Understanding the Effect of Crystalline Structure and Atomic Arrangement in Metal Oxide Electrodes for Sodium Ion Batteries

This dissertation investigates the fundamental understanding in the influences of order-disorder and atomic arrangement on electrochemical properties of electrode materials for sodium ion batteries (SIBs). In specific, TiO2 anode and NaNixFeyMnxO2 cathode materials are studied. Due to their low cost and relatively high abundance of raw materials SIBs are attractive for large-scale energy storage systems for high round trip efficiency and long cycle life. Recent studies suggest that various polymorphs of TiO2 are suitable as anode material. However, the impact of crystallinity on the electrochemical properties of the material has not been explored. Meanwhile, the NaNixFeyMnxO2 cathode exhibits promising performance but detrimental irreversibility at high voltages as well as poor cycling stability and rate capability remain issues for its practical application. This dissertation presents the study which suggests that the increase of crystallinity in anatase TiO2 nanoparticle electrode leads to better electrochemical performance in terms of Coulombic efficiency, rate capability and cycle life. To understand the discrepancy in performance, various structural and electrochemical characterizations are conducted to explore the Na ion diffusion process and the local structural evolution of the material. Metal oxide cathode is also investigated. The atomic rearrangement of transition metals in NaNixFeyMnxO2 at high voltages attributes to irreversibility and instability. It becomes significant with the increase of either Fe composition or upper cut-off operation voltage. X-ray spectroscopy is used to investigate the oxidation state and local environment of the transition metals. The result suggests that vi the redox and bonding activity of Ni-O mainly attributes to irreversibility and instability. In addition, intergrown phases have been shown to improve structural stability and Na mobility in layered cathode materials. A new Li-substituted NaNixFeyMnxO2 intergrowth cathode is designed and synthesized. The improvement in stability and rate capability of the new intergrowth cathode is investigated, which is associated with the mixed layered-spinel phase that possibly offers improved ion diffusion and stability through direct channels between the 2D layered and 3D spinel component