Study of High and Low-Temperature Electrolytes for LiFePO4 Batteries
|School||Shanghai Jiaotong University|
|Course||Chemical Engineering and Technology|
|Keywords||LiFePO4 electrolyte lithium bis(trifluoromethylsufonyl)imide high-temperature performance low-temperature performance|
Olivine LiFePO4 is one of the most promising cathode materials for power battery, which has working voltage of about 3.4 V versus Li/Li+, high theoretical capacity （170 mAh/g） and high safety. In addition, the structure of electrode is stable during cycling. Electrolyte is one of the most important parts of battery, which plays a significant role in delivering of lithium ion and influencing the safety, cycling and rate performances, especially the high and low temperature performances. So it is very important to study electrolytes for LiFePO4 batteries. In this paper, galvanostatic charge-discharge, cyclic voltammetry （CV）, electrochemical impedance spectros-copy （EIS）, inductively coupled plasma （ICP） and differential scanning calorimeter （DSC） were used to investigate the high （low） temperature performances of cells with different lithium salts and solvents.First of all, the effects of lithium salts on the high-temperature performances of LiFePO4 electrode were investigated systematically and the related mechanism was revealed. Results show that lithium bis（trifluoromethylsulfonyl）imide （LiTFSI） is more favorable for the use at high temperature than LiPF6 because of its water insensitivities and excellent thermal stabilities. What’s more, it is found that ferrous ions will be dissolved hardly from LiFePO4 into LiTFSI electrolyte. A small amount of LiBF4 can inhibit the corrosion of aluminum foil current collector from LiTFSI. The electrolyte containing mixed salts of LiTFSI and LiBF4 significantly improves the high-temperature cycling performances of LiFePO4/Li cell and reduces the reduction of capacity. The discharge capacity is still 147.7 mAh/g after 40 cycles at 55℃and 1C rate.Secondly, the effects of electrolytes using different solvents on high and low-temperature performances of LiFePO4 batteries were investigated. Results show that, using quadribasic system, that is, GBL: EA: EC: EMC is 1:1:1:1, the low-temperature performance of LiFePO4/Li is excellent. The retention of capacity is 63% at -20℃and 42% at -40℃. What’s more, using mixed salts LiTFSI and LiBOB, which can not only increase the thermal stability, but also protect aluminum foil, can improve the high-T cycling performances. The capacity of cell retains above 94% after cycling 70 times at 55℃and 1 C. So using 0.9M LiTFSI+0.1M LiBOB/GBL+EA+EC+EMC （1:1:1:1） electrolyte, LiFePO4 cells can be used in a wider temperature.Finally, the performances of protective films of aluminum foils with different contents of LiBOB were investigated respectively. If the content of LiBOB is deficient, the protective films are not compact enough to protect Al foils. With the cycle numbers increasing, the Al foil will be corroded. However, superfluous LiBOB will reduce the conductivity of electrolyte and be ineffectual to form the protective film. So, optimized LiBOB can improve the combination property of batteries.