Understanding the Solvation Structure of Li-Ion Battery Electrolytes Using DFT-Based Computation and ¹H NMR Spectroscopy

Abstract

Molecular dynamics (MD) simulations, density functional theory (DFT) calculations, and 1H NMR spectroscopy were performed to gain a complementary understanding of the concentrated Li-ion electrolyte system, lithium bis(trifluoro­methane­sulfonyl)imide (Li[TFSI]) dissolved in tetraglyme. The computational methods provided the concentration dependence of differing solvation structure motifs by reference to changes in the corresponding NMR spectra. By combining both the computational and experimental methodologies, we show that the various solvation structures, dominated by the coordination between the tetraglyme (G4) solvent and lithium cation, directly influence the chemical shift separation of resonances in the 1H NMR spectra of the solvent. Thus, the 1H NMR spectra can be used to predict the fraction of tetraglyme involved in the solvation process, with quantitative agreement with solvation fraction predictions from MD simulation snapshots. Overall, our results demonstrate the reliability of a hybrid computational and experimental methodology to understand the solvation structure and hence transport mechanism of LiTFSI-G4 electrolytes in the low concentration region.

Infrared Spectroscopy of Li+ Solvation in Diglyme: Ab Initio Molecular Dynamics and Experiment

“…This indicates that the lifetime of bG is longer than the ∼120 ps duration of the individual AIMD trajectories, an observation consistent with many earlier classical MD studies, where the residence times of Li + in glymes and PEOs were found to be in the range of 1−1000 ns. [27][28][29][30][31]33,34,38 These long lifetimes allow us to treat bG and uG as two different species and compute their autocorrelations and cross-correlations unambiguously within the simulation time scale. In addition, the fact that bG structures are relatively rigid enables analysis of its GNMs.…”

“…23−38 Li + solvation structure as well as transport and related dynamics, which require prolonged simulations, were studied mainly using classical MD. 24,25,[27][28][29][30][31][32][33][34][35]38 Ab initio MD (AIMD) was also employed in recent study of Li + solvation structure in glymes 36,37 and in the analysis of Li + transport along a single PEO chain. 26 Various properties of Li + solvates, such as coordination structure, glyme backbone conformations, and normal mode vibrations at local energy minima, have also been analyzed with ab initio methods.…”

“…In parallel, a great deal of computational effort, primarily via molecular dynamics (MD) simulations, has been directed toward a detailed understanding of Li + –PEO and Li + –glyme mixtures. − Li + solvation structure as well as transport and related dynamics, which require prolonged simulations, were studied mainly using classical MD. ,,− , Ab initio MD (AIMD) was also employed in recent study of Li + solvation structure in glymes , and in the analysis of Li + transport along a single PEO chain . Various properties of Li + solvates, such as coordination structure, glyme backbone conformations, and normal mode vibrations at local energy minima, have also been analyzed with ab initio methods. ,− However, the vibrational and spectroscopic properties of Li + –PEO and Li + –glyme mixtures have not been simulated previously via MD.…”

“…In parallel, a great deal of computational effort, primarily via molecular dynamics (MD) simulations, has been directed toward a detailed understanding of Li + –PEO and Li + –glyme mixtures. 23 − 38 Li + solvation structure as well as transport and related dynamics, which require prolonged simulations, were studied mainly using classical MD. 24 , 25 , 27 − 35 , 38 Ab initio MD (AIMD) was also employed in recent study of Li + solvation structure in glymes 36 , 37 and in the analysis of Li + transport along a single PEO chain.…”