Axial Electron Regulation Enables Superior Proton Storage in Aqueous Zinc-Ion Battery Cathodes

The intrinsic low electronic conductivity of manganese dioxide cathodes fundamentally limits the rate capability of aqueous zinc-ion batteries. Here, we demonstrate an axial electron regulation strategy through strategic Nb⁵⁺ doping that induces controlled geometric distortion in MnO₂, directing electron redistribution from high-energy e_g orbitals to low-energy t_2g orbitals. This orbital engineering approach enhances π-type Mn(t_2g)-O(2p) hybridization while suppressing detrimental Jahn-Teller distortions. DFT + U calculations reveal a 42% reduction in π* antibonding occupancy and a 19.9% decrease in Zn²⁺ migration barriers (0.572 eV). Comprehensive characterization confirms enhanced t_2g orbital occupancy and strengthened orbital hybridization (35% improvement via vibrational spectroscopy). The optimized Nb-MnO₂ cathode delivers exceptional electrochemical performance: 366 mAh g^-1 at 0.1 A g^-1 with remarkable rate capability, retaining 225 mAh g^-1 at 5 A g^-1-representing a large improvement over pristine MnO₂. Mechanistic studies reveal that surface-controlled charge storage is dominated by Grotthuss-type proton conduction (92.3% contribution), enabling superior high-rate performance. This work establishes axial electron regulation as a paradigm for orbital-oriented design of high-performance cathode materials.