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

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 pi* antibonding orbital occupancy. This is accomplished through strategic doping with strongly electron-withdrawing Rh3+ (t(2g)(6)e(g)(0)), 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+ t(2g), compared to that between Rh3+ eg and Mn4+ eg, fosters direct electron transfer from the Mn4+ t(2g) to the vacant Rh3+ eg. This electron movement induces an upshift in the Mn-t(2g) orbital energy levels while concurrently diminishing the occupancy of pi* 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.