Structural Design And Lithium Storage Performance Of Pitch-based Carbon Composites

With the rapid development of electric vehicles,hybrid vehicles and portable electronic products,the traditional graphite-based lithium anode materials are difficult to meet the urgent needs of the energy storage market,and there is an urgent need to find active materials that can enhance the capacity of graphite-based electrodes..Active materials such as iron sulfide(Fe_1-xS)and silicon(Si)are expected to be one of the next-generation lithium anode materials because of their high theoretical specific capacity,but they are difficult to be widely used because of their poor electrical conductivity and volume expansion.Researchers generally use four carbon structure design methods,namely,network skeleton design,void space treatment,densification engineering and surface modification,to change the problems of poor electrical conductivity and volume expansion of non-carbon active materials.In this paper,Fe_1-xS embedded interconnected carbon matrix composites and Si embedded mesophase carbon microsphere(MCMB)composites were designed and synthesized by high temperature thermal polycondensation and silicone oil emulsification methods,respectively,mainly using bitumen as raw material,with a view to improving the electrochemical properties of iron sulfide and silicon electrodes.On this basis,the effects of carbon content(Fe_1-xS content)and void structure on the electrochemical lithium storage performance were also investigated in this paper,respectively.In this study,a Fe1-x S embedded interconnected carbon matrix composite(Fe1-x S@C NW)was constructed using petroleum asphalt as the carbon source and a network skeleton structure design method,which improved the charge/discharge specific capacity,multiplicative performance and electrical conductivity of asphalt-based carbon materials,etc.Fe_1-xS@C NW has an Fe_1-xS embedded interconnected carbon matrix structure,and the design features that the Fe_1-xS crystal formation process is accompanied by the simultaneous assembly of the interconnected carbon matrix structure.The interconnected carbon matrix structure helps to increase the charge transfer rate,which in turn improves the electrical conductivity of the Fe_1-xS material and ultimately its electrochemical properties.In addition,carbon coated iron sulfide composites with different iron sulfide contents can be prepared by adding bitumen,iron oleate and sublimated sulfur in different ratios in the autoclave system.The results showed that the 55.26%Fe_1-xS@C NW negative electrode could reach a capacity of 631 m Ah g^-1(higher than the theoretical capacity of graphite of 372 m Ah g^-1)after 120 cycles at 0.2 A g^-1;55.26%Fe_1-xS@C NW negative electrode at 0.2,0.5,1,2 and 4 A g^-1The average reversible specific capacities were 600,548,517,502 and 446 m Ah g^-1at current densities;Rct value(58.2?)of 55.26%Fe_1-xS@C NW negative electrode is much lower than that of Fe_1-xS(77.5?)and the conductivity of the material is improved.A Si-embedded mesophase carbon microsphere composite(Si@MCMB)was constructed by pore space treatment using mesophase asphalt as the carbon source,which improved the charge/discharge specific capacity,multiplicative performance and cycling performance of the asphalt-based carbon material,etc.Si@MCMB has a Si-embedded mesophase carbon microsphere structure,in which the dispersion effect of the silicone oil system makes the formation of MCMB and the embedding of Si MCMB has a unique carbon layer and pore structure,and the embedding of Si increases the charge/discharge capacity of MCMB,while MCMB provides a buffer space for the volume expansion of Si,and the two complement each other to improve the overall multiplicity and cycling performance of the carbon composite.Controlling the Si content in a certain range,Si@MCMB with different pore volumes can be obtained by different carbonization temperature(800?-1300?)treatment,and then the influence of the void structure of Si@MCMB on the electrochemical lithium storage performance can be investigated.Comparing the four different pore volumes of Si@MCMB,the BET specific surface area and pore volume of Si@MCMB-1200°C are 9.884 m²g^-1and 0.003 cm3g^-1,respectively,and they exhibit excellent lithium storage performance when applied to the anode of Li-ion batteries,with a discharge capacity of up to 421.1 m Ah g^-1(after 200 cycles at a current density of 0.2 A g^-1),and even at a high current density of 3.2 A g^-1,it still exhibits a high discharge capacity of 252.5 m Ah g^-1,and the charge and discharge performance of the carbon composites is improved..