Graphene-encapsulated hollow Fe₃O₄ nanoparticle aggregates as a high-performance anode material for lithium ion batteries

Graphene-encapsulated ordered aggregates of Fe₃O₄ nanoparticles with nearly spherical geometry and hollow interior were synthesized by a simple self-assembly process. The open interior structure adapts well to the volume change in repetitive Li⁺ insertion and extraction reactions; and the encapsulating graphene connects the Fe ₃O₄ nanoparticles electrically. The structure and morphology of the graphene-Fe₃O₄ composite were confirmed by X-ray diffraction, scanning electron microscopy, and high-resolution transmission microscopy. The electrochemical performance of the composite for reversible Li⁺ storage was evaluated by cyclic voltammetry and constant current charging and discharging. The results showed a high and nearly unvarying specific capacity for 50 cycles. Furthermore, even after 90 cycles of charge and discharge at different current densities, about 92% of the initial capacity at 100 mA g^-1 was still recoverable, indicating excellent cycle stability. The graphene-Fe₃O₄ composite is therefore a capable Li⁺ host with high capacity that can be cycled at high rates with good cycle life. The unique combination of graphene encapsulation and a hollow porous structure definitely contributed to this versatile electrochemical performance.