Structural Design And Electrochemical Properties Of High-performance Carbon Matrix Composites

With the gradual transformation of all kinds of electronic products and new energy-powered vehicles to high-performance and multi-functional,the lithium battery,as a mobile energy,is also moving towards the direction of high energy density and fast charging.However,the traditional electrode materials are difficult to meet the growing demands of the public for long-range and fast charging of lithium-ion batteries.Electrode materials with high energy density have been paid more and more attention by researchers.However,there are other problems that are difficult to solve for electrode materials with high energy density,which hinder the development of commercialization.The most common problems are the conductivity and theoretical energy density of electrode materials.New carbon matrix composites can often improve the conductivity of electrode materials and improve the electrochemical performance.In this paper,carbon materials with special structure are designed as the base of composite materials to improve the conductivity of electrode materials.New biomass carbon materials and porous fiber carbon materials are used to prepare composite materials for lithium-ion battery anode materials and lithium sulfur battery anode materials.The research contents and results are as follows:(1)Using activated birch as carbon source,N-doped ?-Fe2O3 nanospheres coated carbon paper anode material(NFO/CP)was prepared by hydrothermal method.The nitrogen element existing in birch cells is doped in-situ in birch folded carbon paper after carbonization.The physical adsorption and the chemical adsorption produced by chemical bond of Fe-O-C make ?-Fe2O3 nanospheres anchor on the surface of birch folded carbon paper.The results show that birch carbon paper can improve the electrical conductivity and capacity retention of the composite successfully,which makes it perform excellent discharge performance at high current density.At the current density of 10 A · g-1,the maximum discharge specific capacity reaches 1000 m Ah· g-1,with 3000 cycles,and still remains 500 m Ah· g-1,only 100 m A h· g-1 lower than the first cycle.The calculation of the pseudo capacitance effect shows that the NFO/CP has strong pseudo capacitance characteristics in the electrochemical system used in this experiment;(2)The corn-cob-core porous carbon fiber was prepared by electrospinning,and the porous carbon fiber / sulfur composite CCPCF / S(corn cob borous carbon fiber)was prepared by gradual melting.Combined with one-dimensional carbon fiber structure and porous carbon structure,the porous fiber composite provides an effective "pore container" for sulfur attachment and electrochemical reaction,and effectively improves the sulfur load(70 wt%)and conductivity of the composite.CCPCF/S composite shows good lithium storage performance.The initial discharge specific capacity of 1C cycle is 850 m Ah· g-1.After 300 cycles,the specific capacity is maintained at 660 m Ah· g-1.Even after 400 cycles of 2C,the capacity is maintained at 395 m Ah· g-1.(3)N doped " Wowotou like" concave hollow carbon spheres were prepared by spray drying method as sulfur carriers(N-CHCB/S).In the preparation of materials,biomass carbon lignin was used as carbon source and cyanuric acid was used as nitrogen source.During the pre-firing process at 550?,cyanuric acid was successfully condensed into flake nitrogen carbide(C3N4),which effectively improved the polarity and conductivity of materials.The lone pair electrons of pyrrole nitrogen could form the chemical bond of Li2Sx-Npyrrole with lithium polysulfide,and the "shuttle effect" is effectively reduced by the chemisorption of this chemical bonds.With the increase of nitrogen content,the number of concave structures on the surface of carbon spheres is increased,which causes the increase of sulfur content.The samples with 5% nitrogen content obtained the optimal electrochemical performance,with the sulfur content of 55 wt%,and the initial discharge specific capacity under the current density of 1 C is 1102 m Ah·g-1,the discharge specific capacity remained at 752 m Ah·g-1 after 600 cycles.Compared with the sample without nitrogen doping,the specific discharge capacity of N-CHCB/S-5% is 500 m Ah·g-1 higher under the current density of 1 C,and the specific capacity retention rate is 68.3%.