Preparation And Studying Lithiation/delithiation Performance Of Porous Si@Metal/C Composites For Anode Of LIBs

The growing demand for large-scale energy storage devices with development of mobile electronic device,electric vehicles has promoted to develop high-capacity Lithium ion batterys(LIBs).Increasing the capacity density of the electrode active material is one of the keys to increasing the energy density.Silicon is exhibiting the high theoretical capacity of 4200 mAh/g.However,it shows poor cycles due to the large volume expansion of silicon during the lithium process,which causes the structure of the electrode materials destroyed.In addition,the intrinsic conductivity of silicon is low,which is difficult to achieve rapid transport of lithium ions and electrons,which causes poor rate performance.For these problems of silicon anode materials,in this paper,it is proposed to enhance the anti-expansion and conductivity of silicon materials during delithiation and to improve their electrochemical performance by means of porosification and compositing.This paper mainly prepared porous silicon,silicon@metal/carbon composite materials,porous silicon@tin composite materials and porous silicon@tin/carbon composite materials.On this basis,its morphology,phase,structure,et al.were characterized and its lithiation/delithiation performance was tested.The main research and results of this thesis are as summerized as follows:(1)Using commercial silicon powder(~5 μm)as raw material,Improving the method with ammonia as a pH adjuster.The results show that the pH of the ammonia-adjusted deposition solution and the etching solution affects the deposition density of silver ions on the silicon surface and the catalytic reaction of silver ions.The pH of the deposition liquid and the etching liquid is adjusted to 5.9 by adding ammonia water at 0 ℃,the uniformly distributed and dense of porous silicon channel by the secondary etching method.Prepare a button 2032 half-cell,it shows high first discharge capacity(2326.8 mAh/g)and after 30 cycles at 163 mAh/g,higher than Si(29 mAh/g)at a current density of 100 mA/g.Moreover,the tested porous silicon hasa lower impedance than the silicon powder.(2)The innovative use of sodium alginate as carbon source and nickel chloride as cross-linking agent,and subsequent low-temperature carbonization the Si@Ni/C composites were obtained.To explore the calcination temperature and silicon to carbon ratio,Si@Ni/C composites prepared at a calcination temperature of 480 ℃ and a silicon to carbon ratio of 1:1,which shows an excellent electrochemical properties during the uniformly nickel-doped graphitized carbon layer,it exhibited high first discharge capacity(3137 mAh/g)after 100 cycles at 515 mAh/g,higher than Si(84mAh/g)at a current density of 100 mA/g.Under the same preparation conditions,silver nitrate was used as a crosslinking agent to obtain a Si@Ag/C composite material with dispersed nano-silver particles and a dense graphitized carbon layer.The nano-silver particles and carbon layer act synergistically,which alleviates the volume effect of silicon and greatly improves the electrochemical performance,during the charge-discharge process.it shows high first discharge capacity(1946 mAh/g)and724 mAh/g after 100 cycles at a current density of 200 mA/g.The Si@Ni/C and Si@Ag/C composites shows that the smaller impedance than silicon powder.by EIS testing,because they have a graphitized carbon layer.graphitized carbon layer.(3)Porous silicon@Sn composites were prepared by calcination of the prepared porous silicon and commercial tin powder.The effects of calcination temperature and silicon tin mass ratio on the structure of the material and the lithiation/delithiation performance were investigated.Porous silicon@Sn composites prepared when the calcination temperature is 380 ℃ and the ratio of silicon to Sn is 16:1,which exhibited excellent cycle performance,it shows high first discharge capacity(2435mAh/g)and has 600 mAh/g over 50 cycles at a current of 100 mA/g.The porous Si@Sn/C ternary composite was prepared by coating the surface of the porous Si@Sn composite with the same method as the Si@Ni/C composite.The effects of calcination temperature and silicon-tin mass ratio on the structure of the material and the lithiation/delithiation performance were investigated.Porous silicon @Sn/C composites prepared when the ratio of polysilicon to carbon is 2:1,which shows a high first discharge specific capacity(1421 mAh/g)and excellent cycles performance.After 50 cycles,it has a discharge specific capacity of 173 mAh/g at current density for 200 mA/g.