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

• Published in: Journal of physical chemistry. C 2016-08, Vol.120 (31), p.17196-17204
• Main Authors: Fujii, Kenta, Wakamatsu, Hideaki, Todorov, Yanko, Yoshimoto, Nobuko, Morita, Masayuki
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b04542

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 ( 2.5 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.