Review—Research Progress on P2/O3-Composite Layer Metal Oxide Cathode Materials for Sodium-Ion Batteries

The high cost and uneven distribution of lithium resources have prompted searches for alternatives to lithium-ion batteries. Among various alternatives, the sodium layered oxide cathode materials, have shown significant research potential due to their low cost. Layered oxide materials can be categorized into sodium-rich O3 types and sodium-deficient P2 types, which have different structural features. O3 type materials offer high specific capacities but suffer from complex pathways for Na⁺ de-intercalation, slow Na⁺ diffusion, and poor air stability. P2 type materials are limited in full cell applications due to their lower practical specific capacities. Therefore, researchers conceived the idea of combining the advantages of both to construct P2/O3 composite structure cathode materials (CSMs), utilizing the synergistic effects of the CSMs to overcome the limitations of single structure material, and successfully synthesized CSMs with appropriate specific capacities. These materials effectively suppress unfavorable phase transitions and enhance Na⁺ diffusion coefficient, thereby improving electrochemical performance. This paper reviews the recent advancements in CSMs for sodium-ion batteries, highlighting synthesis strategies that incorporate “cationic potential” theory, element substitution, sodium content adjustment, and control of calcination processes to synthesize diverse CSMs.