Spectroscopic and Density Functional Theory Characterization of Common Lithium Salt Solvates in Carbonate Electrolytes for Lithium Batteries

The structure and composition of lithium ion solvation spheres of electrolyte solutions composed of common lithium salts (LiTFSI, LiPF6, LiBF4, and LiClO4) dissolved in aprotic polar linear and cyclic carbonate solvents (propylene carbonate (PC) or dimethyl carbonate (DMC)) have been investigated via a combination of FTIR, 13C NMR spectroscopy, and density functional theory (DFT). Results from the two different spectroscopic methods are in strong agreement with each other and with predictions from quantum chemistry calculations. The coordination of the carbonyl oxygen of the solvents to the lithium cation is observed by IR spectroscopy. The ratio of coordinated to uncoordinated PC and DMC has been used to determine solvent coordination numbers which range from 2 to 5 depending on salt, solvent, and concentration. The relative stability of the lithium–anion solvates were examined using DFT employing the cluster-continuum approach including changes to the intensity and frequency of the IR bands along with the populations of the cis–cis and cis–trans conformers of DMC in the lithium ion solvation shell. Solvent coordination is dependent upon the nature of the salt. Weakly associating salts, LiTFSI, LiPF6, and LiClO4, dissociate to a similar degree with LiPF6 being the most dissociated, while LiBF4 had significantly less dissociation in both solvents. This investigation provides significant insight into the solution structure of commonly used LIB electrolytes over a wide range of salt concentrations.