Facile Synthesis of a Carbon Encapsulated Metal Oxide Nanocomposite and Its Application As Conversion Anode Material for Lithium Ion Batteries

Metal oxide has been reported to be an active conversion anode material in lithium-ion-batteries (LIBs) owing to its high specific capacity. The large volume change and the poor electronic conductivity during the Li⁺ insertion/extraction process limited its application, leading to capacity fade and poor cycling performance. Great efforts have been devoted on the combination of nanosized metal oxide with versatile carbon materials of special forms, such as carbon nanotubes, graphene, or fibers, in order to improve the cycling behavior. Most strategies to fabricate such functional composite structure involve multi-step processes or the use of certain templates, surfactants, or special solvents, removal of which in the post treatment complicates their practical applications. Thus, it is desirable to develop a facile straightforward synthetic strategy to generate such functional composite materials.

In our work, a simple pyrolysis-oxidation process has been developed to produce metal oxide/C-fiber composite in a solvent-free and template-free way. Metal oxide nanoparticles anchored onto in-situ formed carbon nanofibers were synthesized and have been investigated as promising conversion anode materials for Li-ion batteries. In the fabrication process, metallocene was used as precursor and acts as both metal and carbon source. As an example, Fe₃O₄ encapsulated into helical carbon nanofiber structure was fabricated by pyrolysis of ferrocene and subsequent oxidation of the pyrolysed product. The as-obtained Fe₃O₄/C composite shows a high and reversible capacity, good cycling and rate capability. The synthetic strategy has been further applied to a binary metal oxide CoFe₂O₄/C system through pyrolysis of the mixture of ferrocene and cobaltocene and further oxidation. The final composite shows a high and stable capacity. More than 700 mAhg^-1 based on the total mass of the as-prepared composite was obtained in the composite after long-term cycling of 250 cycles. The synergistic effects by combining metal oxide nanoparticles and carbon and also their intimate contact developed in the synthetic process are suggested to be the major contribution to the observed superior electrochemical properties. This work opens a facile and broadly applicable way for fabrication and utilization of metal oxide/mixed metal oxide-C composites as anode materials for Li-ion batteries.