Synergistic tuning of d/p-band centers in Al-doped δ-MnO2 boosts kinetics and cyclability for aqueous Mg-ion storage

Layered δ -MnO 2 is a potential positive electrode for aqueous magnesium ion batteries (AMIBs), but its wide band gap, limited d/p-band centers interaction, and Mn dissolution during cycling restrict practical application. To address these critical issues, herein, nanoflower-like Al-doped δ -MnO 2 (AlMO) is synthesized via a scalable hydrothermal-calcination method. Density Functional Theory (DFT) calculations reveal that Al substitution simultaneously optimizes three key electronic properties: (1) Reduces band gap (1.44 eV → 0.75 eV), enhancing intrinsic conductivity; (2) Upshifts the O 2p-band center ( ε p ) from −3.24 eV to −2.64 eV and Mn 3d-band center ( ε d ) from −1.79 eV to −1.60 eV; (3) reduces d/p-band centers differences (Δ ε d-p ) from 1.45 eV to 1.04 eV, strengthening orbital hybridization. These adjustments boost Mg 2+ adsorption capacity and accelerate charge transfer/ion diffusion kinetics. Importantly, by constructing the Mn-O-Al bonds to induce the d-band center at the Mn site to upward movement, the O 2p orbital electrons preferentially occupy the Mn-O bonding orbital, thereby significantly strengthening the Mn-O bonds and structural stability. As anticipated, the optimized AlMO positive electrode exhibits exceptional cyclic stability (about 93 % capacity retention after 1500 cycles) and satisfactory discharge capacity (168.3 mAh g −1 , 67.5 % higher than pristine δ -MnO 2 at 0.1 A g −1 ). The assembled AlMO//3,4,9,10-perylenetetracarboxylic diimide full cell also maintains a stable working state. This work establishes a precise electronic structure modulation strategy for δ -MnO 2 , providing an innovative approach to developing high-performance, long-cycle-life positive electrodes for AMIBs, and other aqueous rechargeable batteries.