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 I 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(OCOCF 3 )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 LiTFAB in an ethylene carbonate/ethylmethyl carbonate (3/7 vol/vol) mixed solvent was found to give the highest cycling efficiency.