Inhibiting Jahn-Teller distortion of δ-MnO2 cathode using 3d and 4d block elements doping to improve 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 intermediate Mn3+ during the reduction of Mn4+. Mn3+, unstable in octahedral symmetry due to a single electron in the eg orbital, leads 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 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 by DFT calculation. This doping strategy opens new avenues for developing stable, high-performance aqueous zinc-ion batteries.