| 摘要: | 安全和性能（包括功率和能量密度）是下一代鋰離子電池（LIBs）的關鍵因素，被認為是現今儲能的最佳技術。丁基甲基吡咯烷鎓雙（三氟甲烷磺酰）亞胺Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide，BMP-TFSI）離子液體（ionic liquids，ILs）與鋰鹽，即是LiTFSI、LiPF6，顯示出較高的熱穩定性（>400℃）、不易燃性和在高工作電壓下更加穩定，因此是理想的高安全性和高電壓應用。離子液體電解液相較於傳統的有機電解液，具有高黏度、小鋰離子的移動性使它們不利於高速C rate的應用。為了減輕這些問題，在離子液體電解液引入添加劑和提高溫度能顯著提高電池的容量、高倍率性能和循環性能。對鋰鹽和添加劑對離子液體為基礎的鋰離子電池電化學性能的影響徹底研究。因此，在本研究中，不同的類型和鋰鹽（包括鋰鹽的混合物）和添加劑的濃度被用於在各種溫度下，以優化電池性能。
結果顯示LiTFSI比LiPF6在BMP-TFSI離子液體電解液中更合適。LiFePO4在25oC含有0.5M LiTFSI的離子液體中，在0.1C時有最大的電容值為115 mAh g-1。在50oC含有1M LiTFSI的離子液體電解液中，在0.1 C有最高的電容值為140 mAh g-1。在5 C的時候電容的維持率有45%，100圈的充放電循環後觀察到電容沒有明顯的損失，相對於那些在25oC 0.5 C的時候，有18%速率性能的和77%的循環穩定性。
此外，對三種添加劑：碳酸亞乙烯酯（vinylene carbonate，VC）、γ-丁內酯（gamma-butyrolactone，γ-BL）和碳酸亞丙酯（propylene carbonate，PC）進行了研究。所有添加劑可以顯著提高容量、高速率性能和在25℃時電池的循環性。特別是，γ-BL被認為是最有效的。相反的在上述研究中，添加劑的加入會在高溫時產生負面影響。在75oC 0.1 C時，離子液體電解液顯示的容量為152 mAh g-1。經過100圈充放電循環後，忽略容量的損失，3 C的容量維持率有77%。這些值比無添加劑的離子液體電解液和常見的有機電解液還要好。
有機溶劑和含有1M LiTFSI的BMP-TFSI離子液體在高電壓的時候在基版會有腐蝕的現象產生。在離子液體中加入LiPF6可以有效地抑制鋁的腐蝕，從而提高電池的性能。我們發現，混合0.4M的LiTFSI和0.6 M的LiPF6鹽類在離子液體電解液中顯然優於傳統的有機電解質和且在0.1 C時有最高的放電容量為115 mAh g-1 1 C的容量維持率有25%和50oC 約4.7伏的高電壓下經過30圈的充放電循環還有43%的循環穩定性。我們預計，創新安全鋰離子電池的發展可以儲存可持續能源、延長循環壽命和滿足電子設備的使用環境（手機、筆記本電腦和GPS系統），包括高溫應用，如混合動力電動汽車（HEV）或軍事。
;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 (ɤ-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. |
