Polysulfide cluster formation, surface reaction, and role of fluorinated additive on solid electrolyte interphase formation at sodium-metal anode for sodium–sulfur batteries

Room-temperature sodium–sulfur batteries are promising next-generation energy storage alternatives for electric vehicles and large-scale applications. However, they still suffer from critical issues such as polysulfide shuttling, which inhibit them from commercialization. In this work, using first-principles methods, we investigated the cluster formation of soluble Na2S8 molecules, the reductive decomposition of ethylene carbonate (EC) and propylene carbonate (PC), and the role of fluoroethylene carbonate (FEC) additive in the solid electrolyte interphase formation on the Na anode. The clustering of Na2S8 in an EC solvent is found to be more favorable than in a PC solvent. In the presence of an electron-rich Na (001) surface, EC decomposition undergoes a two-electron transfer reaction with a barrier of 0.19 eV for a ring-opening process, whereas PC decomposition is difficult on the same surface. Although the reaction kinetics of an FEC ring opening in the EC and PC solvents are quite similar, the reaction mechanisms of the open FEC are found to be different in each solvent, although both lead to the production of NaF on the surface. The thick NaF layers reduce the extent of charge transfer to Na2S8 at the anode/electrolyte interface, thus decelerating the Na2S8 decomposition reaction. Our results provide an atomistic insight into the interfacial phenomena between the Na-metal anode surface and electrolyte media.