Fe3O4 nanoparticles encapsulated in one-dimensional Li₄Ti₅O₁₂ nanomatrix: An extremely reversible anode for long life and high capacity Li-ion batteries

Highlights: • [email protected]₃O₄/Li₄Ti₅O₁₂ hybrid NFs showed extremely stable cyclic retention, which enabled to realize a full cell based on transition metal oxides anode for the first time. • Its mechanical flexibility, electric conductivity and thermal stability were surprisingly enhanced. • This enhancement was attributed to zero-strain feature of Li₄Ti₅O₁₂, and the evolution of Li₄Ti₅O_12-δ and metallic Fe. • This novel hybrid composites showed a synergistic effect thanks to high capacity of Fe₃O₄, high thermal and structural stability of Li₄Ti₅O₁₂, and the improved kinetics from dexterous oxidation state and morphology control. Transition metal oxides are very promising electrode materials for lithium-ion batteries that operate through conversion reactions. Energy densities for conversion reactions are higher than for intercalation reactions, but most of transition metal oxides show poor cycling performance and reversibility due to the pulverization of active materials and subsequent volume changes. We here report a facile and scalable synthesis for realizing Li₄Ti₅O₁₂-coated Fe/Fe₃O₄ hybrid nanocomposites in the form of one-dimensional nanofibers ([email protected]₃O₄/Li₄Ti₅O₁₂ hybrid NFs). This is a new class of highly-reversible and safe anode material that can significantly reduce the lithium-ion diffusion length and improve strain tolerance during Li ion insertion/extraction. Its oxidation state was also impressively controlled through a carbothermal reaction during annealing. The precise oxidation state control of [email protected]₃O₄/Li₄Ti₅O₁₂ hybrid NFs simultaneously enabled high capacity due to the conversion reaction of Fe₃O₄ as well as high reversibility and stability resulting from zero-strain characteristics and superb kinetics of Li₄Ti₅O₁₂. This new electrode material appears promising for not only future energy systems but also various electronic devices.