Impedance-Based Analysis of Overpotentials during Fluorination and Defluorination of Cu-Ni Alloys

Electrochemical impedance spectroscopy revealed that Cu electrodes exhibited lower low-frequency resistance and lower polarization than Ni electrodes during both fluorination and defluorination reactions. The impedance response of Cu exhibited a behavior indicative of a diffusion contribution, whereas no such behavior was observed for Ni. In Cu 0.72 Ni 0.28 films, diffusion-related impedance behavior also emerged in the CuF 2 /Cu plateau region. Furthermore, compared with pure Ni films, Cu 0.72 Ni 0.28 films exhibited lower low-frequency resistance during NiF 2 defluorination, resulting in a more positive onset potential for NiF 2 defluorination than that of Ni electrodes. These results suggest two possible mechanisms: (1) F vacancies, which are known to form readily in CuF 2 , may play an important role in promoting F transport, possibly through a mixed ionic/electronic conduction mechanism, thereby enabling ambipolar diffusion in CuF 2 and suppressing the increase in overpotential; and (2) defluorinated metallic Cu may facilitate electron transfer to adjacent NiF 2 , thereby reducing the overpotential for NiF 2 defluorination. Overall, the present findings highlight the potential importance of enhancing F transport through vacancy engineering to reduce low-frequency resistance, as well as introducing electron-conduction pathways into metal fluorides to improve reaction kinetics and rate capability in all-solid-state fluoride shuttle batteries.