Solvation Structure around the Li⁺ Ion in Mixed Cyclic/Linear Carbonate Solutions Unveiled by the Raman Noncoincidence Effect

The solvation structure around the Li⁺ ion in a mixed cyclic/linear carbonate solution, an important factor for the performance of lithium-based rechargeable batteries, is examined by measuring and analyzing the noncoincidence effect observed for the CO stretching Raman band. This technique has the advantage of perceiving relative distances and orientations of solvent molecules clustering around an ion in the first solvation shell and, hence, of developing information on the solvation structure along the wavenumber axis rather than along the intensity axis of the spectra. It is shown that, taking the solution of Li⁺ClO₄^– in the 1:1 mixed solvent of propylene carbonate (PC) and diethyl carbonate (DEC) as an example case, the Li⁺ ion is preferentially solvated by PC molecules [primarily as (PC)₃(DEC)₁Li⁺] and is totally protected from direct interaction (contact ion pairing) with the ClO₄^– ion. The solvation structures in neat PC and neat DEC solvents are also discussed