Controlled synthesis of α-FeOOH nanorods and their transformation to mesoporous α-Fe₂O₃, Fe₃O₄@C nanorods as anodes for lithium ion batteries

α-FeOOH nanorods with varying lengths of 200-500 nm and axis ratios of 2-8 have been successfully synthesized by a simple hydrothermal method with different amounts of hydrazine. Hierarchical nanostructured mesoporous α-Fe₂O₃ and Fe₃O₄@C, which combined both mesoporosity and carbon coating, were fabricated by transformation of these α-FeOOH nanorods. The mechanism of formation has been described in detail. The α-FeOOH, mesoporous α-Fe₂O₃ and Fe₃O₄@C samples with the longest nanorods exhibited excellent cycle and rate performance, as the long nanorods could ensure many fast and convenient electron transport pathways, thus enhancing the electronic conductivity. The as-synthesized mesoporous α-Fe₂O₃ and Fe₃O₄@C nanorods showed large specific surface area and porosity due to the inner mesoporous structure, effectively increasing the contact area between the electrode materials and electrolyte, shortening the diffusion length of Li⁺ and alleviating the stress from volume changes during the charge-discharge process. Mesoporous Fe₃O ₄@C nanorods exhibit high reversible capacities of 1072 mAh g ^-1 after 50 cycles, demonstrating the superior electron transport and fast Li⁺ diffusion ability combination of outer carbon layer and mesoporous microstructure.