Toward new frontiers : Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum beams

Program D01-2	Study on ion conductivity of fluoride ion battery
Principal Investigator	KOBAYASHI, Yoshio (The University of Electro-Communications)

Fluoride ion battery (FIB), which consists of a solid electrolyte, is expected as one of the high energy-storage potentials in the next generation because it offers sufficient electric capacity and energy density, and it has high safety for handling. The longer-lasting batteries require to move much charged. It is difficult to move multiple metal cations, but it is relatively easy to move a lot of monovalent anions. It is important to research and develop a solid electrolyte including fluoride ions, in order to realize the high performance of FIB. The aim of this study is to elucidate the diffusion process of fluoride ions in typical CaF₂-based materials, and to provide basic data for the ion conductivity of FIB. In this study, the nuclear probe technique, such as the time differential perturbed angular distribution (TDPAD) method, the negative muon spin relaxation method (μ^–SR), and the in-beam Mössbauer spectroscopy (IBMS) coupled with a short-lived ⁵⁷Mn (T_1/2 = 89 sec) implantation are carried out complementarily to observe in a atomistic scale the dynamic behavior of fluoride ion in the materials with a fluorite structure.

We investigated previously the final positions and the chemical states of ⁵⁷Fe atoms decayed from ⁵⁷Mn implanted in CaF₂ by means of IBMS. The obtained in-beam Mössbauer spectra of ⁵⁷Fe/⁵⁷Mn implanted in a single crystalline CaF₂ between 13 K and 574 K are shown in Fig. 1. Mössbauer spectrum observed at 13 K could be analyzed with two components of doublets. From the results of

Fig. 1. In-beam Mössbauer spectra of ⁵⁷Fe arising from ⁵⁷Mn implantation in CaF₂. Mössbauer parameters (isomer shifts and quadrupole splittings) and DFT calculations, D1 (blue) and D2 (red) were assigned to the interstitial Fe²⁺ (HS) and substitutional Fe²⁺ (HS) for Ca²⁺, respectively. The area intensity of the D2 increased with an increase of temperature. Above about 150 K, a singlet (orange) was observed as the third component, as the area intensities and the values of the quadrupole splittings of the D2 component was increased and decreased, respectively. It is predicted that the singlet is caused by the relaxation between the substitutional and the interstitial Fe atoms. It is known that F^– diffuse at high temperatures over 500 K. However, it is suggested that F^– might locally oscillate or start local atomic jump at temperatures around room temperature. In our study, PAD, μ^–SR and IBMS coupled with β-γ coincidence measurement will be performed over room temperature to study the dynamic behavior of F^– ions in CaF₂.

Members

Reference Materials

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