Core-shell Structure Porous Carbon Derived From MOF@ZIF For Electrochemical Energy Storage

With the rapid development of society and technology,the demand for high-performance secondary batteries becomes stronger and stronger within various electronic devices,especially electric vehicles.To date,lithium-ion battery has emerged as one of the most widely used secondary energy storage devices in the market.Limited by the low reserve of lithium resources and rising price year by year,it is urgent to find wonderful substitutes.As potassium element,which is the same main group of lithium,has the advantages of high abundance and low cost,and the energy storage principle of potassium ion batteries is similar to that of lithium ion batteries,which enables potassium ion battery to be a new type of secondary battery with great application prospects.Compared with lithium ions,the radius of potassium ions is larger and the diffusion rate is slower,which makes it difficult to deintercalate potassium ions into electrode materials such as traditional graphite anodes;in addition,it will cause large volume expansion of the electrode and structural fragmentation,as a result,the battery has low potassium storage capacity,poor rate performance and short cycle life.Therefore,how to prepare electrode materials with high-efficiency K⁺reversible deintercalation is an important scientific issue that needs to be solved urgently for developing potassium ion batteries.Among the anode materials for potassium ion battery,porous carbon materials derived from metal-organic frameworks（MOFs）exhibit excellent electrical properties stemming from their abundant energy storage active sites,good structural stability and short ion migration channels,which show excellent electrochemical potassium ion storage performance.However,porous carbon derived from a single MOFs cannot fully meet the performance requirements of electrodes.For example,under the same carbonization conditions,Ni/Co-containing MOFs derived porous carbon often have a higher degree of graphitization,higher electronic conductivity and cycling stability,while N-containing MOFs（such as ZIFs,etc.）can produce N-doped porous carbon which have higher potassium ion storage capacity but low conductivity and stability.Constructing a MOF@ZIF core-shell structure that combines the advantages of two different MOFs-derived porous carbons for optimized performance is a desirable strategy.In view of this,in this thesis we successfully prepared MOF@ZIF core-shell materials ranging from three-dimensional particles to two-dimensional nanosheets and one-dimensional nanowires by using a simple synthesis strategy.Subsequently,these MOF@ZIF core-shell structure materials were subjected to simple high-temperature carbonization treatment to obtain MOF-derived core-shell structured porous carbon.Because it actually integrated both merits of Ni/Co-containing MOFs-derived porous carbon core and the ZIFs-derived porous carbon shell,which owned fast charge transport channels,a large number of potassium ion storage active sites,and good structural stability,when coupled with the synergistic effect induced by the core-shell structure,the core-shell porous carbon could be employed as a high-efficiency electrochemical potassium ion storage anode material,showing high specific capacity,excellent rate performance and cycle performance.The main research contents and results are shown as follows:（1）Three-dimensional Ni-MOF-74 and Ui O-66 particles were used as the core MOFs and placed them in 2-methylimidazole solution,and then the metal salt solutions were also added.By controlling the reaction time and concentration of organic ligand,ZIF-8 and ZIF-67 preferentially nucleated and grew on the surface of the core MOFs,and three-dimensional Ni-MOF-74@ZIF-8,Ni-MOF-74@ZIF-67,Ui O-66@ZIF-8 and Ui O-66@ZIF-67 core-shell particles were successfully prepared.（2）Similar to the synthesis of the above-mentioned three-dimensional core-shell structured particles,two-dimensional Zn Co-TCPP nanosheets,one-dimensional Zn Co-MOF-74 nanowires and Ce-BTC one-dimensional nanowires were used as core MOFs and placed in 2-methylimidazole.After adding zinc salt and cobalt salt solution into the former solution,by controlling the reaction time and ligand concentration,ZIF-8 and ZIF-67 nucleated and grew on the surface of the core MOFs,and two-dimensional Zn Co-TCPP@ZIF-8 and Zn Co-TCPP@ZIF-67 core-shell nanosheets,one-dimensional Zn Co-MOF-74@ZIF-8,Zn Co-MOF-74@ZIF-67,Ce-BTC@ZIF-8 and Ce-BTC@ZIF-67 core-shell nanowires could be prepared.（3）The as-prepared three-dimensional Ni-MOF-74@ZIF-8 core-shell nanoparticles,two-dimensional Zn Co-TCPP@ZIF-8 core-shell nanosheets,one-dimensional Zn Co-MOF-74@ZIF-8 core-shell nanowires and four counterparts such as ZIF-8 solid particles,Ni-MOF-74 nanoparticles,Zn Co-TCPP nanosheets and Zn Co-MOF-74 nanowires were used as the precursors to prepare various types of core-shell structured porous carbon materials by carbonization at 900°C for 2 h,including three-dimensional Ni-MOF-74@ZIF-8-C nanoparticles,two-dimensional Zn Co-TCPP@ZIF-8-C nanosheets,and one-dimensional Zn Co-MOF-74@ZIF-8-C nanowires,ZIF-8-C nanoparticles,Ni-MOF-74-C nanoparticles,Zn Co-TCPP-C nanosheets,Zn Co-MOF-74-C nanowires,and detailed and systematic study on their electrochemical potassium storage performance was conducted.It was found that Zn Co-MOF-74@ZIF-8-C porous carbon nanowires derived from one-dimensional Zn Co-MOF-74@ZIF-8 core-shell materials exhibited the best performance for potassium ion storage due to the unique nanowire core-shell structure.At a current density of 0.05 A g^-1,the eletrode delivered a reversible specific capacity of 523.2 m Ah g^-1.Even when the current density was as high as 3 A g^-1,the specific capacity of 138.3 m Ah g^-1 could be achieved;in addition,after 2000 cycles at 1A g^-1,the specific capacity could still retained 80.7% of the initial value.