Beyond Yolk–Shell Nanoparticles: Fe₃O₄@Fe₃C Core@Shell Nanoparticles as Yolks and Carbon Nanospindles as Shells for Efficient Lithium Ion Storage

To well address the problems of large volume change and dissolution of Fe₃O₄ nanomaterials during Li⁺ intercalation/extraction, herein we demonstrate a one-step <i>in situ</i> nanospace-confined pyrolysis strategy for robust yolk–shell nanospindles with very sufficient internal void space (VSIVS) for high-rate and long-term lithium ion batteries (LIBs), in which an Fe₃O₄@Fe₃C core@shell nanoparticle is well confined in the compartment of a hollow carbon nanospindle. This particular structure can not only introduce VSIVS to accommodate volume change of Fe₃O₄ but also afford a dual shell of Fe₃C and carbon to restrict Fe₃O₄ dissolution, thus providing dual roles for greatly improving the capacity retention. As a consequence, Fe₃O₄@Fe₃C–C yolk–shell nanospindles deliver a high reversible capacity of 1128.3 mAh g^–1 at even 500 mA g^–1, excellent high rate capacity (604.8 mAh g^–1 at 2000 mA g^–1), and prolonged cycling life (maintaining 1120.2 mAh g^–1 at 500 mA g^–1 for 100 cycles) for LIBs, which are much better than those of Fe₃O₄@C core@shell nanospindles and Fe₃O₄ nanoparticles. The present Fe₃O₄@Fe₃C–C yolk–shell nanospindles are the most efficient Fe₃O₄-based anode materials ever reported for LIBs