Evolution of high spin state single-atom catalyst active centers in Na-O2 batteries

Due to the abundance and economic viability of Na resources, Na-O-2 batteries are regarded as promising energy storage devices in achieving the carbon neutrality goals, featuring an ultra-high theoretical energy density. Nevertheless, the slow ion diffusion kinetics hinders the applications of batteries. Spin-induced single-atom catalysts (SACs) offer a promising avenue to ameliorate the activation process of the battery reaction. Herein, we study the adsorption-activation mechanism of O-2 on six spin-induced SACs (i.e., MnN3, MnN4, CoN3, CoN4, NiN3, and NiN4) in Na-O-2 batteries. We find that oxygen in mono-vacancy catalysts with high spin states favors the side-on adsorption mode. This mode enhances the coupling between the 3d(xy) orbital of metal and O-2, and alters the active center structures which further reduces the reaction overpotential by cutting down the OER potential. Moreover, we establish the scaling relationship between the oxygen adsorption energy and battery overpotential with a correlation coefficient of 0.98. Our study elucidates the evolution of the SAC active center induced by a shift of spin states and its impact on the oxygen adsorption-activation process which strongly determines the battery performance. The established structure-activity relationship of spin induced SACs may shed light on the catalyst modification involving the oxygen adsorption and activation to achieve a better performance.