Efficient Modulation d/p-Band Center Proximity in Birnessite-Type MnO2 by Cation/Anion Co-Doping for Enhanced Dual-Ion Storage

Abstract

The wide band gap of δ-MnO₂ and the restricted regulation of the p-band center ($\epsilon$_p) of active oxygen atoms hinder electron conduction and ion diffusion during zinc storage. Heteroatom doping has proven effective in improving zinc storage. However, the regulation of $\epsilon$_p solely through cation doping is comparatively constrained. Herein, a cation/anion co-doping strategy is proposed to simultaneously adjust the $\epsilon$_p of oxygen and the d-band center ($\epsilon$_d) of Mn by establishing Mn–O–Ca and Mn─N bonds in Ca/N co-doped δ-MnO₂ (Ca/N-MnO₂). This co-doping effect induces a shift in the predominant stretching vibration mode of Mn─O bonds in [MnO₆] octahedra, resulting in a decreased d/p-band center proximity (Δ$\epsilon$_d-p) of 1.688 eV (1.750 eV for δ-MnO₂). Consequently, both adsorption capacity and adsorption/desorption rates of Zn²⁺/H⁺ ions are enhanced. Theoretical calculations further reveal that co-doping significantly triggers the optimization of the band gap, migration energy barrier, and adsorption energy. As a result, Ca/N-MnO₂ achieves a remarkable reversible capacity of 192.7 mAh g⁻¹ at 1.0 A g⁻¹ after 200 cycles and exceptional rate performance. Furthermore, the energy enhancement mechanisms are thoroughly elucidated. This synergistic d/p-band center regulation strategy offers significant potential for efficient energy storage applications.