Structural Control And Energy Storage Performance Of Carbon-based Materials Baesd On Injection Pyrolysis Strategy

It is significant to change the dependence on fossil fuels and seek more green and efficient energy system.However,most of the clean energy has the characteristics of strong intermittent and uneven distribution.Therefore,the establishment of a supporting energy storage system is very important for its development.Alkali metal-ion batteries are currently the main chemical energy storage media.Electrode material is one of the most critical components in a battery,and its structural design and innovation are the keys to improve performance of battery.Onion-like carbon is a micro-spherical carbon material surrounded by multi-shell carbon layers,prepared by injection pyrolysis in our lab.The material has a highly developed layered structure and good structural stability,exhibiting excellent long-term cycling stability.However,there are shortcomings of ion diffusion hysteresis and low capacity,which restrict its further application.Based on this,this topic proposes a new idea for the hybridization and compound of carbon materials based on injection pyrolysis.Graphite carbon materials with novel structures（soft and hard carbon composite,cage-shaped multi-cavity graphite,polyhedral cage-coated multi-level graphite,and carbon nanotube braided spheres）are designed to solve the problem of ion diffusion retardation;silicon-carbon composite materials with onion-like carbon coated are constructed to solve the problem of low capacity.The main contents and conclusions are summarized as follows:（1）Using hard carbon nanoparticles and pyridine as carbon sources,through injection pyrolysis and graphitization of pyrolysis product,soft and hard carbon hybrid micro-graphite spheres were prepared with a diameter of1-10?m.The hard carbon nanoparticles were embedded in depth of graphitic onion-like carbon sphere,as a hub to connect the separated onion-like carbon layers,and greatly shortening the time for ions to diffuse from the outside into the depth of the sphere.When used as the anode of lithium-ion battery,the hybrid material exhibits the electrochemical curve characteristics of soft carbon and hard carbon with slopes and plateaus coexisting,and the rate performance is significantly improved.（2）Using ferrocene and pyridine as carbon source and catalytic source,through injection pyrolysis and graphitization of pyrolysis product,1.2?m polyhedral cage-shaped multi-cavity graphite spheres were prepared.The outside of the sphere is covered by polyhedral onion-like carbon cage,and the stable cage helps to maintain the structural integrity of the sphere and serve as the growth substrate for the SEI film,enabling long cycle life.Due to the template and catalysis of iron nano particles,the interior is transformed into interconnected defect-rich multi-cavity graphite.The defect-rich carbon structure provides more ion transmission paths.The multi-cavity structure shortens the ion transmission distance and greatly accelerates the diffusion process of ions.The graphite sphere exhibits significantly enhanced rate performance.（3）Using organosilane(C₆H₁₄Si)as carbon source and silicon source both,through injection pyrolysis and graphitization of pyrolysis product,graded graphite spheres with yolk-shell structure,covered by graphite cage with hard carbon core were prepared.The structure transformation mechanism from pyrolysis carbon spheres to graded graphite spheres were studied.Pyrolytic carbon spheres are carbon-rich silicon carbide spheres with a core-shell structure.After graphitization,the carbon layer crystallized and undergoes volume shrinkage to transform into a core of intertwined graphite belts,and finally becoming a graded graphite sphere with a yolk-shell structure.And through the control of the pyrolysis temperature（700℃-1100℃）,the particle size control of the graphite spheres（from micron-sized particles to nanometers）is realized.After graphitization,the control of the content of graphite phase and graphite-like phase is realized,and the quantitative analysis was carried out by XRD and XANES spectroscopy.When used as the anodes for lithium-ion batteries and potassium-ion batteries,the outer cage acts as the growth substrate of the SEI film and maintains the stability of the sphere,the inner carbon core has a wide interlayer spacing,and the gap in the middle can shorten the ion transport path,thus exhibiting excellent ion diffusion kinetics and excellent rate capability.（4）Using thiophene and ferrocene as carbon source and catalyst source,micron carbon nanotube braided spheres were prepared through injection pyrolysis and graphitization of pyrolysis product.The strong elasticity of the woven microstructure can buffer the volume expansion caused by the intercalation of alkali metal ions,and increasing the density of the nanomaterial.After 1000 cycles at 1C,it can still show a reversible potassium storage capacity of 155.5 m Ah g^-1.The capacity does not decay and shows a slight increase.Through HRTEM,ex-situ XRD and Raman analysis of the electrode after cycling,it is found that the carbon nanotube will undergo local amorphization with the repeated intercalation of ions,resulting in an increase of reactive sites.Thus,the reversible capacity rises with the increased cycling number.（5）The content of the previous chapters is devoted to solving the problem of poor kinetics of onion-like carbon.This part,we try to improve the capacity of onion-like carbon through the strategy of preparing silicon-carbon composites.Different volumes of pyridine are used as the carbon source and a certain mass of nano-silicon as the silicon source,the core-shell structure composites of silicon encapsulated by onion-like carbon were prepared.Through the relative adjustment of the proportion of silicon and carbon in source,the silicon content of the composites can be controlled（54%,48%,32%and 14%）,and the particle size was also regulated from micron-size to nano-size.When used as anode for lithium-ion batteries,the onion-like carbon coating exhibits an excellent ability to buffer volume expansion.The most important thing is the synthesis method has only one step,which has the potential of large-scale production,providing an important idea for the large-scale production of silicon anode.