C6N2S monolayer: an auxetic material with ultralow diffusion barrier and high storage capacity for potassium-ion batteries

Abstract

The development of advanced anode materials is crucial for the applications of K-ion batteries (KIBs). In this study, swarm-intelligence structure search calculations have identified an auxetic C₆N₂S monolayer as a promising candidate with a desirable combination of storage capacity, rate capacity, and cycling endurance for KIBs. This monolayer exhibits a unique wavy structure, comprising S atoms forming valleys and interconnecting graphene-like nanoribbons forming peaks, with edge-sharing C₄N₂ rings. Its intrinsic metallicity, stemming from π-electron delocalization, facilitates favorable electronic conduction. The C₆N₂S monolayer spontaneously adsorbs two-layer K atoms, resulting in a stoichiometric composition of C₆N₂SK₄ and a high storage capacity of 812 mA h g⁻¹. The ultralow diffusion barrier of 0.03 eV along the valley ensures ultrafast ion transportation, while a moderate open circuit voltage of 0.28 V enhances battery safety. Furthermore, its auxetic behavior contributes to cycling stability. These remarkable properties of the C₆N₂S monolayer are attributed to its unique structural morphology and electron configuration induced by multiple-bond patterns.