Unravelling the reaction chemistry and degradation mechanism in aqueous Zn/MnO2 rechargeable batteries

Abstract

Aqueous Zn/MnO₂ rechargeable batteries utilizing a near neutral electrolyte have demonstrated great potential for large-scale energy storage applications, due to their safe and sustainable nature. Nevertheless, the reaction chemistry and degradation process associated with the MnO₂-based cathode is not yet fully understood. Herein, a novel reversible Zn/MnO₂ battery with zinc hydroxide sulfate (Zn₄(OH)₆SO₄·5H₂O, ZHS) as the cathode has been designed, where active MnO₂ is formed in situ during the initial charge process from the Mn(II)-containing ZnSO₄ electrolyte. A combination of electrochemical and material characterizations reveal two-step redox reactions (Mn(II) ions ⇌ ZnMn₂O₄ spinel ⇌ layered Zn-birnessite) during the charge–discharge process. Excellent cycling stability with a capacity retention of 100% after 1500 cycles is achieved at 500 mA g⁻¹. The mechanism for long-term capacity fading is also studied. Cycling reversibility is destroyed by the irreversible consumption of Mn(II) to form woodruffite with a tunnel structure and poor electrochemical activity.