Stabilizing the structure of P2-Na_0.67Fe_0.47Mn_0.5Al_0.03O₂ cathodes through Sb-substitution for sodium-ion batteries

Rechargeable sodium-ion batteries (SIBs) are emerging as a viable alternative to lithium-ion batteries, offering comparable electrochemical performance while benefiting from the higher abundance of sodium. Layered oxides are widely regarded as the most promising cathode materials, with significant research focused on phases such as P2- and O3-type materials. However, these layered oxide cathode materials undergo severe structural degradation during cycling, leading to rapid capacity decay. To mitigate this structural instability, Sb-doped Na_0.67Fe_0.47Mn_0.5Al_0.03O₂ P2-type cathode materials are synthesized in this study. It is observed that Sb inclusion in the transition-metal layer improves Na⁺ diffusion kinetics. The optimized material Na_0.67Fe_0.45Mn_0.5Al_0.03Sb_0.02O₂ (NFMAS₂) exhibits 143.8 mAh g⁻¹ at 0.2C discharge capacity, between 1.5 and 4.3 V, and a rate capability of 58.6 mAh g⁻¹ at 5C. High-resolution transmission electron microscopy reveals remarkable structural reversibility for NFMAS₂, confirming that the microstructure remains intact after cycling. In addition, the P2 to O2 reversible phase shift in the sodium de-/intercalation mechanism of NFMAS₂ was studied using in-situ X-ray diffraction, elucidating the origin of the structural stability of the active material, its superior electrochemical performance, and excellent capacity retention. This investigation highlights the effectiveness of doping and provides valuable insights into the development of high-performance cathode materials for SIBs.