Performance Regulation And Energy Storage Mechanism Of Phthalocyanine-based Covalent Organic Framework Electrode Materials

Lithium-ion batteries have been successfully applied to various portable devices and electric vehicles due to their unique advantages such as high capacity and high energy density.In the research of lithium-ion battery anode materials,the commercial graphite anode is difficult to meet the social requirements for energy storage equipment due to its low capacity,while the inorganic anode is often accompanied by serious volume expansion in the process of lithium ion deinterlay,resulting in rapid capacity attenuation.Organic cathode,meanwhile,due to its molecular structure design,environment friendly,low-cost advantages attracted wide attention of the researchers,and no obvious volume expansion after discharge phenomenon,therefore,we can through the design of the functional group and the adjustment of the microscopic morphology design has a higher capacity and rate performance of electrode materials.However,poor electrical conductivity,easy dissolution in electrolyte and limited exposure to active sites are the main challenges to improve the electrochemical performance of organic materials.In this paper,several novel phthalocyanine based covalent organic framework organic compounds(Pc-COFs)were constructed to improve the shortcomings of the covalent organic framework electrode from the following aspects,including the synthesis process,structural design,group and the introduction of nanoparticles and other aspects of organic design.By expanding the diffusion channels of lithium ion and adding abundant active groups and high conductivity materials,we can contribute large specific capacity and achieve long life.It is expected to prepare a new type of battery anode material with excellent cycle life and rate performance.The main contents are as follows:(1)With 4-nitro phthalocyanine as raw material,small molecules such as nitro and amino groups are polymerized into phthalocyanine compounds with large π-π conjugated system,and further polymerized with structural units of different sizes by catalyst free coupling reaction.To form three weakly clustered phthalocyanine azo covalent organic framework compounds with different pore sizes(NA-Ni Pc,PPDA-Ni Pc and DAB-Ni Pc),and explore the differences in the framework structure and microstructure of the three COFs prepared by different synthesis processes.Furthermore,the influence of different sizes of lithium/sodium diffusion channels obtained by adjusting the pore size of covalent organic network on the electrochemical properties of the materials was analyzed.At the current density of 100 m A/g,the negative first Coulombic efficiencies of NA-Ni Pc,PPDA-Ni Pc and DAB-Ni Pc are 131.8%,142.9% and 166%,respectively.After 700 cycles,the specific capacity kept at 557 m Ah/g,670 m Ah/g and 941 m Ah/g,and the capacity retention rates were 45.36%,47.91% and 49.4%,respectively.Even at 2 A/g high current density,it still has 385 m Ah/g,512 m Ah/g,767 m Ah/g high specific capacity.At the same time,NA-Ni Pc,PPDA-Ni Pc and DAB-Ni Pc as sodium ion batteries also show excellent performance as lithium ion batteries.With the increase of skeleton porosity,the performance of lithium-ion battery and sodium ion battery gradually improves,which fully indicates that the skeleton size is the key factor affecting the performance of battery.(2)In order to obtain more active sites of exposed organic active substances,based on the large π conjugated system of phthalocyanine,a multi-active carbonyl group and graphene were introduced into the structural design.Pc-COFs and graphene were combined by in-situ reaction to prepare PMDA-Ni Pc and PMDA-Ni Pc@G covalent organic framework electrode materials.Polyactive organic conjugated carbonyl groups were used to increase the active sites,and graphene was used to improve the conductivity of composite materials.Meanwhile,graphene was used as a dispersive medium to reduce the accumulation degree of bulk covalent organic framework materials,so as to obtain Pc-COFs with small volume and few layers,and shorten the ion diffusion path.Increasing the diffusion rate of lithium ion in two-dimensional meshed layered molecular structure to obtain high capacity and large rate electrochemical performance.At a current density of 100 m A/g,the PMDA-Ni Pc@G electrode can achieve a very large reversible capacity of 1290 m Ah/g after 300 cycles,and there is almost no capacity decay in the next 300 cycles.After 600 cycles,PMDA-Ni Pc can achieve 1861 m Ah/g reversible capacity contribution.It maintains 74.9% specific capacity(812.3 m Ah/g)even at a high current density of 3 A/g.The excellent cycle life and rate capacity are mainly attributed to the two-dimensional small volume and multi-layer porous structure,and the synergistic effect of PMDA-Ni Pc and Graphene can effectively improve the electrochemical performance of PMDA-Ni Pc@G electrode.Introducing carbonyl and Graphene into PMDA-Ni Pc@G electrode can greatly improve the capacity,electronic conductivity and structural stability,and finally obtain good electrochemical performance.(3)In view of the shortcoming of low specific capacity of organic matter,a phthalocyanine-covalent organic framework compound PMDA-Ni Pc@Si coated with nano-Si type was constructed on multi-active carbonyl Pc-COFs using covalent organic framework,and a layer of Pc-COFs with grid layer structure was coated on the surface of nano-Si.The defect of volume expansion of nano-Si in charge-discharge cycle is improved by increasing material specific capacity.This paper mainly studies the influence of covalent organic framework coating nano-Si on the electrochemical properties of materials,mainly on the diffusion rate and specific capacity of lithium ion and high rate performance.The distribution of the covalent organic frame coating was explored,and the effect of COF coating on the lithium ion transport dynamics of Si electrode was studied to observe the improvement of the rate performance.The reversible capacity of 654 m Ah/g can be maintained after 2000 cycles at the current density of 2 A/g,and the retention rate is 80.4%.The stability of structure change of COF coating is increased obviously in the cycle process,and the rate performance and cycle stability of the electrode are improved.