of MnO₂ electronic structure through electrolyte engineering to facilitate rapid and complete MnO₂ dissolution. During the charging and discharging processes, Al³⁺ ions in the electrolyte enter MnO₂ lattice by co-deposition and intercalation, respectively. The incorporated Al³⁺ ions effectively optimize the electronic structure of MnO₂ by lowering the valence state of localized Mn^IV to Mn^III, thereby facilitating the formation of inner-sphere complexes with HQ molecules. This transformation successfully shifts the dominant reaction mechanism between MnO₂ and the redox mediator from outer-sphere electron transfer (Mn^IV-HQ) to inner-sphere electron transfer (Mn^III-HQ). Consequently, complete MnO₂ dissolution can be achieved in the designed electrolyte even at an ultrahigh areal capacity of 50 mAh cm^-2. Furthermore, a 750-mAh electrolytic Zn//MnO₂ battery exhibits a capacity retention rate of 99% after 100 cycles, demonstrating the significance of regulating electron transfer mechanisms during MnO₂ dissolution through electrolyte coupling strategies.