Preparation Of Graphitized Carbon Loaded Transition Metal (Hydroxy) Oxides And Their Electrochemical Energy Storage

To achieve the strategic goal of carbon peaking and carbon neutrality,the proportion of clean energies such as wind energy and solar energy has increased year by year,significantly reducing the use of carbon-containing fossil energy.However,these energy sources have intermittency,volatility and so on.The usage of electrical energy storage technologies（battery,supercapacitor,etc.）can well control these energy sources,achieving continuous and stable power supply.They are also the power sources for mobile devices,new energy vehicles,etc.High energy density,high power density,long lifetime,and low cost are important factors for application of energy storage devices.Reasonable selection and design of phase and microstructure of electrode materials are the keys to realizing high-efficiency energy storage in supercapacitors and batteries.Porous carbon materials have become the preferred materials for super electric double layer capacitors（EDLCs）due to their high specific surface area and porosity,strong adsorption capacity,and good chemical stability.However,obtaining high power density and energy density at the same time poses a great challenge to the structural design and preparation of porous carbons.Highly conductive graphite is used as anode material for lithium-ion batteries（LIBs）,and its dense and ordered layered structure endows stable electrochemical performance,but the low capacity and slow intercalation kinetics hinder the development of high-power batteries.（Hydro）oxides of transition metals（nickel,cobalt,manganese,iron,etc.）show great potential as electrode materials for supercapacitors/LIBs due to their high pseudocapacitance/theoretical capacity for lithium storage.However,their low intrinsic conductivity and lattice distortion and huge volume change during energy storage have serious impacts on the rate capability,and cycling stability of capacitors/LIBs.Therefore,in order to take into account of the energy density and power density of capacitors and LIBs,it is necessary to reasonably regulate the phase,pore structure and morphology of carbon materials and transition metal（hydro）oxides,and construct synergistic structures to optimize their electrochemical performance.Based on this,this thesis controls the phase and pore structure of carbon materials by low-temperature catalytic graphitization using an inexpensive polystyrene as carbon source,and the nanostructured transition metal（hydro）oxides epitaxially grew or in loaded on the obtained graphitized carbon/porous carbon heterostructure to form stable phase heterostructure,thereby improving the electrochemical performance.The main research contents are as follows:（1）Controllable fabrication of few-layer graphene/porous carbon phase heterostructure for supercapacitor applicationsThe few-layer graphene/porous carbon（FLG/PC）heterostructure has the advantages of high electrical conductivity of graphene,high specific surface area and porosity of porous carbon,thus reasonable regulation of the phase and pore structure is critical to synchronously achieve high power density and energy density of EDLCs.The cheap expanded polystyrene（PS）spheres were selected as precursor.The hypercrosslinking structure of PS spheres was achieved by regulating the solvation between the crosslinking agent（formaldehyde diethyl acetal）and the catalyst（anhydrous ferric chloride）,and then the FLG/PC heterostructure materials were prepared through in-situ loading of the catalyst and catalytic graphitization process.The effects of the ratio of crosslinking agent to catalyst and carbonization temperature on the morphology,phase and pore structure were investigated by SEM,XRD,Raman,and BET.The results show that when the molar ratio of crosslinking agent to catalyst is 1:1,the obtained P-F2C2 has the largest specific surface area(1158 cm²·g^-1),micropore volume(0.47 cm³·g^-1)and optimal average pore size（3.17 nm）.The P-F2C2electrode shows excellent capacitive performance.The CV curves maintain quasi-rectangular shapes at 30-200 m V·s^-1,and the CP curves show symmetrical isosceles triangle shapes at 0.5-30 A·g^-1 without obvious IR drop.The specific capacitance（Cs）at 0.5 A·g^-1 is 265 F·g^-1,and the Cs maintains 60%after the current density increases to 30 A·g^-1,showing a good rate performance.Importantly,the P-F2C2 electrode has energy densities of 36.7 Wh·kg^-1 and 21.9 Wh·kg^-1 at power densities of 250 W·kg^-1and 15000 W·kg^-1,respectively.At a high current density of 10 A·g^-1,the Cs retention is as high as 95.0%after 5000 cycles,and the Cs of the symmetric supercapacitor maintains 90.2%after 10,000 cycles at 5 A·g^-1,showing excellent cycling stability and high coulombic efficiency（>99.8%）.（2）Preparation of Ni Co-LDH@FLG/PC heterostructure materials for application in asymmetric capacitorsBased on the above-mentioned performance advantages of FLG/PC heterostructure materials,nickel-cobalt layered double hydroxide（Ni Co-LDH）was dispersed and grown on the FLG/PC heterostructure as matrix by low-temperature hydrothermal method,and the curly-ball shaped LDH@C composite materials were obtained.Meanwhile the porosity and interlayer spacing of hydroxide were regulated by the intercalating agent of potassium acetate.The increase of interlayer spacing significantly improved their electrochemical performance.As a positive electrode,LDH@C shows high Cs of 1950 F·g^-1 at 0.5 A·g^-1 and 1384 F·g^-1 with retention of71.0%even at a high current density of 20 A·g^-1.LDH@C-6KAc//FLG/PC battery-type asymmetric supercapacitor assembled with FLG/PC as anode show significantly higher energy density of 87.1 Wh·kg^-1@800 W·kg^-1 than CNTs@Ni Co-LDH//AC(37.38 Wh·kg^-1@800 W·kg^-1),Co S_x/Ni Co-LDH//AC(35.8 Wh·kg^-1@800 W·kg^-1),Ni Co-LDH/IPC//IPC(29.6 Wh·kg^-1@744 W·kg^-1)and other similar devices.When the power density increases to 16000 W·kg^-1,the corresponding energy density remains48.9 Wh·kg^-1,which is also much higher than the reported similar devices.The Cs still maintains 87.6%after 5000 times of rapid charge and discharge at a high current of 5A g^-1.Furthermore,the microscopic mechanism behind the excellent electrochemical performance of asymmetric capacitors was also investigated.（3）Preparation and lithium storage performance of N-doped graphitized carbon-coated Fe₂O₃ nanoparticles/graphitized carbon hollow fibersCheap Fe₂O₃ with a high theoretical lithium storage capacity is a potential anode material to replace graphite and improve the energy density of LIBs.However,the low coulombic efficiency,rapid capacity fading,poor rate capability and low intrinsic conductivity caused by large volume change during charge-discharge process limit its application in LIBs.Here,we developed a novel structure of hollow fibers through a controllable catalytic graphitization and oxidation process,forming N-doped graphitized carbon-coated Fe₂O₃ nanoparticles/graphitized carbon walls（GF-Fe O@NC）.The internal highly graphitized hollow carbon fiber framework is used to provide structural support,conductive network and buffer space of volume expansion for Fe₂O₃ nanoparticles.The nanometer size of in situ formed Fe₂O₃ particles reduces the expansion stress.The N-doped graphitized carbon coating avoids direct contact of Fe₂O₃ nanoparticles with the electrolyte to form a stable thin SEI film and its rich defects of provide channels for the shuttle of Li ions.The GF-Fe O@NC electrode shows the first discharge/charge specific capacity as high as 1355/974 m Ah·g^-1 and an initial coulombic efficiency（ICE）of 72.0%,which is higher than that of reported similar iron oxide-carbon composites.In the second cycle,the reversible charging capacity retains 964 m Ah·g^-1,and the CE rapidly climbs to 98.0%,showing an excellent reversible lithiation/delithiation reaction.After 200 cycles at 0.1 A·g^-1,the reversible capacity is 918 m Ah·g^-1 with retention of 94.3%,and the CE is as high as99.6%.It is worth noting that the GF-Fe O@NC electrode indicates better cycling stability with higher current density of 2 A·g^-1.After 300 cycles,the capacity retention and CE respectively reach 97.1%(544 m Ah·g^-1)and 99.7%,also demonstrating a good rate performance.These are much more excellent than the lithium storage performance of other reported iron oxide/carbon fibers or nanotube composites.Combining the CV and EIS results,the effect of the material’s structural design on Li-ion kinetics is analyzed.The ICE of the full battery assembled with the pre-lithiated GF-Fe O@NC anode and Li Fe PO₄ cathode is up to 88.5%,and a stable specific capacity of 110m Ah·g^-1 is obtained after 100 cycles at 0.2 C,showing good practical application potential.