Ce-Substituted Nanograin Na₃Zr₂Si₂PO₁₂ Prepared by LF-FSP as Sodium-Ion Conductors

The urgent need for high-performance solid electrolytes has aroused considerable focus on NASICON ceramics. Optimization of processing routes to dense, defect-free materials has yet to receive sufficient attention to date. Although traditional solid-state reaction methods followed by repetitive ball milling and sintering up to 10 h above 1200 °C are common place, the resulting average particle sizes are usually too large to produce dense, robust structures because of excessive grain growth. In this study, nanopowders (NPs) are produced, which offer a superior opportunity to make dense, high-phase-purity sintered bodies. Here, we report on the effect of sintering conditions on the microstructures and phase of Ce⁴⁺-substituted NASICON samples, Na₃Ce_xZr_2-xSi₂PO₁₂ (x = 0, 0.1, 0.2, 0.3). NPs permit processing fine-grained solid-state electrolytes with 98% relative density at 1100 °C/5 h. In addition, Rietveld refinement was applied to evaluate 3-D Na-ion diffusion channels among different NASICON samples. Also, it is found that adding 5 at % Ce⁴⁺ does not change the phase structure but dramatically enlarges the Na⁺ diffusion "bottleneck" from 5.4 to 5.6 Ų. This may be one reason for these samples to exhibit conductivities of 2.4 × 10^-2 S cm^-1 at 140 °C.