Despite its high theoretical energy density,lithium metal faces huge challenges in its implementation as an anode for Li secondary batteries because of its uncontrollable dendritic growth and large volume change during plating/stripping processes.These geometric changes cause degradation in a cell\'s cycle life and performance and can lead to short-circuits and explosions.Here,we report a new approach to producing a LiF-rich phase on the lithium anode surface by employing a“bi-phase”separator.We fabri-cated it by coating polyvinylidene fluoride-co-hexafluoropropylene(PVDF-HFP)onto a cellulose separa-tor.The coverage of coating was adjusted by varying its concentration in the coating solution.The combination of cellulose and PVDF-HFP produced a LiF-rich solid-electrolyte interphase layer on the Li metal surface via an alkyl halide nucleophile exchange in contact with the bi-phase separator during plat-ing.Symmetric cell tests show that the bi-phase separator extends the cycle life by more than 300 h,with overpotentials of less than 100 mV under plating/stripping at 2 mA cm-2 for a capacity of 2 mAh cm-2.The formation mechanism of the LiF-rich phase is suggested from spectroscopic analyses.