Synthesis Of 1D Vanadium Nitride Composite And Perfoarmance Investigation For Lithium Sulfur/Lithium Ion Batteries

With the rapid development of the economy and the application of advanced portable electronic devices and vehicles,the demand of energy is largely increased.Compared with the traditional fuels,lithium ion batteries are more environmental friendly.They have higher energy density with lighter weight,and have already entered into all aspects of our social life.However,despite the wide application,lithium ion batteries now still cannot meet the demand for the development of the society.Therefore,there is an urgent need for new materials with high energy density and long cycle stability.Vanadium nitride(VN)has high electronic conductivity and chemical stability.It has been widely used in energy storage system,which is quite promising for application.In this thesis,we synthesized the vanadium nitride@N-doped carbon composite(VN@C),and investigated their performance in lithium sulfur batteries and flexible lithium ion batteries.The main contents of this thesis are listed as follows:(1)The V2O5 nanowire precursors were synthesized by a hydrothermal method using the commercial V2O5 particles as raw reagents.After they were coated with polymer,the V2O5 nanowires were annealed in ammonia atmosphere,then the as-prepared VN@C was used as sulfur host for lithium sulfur batteries.Due to the excellent conductivity of VN and carbon,the conductivity of the cathode was improved.In addition,after the introduction of VN,the battery showed an enhanced reaction kinetics.The conversion of the polysulfides was accelerated,and therefore the shuttle effect was alleviated.When the composite was used as the host material for lithium sulfur batteries,an enhanced capacity of 592.2 mAh g-1 was achieved at the current density of 1 C,with the columbic efficiency of nearly 100%.Even with high areal sulfur loading of 4.2 mg cm-2,the cathodes still delivered an advanced performance of 1127 mAh g-1 which presented a new orientation for the further application of Li-S batteries.(2)The highly flexible vanadium nitride@N-doped carbon/graphene(VN@C/G)composite was conveniently prepared using a filtration method,and its electrochemical performance was investigated.The flexible electrode could maintain integrity in the bent state,showing a good mechanical property.In addition,due to the good conductivity of graphene,the composite displayed conductive 3D network and the rapid transfer of Li+ ions was achieved.When it was used in half cells,the composite can deliver a high areal capacity of 2.7 mAh cm-2 after 200 cycles with the mass loading of 5 mg cm-2.When used in half cells,the assembled battery can successfully light the red LED,and showed a high capacity of 1.06 mAh cm-2 at 0.1 A g-1 after 300 cycles.Besides,the material also showed high electrochemical performance even with high mass loading as 18.3 mg cm-2.