Structural and Electrochemical Properties of Li Ion Solvation Complexes in the Salt-Concentrated Electrolytes Using an Aprotic Donor Solvent, N,N-Dimethylformamide

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• 作者：Kenta Fujii ; Hideaki Wakamatsu ; Yanko Todorov ; Nobuko Yoshimoto ; Masayuki Morita
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：August 11, 2016
• 年：2016
• 卷：120
• 期：31
• 页码：17196-17204
• 全文大小：494K
• 年卷期：0
• ISSN：1932-7455

文摘

We report the relation between the structural and electrochemical properties of N,N-dimethylformamide (DMF)-based electrolytes containing lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) in the concentration range c_Li = 0–3.2 mol dm^–3. Raman spectroscopy and DFT calculations indicate that Li⁺ ions are solvated by DMF molecules in the form of [Li(DMF)₄]⁺ complexes at low c_Li (<2.0 mol dm^–3, LiTFSA:DMF = 1:4 by mol), whereas the coordination of TFSA^– anions to Li⁺ ions occurs and gradually increases as c_Li increases above 2.0 mol dm^–3. Quantitative Raman data analysis reveals that TFSA^– anions coordinate with Li⁺ ions in a monodentate manner (mono-TFSA) in the c_Li range of 2.0–2.5 mol dm^–3, and mono-TFSA coexists with TFSAs as a bidentate manner (bi-TFSA) in solutions with c_Li > 2.5 mol dm^–3. The high c_Li solutions, in which all the DMF molecules solvate to Li⁺ ions (i.e., no DMF remains in the bulk), make the electrochemical window wider; the oxidative stability is enhanced owing to lower HOMO energy levels of solvated DMF molecules relative to those in the bulk. The salt concentration also controls the reductive stability; coordinated TFSA^– anions within the Li-ion complexes formed in concentrated solutions affect the LUMO energy levels of the electrolyte. The LUMOs located on the TFSA^– anions lead to a preferential reduction of the TFSA component rather than DMF to form a solid electrolyte interphase on graphite negative electrodes, resulting in the Li-ion insertion/desertion into/from graphite in the concentrated solutions.