Computational and Spectroscopic Analysis of Ion Interactions in Sodium and Lithium Ion Battery Electrolytes

Rising interests in sodium ion batteries as low-cost alternatives to lithium ion batteries necessitates a closer look at the interactions between the sodium ion and its anion in electrolyte solution. As demonstrated in lithium ion batteries, solvent and anion species involved in solvation of the lithium ion contribute to the nucleation and growth of the solid electrolyte interphase of the anode. The same is true of sodium ion batteries, which parallel lithium ion batteries in their use of carbonate-based organic solvents and the hexafluorophosphate (PF 6 - ) anion. Infrared spectroscopy (FTIR) was used to probe the interaction between PF 6 - and sodium in various carbonate-based solvent, and these measurements were interpreted using predictions made by density functional theory (DFT) calculations on the model LiPF 6 and NaPF 6 -based solvates. In addition, molecular dynamics (MD) simulations were performed on the EC:DMC/LiPF 6 and EC:DMC/NaPF 6 electrolytes. A revised many-body polarizable APPLE&P force field was used in MD simulations. Solvate structures extracted from MD simulations were found in good agreement with interpretations of FTIR spectra. Specifically, initial MD results supported our previous finding indicating that the Li + and Na + solvation numbers from Raman measurements are found are similar in linear carbonates. Moreover, MD simulations predicted that the PF 6 - anion coordination to Li + and Na + are quite different. Mono-dentate LiPF 6 binding prevailing in LiPF 6 -DMC, and a combination of bi- and tri-dentate binding dominating in NaPF 6 -DMC in agreement with the free energies of solvates estimated from DFT cluster – continuum calculations. MD simulations also predicted solvent, anion, Li and Na self-diffusion coefficients in excellent agreement with pfg-NMR measurements.