Studies On Preparation Of Fe₃O₄/Graphene Electrode Material And Its Electrochemical Properties

The lithium ion secondary battery with the advantages of high specific energy,high operating voltage, good cycling stability, safety and low pollution etc, has beenone of the most important high-energy batteries and developed rapidly. Carbon，as thecommercial lithium-ion battery（LIB） anode material， cannot have met the need oflithium-ion battery applications in high-power devices due to its low specificcapacities. Therefore, it needs urgently to develop new anode materials for LIB. Ironoxide has been a promising anode material for the LIBs owing to its high theoreticalspecific capacity， abundance， low cost and friendly environment. However，thestudies show that Fe₃O₄has the disadvantage of poor cycle stability and rate property.In this paper, in the basis of exploring the technique conditions of synthesizing Fe₃O₄and graphene in the aqueous solution， respectively， Fe₃O₄nanorods/graphenenanocomposites were prepared via in situ self-assembly, Fe₃O₄/C/r-GO compositeswere also synthesized by one-step strategy in hydrothermal conditions and theirelectrochemical properties were analyzed.Pure granular Fe₃O₄nanoparticles could be synthesized with ferrous salts as reactantsand hydrazine as oxidant at pH>10for12h in aqueous solution. The reaction rate wasdifferent with various ferrous salts. The discharge/charge specific capacities were1060and752mAh/g， respectively. The first Columbic efficiency was71%. But it’scycling performance and rate capacity properties were poor.Most of graphite was oxidized and some hydroxyl groups, carboxyl groups andepoxy groups formed when concentrated sulfuric acid intercalation for1h，oxidationreaction with potassium permanganate for2.5h and high temperature reaction for0.25h. The hydrazine could reduce the oxide graphite to graphene. It could be foundthat oxygen-containing functional groups except some epoxide groups of oxidegraphite were reduced and conjugated region were formed at pH>10. PH value,reaction time and heat-treatment temperature had important role in the reducingdegree of oxide graphite. However, epoxy groups could not be reduced completel ywith hydrazine as reducer with changing the reaction conditions. Graphene sheetswere arranged disorderly and curled randomly. Some accumulations occurred amonggraphene sheets. Graphene has mesoporous structure of3-4nm，specific surface areaof373.42m²/g, the total pore volume of0.47mL/g. Fe₃O₄nanorods graphene composites （FNGC） have been successfully preparedwith graphite oxide and （NH4）2Fe（SO4）2in water as reactants at pH>11for12h.Fe₃O₄was inverse spinel magnetite crystal phase. Some conjugated region and someepoxy groups existed in FNGC. The as-formed Fe₃O₄nanorods, about11nm indiameter and more than100nm in length， were uniformly anchored on curledgraphene nanosheets. The distance between graphene nanosheets was about0.41nm.The specific surface area of180m²/g, a total pore volume of0.26mL/g andmesoporous structure existed in FNGC. Through studying the effect of pH, agitationspeed, temperature and reaction time on the morphologies of the products, theformation mechanism of FNGC was supposed.The first discharge/charge specific capacities were1538and925mAh/g，respectively. The Columbic efficiency was60.2%. After80cycles at1C, thedischarge/charge specific capacities sustainities was80.5%and90%, respectively. Atcurrent densities of5C, the sustainities of discharge and charge capacity remain at66.78%and63.80%. The FNGC exhibited improved electrochemical properties incomparison with Fe₃O₄nanoparticles/graphene composites. Through studying the ironoxide morphology, GNS content and the heat treatment temperature on theelectrochemical properties of FNGC, it was found that the surface distribut ion ofFe₃O₄and graphene have an important role in the rate performance and cyclestability of the composites. Fe₃O₄nanostructures evenly covering in the surface ofGNS by bonding force is strong guarantee for excellent electrochemical properties ofiron oxide composite materials.Fe₃O₄/C/r-GO composites were synthesized with ferric chloride, glucose andgraphite oxide by hydrothermal method at15MPa,400℃for12h. Fe₃O₄nanoparticles with some aggregation were mainly anchored on the edge of graphenesheets. Fe₃O₄was crystalline. Many r-GO sheets were stacked together. Someconjugated regions existed in Fe₃O₄/C/r-GO composites but some oxygenated groupwere not reduced.Fe₃O₄/C/r-GO composites have improved electrochemical propertiesin comparison with Fe₃O₄/C.The studies in the paper have laid a good foundation for the further developmentof the transition metal oxide anode material for lithium-ion batteries and also providean effective way for the synthesis of one-dimensional metal oxide/graphenecomposites.