Lithium tetrakis(haloacyloxy)borate: An easily soluble and electrochemically stable electrolyte for lithium batteries

Synthesis and properties of lithium tetrakis(haloacyloxy)borate, Li[B(OCORX)(4)], in which X represents a halogen atom, are described for use as an electrolyte in lithium batteries. In this series of compounds, the anions are of non-chelate type, and X (Cl or F) is introduced as an electron-withdrawing substituent. These salts were easily synthesized from boric acid, lithium halocarboxylate, and halocarboxylic anhydride. The Li[B(OCORX)(4)] salts were easily dissolved in a linear carbonate such as dimethyl carbonate (DMC) and diethyl carbonate (DEC) to give concentrations greater than 1 mol dm(-3). These solutions were highly conductive and stable against oxidation up to 4.5 V vs. the Li/Li+ reference. Lithium tetrakis(trifluoroacetoxy)borate (LiTFAB), Li[B(OCOCF3)(4)], which was found to exhibit the highest conductivity among these salts, did not cause aluminum to corrode. A Li/graphite cell containing 1.0 mol dm(-3) LiTFAB in an ethylene carbonate/ethylmethyl carbonate (3/7 vol/vol) mixed solvent was found to give the highest cycling efficiency. (C) 2003 The Electrochemical Society.