TY - JOUR TI - The solid state conversion reaction of cobalt oxide and iron fluoride thin films DO - https://doi.org/doi:10.7282/T3W0989X PY - 2016 AB - Iron (II) fluoride and cobalt (II) oxide are candidate electrode materials for a new class of rechargeable lithium ion batteries known as conversion batteries. Although the energy storage capacity of these materials represents a significant improvement over that of conventional electrode materials, many challenges must be overcome before conversion batteries can become commercially viable. Chief among these challenges is a loss of energy storage capacity as a function of the number of charge-discharge cycles. In this study, FeF2 and CoO thin films have been studied as solid state analogues for lithium ion battery electrodes. These films were grown with different crystalline orientations and exposed to lithium in an ultra high vacuum chamber in order to simulate the discharge of a conversion electrode. The electronic structure and chemical phase of the films before and after exposure to lithium were characterized using x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPS). The depth and homogeneity of the conversion reaction in the thin films was measured as a function of lithium exposure using angle-resolved XPS (ARXPS). The crystalline structure and morphology were studied using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). For polycrystalline FeF2 and CoO films, the products of the solid state conversion reactions were similar to those observed in electrochemical measurements. However, parasitic reaction pathways were identified for both reactions. The products of these reaction were found to inhibit the full conversion of the thin films, and are possibly responsible for the poor reaction kinetics in electrochemical cells. Furthermore, the diffusion of lithium into the FeF2 and CoO surfaces, and the concomitant conversion reaction, was found to depend strongly on the orientation of the surface. These differences in diffusivity could partially be explained through geometric analyses of the crystalline structure of the films. KW - Physics and Astronomy KW - Lithium KW - Thin films LA - eng ER -