| Silicon is considered to be a promising next-generation anode material.It has the advantages of extremely high theoretical specific capacity,environmental protection and non-toxicity,rich reserves,and low discharge potential.However,the large volume expansion of silicon during the deintercalation of lithium,the formation of unstable solid electrolyte interface(SEI)films,and poor electrical conductivity limit the commercial application of silicon in lithium-ion batteries.In this paper,we combined silicon with carbon nanotubes and aramid fibers to prepare a flexible electrode that integrates active material and current collector.Composite material of polyacrylonitrile coated nano silicon and multi-wall carbon nano tube.The combined nano-silicon and carbon nanotubes through ionic bonding enhance the interface force between the two phases,improve the compatibility of the two and the structural stability of the composite.The following researches have been carried out in this paper:(1)In this work,A aramid structure electrode was prepared by a simple vacuum filtration and hot pressing process.The SEM shown that the active materials were uniformly embedded in the pores of the three-dimensional conductive network of the aramid carbon nanotube(CNTs)conductive paper.The contact interface area of active material and the conductive network significantly increased and the interface resistance is greatly reduced.The porous anode can accommodate the volume expansion of the silicon and effectively alleviated pressed during cycle.The electrode also exhibited good stability in cycles.Electrochemical tests shown that the first discharge specific capacity of the aramid electrode reached 2140 mAh/g with a coulombic efficiency of 85%.After 100 cycles,the specific capacity still maintained at 1450 mAh/g.At a large current density of 1.5 A/g,the specific capacity hold 1000 mAh/g compared with the copper foil electrode of 90 mAh/g.(2)Silicon-carbon electrode composites were prepared by simple high-energy ball milling and high-temperature pyrolysis.Polyacrylonitrile(PAN)-coated nano Si particles(Si@c-PAN)were mixed with multi-walled carbon nanotubes(MWCNTs)to prepare Si@c-PAN/MWCNTs composite.A thin layer of c-PAN was coated on Si nanoparticles to efficiently accommodate a large volume change of Si particles in cycles.The MWCNTs network functioned as a matrix for Si@c-PAN nanoparticles,which not only prevented the agglomeration of Si@c-PAN particles,but also improved the electrical conductivity of electrodes significantly.The electrochemical tests show that the first discharge specific capacity of the Si@c-PAN/MWCNTs electrode was 2098 mAh/g at a current density of 0.2A/g and initial coulomb efficiency was 86%.After 50 cycles,the reversible specific capacity maintained at 1278 mAh/g.When the current rate reached 2 A/g,the capacity kept at 600 mAh/g,exhibiting good cycle stability.(3)Studies on silicon/carbon nanotube composites have been reported,but ionic bond reinforced composites are rare.Carbon nanotubes with carboxylic acid(MWCNTs)and silicon(Si)with amino groups were prepared,and the two solutions were mixed in a water bath for ultrasonication,then dried and heat-treated to obtain a composite material of good electrochemical properties(NMC/Si).The morphology and the electrochemical performance of the composites were analyzed by x-ray diffraction(XRD),x-ray photoelectron spectroscopy(XPS),scanning electron microscopy(SEM)and galvanostatic charge-discharge tests.Specifically,positive charge of amine nitrogen atom in Si-NH2 will electrostatically interact with negative charge of carboxylic acid in critic acid.Silicon and carbon nanotubes are combined by ionic bonding,which is superior to the electrochemical properties of ordinary physical composites.The results show that the NMC/Si battery can stabilize at 1180 mAh/g and the coulombic efficiency is above 99%after 100 charge and discharge cycles at 80 mA/g current density,and has good cycle stability even in the high current density of 2000 mA/g. |
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