Preparation And Electrochemical Performances Of Porous Silicon-based Composites Materials For Lithium-ion Batteries

Due to its high theoretical specific capacity(3579 m Ah g^-1)and low lithiation/delithiationpotential,silicon is considered as the most promising anode materials to replace graphite for lithium-ion battery（LIBs）.However,the huge volume expansion of the silicon anode during the process of lithiation/delithiation leads to unfavable problems,such as particles fragmentation,severe electrical contact loss with the current collector and the fracture of the solid electrolyte interphase（SEI）film,which lead to the rapid performance fading of silicon anodes.The commercialization of silicon-based anodes was therby hindered.Considering the vast volume expansion of silicon,the porous silicon（p-Si）microspheres was designed to buffer the volumetric expansion of silicon by introducing spaces and voids in p-Si microsphere.The p-Si microsphere was synthesized by selective acid etching of Al-Si alloy.Then graphene film was coated on the surface of the synthesized p-Si microspheres via an electrostatic self-assembly strategy.The coated graphene not only can improve the conductivity of the material,but also block the direct deposition of SEI film on inner silicon microsphere and thus slows down the occurrence of side reactions on silicon surface.Al₂O₃ and Ti O₂ layers were deposited on the synthesized p-Si@graphene（p-Si@G）composite to stabilize the SEI film during silicon lithiation/desiliconization.The specific research contentes are listed as follows:In this thesis,with Fe₂O₃-Al₂O₃ as catalyst,carbon nanotubes（CNTs）are successfully grownon the surface of p-Si@G through a chemical vapor deposition method,with different ratios of p-Si@G and catalyst.In order to explore the effect of the catalyst on the capacity of the material,the carbon nanotubes were prepared under the same conditions and removed catalyst,then added to the p-Si@G composite microspheres with different contents to prepare electrodes.The electrochemical performance of graphene and CNTs on p-Si were systematically evaluated.Results suggest that,when the ratio of p-Si@G to catalyst reaches 5:1,and the adding dose of CNTs reaches 5 wt%,the as-synthesized 5-p-Si@G-CNT composite microspheres can deliver a high specific capacity of 1410.9 m Ah g^-1 after 100 cycles at a current density of 0.5 A g^-1.Meanwhile,the as-prepared 5-p-Si@G-CNT composite microspheres also have good rate capability and can deliver a high specific capacity of 536.2 m Ah g^-1at a high current density of8 A g^-1.It was also found that directly dispersing the CNTs in the slurry during electrode preparation process can endow p-Si@G microsphere the better electrochemical performance than that growing the CNTs on p-Si@G microsphere.Results indicate that,when the dosage of CNTs reaches 5 wt%,the p-Si@G microsphere can deliver a high specific capacity of 1495.6m Ah g^-1,after testing for 100 cycles at 0.5 A g^-1,and also can possesses a high 632.1 m Ah g^-1at a high current density of 8 A g^-1,superior to those of 5-p-Si@G-CNT composite microspheres.Considering the significant role of solid electrolyte interphase（SEI）layer in improving the electrochemical performance of Si anode,in this thesis,an artificial SEI layer,Al₂O₃ was deposited on p-Si@G microsphere via a modified sol-gel process.The adding of glycerin during the sol-gel process can limit the evaporation of solvent and slow down the nucleation growth rate of Al（OH）₃,and finally enable the uniform deposition of Al（OH）₃ which was thermally converted into Al₂O₃ after thermal annealing process.The as-prepared p-Si@G@Al₂O₃possesses the high capacity of 1392 m Ah g^-1 at a current density of 0.5 A g^-1 after testing for100 cycles and also demonstrate good rate capability with a high capacity of 496 m Ah g^-1at a high current density of 8 A g^-1.When highly conductive CNTs with dosage of 5 wt%was added during the slurry preparation process,the capacity of p-Si@G@Al₂O₃ composite can reach up to 1514 m Ah g^-1 after testing for 100 cycles at 0.5 A g^-1,and the capacity value at 8 A g^-1 can increase up to 571 m Ah g^-1.In this thesis,the effect of another artificial layer（Ti O₂）on the electrochemical performance of p-Si@G microspheres was also explored.The coating of Ti O₂layer on the p-Si@G microspheres was performed via a modified hydrothermal-induced solvent-confined monomicelle assembly strategy.Ti O₂ not only had high conductivity and ion mobility,but also can stabilize the formation of SEI film.Electrochemical results show that,at a current density of 0.5 A g^-1,the synthesized p-Si@G@s-Ti O₂ composite microspheres can still deliver a high capacity of 1486.5 m Ah g^-1 after 100 cycles.When the current density is further increased up to 8 A g^-1,the composite microspheres also showed a high specific capacity of 802.53 m Ah g^-1.In order to explore the application of the synthesized p-Si@G@s-Ti O₂ in LIBs,the graphite/p-Si@G@s-Ti O₂ composite electrode is prepared by mixing 10 wt%p-Si@G@s-Ti O₂composite microspheres with graphite.After testing for 100 cycles at current densities of 0.1and 0.2 C,the composite electrode can still deliver the high specific capacity of 450 and 420m Ah g^-1,respectively,two of which value are much high than 372 m Ah g^-1 of graphite,suggesting the promising application of the synthesized p-Si@G@s-Ti O₂composite in LIBs.