Lithium metal is a promising anode candidate for the next generation rechargeable battery due to its highest specific capacity (3860 mA h g^-1) and lowest potential, but low Coulombic efficiency (CE) and formation of lithium dendrites hinder its practical application. Here we report a self-formed flexible hybrid solid-electrolyte interphase (SEI) layer through co-deposition of organosulfides/organopolysulfides and inorganic lithium salts using sulfur-containing polymers as additive in the electrolyte. The organosulfides/organopolysulfides serve as “plasticizer” in the SEI layer to improve its mechanical flexibility and toughness. The as-formed robust SEI layers enable dendrite-free Li deposition and significantly improve CE (99% over 400 cycles at a current density of 2 mA cm^-2). A lithium-sulfur battery based on this strategy exhibits long cycling life (1000 cycles) and good capacity retention. This study reveals a new avenue to effectively fabricate stable SEI layer for solving the issues associated with lithium metal anodes.