Preparation And Electrochemical Lithium Storage Performance Of Fe₃O₄@ Carbon Based Composites

With the development of electric vehicles,energy storage systems and rail transit,the traditional lithium-ion batteries cannot fully meet high requirements for mobile power at the current market.Finding and developing new anode materials is the key to solving this problem.Iron oxide（Fe₃O₄）has attracted much attention due to its high theoretical lithium-storage capacity,low cost,toxicity and abundant resources and it is expected to become one of the alternative materials for new generation anode of lithium battery.However,the Fe₃O₄ electrode tends to suffer from a large volume effect during charging and discharging,causing structural instability and resulting in a significant decrease in cycle performance.Moreover,the inherent poor conductivity of the electrode material blocks the effective transmission of electrons,thus,the reversible capacity of the electrode material is affected,further limiting the practical application of the Fe₃O₄ electrode.Therefore,a ferro-based composite with a special structure is prepared by nanocrystallization of Fe3O4 and being compounded with carbon-based materials in this study,and the electrochemical lithium-storage performance of the composite as an electrode material is researched.A simple,effective and batch-operable"one-pot"co-precipitation method is used to prepare Fe₃O₄@PF precursor,followed by thermal-treatment to obtain Fe₃O₄@C composite.It can be seen from the electron micrograph that 10¹5 nm of Fe₃O₄ particles are uniformly embedded in the ultra-thin carbon layer,which acts as an electrode material and is carried out at a current density of 92.4 mA g^-1（0.1 C）with the charge-discharge test,the Fe₃O₄@C composite electrode exhibits high cycle stability and retains 980 mAh g^-1 after 100 cycles,much higher than the reversible specific capacity of pure Fe₃O₄(170 mAh g^-1).Analysis shows that this is mainly due to the ultra-thin amorphous carbon slowing down the agglomeration of Fe₃O₄ nanoparticles,shortening the lithium ion transport path and maintaining the structural stability of the material,while constructing 3D conductive network between the particles to accelerate the electron transfer rate,thereby improving the electrochemical lithium storage characteristics of the Fe₃O₄@C composite.In order to further improve the cycle stability of Fe₃O₄ electrode at high current density,a new composite structure of Fe/Fe3O4@r-GO is designed in the research.With the help of PVP,the Potassium ferricyanide is thermally decomposed to form Prussian blue（PB）,followed by high temperature heat treatment to obtain Fe/Fe₃O₄@r-GO composite.As seen in the SEM image,the Fe/Fe3O4 nanocage with hollow heterogeneous porous structure is uniformly wrapped in reduced graphene oxide,looking like the"flower cluster"structure,which will greatly alleviate the structural collapse caused by the volume effect of Fe₃O₄ electrode in the cycle.At the same time,the larger specific surface area exposed by the heterogeneous hollow porous structure is beneficial to shorten the transport path of lithium ions in the active material,and the electrochemical performance of the electrode material is remarkably improved.The charge/discharge test of Fe/Fe₃O₄@r-GO composite electrode shows that the discharge specific capacity of the electrode increases to 1200 mAh g^-1 after 160 cycles,and stabilizes at 1050 mAh g^-1 in the next cycles,exhibiting high cycle stability.