Phase‐Field Simulation for the Microstructural Evolution of CuPb ₂ F ₆ During Defluorination

The microstructural evolution of CuPb 2 F 6 , a cathode material for fluoride‐ion batteries, during defluorination was investigated using phase‐field simulation coupled with a CALPHAD‐based thermodynamic database. By employing a nonstoichiometric face‐centered‐cubic interstitial solid solution (FCC‐int) description, the simulations capture the nucleation and growth of Cu nanoparticles and reproduce the experimentally observed ∼ 100 nm grain size by tuning the effective nucleation density. A strong correlation among discharge rate, nucleation density, and fluorine diffusion coefficient is revealed, demonstrating that faster discharge rates and higher nucleation densities lead to finer Cu nanoparticles. These trends are consistent with electrochemical measurements conducted at different current densities, indicating that Cu grain size is governed by the concentration gradient within the FCC‐int matrix, which controls nucleation frequency and early‐stage growth. This work elucidates the defluorination mechanism of CuPb 2 F 6 and provides a predictive framework for controlling microstructure evolution, offering design guidelines for next‐generation fluoride‐ion batteries.