Phase Evolution of NaSICON Materials during Temperature-Dependent Conventional and Cold Sintering

In this study, we investigated the impact of sintering temperature on the densification, phase formation, microstructure, crystallinity, and ionic conductivity of NaSICON materials with varying nominal Zr deficiency and a varying Si/P ratio. Several powder batches were synthesized and resulted in substantially different sintering abilities using conventional sintering. For most of the powder batches, the conventionally sintered specimens reached maximum ionic conductivities between 2 and 3 mS cm-1after sintering at 1200-1300 °C. Cold sintering was explored using one of the powder batches with different sintering additives. After cold sintering, an annealing step at 900 °C yielded similar conductivities. Without postannealing, a maximum ionic conductivity of 0.55 mS cm-1was reached at temperatures as low as 275 °C. There is clear evidence that (a) the densification temperature can be significantly reduced with increasing glass fraction in the specimens and (b) the total conductivity increases with increasing sintering temperatures due to increasing density and crystallinity.