Structure, Disorder, and Crystallization; Lessons Learned from Analysis of Lithium Trifluoromethanesulfonate

Batteries are one of the most impactful inventions humankind has ever developed. Lithium-ion energy cells represent a significant advance in battery technology. Electrolytes used in batteries are still not well understood in terms of solution structure. We investigate electrolyte solvate structure with the aid of differential scanning calorimetry, Raman spectroscopy, and X-ray diffraction. Here is a report on the solvate structure(s) present in mixtures of propylene carbonate (PC) and lithium trifluoromethanesulfonate (LiTf). Mixtures of racemic PC with LiTf form glasses whereas enantiomerically pure PC, both R and S isomers, forms mixtures with more complex phase behavior. At concentrations approaching a 1:1 molar ratio of solvent to salt, a solvate that melts at ~150 °C was detected. A crystal of the 1:1 solvate of the R-(+)-PC:LiTf was successfully grown and analyzed by X-ray diffraction. This solvate has an aggregated structure where the Li + cations are tetrahedrally coordinated. Three directions of coordination come from Tf – anions acting as a bridging ligand between three different Li + cations and the 4 th direction of coordination comes from the carbonyl oxygen of the solvent molecule.