Lithium extraction is a major concern in view of the increase in battery manufacturing. Separation of lithium from sodium is still challenging due to the similar chemical behavior of the two alkali metals. Sources such as brines or seawater usually contain large amounts of sodium which requires a high selectivity for the lithium extraction process. In this study, we demonstrate the application of a supported liquid membrane (SLM) with very high selectivity for the separation of lithium from sodium. A synergistic system made of heptafluoro-dimethyloctanedione (HFDOD) and tri-octyl phosphine oxide (TOPO) was selected based on its extraction efficiency (>99% of lithium extracted) and selectivity for lithium over sodium (separation factor≈400). Several parameters were studied using a SLM made of HFDOD and TOPO (lithium concentration, sodium to lithium ratio, HFDOD: TOPO ratio). It was found that high lithium permeation rates can be obtained even for low lithium concentrations and high 2 sodium concentration. Membrane stability was evaluated and was found to be poor (loss of performances after one cycle of use), due to the leakage of the organic phase and change in HFDOD: TOPO ratio. This SLM system is suitable for the extraction of lithium from brines and seawater.