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

Among all anode materials in lithium-ion batteries,silicon has attracted a wide attention for its high theoretic specific capacity(4200 mAh g^-1)and volume ratio capacity(9786 mAh cm^-3).However,the large volume expansion of Si?³00%?leads to the serious capacity performance deterioration due to the pulverization of the Si particles and thus induced electrical contact loss of Si with current collector during the delithiation/lithiation processes.The above problems can be effectively solved by designing the silicon-based porous structures and introducing the highly conductive carbon material?such as graphene and carbon nanotube?into the silicon-based composites so that the electrochemical performance of silicon can be greatly improved.In this thesis,by using the commercial AlSi alloy as raw material,hierarchical structured silicon microsphere with different residual Al contents was prepared through a selective chemical etching process.The prepared silicon microsphere possesses large abundant pores and provides the volume expansion for silicon during the lithiation/delithiation processes.To further improve the electrochemical performance of the prepared silicon microsphere,especially its cycling performance,series silicon microsphere-based composites,including heterohierarchiacal silicon@graphene?HH-P-Si@G?,silicon@graphene@carbon nanotube?P-Si@GNF@CNTs?and silicon@Al₂O₃@C hybrid?P-Si@GNF@CNTs?,were successfully fabricated by adopting the chemical vapor deposition strategy.The structure and electrochemical performances of the aforementioned silicon-based composites were systematically characterized and evaluated.The main results are listed as follows:?1?The commercially available aluminum-silicon alloy is used as the raw material to selectively etch the porous silicon material with abundant internal pores with dilute hydrochloric acid.The abundant internal pores can accommodate the volume expansion of silicon in the process of lithiation,prevent the structural damage caused by volume expansion,and improve the energy density and cycle life of the battery.?2?A layer of graphene was deposited on the porous silicon surface by chemical vapor deposition?CVD?.This layer of graphene as a packaging layer to block the direct contact between silicon and electrolyte,prevent the occurrence of side reactions,reduce the irreversible capacity caused by excessive consumption of lithium ions,and reduce the formation of unstable SEI.The first coulomb efficiency of the synthesized HH-P-Si@G composite material reached 77.8%.After circulating for 120 cycles at the current density of 0.4 A g^-1,the capacity could be maintained at 1124 mAh g^-1.When the current density increased to 3.2 A g^-1,the specific capacity of 808.9 mAh g^-1 could still be maintained.?3?The surface of porous silicon balls with flexible graphene oxide film as the cladding layer,long on the face a layer of carbon nanotubes,highly conductive graphene and carbon nanotubes are introduced to further improve the electrical conductivity of electrode,graphene oxide layer block direct contact of the electrolyte and silicon,reducing the excessive consumption of lithium ion,and alleviate the volume expansion in the process of lithiation,and that carbon nanotubes provides a channel for lithium ion diffusion.After 100 cycles,the composite material has a capacity of 1475.5 mAh g^-1,and still maintains the capacity of 804.2 mAh g^-1 at a high rate of 5A g^-1.?4?During the selective etching of commercially available aluminum-silicon alloy,an appropriate amount of aluminum is retained,and CO₂ gas is injected into the tube furnace.The excess aluminum is oxidized to alumina to form a protective layer on the silicon surface to prevent structural damage caused by volume expansion.Meanwhile,CO₂ is reduced to C,which increases the conductivity of the electrode.The P-Si@Al₂O₃/C composite material finally prepared still has the specific capacity of842.1 mAh g^-1 after 300 long cycles,and under the large current density of 3.2 A g^-1,the reversible capacity can still reach 906.9 mAh g^-1.