Study Of Low Temperature Electrolyte For Lithium Ion Battery

As one of the core materials of lithium ion battery, electrolyte has an extreme impact on capacity and temperature performance. In this thesis, effects of the electrolyte respectively composited of three lithium salts (four fluorine boric acid lithium (LiBF4), lithium hexafluorophate (LiPF6), and2-three fluorine sulfonylurea imide lithium (LiTFSI)), and cosolvent butyric acid ethyl ester (EB) on charge-discharge capacities of graphite and lithium iron phosphate half cells (AG/Li, LiFePO4/Li) are investigated. The reasons for the poor low temperature performance are studied by a series of electrochemical measurements. Conclusions as follows:(1) At25℃, discharge capacities of AG/Li half cells with the three lithium salts are all330mAh/g. At0℃, the capacity of the AG/Li half cells with LiBF4, LiPF6and LiTFSI is57.7%,24.4%,52.7%of the room temperature one, respectively. At-20℃, the discharge capacity decreases to20%. At-40v, the cells cannot charge-discharge. At0℃18w%EB has a17.9%increase in capacity. At-20℃and-40℃, the capacity changes faintly when EB concentration changes. Results from electrochemical impedance spectroscopy (EIS) indicate that the resistances of the solid electrolyte interface increase when temperature decreases. This may induce the deposition of Li+at the interface, and make the insertion of lithium ion more difficult, resulting in increases of the charge transfer resistance and polarization, and decreases of low temperature capacities of AG/Li half cells.(2) The capacities of LiFePO4/Li half cells with the three lithium salts almost equal to145mAh/g at25℃. At0℃,-20℃,-40℃, the discharging capacity of the cells is85%,60%, and35%of the room temperature one, respectively. The discharge capacity at room temperature is not changed by EB. However, the polarization increases as EB increases at low temperature. Results of EIS show that the Li+conductivity reduces, the charge transfer resistance increases, and the polarization increases when the temperature decreases. These may are the reasons for the poor capacity performance of LiFePO4/Li half cells at low temperature.