Fluorination of single-crystal copper electrodes in fluoride-ion batteries

Abstract

Fluoride-ion batteries are promising candidates for next-generation energy storage systems owing to their high theoretical energy density. However, several fundamental challenges hinder their practical implementation, including the need for optimized active material design. Addressing these challenges requires a detailed understanding of the fluorination process in metal electrodes. In this study, we employed single-crystal copper (Cu) as the model electrode to investigate the formation behavior of copper fluoride (CuF₂) under well-defined crystallographic conditions and to evaluate the effect of the surface oxide layer (Cu₂O) using scanning transmission electron microscopy. By analyzing this well-defined single-crystal model system, we have gained the first comprehensive insights into the multi-scale fluorination behavior of Cu electrodes. Fluoride ions migrated through the Cu₂O layer, enabling the fluorination reaction to proceed. Despite the significant lattice mismatch between Cu and CuF₂, atomic-resolution analysis of the Cu/CuF₂ interface revealed no intermediate phases or misfit dislocations. Although a single-crystal Cu electrode was used, the resulting CuF₂ grains exhibited random crystallographic orientations. Additionally, within the CuF₂ region, residual metallic Cu was detected at grain boundaries, and specific orientation relationships between adjacent CuF₂ grains were observed. These observations highlight the complex structural evolution that occurs during the fluorination of metal electrodes and underscore the importance of local crystallographic interactions in accommodating the large strain associated with conversion reactions. Understanding these structural features provides valuable insights for the design of more efficient electrode materials for fluoride-ion batteries.