Preparation Of Silicon-based Negative Electrode Materials For Lithium-ion Batteries And Research On Their Lithium Storage Performanc

The advantages of Li-ion batteries such as high specific capacity,long cycle life,and high operating voltage make them ideal for mobile electronic devices,electric vehicles,and energy storage devices.Presently,graphite serves as the most utilized negative electrode material in Li-ion batteries;however,its lower limit of specific capacity(372 m Ah g^-1)proves inadequate for imminent demands of high-specific-capacity Li-ion batteries.With its high theoretical specific capacity(4200 m Ah g^-1),environmental friendliness,abundant raw material sources and low price,silicon is considered to be the ideal material for the next generation of lithium-ion anode.However,due to the poor conductivity of silicon anode and the rapid attenuation of specific capacity caused by serious volume expansion effect,it is difficult to realize large-scale application.In order to solve the volume effect of silicon and improve the electrical conductivity,this study obtained a variety of silicon-based composite anode materials by optimizing the structure of silicon,preparing silicon/silver and silicon/graphite composites,etc.The main research contents of this paper are summarized as follows:（1）Study on the preparation processes of porous Si micropowders and Si/Ag micropowders.In order to realize the recovery and high value utilization of quartz wastes,silica waste was served as raw material,Si O₂ micropowders was prepared by dry high-energy ball milling method,and porous Si anode material was successfully prepared by magnesiothermic reduction method.The effect of ball milling processes on the particle sizes and morphologies of the materials were studied,and the lithium storage performance of the porous Si anode prepared under different ball milling parameters was compared and analyzed.Material characterizations indicated that augmenting the ball milling speed and duration positively influenced the reduction of the particle sizes of Si O₂ micropowders and porous Si,and concurrently increased the specific surface area of porous Si.Researches into the mechanism of magnesiothermic reduction processes for Si O₂ suggested that the Si O₂ in smaller particle sizes can be entirely reduced,with the reduced product Mg O and porous Si existing as discrete particles.Conversely,Mg O and by-product Mg₂Si generated during the reduction of larger Si O₂ particles enveloped the particle surface densely and impeded diffusion of substances,leading to an incomplete reduction process.Electrochemical tests indicated that the Si O₂ residue within porous Si significantly contributed to the degradation of lithium storage performance of anode;the porous Si produced under the condition of ball milling operation speed of 900 r/min and duration of 1 h presented optimal lithium storage performance.The performance of the porous Si prepared by further extending ball milling time paradoxically deteriorated,owing to an increase in the specific surface area of the porous Si,which then leaded to formation of more SEI film on the surface of active material particles during the initial cycle.In addition,the porous Si/Ag composite powder was prepared by ball milling with Ag NO₃ and quartz wastes.The existence form of Ag in Si matrix was investigated experimentally,and the effect of Ag NO₃ additive amount on the composite structure and lithium storage performance of the anode was analyzed.Results revealed that Ag existed as nanoparticles and served as the conductive medium within the composite.Electrochemical tests indicated that adding 5 wt%Ag NO₃ to the porous Si/Ag composite powder anode（m Si/Ag-5）delivered optimal lithium storage performance,achieving a specific charge capacity of1010.7 m Ah g^-1 after 300 cycles at 1 C.（2）Study on the preparation processes of porous Si microspheres and Si/Ag microspheres.In order to further improve the initial Coulombic efficiency and cycle stability of porous Si,Si O₂ secondary particle microspheres were prepared by wet high-energy ball milling combined with spray drying.Then a novel porous Si microsphere（p Si）with three-dimensional structure was designed and prepared by magnesiothermic reduction.The influence of slurry solid content on the microstructure of materials was explored,with a focus on comparing lithium storage performance of p Si.With the increase of slurry solid content,the p Si structure tended to be complete,the particle size tended to increase,and the initial Coulombic efficiency of the electrode gradually increased.At a slurry solid content of 30 wt%,the p Si delivered an initial Coulombic efficiency of 88.8%with the best cycle performance.Based on this,porous Si/Ag composite microspheres were fabricated by adding Ag NO₃ during high-energy ball milling.The existence form of Ag in Si matrix was investigated experimentally,and the effect of Ag NO₃ additive amount on the composite structure and lithium storage performance of the anode was analyzed.Results suggested that Ag existed as nanoparticles and served as the conductive channels inside porous Si microspheres.Electrochemical tests demonstrated that the p Si/Ag composite microsphere anode（p Si/Ag-5）exhibited optimal lithium storage performance when Ag NO₃ addition was5 wt%,achieving a specific charge capacity of 1251.4 m Ah g^-1 after 300 cycles at 1 C.Furthermore,Ag nanoparticles inside p Si/Ag provided rigid support,which was conducive to maintaining a stable structure of porous Si microspheres.（3）Study on the preparation processes of amorphous Si/G and polycrystalline Si/G matrix composites.Amorphous Si powders were prepared by dry high-energy ball milling to destroy the crystal structure of Si,and then amorphous Si/G composites were prepared by ball milling mixed with natural graphite.The effects of ball milling time and Ti C addition on amorphization of Si and lithium storage performance of anode were investigated.Research indicated that the inclusion of 50 wt%Ti C effectively accelerated the amorphization process of micron-sized silicon via high-energy ball milling.The amorphous Si/Ti C/G anode obtained by ball milling amorphous Si/Ti C with graphite exhibited superior cycle performance and rate performance in electrochemical tests due to the synergistic effect of the isotropic expansion of amorphous Si,high conductivity of Ti C nanoparticles,and rigid support of the graphite matrix.Subsequently,to extend the use of porous Si in silicon-carbon anode,polycrystalline Si/G composites were fabricated using dry high-energy ball milling,employing porous Si and natural flake graphite.The effects of milling time and Si content on the structure and lithium storage performance of the composites were studied.Research suggested that the m-Si/G composite fabricated with a 1 h milling duration and a 30 wt%Si content exhibited uniform dispersion of silicon and carbon,with graphite formed a uniform coating layer onto the Si particle surface.Electrochemical tests showed that the anode materials exhibited optimal lithium storage performance.Further expansion of the milling time leaded to serious destruction of the graphite structure,resulting in a decrease in the specific capacity of the anode at high current density.The further increase in Si content was not conducive to uniform dispersion of Si and graphite,leading to reduced anode cycle properties.