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.

Microwave synthesis of FeF2@C composites as conversion cathodes for li metal batteries

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.