Adapting Crystal Structure and Grain Boundaries through Sm³⁺ Doping in Na₃Zr₂Si₂PO₄ for Boosting Applicability in Sodium Solid-State Batteries

Solid-state sodium batteries represent a highly promising option for future electrochemical energy storage applications. The ionic conductivity of solid-state electrolytes is one of the significant factors limiting the development of solid-state batteries. In this study, we establish that Sm 3+ doping effectively boosts the ionic conductivity of Na 3 Zr 2 Si 2 PO 12 (NZSP). The optimal composition, Na 3.2 Zr 1.8 Sm 0.2 Si 2 –PO 12 (NZSP-S20), exhibits a total conductivity of 1.87 mS cm –1 at 23 °C. Structural and microscopic morphology analyses reveal that Sm 3+ doping enhances the ionic conductivity of NZSP through structural modulation, phase fraction adjustment, and grain size reduction. High-frequency impedance spectroscopy (40 Hz to 110 MHz) demonstrates that bulk and grain boundaries contribute 49.4 and 50.6%, respectively, to the total conductivity. The structural and microscopic morphology analyses reveal that Sm 3+ doping enhances the ionic conductivity of NZSP. Furthermore, the critical current density (CCD) attained in the symmetric cell, assembled by using NZSP-S20 as the solid-state electrolyte and NaSn alloy as the electrode, reaches 2.2 mA cm –1 . These results furnish a theoretical foundation for comprehending the modification of solid-state electrolytes.