Synthesis Mechanism And Energy Storage Performance Of Carbon Materials

Carbon materials have been widely used in various fields of the national economy due to their adjustable physical and chemical properties.However,the green synthesis technology of carbon materials has become one of the bottlenecks in the development of circular economy in China.Carbonization of organic matters with the emission of greenhouse gases and noxious gases is the traditional approach to synthesize carbon materials.In this work,a new green method for synthesizing carbon materials by using the greenhouse gas CO₂ as the carbon source was proposed,in which CO₂ could be efficiently converted into carbon materials.The following scientific problems were systematically studied,including the preparation of pyrolytic carbon and the effect of introducing Si O₂ on the electrochemical performance of pyrolytic carbon,the synthesis of heterogeneous atoms doped porous carbon,and the relationship between the morphology,microstructure,doping heterogeneous atoms and the electrochemical properties of carbon materials.Furthermore,the green synthesis mechanism and electrochemical energy storage mechanism of carbon materials were revealed in this paper,which has important reference value for developing green synthesis technology and large-scale energy storage applications of carbon materials.Main research contents and results are as follows:Pyrolytic carbon derived from PS showed low reversible lithium storage capacity of only 140 m Ah g^?1 at 0.1 A g^?1 and poor capability performance.Thus,pyrolytic carbon was combined with Si O₂ to form C/Si O₂ composites in order to improve its electrochemical performance.The morphology,microstructure and electrochemical performance were found to change significantly before and after introducing Si O₂ into pyrolytic carbon.The stable lithium storage capacity increased from 140 m Ah g^?1 for the pyrolytic carbon to 555 m Ah g^?1 for the C/Si O₂ composte at 0.1 A g^?1.The pyrolytic carbon was coated on the surface of Si O₂,in which the pyrolytic carbon served the function of an interconnected frame structure.This structure not only enjoys the capacity increase brought by the Si O₂,but also alleviates the volume effect of Si O₂ and improves the electronic conductivity.This is the main reason for the improved electrochemical performance of pyrolytic carbon.A new coordination compound 2LiBH₄·CO₂ was synthesized by using LiBH₄ and CO₂ as raw materials.Boron doped porous carbon?BPC?could be synthesized by heating 2LiBH₄·CO₂ under CO₂.The products of the reaction between 2LiBH₄·CO₂and CO₂ were proved to be hydrogen,LiBO₂,Li₂CO₃ and Li₅B₇O₁₃ by XRD and MS analyses.The new synthesis reaction of BPC was elucidated and the reasons for producing porosity in BPC was discussed as well.The BPC exhibits flake-like porous structure with a high specific surface area of 520 m² g^-1.The pore diameter of BPC is mainly distributed around 4 nm and 34 nm.The BPC synthesized under 60 bar CO₂delivered a reversible lithium storage capacity of as high as 890 m Ah g^-1 after 660cycles at the current density of 1 A g^-1.The excellent electrochemical performance of BPC can be attributed to the following two points:?1?doping boron atoms improves the electronic conductivity of carbon materials and accelerates the transportation of electrones,?2?the porous structure of porous carbon shortens the transmission path of Li ions,and increases lithium storage active sites.The theoretical calculation shows that the excellent lithium storage performance of BPC arised from its high capacitance contribution.In addition,BPC synthesized under different CO₂ pressure showed different electrochemical performance.CO₂ pressure dependent morphology of BPC is found to be the main reason for the different electrochemical performance.On the basis of BPC,boron and nitrogen co-doped porous carbon materials?BNPC?were synthesized by heating Li₂BNH₆ under CO₂.Li₂BNH₆ was obtained after a ball milling process of LiBH₄ and Li NH₂.The products of the reaction between Li₂BNH₆and CO₂ were proved to be hydrogen,LiBO₂,Li₂CO₃ and NH₄NO₃.Based on the analysis of composition and microstructure,the new synthesis reaction of BNPC was elucidated and the reasons for producing porosity in BPC was discussed as well.Similar to BPC,BNPC shows flake-like porous structure with a specific surface area of 431 m²g^-1.The pore diameter is mainly distributed around 1.5 nm and 28.7 nm.Compared with BPC,BNPC showed more stable electrochemical performance(680 m Ah g^-1 at 1 A g^-1),with almost no significant capacity attenuation after 1000 cycles at the current density of 1 A g^-1.Boron and nitrogen co-doping can not only improve the conductivity of carbon materials,but also have profound ramifications on their morphology and microstructure,which leads to their different electrochemical performance.