Enabling Reversible MnO2/Mn2+ Transformation by Al3+ Addition for Aqueous Zn–MnO2 Hybrid Batteries

Aqueous rechargeable Zn–manganese dioxide (Zn–MnO2) hybrid batteries based on dissolution–deposition mechanisms exhibit ultrahigh capacities and energy densities due to the two-electron transformation between MnO2/Mn2+. However, the reported Zn–MnO2 hybrid batteries usually use strongly acidic and/or alkaline electrolytes, which may lead to environmental hazards and corrosion issues of the Zn anodes. Herein, we propose a new Zn–MnO2 hybrid battery by adding Al3+ into the sulfate-based electrolyte. The hybrid battery undergoes reversible MnO2/Mn2+ transformation and exhibits good electrochemical performances, such as a high discharge capacity of 564.7 mAh g–1 with a discharge plateau of 1.65 V, an energy density of 520.8 Wh kg–1, and good cycle life without capacity decay upon 2000 cycles. Experimental results and theoretical calculation suggest that the aquo Al3+ with Brønsted weak acid nature can act as the proton-donor reservoir to maintain the electrolyte acidity near the electrode surface and prevent the formation of Zn4(OH)6(SO4)·0.5H2O during discharging. In addition, Al3+ doping during charging introduces oxygen vacancies in the oxide structure and weakens the Mn–O bond, which facilitates the dissolution reaction during discharge. The mechanistic investigation discloses the important role of Al3+ in the electrolyte, providing a new fundamental understanding of the promising aqueous Zn–MnO2 batteries.