LI-ION BATTERY ELECTROLYTE FORMULATED FOR LOW-TEMPERATURE APPLICATIONS

Low-temperature (<0 degrees C) applications of Li-ion batteries have p rompted the search for improved, high-conductivity electrolytes. Becau se the performance of the carbonaceous anode is highly sensitive to ch anges in electrolyte composition, we focused our efforts on this elect rode. Electrolytes containing LiAsF5, LiPF6, LiN(SO2CF3)(2) [lithium b is(trifluoromethanesulfonyl)imide] , or LiIm, and LiC(SO2CF3)(3) [lith ium tris(trifluoromethanesulfonyl)methide], or LiMe, in methyl formate (MF)-ethylene carbonate (EC) solvent mixtures were tested in lithium- graphite half-cells. The graphite electrodes could be cycled at ambien t temperature with high reversible capacity. The best supporting elect rolyte was found to be LiAsF6, and the presence of a high concentratio n of ethylene carbonate and up to 300 ppm H2O in the solution consider ably increased the reversible capacity upon cycling. The conductivity values of a binary solvent mixture of methyl formate and ethylene carb onate containing LiAsF6 or LiMe were measured between -40 degrees C an d room temperature. Graphite electrodes cycled at -2 degrees C in thes e electrolytes obtained reasonable reversible capacity, approaching 50 %.