Expediting layered oxide cathodes based on electronic structure engineering for sodium-ion batteries: Reversible phase transformation, abnormal structural regulation, and stable anionic redox

With the growing demand for energy storage, layered oxide cathodes (NaxTMO2) for sodium-ion batteries (SIBs) have become the spotlight for researchers. However, irreversible multiphase transformation and structural degradation, as well as lattice oxygen loss, hindered their commercialization. Electronic structure modulation based on the orbital hybridization concept is an important way to solve key scientific problems. Herein, due to its unique electronic structure, Sn is chosen as the proof of the conceptual element, and its effect on layered oxide cathode is summarized in three aspects: reversible phase transformation, abnormal structural regulation, and stable anionic redox. Firstly, the large size of Sn4+ suppresses the sliding of the transition metal oxide (TMO2) layer and Na+/vacancy ordering as well as enhances the delocalization of electrons. Secondly, Sn with a similar ionic radius to other TM ions in the structure promotes the stacking of the O3 phase. What's more, the distinctive electronic structure of Sn4+ will enhance the operating voltage. Thirdly, a strong Sn-O bond stabilizes the lattice oxygen, promotes stable anion redox, and improves the energy density of the battery. Therefore, electronic structure modulation can provide technical direction for the development and industrialization of highperformance SIBs.