Stabilizing the structure of P2-Na0.67Fe0.47Mn0.5Al0.03O2 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 identified as the most promising cathode materials for their application, with significant research focused on phases including 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 Na0.67Fe0.47Mn0.5Al0.03O2 P2-type cathode materials are synthesized in this study. It is noticed that Sb inclusion into the transition metal layer improves Na+ diffusion kinetics. The optimized material Na0.67Fe0.45Mn0.5Al0.03Sb0.02O2 (NFMAS2) exhibits 143.8 mAh g–1 at 0.2 C discharge capacity, between 1.5 and 4.3 V, and a rate capability of 58.6 mAh g–1 at 5 C. High-resolution transmission electron microscopy reveals remarkable structural stability 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 in the active material, superior electrochemical performance, and excellent capacity retention. This investigation highlights the effectiveness of doping and offers valuable insights into developing high-performance cathode materials for SIBs.