Porous Fe3O4-NCs-in-Carbon Nanofoils as High-Rate and High-Capacity Anode Materials for Lithium-Ion Batteries from Na-Citrate-Mediated Growth of Super-Thin Fe-Ethylene Glycolate Nanosheets.

Porous Fe3O4/C composite nanofoils, characterized by a thickness of ∼20 nm and with ∼8 nm open pores and ∼5 nm Fe3O4 nanoparticles embedded in the carbon matrix, were prepared for the first time using Na-citrate to mediate the growth of hexagonal Fe-ethylene glycolate nanosheets and subsequently annealing them at 350 °C in N2. It has been found that the Fe-ethylene glycolate nanosheets can be effectively slimmed by increasing the concentration of Na-citrate, and the microstructures of Fe3O4/C nanocomposites may be tailored by the annealing temperature. When tested as the anode materials in LIBs, the Fe3O4/C nanofoils obtained after annealing at 350 °C were found to exhibit superior electrochemical performance due to its optimal microstructure, featured by a reversible capacity of 1314.4 mAh g(-1) at 0.4 A g(-1) over 100 cycles, 1034.2 mAh g(-1) at 1 A g(-1), and 686.4 mAh g(-1) at 5 A g(-1) after 500 cycles, whereas the annealing treatments at 450 and 550 °C render the Fe3O4/C nanocomposites with the inferior electrochemical performances as a result of shrinking porous microstructures and coarsening of Fe3O4 nanoparticles in the carbon matrix. With a particle-size control model proposed herein, the cycle discharging behaviors of the Fe3O4/C nanocomposites with different microstructures are well explained from the perspective of the local confinement of Fe3O4 nanoparticles inside the carbon matrix and their evolution in size and composite microstructure during the charge/discharge cycling.