C. S. Hausner ¹ * , M. Rohde ¹ , H. J. Seifert ¹

¹ Karlsruhe Institute of Technology (KIT)
^* clemens.hausner@kit.edu

In recent years sodium-based All-Solid-State-Batteries (ASSBs) have attracted attention, as they have
the potential to combine the advantages of ASSBs like increased safety with abundant raw materials.
In 2019 Ma et al. identified the composition Na_3.4Zr_2.0Si_2.4P_0.6O_12.0 to be the best Na-ion conductor
within the crystal series Na_1+xZr₂Si_xP_1-xO₁₂ , with a total ionic conductivity of 5 x 10 ^-3 S/cm at 25 °C. ^[1]
The most intuitive approach to maximize the ionic conductivity of a ceramic ion-conductor would be
to enhance the material densification by raising the sintering temperature. In this context the
question arises, what criterion actually defines an upper limit for the sintering temperature in the
given material system. For these ceramics some authors suspect a beginning melt formation even at
relevant sintering temperatures. ^[2] Although this hypothesis would strongly affect the sintering and
densification behaviour of these materials, this phenomenon hasn‘t been investigated in more detail
so far. To clarify to what extent a potential liquid phase formation might take place at relevant
sintering temperatures applied in literature and therefore might influence the densification of these
materials, the melting behaviour of the given material is investigated in this study.

Na_3.4Zr_2.0Si_2.4P_0.6O_12.0 was synthesized by a solution-assisted solid state reaction. ^[3] Samples were
sintered at temperatures of 1230, 1280, 1300 and 1330 °C for 5 h. The material density passes
through a maximum for the temperature 1300 °C, as does the total ionic conductivity. Cross sections
of samples, that were quenched from 1300 and 1330 °C were analysed by Scanning Electron
Microscopy (SEM). First evidence for the existence of a melt phase in the microstructure at the
sintering temperature 1330 °C could be found.

To get an even closer insight into the melting behaviour of the investigated material Differential
Thermal Analysis (DTA) was applied. Based on the DTA signal the temperature of first melt formation
in the compound is located at 1320 °C.

This finding suits the identified frozen melt phase in the microstructure quenched from 1330 °C by
SEM. The formation of this melt phase could be part of an explanation for the observed decrease of
the material density and ionic conductivity from 1300 to 1330 °C.

Setting the determined temperature of the first melt formation of 1320 °C and the identified
optimum sintering temperature of 1300 °C into relation, it can be concluded, that the sintering
temperature of the investigated composition can be set as high as 20 °C below the temperature of
first melt formation. Despite the related high thermal load on the material at the given temperature,
no clear indications of increased material degradation could be identified.

References:
[1] Q. Ma, C. Tsai, X. Wei, M. Heggen, F. Tietz, J. Irvine, Journal of Materials Chemistry A, 2019, 7(13), 7766-
7776
[2] A. K. Kuriakose, T. A. Wheat, A. Ahmad, J. Dirocco, Journal of the American Ceramic Society, 1984, 67(3),
179-183
[3] S. Naqash, Q. Ma, F. Tietz, O. Guillon, Solid State Ionics, 2017, 302, 83-91