Lithium-ion batteries are usually used in power tools, electric vehicles, and portable electronics because of their high capacities, high voltages, and long lifetime. However, the conventional organic liquid electrolyte is highly flammable and there is a leakage risk. As a result, solid-state electrolytes are widely studied for next-generation batteries. Herein, an electrolyte with both high conductivity and mechanical strength is developed by using a composite electrolyte. The lithium lanthanum titanate (LLTO)/poly(ethylene oxide) electrolyte (PEO) with the addition of porous polyvinyl alcohol (PVA) as a polymer framework was used to increase mechanical strength.