Structure of hollow iron oxide nanoparticles modified with Mo addition studied by XAFS and SAXS

Full text: The large capacity resulting from reducing iron oxide to iron is promising for lithium battery anodes; however, this conversion during the electrochemical cycling is accompanied by fading of the electrode performance due to material pulverization and subsequent loss of electrical contacts between the active material and current collector. On the other hand, cycling of iron oxide-based electrodes in the high voltage range where only intercalation of Li ions occurs has been recently found to be promising to design cathodes. The unique nature of hollow metal oxide nanoparticles (NPs) such as a thin shell, large internal void, and doubled surface area and cation vacancies has raised a lot of interest for their applications in energy storage. Recently Shevchenko et al. reported the role of the high concentration of cation vacancies in hollow iron oxide NPs in lithium intercalation reactions. This report explains significantly higher capacities observed in the case of hollow NPs as compared with their bulk analogues and emphasize the importance of morphology of NPs for targeted applications. Very recently it was found that Mo-substituted γ -Fe₂O₃ also enhanced lithium ion battery performance by generating additional cation vacancies[1]. These studies motivated to analyze the structure and morphology of hollow iron oxide NPs similar to the ones reported by Shevchenko et al. modified by the addition of Mo for lithium ion storage. In this work we report the analysis of samples before and after annealing process and after Li cycling by transmission electron microscopy (TEM), small angle X-ray scattering (SAXS) and X-ray absorption fine structure (XAFS). SAXS experiments were performed in order to follow changes in morphology and NPs arrange with the Mo addition and after Li intercalation process. XAFS experiments at the Fe K edge and Mo L23 edges were performed in order to understand the origin of the change in Fe vacancies with Mo addition. (author)