Properties of trifluoromethylated lithium borates for lithium-ion battery electrolytes

Abstract

Lithium salts govern important characteristics of lithium-ion batteries, including their efficiency (ion conductivity), operating voltage (potential window), and thermal stability. Herein, a series of lithium borates (lithium difluoro(perfluoropinacolato)borate (PFP-F2), lithium difluoro(2-hydroxy-3,3,3,3ʹ,3ʹ,3ʹ-hexafluoroisobutirato)borate (HHIB-F2), lithium (perfluoropinacolato)(oxalato)borate (PFP-Ox), lithium bis(2-hydroxy-3,3,3,3ʹ,3ʹ,3ʹ-hexafluoroisobutirato)borate (HHIB2), and lithium (2-hydroxy-3,3,3,3ʹ,3ʹ,3ʹ-hexafluoroisobutirato)(oxalato)borate) containing CF3 and C=O groups were developed as potential electrolytes for lithium-ion batteries. The proposed lithium borates were synthesized in good purity from lithium tetrafluoroborate and lithium difluoro(oxalato)borate using trimethylchlorosilane and three types of bidentate ligands in ethylmethylcarbonate. The applicability of the novel lithium borates as electrolytes for lithium-ion batteries was demonstrated based on thermal and electrochemical stability evaluations. In addition, lithium borates with >4 CF3 groups (PFP-F2, PFP-Ox, and HHIB2) exhibited outstanding stability against hydrolysis (water contamination). HHIB-F2 showed the best ionic conductivity owing to the balanced incorporation of increased Li+ dissociation and mobility by introducing the CF3 group and reducing the anion size. HHIB-F2 and HHIB2-containing electrolytes showed better cycle performance than their conventional BF4 counterparts. This study suggests that the new lithium borates, HHIB2, and HHIB-F2, are promising lithium salts for lithium-ion batteries, providing a new direction for the lithium salt molecular design.