Synthesis And Electrochemical Performance Of Vanadium Compounds/graphene Composites For Lithium-ion Batteries

Lithium-ion battery with high energy density,high open-circuit voltage and without memory effect has become the most ideal energy storage devices,which has been widely used in portable electronics and electric vehicles.Greater demands were being placed on the performance of lithium-ion battery by the miniaturization of electronic devices and the growing driving mileage of electric vehicles.The electrode material plays a key role in the lithium-ion battery,so the development of new electrode materials with highperformance becomes the frontiers in the research field of energe storage.In this thesis,the synthesis and electrochemical performance of vanadium pentoxide(V2O5)and vanadium sulfide(VS4)were systematically studied.The main contents are as follows.1.The synthesis and electrochemical performance of V2O5,V2O5/graphene(VG)and V2O5/carbon nanotubes(VCM)have been investigated.The initial specific capacities of the three materials at 0.2C are 140.5,227.5 and 242.3 mAh g-1,respectively.After 50 cycles,the specific capacities of bare V2O5 and VG retain 98.8 and 187.0 mAh g-1,respectively,which suggest that the electrochemical performance of V2O5 could be greatly improved by graphene.The capacity of VCM remains at 223.8mAh g-1 after 80 cycles.The discharge capacities of bare V2O5 powder,VG and VCM at 10 C were 5.1,75.2 and 109.3 mAh g-1,respectively.Compared with bare V2O5 particles,the graphene can improve the conductivity of VG and provide a path for the migration of lithium ions and electrons.Compared with VG,the porous microsphere on VCM surface can help the electrolyte penetrate into the whole structure to ensure the contact of active material and electrolyte.So VCM has better electrochemical performance.2.The synthesis and electrochemical properties of vanadium pentoxide microspheres(VM)and vanadium pentoxide microspheres/graphene(VMG)cathode materials were studied.At the current density of 0.2C,the initial discharge capacity of VM is 243.6 mAh g-1,and the specific capacity remains at 198.4mAh g-1 after 50 cycles.The VMG displays a intial capacity of 256.7 and remains 227.2 mAh g-1 after 50 cycles.The discharge capacities of VM at 5C,10 C and 20 C are 115.0,26.8 and 14.6 mAh g-1,respectively,and corresponding capacities of VMG are 217.2,191.6,156.2,and 117.1 mAh g-1,respectively.Compared with bare V2O5 particles,VM is composed of nanoparticles with uneven surface,so it has a larger surface area;moreover,the contact area between electrolyte and active substances have increased as the result.During the hydrothermal process of VMG,many oxygen-containing functional groups on GO act as nucleation sites to reduce the size of the particles and provide more active sites to improve its reversible capacity during the electrochemical reaction.Furthermore,the graphene nanosheets increase the conductivity,prevent the restacking of V2O5 microspheres and further suppress the volume change during charging/discharging process,resulting in further improving the electrochemical performance.3.VS2,VS4/graphene(VSG)and three-dimensional VS4/graphene(3DVG)were prepared by one-pot hydrothermal method.The capacities of the three materials are 74.7,502.4 and 985.1 mAh g-1 after 50 cycles at 200 mA g-1,and the discharge capacities at 2 A g-1 were 3.3,397.5,and 588.8 m Ah g-1,respectively.During the reduction process of GO,VS2 becomes VS4.Graphene with excellent conductivity can be used as a protective layer to prevent VS4 from volume expansion in the charge/discharge process to improve the performance.The 3D porous graphene can further enlarge the specific surface area to increase the infiltration of the electrolyte and provide more space for the volume expansion of active material in the charge/discharge process.