Thermally durable electrolyte for lithium-ion battery

Abstract

For ensuring the safety of lithium-ion batteries (LIBs), we extensively investigated a composite electrolyte composed of a less volatile Li conductive electrolyte solution, silica (SiO2) particles, and fluorine-based binder as a thermally durable electrolyte. In this article, we review the roles of the electrolyte constituents. For the less volatile solution, we applied an equimolar complex of lithium bis(trifluoromethanesulfonyl)amide (LiTFSI) and tetraethylene glycol dimethyl ether (G4), Li(G4)TFSI, which was diluted with a low-viscosity solvent (propylene carbonate, PC) to enhance the ionic conductivity. The resultant electrolyte exhibited a favorable volatilization temperature higher than 373 K, and was applied to a 100-Wh-class laminated LIB. The cell generated neither fire nor smoke in a nail-penetration test, where the composite of SiO2 and fluorine-based binder had evidently covered the electrodes at the short-circuit region. This insulation phenomena effectively suppressed the joule heat generation and thereby kept the battery safe. One of the challenges with Li(G4)TFSI-based electrolytes is how to improve the durability during the charge and discharge cycles. The continuous decompositions of G4 and PC were a major reason for the growth of a solid-electrolyte interphase, which caused the capacity fade during charge-discharge cycles. We therefore replaced the G4-based liquid with a highly concentrated sulfolane-based liquid featuring a high Li+ transport number and a lower-viscosity solvent and found that it demonstrated a higher capacity retention after a charge-discharge cycle test.