Enhancing ionic conductivity with fluorination in organosilyl solvents for lithium-ion battery electrolytes
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Research Letter
Enhancing ionic conductivity with fluorination in organosilyl solvents for lithium-ion battery electrolytes Leslie J. Lyons, Tom Derrah, Steven Sharpe, Seiyoung Yoon, and Scott Beecher, 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 13 September 2019)
Abstract Increasing fluorination of organosilyl nitrile solvents improves ionic conductivities of lithium salt electrolytes, resulting from higher values of salt dissociation. Ionic conductivities at 298 K range from 1.5 to 3.2 mS/cm for LiPF6 salt concentrations at 0.6 or 0.7 M. The authors also report on solvent blend electrolytes where the fluoroorganosilyl (FOS) nitrile solvent is mixed with ethylene carbonate and diethyl carbonate. Ionic conductivities of the FOS solvent/carbonate blend electrolytes increase achieving ionic conductivities at 298 K of 5.5–6.3 mS/cm and salt dissociation values ranging from 0.42 to 0.45. Salt dissociation generally decreases with increasing temperature.
Introduction Lithium-ion batteries (LIBs) have numerous consumer electronic devices and electric vehicle applications due to their high energy density. This high energy density results from LIB operating voltages of 3 V or higher and lithium’s low mass.[1] The electrolyte in a LIB is a key component since it provides the pathway for the lithium cation’s transport between the anode and the cathode during battery operation. While numerous lithium salts can be used in the electrolyte, lithium hexafluorphosphate, LiPF6, is the conventional salt due to its range of suitable properties. However, LiPF6 has safety problems due to its chemical dissociation equilibrium which produces LiF (lowering the charge transport) and PF5 (a safety risk due to its chemical reactivity with water) leading to its thermal and chemical instability.[2] The traditional carbonate solvents used as the liquid component of the electrolyte in LIBs are flammable, have high vapor pressures, and and are potentially toxic.[3] In particular, linear carbonates, a frequent co-solvent, have a low flash point; one common linear carbonate diethyl carbonate’s (DEC’s) flash point is 25 °C.[4] Organosilyl nitrile (OSN) solvents are attractive alternatives to the linear carbonates in electrolyte formulations for LIBs. In comparison to carbonates, organosilyl compounds generally have lower vapor pressures, higher flash points, and tolerate higher operating voltages.[5] The nitrile functional group acts as the polar moiety in the molecule and solvates the lithium cation, while the organosilyl fragment prevents thermal decomposition.[6,7] There have been a number of reports recently on nitrile-based electrolytes for LIB applications. Ma and Mandal[8] have reported on etheric nitriles/LiTFSI-based
electrolytes, while Xie et al.[9] have used the more conventional LiPF6
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