The temperature dependent self-diffusion coefficients of lithium cations (DLi), fluorine anions (DTFSI) and solvent molecules (Dproton) were measured by 7Li+, 19F- and 1H pulsed-field gradient (PFG) NMR spectroscopy for six electrolytes, using three silyl solvents doped with the lithium salt LiTFSI at two different salt concentrations. The silyl solvents, prepared and patented by Silatronix® (Madison, WI), vary in the degree of fluorination of the solvent. The effects of salt concentration, solvent composition, and temperature on the self-diffusion coefficients, transference numbers, molar conductivities, and α values are discussed. Diffusion is faster for both ions in the dilute electrolytes compared to the concentrated salt electrolytes for all temperatures. Lithium transference numbers ranged from 0.42 to 0.52 at 295 K. These values are considerably higher than conventional ethylene carbonate electrolytes and quite promising for lithium ion battery applications. Using the diffusion measurements from 295 to 330 K, the molar conductivities for the electrolytes were calculated with the Nernst-Einstein equation and then compared to electrochemical impedance measurements of ionic conductivity to calculate the degree of salt dissociation, α. As fluorination of the samples increased, α values at 300 K increased from 0.20 to 0.37 in the dilute samples, and from 0.18 to 0.31 in the concentrated samples.

NMR diffusion studies of LiTFSI/organosilyl electrolytes

255th National Meeting (2018)

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