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

Abstract

AbstractIn 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+ (t2g6eg0), 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+ eg and Mn4+ t2g, compared to that between Rh3+ eg and Mn4+ eg, fosters direct electron transfer from the Mn4+ t2g to the vacant Rh3+ eg. This electron movement induces an upshift in the Mn‐t2g orbital energy levels while concurrently diminishing the occupancy of π* antibonding orbitals formed via Mn t2g‐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.