Conversion reaction of FeF ₂ thin films upon exposure to atomic lithium

The chemical and structural phase evolution of ultrathin (∼5 nm) FeF ₂ films upon deposition of atomic lithium in an ultrahigh vacuum environment has been studied using X-ray and UV photoemission spectroscopies, inverse photoemission spectroscopy, and transmission electron microscopy in an effort to explore the fundamental properties of the conversion reaction of this promising Li battery cathode material. Spectroscopic measurements show reduction of FeF ₂ into a metallic Fe ⁰ phase and a LiF phase upon Li deposition. No other phases are detected. Transmission electron microscopy reveals extensive changes in the film's morphology and material reorganization upon full lithiation. The initial FeF ₂ film, with grains on the order of 10 nm in diameter, phase separates into smaller (∼3 nm) interconnected Fe ⁰ regions surrounded by LiF. This structural modification is attributed to the large Li ⁺ ionic mobility with respect to Fe ²⁺. The intrinsic nanoscale texture of the final phases is believed to aid in accommodating the extensive structural transformations that occur in this conversion reaction material during an electrochemical cycle in battery applications.