The development of better energy storage technologies is required for a sustainable growth of the world population. Numerous different technologies have been developed and applied for energy storage in the last decades. Currently, the most utilized technology relies on electrochemical reduction and oxidation of lithium. Although the lithium technology has been developed for more than 10 years, the structure of the lithium ion solvation shell is still under debate. It has been previously suggested that the coordination number of organic carbonate molecules around the lithium ion can be 4 and 6, but no definitive conclusions have been derived from these experiments. Previous infrared studies on lithium ion battery electrolytes have been limited by the high optical density of the carbonate solvents, resulting in researchers studying dilute systems instead of the true lithium ion battery electrolyte. Using a novel spectroscopic sample cell, we have used linear infrared (FTIR) and two-dimensional infrared (2DIR) spectroscopies in combination with density functional theory (DFT) calculations to study lithium hexafluorophosphate salt (LiPF₆) dissolved in pure carbonate solvents (dimethyl carbonate and diethyl carbonate) at concentrations of ca. 1.5 M.

In agreement with previous studies our infrared experiments show the presence of two infrared bands in the carbonyl stretching region: low and high frequency bands. These two bands have been traditionally assigned to the lithium-coordinated carbonyl stretch (low frequency band) and to the free carbonyl stretch (high frequency band). However, in the 2DIR spectra of LiPF₆in either linear carbonate solvent, the spectrum shows the presence of a cross peak between the “free” and “coordinated” carbonyl stretches at a waiting time of 0 fs indicating coupling between the two vibrational states. Thus, a new assignment of the peak is proposed in which the lithium ion has a tetrahedral arrangement of linear carbonate molecules. In this molecular arrangement, the low frequency band is due to the overlap of three asymmetric carbonyl stretches arising from the tetrahedral structure of the carbonates around the Li ion, while the high frequency band is due to both the uncoordinated carbonate molecules as well as the symmetric carbonyl stretching mode arising from the tetrahedral complex, which for a perfect tetrahedral is not IR active. This assignment is perfectly consistent with the experimental observations of both this work and previous studies. Notably, the use of 2DIR spectroscopy also demonstrates that the lithium ion solvation shell is not a perfect tetrahedron as seen by the presence of a cross peak at 0 fs waiting time. This cross peak arises from the imperfect geometry of the tetrahedral solvation shell of the complex that makes the symmetric stretch infrared active since it is not expected that the carbonates of the lithium solvation shell, which have the carbonyl groups pointing towards the cation, have a strong coupling with the surrounding “free” carbonates. Thus, in the context of a symmetrically broken tetrahedral structure, the “dark” symmetric stretching mode will be overlapped with the “free” carbonyl stretching mode. Ab-initio calculations confirm the existence of the observed infrared bands, and they assign them to three frequency overlapped asymmetric carbonyl stretching modes and the weaker symmetric carbonyl stretching mode frequency, which overlaps with the “free” carbonyl stretch. Moreover, these theoretical calculations present an excellent agreement for the ratio of transition dipoles, strongly supporting the new assignment of the bands and the solvation structure consisting of four coordinated carbonates in a tetrahedron arrangement.

Our studies also provide dynamical information of the system. In these electrolytes, the asymmetric stretching band of the diethyl carbonate solution shows significantly longer frequency frequency correlation time than that of the dimethyl carbonate solution which indicates that the diethyl carbonates coordinated to the Li ion have slower structural motions than the Li ion coordinated dimethyl carbonate molecules. In addition, our results show that  the difference observed in the dynamics are not likely to arise from the motions of the solvent molecules since the solvent motions for both carbonates have very similar time scales. Thus, the difference in the dynamics is belived to be associated with either the energetics of the Li-carbonyl interactions or the presence of solvent separated ion-pairs.