五种1-烷基-2,3-二甲基咪唑型离子液体的合成及作为Li/LiFeO_4电池电解液的研究

合成了五种新的1-烷基-2,3-二甲基咪唑二(三氟甲基磺酰)亚胺离子液体(alkyl-DMimTFSI).以离子液体作为Li/LiFeO4电池电解液,分别考察不同烷基(正丁基、正戊基、正辛基、异辛基和正癸基)对电解液理化性质、界面性质和电池行为的影响.结果表明离子液体的电化学窗口都可以达到5.6V(-0.4～5.2Vvs.Li+/Li),显示它们具有较好的电化学稳定性.加入碳酸亚乙烯酯作为添加剂后,离子液体电解液在Li负极形成稳定的固体电解质相界面膜(SEI),从而提高了Li负极的稳定性,保护了Li片不受腐蚀.电化学阻抗和循环伏安分析进一步揭示LiFeO4正极与离子液体电解液也有良好的兼容性.此外,研究还表明离子液体中烷基种类严重影响它们的电池行为.采用butyl-DMimTFSI和amyl-DMimTFSI电解液体系的电池充放电容量和可逆性明显优于另外三种离子液体,它们的首次放电容量分别达到145和152.6mAh/g,并表现出良好的充放电循环性能.因粘度最大,采用isooctyl-DMimTFSI电解液的电池首次放电容量仅为8.3mAh/g,但添加碳酸丙烯酯(质量比1∶1)稀释后首次放电容量上升至132.4mAh/g.

Study on the Synthesis of Five 1,2-Dimethylimidazolium Type of Ionic Liquids and Their Application to Li/LiFeO4 Battery as Electrolytes

We designed and synthesized five new 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)-imide room temperature ionic liquids in the work. Subsequently, these ionic liquids were developed as electrolytes for Li/LiFeO₄ batteries to study on effect of the physical properties, interfacial and electrochemical performance by changing the alkyl chain: -butyl, -amyl, -octyl, -isooctyl and -decyl group. The results showed electrochemical windows of all ionic liquid are up to 5.6 V (－0.4～5.2 V vs. Li^＋/Li). The fact reveals the ionic liquid electrolytes are of excellent electrochemical stability. The ternary electrolyte will form fine solid electrolyte interphase film (SEI) on the surface of metallic lithium electrode in the presence of vinylene carbonate. The SEI protects lithium foil to corrosion and enhances the stability of the Li electrode. Ac impedance spectroscopy and cyclic voltammetry analysis further demonstrate the ionic liquid electrolytes have also excellent compatibility with LiFeO₄ electrode. Moreover, obvious dependence of the electrochemical properties on the alkyl in the ionic liquid was observed. Li/LiFePO₄ batteries with butyl-DMimTFSI and amyl-DMimTFSI as electrolytes exhibited better charge/discharge capacity and reversibility than that of other three ionic liquids. Their initial discharge capacity is about to 145 and 152.6 mAh/g and indicates excellent cycle reversibility. For the batteries with the electrolyte-based isooctyl-DMimTFSI ionic liquid, the initial discharge capacity is only 8.3 mAh•g^－1 due to its biggest viscosity. However, it added dramatically to 132.4 mAh•g^－1 when 50% (w/w) propylene carbonate was introduced into the ternary electrolyte as diluent.