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.