In spite of the extremely high ionic conductivity, the commercialization of Li₁₀GeP₂S₁₂ (LGPS)-type materials is plagued by the poor stability against Li metal. Herein, to address that issue, a novel strategy has been proposed and demonstrated for the first time to in-situ modify the interface between Li metal and Li₁₀SnP₂S₁₂ by employing ionic liquid additives. And the effects of lithium salt on the interfacial modification is further investigated too. X-ray photoelectron spectroscopy (XPS) results reveal that the addition of a small volume of 1.5 M LiTFSI/ N-propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr₁₃TFSI) ionic liquid can form a stable SEI layer at the interface between Li and solid electrolyte simultaneously during cycling whereas the presence of LiFSI salt causes the enrichment of LiF in SEI layer and results in the higher resistance of the cell upon a long cycling life. A stable SEI layer instead of mixed conducting layer can prevent sulfide solid electrolyte from further decomposition and the Li/Li symmetric cell with 1.5 M LiTFSI/ Pyr₁₃TFSI ionic liquid shows stable cycle performance for more than 1000 h with both the charge and discharge voltages reaching around 0.05 V at 0.038 mA cm^-2.