Enhancing Two-Electron Reaction Contribution in MnO₂ Cathode Material by Structural Engineering for Stable Cycling in Aqueous Zn Batteries.

Abstract

MnO₂ is a promising cathode for aqueous Zn batteries. However, the cycling stability is seriously hindered by active material dissolution, and the pre-addition of Mn²⁺ salts in electrolytes is widely required. Herein, we propose a structural engineering strategy for MnO₂ to enhance the capacity contribution from the reversible two-electron transfer reaction of MnO₂/Mn²⁺ and realize stable cycling in Mn²⁺-free electrolytes. By compositing with MoO₃, MnO₂ 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 Mn²⁺, 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 MnO₂ to 1.68 in the MnO₂/MoO₃ composite material. In the ZnSO₄ electrolyte, the MnO₂/MoO₃ cathode achieves 92.6% capacity retention after 300 cycles at 0.1 A g^-1 (>1900 h), superior to 62.7% for MnO₂. MnO₂/MoO₃ 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 MnO₂ in Zn batteries.