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

Abstract

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 + ClO 4 − 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) 1 Li + ] and is totally protected from direct interaction (contact ion pairing) with the ClO 4 − ion. The solvation structures in neat PC and neat DEC solvents are also discussed.I on solvation is a central, and still an open, issue in many chemical, biochemical, and electrochemical processes. One of those important processes would be the functioning of lithium-based rechargeable batteries. 1−3 Their performance depends on the electrode materials and processes on the one hand and on the charge carrier concentration and mobility in the electrolyte solution on the other hand. With regard to the latter, high charge density of the Li + ion should be sufficiently stabilized, and at the same time, the electrolyte solution should have sufficiently high fluidity. A usual practice to make these two factors compatible is to employ a mixed solvent, consisting of a highly dipolar liquid such as a cyclic carbonate stabilizing the high charge density (but highly viscous) and a liquid of lower viscosity such as a linear carbonate (being less dipolar). Quite often ethylene or propylene carbonate (with dielectric constant ε = 65−90 and viscosity η ≅ 2.5 cP, abbreviated as EC and PC) is used for the former, and dimethyl, diethyl, or ethyl methyl carbonate (with ε ≅ 3 and η = 0.6−0.9 cP, abbreviated as DMC, DEC, and EMC) is used for the latter.The solvation structure around the Li + ion, especially that of the first solvation shell, has been suggested to be important for the interphase chemistry on the electrodes. 4−6 The use of a mixed solvent introduces a complexity in this. One controversial subject in this regard is the presence/absence of the preferential solvation and (if present) its nature for the Li + ion in a mixed cyclic/linear carbonate solution. 7−19 On the basis of electrospray ionization mass spectroscopy (ESI-MS), 7,8 it has been suggested that there is a strong preferential solvation for Li + in EC/EMC, with the Li + (EC) 2 species as the main ingredient. 7 The same type of preferential solvation (i.e., with a higher population of cyclic carbonate around the ion than in the bulk) has also been suggested in some NMR studies 9−11 but with a much larger total solvation number (≥6). 9,20 It has been argued that some...

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The Proton Trap Technology—Toward High Potential Quinone‐Based Organic Energy Storage

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