We report on the 7Li, 19F, and 1H diffusion of six organosilyl/LiPF6 electrolytes based on different organosilyl solvents. Three solvents are organosilyl esters, one is an organosilyl sulfone and two are organosilyl molecules with two lithium coordinating groups. One of the bifunctional solvents combines nitrile and sulfone moieties while the other solvent has both ester and sulfone functional groups in the solvent molecule. LiPF6 concentrations based on 5 moles of solvent per salt were used (0.8-1.1 M LiPF6 varying with the density of the solvent). In each of these six electrolytes we have measured lithium, anion (fluorine) and solvent (proton) diffusion over a temperature range from 300 K to 330 K using PFG-STE NMR techniques. From our cation and anion diffusion measurements we calculate cation transport and transference numbers to explore issues of ion pair formation in these electrolytes. We also assess the extent of salt dissociation, α, in these electrolytes by calculating the molar conductivities based on the diffusion of the cation and anion and compare these to the electrochemical impedance measurements of the molar ionic conductivities of these six electrolytes. Generally, the organosilyl ester/LiPF6 electrolytes have higher diffusion than those based on the other solvents largely due to the lower viscosity of the ester solvents. As typically observed in electrolytes, the solvent (1H) diffuses faster than the anion (19F) while the cation (7Li) is the slowest diffusing species. These higher concentration electrolytes have slower diffusion than the more dilute electrolytes reported by Sharpe and Lyons. Neutral ion pair formation is extensive in these electrolytes demonstrated by both low salt dissociation values and the transference numbers calculated. The organosilyl ester electrolytes have more ion aggregation than the organosilyl sulfone electrolytes due to the different dielectric constants of these solvents.

NMR diffusion studies of organosilyl ester and sulfone electrolytes for applications in lithium-ion batteries at higher salt concentration

ACS Spring Meeting 2021

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