Formulation of a New Type of Electrolytes for LiNi<sub>1/3</sub>Co<sub>1/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> Cathodes Working in an Ultra-Low Temperature Range

A new type of electrolytes for low temperature operation of Li-ion batteries was formulated in this work. Instead of LiPF 6 , LiBF 4 and LiODFB were used to form this new type of electrolytes, although LiPF 6 is the mostly chosen solute in the state-of-the-art Li-ion electrolytes. It was found although a LiBF 4 -based electrolyte had a lower ionic conductivity than that of a LiODFB-based electrolyte, a LiODFB-based electrolyte demonstrated improved low temperature performance. In particular, at-30°C, a Li-ion cell with 1M LiODFB dissolved in a 1:2:5 (wt.) propylene carbonate (PC)/ethylene carbonate (EC)/ethyl methyl carbonate (EMC) mixed solvent delivered 86% of the capacity obtained at 20°C. Furthermore, the cells with a LiODFB-based electrolyte showed lower polarization at-30°C. The above results suggest that beside the ionic conductivity of an electrolyte as a limitation to the low temperature operation of Li-ion batteries, there was interface impedance having effect on it. Analysis of cell impedance revealed that reduced charge-transfer resistance by using LiODFB resulted in improved low temperature performance of Li-ion batteries.