Improve The High Temperature Performance Of Lithium-Ion Battery Based On Novel Electrolytes

With the development of the economy and society, much more attention has been paid to the utilization of the new energy for the shortage of traditional energy. Due to its high energy density, high voltage and long cycle life, lithium-ion battery becomes the most popular secondary battery since it was firstly introduced in 1990. After many years'development, lithium-ion batteries have been widely used as power source of cell phones, laptops and digital cameras, as well as other potable electric devices. However, lithium-ion battery still cannot be used as the power source for electric vehicles (EV) or plug-in hybrid electric vehicles (PHEV) because of safety concerns, short cycle life and high cost. For example, the battery can only work less than 5 years at room temperature, which is much shorter for the requirements of electric vehicles, 10 to 15 years; the battery will cause safety problems such as explosion under abused conditions; and the battery decays quickly at elevated temperatures (≥50oC). These problems are closely related to the electrolyte used in the battery. In this paper, we try to use electrolyte additives and develop a new polymer electrolyte to solve the existing high temperature cycle stability problems of the lithium ion battery.Tris(trimethylsilyl) borate (TMSB) was used as a new electrolyte additive to improve high temperature performance of LiFePO4 based lithium-ion battery. The effects of the TMSB on the LiFePO4 electrode are investigated via a combination of electrochemical impedance spectroscopy (EIS), cyclability, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). It is found that the LiFePO4 battery with a composite LiPF6-based electrolyte containing 1 wt% TMSB additive exhibits higher discharge retention and better cycling performance than the battery without TMSB additive at both 30°C and 55°C. SEM and XPS measurements show the changes of surface morphology and formation of solid electrolyte interface (SEI). EIS results indicate that the interfacial impedances of the batteries after cycled at 55°C with the electrolyte containing TMSB additive are significantly smaller than the batteries without additive. The improved performances are ascribed to the enhancement of the thermal stability of the electrolyte and the modification of SEI component on the LiFePO4 electrode. New polymer electrolyte based on lithiated perfluorinated sulfonic ionomer with high ion exchange capacity for lithium-ion battery is investigated.Through polymerization, solution-casting, salification and dehydration/swelling processes, lithiated perfluorinated sulfonic polymer electrolyte swelled with PC (PC-PFSA-Li) is prepared. The PC-PFSA-Li polymer electrolyte shows ionic conductivity of 4.63×10-4 S/cm at room temperature and 1.03×10-3 S/cm at 60 oC, respectively. Cyclic voltammetry indicates that the PC-PFSA-Li polymer electrolyte possessed good electrochemical stability in the range 2.5-4.5 V (vs. Li/Li+). EIS shows that PC-PFSA-Li polymer electrolyte is compatible with lithium foil. The LiFePO4/PC-PFSA-Li/Li batteries show higher capacity retention at 80 oC after 100 cycles than that of the battery using the LiPF6-EC-DMC conventional liquid electrolyte. The replacement of conventional liquid electrolytes and the use of polymer electrolytes with the ion-exchange membranes not only gives an extra contribution in improving the performance and safety of the lithium-ion battery, but also reduces the cost of the battery system. Therefore, this novel lithium-ion polymer battery would be used as new, advanced types of power sources for hybrid and electric vehicles.