New concepts for electrochemical energy storage devices are required to handle the physicochemical energy density limit that Li-ion batteries are approaching. All-solid-state fluoride-ion batteries (FIBs), in which monovalent fluoride anions are employed as charge carriers, are regarded as attractive options, and metallic Cu has been proved to be a promising cathode material. However, the rate capability is currently low and kinetic factors associated with the Cu/CuF2 reaction are not clearly understood, and the rate-determining step has not yet been identified. Herein, we present the kinetic analyses of a Cu thin-film cathode with a phase-boundary-controlled one-dimensional phase transition process via the Kolmogorov–Johnson–Mehl–Avrami equation. Concerning the capacity fading caused by the repeated volume expansion/contraction and the consequent interfacial contact loss, a Cu–Au alloy with a reduced lattice mismatch was designed and verified to be efficient to enable fast phase-transition kinetics along with stable cyclabilities, which opens new possibilities in cathode design for all-solid-state FIBs.