Session: Innovative Chemistry and Materials for Electrochemical Energy Storage:

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. LiPF₆concentrations based on 10 moles of solvent per salt were used (0.4-0.6 M LiPF₆varying 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 lower concentration electrolytes have faster diffusion than the more concentrated electrolytes reported by Dixon-Anderson 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.

Steven Sharpe

Appearances