Preparation And Electrochemical Performance Of MOF-based CoFe₂O₄ Composite Fibers

Rechargeable batteries have become one of the most popular energy storage systems due to their high conversion efficiency and environmental protection.Lithium-ion batteries（LIBs）are being applied to energy storage such as new energy vehicles due to their outstanding advantages.Meanwhile,sodium-ion battery（SIBs）has attracted wide attention in recent years due to its advantages such as wide source of raw materials,low price and electrochemical performance close to lithium-ion battery,which brings a new choice for electric energy storage.The development of traditional commercial graphite carbon anode materials is restricted due to the low theoretical capacity.Therefore,it is the key to improve the energy density of LIBs and SIBs to seek anode materials with high specific capacity,high natural abundance and long cycle life.CoFe₂O₄has about 2.5 times the theoretical capacity of commercial graphite,but the volume expansion and structure pulverization during the cycle seriously hinder its application.Prussian blue analogue（PBA）,as a metal-organic framework（MOF）with structurally controllable and porous properties,can alleviate the above problems to a certain extent.However,the electrochemical properties of long cycles and high magnification still need to be improved.Therefore,in view of the above problems,three kinds of MOF-CoFe₂O₄@C,MOF-CoFe₂O₄@C@alkalized MXene and MOF-CoFe₂O₄@MXene@CNFs composite nanofibers with different structures were prepared by using Prussian blue analogues as precursors.As the anode material of LIBs and SIBs,the research content is as follows:（1）By electrospinning,in situ growth combined with high temperature calcination,different in situ growth times（12 h,24 h and 36 h）were regulated,PAN nanofiber membrane as the growth base,porous structure MOF-CoFe₂O₄@C composite nanofibers derived from PBA were prepared.By SEM and XRD characterization,it can be seen that CFO@C-24 grown in situ for 24 h has the most MOF-CoFe₂O₄particles and the most complete hexahedral metal-organic framework structure.As LIBs half cell anode material,at 1 A g^-1 current density,the reversible capacity of 400 cycles still has623 m Ah g^-1.For the LIBs full cell,showed excellent cycling performance,with a reversible capacity of 434 m Ah g^-1 after 200 cycles.The results show that the MOF structure derived from PBA can effectively relieve the volume expansion phenomenon during charging and discharging.The continuous fiber network structure provides a fast channel for the transmission of lithium ions and alleviates the agglomeration of MOF-CoFe₂O₄.（2）In order to further improve the electrochemical performance of CFO@C at high current density,alkali treatment and freeze drying were used to form a pillar-layer structure of Ti₃C₂T_x MXene,which was combined with CFO@C-24.MOF-CoFe₂O₄@C@alkalized MXene（CCM）composite nanofibers with pillar-layer structure were prepared.By exploring the mass ratio of Ti₃C₂T_x MXene to CFO@C-24（1:3,2:3 and 3:3）,CCM1,CCM2 and CCM3 were obtained.The pillar-layer structure and mesoporous structure of CCM were demonstrated by TEM and BET characterization.Among them,CCM2,as a LIBs/SIBs half cell anode material,has the best performance.the LIBs half cell has a reversible capacity of 807 m Ah g^-1 after 400cycles at a high current density of 2 A g^-1.SIBs half cell showed excellent magnification performance of 149 m Ah g^-1 at 5 A g^-1 current density.Application tests showed that Li CoO₂//CCM2 and Na₃V₂（PO₄）₃//CCM2 full cell maintain excellent reversible capacity of 295 m Ah g^-1 and 94 m Ah g^-1 at current density of 10 A g^-1,Respectively.Electrochemical simulation shows that CCM2 is a lithium/sodium storage mechanism based on pseudo-capacitance and exhibits excellent capacity contribution rate.As the LIBs/SIBs anode material,the pillar structure（CFO@C-24）and the layer structure（a-Ti₃C₂T_x）can effectively alleviate the instability of the structure under high current density.（3）In order to solve the problem of low capacity of MOF-CoFe₂O₄ on PAN fiber due to low loading capacity,the loading capacity of MOF-CoFe₂O₄ on Ti₃C₂T_x was increased by using polydopamine self-assembly,and the mass ratio of Ti₃C₂T_x MXene to MOF-CoFe₂O₄ was regulated（1:5,1:7 and 1:9）.The mass ratio of Ti₃C₂T_x MXene:MOF-CoFe₂O₄ is 1:7,which has the best lithium storage performance.Carbon nanofibers（CNFs）obtained from PAN nanofibers prepared by electrospinning after high temperature heat treatment were combined with CM2（mass ratios of 1:3,1:5 and1:7,respectively）by filtration to improve the electrical conductivity of the composites.The final result is 3D ant nest-like structure MOF-CoFe₂O₄@MXene@CNFs（CMC）composite nanofibers.SEM and BET physicochemical characterization proved that CMC has 3D ant nest-like structure and large specific surface area.Among them,the mass ratio of CNFs to CM2 of 1:5（CMC2）has the best lithium/sodium storage performance the LIBs half cell still has a high reversible capacity of 970 m Ah g^-1 after1000 cycles at the current density of 2 A g^-1 with CMC2 as the cathode material.The SIBs half cell also maintains reversible capacity of 97 m Ah g^-1 after 4000 cycles at a current density of 1 A g^-1.Li CoO₂//CMC2 and Na₃V₂（PO₄）₃//CMC2 full cell have excellent cycle stability,at 1.0 A g^-1 current density Li CoO₂//CMC2 still maintains a reversible capacity of 533 m Ah g^-1 after 200 cycles.Na₃V₂（PO₄）₃//CMC2 maintained a specific discharge capacity of 74 m Ah g^-1 after 500 cycles.Electrochemical simulation tests show that CMC2 also exhibits excellent capacity contribution rate and the mechanism of lithium/sodium storage based on pseudo-capacitance.The results show that the reasonable composition mass ratio makes the CMC2 composite nanofibers better show the structural advantages of 3D ant nest-like structure,which greatly alleviates the volume expansion during the electrode reaction process,and solves the problem of low capacity caused by less load of MOF-CoFe₂O₄ in PAN fibers.