Enhanced lithium-ion transport in organosilyl electrolytes for lithium-ion battery applications
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esearch Letter
Enhanced lithium-ion transport in organosilyl electrolytes for lithium-ion battery applications Leslie J. Lyons, Department of Chemistry, Grinnell College, Grinnell, IA 50112, USA Scott Beecher, 1252 University of Oregon, Eugene, OR 97403-1252, USA Evan Cunningham, Tom Derrah, Shengyi Su, and Junmian Zhu, Department of Chemistry, Grinnell College, Grinnell, IA 50112, USA Monica Usrey, Adrián Peña-Hueso, Tobias Johnson, and Robert West, Silatronix Inc., 3587 Anderson Street, Suite 108, Madison, WI 53706, USA Address all correspondence to Leslie J. Lyons at [email protected] (Received 18 June 2019; accepted 14 August 2019)
Abstract The authors report on 7Li, 19F, and 1H pulsed field gradient NMR measurements of 26 organosilyl nitrile solvent-based electrolytes of either lithium bis(trifluorosulfonyl)imide (LiTFSI) or lithium hexafluorophosphate. Lithium transport numbers (as high as 0.50) were measured and are highest in the LiTFSI electrolytes. The authors also report on solvent blend electrolytes of fluoroorganosilyl (FOS) nitrile solvent mixed with ethylene carbonate (EC) and diethyl carbonate. Solvent diffusion measurements on an electrolyte with 6% FOS suggest both the FOS and EC solvate the lithium cation. By comparing lithium transport and transference numbers, the authors find less ion pairing in FOS nitrile carbonate blend electrolytes and difluoroorganosilyl nitrile electrolytes.
Introduction Lithium-ion batteries have abundant applications in consumer electronic and electric vehicles because of their high specific capacity and energy density.[1] The electrolyte is essential for lithium-ion batteries since it functions as the medium for the lithium cation’s movement between electrodes during charging and discharging processes. Lithium hexafluorophosphate, LiPF6, is used as the salt in many commercially available lithium-ion batteries due to its well-balanced properties. However, LiPF6 has safety problems due to its chemical dissociation equilibrium producing LiF (less charge transport) and PF5 (a reactive safety risk) which further lead to its chemical and thermal instability.[2] The traditional carbonate solvents, with high vapor pressures, used in lithium-ion batteries are flammable and potentially toxic.[3] In particular, linear carbonates have a low flash point; for example, diethyl carbonate’s (DECs) flash point is 25 °C.[4] We seek to replace the linear carbonates in electrolyte formulations with organosilyl nitrile (OSN) solvents. Organosilyl compounds generally have lower vapor pressures, higher flash points, and tolerate higher operating voltages than carbonates.[5] The nitrile functional group is able to solvate the lithium cation, while the organosilyl moieties prevent thermal decomposition.[6,7] Nitrile-based electrolytes have been reported recently by several groups. Ma and Mandal[8] have reported on lithium bis(trifluorosulfonyl)imide (LiTFSI)-based electrolytes in solvents of etheric nitriles, while Xie et al.[9] have used the LiPF6 salt in etheric nitrile electrolytes. The dinitrile adip
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