Fe₃O₄ nanoparticles confined in mesocellular carbon foam for high performance anode materials for lithium-ion batteries

Fe₃O₄ nanocrystals confined in mesocellular carbon foam (MSU-F-C) are synthesized by a "host-guest" approach and tested as an anode material for lithium-ion batteries (LIBs). Briefly, an iron oxide precursor, Fe(NO₃)₃·9H₂O, is impregnated in MSU-F-C having uniform cellular pores ∼30 nm in diameter, followed by heat-treatment at 400 °C for 4 h under Ar. Magnetite Fe₃O ₄ nanocrystals with sizes between 13-27 nm are then successfully fabricated inside the pores of the MSU-F-C, as confirmed by transmission electron microscopy (TEM), dark-field scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and nitrogen sorption isotherms. The presence of the carbon most likely allows for reduction of some of the Fe³⁺ ions to Fe²⁺ ions via a carbothermoreduction process. A Fe₃O₄/MSU-F-C nanocomposite with 45 wt% Fe₃O₄ exhibited a first charge capacity of 1007 mA h g^-1 (Li⁺ extraction) at 0.1 A g ^-1 (∼0.1 C rate) with 111% capacity retention at the 150 ^th cycle, and retained 37% capacity at 7 A g^-1 (∼7 C rate). Because the three dimensionally interconnected open pores are larger than the average nanosized Fe₃O₄ particles, the large volume expansion of Fe₃O₄ upon Li-insertion is easily accommodated inside the pores, resulting in excellent electrochemical performance as a LIB anode. Furthermore, when an ultrathin Al₂O ₃ layer (<4 Å) was deposited on the composite anode using atomic layer deposition (ALD), the durability, rate capability and undesirable side reactions are significantly improved. Fe₃O₄ nanocrystals confined in mesocellular carbon foam (MSU-F-C) exhibit capacities larger than 1000 mA h g^-1 at 0.1 A g^-1 with excellent cycling performance and rate capability as an anode material for lithium-ion batteries. The combined effect of nanosized Fe₃O₄, extra pore volume to accommodate volume expansion of Fe₃O₄, and large three dimensional interconnected mesoporous carbon pores with excellent electronic connectivity accounts for the excellent performance.