A new lithium salt with 3-fluoro-1,2-benzenediolato and oxalato complexes of boron for lithium battery electrolytes

A new unsymmetrical lithium salt, lithium [3-fluoro-1,2-benzenediolato(2-)-o,o′ oxalato]borate (FLBDOB), is synthesized and characterized. The thermal characteristics of FLBDOB and its counterparts lithium bis[3-fluoro-1,2-benzenediolato(2-)-o,o]borate (FLBBB) and lithium bis[oxalate]borate (LBOB) are examined and compared by thermogravimetric analysis (TG). The thermal decomposition of these salts in air is found to begin at 302, 262 and 256 °C for LBOB, FLBDOB and FLBBB, respectively. The order of the stability toward oxidation of these organoborates is LBOB > FLBDOB > FLBBB, which is in the same order of the thermal stability. The cyclic voltammetry study shows that the FLBDOB solution in PC is stable up to 4.0 V versus Li⁺/Li. It is soluble in common organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), 1,2-dimethoxyethane (DME), and tetrahydrofuran (THF). Ionic dissociation properties of FLBDOB and the counterparts in PC, PC + THF, PC + DME, EC + DME, EC + THF (molar ratio 1:1) solutions are also examined by conductivity measurements. The conductivity values of the 0.10 mol dm⁻³ FLBDOB electrolyte in PC, PC + THF, PC + DME, EC + DME, EC + THF solutions are higher than those of FLBBB, but lower than those of LBOB electrolytes.