Synthesis and Ion Conduction Properties of Rb3AlF6-Na3AlF6 Solid Solution
Authors: Kaneda Yudai , Miyazaki Reona , Hihara Takehiko 
Published: December 2024

Introduction
Inorganic solid electrolytes in which ions can smoothly migrate are actively studied for the development of all-solid-state batteries. Recently, fluoride-based solid electrolytes are attracting attention because of their high electrochemical stability1. Previous studies have shown that the solid solution of Li3AlF6-Li2SiF6 has a relatively high Li+ conductivity. The charge-discharge properties of all-solid-state batteries with Li3AlF6-Li2SiF6 electrolytes were excellent2. However, due to the uneven distribution and supply instability of lithium, attention has turned to sodium, which is more abundant metal in earth3. In this study, Na+ conductivity of Na3AlF6, which has the same composition with Li3AlF6 is investigated. It is known that Na3AlF6 undergoes a phase transition from monoclinic to cubic at 570°C, which improves the conductivity4.Hence, if the cubic Na3AlF6 is stabilized, high Na+ conductivity at lower temperature is expected. It has been reported that Rb3AlF6 also has the cubic polymorph5. In this study, Na3AlF6 is doped to cubic Rb3AlF6 with the aim of stabilizing cubic Na3AlF6. If the Na concentration in Rb3AlF6-Na3AlF6 solid solution exceeds 50 mol%, it is said that cubic Na3AlF6 is stabilized. The solubility limit of Rb3AlF6-Na3AlF6 is investigated and the occupancies of Rb sites in cubic Rb3AlF6 are determined. The compositional dependence of the conductivity of Na-doped Rb3AlF6 was investigated.
Experimental Methods
Rb3AlF6 was synthesized by planetary ball milling. RbF (SIGMA-ALDRICH 99.8%) and AlF3 (HIGH PURITY CHEMICALS) were mixed in a molar ratio of 3:1 in the glove box. The mixed powder was sealed in ZrO2 milling vessel with 10 pieces of balls. Ball-milling was performed at 400rpm for 50 hours.
Na3AlF6 was prepared by melting. Rb3AlF6- Na3AlF6 solid solutions were prepared by ball-milling by mixing a given molar ratio of Rb3AlF6 and Na3AlF6. To investigate the crystal structure of the samples, XRD measurements were performed with Cu-Kα radiation source (MiniFlex, Rigaku Co.). The crystal structures of Rb3AlF6-Na3AlF6 were refined by Rietveld analysis using PDXL software (Rigaku Co.)
To introduce Na vacancies, Na2SiF6 was added to the Rb3AlF6-Na3AlF6 solid solution by planetary ball milling. For the conductivity measurement, the sample was grounded and pelletized at about450 MPa in the PEEK cylinder. The impedance was measured in a frequency range between 1 MHz to 1 Hz.
Results and discussions
As shown in Figure 1(a), Rb3AlF6 synthesized by planetary ball milling showed the cubic diffraction pattern with the space group of Fm-3m. From the previous report, the stable phase of Rb3AlF6 was orthorhombic5. Hence, it is said that the cubic Rb3AlF6 obtained in this study is the meta-stable phase.
The compositional dependence of the lattice volumes of Rb3AlF6-Na3AlF6 is presented in Figure (b). The Rb3AlF6 lattice size was decreased with increasing Na composition, which remained unchanged above 50 mol%. Hence, the solubility limit of Na would be 50 mol%. The shrinkage of Rb3AlF6 lattice is possibly due to the difference of the ionic radii of Na=1.02 Å and Rb=1.52 Å6. As shown in the inset of Figure 1(c), there are two types of Rb sites in cubic Rb3AlF6; Rb1 (1/4, 1/4, 1/4) (yellow) and six-coordinated Rb2 (0 , 0 , 1/2)(magenta). Rietveld analysis was performed to determine the occupancies of Rb1 and Rb2 (Occ
Rb1 and Occ
Rb2, respectively) for each Na composition. As shown in Figure (c), both Occ
Rb1 and Occ
Rb2 decreased with Na concentration, however, the Na substitution was not even among two Rb sites. The substitution preferentially occurred for Rb2 site. For 25 mol% Na3AlF6, Occ
Rb2 was decreased to 0.136 while Occ
Rb1 was almost unchanged. Na+ ions occupied Rb1 site after completing the substitution of Rb2 site.
As a result of conductivity measurement, the sample with Rb3AlF6-Na3AlF6 -Na2SiF6 of 2:1:3 had the maximum conductivity of 4.65x10-7S/cm at 30℃.
Acknowledgement
This work was supported by the NGK Environment Innovation Laboratory and JSPS KAKENHI Grant JP23H23447.
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Figure 1