Synergistic Impact of Alloying with Ni on Cu Cathode Interfaces for Fluoride Batteries

All-solid-state fluoride batteries have the potential to achieve energy densities significantly higher than those of lithium-ion batteries. A common cathode material for fluoride batteries is Cu. Cu has a low polarization, but its rapid capacity degradation due to grain growth and subsequent delamination from the solid-state electrolyte are critical issues. To enhance the performance of Cu-based cathodes in all-solid-state fluoride batteries, we explore alloying of Cu with Ni to create metastable solid solution phases (CuNi_1- with x = 0, 0.32, 0.52, 0.72, 0.89, and 1.0). Compared to Cu, Ni has a higher polarization but exhibits superior capacity retention. The Cu_0.72Ni_0.28 alloy demonstrates a polarization as low as Cu, but it has a significantly improved capacity retention, which is comparable to Ni. Transmission electron microscopy observations demonstrate that the thin Ni-rich region formed near the interface inhibits Cu grain growth and delamination from the LaF₃ electrolyte. By incorporating an appropriate amount of Ni into Cu, Cu-Ni alloy films combine the advantages of both metals, improving the performance of fluoride batteries.