Lithium oxalyldifluoroborate/carbonate electrolytes for LiFePO4/artificial graphite lithium-ion cells

The electrolytes based on lithium oxalyldifluoroborate (LiODFB) and carbonates have been systematically investigated for LiFePO4/artificial graphite (AG) cells, by ionic conductivity test and various electrochemical tests, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge test. The conductivity of nine electrolytes as a function of solvent composition and LiODFB salt concentration has been studied. The coulombic efficiency of LiFePO4/Li and AG/Li half cells with these electrolytes have also been compared. The results show that 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte has a relatively higher conductivity (8.25 mS cm(-1)) at 25 degrees C. with high coulombic efficiency, good kinetics characteristics and low interface resistance. With 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte, LiFePO4/AG cells exhibit excellent capacity retention similar to 92% and similar to 88% after 100 cycles at 25 degrees C and at elevated temperatures up to 65 degrees C, respectively; The LiFePO4/AC cells also have good rate capability, the discharge capacity is 324.8 mAh at 4 C, which is about 89% of the discharge capacity at 0.5 C. However, at - 10 degrees C, the capacity is relatively lower. Compared with 1 M LiPF6 EC/PC/DMC (1:1:3, v/v), LiFePO4/AG cells with 1 M LiODFB EC/PC/DMC (1:1:3, v/v) exhibited better capacity utilization at both room temperature and 65 degrees C. The capacity retention of the cells with LiODFB-based electrolyte was much higher than that of LiPF6-based electrolyte at 65 degrees C. while the capacity retention and the rate capacity of the cells is closed to that of LiPF6-based electrolyte at 25 degrees C. In summary, 1 M LiODFB EC/PC/DMC (1:1:3, v/v) is a promising electrolyte for LiFePO4/AG cells. (C) 2010 Elsevier B.V. All rights reserved.