The interfacial resistance between the composite electrode and PEO-based solid polymer electrolyte was examined using two different symmetrical cells and AC impedance measurements in the temperature range of 30–60°C. Of the four major resistance components identified in our previous study, three components due to the resistances of the polymer electrolyte and the charge transfer resistance between LiFePO4-C and the electrolyte were assigned; however, one resistance component, in the medium frequency range has not yet been assigned. In this report, the medium frequency range resistance component was investigated using a symmetrical cell with composite electrodes composed of active and inactive materials (LiCoO2, LiMn2O4 and SiO2), solid polymer electrolyte, and carbon fiber as an electron conductor. The resistance was not affected by the carbon coating in LiFePO4-C and the conductive carbon additive (VGCF). An electron blocking cell was also prepared and the resistance was measured in the temperature range of 30–60°C to confirm the effect of the electrode thickness and the electrode composition. The resistance was not affected by the thickness, and decreased with decreasing PEO-LiTFSI electrolyte content in the composite electrode. However, no significant change in the activation energy was evident with the change in the electrode composition.