Microstructure and electronic band structure of pulsed laser deposited iron fluoride thin film for battery electrodes

Battery electrodes in thin-film form are free of the binders used with traditional powder electrodes and present an ideal platform to obtain basic insight to the evolution of the electrode-electrolyte interface passivation layer, the formation of secondary phases, and the structural underpinnings of reversibility. This is particularly relevant to the not yet fully understood conversion electrode materials, which possess enormous potential for providing transformative capacity improvements in next-generation lithium-ion batteries. However, this necessitates an understanding of the electronic charge transport properties and band structure of the thin films. This work presents an investigation of the electron transport properties of iron fluoride (FeF ₂) thin-film electrodes for Li-ion batteries. FeF₂ thin films were prepared by pulsed-laser deposition, and their phase purity was characterized by electron microscopy and diffraction. The grown materials are polycrystalline FeF₂ with a P4₂/mnm crystallographic symmetry. Room-temperature Hall measurements reveal that as-deposited FeF ₂ is n-type: the Hall coefficients were negative, electron mobility was 0.33 cm²/(V s) and resistivity was 0.255 Ω cm. The electronic band diagram of FeF₂ was obtained using a combination of ultraviolet photoelectron spectroscopy, photoluminescence, photoluminescence excitation and optical absorption, which revealed that FeF₂ is a direct bandgap, n-type semiconductor whose band structure is characterized by a 3.4 eV bandgap, a workfunction of ∼4.51 eV, and an effective Fermi level that resides approximately 0.22 eV below the conduction band edge. We propose that the shallow donor levels at 0.22 eV are responsible for the measured n-type conductivity. The band diagram was used to understand electron transport in FeF₂ thin film and FeF₂-C composite electrodes.