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The NASICON-type (Sodium Super Ionic Conductor) Na₃Zr₂Si₂PO₁₂ solid electrolyte is one of the most promising electrolytes for solid-state sodium metal batteries. When preparing Na₃Zr₂Si₂PO₁₂ ceramic using a traditional high-temperature solid-state reaction, the high-densification temperature would result in the volatilization of certain elements and the consequent generation of impurity phase, worsening the functional and mechanical performance of the NASICON electrolyte. We rationally introduced the sintering additive B₂O₃ to the NASICON matrix and systemically investigated the influence of B₂O₃ on the crystal structure, microstructure, electrical performance, and electrochemical performance of the NASICON electrolytes. The results reveal that B₂O₃ can effectively reduce the densification sintering temperature and promote the performance of the Na₃Zr₂Si₂PO₁₂ electrolyte. The Na₃Zr₂Si₂PO₁₂-2%B₂O₃-1150 ℃ achieves the highest ionic conductivity of 4.7 × 10⁻⁴ S cm⁻¹ (at 25 °C) with an activation energy of 0.33 eV. Furthermore, the grain boundary phase formed during the sintering process could improve the mechanical behavior of the grain boundary and inhibit the propagation of metallic sodium dendrite within the NASICON electrolyte. The assembled Na/Na₃Zr₂Si₂PO₁₂-2%B₂O₃/Na₃V_1.5Cr_0.5(PO₄)₃ cell reveals the initial discharge capacity of 98.5 mAh g⁻¹ with an initial Coulombic efficiency of 84.14% and shows a capacity retention of 70.3% at 30 mA g⁻¹ over 200 cycles.