Composite solid polymer electrolytes (CSPEs) consisting of polyethyleneoxide (PEO), LiClO4, organic acids (malonic, maleic, and carboxylic acids), and/or Al2O3 were prepared in acetonitrile. CSPEs were characterized by Brewster Angle Microscopy (BAM), thermal analysis, ac impedance, cyclic voltammetry, and tested for charge–discharge capacity with the Li or LiNi0.5Co0.5O2 electrodes coated on stainless steel (SS). The morphologies of the CSPE films were homogeneous and porous. The differential scanning calorimetric (DSC) results suggested that performance of the CSPE film was highly enhanced by the acid and inorganic additives. The composite membrane doped with organic acids and ceramic showed good conductivity and thermal stability. The ac impedance data, processed by non-linear least square (NLLS) fitting, showed good conducting properties of the composite films. The ionic conductivity of the film consisting of (PEO)8LiClO4:citric acid (99.95:0.05, w/w%) was 3.25×10−4 Scm−1 and 1.81×10−4 Scm−1 at 30°C. The conductivity has further improved to 3.81×10−4 Scm−1 at 20°C by adding 20w/w% Al2O3 filler to the (PEO)8LiClO4 +0.05% carboxylic acid composite. The experimental data for the full cell showed an upper limit voltage window of 4.7V versus Li/Li+ for CSPE at room temperature.