Enhancing Two-Electron Reaction Contribution in MnO 2 Cathode Material by Structural Engineering for Stable Cycling in Aqueous Zn Batteries.

MnO 2 is a promising cathode for aqueous Zn batteries. However, the cycling stability is seriously hindered by active material dissolution, and the pre-addition of Mn 2+ salts in electrolytes is widely required. Herein, we propose a structural engineering strategy for MnO 2 to enhance the capacity contribution from the reversible two-electron transfer reaction of MnO 2 /Mn 2+ and realize stable cycling in Mn 2+ -free electrolytes. By compositing with MoO 3 , MnO 2 exhibits weakened Mn-O bonds, more oxygen vacancies, spontaneous generation of structural water, and thus a lowered energy barrier for Mn release during discharge. Meanwhile, the composite material presents stronger electrostatic attractions for dissolved Mn 2+ , which ensures highly reversible re-deposition during charge. As a result, the mass ratios between materials undergoing reversible two-electron and one-electron transfer reactions increase from 0.85 in MnO 2 to 1.68 in the MnO 2 /MoO 3 composite material. In the ZnSO 4 electrolyte, the MnO 2 /MoO 3 cathode achieves 92.6% capacity retention after 300 cycles at 0.1 A g -1 (>1900 h), superior to 62.7% for MnO 2 . MnO 2 /MoO 3 also retains 80.1% capacity after 16 000 cycles at 1 A g -1 (>3200 h). This work presents an effective path to realize stable cycling of MnO 2 in Zn batteries.