The structure and dynamics of electrolytes composed of lithium hexafluorophosphate and either butylene carbonate or dimethyl carbonate were investigated using steady state and two-dimensional infrared spectroscopies. This study was focused on lithium ion compositions similar to that of commercial batteries (i.e., X(Li + ) = 0.09) and higher. Experiments provide sufficient evidence to demonstrate that both organic carbonates form tetrahedral solvation complexes around the lithium ion. Ab initio computations confirmed that the IR spectroscopic signatures derived from experiments correspond to a tetrahedral arrangement of carbonate molecules in the lithium ion solvation shell. Time resolved experiments further revealed that the solvation shell formed by cyclic carbonates is more rigid than that of its linear carbonate analogue. In addition, butylene carbonate was found to present a more organized "overall" solvent structure than dimethyl carbonate. At lithium salt concentrations beyond that of a conventional electrolyte, the electrolytes displayed changes in the dynamics and structure of their molecular components due to the presence of ion pairs. Cyclic and linear carbonates were found to preferentially form ion pairs with two distinct structures: contact and solvent separated ion pairs, respectively. The formation of distinct ion pairs by butylene carbonate and dimethyl carbonate is predicted to arise from the different solvation shells formed by the two carbonates; this was confirmed by ab initio frequency calculations and FTIR. Results of this study shed some light on the characterization of the solvation structure of the lithium ion in the electrolyte which could help to rationalize the importance of the electrolyte composition on the performance of the lithium ion battery.

The structure and dynamics of electrolytes composed of lithium hexafluorophosphate (LiPF) in dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate were investigated using a combination of linear and two-dimensional infrared spectroscopies. The solutions studied here have a LiPF concentration of X(LiPF) = 0.09, which is typically found in commercial lithium ion batteries. This study focuses on comparing the differences in the solvation shell structure and dynamics produced by linear organic carbonates of different alkyl chain lengths. The IR experiments show that either linear carbonate forms a tetrahedral solvation shell (coordination number of 4) around the lithium ion irrespective of whether the solvation shell has anions in close proximity to the carbonates. Moreover, analysis of the absorption cross sections via FTIR and DFT computations reveals a distortion in the angle formed by Li-O[double bond, length as m-dash]C which decreases from the expected 180° when the alkyl chains of the carbonate are lengthened. In addition, our findings also reveal that, likely due to its asymmetric structure, ethyl methyl carbonate has a significantly more distorted tetrahedral lithium ion solvation shell than either of the other two investigated carbonates. IR photon echo studies further demonstrate that the motions of the solvation shell have a time scale of a few picoseconds for all three linear carbonates. Interestingly, a slowdown of the in place-motions of the first solvation shell is observed when the carbonate has a longer alkyl chain length irrespective of the symmetry. In addition, vibrational energy transfer with a time scale of tens of picoseconds is observed between strongly coupled modes arising from the solvation shell structure of the Li which corroborates the modeling of these solvation shells in terms of highly coupled vibrational states. Results of this study provide new insights into the molecular structure and dynamics of the lithium ion electrolyte components as a function of solvent structure.

The optimal salt concentration used in metal-ion energy storage devices has long focused heavily on 1 M electrolytes; however, recent evidence suggests taking a deeper look at electrolyte properties as a function of salt concentration. Toward that goal, the effect of concentration on solvation properties for a prototype sodium electrolyte is considered with potential applications for sodium-ion and sodium−air technologies. An empirical force field for sodium triflate in digylme, an electrolyte already in use with sodium−air systems, was developed from ab initio molecular dynamics simulations in conjunction with the variational force-matching method. Atomistic simulations of this electrolyte along with Fourier transform infrared (FTIR) experimental studies validate the qualitative accuracy of the model and demonstrate its transferability across different concentrations. The solvation structure and the extent of ion pairing effects in the electrolyte were considered for concentrations ranging from 0.25 to 2.0 M in the sodium salt. Ion pairing effects are seen even at dilute concentrations of 0.5 M in both simulations and experiments, with a transition from solvent-separated species to direct contact ion pairs as the concentration increased to 1.5 M. With further increase in the concentration, evidence for ion aggregation is also presented.

Solvation of the thiocyanate ion in three different deep eutectic solvents (DES) was investigated by linear FTIR spectroscopy, and Two Dimensional IR spectroscopy. Linear infrared spectroscopy reveals that the thiocyanate ion forms a hydrogen bond through its sulphur atom, while its nitrile end remains free. Photon-echo vibrational spectroscopy shows that the thiocyanate has a frequency-frequency correlation function (FFCF) with two distinct dynamics occurring on the picosecond time scale in all of the studied solvents. The observed dynamics is assigned to in-place and diffusional motions of the components within the thiocyanate solvation shell. Molecular dynamics simulations and ab initio calculations confirm the experimental findings and their molecular interpretation. In addition, theoretical modeling of the thiocyanate nitrile stretch lineshape suggests that alcohol-based DES are more structurally disorganized than the amide-based analogue. However, the organization observed in the different DES is not sufficient to explain physical properties, such as density, indicating that the amount of defects (i.e., hole theory) is not sufficient to fully describe the properties of DES.