Universal Single Atom Engineering Enhances Coulombic Efficiency of Ion Storage in Carbon Materials

Metal single atoms are widely used to optimize the microstructure of carbon materials to improve their ion storage capacity and rate performance, but the impact on another key parameter, Coulombic efficiency (CE), is not sufficiently addressed and confirmed. Herein, a universal phenomenon is reported that carbon-loaded asymmetric sulfur-modified metal-N₄ moiety (MN₄-S, M = Zn, Fe, Cu, and Ni) possesses higher CE than the symmetric MN₄ moiety, and this phenomenon is applicable to various of carbon matrices, ions (Li⁺, Na⁺, and K⁺), charge and discharge rates, and electrolyte formulations. The carbon-loaded asymmetric MN₄-S moiety exhibits larger CEs (0.03-0.46% higher of average CEs, 4.2-28.4% higher of the initial CEs) and smaller variance compared to the MN₄ moiety, implying better reversible stability. The mechanism driving this phenomenon is revealed by the ZnN₄-S sodium storage process. The asymmetric MN₄-S coordination promotes the rapid ions diffusion kinetics by changing the charge density. Meanwhile, the MN₄-S moiety can reduce the adsorption energy of ions and regulate the surface chemical reactivity of the material to increase the reversibility of surface ion storage, thereby achieving higher CE and better stability.