Preparation Of Graphene Based Composites And Their Applications In Energy Storage Devices

The problem of energy shortage and environmental pollution is becoming more and more serious,which promotes the rapid development of new energy development and utilization technology.Research on energy storage devices such as supercapacitors and lithium ion batteries has attracted extensive attention.The electrode material of energy storage device is the key factor to determine its performance.Due to its high conductivity,high specific surface area and excellent electrochemical stability,graphene has shown broad application prospects in the electrode materials of energy storage devices.However,if a single graphene material is used as the electrode material of an energy storage device,the problem of low energy storage capacity will occur.In contrast,Cu（OH）₂ and silicon have the advantages of high theoretical energy storage capacity and environmental friendliness,but they will undergo volume expansion during the charging and discharging process,which will damage the structure of electrode material,leading to poor cycling performance.Therefore,Cu（OH）₂/graphene composite and porous silicon/graphene composite were prepared to improve their electrochemical properties in energy storage devices.The detailed research content is as follows:（1）Preparation of Cu（OH）₂/graphene composites and their applications in supercapacitors.Cypress-like Cu（OH）₂ nanostructures/graphene nanosheet composites were synthesized in situ on the surface of copper foam using a solution based two-electrode electrochemical method.This composite structure will promote charge transfer and effectively reduce the volume change in the long-term charge-discharge process.The experimental results show that the composite material,as the working electrode of the supercapacitor,has excellent specific capacitance,remarkable rate performance and extremely long cycle life.At the current density of 1 mA cm^-2,Cu（OH）₂/graphene nanocomposite has a high reversible specific capacitance of 317 mF cm^-2,and has excellent cycling stability when the current density is 2 mA cm^-2.After 20,000 cycles,the initial capacity retention rate of the composite material reached 100%.Moreover it has excellent rate performance at increased current densities.This method opens a new way for the rapid preparation of other hydroxides and graphene composites.（2）Preparation of porous silicon/graphene composites and their applications in lithium ion batteries.Porous silicon nanoparticles were prepared by chemical etching,and graphene oxide was prepared by improved Hummers method.Porous silicon nanoparticles coated with graphene composites were prepared by high temperature thermal reduction.As the working electrode of lithium ion battery,the specific capacity of this composite material can still reach 854 mAh g^-1 after 50 cycles under the current density of 0.1 A g^-1,showing better lithium storage performance and cycling stability.This is mainly due to the synergy between the porous structure of silicon and graphene coating.The porous structure of silicon can alleviate the volume change during the cycle and provide large specific surface area to facilitate the rapid transmission of lithium ions.Graphene coating can not only improve the electrical conductivity of the composites,but also inhibit the volume expansion of silicon and improve the structural stability of the composites.In addition,the porous silicon/graphite composites prepared by ball grinding method can maintain the specific capacity of 628 mAh g^-1 after 50 cycles at the current density of 0.1 A g^-1.The preparation method of ball milling porous silicon/graphite composites is simple and the cost is very low,which has a broad application prospect.