Solid state batteries are advantageous and attractive in terms of alleviating safety hazard and enabling thin, miniature, and flexible applications. Polymer-ceramic composites with appropriate chemistry and composition have potentials to transcend individual constituent’ electrical, thermal, and/or mechanical properties. Our research has been directed to developing free-standing electrolyte membrane system made up of lithiated polymer and active lithium-ion conducting ceramic. This study is to present the static and dynamic mechanical behaviors of the Li-salt/PEO/LAGP membranes in relation to temperature, Li-salt, and ceramic loadings. The differences origincated from interactions among the polymer matrix, anions in the lithium salt, and LAGP ceramic in the composite setting.