| Silicon is widely regarded as the most promising cathode material for the next generation of lithium batteries due to its high specific capacity(4200 m Ah/g)and abundant reserves(the second highest)in the earth.However,in the process of charging and discharging,the volume expansion of silicon-based materials is intense,the huge volume strain and low conductivity will inevitably cause the crushing of micron silicon particles,resulting in the continuous destruction and reconstruction of the solid electrolyte interphase(SEI),resulting in electrode rupture,rapid capacity attenuation and poor performance.Providing a stable conductive network and effective buffer space for silicon,and designing and preparing silicon-carbon anode materials with reasonable structures are fundamental and effective ways to solve the problem of silicon-based materials.It is an inevitable requirement for the practical application of silicon carbon anode materials to find a method that can meet the needs of low cost and large-scale production of silicon carbon anode materials with a reasonable structure.Based on the above considerations,the following researches are carried out in this thesis.The main research contents are as follows:(1)Using nanometer silicon as a source of silicon,silicon dioxide as sacrifice template,asphalt as coated carbon source,through improved St?ber method,spray drying,heat treatment and etching process preparation of the novel raspberry-like yolk-shell structured(R-YS)Si/C micro/nano-spheres,The nano-cavity constructed by cross-linked carbon shell derived from low-cost pitch provides buffer space for nano-silicon,while the micron cavity in the middle of the raspberry structure not only provides buffer space for the entire micro-nano composite microsphere,but also shortens the diffusion distance of lithium ions in the electrolyte storage.Through systematic microstructural and electrochemical analysis,R-YS Si/C micro/nano-spheres with raspberry-like morphology and cross-linked carbon shell exhibit a better structural and cyclic stability than that of C-YS Si/C nanospheres composite.Electrochemical test results show that the reversible capacity of R-YS Si/C micro/nano-spheres presents 935 m Ah/g at 200 m A/g.Along with structural adjustment during the cycling process,the reversible capacity decreased at the beginning,but recovered to 762.8 m A/g after 105 cycles,with a high capacity retention rate of 81.6%.The results show that the structure design of the middle cavity of raspberry-like structured and the structure design of the yolk-shell provides sufficient buffer space for the silicon-carbon composite and is the fundamental guarantee for the better cycling performance of the composite.This research supply new strategy to fabricate Si/C composite as anode materials with low-cost,facile preparation method and high performances simultaneously,which is beneficial to promote the practical application of Si/C materials.(2)Fly ash acid leaching residue was selected to prepare sodium silicate solution,hollow silica was obtained after spray drying and pickling,magnesium thermal reduction as the silicon source,phenolic resin as the carbon source to prepare Si/C composite microspheres of silicon and carbon double shell hollow structure.Through electrochemical analysis,it was found that the Si/C composite prepared with phenolic resin as coated carbon source had better cyclic stability than the silicon obtained by direct magnesium reduction of silica.The electrochemical test results showed that at 200 m A/h,the reversible specific capacity of Si/C composites slowly decayed from 612 m Ah/g to 478 m Ah/g in the first 55 cycles.To 105 cycles,the reversible capacity was 470.55 m Ah/g,much higher than the reversible capacity of Si.The results show that the sufficient buffer space inside and the outer carbon shell are the important reasons for the better electrochemical performance of Si/C composite microspheres with hollow structure of Si/C double shells.The anode material can not only greatly improve the energy density of lithium ion battery,but also utilize the solid waste from coal power generation and coal chemical industry with high value,which has high social,environmental and economic benefits.(3)In view of the low carbon residue characteristics of melamine-formaldehyde(MF)resin at high temperature,it can be used to prepare hollow carbon spheres.After the preparation of Si@MF,the structure of Si@void@MFC can be further obtained.Therefore,the controllable preparation of monodisperse melamine resin microspheres and nitrogen-doped carbon microspheres is the key step of the above design idea.By means of scanning electron microscope(SEM),transmission electron microscopy(TEM),thermal gravimetric analysis(TGA),fourier transform infrared spectroscopy(FT-IR)and statistical distribution of particle size,the authors find the following unreported phenomena and rules.In this work,for the first time,without any surfactant,monodisperse MF microspheres with the particle size(D50)ranging from 200 nm to 1.8?m could be prepared by using only appropriate amount of acetic acid as catalyst.In addition,it was firstly found that with the increase of stirring time,D50 of MF microspheres did not simply increase and then remain stable,but further decreased and became stable again with 40% shrinkage.Under the proper heat treatment conditions,the corresponding monodisperse nitrogen doped carbon microspheres can be obtained and the shrinkage rate is 81~65% at 800 o C,which is depending on the stirring time.The above phenomena and rules found in the formation process of MF resin microspheres and nitrogen-doped carbon microspheres were discussed from the perspective of molecular formation.Thus,this work lays a good practical and theoretical foundation for the subsequent preparation of functionalized MF resin microspheres and nitrogen-doped carbon microspheres with different particle sizes.In addition,it also lays a solid foundation for the preparation of silicon carbon anode materials with Si@void@MFC structure and composition.However,the experimental results show that Si@void@MFC yolk-shell structure is not ideal,Si@MF and water heat treatment conditions need to be studied systematically,which is expected to synthesize a kind of silicon-carbon composite material without special etching treatment,which will make a positive contribution to the development of the whole silicon-carbon composite material. |
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