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

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.