Recent progress on layered oxide cathode materials for sodium-ion batteries

Driven by the national strategic goal of “emission peak and carbon neutrality”, developing grid-scale energy storage systems (ESSs) for high-efficiency utilization of renewable clean energy is of great importance and urgency. Currently, lithium-ion batteries (LIBs) are being widely used in portable electronics and electric vehicles markets due to their high energy density and long cycling life. Nevertheless, the ever-increasing price and uneven distribution of lithium resources limit the further applications of LIBs for large-scale ESS. Recently, sodium-ion batteries (SIBs) have gained tremendous attention as promising large-scale energy storage devices and low-speed electric vehicle power sources, owing to the low-cost and abundant sodium reserves. However, the larger size and heavier mass of Na⁺ than those of Li⁺ commonly lead to sluggish reaction kinetics, severe volume expansion, and the undesirable structural failure of electrode materials upon charge/discharge, which hinder the commercial value of SIBs. Leveraging high-performance cathode materials is expected to boost the development of SIBs because cathodes largely determine the cost and electrochemical performance of batteries. Among the reported cathode candidates, layered oxide materials hold great potential due to their high capacity and a facile synthesis process; however, these materials face some challenges such as low capacity retention and poor air stability. Recently, exploring appropriate methods to strengthen the structural stability and further enhance the energy density of layered oxides has become an emerging research hotspot. In this regard, various strategies, such as element composition and relative content manipulation and microstructure and surface/interface modulation, have been proposed. In this review, typical modification methods for improving the Na-storage performance of layered oxide cathodes are comprehensively summarized. From the perspective of component design, the effects of different doping elements and doping sites on the capacity and cycling life are discussed. In addition, the basic principle of anionic redox reaction to offer extra capacity is elucidated, and the doping strategies for enhancing the anionic redox reversibility are outlined. From the perspective of structure design, the recent progresses on the preparation of composite phase materials and microstructures design are introduced. From the perspective of surface design, the functional mechanism of metal oxides and phosphates as coating layers to improve the structural stability and rate performance is explored. Finally, the challenging issues facing layered sodium oxide cathodes and possible remedies in the future are discussed. We believe that this review will shed light on the development of advanced layered oxide cathode materials for SIBs.