Preparation Of Si/C Anode Materials From Waste Silicon Powders Of Photovoltaic Industry And Their Li-storage Properties

The low enegy density of Li-ion batteries（LIBs）has become the major obstacle to the improvement of electric vehicles’driving range.The low specific capacity of graphite anode is a main factor that restricts the energy density of LIBs.Silicon has been considered as a promising anode material for next-generation high energy LIBs due to its high specific capacity（up to 4200m Ah/g）,low discharge platform,good safety and abundant raw materials.However,its large volume expansion during lithiation/delithiation will not only lead to pulverization of the electrode,shedding of the active material,but also result in repeated destruction and reformation of the solid electrolyte interface（SEI）films,which in turn lead to serious decomposition of electrolyte and continuous capacity loss.In addition,the cost of current Si/C anode materials is still too high due to the use of nano-silicon.Therefore,how to prepare high performance Si/C anode materials with cheap raw materials is still a research focus in the field of LIBs.In this thesis,we tried to improve the performance of silicon anode materials through carbon coating,porous structuring,and interface designing by using waste silicon powder from photovoltaic industry as a low cost silicon source.The major research outcomes are as below:（1）The Si/C composite anode materials with similar carbon content were prepared by using waste silicon powder as silicon source,and glucose,phenol-formaldehyde resin and poly-dopamine as carbon source,respectively.The micro-structural morphology,phase composition and electrochemical performance of the pristine waste silicon powder and the different Si/C composite anode materials were compared.The thickness of carbon coating layer derived from glucose was uneven in addition to agglomeration.Instead,the carbon coatings obtained by in situ polymerization of phenol-formaldehyde resin and poly-dopamine were more uniform.As a result,the initial Coulomb efficiency,rate performance and cycle stability of the prepared Si/C composite anode materials were relatively better.Especially,the sample with phenol-formaldehyde resin as the carbon source showed the best performance,including a high discharge specific capacity of 379.0 m Ah/g at 3 A/g and a high remaining capacity of 311.6m Ah/g after 200 cycles at 0.4 A/g.（2）By adjusting the mixing mode between waste silicon powder and magnesium powder,Mg₂Si with different particle sizes were obtained after a heat treatment.Porous silicon/silicon carbide/carbon（pSi/SiC/C）or porous silicon/carbon（pSi/C）composite anode materials was obtained by heating the different Mg₂Si precursors at 650 ^oC under CO₂atmosphere,followed by acid washing to remove the Mg O.Differential scanning calorimetry（DSC）curves showed that after decreasing the particles size of Mg₂Si,the igniting temperature for the exothermic displacement reaction between Mg₂Si and CO₂was decreased,and a more concentrated exothermic intensity was achieved,which might have led to a temperature rise up to the formation temperature of SiC and converted part of the Si and C layers to SiC,forming a SiC/C dual interface structure.The results of XRD,XPS and HRTEM well confirmed the existence of SiC interface layer.As compared to the pSi/C sample,the pSi/SiC/C composite anode showed higher initial Coulomb efficiency（75.0%vs.72.3%）,better cycle stability（731.1 vs.494.0m Ah/g after 200 cycles at 0.4 A/g）and rate performance（903.1 vs.375.5 m Ah/g at 3 A/g）.This thesis provided a facile process to construct SiC/C dual interface on the surface of porous silicon,which is low energy consumption and conducive for industrial production and application,as compared to the methods reported in literatures.（3）Spherical pSi/SiC/C@C was prepared by spray-drying the pSi/SiC/C with glucose as the carbon source and binder,followed by carbonization.As compared to pSi/SiC/C,the spherical pSi/SiC/C@C composite anode material showed better cycle performance and rate performance.For example,a reversible specific capacity of 515.9 m Ah/g was maintained after 200 cycles at0.4 A/g,and a high discharge capacity of 620.4 m Ah/g was achieved at a current density of 3A/g.Microstructural analysis showed that the volume expansion rate of spherical pSi/SiC/C@C composite electrode decreased significantly after cycling,which suggested that the synergetic effect of porous spherical structure and carbon coating might have effectively enhanced the conductivity and structure stability of the electrode.