(342p) First-Principles Study of the Role of Carbon Host in the Sodiation of Selenium

Sodium−selenium (Na−Se) batteries are considered as an emerging alternative for the traditional Li-ion batteries owing to the high theoretical volumetric capacity (~ 3250 mAh/cm³) with high electronic conductivity of Se (~ 10^-3 S/m). However, Na-Se batteries suffer from massive volume changes associated with the sodiation/desodiation and the formation of soluble polyselenides. Carbon composites could be employed as host materials to buffer the sodiation-induced expansion, provide conductive channels, and physically trap the high-order polyselenides. In this talk, we evaluate the role of carbon host in the sodiation of Se as the fundamental understanding of the sodiation of the Se-C composites is still limited. Based on first-principles simulations, we present comprehensive analyses on the sodiation of Se-graphene (C), in terms of the interfacial structure, charge transfer, diffusion characteristics of Na, and electronic properties. Our calculations demonstrate a distinct Na enrichment at the Se/C interface, donating electrons to both Se and C. The interfacial strength is enhanced with sodiation, which could lead to compressive stresses at the interface during Na incorporation and mechanical failures. Longer Se chains are more populated with the presence of graphene, as less reduced Se ions (due to the electron flows to graphene) are more likely to cluster. Our calculations demonstrate that the interfacial Na ions shows a higher mobility along the graphene surface, compared to the bulk diffusion in Se, which suggests the enhanced Na diffusion could promote the sodiation/desodiation rate. The density of states (DOS) analysis clearly illustrates that the Na−Se/C system shows n-type metallic behavior, which could provide facile electronic pathways for electrochemical reaction process of Na−Se batteries. This work highlights the role of carbon host in establishing the sodiation kinetics, which may provide a framework for the development of the Na−Se batteries.