FeF₂ is a promising earth-abundant conversion cathode material for use in Li metal batteries, due in part to its large theoretical capacity (571 mAh/g) and high theoretical discharge voltage (2.66 V). Unfortunately, FeF₂ cathodes frequently suffer from poor (electro)chemical stability, slow lithiation/de-lithiation kinetics, and poor electrical conductivity that has thus far limited their application in batteries. To address these problems, we report the use of microwave synthesis to produce a core@shell FeF₂@C composite consisting of approximately 35 nm FeF₂ particles surrounded by a thin (roughly 2-3 nm) thick shell of semi-amorphous N-doped carbon. The small size of the FeF₂ nanoparticles improves the lithiation kinetics of the material, while the carbon shell improves the conductivity and (electro)chemical stability of the underlying FeF₂. Together, this allows the composite to cycle within 10% of theoretical capacity at a rate of C/20 for 50 cycles and retain this stability over 200 cycles, even when cycled at C/2. Electrochemical characterization of the composite using cyclic voltammetry (CV) shows little impact from the C-shell on the redox behavior of FeF₂, while electrochemical impedance spectroscopy (EIS) shows that the composite exhibits both low charge transfer resistance and low resistance across the carbon shell. Post-cycling characterization using scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) shows that the composite retains both its small particle size and much of its FeF₂-like character, with a Li₂O-rich cathode electrolyte interphase (CEI) forming atop the carbon shell. In combination, these properties likely contribute significantly to the observed cycling stability. Altogether, our results indicate that core@shell composites may allow for high capacity, high stability FeF₂ cathodes for use in future Li metal batteries.

Igor Kolesnichenko

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