Mo doping provokes two electron reaction in MnO₂ with ultrahigh capacity for aqueous zinc ion batteries

Rechargeable aqueous zinc ion batteries (AZIBs) based on manganese dioxide (MnO₂) have received much attention for large-scale energy storage applications, however, their energy density is mainly limited by the one-electron reaction of Mn⁴⁺/Mn³⁺ redox. Herein, Mo doped δ-MnO₂ (Mo-MnO₂) is prepared and used as a high-performance cathode for AZIBs, which delivers an ultrahigh specific capacity of 652 mAh·g⁻¹ at 0.2 A·g⁻¹ based on the two-step two-electron redox reaction of Mn⁴⁺ $\rightleftharpoons$ Mn³⁺ $\rightleftharpoons$ Mn²⁺. Ex-situ structural analysis and density functional theory calculation reveal that the Mo⁵⁺ dopant plays an important role in enhancing the electronic conductivity of Mo-MnO₂ and promoting Jahn–Teller distortion of octahedral [MnO₆] in ZnMn₂O₄, which facilitates the second step redox reaction of Mn³⁺/Mn²⁺. This work provides a novel cathode materials design with multi-electron redox chemistry to achieve high energy density in AZIBs.