Ultrafast Electron–Ion Transport in Micron-Scale Iron Difluoride Cathode by Self-Assembly of Nanoconfined Carbon Black

Conversion-type iron difluoride (FeF 2 ) has attracted considerable attention as a promising lithium-free cathode candidate for next-generation high-energy-density batteries owing to its high theoretical capacity and low cost. However, its practical application is severely limited by poor electronic/ionic conductivity, large volume expansion, and instability of the cathode electrolyte interphase (CEI). In this work, a FeF 2 /C composite with a unique dice-like micron-scale morphology was successfully synthesized via a simple solvothermal approach using ferrocene, ammonium fluoride, and nanoconfined Ketjen black (KB). Structural characterizations reveal that FeF 2 nanocrystals (∼15 nm) are uniformly encapsulated by a 3–5 nm amorphous carbon layer and self-assembled into micron-sized secondary particles. This hierarchical architecture integrates nanoscale fast electron/ion transport pathways with microscale structural integrity, significantly improving the reaction kinetics of the entire material. Benefiting from the nanocarbon confinement effect, the FeF 2 /C cathode shows a high reversible capacity of 229.5 mAh g –1, excellent rate capability, and outstanding cycling stability with 95% capacity retention after 200 cycles at 0.6 A g –1 . Electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) indicate that KB remarkably reduces interfacial charge-transfer and CEI resistances, bringing with it a 2.5 times enhancement in the Li + diffusion coefficient. Meanwhile, this structure effectively mitigates structural collapse and volume expansion (65.6 vs 287.1% for pristine FeF 2 ), ensuring robust mechanical stability during cycling. This work demonstrates that synergistic integration of micron-scale structural integrity and conductive carbon engineering provides an effective strategy for advancing a lithium-free high-energy FeF 2 -based cathode.