4182656 - Variable temperature diffusion in carbonate and cosolvent blend electrolytes for lithium and lithium-ion batteries | Poster Board #437

Seeking better low temperature electrolyte performance, we report on multinuclear pulsed field gradient (PFG) NMR experiments over the temperature range of -20 °C to 40 °C. We also have used 3 different cosolvents, methyl propionate (an ester), a fluorinated dioxolane, and a hydrofluoroether (HFE), in an effort to improve lithium ion diffusion over this temperature range. The base electrolyte was 1 M lithium hexafluorophosphate in a solvent blend of equal proportions of ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate with 20% of the cosolvent. Orbia Fluor & Energy Materials (Madison, WI) provided the electrolytes reported here. Multinuclear ¹H, ⁷Li, and ¹⁹F pulsed field gradient NMR experiments over this temperature range measure the slowest diffusion for the lithium cation and increasing diffusion coefficients, respectively, for the anion, the ethylene carbonate protons, the diethyl carbonate protons and the fastest diffusion coefficient for the ethyl methyl carbonate solvent component in the control electrolyte. In the cosolvent electrolytes the diffusion coefficients for the ester and fluorodioxolane are consistently larger than the carbonate protons while the hydrofluoroether diffuses at the same rate as the ethylene carbonate protons. Lithium ion diffusion is fastest in the electrolyte with the ester cosolvent and much faster than the electrolyte with the HFE cosolvent, especially at the lowest temperature of -20 °C. These multinuclear NMR results show better diffusion performance for the electrolyte with the ester cosolvent in comparison to the carbonate blend electrolyte (control). We will also report on the salt dissociation values, lithium transport numbers, and cycling performance in lithium-ion batteries for these electrolytes. Overall, the increased diffusion characteristics of carbonate blend electrolytes with the ester or fluorodioxolane cosolvents enhance electrolyte performance, and blends with these solvent additives may optimize lithium metal and lithium-ion battery results, especially at low temperatures.