A Material Study on Na-SrSiO3 Ionic Conductor
Authors: Jee Youngseok , Zhao Xuan , Huang Kevin 
Published: February 2020

In order to find a fast oxygen ion conducting material which is essential for solid oxide fuel cell (SOFC) commercialization, many researches have been conducted for last couple of decades. But most of perovskite and fluorite materials have been able to facilitate SOFC system above 600°C. Recent reports by Singh et al. on the layered / monoclinic ionic conductors with the formula of Sr1-xAxMO3-0.5x (A=K or Na and M=Si or Ge) have been issued for its high conductivity (in the order of 10-2 S/cm at 550°C) and the nature of ionic conduction.[1-2] After the first report of this material, a lot of studies argued on the ionic conduction mechanisms. NMR spectroscopy and neutron diffraction study by Xu et al. confirmed the local crystalline structure of Sr0.7K0.3SiO2.85.[3] However, others insisted this material is a mixture of a crystalline and amorphous phase.[4-8] A critical work by Bayliss et al. showed from an experimental and theoretical perspective that substitution of Na or K into Sr sites is an energetically unfavorable process.[4-5] A subsequent oxygen isotope exchange and SIMS measurement on Sr0.8K0.2Si0.5Ge0.5O2.9 indicated a very low oxygen diffusivity.[4] Evans et al. found the composition of the crystalline phase being SrSiO3 and amorphous phase being Na2Si2O5.[6] A strong correlation among conductivity, the amount of amorphous Na2Si2O5 and Na content was also found. Similarly, Tealdi et al. and Fernandez-Palacios et al. confirmed that the amorphous phase was responsible for the ionic conduction of the material.[7-8] In spite of our previously reported result, the inconsistency encountered during the material synthesis and subsequent characterizations has prompted us to revisit here with a more systematic approach for better understanding of the phase relationship and ionic conduction in this material.
In this study, a full range of Sr1-xNaxSiO3-0.5x (x=0.0–1.0) was prepared using solid-state reactions. The two-phase nature of the x=0.45 sample is reconfirmed by Fig. 1; there are clearly a liquid-like dark phase mixed with a crystalline-like bright phase in the SEM image. The EDX analysis indicates that the dark phase has an average chemical composition of Na:Si=1:1 implying Na2Si2O5 composition. By contrast, the bright phase has a very low concentration of Na, but a high and similar concentration of Sr and Si, suggesting that SrSiO3 with a minor doping by Na be the composition.
The Arrhenius plot of electrical conductivity is shown in Figs. 2 (a) and (b) for all the compositions investigated. A general trend is found in that the conductivity increases systematically with x for a temperature below 600°C. The highest and lowest conductivities at ≤500°C are observed for the AM(amorphous)-Na2Si2O5 and x=0.1 sample, respectively. The highest conductivity observed in the AM-Na2Si2O5 reaches 0.028 S/cm at 500°C. It is also interesting to see that the C(crystalline)-Na2Si2O5 is virtually an insulator, yielding the second lowest conductivity among all the compositions studied. At x≥0.40, the conductivity exhibits a “bend-over” behavior at ~600°C, whereas the bend-over is at lower ~500°C for the AM-Na2Si2O5. We show below that this phenomenon is associated with the crystallization of the AM-Na2Si2O5 during the sample synthesis. Furthermore, the combined results of Figs. 1-2 indicate that the more the dark phase in the microstructure, the higher the conductivity of the material.
Fig. 3 displays high-temperature XRD patterns collected from AM-Na2Si2O5 and x=0.45 samples as a function of temperature. For the AM-Na2Si2O5, Fig. 3 (b), crystallization clearly occurs at >500°C as evidenced by the appearance of diffraction peaks related to a crystalline Na2Si2O5 phase. A close examination of the patterns suggests that the crystallized Na2Si2O5 is a new polymorph, RC-Na2Si2O5 (PDF No. 19-1233) Similarly, extra peaks in Fig. 3(a) related to RC-Na2Si2O5 phase are also observed at 600°C for the x=0.45 sample as indicated by the asterisks. These XRD analysis explicitly demonstrate that the conducting AM-Na2Si2O5 is unstable at a temperature >500°C, crystallizing into an insulating RC-Na2Si2O5 phase. The gradual crystallization of the AM-Na2Si2O5 into RC-Na2Si2O5 is, therefore, the root cause for the conductivity “bend-over” behavior in Fig. 2. In other words, the real conducting phase in all the samples is the AM-Na2Si2O5 phase, not the previously perceived “Na-doped” SrSiO3. Since the residual amount of conductive AM-Na2Si2O5 in a sample is sensitive to both Na content and thermal history, the conductivity reproducibility among different research groups has been poor.
[1]P.Singh and J.B.Goodenough, Energy Environ. Sci.,5,9626–31(2012).
[2]P.Singh and J.B.Goodenough, J. Am. Chem. Soc.,135,10149–54(2013).
[3]J.Xu et al., Inorg. Chem.,53,6962–8(2014).
[4]R.D.Bayliss et al., Energy Environ. Sci.,7,2999–3005(2014).
[5]R.D.Bayliss et al., J. Mater. Chem. A,2,17919–24(2014).
[6]I.R.Evans et al., Chem. Mater.,26,5187–9(2014).
[7]C.Tealdi et al., Chem. Commun.,50,14732–5(2014).
[8]S.Fernandez-Palacios et al., Ceram. Int.,41,6542–51(2015).

Figure 1