Composite electrolytes (CEs) containing polyethylene oxide (PEO), polytetram-ethylene glycol based waterborne polyurethane (WPU(PTMG)), LiClO4/propylene carbonate (PC), and aluminum oxide (Al2O3) were prepared by blending. The influences of the addition of Al2O3 in CEs were investigated using the following methods. Differential scanning calorimetry (DSC) and polarizing microscopy (PM) were employed for material characterization. The swollen weight of CEs was measured to appraise the maximum tolerated intake. Alternating current (AC) impedance was employed to obtain the ionic conductivity. Furthermore, Li/CEs/LiCoO2 laminated cells were assembled to measure open-circuit voltages (V proportional to) for the application evaluation. Accordingly, the CE consisting of 20% PEO, 20% WPU(PTMG), 10% Al2O3, and 50% LiClO4/PC was found to have the best performance for all the investigated characteristics.