The Preparation Of Properties Of Vanadium-based Cathode Materials For Rechargeable Lithium-ion Batteries

Lithium-ion batteries were widely used in portable electronic devices,mainly due to their high energy density,good cycle performance and charge retention ability.Moreover,as the potential power sources of the hybrid vehicles（HV）and electric vehicles（EV）,LIBs were widely studied.But at present their electrochemical properties cannot fully meet the requirements of high energy density,high power for power sources of HV and EV.This is mainly because most commercial and studied cathode materials,such as LiCoO₂、LiMn₂O₄、LiFePO₄ and LiNi_1/3Co_1/3Mn_1/3O₂,which have an intrinsic constraint,i.e.low capacity.V-based cathode materials,such as V₂O₅,LiV₃O₈,VO₂（B）and Li₃V₂（PO₄）₃,possess relatively high theoretical specific capacity because of their abilities to intercalate more Li⁺ions per formula.However,due to the structure limitation of these materials,their actual capacity is much lower than the theoretical value.Synthesis of these materials with nanostructures can greatly enlarge their surface areas and reduce the Li⁺ion diffusion distance significantly,resulting in the fact that the actual specific capacity is closer to the theoretical value.Such V-based nanomaterials may make LIBs play an important role in the high efficiency store of energy,especially for power sources of HV and EV.This review focuses on the research development of synthesis of V-based nanomaterials,characterization and their corresponding electrochemical properties.In this thesis,V-based nanomaterials modified with carbon have been synthesized to enhance the lithium storage capability.The detailed contents could be seen as follows:（1）A novel graphene-nanosheet-wrapped LiV₃O₈ nanoflakes（GNS/LiV₃O₈）nanocomposite were generated by sheet-to-sheet self-assembly of ultrathin LiV₃O₈ nanoflakes and graphene nanosheets.When used as a cathode material for lithium-ion batteries,the GNS/LiV₃O₈ nanocomposites show superior rate capability and excellent cycling stability.Discharge capacities of approximately 328.7,305.3,276.9,251.4,and 209.3 mAh/g are achieved at current densities of 2,5,10,20,and 50 C,respectively.A reversible capacity of approximately 287.2mAh g^-1 is retained even after 100 cycles at 1.0 A/g（about 3 C）,approximately 88.3%of the initial discharge capacity.It is believed that the unique nanoflake morphology of LiV₃O₈ and the surface modification by graphene nanosheets contribute to the improved kinetics of lithium-ion diffusion,excellent structural stability and superior electrochemical performance.The structural evolution of LiV₃O₈ species upon charging and discharging is investigated by in situ X-ray diffraction technique.Anisotropic lattice expansion is found occurring along a,b and c axes upon the insertion of lithium ions into the crystal structure of LiV₃O₈.（2）LiVO/C nanoflakes nanocomposite were successfully synthesized through one step of simple hydrothermal treatment by using the graphene oxide and NH₄VO₃ as sources of carbon and vanadium.After XRD,SEM,TEM and other analytical approaches,we concluded that the phase of LiVO/C is the LiV₃O₈/Li_0.3V₂O₅/C nanoflakes nanocomposite material.LiVO/C nanoflakes composite shows quite excellent electrochemical properties.Approximately 99.2%of the initial capacity is retained after 50charge/discharge cycles at 200 mA/g.It is believed that graphene carbon film incorporated can solve the inherent properties of vanadium-based materials with poor electrical conductivity.The high performance is attributed to its unique nanoflakes compound structure,which can provide extremely large electrodes’surface area for high capacity density and ultrafast charge/discharge rate.This method of operation is simple,and the mole number of chemicals is easy to control.Moreover,compared to the conventional ion exchange method,this synthetic path has not any impurity ions to be introduced.Thus,we believe that it will be conducive to make a difference of commercialization.