Preparation of LaF3-Based Composite Electrolytes for Solid-State Fluoride Shuttle Batteries
Authors: Katakura Seiji , Imaeda Yuya , Abe Takeshi , Fukutsuka Tomokazu 
Published: December 2024

Fluoride shuttle battery (FSB)1 is expected as one of the candidates for the post lithium-ion battery. In FSBs, fluoride ions are charge carriers in the electrolyte, and fluorination and defluorination reactions of transition metal compounds are the charging and discharging reactions. Since many transition metals can be multivalent, FSBs are expected to have high theoretical capacities. All-solid-state FSBs have the advantage of high safety due to the non-flammability of the inorganic solid electrolytes. However, inorganic solid electrolytes have problems in that the effective ionic conductivity is lowered because of the poor contact between solid electrolyte particles. Although high-temperature sintering increases the effective ionic conductivity, it is energy-consuming and might transelement the interface between electrode and electrolyte in the case of integral sintering. This becomes an obstacle to constructing all-solid-state batteries.
From the view of the practical process, it may be effective to fill the voids between electrolyte particles with a flexible electrolyte, such as a polymer electrolyte, to construct an electrolyte layer without sintering. Research on electrolytes for FSB is limited, not only on composite electrolytes but also on polymer electrolytes.2 In this study, we synthesized a new fluoride-ion conductive polymer electrolyte and prepared composite electrolytes by adding the polymer electrolyte to the lanthanum fluoride (LaF3) as the sintering-free electrolytes.
Acetonitrile solutions of a polymer electrolyte using polyethylene oxide (PEO, M
w 600,000) and potassium fluoride (KF) were prepared with/without the addition of an oligomer having boroxine rings as a nonvolatile anion acceptor (AA). The polymer solutions were vacuum-dried to obtain a polymer electrolyte membrane. Differential scanning calorimetry (DSC) and ionic conductivity measurements were performed for the polymer electrolytes. Composite electrolyte powder was obtained by mixing the LaF3 powder and polymer solution containing AA, followed by vacuum drying. The composite electrolyte powder was pelletized using a uniaxial press to obtain a composite electrolyte pellet. Ionic conductivity measurements were performed for the composite electrolyte pellet, pellet of LaF3 powder, and dense LaF3 disk.
In the case of the polymer solution without adding AA, KF remained undissolved, but by adding AA, it completely dissolved. DSC of the polymer electrolyte showed that the softening point of the polymer electrolyte decreased from 69°C to 57°C with the addition of AA (Fig. 1a). The ionic conductivity of the polymer electrolyte is shown in Fig. 1b. The slope of ionic conductivity with respect to the temperature changed at 50–70°C, corresponding to the softening point. The polymer electrolyte with AA had higher ionic conductivity by three orders of magnitude and lower activation energy in the low-temperature range than without AA. The AA works to increase carrier density by increasing the solubility of KF to the polymer and to activate carrier conduction by plasticizing the polymer electrolyte.
Ionic conductivities of the composite electrolyte and LaF3 pellet are shown in Fig. 1c. The behavior of the ionic conductivity of the composite electrolyte changed at 50–60°C. This was influenced by the softening point of the PEO-based polymer electrolyte, indicating that the polymer electrolyte is involved in ion conduction in the composite electrolytes. The main ion conduction path in the composite electrolyte is probably the polymer electrolyte/LaF3 interfacial layer. Although the composite electrolyte increased the ionic conductivity by more than an order of magnitude compared with the LaF3 pellet, it was still an order of magnitude lower than the dense LaF3 disk. Further investigation to form an optimal ionic conduction path is necessary to construct a sintering-free solid electrolyte for all-solid-state FSBs.

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K. Takahashi, A. Yokoo, Y. Kaneko, T. Abe, and S. Seki, Electrochemistry, 88, 310 (2020).

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

Figure 1