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

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 in situ 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 MnIV to MnIII, 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 (MnIV-HQ) to inner-sphere electron transfer (MnIII-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.