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Manganese-based layered oxides are prospective cathode materials for sodium-ion batteries (SIBs) due to their low cost and high theoretical capacities. The biphasic intergrowth structure of layered cathode materials is essential for improving the sodium storage performance, which is attributed to the synergistic effect between the two phases. However, the in-depth formation mechanism of biphasic intergrowth materials remains unclear. Herein, the layered/tunnel intergrowth Na0.6MnO2(LT-NaMO) as a model material was successfully prepared, and their formation processes and electrochemical performance were systematically investigated. In situ high-temperature X-ray diffraction displays the detailed evolution process and excellent thermal stability of the layered/tunnel intergrowth structure. Furthermore, severe structural strain and large lattice volume changes are significantly mitigated by the interlocking effect between the phase interfaces, which further enhances the structural stability of the cathode materials during the charging/discharging process. Consequently, the LT-NaMO cathode displays fast Na+transport kinetics with a remarkable capacity retention of ∼70.5% over 300 cycles at 5C, and its assembled full cell with hard carbon also exhibits high energy density. These findings highlight the superior electrochemical performance of intergrowth materials due to interlocking effects between layered and tunnel structures and also provide unique insights into the construction of intergrowth cathode materials for SIBs.

Because of the low price and abundant reserves of sodium compared with lithium, the research of sodium-ion batteries (SIBs) in the field of large-scale energy storage has returned to the research spotlight. Layered oxides distinguish themselves from the mains cathode materials of SIBs owing to their advantages such as high specific capacity, simple synthesis route, and environmental benignity. However, the commercial development of the layered oxides is limited by sluggish kinetics, complex phase transition and poor air stability. Based on the research ideas from macro- to micro-scale, this review systematically summarizes the current optimization strategies of sodium-ion layered oxide cathodes (SLOC) from different dimensions: microstructure design, local chemistry regulation and structural unit construction. In the dimension of microstructure design, the various structures such as the microspheres, nanoplates, nanowires and exposed active facets are prepared to improve the slow kinetics and electrochemical performance. Besides, from the view of local chemistry regulation by chemical element substitution, the intrinsic electron/ion properties of SLOC have been enhanced to strengthen the structural stability. Furthermore, the optimization idea of endeavors to regulate the physical and chemical properties of cathode materials essentially is put forward from the dimension of structural unit construction. The opinions and strategies proposed in this review will provide some inspirations for the design of new SLOC in the future.