Lithium argyrodites with high ionic conductivity and low cost are considered as one of the most prospective solid electrolytes for all-solid-state lithium batteries. However, the poor chemical stability and compatibility with lithium metal limit their application. Herein, Li5.4PS4.4Cl1.4I0.2 solid electrolyte with high ionic conductivity of 11.49 mS cm−1 and improved chemical stability is synthesized by iodine doping. An ultra-thin Li5.4PS4.4Cl1.4I0.2 membrane with thickness of 10 μm can be obtained by wet coating process, exhibiting a high ionic conductivity of 2.09 mS cm−1 and low areal resistance of 1.17 Ω cm−2. Moreover, iodine doping could in-situ form LiI at the lithium/electrolyte interface and improve the critical current density of Li5.4PS4.4Cl1.6 from 0.8 to 1.35 mA cm−2. The resultant LiCoO2/Li5.4PS4.4Cl1.4I0.2/Li battery shows excellent cycling stability at 1 C, with a reversible specific capacity of 110.1 mA h g−1 and a retention of 80.5% after 1000 cycles. In addition, the assembled LiCoO2/Li5.4PS4.4Cl1.4I0.2 membrane/Li pouch cell delivers an initial discharge capacity of 110.4 mA h g−1 and 80.5% capacity retention after 100 cycles.