Unraveling the deposition/dissolution chemistry of MnO2 for high-energy aqueous batteries

Abstract

Aqueous rechargeable batteries based on the deposition/dissolution of MnO₂ are drawing significant attention because of their record-high theoretical capacity and redox potential in addition to their low cost and high safety. However, the deposition/dissolution chemistry of MnO₂ remains elusive, which must be overcome for it to be used in batteries. Herein, we used an in situ electrochemical quartz crystal microbalance technique to reveal the deposition/dissolution process, and unequivocally identified that it has pH-dependent Mn(III)-intermediates (MnOOH and Mn³⁺). In particular, the dissolved Mn³⁺ results in a loss of active species and thus greatly decreases the discharge capacity, Coulombic efficiency and cycling stability. As proof of this new understanding, introducing some redox mediators into the electrolyte effectively addresses this problem in a prototype Cu//MnO₂ battery. Our work provides a new and significant insight into the deposition/dissolution chemistry of MnO₂, which will promote the further exploration of high-energy aqueous batteries.