| In recent years,the development of portable electronics,renewable energy and electric vehicle demand better performance for lithium ion battery(LIB).However,the high cost and low energy density of tranditional cathode material have impeded the further development of LIB.Therefore,the exploitation of cathode materials with higher energy density,better safety and longer cycle life has become the hot area of research.Compared with traditional cathode materials,V2O5 possesses higher specific capacity and better safty,thus attracting extensive attention.However,the inherent inferior conductivity of V2O5 leads to the unsatisfied cycle life and rate capability in LIB.In order to overcome the shortcoming of the inferior conductivity in vanadium-based cathode materials,four kinds of vanadium-based oxides/sulfides electrodes have been fabricated in this work.The electrochemical performance of V2O5 cathode materials have been improved,and the mechanisms were discussed as well.The electrochemical performance and reaction mechanism of VS4 as a new cathode materials were also studied.V2O5 hollow spheres(V2O5-HS)are prepared by a template-free method.The V2O5 nanosheets in V2O5-HS can shorten the diffusion path of lithium ion in electrode effectively,while the hollow structure of V2O5-HS can provide more active reaction sites and increase the contact area between electrode and electrolyte.The synergistic effect of nanosheets and hollow structure guarantees the better cycle performance and rate capacity in such V2O5-HS electrode.Typically,V2O5-HS demonstrate the specific capacity of 141 mAhg-1 after 200 cycles at current density of 0.5 C.Moreover,the specific capacity can be maintained at 91 mAhg-1 after 500 cycles at 1.5 C.Additionally,the V2O5-HS deliver the specific capacity of 258,216,171,139 and 86 mAhg-1 at current densties of 0.3,0.6,1.7,3 and 6 C.Self-supported N doped carbon coated V2O5 nanobelt arrays(V2O5@C-N)directly grow on a flexible carbon cloth through a template-free hydrothermal method and followed calcination,while the N doped carbon derived from the carbonization of coating polydopamine layer.The self-supported electrodes avoid the addition of conductive agent and binder,simplifying the manufacture process of battery.And the coating N doped carbon improves the structure stability of the V2O5 electrode and reduces the charge transfer resistance between electrode and electrolyte.Therefore,superior electrochemical performance is exhibited in such self-supported V2O5 cathode.Specifically,V2O5@CN/carbon cloth delivers the initial discharge specific capacity of 272 mAhg-1,and maintains the specific capacity of 215 mAhg-1 after 50 cycles at a current density of 0.5 C.Moreover,a reversible capacity of 130 mAhg-1 at high current density of 10 C.In addition,V2O5@C-N/carbon cloth displays capacitive capacity contribution according to the further electrochemical kinetics analysis.Typically,the contribution rate of capacitive capacity is 6 %,10 %,17 % and 24 % at scan rates of 0.1,0.2,0.5 and 1 m V s-1 respecitively.V2O5@graphene composites(V2O5@G)are synthesized through liquid exfoliation of V2O5 and in situ compositing with graphene nanosheets.Compared with bulk V2O5,V2O5@G shows improved cycle performance and rate capacibility.Specifically,V2O5@G composites exhibit the specific capacity of 203 mAhg-1 after 245 cycles at current density of 0.5 C.Furthermore,a stable capacity of 136 mAhg-1 is obtained after 990 cycles at high current density of 2 C.Besides,the composites deliver specific capacity of 275,250,230,200 and 165 mAhg-1 at current densities of 0.5,1,2,5 and 10 C.The superior cycle stability and rate performance of V2O5@G composites can be attributed to the typical stacked layer structures between V2O5 nanosheets and graphene,which not only lower the charge transfer resistance and electrochemical polarization,but also stabilize the structure of electrode.Moreover,the addition of graphene can improve the conductivity of the composite as well,leading to a good electrochemical performance.VS4@graphen aerogels composties(VS4@GAs)are fabricated via hydrothermal reaction and following freeze-drying method.And the VS4@GAs composites are firstly applied as cathode materials in lithium ion batteries with excellent rate performance.Specically,VS4@GAs composites exhibit the specific capacity of 486,442,379,325,243 and 190 mAhg-1 at current densties of 0.1,0.2,0.5,1,2 and 5 C.VS4@GAs composites display the initial specific capacity of 511 mAhg-1,and a reversible capacity of 112 mAhg-1 is obtained after 30 cycles at current density of 0.1 C.Benefiting from the good conductivity of graphene,nanosized material and obvious capacitive effect,V2O5@G delivers the best cycle performance and rate capability among the prepared V2O5 materials.The cycle performance and rate capability of V2O5@C-N/carbon cloth is inferior than that of V2O5@G for the larger size of material,but the electrochemical performance is still favorable due to the carbon coating. |
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