The Preparation And Electrochemical Performance Of Graphene-wrapped Natural Spherical Graphite As Anode Material For Lithium-ion Batteries

Natural graphite is considered as an important commercially available anode material due to its flat voltage profile,low cost,and high reversible capacity.However,natural graphite anode suffers from some disadvantage such as its high sensitivity to electrolytes and easy exfoliation,resulting in low initial coulombic efficiency,poor cycling stability and bad rate performance.Graphene,a two-dimensional monolayer of sp2 carbon atoms,is regarded as the building unit of graphite.Graphene has been introduced into LIB system due to its excellent electrical conductivity,high specific surface area,wonderful structural flexibility,superior thermal conductivity and good chemical stability.In this paper,graphene-wrapped natural spherical graphite(SG)was successfully prepared using a simple two-step approach starting from graphene oxide.The structure of the composites is revealed by scanning electron microscope(SEM)and X-ray diffraction(XRD).The electrochemical performance of the composites was investigated by a variety of electrochemical testing techniques.At the same time,we investigated the ffects of reduced graphene oxide(rGO)addition on the electrochemical performance of the natural spherical graphite electrode by mechanical lapping.And we also investigated the effects of acetylene black addition on the electrochemical performance of the graphene-wrapped natural spherical graphite for comparison.We found that the graphite electrode loaded with rGO exhibits superior electrochemical performance compared with that of adding traditional conducting agent-acetylene black(AB).Without the addition of conductive agent,the composite electrodes exhibit not only high specific capacities with higher first-cycle coulombic efficiencies but also good cycle stability and rate performance.Compared with SG+10%AB electrode,when the amount of graphene additive is 1%,the reversible capacity after 50 cycles is equal to that of SG+10%AB electrode,and when the amount of graphene additive is 2.5%,both the initial charge capacity and reversible capacity during 50 cycles are higher than that of SG+10%AB electrode.The significant improvement of the electrochemical performance for graphene-wrapped SG could be probably attributed to the graphene wrapping.On the one hand,the graphene shell enhances the structural integrity of the SG particles during the lithiation and delithiation process,further improving the cycle stability of the materials.On the other hand,by bridging adjacent SG particles together,it provides a highly conductive network for electron transfer among the SG particles.The graphene in the composite materials could act not only as an active material,but also as an excellent conductive agent to improve the electrochemical performance.We found that conductive agent is still needed for graphene-wrapped SG electrodes.The reversible capacities and rate performance of graphene-wrapped SG electrodes increase when 5 wt.%AB was added.It can be assigned to a better conductive network enhanced by graphene and AB synergistically.