Synthesis Of Three-Dimensional Carbon Network Materials And Their Electrochemical Energy Storage Mechanisms

The demand for electrochemical energy storage devices,such as supercapacitors and lithium ion batteries,is increasing.Three-dimensional carbon network nano-materials have become the research focus of high-performance electrode materials because of their high specific surface area,high conductivity and high ion transfer ef-ficiency.At present,most of the three-dimensional carbon materials synthesized by traditional methods have some problems such as uncontrollable structure,complex production process and so on,which leads to the limitation of their applications.Therefore,it is of great significance to develope the green and controllable prepara-tion strategies of three-dimensional carbon materials,and reveal their electrochemical energy storage mechanism.In the dissertation,a series of three-dimensional carbon network materials with high specific surface areas and controllable pore structures were prepared by using water-soluble salts as templates.The electrochemical performance and energy storage mechanism of these materials in the supercapacitors and lithium-ion batteries were studied systematically.The specific research contents are as follows:1)A series of three-dimensional hierarchical porous carbon networks(HPC)were prepared by freeze-drying and high-temperature carbonization,using wa-ter-soluble salts(sodium chloride,sodium carbonate,sodium silicate)as sin-gle-component or multi-component templates and glucose as carbon source.The in-fluences of salt template type and the ratio of salt to carbon source on the structure and the capacitive performance of HPC were studied.It is found that the crystalline grain sizes of different salts are different.Therefore,the HPC with controllable pore size from 10 nm to 1?m can be obtained by selecting different salt templates.Using multi-kinds of salts,the obtained three-dimensional carbon network has hierarchical pore structures and large specific surface area.As the electrode of supercapacitors,HPC can not only achieve high specific capacitance(320 F g^-1@0.5 A g^-1),but also show the excellent rate performance(126 F g^-1@200 A g^-1).2)N-doped three-dimensional mesoporous carbon network materials(N-MCN)with controllable pore size and composition were prepared by freeze-drying and cal-cination,which used sodium chloride and sodium silicate as double templates,urea and glucose as mixed precursors.The effects of the ratio of double templates and the composition of precursors on the pore structure and doping content of the materials were studied.Meanwhile,the electrochemical performances of N-MCN were investi-gated.It is found that N-MCN shows a three-dimensional network structure composed of ultrathin mesoporous nanosheets.By adjusting the ratio of sodium chloride to so-dium silicate,the mesopores can be controlled in the range of 5?32 nm.By adjusting the ratio of glucose to urea,the doping ratio of nitrogen and oxygen can be controlled as well.Thanks to the mesoporous network structure and doping effect,N-MCN shows the high specific capacitance(225 F g^-1@1 A g^-1)in the supercapacitor and the high stability(no obvious attenuation after 1000 cycles)in the lithium-ion battery.3)Nitrogen-doped carbon network/graphene hybrid materials(N-GPC)were prepared by using copper/nickel sheets as catalytic matrix,sodium carbonate as tem-plate,glucose and urea as precursors.The effect of precursor on the morphology and composition of carbon hybrid was investigated.Meanwhile,the formation mechanism and the energy storage characteristics of N-GPC were studied.The results show that the prepared N-GPC is a self-supporting bilayer membrane structure:the upper layer is a three-dimensional network carbon structure prepared by salt template,which has high specific surface area,connected pores and nitrogen doping;the lower layer is wave-like graphene,which is closely combined with the carbon network and im-proves the conductivity and mechanical stability of N-GPC.The simulation results reveal that the hybridization of carbon atoms tends to be in the form of sp² under the dual function of salt template space-confinement effect and metal catalytic effect.As the electrode of supercapacitor,the specific capacitance of N-GPC can reach 118 F g^-1@200 A g^-1.As the anode of lithium ion battery,the capacity of N-GPC can be maintained at 365 m Ah g^-1 after 1000 cycles under the condition of 2 A g^-1.4)The macroscopic morphology of the three-dimensional carbon networks,which were prepared by sodium chloride template method,were transformed among powder,monolith and clay by nitrogen doping and introducing aniline molecules.The mechanism of intermolecular interactions and binding energies on the formation of macroscopical bodies of different carbon materials was studied.The results show that the doping of nitrogen can enhance the van der Waals force between the carbon net-works,so that the original powders can be self-assembled into a monolith.Aniline molecules will further introduce hydrogen bonds into the surface of the materials,which will greatly enhance the binding force between the three-dimensional carbon networks,so that the product will be transformed into a highly plastic clay-like state.The theoretical calculation based on density functional theory shows that the change of binding energy(Er)follows the order of:Er_clay>Er_monolith>Er_powder,which provides theoretical evidence for the morphological change trend of carbon network macro-bodies at atomic scale.5)Self-supporting mesoporous carbon monolith(MCM)was fabricated through the freeze-drying and high temperature carbonization using source and sodium silicate as raw materials.The influence of carbon source type and salt/carbon ratio on the formability and microstructure of MCM was studied,and the capacitance characteris-tics of MCM in various electrolytes were investigated.Using the mass ratio of sucrose to salt at 1:2 as raw material,the obtained MCM shows a high specific surface area(769 m² g^-1)and a large pore volume(2.0 cm³ g^-1).By adjusting the ratio of salt tem-plate to carbon precursor,the mesoporous of the product can be controlled in the range of 15?30 nm.Solid-state-like supercapacitors were assembled by combining MCM with ionogels,MCM shows high ion diffusion ability,which can ensure its ca-pacity retention at high rate.