Size Tailoring of CuF2 Nanoparticles for Enhanced Conversion Cathode Performance in Sodium-Ion Batteries

Conversion-type cathodes, exemplified by CuF2, have garnered significant attention for next-generation sodium-ion batteries (SIBs) due to their superior theoretical capacities compared to conventional intercalation materials. However, research on CuF2 has predominantly focused on conductive matrix composites, leaving the critical role of particle size in electrochemical performance largely unexplored. Herein, a precision-engineered synthesis strategy integrating room-temperature coprecipitation with low-temperature thermal treatment was developed to fabricate CuF2 nanoparticles with minimized agglomeration. By systematically balancing temperature-induced agglomeration against degassing-driven antiagglomeration effects, CuF2 nanoparticles (CuF2-270) with an average size of 89.96 nm were successfully synthesized. Electrochemical characterization revealed that CuF2-270 delivered an initial discharge capacity of 334.6 mAh g–1 at 0.2 C, an 88.4% improvement over CuF2-285 (microscale), along with a significantly reduced charge transfer resistance and enhanced sodium-ion diffusion kinetics. However, capacity decay remained a challenge during long-term cycling. In situ FTIR analysis tracked the dynamic evolution of the electrode surface composition in real time during the initial charge–discharge cycle. Concurrently, the generation and dissolution of Cu were characterized via TEM and ICP-OES postcycling. These findings reveal that the rapid capacity decay of the CuF2 cathode is primarily driven by Cu dissolution and SEI film thickening induced by severe parasitic reactions. This study provides insights into the optimization of CuF2 nanoparticle synthesis and the failure mechanisms of conversion-type cathodes in sodium-ion batteries.