Lithium-sulfur battery with partially fluorinated ether electrolytes: Interplay between capacity, coulombic efficiency and Li anode protection

Seven partially fluorinated ethers (PFE) were studied as co-solvents for lithium-sulfur battery with the goal to understand the interplay between the molecular structure and the electrochemical properties, including capacity, Coulombic efficiency and SEI formation on the Li anode. Electrochemical cycling indicates a loose correlation between the Coulombic efficiency observed over 200 cycles and the fluorination degree of the PFE, which correlates with the computed pKa of the C-H bond of the ethers. PFE with lower pKa (<36) tends to chemically react with lithium anode forming a solid-electrolyte-interphase (SEI) layer thus preventing the shuttling effect of the lithium polysulfides. The cycled lithium anodes with DOL/PFE electrolyte was found to produce a protective SEI analyzed by FT-IR and SEM/EDS. Computations of the interactions between the PFEs of interest and a lithium surface suggest the generation of LiF and unsaturated PFE derivatives which are easily reduced and polymerized on the Li surface. The PFE decomposition is more thermodynamically likely with increasing fluorination degree. This research sheds light on the role that fluorine substitution and structure play in the high Coulombic efficiencies observed for Li-S cells.