(264h) Investigation on the Sodiation of Selenium-Graphene for Na-Ion Batteries: A First-Principles Study

Sodium−selenium (Na−Se) batteries have recently attracted attention as an emerging alternative for the traditional Li-ion batteries owing to their high theoretical volumetric capacity (~ 3250 mAh/cm³) and electronic conductivity of Se (~ 10^-3 S/m). Carbon composites are proposed as promising host materials to buffer the volume fluctuation of Se during charge/discharge, provide conductive channels, and physically trap the high-order polyselenides. In this talk, we evaluate the role of carbon host in establishing the reaction kinetics as the fundamental understanding of the sodiation of the Se-C composite is still limited. Based on first-principles simulations, we first present interfacial structure, charge transfer, and diffusion characteristics of Na atoms at the interface between Se and graphene (C). Our calculations demonstrate that there is a substantial enrichment of Na atoms at the interface and corresponding electron transfer to both Se and C nearby. The interactive strength of Se with graphene is found to increase with sodiation, which could lead to mechanical failures as compressive stress build-up at the interface. The presence of C host results in a more populated distribution of longer Se chains, as less reduced Se (due to electron transfer toward graphene) tends to be clustered. We observed a faster mobility of interfacial Na atoms along the graphene surface when compared to the diffusion of Na atoms in Se. Our calculations also show that the Na−Se/C system exhibits n-type metallic characteristic, which could provide fast electron flux for the sodiation reaction.