Abstract
Several high-purity methyl perfluorocarboxylates were prepared (>99.5% purity by mole) and investigated as potential fluorine-rich electrolyte solvents in Li-ion batteries. The most conductive electrolyte, 0.1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethyl perfluoroglutarate (PF5M2) (ionic conductivity = 1.87 × 10−2 mS cm−1), is investigated in Si thin-film half-cells. The solid-electrolyte-interphase (SEI) formed by the PF5M2 electrolyte is composed of similar organic and inorganic moieties and at comparable concentrations as those formed by ethylene carbonate/dimethyl carbonate electrolytes containing LiPF6 and LiTFSI salts. However, the SEI formed by the PF5M2 electrolyte undergoes reversible electrochemical defluorination, contributing to the reversible capacity of the cell and compensating in part for capacity fade in the Si electrode. While far from ideal these electrolytes provide an opportunity to further develop predictions of suitable fluorinated molecules for use in battery solvents.
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