Nanoparticulate FeF₂@C as a Li Battery Conversion Cathode

The high theoretical capacity (571 mAh/g) and energy density (1519 Wh/kg) of iron difluoride (FeF2) make it a promising conversion cathode material for use in Li-based batteries, provided inherent limitations related to material conductivity and reactivity are surmountable. In this work, we report a simple synthesis to produce crystalline FeF2 particles approximately 35 nm in diameter surrounded by a thin carbon shell (FeF2@C) and demonstrate its excellent performance as a cathode in Li metal batteries. Characterization of the FeF2@C shows that the C-shell is 2–3 nm thick and composed of amorphous conjugated carbon with a nitrogen content of 3.8%, largely in the form of pyridinic moieties. When paired with a Li metal anode, the FeF2@C composite cathodes exhibit excellent specific capacity and retention, 634 mAh/gFeF2@C after 50 cycles at C/20, compared to 234 mAh/gFeF2 when a cathode containing commercial FeF2 was used. The material also shows excellent rate performance and, at a 1C charge/discharge rate, demonstrates a capacity greater than that of common intercalation cathodes like LiFePO4. We attribute the performance of the FeF2@C to improved lithiation/delithiation behavior due to the nanoscale FeF2 particles, increased protection from chemical and electrochemical damage, improved conductivity and capacity granted by the C-shell, and additional capacity from the in situ formation of FeF3 during cycling. After electrochemical cycling, ex situ analysis of the FeF2@C material shows that while a roughly 2–8 nm cathode electrolyte interphase (CEI) forms on the surface of the particles, the underlying material retains its initial nanostructure and FeF2-characteristics.