Transition metal fluorides have recently gained interest as possible electrode materials in lithium ion conversion batteries. Owing to their large band gaps, they operate at high voltages and enable high energy densities. However this large band gap inhibits charge conduction and thus impedes efficient charge and discharge. One path to overcome this limitation is the use of metal oxyfluorides, which are characterized by a slightly smaller energy gap and thus a higher electronic conductivity. Currently, little is known about the electronic structure of metal oxyfluorides, particularly the relation between chemical structure, composition, and energy gap. Hence, we have produced model oxyfluoride systems in order to characterize the conversion mechanism using surface science tools.

Of all metal fluorides, iron-based compounds are the most promising to maximize energy density. Ultra-thin FeF₂ films have been synthesized via the fluorination of clean Fe foil exposed to XeF₂, following a self-limited Mott-Cabrera mechanism. The FeF₂ films have then been sequentially exposed to a partial pressure of O₂ of 2x10^-6 Torr at 285^oC in order to produce iron oxyfluoride. Using x-ray and ultraviolet photoemission as well as inverse photoemission, we have probed the electronic structure of these FeO_xF_y samples and characterized the occupied and unoccupied states near the band gap of the material.

It has been found that oxygen insertion into the FeF₂ matrix can be controlled until complete oxidation occurs. As expected for a Mott-Hubbard insulator, the valence band and conduction band of FeF₂ can be interpreted using a simple crystal field approach. In the case of Fe₂O₃, strong charge transfer effects need to be taken into account in order to interpret the band edges. To explore the conversion process, Li has been evaporated onto these iron oxifluorides in-situ. Preliminary results addressing the reactivity of lithium at the surface of these materials will also be presented.