Lithium Storage Properties Of Carbon Nanotubes And Tin-based Carbon Nanotube Composites

The development of electric and hybrid vehicles put up new demands ofhigh power density and long cycling life to lithium ion batteries. Owing to itshigh specific capacity (Sn991mAhg^-1, SiO₂781mAhg^-1, SiS₂645mAhg^-1,),controllable synthesis strategy, no solvent intercalation and low cost, Sn-basedmaterials have been taken as one of the most promising anode materials.While like other high specific materials, the problem on large-scaleapplication of Sn-based materials is the volume change with the lithium ionalloying and de-alloying, leading to the pulverization and electronicdisconnection of the electrode as well as capacity fading. By controlling thenanosize of Sn, creating different morphology and multi-dimensional structuredesign, its electrochemical performance can be enhanced. Another effectivestrategy is to prepare Sn-Carbon composites. Because of its excellentelectronic conductivity, high specific area and nanoscale morphology, carbonnanotubes （CNTs） have been greatly investigated as electrode materials andused as the ideal matrix to restrict the volume expansion of Sn.In this thesis, we prepared nitrogen-doped carbon nanotubes （N-CNT）,spindle-like graphene rods, CNTs encapsulated tin oxide, CNTsencapsulated/coated tin, flower-like SiS₂sheets reinforced by CNT network,and investigated their lithium ion storage properties. We found the effect ofnitrogen types of N-CNT on lithium ion storage property; the assemblymechanism from CNTs array to graphene rods under high temperaturetreatment; the volume restriction inside CNTs during cycling; the capacity gapbetween two different strcture of Sn-CNT composites and the enhanced capaity of SiS₂sheets compositing with CNT.Nitrogen-doped carbon nanotubes （N-CNTs） were prepared by catalyticchemical vapor deposition, and the nitrogen contents increased from1.23,1.71,2.36to2.89at.%for4samples. The nitrogen types for each N-CNTs aregraphitic-N and pyridinic-N. The structure of N-CNTs was like bamboo, andthe ordered graphitic structure of CNTs was broken associated with moreedges and vacancy. Compared with CNTs, N-CNTs exhibited an enhancedfirst discharge capacity which was directly proportional to the nitrogencontents, but most of them was irreversible. After30cycles at the currentdensity of50mAg^-1, N-CNTs with the lowest nitrogen contents （1.23at.%）displayed the highest reversible capacity of270mAhg^-1. AC impedanceindicated that the contact resistance and charge-transfer resistance of thisN-CNTs （1.23at.%） was lower than other N-CNTs which was very helpful tolithium ions rapid transport. Local-density approximation of C+175N4Liindicated that the lithium ions preferred locating at the vacancy surrounded bythree pyridinic nitrogen atoms and binding with them which was veryresponsible for the large irreversible capacity, while the graphitic nitrogenatoms enhanced the electronic conductivity as well as the reversible lithiumions storage capacity.The thermal assembly of CNTs array was studied. After heat treatment at2800℃, the CNTs array was assembled into spindle-like rods. SEM andTEM analysis showed that these spindle-like rods are composed of graphenesheets which were vertically arranged into the rod. These spindle rods couldbe easily exfoliated into worm-like graphene sheets, and the later exhibitedexcellent lithium ion storage property. The reversible capacity retention was:385.6mAhg^-1after100cycles at50mAg^-1;267.9mAhg^-1after500cycles atthe current density of1Ag^-1;216mAhg^-1after500cycles at the currentdensity of4Ag^-1;180.4mAhg^-1after500cycles at10Ag^-1. These excellentrate capacity performances were subscribed to the worm-like structure ofgraphene sheets which could leave lots of porous channels for lithium ionsinsertion/extraction. The assembly mechanism of CNTs array were exploredby heat treatment at1500,1800,2100,2650,3000℃, which indicated CNTsarray experienced the coalescence, splitting and surface reconstruction, andfinally assembled into spindle-like rods. Carbon nanotube-encapsulated SiO₂（SiO₂-in-CNT） core-shellcomposites with SiO₂contents of25%,44%, and65%were prepared by wetchemical filling and selective washing. The SiO₂-in-CNT compositesexhibited excellent electrochemical performance. After50cycles at50mAg^-1（0.1C）, the reversible capacity was:401mAhg^-1（SiO₂-in-CNT-25），490mAhg^-1（SiO₂-in-CNT-44） and627.8mAhg^-1（SiO₂-in-CNT-65）,corresponding to84.6%（SiO₂-in-CNT-25）,88.7%（SiO₂-in-CNT-44） and98.4%（SiO₂-in-CNT-65） of the theoretical capacity. When the current densityincreased to250,500and1000mAg^-1, SiO₂-in-CNT-65still exhibited highcapacity retention which was also better than SiO₂-in-CNT-25andSiO₂-in-CNT-44. The excellent electrochemical performance of SiO₂-in-CNTcomposites were closely related the encapsulation structure of SiO₂intointerior cavity of CNTs.Based on the excellent electrochemical performance of SiO₂-in-CNTencapsulation structure, we further prepared encapsulation （Sn-in-CNT） andcoating （Sn-out-CNT） structure of Sn-CNT composites with40%Sn contentsand studied their lithium ion storage property. After170cycles at the currentdensity of50mAg^-1, Sn-out-CNT showed capacity retention of351.1mAhg^-1,equals to56.8%of theoretical capacity, while Sn-in-CNT remained at639.7mAhg^-1and the fade rate was only0.074%per cycle. It was surprising that thereversible capacity gap between two different structure reached as high as290mAhg^-1. Raman spectra indicated that, in interior CNTs the charge transferedfrom Sn to electron-deficient interior surface and leave Sn in a reduced statewhich was very helpful to absorb lithium ions; on the contrary, in exteriorCNTs, the charge would transfer from CNTs to Sn. DSC results showed thechemical bond between Sn and interior CNTs was stronger than that of Snwith exterior CNTs. AC impedance also showed Sn-in-CNT displayed smallercontact resistance and charge transfer resistance than Sn-out-CNT. The spatialrestriction inside CNTs was also helpful to hinder Sn from aggregation,prolong their reaction time and keep Sn-C bonds alive.The flower-like SiS₂sheets reinforced by CNTs network were preparedby hydrothermal reaction, and Sn-out-CNT was used as precursor. Thecontents of Sn and reaction time affected the flower-like morphology. Theelectrochemical performance of SiS₂-CNTs composites was excellent that the reversible capacity slightly increased with cycling and the capacity retentionafter100cycles was940mAhg^-1at50mAg^-1. The excellent lithium ionstorage property could be ascribed to the CNT network and layered structureof SiS₂. The growth mechanism of flower-like SiS₂-CNT composites was:SiS₂sheets grew up around Sn （the nucleation）; SiS₂sheets rolled up intoflower-like structure; CNTs network were wrapped up into SiS₂sheets.