Synthesis And Electrochemical Performances Of High Specific Capacity As Anode Materials For Lithium-ion Batteries

Lithium-ion batteries?LIBs?have attracted much attention and widely used as electrochemical energy storage devices,becaused of the high energy density,longer cyclic performance,light weight,lost-cost and environment friendly.In order to overcome the stagnant Li⁺diffusion kinetics,low conductivity and short cycling stability of LIBs.Consequently,the new electrodes crosslinked architectures that can allow more efficient ionic and charge transport beyond the limits of traditional structure.Because of their unique morphologies and structures with low-dimensioncal,orientated charge and ionic transport in the confinement space,and strong tolerance to volume change.The aim of this thesis is design high specific capacity LIBs anode materials via sol-gel electrospinning technique.Electrode materials with increasing specific capacity were designed and prepared successively such as carbon nanofibers,NiO/Co₃O₄ CNFs,hollow nanotubes,necklace-like nanowirse and Si CNFs have fabricated.Moreover,some of scientific results are obtained to enhance the energy density of LIBs,in terms of the investigation of insertion and deinsertion,convert reaction and alloying and dealloying electrochemical reaction.?1?Ultro-long and diameter uniformly carbon naonfibers were synthsised by electrospinning and subsequently annealing process.The surface smoothness of carbon nanofibers can be effectively modify with the special sample clamping in the pyrolysis process,which improved the machinability of electrodes and dramatically reduce the cost.The electrochemical perforamce were further investigated when directely applied as flexible and binder-free anode materiasl for LIBs.Carbon nanofibers deliver specific capacity of 205 mAh g^-1 at current density of 400 mA g^-1 after 1910 cycles,which corresponds to the capacity fading of 0.02%per cycle.It is indicting that the indertion/deinsertion electrochemical proess can be improved by the ordered electronic transport pathway and high mechanical of one-dimensioncal nanostructrure.?2?NiO NWs,NiO NTs,NiO embedded in the carbon nanofibers?NiO CNFs?were elaborately designed and synthesized through electrospinning and controlled annealing process.When NiO CNFs applied as flexible electrodes,it delivered a specific capacity of 481 mAh g^-1at 0.5C after 200 cycles,which is more than double NiO NWs and NiO NTs.The result indicats that carbon nanofiber could effectively improve the electronic conductivity of NiO and prevent aggregation,resulting enhanced electrochemical performance.?3?Co₃O₄ NTs and Co₃O₄ CNFs were synthsised by electrospinning and subsequently annealing in air and Ar atomosphere,respectivlty.The formation mechanism of the two morphological characteristics have been discussed.Meanwhile,it was also demonstrated that the continuity of microstructure and porosity related to the rate of pyrolysis heating significantly.Co₃O₄ CNFs delivered Li storage capacity(426 mAh g^-1 at 0.5 C)could exceed Co₃O₄ NTs after 500 cycles.More importantly,the internal resistance can be decreased by increasing electronical conductivity through electrospinning carbon coating layer,resulting in an enhanced energy storage and electrochemical cyclic performance.?4?Carbon coated Si nanoparticles?Si CNPs?with high specific capacity and Si nanoparticles embedded in the carbon nanofibers?Si CNFs?were prepared by hydrothermal method and electrospinning,respectively.The specific capacity of the Si CNFs as anode for LIBs can reach 951 mAh g^-1 at a current density of 200 mA g^-1 after 150 cycles,whereas the Si CNPs only displayed a lower capacity of 592 mAh g^-1.A novel one-dimensional structural material?Si CNPs/CNFs?was further fabricated through hydrothermal method combining with electrospinning.It exhibited reversible capacity of 1370 mAh g^-1 after 200 cycles.Meanwhile,EIS results show that the charge transfer resistance of Si CNPs/CNFs?70.7??significantly below Si CNFs?100??and Si CNPs?175??.The superior electrochemical performance of the Si CNPs/CNFs could be ascribed to the carbon layer can largely improve the conductivity of Si nanoparticles.Furthermore,three-dimensional interconnected and porous structure could not only effectively prevent the self-aggregation of Si nanoparticles,but also buffered the volumetric changes upon repeated lithiation/delithiation processes.All these results show the potential application of carbon nanofibers in the LIBs anode materials.