Selective　extraction　of　Li+　from　high　Mg2+/Li+　ratio　brines　with　ionic　liquid　(IL)　based　collaborative　extractants　was　investigated　by　experiments,　thermodynamic　analyses,　and　quantum　chemical　(QC)　calculations.　Effects　of　different　IL　cationic　structures　and　organophosphorus　ligands　on　extraction　performances　were　studied.　The　results　demonstrated　that　the　system　1-(2-hydroxyethyl)-3-methylimidazolium　bis(trifluoromethylsulfonyl)　imide　+　trioctyl　phosphate　([HOEMIM][Tf2N]　+　TOP)　was　considered　as　the　best　extractant,　with　the　very　high　extraction　efficiency　of　Li+　(83.16　%)　and　separation　selectivity　of　Li+/Mg2+　(742.11),　which　is　higher　than　values　reported　in　literature.　The　thermodynamic　model　ePC-SAFT　was　first　extended　to　quantitatively　predict　the　phase　equilibria　of　the　so-called　＂organic-inorganic　complex　strong　electrolyte　system＂　presented　in　this　work.　The　molecular-level　extraction　mechanism　was　explored　by　QC　calculation,　indicating　that　the　strong　multi-site　intermolecular　interactions　between　Li+　and　[HOEMIM][Tf2N]　+　TOP　break　the　Li+　hydration.　This　work　provides　guidance　to　rationally　design　novel　IL-based　extractants　for　efficient　extraction　of　Li+