Ceramic-polymer film includes a polymer matrix, plasticizers, a lithium salt, and a ceramic nanoparticle, LLZO: AlxLi7-xLa3Zr1.75Ta0.25O12 where x ranges from 0 to 0.85. The nanoparticles have diameters that range from 20 to 2000 nm and the film has an ionic conductivity of greater than 1×10−4 S/cm (−20° C. to 10° C.) and larger than 1×10−3 S/cm (≥20° C.). Using a combination of selected plasticizers to tune the ionic transport temperature dependence enables the battery based on the ceramic-polymer film to be operable in a wide temperature window (−40° C. to 90° C.). Large size nanocomposite film (area ≥8 cm×6 cm) can be formed on a substrate and the concentration of LLZO nanoparticles decreases in the direction of the substrate to form a concentration gradient over the thickness of the film. This large size film can be employed as a non-flammable, solid-state electrolyte for lithium electrochemical pouch cell and further assembled into battery packs.