Preparation And Electrochemical Properties Of Nickel-Based Metal Organic Framework And Their Composites

Supercapacitors,as a kind of clean electrochemical energy storage devices,exhibit excellent electrochemical performance such as high power density,long cycle and fast charge/discharge,which is attractive for the application s in portable(wearable)electronic products,wind power generation and military industry.However,due to the limited specific capacity and potential window,supercapacitors have a low energy density,which prevent their application in electric vehicles and large energy storage devices.Lithium-ion batteries,were widely used in mobile phones,laptops and electric vehicle due to their high energy density and environmental friendliness.However,commercial graphite anode can not meet the high requirements in lithium batteries due to the low theoretical capacity.Therefore,it is urgent to search a new electrode material with both high energy and power density.Metal organic frameworks(MOFs)is a coordination polymer formed by metal ions and organic.In recent years,MOFs has been used in electrochemical energy storage devices due to their adjustable structure and morphology,multiple electronic reactions and large ion diffusion channels.However,the MOFs exhibit low cycling stability and rate performance in the application of electrochemical energy storage devices caused by poor electrical conductivity and sta bility.Herein,we chose a high capacity nickel-metal organic framework and study the relationship between the morphology,structure and electrochemical performance by adjusting its growth reaction kinetic.In addition,the poor conductivity and stability of MOFs was significantly improved by composing metal oxides,graphene and carbon materials with MOFs.The results are as follows:(1)Self-supporting Ni-MOF/NF were prepared by one-step solvothermal method using nickel chloride hexahydrate(NiCl₂·6H₂O)and terephalic acid(PTA)as metal salts and organic ligands,respectively.The morphology and loading of Ni-MOF/NF were regulated by adjusting the reaction time and solvent type.The results showed that the area capacity and mass loading of Ni-MOF/NF-D/W/E(DMF/Water/Et OH,volume ratio 1/1/1)were firstly increased and then decreased with the increase of reaction time.Different morphologies such as spherical clusters,sheet and bulk were formed by changing the solvent type.The obtained Ni-MOF/NF-D/W/E electrode material delivers a high area capacity of 8780 m F cm^-2 at the current density of 1 m A cm^-2 in 3 mol L^-1 KOH electrolytes with a high mass loading of 10 mg cm^-2 on the nickel foam.Furthermore,the area capacity still reached 5544 m F cm^-2 at 5 m A cm^-2with a capacity retention of 63%.Then,the Ni-MOF@manganese dioxide composite(Ni-MOF@MnO₂/NF)was synthesized by a two-step method.The results show that the Ni-MOF@MnO₂/NF exhibits an improved capacity retention of 63.25%at 20 m A cm^-2 after 5000 cycles compared to Ni-MOF/NF,which is only 8%.In addition,an asymmetric supercapacitor device was assembled with Ni-MOF@MnO₂/NF,activated carbon electrode as cathode and anode,respectively.The device achieves an energy density32 Wh kg^-1 at a high power density of 6071 W kg^-1.(2)Ni-MOFs with different morphology and structure were employed as anode materials for lithium ion batteries(LIBs).The Ni-MOF displays the fold block,spherical and stacking bulk morphology in DMF,DMF/Et OH=1/1 and DMF/Et OH/Water=1/1/1,respectively.Among them,the Ni-MOF(D/E)spheres were composed of nanosheets and has a monoclinic crystal structure with almost no guest molecules.As a result,the Ni-MOF(D/E)exhibits a high discharge/charge capacity during the first cycle of 2196.4/1762.3 m Ah g^-1,and a high reversible capacity of1300 m Ah g^-1 at 100 m A g^-1 after 50 cycles.Even at the current density of 4 A g^-1,the Ni-MOF(D/E)maintains a capacity of 452 m Ah g^-1,showing a good rate capability.Moreover,in order to improve the long cycle stability of Ni-MOF(D/E),the N-doped graphene(NG)was introduced into Ni-MOF(D/E)to form a NG@Ni-MOF(D/E)composite.On the one hand,the NG can provide the nucleation sites and control the growth direction of MOFs as a guiding agent;On the other hand,NG can also act as a substrate material to improve the cycle stability of MOFs.As a result,the NG@Ni-MOF(D/E)composite demonstrates an ameliorative long-term cyclability of 802 m Ah g^-1 at 1 A g^-1 after 420 repeated cycles.