Metal fluorides generally have low electric conductivities. Hence, if a single metal (e.g. Cu) is used as the cathode active material for a fluoride-shuttle battery (FSB), the electric resistance in the cathode increases after the fluoridation reaction (charging).^1,2 Hence, repeating charging and discharging leads to significant drop in capacity. If an alloy consisting of two metal elements with different fluorination/defluorination potentials is used as the cathode active material, one metal element behaves as a conductive metal even when the other metal element is fluorinated, and the increase in electric resistance can be suppressed. So, we evaluated Cu-Ni binary alloys as cathode active materials for FSB in this study.

The standard redox potentials (25 °C) of NiF₂/Ni and CuF₂/Cu are +0.11 V and +0.72 V (vs. PbF₂/Pb), respectively. ³ Although NiF₂ is an insulating compound, Cu behaves as a metal, providing electron conduction pathways. Additionally, the crystal structures of Cu and Ni are commonly fcc structures, with lattice constants of 3.597 Å and 3.499 Å, respectively, so that Cu-Ni alloys exhibit solid solutions in the full composition range above 380 °C in the phase diagram. Hence, the Cu-Ni alloy is considered to be an ideal alloy system that hardly separates different phases and can provide highly-dense-interconnecting-electron-conduction pathways within the whole cathode like veins.

Cu_xNi_1−x (x = 0, 0.3, 0.5, 0.7, 1.0) films with thicknesses of approximately 3 nm were sputter-deposited onto 0.5-mm-thick LaF₃ solid electrolytes to form all-solid-state PbF₂/LaF₃/Cu_xNi_1−x cells. Charging/discharging curves and Nyquist plots of the cells were measured under high vacuum (< 10⁻³ Pa) at 140 °C.

Acknowledgment

This paper is based on the results obtained from projects, JPNP16001 and JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

1. Okazaki et al., ACS Energy Lett., 2, 1460 (2017).
2. Yamamoto et al., ACS Appl. Energy Mater., 2, 6153 (2019).
3. Gschwind et al., J. Fluor. Chem., 182, 76 (2016).