Research and Development of Thermally Durable Electrolyte for Lithium Ion Battery

Abstract

For ensuring safety of lithium ion batteries (LIBs), we have extensively investigated the quasi-solid electrolyte where lithium ion conducive liquid is quasi-solidified at silica surfaces as thermally durable electrolyte, and applied it to high capacity and high energy density LIB. For the liquid phase, a solvate ionic liquid, which is an equimolar complex of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraethylene glycol dimethyl ether (G4), Li(G4)TFSA, was used. For enhancing discharge capability at a higher rate, Li(G4)TFSA was diluted by low viscos solvent such as propylene carbonate (PC). The developed electrolyte possessed a favorable volatilization temperature higher than 373 K. A 100-Wh-class laminated LIB with energy density of 363 Wh L−1 was fabricated by employing the electrolyte to graphite-LiNixCoyMnzO2 chemistry, and it generated neither fire nor smoke in a nail-penetration test. The result suggest that the developed LIB has high safety compared to a LIB comprised of a conventional organic liquid electrolyte. In addition, to enhance the cycle life of the LIB, the formation and growth mechanism of a solid-electrolyte interphase on a graphite-based negative electrode was investigated. Nuclear magnetic resonance and hard x-ray photoelectron spectroscopy revealed that the decompositions of LiTFSA, PC, and G4 contributed to the SEI formation at the initial charge, and that continuous decompositions of G4 and PC were a major reason for the SEI growth during charge-discharge cycles. Based on these analysis, we have substituted a highly concentrated sulfolane based liquid which exhibits a high Li ion conductivity with less amount of the low viscos solvent, for the G4 based liquid. The modification effectively improved the electrochemical durability of the electrolyte, leading to a higher capacity retention after charge-discharge cycle test.