Phase‐field Determination of NaSICON Materials in the Quaternary System Na2O−P2O5−SiO2−ZrO2: The Series Na3Zr3–xSi2PxO11.5+x/2

Two types of solid electrolytes have reached technological relevance in the field of sodium batteries: ß/ß”‐aluminas and NaSICON‐type materials. Today, significant attention is paid to room‐temperature stationary electricity storage technologies and all‐solid‐state Na batteries used in combination with these solid electrolytes are an emerging research field besides sodium‐ion batteries. In comparison, NaSICON materials can be processed at lower sintering temperatures than the ß/ß”‐aluminas and have a similarly attractive ionic conductivity. Since Na2O−SiO2−ZrO2−P2O5 ceramics offer wider compositional variability, the series Na3Zr3–xSi2PxO11.5+x/2 with seven compositions (0≤x≤3) was selected from the quasi‐quaternary phase diagram in order to identify the predominant stability region of NaSICON within this series and to explore the full potential of such materials, including the original NaSICON composition of Na3Zr2Si2POl2 as a reference. Several characterization techniques were used for the purpose of better understanding the relationships between processing and properties of the ceramics. X‐ray diffraction analysis revealed that the phase region of NaSICON materials is larger than expected. Moreover, new ceramic NaSICON materials were discovered in the system crystallizing with a monoclinic NaSICON structure (space group C2/c). Impedance spectroscopy was utilized to investigate the ionic conductivity, giving clear evidence for a dependence on crystal symmetry. The monoclinic NaSICON structure showed the highest ionic conductivity with an optimum ionic conductivity of 1.22×10−3 at 25 °C for the composition Na3Zr2Si2PO12. As the degree of P5+ content increases, the total ionic conductivity is initially enhanced until x=1 and then decreases again. Simultaneously, the increasing amount of phosphorus leads a decrease in the sintering temperatures for all samples, which was confirmed by dilatometry measurements. The thermal and microstructural properties of the prepared samples are also evaluated and discussed. Stability region of NaSICON materials is larger than the known solid solutions suggest. A series of compositions were prepared to explore the real stability region and to investigate the physical properties as well as the phase relations of these materials. Within this series, the composition Na3Zr2Si2PO12 showed the highest ionic conductivity at 25 °C (1.22 mS cm−1).