| The sulfur is one of the most potential cathode materials in the next generation of the lithium ion batteries because of its high theoretical specific capacity,low environmental pollution and low price.However,the low conductivity of battery material?sulfur and polysulfides?,the dissolution of polysulfides during charge and discharge of lithium-sulfur batteries,the shuttle effect and the volume change of sulfur positive plates cause severe damage to the cycling performance of lithium-sulfur batteries.In this paper,vanadium nitride composites with different structures were designed and fabricated.The shuttle effect of polysulfide was weakened,and the cycling performance of lithium-sulfur battery was improved obviously.The main results are as follows:?1?The preparation of flower-like vanadium nitride with nitrogen-doped carbon nanosheets and its electrochemical performance in lithium-sulfur batteries.Flower-like polyaniline-vanadium oxide was prepared by hydrothermal method.The precursor was converted to flower-like vanadium nitride with nitrogen-doped carbon nanosheets by nitridation of ammonia,which comes from the thermal decomposition of dicyandiamide The material was used as a sulfur host to form an electrode loaded with0.7 mg cm-2sulfur,which was assembled into the lithium-sulfur batteries.After 100cycles at the current density of 0.2 C,the specific capacity of the batteries was 511.3mAh g-1.As a coatings of separator for lithium-sulfur batteries,the flower-like vanadium nitride with nitrogen-doped carbon nanosheets and the cathode electrodes of sulfur-carbon composite with 1.2 mg cm-2sulfur form the batteries,which achieve a specific capacity of 654.1 mAh g-11 after 100 cycles of charge and discharge at the current density of 0.2 C.Finally,the different vanadium compounds were prepared by different high temperature and nitridation treatment conditions.After preparing the coatings of the separators,it was found that the best batteries came from the vanadium nitride coatings,which were nitrided at 700?C.The lithium-sulfur batteries with a sulfur load of 0.7 mg cm-2can achieve a long cycle of more than 500 cycles at the current density of 0.2 C,and the batteries can still maintain the specific capacity of504.0 mAh g-1.?2?The electrochemical properties of vanadium nitride@carbon nanotube composites and lithium-sulfur batteries were studied.Using commercial carbon nanotubes as precursor,vanadium oxide ions in the solution were adsorbed on the surface of carbon nanotubes by surface active agents.Vanadium nitride and carbon nanotubes composites were obtained by nitridation at high temperature.When the material is used as the sulfur hosts,the lithium-sulfur batteries at the current density of0.2 C and with 1.0 mg cm-22 load of sulfur and 60/40?L of electrolyte can achieve the specific capacity of 378.1/521.1 mAh g-1after 100 cycles,respectively.In addition,by loading 80%,70%and 60%of sulfur in vanadium nitride and carbon nanotube composites,the specific capacities of the lithium-sulfur batteries decreased by 2.10%,1.68%and 1.57%per cycle during the first 20 cycles,respectively.The experimental results show that properly reducing the load content of sulfur and electrolyte can reduce the decline of capacity and improve the cycle stability of Li-S batteries during charge-discharge process. |
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