Inner-Sphere Electron Transfer Enabling Highly Reversible Mn/MnO Conversion toward Energy-Dense Electrolytic Zinc-Manganese Batteries.

High-voltage electrolytic Zn//MnO 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 MnO dissolution. Herein, we propose the strategy by coupling a 1,4-benzoquinone (1,4-BQ)/hydroquinone (HQ) redox mediator pair with modulation of MnO electronic structure through electrolyte engineering to facilitate rapid and complete MnO dissolution. During the charging and discharging processes, Al ions in the electrolyte enter MnO lattice by co-deposition and intercalation, respectively. The incorporated Al ions effectively optimize the electronic structure of MnO by lowering the valence state of localized Mn to Mn, thereby facilitating the formation of inner-sphere complexes with HQ molecules. This transformation successfully shifts the dominant reaction mechanism between MnO and the redox mediator from outer-sphere electron transfer (Mn-HQ) to inner-sphere electron transfer (Mn-HQ). Consequently, complete MnO dissolution can be achieved in the designed electrolyte even at an ultrahigh areal capacity of 50 mAh cm. Furthermore, a 750-mAh electrolytic Zn//MnO battery exhibits a capacity retention rate of 99% after 100 cycles, demonstrating the significance of regulating electron transfer mechanisms during MnO dissolution through electrolyte coupling strategies.