2,2,2-Trifluoroethyl Acetate as an Electrolyte Solvent for Lithium-Ion Batteries: Effect of Weak Solvation on Electrochemical and Structural Characteristics

We report the characteristics of 2,2,2-trifluoroethyl acetate (TFEAc) as a new type of electrolyte solvent for lithium (Li)-ion batteries. TFEAc-based electrolyte solutions containing 1.0 mol dm–3 LiTFSA salt [TFSA: bis(trifluoromethanesulfonyl)amide] exhibited a Li-ion insertion reaction into the negative graphite electrode in the first cycle; however, the performance was noticeably degraded during subsequent cycles due to the lack of solid electrolyte interphase (SEI) formation on the electrode. The electrode reaction was markedly improved when a small amount of ethylene carbonate (EC) was added into the LiTFSA/TFEAc solution, which demonstrated a high-rate charge/discharge performance superior to that of the conventional carbonate-based Li-ion battery electrolyte, that is, 1.0 mol dm–3 LiPF6 in the EC + dimethyl carbonate mixture. Quantitative Raman spectral analysis and density functional theory calculations revealed that TFEAc could be categorized as an organic solvent with low solvation power (i.e., with a predicted Gutmann donor number of 9.1); thus, Li ions mainly formed contact ion-pair complexes, [Li(TFEAc)2(TFSA)], in binary LiTFSA/TFEAc solutions. Adding EC into the TFEAc electrolyte modified the Li-ion complex structure; namely, Li ions were coordinated with each TFEAc, EC, and TFSA component to yield [Li(TFEAc)(EC)(TFSA)] as the major species, which coexisted with the ion pair [Li(TFEAc)2(TFSA)]. We discuss the effect of the weak coordination solvent and EC additive on the graphite electrode reaction from the aspects of Li-ion desolvation and SEI formation.