Design And Preparation Of High-Performance Carbon-Based Composites And Investigation Of Their Electrochemical Properties

In recent years,with the rapid development of economy and science in the world,environmental pollution has become more and more serious,accompanied by the depletion of non-renewable fossil energy.In order to solve above problems,renewable new energy sources such as wind energy,solar energy and ocean energy have gradually attracted extensive attention.However,these renewable energy sources have the problems of instability and discontinuity.Therefore,these renewable energy can be converted into green,clean and pollution-free electricity using the conversion system,which is an effective way to make full use of renewable energy in the fields of portable electronic products and electric vehicles.Among the energy storage systems,lithium-ion batteries（LIBs）have been extensively studied due to their high charge-discharge efficiency,long cycle life and environmental friendliness.The electrochemical performance of LIBs including capacity,charge-discharge efficiency and cycle life mainly depends on the choice of electrode materials.Among many anode materials for LIBs,carbon materials,such as one-dimensional carbon nanotubes,two-dimensional graphene and three-dimensional porous carbon,have attracted wide attention because of their abundant reserves,good conductivity,stable structure and environmental friendliness.Graphite is the most commonly used electrode material for the commercialized LIBs,but its theoretical specific capacity is only 372 mAh g^-1,which is difficult to meet the needs of high energy density of LIBs.Therefore,surface control,structural design and composite modification are effective ways to solve the low capacity of single carbon materials.In this dissertation,four kinds of carbon-based materials have been prepared by surface control,structural design and composite modification.Both the energy density and power density of carbon materials have been effectively improved.The main research results are listed below:（1）The preparation of nitrogen-riched mesoporous carbon.By surface control,nitrogen atoms can effectively increase the wettability of carbon materials.In this work,egg yolk is used as carbon source,and the precursor of carbon materials is obtained by hydrothermal method.After annealing,nitrogen-riched mesoporous carbon materials are obtained.The main reason for choosing yolk as carbon source is that yolk contains abundant nitrogen elements,which can be doped in situ.At the same time,the electronegativity of nitrogen atom is lower than that of carbon atom,which is more conducive to adsorb lithium ion and provide more active sites for electrochemical reactions,so as to improve the capacity of carbon materials.In addition,the yolk contains abundant inorganic molecules,which forms abundant mesoporous channels on the surface of the material during carbonization,facilitating the rapid diffusion and transfer of lithium ions and charges.The nitrogen-rich mesoporous carbon materials used as anodes for LIBs exhibit excellent electrochemical performance.The capacity is711.6 mAh g^-1 after 100 cycles at 100 mA g^-1.When the current density increases to500 mA g^-1,the capacity is still as high as 601.0 mAh g^-1 even after 300 cycles with the Coulombic efficiency of 100%,showing excellent rate performance.This simple synthesis method and excellent electrochemical properties provide an effective way for the preparation of anode materials for LIBs.（2）The preparation of graded porous and nitrogen-rich graphene.Graphene is easy to accumulate resulting from strong interlayer van der Waals attractions.This accumulation structure is not beneficial to the diffusion and transmission of lithium ions and electrons,but also reduces the contact area with electrolyte,resulting in the loss of active sites.In order to solve the above problems,porous and nitrogen-rich g-C₃N₄@RGO is prepared using urea as heteroatom dopant.A large amount of NH₃ and CO₂ are produced during urea decomposition,which alleviates the accumulation of graphene,at the same time formed g-C₃N₄.Finally,rich microporous are fabricated on the surface of the material by potassium hydroxide activation.Firstly,g-C₃N₄ will provide abundant active sites and increase the adsorption capacity of Li⁺,which is beneficial to the increase of capacity;secondly,the large pore structure is conducive to accelerating the infiltration of electrolyte and improving the transport rate of Li⁺;finally,the abundant microporous provides a fast ion diffusion channel and facilitates the Li⁺transfer,which benefits the improvement of rate performance.The layered g-C₃N₄@RGO composites exhibit excellent electrochemical properties,especially the rate performance.The electrochemical test shows that the capacity of g-C₃N₄@RGO is still as high as 595.1 mAh g^-1 after 1000 cycles at 1 A g^-1 with the Coulombic efficiency of 100%.The electrochemical kinetics study shows that the charge transfer resistance decreases with increase the cycles.It proves that with the electrochemical reaction,the electrolyte is gradually immersed into the porous materials,and the electrochemical process becomes more and more completely.The simple synthetic method provides a new idea for improving the electrochemical properties of graphene.（3）The preparation of NiMoO₄ carbon nanofiber composites（NiMoO₄-NFs）.By structural design and composite modification,NiMoO₄ nanorods are successfully transformed into NiMoO₄ nanocrystals by electrospinning technology,in which NiMoO₄ nanocrystals are embedded in the framework of one-dimensional carbon nanofibers（NiMoO₄-NFs）.Carbon nanofibers not only improve the conductivity of NiMoO4,but also play an important role in maintaining the stability of NiMoO4nanocrystals.The volume expansion of materials will be alleviated and the diffusion path of Li⁺will be shortened after nano-treatment,so that the cycle life and rate performance of materials are improved simultaneously.The nanofibers obtained by electrospinning can be directly used as anode materials for LIBs without any collectors,conductive agents and binders,which can greatly reduce the quality of the whole battery and can be used as flexible electrode materials for wearable electronic products.The electrochemical tests show that the electrochemical performance of NiMoO₄-NFs is greatly improved compared with pure NiMoO₄,especially the cyclic stability.The capacity of NiMoO₄-NFs is still as high as 893.0 mAh g^-1 at 100 mA g^-1 after 200 cycles.The electrochemical mechanism of NiMoO₄-NFs is investigated by ex-situ transmission electron microscopy.The tests show that NiMoO₄ is actually composed of two-phase phase conversion reaction,so the composite exhibits high capacity.（4）The preparation of C-SnS₂@rGO composites.To address the problems of poor conductivity,slow ion diffusion and volume expansion after cycling of SnS₂ as anode materials for LIBs,C-SnS₂@rGO composites with double carbon layer are prepared by one-step hydrothermal method using graphene as carbon skeleton and glucose as soft template.Carbon layers are formed on the surface of SnS₂ nanocrystals in situ after carbonization of glucose,and stable C-S bonds are formed between the nanocrystals and the carbon layers.Here,glucose plays the key roles in the formation of C-SnS2@rGO:firstly,carbon layer not only alleviates the volume expansion of SnS2,but also reduces the interface contact resistance of electrode materials,thus accelerating the diffusion of Li⁺and electrons;secondly,glucose also controls the size of SnS₂nanocrystals by adjusting the amount of glucose;finally,the addition of glucose can also make the distribution of SnS₂ nanocrystals on graphene more uniform,thus solving the problem of easy agglomeration of nanoparticles.The C-SnS₂@rGO exhibits excellent electrochemical performance,especially long cycle stability,in both lithium-ion and sodium-ion batteries.The Li-ion storage reversible capacity is still as high as710.0 mAh g^-1 at 2 A g^-1 even after 1000 cycles.The Na-ion storage reversible capacity is 450.0 mAh g^-1 at a current density of 100 mA g^-1 after 1000 cycles,showing excellent cyclic stability.