Graphene/carbon-coated Fe3O4 nanoparticle hybrids for enhanced lithium storage

A novel hierarchical nanostructure composed of carbon coated Fe3O4 nanoparticles with seed-like morphology distributed on graphene (denoted as G/Fe3O4@C) is prepared as a high-capacity anode electrode for LIBs. beta-FeOOH nanoseeds were first assembled on graphene by solvothermal treatment, followed by coating b-FeOOH nanoseeds with polydopamine via immersion in dopamine aqueous solution. Finally, G/Fe3O4@C is obtained after in situ phase transformation of b-FeOOH into Fe3O4 and simultaneous carbonization of the polydopamine nanocoating through thermal annealing at 500 degrees C. The thickness of the uniform and continuous carbon layer can be easily tailored by varying the polymerization time and the concentration of dopamine to balance the concurrent needs for high active material content and structural stability. The carbon layer can effectively prevent the agglomeration of Fe3O4 nanoparticles, which enables the reversible conversion reaction between Fe3O4 and lithium, and significantly improves the mechanical stability of electrodes by accommodating volume expansion of Fe3O4 nanoparticles during the electrochemical cycling. Meanwhile, the combination of graphene and the carbon shell improves the electrochemical reaction kinetics of the electrode. As a result, the obtained G/Fe3O4@C nanocomposites with the optimal carbon shell thickness of about 1.2 nm exhibit high reversible capacities with remarkable cyclic retention at different current rates (1344 mA h g(-1) after cycling at 0.5 C for 200 cycles, 743 mA h g(-1) after further cycling at 2 C for another 200 cycles) and excellent rate performance (150 mA h g(-1) at 20 C) as anodes in lithium ion batteries.