Research progress on the mechanism of oxygen redox in sodium cathodes

<p indent="0mm">Sodium-ion batteries, as an emerging secondary battery technology, have rapidly gained prominence in the electrochemical energy storage device market in recent years, largely due to their distinctive cost advantages and the abundant availability of sodium resources. These factors make sodium-ion batteries highly promising for large-scale applications, particularly in the energy storage sector, where cost and resource considerations are crucial. Among the various cathode materials for sodium-ion batteries, layered transition metal oxides have emerged as one of the most promising options. These materials have demonstrated significant achievements in laboratory research and have begun to show potential for commercialization. However, layered sodium transition metal oxides still face several challenges in practical applications, with the most notable bottleneck being the complex phase transition processes and their limited energy density, which hinder their performance and longevity. In the field of lithium-ion battery research, the development of lithium-rich manganese-based cathode materials has provided valuable insights for advancements in battery technology. Specifically, the involvement of lattice oxygen in redox reactions for charge compensation can significantly enhance the capacity and discharge voltage of electrodes. This mechanism not only lays the foundation for increasing the energy density of lithium-ion batteries but also offers new opportunities for the development of high-energy-density sodium-ion batteries. Compared to lithium ions, sodium ions have a larger ionic radius, leading to more diverse and complex structures and elemental compositions in sodium-layered oxides. For instance, sodium-deficient P2/P3 phase materials also exhibit oxygen redox behavior, indicating that the mechanism triggering oxygen redox is no longer limited to lithium elements but can also be achieved using more economically viable elements such as magnesium, zinc, aluminum, and iron. This broadens the possibilities for sodium-ion batteries in terms of cost control and performance enhancement. Nevertheless, while the lattice oxygen redox mechanism offers significant advantages in terms of increased energy density, it also introduces a series of potential issues. These challenges include irreversible lattice oxygen loss, complex structural changes, and ultimately, capacity loss, voltage decay, and voltage hysteresis. These phenomena severely impact the energy storage and conversion efficiency of the batteries, thereby limiting the widespread application of these materials in practice. Therefore, in-depth research on the oxygen redox mechanism in sodium-layered oxide materials is essential not only for understanding its fundamental principles but also for addressing the challenges encountered in practical applications. This review aims to systematically summarize the overall research progress on the oxygen redox mechanism, tracing developments from lithium-rich manganese-based materials to sodium-layered oxides. The research experiences gained from Li-rich cathode materials are also introduced to provide a comprehensive overview of the anionic redox mechanism’s research history. Investigations into sodium-based materials have contributed new perspectives, deepening the understanding of anionic redox fundamentals. Typical oxygen redox-based sodium cathodes are classified by their structure and elemental composition. We also focus on discussing the key issues and challenges faced by these cathode materials in practical applications. Through the analysis and comparison of existing studies, this paper seeks to provide a reference for the future development of sodium-ion battery cathode materials and proposes potential solutions for enhancing their energy density and cycle stability, ensuring their viability in real-world applications.