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

A new unsymmetrical lithium salt containing C2O42- [dianion of oxalic acid], C6O2F42- [dianion of tetrafluoro-1,2-benzenediol], lithium [tetrafluoro-1,2-benzenediolato(2-)-o,o' oxalato]borate (4FLBDOB) is synthesized and characterized. The thermal characteristics of it, and its derivatives, lithium [3-fluoro-1,2-benzenediolato(2-)-o,o' oxalato]borate (FLBDOB), lithium [1,2-benzenediolato(2-)-O,O' oxalato]borate (LBDOB), and lithium bis[oxalate]borate (LBOB) are examined by thermogravimetric analysis (TG). The thermal decomposition in air begins at 302 degrees C, 272 degrees C, 262 degrees C and 256 degrees C for LBOB, 4FLBDOB, FLBDOB and LBDOB respectively. The order of the stability toward oxidation of these organoborates is LBOB > 4FLBDOB > FLBDOB > LBDOB, which is in the same order of the thermal stability. The cyclic voltammetry study shows that the 4FLBDOB solution in PC is stable up to 4.0 V versus Li+/Li. They are soluble in common organic solvents. Ionic dissociation properties of 4FLBDOB and its derivatives are examined by conductivity measurements in PC + THF, PC + DME, EC + THF, and EC + DME (molar ratio 1:1) solutions. Among the series of LBDOB, FLBDOB and 4FLBDOB, the new lithium salt (4FLBDOB) has the best battery performances. (C) 2013 Elsevier B.V. All rights reserved.