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.

中文翻译：

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2,2,2-三氟乙酸乙酯作为锂离子电池的电解质溶剂：弱溶剂化对电化学和结构特性的影响

我们报告了 2,2,2-三氟乙酸乙酯 (TFEAc) 作为一种新型锂 (Li) 离子电池电解质溶剂的特性。含有 1.0 mol dm ^–3 LiTFSA 盐 [TFSA: 双（三氟甲磺酰基）酰胺]的基于 TFEAc 的电解质溶液在第一次循环中表现出锂离子嵌入石墨负极的反应；然而，由于电极上没有形成固体电解质中间相 (SEI)，因此在随后的循环中性能明显下降。当在 LiTFSA/TFEAc 溶液中加入少量碳酸亚乙酯 (EC) 时，电极反应显着改善，表现出优于传统碳酸酯基锂离子电池电解液的高倍率充放电性能，即 1.0 mol dm ^–3EC+碳酸二甲酯混合物中的LiPF ₆。定量拉曼光谱分析和密度泛函理论计算表明，TFEAc 可归类为溶剂化能力低的有机溶剂（即预测的古特曼供体数为 9.1）；因此，锂离子主要在二元 LiTFSA/TFEAc 溶液中形成接触离子对复合物 [Li(TFEAc) ₂ (TFSA)]。在TFEAc电解液中加入EC改变了锂离子络合物结构；即，锂离子与每个 TFEAc、EC 和 TFSA 组分配位以产生 [Li(TFEAc)(EC)(TFSA)] 作为主要物质，其与离子对 [Li(TFEAc) ₂(TFSA)]。我们从锂离子去溶剂化和 SEI 形成方面讨论了弱配位溶剂和 EC 添加剂对石墨电极反应的影响。