Preparation And Electrochemical Performance Of MOF-based NiCo₂O₄ Fibers

Due to the increasingly severe environmental degradation and energy consumption,the research and development of green,pollution-free and renewable energy have become the focus of researchers.Lithium-ion battery,as a popular clean energy storage device,plays a pivotal role in new energy vehicles,smart wearable electronic equipment and other industries.Nevertheless,the anode material graphite currently used in the lithium-ion battery,which has widely occupied the market,has always been a key factor restricting the electrochemical performance of the lithium-ion battery.Metal oxides derived from metal-organic frameworks generally have high specific capacity,large specific surface area,and rich pore structure,and have great potential in the application of negative electrode materials for lithium ion batteries.Therefore,this study successfully prepared NiCo₂O₄ nanofibers,NiCo₂O₄@Carbon composite nanofibers and NiCo₂O₄/RGO/C composite nanofibers by electrospinning,in-situ growth,ion exchange,and high temperature calcination.The research contents include:（1）ZIF-67/PAN/Ni（CHOO）₂·H₂O precursor nanofiber membrane was prepared by electrospinning technology and in-situ growth process accompanied by ion exchange reaction,and NiCo₂O₄ nanofibers were calcined in air at 330℃.Keeping the other experimental conditions unchanged,using in-situ growth time as a variable（set to 2 h,6 h,8 h,and 10 h,respectively）,four comparative samples were prepared.Through SEM,TEM and it can be seen that only the samples grown in-situ for 6 h and 8 h retain the MOF structure,and XRD shows that the composition is NiCo₂O₄.Therefore,the two qualified samples of NiCo₂O₄-6and NiCo₂O₄-8 nanofibers were further characterized.BET tests show that NiCo₂O₄-6nanofibers have a larger specific surface area and rich pore structure.The charge-discharge cycle test showed that at a current density of 0.3 A g^-1,the discharge specific capacity of NiCo₂O₄-6 nanofibers in the first week was 1583 mAh g^-1,and the discharge specific capacity after 300 cycle was 877 mAh g^-1.（2）On the basis of preparing the MOF-derived NiCo₂O₄-6 nanofibers,change the experimental parameters to obtain more unique and excellent nanofiber materials,and the step of carbonization is added to prepare a composite anode material of NiCo₂O₄ and carbon nanofibers.Keeping the other experimental conditions unchanged,using the molar ratio of metal salt to ligand in the in-situ growth solution as variables（respectively set to 1:8 and 1:4）,two comparative samples NCO@CNFs-8 and NCO@CNFs-4 were made.It can be seen from SEM and TEM that when the molar ratio of metal salt to ligand is 1:8,MOF-derived necklace-shaped fiber can be obtained,which retains the porous,large specific surface area characteristics and stable structure of the MOF itself;while the molar ratio of metal salt to ligand is 1:4,MOF particles are scattered in the fiber web.The charge and discharge cycle test shows that the cycle and rate performance of NCO@CNFs-8 anode material is much better than NCO@CNFs-4.At a current density of 0.3 A g^-1,the specific discharge capacity was 1953 mAh g^-1 in the first week,and the discharge specific capacity of 1388 mAh g^-1could be maintained after 500 cycles.（3）On the basis of the NCO@CNFs-8,keeping the other experimental conditions unchanged,change the amount of graphene added（set to 2 mg/m L,3 mg/m L,and 4 mg/m L,respectively）to prepare three comparative samples.According to SEM and TEM,it can be seen that adding too much graphite will cause in-situ growth failure,only the two samples NCO/RGO/C-20 and NCO/RGO/C-30 retain the MOF structure.The charge-discharge cycle test shows that the performance of NCO/RGO/C-30 composite nanofibers is the best.At a current density of 0.3 A g^-1,the discharge specific capacity was 2048 mAh g^-1 in the first week,and the ultrahigh discharge specific capacity of 1712 mAh g^-1 was retained after 500 cycles.