Enabling Reversible MnO 2 /Mn 2+ Transformation by Al 3+ Addition for Aqueous Zn-MnO 2 Hybrid Batteries.

Aqueous rechargeable Zn-manganese dioxide (Zn-MnO ₂ ) hybrid batteries based on dissolution-deposition mechanisms exhibit ultrahigh capacities and energy densities due to the two-electron transformation between MnO ₂ /Mn ²⁺ . However, the reported Zn-MnO ₂ 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-MnO ₂ hybrid battery by adding Al ³⁺ into the sulfate-based electrolyte. The hybrid battery undergoes reversible MnO ₂ /Mn ²⁺ 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 Al ³⁺ 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 Zn ₄ (OH) ₆ (SO ₄ )·0.5H ₂ O during discharging. In addition, Al ³⁺ 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 Al ³⁺ in the electrolyte, providing a new fundamental understanding of the promising aqueous Zn-MnO ₂ batteries.