Disclosed herein is an integrated electrode assembly including a cathode, an anode, and a separation layer disposed between the cathode and the anode. The cathode, the anode, and the separation layer are integrated with each other. The separation layer includes 3 phases including a liquid-phase component containing an ionic salt, a solid-phase component supporting the separation layer between the cathode and the anode, and a polymer matrix in which linear polymers and cross-linked polymers form a viscoelastic structure with the liquid-phase component and the solid-phase component being incorporated in the polymer matrix. The polymer matrix is coupled to each of the cathode and the anode. The liquid-phase component of the separation layer flows into the electrodes (i.e., the cathode and anode) during preparation of the integrated electrode assembly to greatly improve wetting properties of the electrodes and to increase ionic conductivity of the electrodes.