Research On Preparation Of Graphene Composites Generated From Waste Graphite And Its Electrochemical Performance

At present,a large number of new energy equipment represented by lithium-ion batteries have been put into use.Most of these batteries use graphite as anode.Graphite is an intercalation anode.During the charging process,lithium ion intercalates to graphite in 002 plane,and removed during discharging.As battery cycling,the lithium ion intercalated in the charging process will partly stay in the basal plane,resulting to larger interlayer spacing than fresh graphite.Graphite is a main precursor for the preparation of graphene.Graphene is a two-dimensional material with good conductivity and mechanical properties.However,graphene is apt to restack when dispersed in solution because ofπ-πinteraction,resulting in inconvenient subsequent application.Waste anode graphite has a larger interlayer spacing than commercial graphite,apply waste graphite as precursor,GO with larger interlayer spacing can be prepared under the same condition,which is more appropriate to the assembly of composite materials.In this paper,waste graphite is used to prepare GO,then the composite electrode material with better electrochemical performance can be obtained by combining GO with other materials,thus realizing the conversion and utilization of waste material.Sinano particles（SiNPs）and ferrous fluoride（FeF₂）are chosen to the combination with GO respectively.The electrochemical performance of these two composites is further studied,including specific capacity,capacity retention,rate performance,A.C.impedance,cyclic voltammetry and other tests.The specific research processes and results are as follows:1.Preparation of GO:Employ waste battery graphite and commercial graphite as precursors,apply concentrated sulfuric acid（H₂SO₄）and concentrated phosphoric acid（H₃PO₄）as intercalating agents,potassium permanganate（KMn O₄）as oxidant to prepare GO and test its water dispersion stability.The results show that the layer spacing of s GO prepared from waste graphite is 0.906 nm,while the layer spacing of c GO prepared from commercial graphite is0.828 nm.After sonication and settlement,the c GO dispersion shows obvious sedimentation,while s GO is more stable,indicating that from cycled graphite GO dispersion with better stability can be generated.2.The silicon-graphene composite is prepared by electrostatic attraction.The surface of silicon is modified to make it suitable to combine with the carboxyl group on the surface of GO.The green reducing agent ascorbic acid is used to reduce GO.After freeze-drying,the composite is added to the GO aqueous solution again.After full dispersion,a self-assembly process is carried out to form aerogel with two layers of rGO coating SiNPs,denoted as Si@GA1@GA2.After annealing,the electrochemical test is conducted and the results show that the cycling performance of the composite is better than that of the pure silicon.The specific capacity of the composite is 800 m Ah g^-1 after 400 cycles at 1 A g^-1,and 400 m Ah g^-1 after 300cycles at 2 A g^-1.3.For the production of FeF₂@rGO,FeSiF₆ solution is first obtained by mixing iron powder and fluorosilicic acid（H₂SiF₆）solution,after centrifugation the supernatant is mixed with GO aqueous dispersion,GO layers will agglomerate and precipitate as a result of charge balance changing between GO layers caused by Fe²⁺.Subsequently,collect and freeze-dry the precipitation,after high-temperature conversion and reduction.FeF₂@rGO composite is finally obtained.Profiting from the excellent electrical conductivity and mechanical property,rGO effectively improved the cycling performance and rate performance of FeF₂.The specific capacity is 298 m Ah g^-1 after 200 cycles at 0.1 A g^-1,and 150 m Ah g^-1 at 0.5 A g^-1.