Modification And Energy Storage Mechanism Of Metal-Organic Framework For Secondary Batteries

With the widespread use of electronic devices and the urgent need for clean energy,the demand for energy storage batteries is increasing,and the performance requirements for batteries are also increasing.On the one hand,researchers are constantly looking for electrode materials with higher specific capacity to increase energy density,and on the other hand,developing new battery systems to reduce the excessive cost caused by excessive dependence on lithium resources.Lithium-ion batteries are widely used in portable devices because of their excellent electrochemical performance.However,due to the high cost and limited storage of lithium resources in the earth’s crust,they are not suitable for use in large-scale energy storage.The metal potassium resource is rich in reserves and has similar electrochemical characteristics as the first main group of lithium,and is expected to be applied in energy storage batteries.However,due to the large ionic radius of potassium ions,it is required that the electrode material has a larger pore structure to provide diffusion channels and to alleviate the volume expansion effect caused by the deintercalation process.In recent years,scientists have.reported a class of metal-organic framework materials(MOFs)with large specific surface area and porous structure,whose structure is composecd of organic ligands and metal ions,and has the flexibility of organic ligands and the rigidity of metal io:ns.Therefore,it is a kind of electrode material with development potential.In this paper,a class of Fe-based MOF-235 materials was synthesized and modified for its weak conductivity to study its potassium ion storage characteristics.The specific work is as follows:(1)The Fe-based metal-organic framework material MOF-235 was successfully prepared by solvothermal method using cheap and environmentally friendly terephthalic acid and ferric chloride hexahydrate.It was found that MOF-235 material is a kind of microporous material.BET specific surface area 710m2/g,pore diameter 0.83nm,pore volum 0.3cm3/g.It has good ion storage characteristics and is used for Li+ Na+,and K+ studies.The reversible capacity of lithium and potassium storage at 50mA g-1 current density is 220 and 80mAh g-1,respectively,and it has high coulombic efficiency.It indicates that MOF-235 is a kind of electrode material capable of reversible potassium storage.(2)A composite of one-dimensional multi-walled carbon nanotubes(MWCNTs)and MOF-235 was prepared in situ for storage of potassium.Due to the excellent electrical conductivity of the multi-walled carbon nanotubes,the reversible specific capacity of the composite material is remarkably improved,and the excellent cycle performance is attributed to the good combination of the carbon nanotubes and the MOFs crystals,thereby having a fixed effect on the active material.The electrochemical performance was optimal when 20%MWCNTs were added.The capacity reached 143 mAh g-1 after 50 cycles at 50 mA g-1 current density,132 mAh g-1 for 200 weeks at 200 mA g-1 high current density,capacity reation rate is 52.8%and has excellent coulombic efficiency and rate performance.The study of energy storage mechanism found that iron ions do not participate in redox reaction during charging and discharging.The reversible potassium storage capacity is mainly derived from the reversible opening and closing of C=O double bonds in organic ligands.(3)A composite electrode material having a two-dimensional conductive network is further constructed by combining a two-dimensional conductive material graphene material with MOF-235 material.The conductivity test shows that the graphene has higher conductivity than the multi-walled carbon nanotubes under the same conditions,because the MOFs crystals are embedded in the graphene surface or the interlayers increase the contact area.When the addition of 20%graphene,the composite material achieves the best potassium storage performance.The reversible potassium storage capacity is 169 mAh g-1 at 50 mA g-1 current density,and 145 mAh g-1 at 200 mA g-1 high current,capacity retention rate is 72%and better than the Capacity properties of 20%MWCNTs composites.Through the variable-speed CV test of the MOF-235+20G composite maternal,it was found that the reaction process of the electrode material was mainly controlled by the potassium ion diffusion process,that is,the battery behavior.