A novel way of in situ forming ionic liquids (ILs) in synthetic esters is presented. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) can form ILs with synthetic esters, [Li(synthetic ester)]TFSI, by the donation of lone pairs on carbonyl oxygen atoms of an ester molecule to a lithium ion to form a weakly Lewis-acidic complex cation [Li(synthetic ester)](+) and following interaction with the weakly Lewis-basic anion TFSI(-) to generate [Li(synthetic ester)]TFSI. LiTFSI has high solubility in synthetic esters because of the complex formation. The physicochemical and tribological properties of [Li(synthetic ester)]TFSI as lubricant additives were investigated. The easy preparation, extremely good solubility and excellent tribological properties of the type of ILs make them great advantages as compared with conventional ILs that can hardly be used as lubricant additives in synthetic esters due to their low solubility. The lubrication mechanism of these ILs is tentatively discussed.