Inhibiting Jahn–Teller distortion of δ-MnO2 cathode using 3d and 4d block elements doping to improve the aqueous zinc ion battery performance

Abstract

The δ-MnO₂ polymorph is a safe, low-cost cathode with high energy density for rechargeable aqueous zinc-ion batteries. However, its structural instability arises from the formation of the intermediate Mn³⁺ during the reduction of Mn⁴⁺. Mn³⁺ is unstable in octahedral symmetry due to the presence of a single electron in the e_g orbital, leading to Jahn–Teller distortion and substantial capacity fading. To address this, we adopted a doping strategy using suitable 3d (Fe³⁺) and 4d (Mo⁶⁺, Rh³⁺) block elements to substitute Mn³⁺ and suppress Jahn–Teller distortion. This approach aims to ensure morphological stability, expand interlayer spacing, and facilitate Zn²⁺ diffusion for enhanced electrochemical performance. Fe³⁺ (0.55 Å) and Mo⁶⁺ (0.59 Å), with ionic radii close to that of Mn³⁺ (0.58 Å), are more likely to substitute Mn³⁺ effectively than Rh³⁺ (0.67 Å). Electrochemical results reveal that Rh³⁺-doped electrodes show rapid capacity decay, indicating failed substitution and persistent distortion. Mo⁶⁺-doped δ-MnO₂ maintains high capacity up to 600 cycles but declines gradually with further cycling. In contrast, Fe³⁺-doped δ-MnO₂ demonstrates excellent stability, high capacity at 2C, and low charge transfer resistance, confirming successful mitigation of Jahn–Teller distortion, which is also verified by theoretical evidence obtained via DFT calculation. This doping strategy opens new avenues for developing stable, high-performance aqueous zinc-ion batteries.