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The ionic conductivity, bend strength, and electrochemical performance in a seawater battery (SWB) of an Na 3.1 Zr 1.55 Si 2.3 P 0.7 O 11 (vA-NASICON) solid electrolyte were compared to those of Na 3 Zr 2 Si 2 PO 12 (H-NASICON). vA-NASICON exhibited three times higher total ionic conductivity (8.6 × 10 -4 S/cm) than H-NASICON (2.9 × 10 -4 S/cm). This is due to the higher bulk ionic conductivity and lower grain boundary resistance of vA-NASICON. The higher bulk conductivity of vA-NASICON is a result of its higher Na content, leading to a larger concentration of charge carriers and/or the formation of a higher conductive rhombohedral phase. The lower grain boundary resistance of vA-NASICON is a result of its larger grain size and reduced ZrO 2 content. The bend strength of vA-NASICON (95 MPa) was 30% higher than that of the H-NASICON ceramic. The higher bend strength of vA-NASICON was attributed to its reduced ZrO 2 secondary phase (1.1 vol %) compared to that of H-NASICON (2.6 vol %). When the vA-NASICON ceramic was tested in the SWB as a solid electrolyte, an 8.27% improved voltage efficiency and 81% higher power output were demonstrated, compared to those of H-NASICON, as a result of its higher total ionic conductivity and mechanical strength. At the same time, the vA-NASICON membrane revealed comparable cycle life (1000 h) to that of H-NASICON. These results suggest that vA-NASICON can be a better alternative than H-NASICON for use in the SWB.