Synthesis and Ionic Conductivity of K2Bi1-X
Sn
x
F5-X
Authors: Morinaga Sumiko , Hamaguchi Kaisei , Taminato Sou , Imanishi Nobuyuki , Mori Daisuke 
Published: December 2024

Introduction

In recent years, xEVs have been spreading rapidly. In order to extend the driving range, automotive batteries with high energy density and high safety exceeding lithium-ion batteries are required. Solid-state fluoride batteries (SSFBs), in particular, are currently attracting attention as one of the innovative storage batteries with a high theoretical energy density due to multi-electron reactions. Since organic electrolytes are not used, SSFBs have no risk of leakage and firing and a high level of safety.

The fluoride ion is expected to be suitable for ion diffusion at a high rate because it is monovalent and has a small atomic mass. However, the operating temperature of SSFBs is still high at about 150 ˚C. The challenges of SSFBs to lower the operating temperature are especially in electrolytes. Currently, PbSnF4 compounds and tysonite-type fluorides are representative examples of fluoride ionic conductors. However, these electrolytes have issues such as a narrow electrochemical potential window or insufficient ionic conductivity. New fluoride ionic conductors with high ionic conductivity and wide electrochemical potential windows are required.

In this study, we focused on K2BiF5-type fluoride with one-dimensional chains of edge-shared BiF7 polyhedra acting as an ionic conductive pathway. Previously we demonstrated that the ionic conductivity of K2-x
Rb
x
BiF5 (0.0 ≤ x ≤ 0.4) increases with increasing Rb content and the activation energy decreases. K2-x
Rb
x
BiF5 with x = 0.4 exhibited the ionic conductivity of 1.0 × 10-5 S-cm-1 at 150°C and the activation energy of 0.68 eV. The expansion of bottle neck size by the substitution of Rb with larger ionic radius than that of K is effective to enhance fluoride ion conductivity. To further improve the ionic conductivity we synthesized Sn-substituted K2BiF5 to introduce fluorine defects and evaluated their ionic conductivity.

Experimental

K2Bi1-xSnxF5-x with x = 0, 0.05 and 0.10 were synthesized using potassium fluoride KF, bismuth fluoride BiF3, and tin fluoride SnF2 as starting materials. The mechano-chemical treatment was carried out at 600 rpm for 3 - 24 h using a FRITSCH planetary mill with a mill jar and balls made of zirconia. The initial loading of the mixture was 1.5 g. K2Bi1-x
Sn
x
F5 with x = 0, 0.05, and 0.10 were synthesized by mechano-chemical treatment and subsequent heating. The ionic conductivity was evaluated by the AC impedance measurement using Au sputted electrode.

Results and discussion

The XRD measurements demonstrated that the target phase was obtained after ball-milling for the sample with x = 0 and 0.05, while a small amount of unknown phase was contained. After heating, a single phase of K2Bi1-x
Sn
x
F5 was obtained. For K2Bi1-x
Sn
x
F5 with x = 0.10 K2BiF5 phase did not appear after ball-milling for 3-12 h but appeared after heating at 200 ˚C for 10 h twice. The sample contained a large amount of impurity phase. Considering the lattice parameter of K2Bi1-x
Sn
x
F5, since the Sn2+ ion has a smaller ionic radius than that of the Bi3+ ion, the lattice is expected to shrink with Sn substitution. However, the lattice parameters of a-, b- and c-axes and the lattice volume did not change systematically with Sn content. It would be due to compositional deviation caused by the formation of impurities.

The ionic conductivity was evaluated by the AC impedance measurement. With increasing Sn content in the K2Bi1-x
Sn
x
F5, the ionic conductivity was increased and the activation energy was decreased. K2Bi1-x
Sn
x
F5 with x =0.10 showed the ionic conductivity of 3.59× 10-8 S-cm-1 at 25°C and 2.05 × 10-5 S-cm-1 at 150°C. The activation energy was 0.55 eV. They indicate that aliovalent Sn substitution is effective in improving the ionic conductivity of K2BiF5.

Acknowledgements

This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).