Preparation Of Thin-walled Porous CNTs And Its Application In Rechargeable Lithium Batteries

Carbon nanotubes(CNTs)are widely used in the fields of electronics,biomedicine,energy storage,adsorption and catalysis due to their unique one-dimensional tubular structure and excellent optical,thermal,electrical and mechanical properties.There are requirements for the properties of carbon nanotubes in different fields.However,abundant pore structure,moderate wall thickness and easy modification are the necessary conditions for the application of CNTs in many fields.Therefore,to meet the needs of different fields,it is very important to develop CNTs with suitable morphology and structure which can be easily modified.At present,CNTs are mainly prepared by arc discharge,laser ablation and chemical vapor deposition.However,it is difficult to regulate the pore structure,diameter,wall thickness and and properties of CNTs by these methods.It is worth noting that CNTs with appropriate diameter,wall thickness and pore structure can be prepared by a polymer pyrolysis method,which involves in the design of the precursor for polymer nanotubes and the regulation of pyrolysis temperature,pyrolysis atmosphere and other factors.Nevertheless,most polymer precursors have the disadvantages of high cost and low yield,which limit their mass production.Here,a kind of carbon nanotube with large diameter,large specific surface area and thin tube wall was prepared by using a low cost and high yield polymer nanotube as the precursor,through sol-gel method and subsequent pyrolysis process.Based on the requirement of easy modification and porous structures for CNTs,the thin-walled carbon nanotubes were modified with different calcining atmosphere and H₂O₂oxidation treatment.It shows a good application prospect in the field of lithium ion batteries and lithium sulfur batteries.Specific research contents are as follows:1.Preparation and elcetrochemial performance of thin-walled CNTs in lithium ion batteries:Taking polymer nanotubes as the precursor,a polymer nanotube/Si O₂complex was constructed by the sol-gel method,subsequent pyrolysis and chemical etching,the thin-walled porous CNTs with large diameter were prepared(inner diameter of 80 nm,tube wall thickness of 10 nm,specific surface area of 410.6m²g^-1).The formation mechanism of the thin-walled CNTs was analyzed.During the pyrolysis process of the polymer nanotubes/Si O₂complex,the shrinkage of the polymer nanotubes and the extrusion of Si O₂on both sides of the tube walls resulted in the thinning of the tubes walls.Meanwhile,the effects of pyrolysis atmosphere on morphology,pore structure,surface functional groups,graphitization degree and electrochemical properties of thin-walled CNTs were investigated.The experimental results show that the one-dimensional thin-wall structure of carbon nanotubes(NHCNTs)obtained under the atmosphere of N₂-H₂is the most complete.NHCNTs exhibit the largest specific surface area,and the abundant defect sites.When applied as the anode of lithium ion batteries,NHCNTs show excellent lithium storage capacity and rate performance.Accordingly,NHCNTs attain a reversible capacity of400.6 m A h g^-1after 200 cycles at 2 A g^-1,a level of 268 and 212.1 mA g^-1after 400cycles at 5 and 10 A g^-1,respectively,which is much higher than that of traditional CNTs.2.Study on application of thin-walled CNTs in lithium sulfur batteries:It is an effective method for non-polar carbon materials to enhance its adsorption to polar polysulfide by introducing polar functional groups on the surface of it.Here,in order to enhance the binding force with polysulfides,the polar oxygen-containing functional groups O-H/C-O were introduced into thin-walled CNTs by a simple H₂O₂oxidation treatment.The modified thin-walled carbon nanotubes(NHOCNTs)were used as the host materials of sulfur,and the S/NHOCNT composite with sulfur encapsulated in the cavity of CNTs and embedded in the tube walls was constructed by simple melt diffusion.As the cathode of lithium sulfur batteries,S/NHOCNT shows a high reversible capacity and excellent rate performance.At the current density of 0.2 C,S/NHOCNTs still has a reversible capacity of 753 m Ah g^-1after 200charge and discharge cycles,and a reversible capacity of 511.6 mAh g^-1after 500cycles even at 1 C.This is mainly attributed to the fact that the thin-walled porous structure is conducive to the penetration of sulfur.Sulfur is encapsulated in the CNTs,which can enhance the conductivity of the sulfur cathode,and also inhibit the volume expansion of sulfur.In addition,oxygen-containing functional groups(O-H/C-O)can enhance the binding of CNTs to polysulfides,thus limiting the shuttle effect.