TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The sulfide-based electrolyte comprises at least one selected from Li2S-P2S5, Li2S-B2S3, Li2S-Si2S3, Li2S-SiS2, LiI-Li2S-B2S3, and LiI-Li2S-SiS2 (preferably Li2S-P2S5). The Li2S-P2S5 has a Li2S to P2S5 molar ratio of 1:9 to 9:1.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The water-resistant, lithium conductive polymer comprises at least one selected from polyethylene oxide, polyethylene glycol, polypropylene oxide, polymethyl methacrylate, polydimethylsiloxane, polyvinylpyrrolidone, and polyacrylonitrile. The coating film comprises polyethylene oxide. The contact angle of a surface of the solid electrolyte is 90-120 degrees C. The coating film has a thickness of 5-50 nm. The weight average molecular weight of the water-resistant, lithium conductive polymer is 5000-8000000 g/mol. Preferred Composition: The electrolyte comprises water-resistant, lithium conductive polymer (0.5-10 parts by weight). Preferred Method: The drying the mixture involves drying the mixture at 30-70 degrees C. The solvent is included at 1-50 parts by weight based on 100 parts by weight of the water-resistant, lithium conductive polymer. EXAMPLE - Polyethylene oxide (10 g) was dissolved in acetonitrile (90 g) in a dry room to prepare polyethylene oxide solution (10 wt.%). The weight average molecular weight of polyethylene oxide was 100000 g/mol. Li2S-P2S (10 g), having a molar ratio of 7:3, was added to the polyethylene oxide solution and stirred for about 6 hours. Then, the mixture was dried at 60 degrees C under a vacuum for 24 hours, thereby preparing a solid electrolyte including a Li2S-P2S electrolyte and a polyethylene oxide coating film formed on a surface of the Li2S-P2S electrolyte. The polyethylene oxide coating film was included in the solid electrolyte at 5 parts by weight based on 100 parts by weight of the Li2S-P2S electrolyte. The solid electrolyte showed contact angle of 110.5 degrees , when tested using a contact angle measuring instrument.