Low-temperature performance of Li-ion cells with a LiBF4-based electrolyte

The effect of LiBF4 on the low-temperature performance of a Li-ion cell was studied by using a 1:1:1 (wt) EC/DMC/DEC mixed solvent. The results show that the LiBF4-based electrolyte has a 2- to 3-fold lower ionic conductivity and shows rather higher freezing temperature compared with a LiPF6-based electrolyte. Owing to electrolyte freezing, cycling performance of the Li-ion cell using LiBF4 was significantly decreased when the temperature fell below -20 degreesC. However, impedance data show that at -20 degreesC the LiBF4 cell has lower charge-transfer resistance than the LiPF6 cell. In spite of the relatively lower conductivity of the LiBF4-based electrolyte, the cell based on it shows slightly lower polarization and higher capacity in the liquid temperature range (above -20 degreesC) of the electrolyte. This fact reveals that ionic conductivity of the electrolytes is not a limitation to the low-temperature performance of the Li-ion cell. Therefore, LiBF4 may be a good salt for the low-temperature electrolyte of a Li-ion cell if a solvent system that is of low freezing temperature, high solubility to LiBF4, and good compatibility with a graphite anode can be formulated.