The Synthesis And Application Of Heteroatom-doped Carbon Nanotubes

Carbon nanotubes（CNT）have attracted much attention,because of its unique geometry property,remarkable electronic conductivity and extraordinary chemical and mechanical stability.CNTs have been applied as excellent conductive additive in the electrodes for lithium ion batteries（LIBs）to build 3D conductive network,due to its unique one dimensional morphology and remarkable electronic conductivity.In this work,N-doped CNT（NCNT）have been successfully synthesized by chemical vapor deposition（CVD）processes,and S doping of CNT has been realized by heat treatments of CNT using MgSO₄ or SO₂ as S source.Applications of the heteroatom-doped CNT have been explored by using the CNT as conductive additives in the cathodes for LIBs or to construct SnO₂-CNT composite materials.（1）NCNT were prepared by a chemical vapor deposition（CVD）using ethylene as C source and ammonia as N source in a fluidized bed reactor.S-doped CNT（SCNT）and S,N duel doped CNT（SNCNT）were prepared by applying S-doping treatments on CNT or NCNT,using MgSO₄ as S source.The contact angle test shows that heteroatom doping is capable of enhancing the hydrophilicity of CNT.Among all the CNT samples,the LiFePO₄（LFP）cathode using SNCNTs as conductive additive exhibits the highest capacity(168.2 mAh·g^-1 at 0.2 C).The capacity of the electrode increases as the nitrogen content of NCNTs increasing,especially at 1 C and 2 C.As the loading amount increasing,the capacity of LFP significantly decreases.（2）Thermal treatments of CNT in a SO₂ flow at 700⁹00oC were carried out to synthesize SCNT.Higher S doping level was obtained at higher temperature,and the introduction of a small amount of water vapor was found to be capable of enhancing the sulfur doping level in SCNT.（3）To reduce the agglomeration of CNT,ball-milling,acid pickling with mixture of HF and H₂O₂,decreasing the growth time,synthesis of vertical aligned carbon nanotubes have been carried out.CNT-NMP suspension liquids with ultra-low CNT concentrations（0.2%）were prepared and used to construct conductive networks in cathodes.The cathodes with adding of low concentration CNT exhibited superior capacity and rate capability as compared to those without CNT.（4）SnO₂-CNT composites were synthesized under hydrothermal conditions using SnCl₂·2H₂O as precursor.Sn-CNT composites were synthesized by ball mill of the mixture of Sn and CNT powders.Uniform SnO₂-CNT composite fibres were obtained by the hydrothermal synthesis at the CNT concentration of 0.1%,showing good dispersibility of the CNTs.As anodes for LIBs,the SnO₂-CNT composite exhibits high capacity at the voltage platform around 0.4 V.