Investigation on transition metal oxide cathodes for sodium-ion batteries

The ever-increasing demand for sustainable energy resources has driven the development of large-scale electrochemical energy storage systems (EESs). Among the various alternative EESs, room-temperature sodium-ion batteries (SIBs) have drawn considerable attention owing to their similar electrochemical storage mechanism to lithium-ion batteries and the much larger abundance of Na resources. Nevertheless, the development of SIBs has been restrained by the inadequate advancement of cathode host materials for SIBs, as the cathode plays a critical role in determining the electrochemical performance of the battery. An advantageous cathode material should exhibit high Na-storage capacity, excellent rate capability, and a stable cycling lifespan. Among the leading cathode candidates, transition metal oxide cathodes have aroused much interest over recent years due to their large specific capacity, high ionic conductivity, environmental benignity, and feasible synthesis. However, most transition metal oxides suffer from structural instability during Na-insertion/extraction, which usually leads to complicated multiphase transitions and severe structural degradation of the host materials during charge/discharge processes. Therefore, rationally manipulating their structural evolution to inhibit unfavourable phase transition as well as constructing beneficial phase-interfaces could favourable for building stable host materials for Na+ storage. In this doctoral work, multiphase intergrowth structures have been constructed, in which the synergistic effects dependent on the different phases could contribute to outstanding electrochemical capability. Specifically, the multiphase structural evolution and phase transitions during the Na-insertion/extraction, as well as the sodium storage mechanism, are clearly articulated and confirmed, via combined analyses including scanning transmission electron microscopy (STEM) with atomic resolution, in-situ/ex-situ synchrotron-based X-ray absorption spectroscopy (XAS) and in-situ synchrotron-based X-ray diffraction (XRD), which provide an in-depth understanding of the structure-performance correlations in multi-phase structures and opens up a novel field via manipulating the structural evolution for the design of high-performance SIB cathodes. Moreover, in order to further enhance the energy density of the transition metal oxide cathode, anionic redox reactions based on oxygen charge compensation have been evoked and investigated