Engineering Antibonding Orbital Occupancy for Enhanced Sodium‐Ion Intercalation Kinetics in Transition Metal Oxides

Abstract In the quest to elevate the sodium‐ion intercalation kinetics of transition metal oxide electrodes, the intrinsic low conductivity of these materials often acts as a bottleneck, restricting Na⁺ storage. Herein, the mechanism behind sodium‐ion diffusion kinetics in MnO₂ is explored, specifically focusing on the manipulation of π* antibonding orbital occupancy. This is accomplished through strategic doping with strongly electron‐withdrawing Rh³⁺ (t_2g⁶e_g⁰), enhancing the hybridization of Mn 3d‐O 2p orbitals and significantly increasing the electrical conductivity of MnO₂. Density functional theory (DFT) calculations and X‐ray absorption spectroscopy (XAS) results demonstrate that the smaller orbital energy difference between Rh³⁺ e_g and Mn⁴⁺ t_2g, compared to that between Rh³⁺ e_g and Mn⁴⁺ e_g, fosters direct electron transfer from the Mn⁴⁺ t_2g to the vacant Rh³⁺ e_g. This electron movement induces an upshift in the Mn‐t_2g orbital energy levels while concurrently diminishing the occupancy of π* antibonding orbitals formed via Mn t_2g‐O 2p hybridization. The resultant Rh‐MnO₂ electrode exhibits an impressive specific capacity of 335 F g⁻¹ at 1 A g⁻¹ and a substantial rate capacity of 224.8 F g⁻¹ at 20 A g⁻¹. This investigation elucidates the intricate mechanism underlying the sluggish kinetics of sodium ion intercalation within transition metal oxide frameworks.