Phase transition and targeted modulation mechanisms of layered cathodes for sodium-ion batteries

Na-based layered transition metal oxides (Na_xTMO₂) are considered as the promising cathodes for high-performance sodium-ion batteries (SIBs) because of the scalable synthesis and high theoretical capacity. Most attention thus far has been focused on activating anionic redox and addressing irreversible lattice oxygen loss to enhance the energy density of Na_xTMO₂. However, the poor cycling stability of Na_xTMO₂ remains a tough problem in commercial application, and the essential phase transition mechanism closely related to the cycling stability is also still being investigated and puzzling. Herein, we provide a thorough and comprehensive overview on the phase transition mechanisms from the perspectives of chemical potential, thermodynamic, kinetic and electrochemical driving force. Notably, the connection between the different driving force is then identified, which is the crux of clarifying phase transition mechanism. Moreover, the corresponding regulation strategies is also presented to guide the construction of high-performance Na_xTMO₂ cathodes. Finally, the future prospects and the challenges are proposed to navigate the practical application of SIBs.