Lanthanum-Induced Phase Engineering Enables Enhanced Structural Stability and Fast Na⁺ Transport in P2/O3 Layered Cathodes for Sodium-Ion Batteries

Fe/Mn-based P2-type layered oxides are promising cathodes for sodium-ion batteries (SIBs), yet suffer from poor cycling stability due to Jahn–Teller distortion of Mn³⁺ and irreversible P2–Z phase transitions. In this study, a multi-element doping strategy involving Cu, Mg, and La is employed to construct a structurally stable P2/O3 composite, Na_0.67Fe_0.19Mn_0.5Cu_0.2Mg_0.1La_0.01O₂ (FMCML), via a sol–gel method and high-temperature calcination. The synergistic doping suppresses Mn³⁺ redox activity, mitigates lattice distortion, and enhances Na⁺ diffusion kinetics. Notably, La³⁺ incorporation induces LaO₆ octahedral formation, which refines particle size and boosts crystallinity. As a result, FMCML exhibits outstanding rate performance and long-term durability, with capacity retentions of 99.11%, 87.21%, and 74.92% after 100, 300, and 1600 cycles at 200, 1000, and 2000 mA·g⁻¹, respectively. The dual-phase structure promotes structural reversibility, suppresses Na⁺/vacancy ordering, and enhances ambient air stability. This work offers a practical approach to designing robust layered oxide cathodes through rational phase and composition engineering, paving the way for next-generation high-performance SIBs.