Effect of lithium bis (trifluoromethylsulfonyl) imide on the hightemperature behavior of LiFePO₄ positive electrode

The effect of lithium salts in electrolytes on the high-temperature performance of LiFePO₄ electrode is investigated systematically and the related mechanism is revealed. Differential scanning calorimetry measurements show that lithium bis (trifluoromethylsulfonyl) imide (LiTFSI) and LiBF₄ are more favorable for the use at high temperature than LiPF6 because of their water insensitivities and excellent thermal stabilities. Inductively coupled plasma is used to examine the dissolving degree of ferrous ions from LiFePO₄ in the different electrolytes at 55°C. It is found that little ferrous ions can be dissolved from LiFePO₄ in LiTFSI and fluorine-free lithium salt electrolytes. On the contrary, it is quite serious in LiPF6 electrolyte. In addition, FePO₄ is easier to release iron ions than LiFePO₄ in the LiPF6 electrolyte. The experimental results of cyclic voltammetry and light microscope indicate that 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 performance of LiFePO₄/Li cell. The discharge capacity is still 147.7 mAh/g after 40 cycles at 55°C and 1 C rate.