The invention relates to a solid composite electrolyte comprising: i) at least one solid inorganic particle, ii) at least one ionic liquid electrolyte, and iii) at least one ionically non-conductive polymer, wherein the at least one solid inorganic particle i) is ionic conductive and is blended with the at least one ionic liquid electrolyte ii). The invention also relates to a process for manufacturing the solid composite electrolyte, to a solid state battery comprising the solid composite electrolyte, and to the use of said solid composite electrolyte in a solid state battery for improving ionic conductivity and mechanical properties.