Preparation Of Defective Graphene And Electrochemical Energy Storage Properties

Graphene materials draw attentions in the field of electrochemical energy storage thanks to good electrical conductivity,huge surface area and excellent ions transport,whereas the advantages stemming from their physical and chemical properties fail to be effectively utilized in the actual preparation process owing to the stacked lamellas.The defects are included in graphene to overcome the deficiency.The introduction of defect structure enables to enhance the specific capacity,perfect rate performance and improve the life performance of graphene electrode materials.Recent concerns focused on effects of defects on the electrochemical energy storage mechanism,improving defect quality,and solving the practical application problems of graphene electrode materials while great interests are shown in improved electrochemical energy storage performance of graphene materials.However,the complex defect environment in graphene materials makes it difficult to accurately study the structure-activity relationship between defects in graphene and electrochemical properties.The in-depth study of regulation on graphene defects and corresponding energy storage mechanism is critically significant.This paper introduces the folds,micro-pores and ribbon defects into graphene through different physical and chemical techniques,and looks at the properties of defective graphene on sodium-ion batteries and supercapacitors.The quantity and efficiency of defect structures are analyzed.The structure-activity relationship between the structure and electrochemical properties of defective graphene are analyzed by electrochemical analysis methods,and the energy storage mechanism is explored.First,graphene materials are prepared by reducing graphene oxide with active metal Al.By effecting the pulsation of hydrogen bubbles generated by the reaction of Al and hydrochloric acid in the reaction process,the alternating stress is applied to graphene and the folded structure is prepared.The control variable method reveals that the bubble pulsation impact plays a critical role in the generation of fold structure.The generation rate of bubbles is controlled by changing the degree of folds.The graphene platelets will bound after bubbles appear,having the mesopores size concentrated at the site in 4.52 nm and forming the“soft fold”structure.The graphene with“hard fold”structure without bounding will be be prepared by“quick-freezing method”accordingly,and the effects of soft and hard folds on the sodium storage properties of graphene will be compared.The excessive fold structure of hard fold worsens the transmission of electrolyte and impairs the mass transfer process.Furthermore,the adaptability of“mesopore-neck”in the mesoporous network is reduced owing to failed bounding,and decreasing the transmission efficiency of sodium ions.Graphene with soft folded structure exhibits better capacity,rate performance and cycle life.Moreover,excessive fold structure will increase the pore volume and reduce the volume performance of materials.Finally,the physical quantity of fractal dimension is applied in this paper to characterize the folding degree of graphene,and explore a more efficient folding structure of sodium-ion storage,with the aim of providing reference for subsequent folding engineering.The results show that crumpled graphene with fractal dimension of 2.74 is beneficial to sodium ion storage.Then,graphene block with abundant pores on the surface is prepared by thermal reduction,and appropriate sodium ion storage properties are tested.The block structure further intensifies the stacking phenomenon of graphene platelets,the synthesis process introduces abundant on-plane micropores and internal space,leaving sodium storage active sites for sodium ions.By studying the effects of different reduction temperatures and atmospheres on the microstructure and sodium storage properties of graphene block,it is found that the quantity and quality of micropores in graphene block have effects on the sodium-ion storage capacity of graphene block.By feat of the regulation on microporous microstructure,the stacking phenomenon of graphene block is extremely severe,significantly considerable specific capacities and excellent cycle life are also presented,and the laminated structure supports effectively improving the volume properties of materials.The graphene block under 800℃and H₂ atmosphere heat treatment condition shows the best effective micropore volumes(7.7×10^-2 cm³ g^-1)and capacities(286 m Ah g^-1/347 m Ah cm^-3 at 0.1 A g^-1).In the end,the dynamic sodium storage process of sodium ions in graphene block is revealed with ex-situ X-ray diffraction,Raman spectroscopy,transmission electron microscopy as well as electrochemical impedance spectroscopy and galvanostatic intermittent titration technique analysis of graphene block at different potentials,and the sodium storage mechanism of graphene block is studied,casting reference significant for micropore design in the future.Graphene ribbons are prepared by cutting graphene platelets with“spray-rapid freezing”technique.The unique one-dimensional structure of graphene ribbons adds the geometric types in the stacking process of graphene ribbons,prevents graphene agglomeration,generates satisfactory“microporous-mesoporous-microporous”structure,and reduces the transmission resistance of electrolytes and ions.Besides,graphene ribbons mutually overlap in the stacking process thanks to one-dimensional structure,and generate on-plane micropores,providing a channel for ions to traverse the active material layer.The geometric characteristics of graphene ribbon partially neutralize the reduction of mass transfer performance and deteriorated ion transport caused by the increase in the load.The graphene ribbon is coated on nickel wire by means of the cycle with“dipping-drying”step in this paper,solving intrinsic disadvantage of low load of metal fiber active materials.Using graphene ribbon increases the mass loading of fiber-shaped electrodes and provides excellent capacitance(73.8 m F cm^-1at 2 m V s^-1),and rate performance(36%at 200 m V s^-1).However,the graphene only provides 14%capacitance retention at the same measured condition.The fiber supercapacitor coming with metal fiber loaded with active materials enables to fully utilize high conductivity,weldability and a certain degree of flexibility of metal wire,inspiring the design of maintaining electrochemical performance under high load and improving the adaptability of energy supply elements in microcircuits.