The lithium ion conductivities of as-prepared composite membranes consisting of a polyethylene oxide (PEO) matrix with various contents of tetragonal Li7La3Zr2O12 (LLZO) were evaluated, and the optimum composition (52.5% LLZO) was determined by performing AC impedance measurements. The ionic conductivities of the composite membranes pass through a maximum as the LLZO content varies. Therefore, the hybridization of the organic and inorganic components of these membranes results in synergetic effects on their lithium ionic conductivity. In addition, tests of Li/composite membrane/LiNi0.6Co0.2Mn0.2O2 half-cells found that their charge/discharge properties are better than those of a PEO-only membrane and a membrane containing 52.5% Al2O3 instead of LLZO.