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.

Key words: 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethane sulfonyl)imide, ionic liquid, electrolyte, Li/LiFeO₄ battery