Preparation And Study On Electrochemical Properties Of Graphene/transition Metal Compound Nanomaterials

With the global energy shortage and the increasing pollution of the traditional energy,the development of new sustainable energy has become a crucial task.At present,lithium-ion batteries have been widely used in portable electronic devices,electric vehicles,smart grids and other fields due to their excellent energy density and cycling stability.However,the shortage and unreasonable distribution of lithium resources greatly hinder the large-scale use of lithium ion batteries.Recently,new electrochemical energy storage devices such as supercapacitors,sodium ion batteries and potassium ion batteries have been widely explored as suitable and potential alternatives to lithium ion batteries to promote the development of new and efficient energy storage.Among many new energy storage materials,transition metal compounds have become a new research focus due to their high energy density and conversion efficiency.However,such materials generally have some inherent properties such as low specific surface area,few active sites,poor conductivity,large volume expansion and easy crushing,which seriously restrict their application in new electrochemical energy storage.Therefore,the above problems can be effectively solved through the coordination and composition of the transition metal compound active material and the structure of the whole electrode.Firstly,the morphology and structure of transition metal compounds can be synthesized at the nanoscale to enhance the structural strength and stability of the materials,expand the specific surface and improve the dynamic performance.Secondly,substrate materials,such as graphene and conductive polymer,are provided based on the new nanostructure design of transition metal compounds.As a new kind of two-dimensional multi-functional material of single carbon atom,graphene has excellent optical,electrical and mechanical properties.The 3D graphene formed by furtherassembly has higher strength and flexibility,and its application in the electrode material can reduce the structural strain of the active material in the electrochemical storage process,enhance the conductivity and improve the efficiency of electronic transmission.In this paper,a novel transition metal/polymer nanomaterial and a nanomaterial electrode with three-dimensional graphene tightly wrapped transition metal compound were designed and synthesized based on the solution of the above problems.The application of these materials in new solid electrolyte or flexible energy storage device is developed.The specific research contents are as follows:（1）We successfully synthesized the porous multi-shelled cobalt tetroxide hollow microsphere protected by polypyrrole（pMS-Co₃O₄/PPy）composite material.pMS-Co₃O₄ hollow microspheres are composed of interconnected transition metal oxide nanoparticles with rich porosity and hollow shell layer,which can maximize the use of active sites,promote ion diffusion,and relieve structural strain.The polypyrrole coating not only significantly improves the conductivity,but also effectively protects the Co3O4 nanoparticles from structural crushing and chemical dissolution.Therefore,the composite electrode obtained has A high specific capacitance(1292.2 F g^-1 at 1 A g^-1),excellent multiplier performance(1205.8 F g^-1 at 10 A g^-1)and cyclic stability(91.5% of 5000 cycle retention at 10 A g^-1).The subsequently assembled all-solid asymmetric supercapacitor has an energy density of 40.2 Wh kg^-1 at a power density of 761.7 W kg^-1.At 10 A g^-1,the capacitance retention rate of5000 cycles is 90.6%.This study provides an effective structural design strategy to solve the electrochemical energy storage problem of transition metal oxides and promote the development of high-performance energy storage devices.（2）The three-dimensional graphene capsulated porous nickel oxide hollow microspheres（3DG/pMS-NiO）aerogel was successfully prepared through the improved self-template synthesis of porous nickel oxide multi-shelled hollow microspheres and the subsequent dopamine（DA）-assisted self-assembly method.Aerogel maintains the three-dimensional porous network structure of 3DG and the porous multi-shelled structure of p MS-NiO.The porous multi-shelled shell layer is composed of a large numberof NiO nanoparticles,providing a large area of active site with high porosity for electrochemical reaction.By simply pressing aerogel mechanically,the composite electrode film can be used as a flexible self-supporting electrode without adhesive.After testing,the specific capacitance of the flexible3DG/p MS-NiO composite film at 0.5 A g^-1 was significantly higher than that of 710.4 F g^-1,and the specific capacitance retention rate was 92.5% at 10 A g^-1.The new 3DG/p MS-NiO//AC all-solid flexible asymmetric supercapacitor showed high specific capacitance and excellent electrochemical performance(34.4 F g^-1 at 1 A g^-1).Meanwhile,the high energy density of 12.3Wh kg^-1 was obtained at the power density of 815.3 W kg^-1,and the capacity retention rate was 74.6% after 5000 cycles.This work has profoundly inspired material design and electrode preparation,and even opened a way for the development of high-performance and flexible flexible self-supporting energy storage systems.（3）A flexible and hierarchically porous anode consisting of discrete FeP hollow nanospheres well encapsulated within three-dimensional graphene（3DG）skeleton via a novel spatially confined one-step thermal transformation strategy using the exquisite 3DG wrapped metal-organic framework（MOF）nanoparticles as precursor.In this strategy,the confinement of 3DG could promote the microphase separation of MOF nanoparticle and nanoscale Kirkendall effect under the thermal treatment,which could not be achieved in MOF alone and had never been reported before.The obtained monolithic3DG/FeP composite aerogel integrated the 3D interconnected conductive network,hollow nanostructures and graphene encapsulation effect together,which was highly beneficial to the electron/ion transport and the alleviation of volume change.As a result,the 3DG/FeP composite aerogel could be directly used as flexible anode for KIBs upon simple mechanical pressing and delivered high reversible capacities of 332 and 323 m A h g^-1 at 0.1 A g^-1 for the first and300 th cycles,respectively,and excellent rate capability as well as ultrastable cycle performance with a capacity retention of 97.6% after 2000 cycles at 2 A g^-1,which was almost the best result in all reported FeP anodes for KIBs.The electrochemical storage mechanism was also systematically studied and confirmed multistep intercalation and conversion reversible reactions betweenthe FeP and K⁺（FeP ? K_xFeP ? Fe + K₃P）,which provided important insights into the development of high-performance TMP-based anode materials for KIBs.In this study,novel polypyrrole/transition metal compounds and graphene-based transition metal compound electrode materials were prepared and applied to high-performance supercapacitors and potassium ion batteries.Some problems in electrochemical energy storage applications of transition metal compounds are solved,and a new scheme and perspective for structural design of new energy storage materials are proposed.