A liquid silicone was used as a binder to make composite solid electrolytes from lithium-ion conductive inorganic solid electrolytes (ISEs): an oxysulfide glass, 0.01Li3PO4–0.63Li2S–0.36SiS2 and/or a lithium germanium thio-phosphate, Li3.25Ge0.25P0.75S4. Ionic conductivities of the composites were of the order of 10−4 Scm−1, even when the silicone was enriched to 10% (v/v). On the other hand, the composite with styrene–butadiene block co-polymer (SBR) or polypropylene oxide–polyethylene oxide (PO–EO) co-polymer as binder showed much lower conductivity. In the composite electrolyte, the silicone rubber must partly cover the surface of the ISE particles because the composite electrolyte is molded before the vulcanization of the fluid liquid silicone; and thus, it must rarely interfere with the conduction between the ISE particles. Hydrocarbons were found to be suitable in the preparation process of the composite solid electrolyte (CSE).