Modern Li-ion intercalation batteries use electrode materials that accommodate Li intercalation, enabling the exchange of Li ions during battery cycling without extensive alteration of the electrode's crystalline structure. Despite the stability of these materials to the intercalation process, the ability of such batteries to store energy is limited by the number of host sites in the electrodes to approximately one electron per formula unit. Conversion reaction materials could potentially store several times more energy than current generation batteries by utilizing the full range of charge states available to their constituent metal ions. For example, the following reaction occurs in a FeF2 cathode:

Fe(2+)F2 + 2Li+ + 2e- --> Fe(0) + 2LiF.

Although conversion reaction materials have shown the promise of high energy storage density in electrochemical cells, their cycling stability is poor, and relatively little is known about the phase evolution and structural changes that occur during charge and discharge.

In order to study the fundamental properties of these materials, we have grown high purity thin films of conversion materials FeF2, FeF3, FeOF, FeOx, and CoO. By exposing these films to atomic Li in vacuum, can follow the evolution of these materials as they approach the reaction products reached in the discharge of a conversion battery. We have used UV and inverse photoemission to measure the electronic structure of the valence and conduction bands respectively. Using these techniques in tandem, we are able to measure the band gap of these materials, which can then be related to their electronic conductivity. Using x-ray photoemission, we have measured the stoichiometry and valence states of the compounds involved in these reactions. In addition, we have examined the structure of these nano-scale materials using TEM and LEED.

For FeF2, our XPS results show immediate reaction upon exposure to Li, fully reducing the Fe to the metallic state and forming LiF, with no evidence of intermediary phases in the film due to the high mobility of lithium. TEM of the initial and final films indicates a drastic morphology alteration, leading to a local precipitation of Fe0 and LiF formation, with an overall particle size reduction from 10nm to 2nm, consistent with what is found in electrochemical studies.

However, the Li-CoO and Li-FeOx reactions appear to diverge from the results of Li+ electrochemical reactions, leading to the simultaneous formation of both Li2O and Li2O2, the latter of which hinders further reduction of the Fe and Co. These observations will be contrasted with results obtained from the related conversion reaction compounds FeOxFy and FeF3.