A comparison of the solvation structure and dynamics of the lithium ion in linear organic carbonates with different alkyl chain lengths

Abstract

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.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction

“…However, recent linear and 2D IR experiments of the carbonyl stretch mode of carbonates in LiPF6/carbonate solutions revealed interesting structure and dynamics of lithium ion-solvent complexation in linear and cyclic carbonate solvents. 101,[373][374][375][376][377] To extract quantitative information about the solvation structure and chemical exchange dynamics from the experimental results, it is necessary to numerically simulate the steady-state and timeresolved IR spectra using the vibrational frequency map of the carbonate CO stretching mode and carrying out numerical calculation of the vibrational Schrödinger equation. 110 Liang et al 375 employed the electric field vectors located at the carbonate group to estimate the timevarying frequency and transition dipole moment, which were then used to simulate the carbonyl stretch IR spectra and the time-resolved 2D IR spectra of various lithium salt/carbonate solutions.…”

Structure and dynamics in the lithium solvation shell of nonaqueous electrolytes

“…In addition to the average binding distance of the solvents, we consider the binding direction of EC and DMC with a Li + ion to fully understand the nature of the lithium solvation structure as a function of χ EC 36 . Specifically, we investigate the distribution P ( θ ) of a binding angle θ between a Li + ion and the carbonyl group of EC and DMC for various χ EC s. Here we consider an angle θ ≡ ∠Li + O c C, where O c =C is the carbonyl group of EC and DMC.…”

Operando Raman analysis of electrolyte changes in Li-ion batteries with hollow-core optical fibre sensors

“…[54][55][56] In the carbonyl groups being largely responsible for solvating the lithium ions. [57][58][59][60][61] The Raman bands in the spectral region observed at 1700−1850 cm -1 (Fig. 4) illustrate the evolution of cyclic and linear carbonate C=O stretches during the formation cycle (Fig.…”