Asymmetric cross-orbital coupling in Fe–Mn spinels decouples structural stability and kinetics in sodium-ion storage

Abstract

The aqueous energy storage potential of transition metal oxides (TMOs) has long been hampered by the inherent trade-off between structural stability and reaction kinetics—a dilemma rooted in their antagonistic dependence on metal–oxygen (M–O) hybridization. Conventional strategies, constrained by the rigid linear correlation between M–O covalency and performance metrics, fail to decouple these competing properties. Here, we demonstrate an asymmetric cross-orbital coupling strategy to modulate electron distribution by precisely engineering Fe–O–Mn interactions in K_xFe_yMn_1−yO_z spinels. Combining density functional theory calculations with in situ spectroscopic characterization, we unveil the orbital coupling mechanism between Mn e_g and Fe t_2g states: selective redistribution of antibonding electron occupancy through orbital energy-level and spatial distribution differences, alongside an upshift of the O 2p band center that narrows the M–O orbital energy gap. This dual modulation effectively decouples structural and kinetic limitations. The optimized KFe_0.15Mn_0.85O₂ electrode demonstrates a remarkable specific capacitance of 355.7 F g⁻¹ at 1.0 A g⁻¹, a 147% increase over the pristine material, with 86% retention after 30 000 cycles and a 40% lower Na⁺ migration barrier. This work provides a paradigm-shifting solution to the stability-kinetics dilemma in TMOs and opens new avenues for designing advanced energy materials that transcend classical hybridization constraints.