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 9.54 x 10(-3) S/cm in DME and 4.79 x 10(-3) S/cm 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 Li/LiMn2O4 and Li/LiNi0.8Co0.2O2 cells. (C) 2002 The Electrochemical Society.