Solvation Structure Analysis of Lithium Ion in Concentrated Lithium Salt Solutions Using Raman Spectroscopy

Electrolyte solutions are one of the key liquid materials in the development of high-performance lithium-ion batteries (LIBs). Recently, a new concept, "superconcentrated electrolyte", i.e., solvent-in-salt in liquid state, was proposed and widely investigated in terms of not only electrochemistry, but also physical chemistry and/or solution chemistry. In this work, we report on a structural study of lithium-ion complexes in highly salt-concentrated electrolytes, lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt in N,N-dimethylpropionamide (DMPA), using Raman spectroscopy with the aid of density functional theory (DFT) calculations. In the range of the Li salt concentration, c(Li) = 0 - 3.2 mol dm(-3), the individual solvation numbers of DMPA and TFSA species around Li ions [n((DMPA)) and n((TFSA)), respectively] were quantitatively determined and the relation between the Li ion complex structure and c(Li) was discussed at the molecular level.