|dc.description.abstract||Both safety and performance (including power and energy density) are key factors for next-generation Lithium-ion batteries (LIBs), which are considered today the best technology for energy storage. Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI)-based ionic liquids (ILs) with Li salts, namely LiTFSI, LiPF6, show high thermal stability (>400 oC), nonflammability and more stable at high operating voltage, thus is ideal for high-safety and high voltage applications. Compared to conventional organic electrolytes, high viscosity and small Li+ mobility of IL electrolytes make them unfavorable for high-C-rate applications. To mitigate these issues, the introduction of additive in IL electrolytes and increasing temperature can significantly improve the capacity, high-rate performance and cyclability of the cells. However, the effects of Li salts and additives on the electrochemical properties of IL-based LIB are not thoroughly investigated. As a consequence, in this study, different types and concentrations of Li salts (including mixtures of Li salts) and additives were used to optimize the cell performance at various temperatures.
The results showed that LiTFSI is more suitable than LiPF6 in the BMP-TFSI IL electrolyte. The maximum capacity was found to be 115 mAh g-1 (at 0.1 C) for LiFePO4 in the IL with 0.5 M LiTFSI at 25 °C. At 50 oC, 1 M LiTFSI-doped IL electrolyte shows highest capacity of 140 mAh g-1 at 0.1 C. 45% of the capacity can be retained at 5 C No obvious capacity loss was observed after 100 charge-discharge cycles, as opposed to those at 25 °C, which are 18% rate capability at 0.5 C and 77% cyclic stability
Furthermore, three kinds of additives (vinylene carbonate (VC), gamma-butyrolactone (?-BL) and propylene carbonate (PC)) were investigated. All the additives can significantly improve the capacity, high-rate performance, and cyclability of the cells at 25 °C. In particular, γ-BL was found to be the most effective. In contrast to the aforementioned studies, the addition of additives had a negative effect at elevated temperatures. At 75 °C, the plain IL electrolyte showed a capacity of 152 mAh g-1 at 0.1 C. 77% of this capacity can be retained at 3 C and negligible capacity loss is measured after 100 charge–discharge cycles. These values are superior then the additive-incorporated IL and conventional organic electrolytes.
It is well-known that the organic solvent and 1 M LiTFSI in BMP-TFSI IL have substrate corroded at high voltage. The introduction of LiPF6 in the IL can effectively suppress Al pitting corrosion and thus improves cell performance. We found that 0.4 M LiTFSI/0.6 M LiPF6 mixed-salt IL electrolyte clearly outperform the conventional organic electrolyte and show highest discharge capacity of 115 mAh g-1 (at 0.1 C), 25% of the capacity can be retained at 1 C and 43% of the cyclic stability after 30 charge-discharge cycles are obtained at 50 °C with a high cell voltage of ~4.7 V. We anticipated that the development of innovative safe lithium-ion batteries can store sustainable energy, prolonged cycle life and meeting environmental use in electronic devices (cellular phone, laptop and GPS system) including high temperature application such as hybrid electric vehicle (HEV) or military.