Atmosphere and Phosphorus-Doping Effects on FeOOH-Derived Iron Oxides for Enhanced Lithium Storage Performance

Iron oxide-based anodes are promising for lithium-ion batteries due to their high theoretical capacity, yet their practical performance is often limited by structural instability and poor cycling efficiency. Here, a combined strategy of controlled thermal treatment and phosphorus doping is employed to regulate the phase composition and morphology of FeOOH-derived iron oxides. Annealing atmosphere significantly impacts particle evolution, with air promoting porous-to-hollow structures and nitrogen favoring dense morphologies. Phosphorus incorporation enables direct conversion to Fe₃O₄ and, at elevated levels, induces phosphate formation. Density functional theory calculations indicate that phosphorus doping improves electronic conductivity, increases Li⁺ insertion energy, and lowers diffusion barriers, facilitating faster lithium transport. Benefiting from these synergistic effects, the optimized P-doped Fe₃O₄ (P-Fe₃O₄) electrode delivers a high reversible capacity of ∼469.2 mAh g⁻¹ after 300 cycles at 0.2 A g⁻¹ and achieves an enhanced initial Coulombic efficiency of ∼85.3% compared to ∼58.8% for the pure FeOOH anode.