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• A novel ionic liquid of [Bmim]Bph 4 was used as co-extractant for extraction of Li+. • The single-stage Li/Mg separation factor was higher than 2500. • Li 2 CO 3 was obtained and the process flow was reduced with Na 2 CO 3 as stripping agent. • The binding capacity of TBP to metal ion followed the sequence Li+ ≫ Na+ > K+ ≈ Mg2+. • The whole extraction process with novel extraction system was green and efficient. A novel ionic liquid named 1-butyl-3-methylimidazolium tetraphenylboron ([Bmim]Bph 4) containing a stereo symmetric structure of Bph 4 - was synthesized and used as co-extractants with TBP and CH 2 BrCl as extractant and diluent for selective extraction of lithium ions from salt lake brines with high molar ratio of magnesium to lithium. The effects of extraction conditions containing molar ratio of co-extractant to lithium ion, volume percentage of TBP and O/A phase ratio on extraction performance were investigated. Under the optimal extraction conditions, the maximum separation factor of lithium to magnesium reached up to 2754 after one single-stage extraction process. After four stages of counter-current extraction process, the extraction efficiency of Li+ reached up to 99.47%. Then the washing and stripping conditions were optimized with the combination of LiCl and NaCl solution as the washing agent and Na 2 CO 3 solution as the stripping agent. After that, 10 cycles of the whole extraction experiments were carried out and the organic phase exhibited excellent reusability and stability. Since no acid and alkaline were used throughout the extraction process, the novel extraction process is greener and more environmentally friendly than the traditional solvent extraction process. [ABSTRACT FROM AUTHOR]