context: Fig. 3(a) and (d) show the results of ion conductivity through EIS measurements of the porous NASICON pellets and epoxy-NASICON pellets, respectively. The NASICON ceramics sintered at 1100 °C had an ionic conductivity of 1.76 × 10−4 S cm−1. A cross-sectional SEM image of the NASICON pellet sintered at 1100 °C is shown in Fig. 3(b). Here, the solid electrolyte secures the ion transfer channel after sintering, leaving pores to remain inside. When the epoxy polymer is added to the sample, the polymer material sufficiently fills the internal pores (Fig. 3(e)). In the SEM image, it was confirmed that the empty space between the NASICON crystals was filled with a black material, which was identified as a polymer material containing C through the EDS analysis shown in Fig. 3(c) and (f). The detailed EDS analysis results for each component can be seen in ESI Fig. S4.† Comparing the components of Na, Si, Zr, and P, which are all contained in NASICON, with the components of C, which are only contained in the epoxy polymers, we can see that the epoxy polymer is completely contained in the inner pores. In the same sample, Na-ions only move through the connected ceramics, which means that adding epoxy polymers does not affect the ion conduction. The ionic conductivity after epoxy treatment of the NASICON pellet is determined to be 1.45 × 10−4 S cm−1. Therefore, epoxy polymer is not involved in the ion transport mechanism, so it can increase the physical strength while maintaining the existing ion conductivity.