Room-temperature ionic liquids based on the bis ((trifluoromethyl)sulfonyl)-imide anion with the tributyl phosphate (TBP) system employed to extract lithium ion from salt lake brine with a high Mg/Li ratio. The influences of cation structure, concentration of ionic liquid and phase ratio on extraction efficiency were investigated. Results indicated that N-butyl pyridinium bis((trifluoromethyl)sulfonyl)imide (BD) has high extraction efficiency for lithium; tributylmethylammomium bis((trifluoromethyl)sulfonyl)imide (JA) has lower extraction efficiency for lithium and impurity ions but exhibits better selectivity; when cation exchange mechanism occurs during solvent extraction by using ionic liquids as a synergistic extraction agent, the cation more hydrophobic was, the more difficult for it to enter the solution exchange process. (C) 2018 Elsevier B.V. All rights reserved.