Stabilizing P2-type layered oxides for sodium-ion batteries: Phase behaviour, interface engineering chemistry, and design strategies

The increasing demand for large-scale energy storage has intensified interest in sodium-ion batteries (SIBs) as cost-effective and sustainable alternatives to lithium-ion systems. P2-type layered transition metal oxides are among the most promising cathode materials due to their fast Na + diffusion kinetics, favourable rate capability, and structural tunability. However, their practical implementation is limited by challenges including structural instability, P2→O2 phase transitions, Jahn-Teller distortion, anionic redox-induced oxygen release, voltage hysteresis, and limited cycling stability. This review systematically examines the electrochemical performance of P2-type materials and critically analyses strategies to enhance their stability and functionality, including elemental doping, surface modification, nanostructure engineering, and electrolyte optimization. Special emphasis is placed on interfacial chemistry, including sacrificial additives, pre-sodiation techniques, in situ interface formation, and operando characterization tools to provide real-time insights into structural and interfacial evolution. Finally, practical applications, manufacturing considerations, and future perspectives are discussed, offering guidance for the rational design of durable, high-performance P2-type cathodes and the advancement of next-generation SIBs.