Unraveling the Key Atomic Interactions in Determining the Varying Li/Na/K Storage Mechanism of Hard Carbon Anodes

Hard carbons have been identified as competitive anodes for Li/Na/K-ion batteries but their Li/Na/K-ion storage mechanisms significantly vary in different batteries. It is fundamental to understand the basic science behind the difference. Herein, it is theoretically revealed that defects on the carbon layers generally have an influential impact on the atomic interactions including the metal-metal (M-M) and metal-carbon (M-C) interactions, thereby determining whether the stored alkali-metal atoms are in ionic or quasi-metallic states. Upon increasing the number of metal atoms on a carbon layer composed of only hexatomic rings, K tends to be stored in an ionic state similar to Li due to the dominant M-C interaction, while on a carbon layer with defects, K tends to be stored in a quasi-metallic state similar to Na due to the dominant M-M interaction. For experimental verification, a glassy carbon, the extreme form of hard carbon with dominant sp(2) hybridization and only Stone-Wales defects, is selected as a model anode, and its Li/Na/K-ion storage mechanisms are exactly consistent with the theoretical prediction. More profoundly, for the first time, the quasi-metallic K cluster information is captured by ex situ electron paramagnetic resonance.