Design And Synthesis Of The Carbon And Its Nano-micro Composites For Energy Storage

Since the 21 st century,the serious energy crisis makes humans strive out of fossil fuels.New clean energy,such as wind energy,solar energy and tidal energy,etc.,has attracted a lot of attention from researchers.However,because of the instability and discontinuity,a series of energy conversion and storage devices have been developed for energy output in the form of electric energy,which have been widely applied in portable electronic devices,electric vehicles and even smart grid.Supercapacitor（SC）,as a new energy storage device between traditional electric double layer capacitors and batteries,exhibits a rapid charge and discharge,high power density and the excellent cycle stability.Among numerous energy storage devices,supercapacitors stand out and have been paid much attention,showing a rapid development.Moreover,due to the low capacity,narrow voltage and low energy density of the first-generation supercapacitor,people develop the second-generation hybrid metal ion capacitor,broadening their application in energy storage field.Their composition and structure of electrode materials directly determine the electrochemical properties of supercapacitors.In the current study,different kinds of electrode materials show their unique strengths.Among them,carbon materials show a large specific surface area,high conductivity and excellent electrochemical stability,which is a vital electrode material in supercapacitors and hybrid metal ion capacitor.Unfortunately,relatively low specific capacity and narrow voltage window limit the practical applications of supercapacitors.Therefore,designing and synthesizing high-performance carbon-based materials are critical to develop supercapacitors.In this dissertation,based on structure design,in-situ interface control and composite modification,the active sites were introduced by heteroatom doping,microstructure was designed to increase the electrode surface area and transition metal compounds were introduced to increase the capacity of electrode,which was effectively improve the properties of supercapacitors.Therefore,four kinds of carbon materials with high power density,high energy density and good cycle stability were prepared.Moreover,by introducing high voltage electrolyte,a series of high output supercapacitors and lithium ion capacitors were successfully assembled.The detailed research content includes the following four aspects:（1）Porous carbon with willow-leaf-shaped pores for high-performance supercapacitors.In this work,a novel willow-leaf-shaped porous carbon（HPC-X,X represents the activation temperature）was derived from gelatin and citric acid via interface control and structure optimization.A large number of N atoms and O atoms realized the in-situ heteroatom doping modification on the surface of carbon materials,which not only introduced Faraday reactive sites and provided pseudocapacitance contribution,but also enhanced the wettability of electrode materials and effectively improved the capacity of carbon materials.The introduction of pore-forming agent（FeCl₃）successfully promoted the formation of 3D porous carbon structure,increased the specific surface area of the electrode material,exposed more reactive sites and shortened the ion transport path,further optimizing the rate performance of the carbon material.As the electrode material in supercapacitors,HPC-650 shows a specific capacitance of 312.3 F g^-1at 1 A g^-1.When the current density increases to 20 A g^-1,its specific capacitance only loses 23.5%,showing excellent rate properties.The assembled symmetric supercapacitor has an energy density of up to 50.22 Wh kg^-1at a power density of 1.19 kW kg^-1,demonstrating a great potential for practical applications in energy storage.（2）N,B doped lamellar porous carbon electrode for high-performance Li ion capacitor.Although designing porous structure and heteroatoms modification can increase the capacity of carbon materials and the energy density of supercapacitors in the first work,inorganic electrolytes in supercapacitors limit the operating voltage,which cannot provide a high energy output.Therefore,a kind of layered porous carbon material doped with B,N and O heteroatom（C-B-X,X represents the addition content of borax）was successfully prepared via introducing B atoms,further modifying the carbon surface.With the help of organic electrolytes,the operated voltage range was broadened to increase the energy density of lithium ion supercapacitors.Firstly,we utilized the characteristics of heat decomposition and gas escaping during the calcination process of borax,successfully constructed two-dimensional lamellar structure.Then,the pore-forming agent FeCl₃ etched the carbon,further forming the porous structure inside and greatly increasing the specific surface area and pore volume of carbon materials.Moreover,B and N atoms were modified on carbon surface,increasing the Li⁺ active storage sites for more Li⁺ adsorption,optimizing surface properties of C-B-X materials and improving the interface contact between the electrode and electrolyte.As an anode for Lithium ion capacitors,the C-B-2 shows a capacity of 780 mAh g^-1at 100 mA g^-1.As a capacitive cathode,it also displays a high capacitance of 125 mAh g^-1at 100 mA g^-1.Furthermore,the symmetric Li ion capacitors with C-B-2 as both cathode and anode（C-B-2//C-B-2 LICs）can achieve a capacity of 61.6 F g^-1at 100 mA g^-1with a high energy density of 173.1 Wh kg^-1at a power density of 225 W kg^-1,showing the potential practicability in energy storage field and providing an important idea for designing high-performance double-carbon LICs.（3）Fe₃O₄ nanoflakes-RGO composites electrode for high-performance lithium ion capacitors.From the aboved two works,we found that structural design and interface modification can only increase limited capacity of carbon materials.Therefore,we combined transition metal compounds and carbon as composites,effectively enhancing the capacity,conductivity and stability of composite electrodes.Different from the one-spot synthesis method,a broccoli-like FeCO₃ material was prepared via a simple solvothermal method at first.Then,a kind of Fe₃O₄ nanoflakes-RGO composites were synthesized utilizing the hydrolysis reaction of broccoli-like FeCO₃ in the solution of graphene and hydrazine hydrate,realizing an interesting phase and morphology transformation from FeCO₃to Fe₃O₄.This multichannel structure of FeCO₃ benefited for the permeation of of alkaline solution and improved the dispersion of Fe²⁺.Finally,the Fe₃O₄ nano-flakes were uniformly anchored onto the reduced graphene oxide（RGO）.The introduction of graphene increased the reactive sites,shortened the ion transmission path and promoted the Li⁺ transmission,endowing Fe₃O₄-RGO composites fast Li reaction kinetics and satisfied rate properties.A further theoretical calculation also proved this point.The Li adsorption energy of Fe₃O₄-RGO composites（-3.24 eV）is lower than that of Fe₃O₄（-1.39 eV）.Moreover,when Li is in contact with Fe₃O₄,the electrons will rearrange and accumulate at their interface.The addition of RGO leads to both increase for electron accumulation area and electron loss area in Fe₃O₄-RGO composites,indicating that the existence of RGO enhances the charge transport between Fe₃O₄ and Li.In addition,RGO protected Fe₃O₄ from expansion,which effectively improved the capacity,conductivity,rate properties and cyclic stability of Fe₃O₄-RGO composites.As anode materials,the Fe₃O₄-RGO composites deliver a desired capacity of 925.3 mAh g^-1at 100 mA g^-1.When Fe₃O₄-RGO composites and commercial activated carbon are assembled into lithium-ion capacitors（Fe₃O₄-RGO//CAC LICs）,a “4.2 V” LICs can show a high energy up to 0.41 Wh,further demonstrating the excellent electrochemical performance of the LICs.This novel-innovative synthetic route abandons the high-temperature calcination and successfully decreases the energy consumption,which will promote the development of practical application for iron-based materials.（4）Micron-scaled MoS₂/N-C particles with embedded nano-MoS₂ for high-performance lithium ion capacitors.Compared with the lattice spacing of graphite（0.34nm）and Fe₃O₄（0.254nm）,the layered MoS₂ with unique sandwiched S-Mo-S layers are connected together by weak van der Waals with the adjacent layer spacing（0.615 nm）,in favour of accommodating more Li ions.In this work,from the view of structural design and composite modification,a micron-scaled N-doping porous carbon skeleton with embedded nano-MoS₂ material was synthesized.Biomass carbon sources（egg yolk）contained a large number of N and O atoms,which successfully modified the carbon surface of composites,achieved in-situ heteroatom doping and improved the conductivity and electrochemical activity.The porous carbon as the skeleton limited the volume expansion of embedded MoS₂.MoS₂ with one or few layers shortened the ion/electron transport path,showing excellent rate performance and long cycle stability.In addition,combining their advantages of nano MoS₂ and micron carbon,the poor conductivity,easy sulfur loss and serious agglomeration in the lithiation/delithiation process have been improved successfully.As anode materials,MoS₂/N-C composites maintain at 805 mAh g^-1after 100 cycles at 100 mA g^-1.Also,the assembled “4.2 V” MoS₂/N-C//CAC LICs can deliver a high energy of 0.55 Wh,demonstrating the high energy output and potential application value of the LICs.