Synthesis and Study of New Cyclic Boronate Additives for Lithium Battery Electrolytes

Two novel boronate compounds, 2-(pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole ( 1 ) and 2-(pentafluorophenyl)-4,4,5,5-tetrakis(trifluoromethyl)-1,3,2-dioxaborolane ( 2 ), have been synthesized as additives for lithium battery electrolytes. These cyclic boronate compounds have a much more significant effect on conductivity enhancement of LiF salt in dimethoxyethene (DME) or ethyl carbonate-dimethyl carbonate (EC-DMC) than either borane or borate additives we previously synthesized. The conductivity of a composite electrolyte containing compound 1 and LiF has reached in DME and in EC-DMC (1:2). This is due to the lower molecular weight and less steric hindrance effects of compound 1 . In the case of compound 2 , the enhanced performance also comes from the improved solubility in polar solvents. Composite electrolytes containing LiF and either compound 1 or compound 2 have excellent electrochemical stability in the EC-DMC solvent, with respective electrochemical windows of 4.05 and 5.1 V. The composite electrolyte containing LiF and compound 2 shows high cycling efficiency and cyclability in both and cells. © 2002 The Electrochemical Society. All rights reserved.