Preparation And Lithium Storage Properties Of Ferrite Nanofiber Composite Anodes

The environment pollution and energy crisis are becoming more and more serious,which leads to the rapid development of green new energy.Due to the advantages of high energy density,long cycle life,low environment pollution and high security,lithium-ion batteries(LIBs)have been widely used in portable electronic devices and electric vehicles.However,because of the low theoretical capacity(~372 mAh g-1),it is difficult for carbon anodes to meet the requirements of high energy density and high capacity in current potential applications.Mixed transition metal oxides are considered as the promising anode candidates for LIBs to replace conventional carbon anodes with lots of virtues,such as low cost,abundant resources and high theoretical capacity(~1000 mAh g-1).In this work,three different kinds of ferrites(MFe2O4,M=Ni,Cu,Mg)nanofibers were fabricated by electrospinning combined with subsequent calcination technology and employed as anode materials for LIBs.One kind of the nanofibers above with the best electrochemical performance was chosen to be further modified with carbon and graphene,respectively,to improve its lithium storage performance and enhance reversible capacity.The electrochemical performance enhancement mechanism of the composite anodes was also discussed.The main contents and conclusions are shown as following:Three different kinds of MFe2O4(M=Ni,Cu,Mg)nanofibers were prepared via electrospinning and calcination precesses.The effect of calcination temperature on the crystal structure of MFe2O4 nanofibers was investigated.The structure and morphology of MFe2O4 nanofibers were also analyzed by thermogravimetric analysis(TG),X-ray Diffraction(XRD),fourier transform infrared spectroscopy(FTIR),surface area and pore size analyzer(BET),scanning electron microscopy(SEM)and transimission electron microscopy(TEM).The results showed that these three samples exhibited high crystallinity and good fibrous morphology.The surfaces of the nanofibers were rough with highly porous structure inside.When used as anode materials for LIBs,one dimensional fibrous structure can effectively shorten the diffusion path of ion and electron,increase the contact area between electrode and electrolyte,promote ion and electron transfer.The initial charge/discharge capacities of NiFe2O4,CuFe2O4 and MgFe2O4 nanofibers were 935/1301,891/1226 and 971/1304 mAh g-1,respectively,with the stable reversible capacities of 347,572 and 714 mAh g-1.MgFe2O4 nanofibers exhibited the best electrochemical performance in these three samples,which is mainly ascribed to its unique charge-discharge mechanism.The obtained MgO during the first discharge process does not participate in the reversible reactions due to the electrochemical inactivity,which can act as a beneficial dispersant to inhibit the agglomeration of nanoparticles and alleviate the volume expansion of the active materials.In order to further improve the lithium storage performance of MgFe2O4 nanofibers,the surface of the nanofibers were coated with uniform carbon layer combining in-situ polymerization and carbonization technology,using dopamine as the carbon precursor.MgFe2O4@C(MFO@C)composite nanofibers were obtained with different thickness of carbon layers by changing the mass ratio of MgFe2O4 nanofibers and dopamine.The composition and structure characteristics of the composite nanofibers were analyzed by XRD,Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),TG and BET.SEM,TEM,element mapping and HRTEM were employed to investigate the micro morphology of the composite nanofibers.It is found that MgFe2O4 nanofibers were well coated with carbon layers derived from polypodamine.The thickness of carbon layers was 4-15 nm and nitrogen was introduced into the carbon layers.The electrochemical tests results showed that both the cycling performance and rate capability of MFO@C composite nanofibers were better than those of pure MgFe2O4 nanofibers.This may be attributed to the nitrogen-doped carbon layers which increase the electrical conductivity,induce more active sites and defects leading to easier transport and storage of lithium ion.In addition,MgO and carbon layers make double contributions to buffer the volume changes of the active materials during repeatable charge-discharge processes.Among them,MFO@C-2 with a 7 nm carbon layer exhibited best lithium storage performance.The initial charge-discharge capacities of MFO@C-2 at 0.1 A g-1 were 1044/1384 mAh g-1,with an initial coulombic efficiency of 75.5%.The reversible capacity after 200 cycles was 926 mAh g-1,with a capacity retention of 88.8%.Even at a high current density of 1 A g-1 for 500 cycles,the reversible capacity of 610 mAh g-1 was obtained.The excellent electrochemical properties of MFO@C-2 is mainly related to its appropriate carbon coating layer.Finally,MgFe2O4/graphene(MFO/rGO)composite aerogels with three different structures were prepared by hydrothermal method and freeze-drying technique to combine MgFe2O4 nanofibers and three dimensional graphene network.XRD,Raman spectroscopy,XPS,TG,BET,SEM and TEM were respectively used to investigate the structure and morphology of the composite aerogels.It is found that the crystal structure of MgFe2O4 in the composites did not change,and the graphene oxide(GO)was successfully reduced into rGO.In addition,MgFe2O4 nanofibers were well encapsulated in the three-dimensional network structure of rGO aerogels.With the increase of MgFe2O4 content,the thickness of rGO aerogels decreased and MgFe2O4 dispered more evenly.During the half cell test,the MFO/rGO-3 with 90 mg MgFe2O4 nanofibers inside exhibited the best electrochemical performance,which delivered an initial charge and discharge capacities of 1232 and 1583 mAh g-1 at a current density of 0.1 A g-1,respectively,with the coulombic efficiency of 77.8%.After 200 cycles,a reversible capacity was found to be 1104 mAh g-1.Even at 1 A g-1,MFO/rGO-3 exhibited a reversible capacity of 686 mAh g-1 after 500 cycles,with a capacity retention of 90.6%.Additionally,in order to further evaluate the potential application of MFO/rGO-3,the full cell was assembled with the anode of MFO/r GO-3 and the cathode of commercial lithium cobalt oxide(LiCoO2).The first charge and discharge capacities of the full cell were 915 and 824 mAh g-1 at a current density of 0.1 A g-1,respectively,with the coulombic efficiency of 90.0%.The reversible capacity of LiCoO2//MFO/rGO-3 full cell was 714 mAh g-1 after 100 cylces.Even at high current density of 2 A g-1,a reversible capacity of 400 mAh g-1 was achieved,indicating good cycling performance and rate capability of LiCoO2//MFO/rGO-3 full cell.The excellent electrochemical performance of MFO/rGO-3 anode is due to the unique three-dimensional network structure of aerogels.On the other hand,MgO and rGO can play a double buffering effect on the volume change during the process of lithium intercalation/deintercalation,which has a synergistic enhancement for the improvement of the reversible capacity.