Preparation Of Three-dimensional Graphene-like Based Materials And Their Application In Supercapacitors And Lithium Metal Batteries

With the development of the global economy,the increasing demand for energy has led to the rapid consumption of non-renewable resources such as oil,coal,and natural gas,thus placing higher demands on advanced energy recovery,conversion,and the development of storage equipment.This is a major issue of global concern for storage devices.Therefore,researchers have invested a lot of energy in designing environmentally friendly energy storage devices,such as secondary batteries,fuel cells,and supercapacitors.But overall,the problems of low energy density,short service life,and high production costs still restrict the practical application of these devices.As one of the core components of the new energy storage battery,energy storage materials,in order to meet the needs of actual use,we hope to find an energy storage material with high energy density,long service life and low production cost.Since graphene was discovered,it has been paid much attention because of its excellent performance in many fields for its wide application potential application and revolutionary changes.However,due to its large specific surface area,graphene often causes the agglomeration together,which reduces its own adsorption capability.As a result,the theoretical performance cannot be achieved in practical application,thereby limiting its large-scale practical application.As a new type of graphene,three-dimensional（3D）graphene not only maintains the intrinsic characteristics of graphene,but also has unique interconnection,self-supporting structure and porosity,and can effectively prevent the agglomeration or re-stacking between 2D graphene nanosheets.In addition,it can also provide a large specific surface area,high mechanical strength,and fast electron and ion transport.It played an important role in promoting the practical application of graphene and further enhancing the energy density,cycling stability,and rate performance of energy storage materials.This work explores the preparation of novel three-dimensional graphene-based energy storage materials,and was creatively adopted water-soluble polyvinyl alcohol to prepare three-dimensional hierarchical porous 3D graphene-like materials（3D GP）with good conductivity,stable electrochemical properties,and adjustable specific surface area and pore structure.The use of three-dimensional multi-stage pore-like graphene structural materials as the base material,combined with its own structural characteristics and the addition of lithium-philic nitrogen carbide,provides the possibility of commercial application of lithium metal batteries.The specific research contents are as follows:（1）We use water-soluble polyvinyl alcohol as a carbon source,catalyzed by transition metal ions,and activated by an activator,prepare a new three-dimensional hierarchical graphene-like material（3D GP）.As a results,the as-prepared three-dimensional graphene has a specific surface area of up to 1370 m²g^-1,a conductivity of 1.0×10³S m^-1,and a rich pore structure.By adjusting the reaction temperature and the ratio of reactants,three-dimensional porous graphene with pore walls less than 5 nm can be obtained.By optimizing the heating time and the ratio of reactants,the thinnest walls（<5 nm）was obtained.（2）This material is applied in organic electrolyte for supercapacitors,and its specific capacitance reaches up to 170 F g^-1at 1 A g^-1.When the discharge current density increases to 10 A g^-1,it still maintains at 160 F g^-1,which is about 94%capacity retention.At a current density of 1A g^-1,the specific capacitance did not decline significantly after 10,000 cycles.The electrochemical performance of the supercapacitors is far superior to commercial activated carbon powder.In addition,the material is applied to aqueous supercapacitors when the discharge current density is 0.5 A g^-1,its specific capacity is calculated to be as high as 288 F g^-1.When the discharge current density is 10 A g^-1,the specific capacitance still reaches 248 F g^-1,which is about 93%capacity retention,indicating a higher specific capacitance and fast,ultra-stable electrochemical performance.The results confirmed that3D GP has a high specific capacity,rapid charge and discharge capability,and stable electrochemical performance in both aqueous and organic capacitors.3D GP provides a new idea for the application of carbon materials in high-power devices.（3）Using 3D GP as a matrix and mono-cyanamide as a raw material,g-C₃N₄was uniformly coated on the surface of 3D GP through in-situ polymerization and followed carbonization processes to form a g-C₃N₄/3D GP composite.Due to the high specific surface area and good conductivity of 3D GP,it is helpful to decrease the local current density.At the same time,the low over-potential of lithium nucleation in g-C₃N₄induced the uniform deposition of lithium on the surface of g-C₃N₄/3D GP.The high specific surface area combined with the abundant pore structures of 3D GP form a reservoir for the deposition of lithium metal,which can deposit a large amount of lithium metal and increase the energy density of the device.The current density of g-C₃N₄/3D GP composite is 0.5 m A cm^-2,and the area capacity of the deposition of lithium metal is 1.0 m A h cm^-2,the Coulomb efficiency of more than 99%is maintained after 250 cycles.The overpotential of lithium nucleation is almost 0 V.When the current density is 0.5 m A cm^-2and the area capacity of the deposition of lithium metal is 4.0 m A h cm^-2,the Coulomb efficiency was remained at above 98%.When the Li@g-C₃N₄@3D GP electrodes were used as a symmetrical cell,it has good cycle stability for over 500 hours.