Characterizing the Effect of Sodium Concentration in NaSICON (Na 1+ x Zr ₂ Si x P 3– x O ₁₂ , 2.0 ≤ x ≤ 2.8): 36 mS/cm Bulk Conductivity Achieved at x = 2.6 and 2.8

NaSICON (Na Super-Ionic CONductor) is a promising family of Na-ion solid electrolyte materials for Na-based all-solid-state and aqueous redox-flow batteries, with the most simple and well-studied compound being Na 1+ x Zr 2 Si x P 3– x O 12 (0 ≤ x ≤ 3), or NZSP. It has been recently shown through simulation and experiment that the bulk and total conductivities are maximized at x = 2.4. In the present study, NZSP of varying Na contents (2.0 ≤ x ≤ 2.8) has been synthesized through a solution-assisted solid-state reaction and densified using rapid-induction hot pressing (RIHP). The bulk and total conductivities, phase purity, and elemental composition were characterized with low-temperature impedance spectroscopy, synchrotron X-ray diffraction, and wavelength dispersive X-ray spectroscopy. For the first time, we report a maximum bulk conductivity at x > 2.4: an unprecedented average room temperature bulk conductivity of 3.6 × 10 –2 S cm –1 is achieved at x = 2.6 and 2.8, and a remarkably high average total conductivity of 8.7 × 10 –3 S cm –1 is achieved at x = 2.8. This trend deviates from the literature and was previously thought to be improbable due to the low vacancy concentration at high Na content. We showed that the bulk conductivity of an x = 2.8 sample decreased significantly after altering its thermal history, providing indirect evidence that cation disorder greatly affects Na transport, although further characterization is needed to fully understand the cause. The results of this study show how formulation, synthesis, and processing affect the conductivity and performance of NaSICON and help guide future research on NaSICON-based ceramic ion conductors.