Metal-assisted Etching To Strengthen The Removal Of Impurities In Industrial Silicon And The Preparation Of Porous Silicon

At present,silicon material has been widely used in energy conversion and storage fields because of its unique physical properties,abundant reserves and mature manufacturing technology.However,impurities in silicon not only affect the conversion efficiency of silicon solar cells,but also threaten the safety of lithium batteries.In addition,the semiconductor properties of silicon(low conductivity),volume expansion effect and other issues in the field of lithium battery applications face great challenges.Based on this,this thesis puts forward using cheap metallurgical-grade silicon(MG-Si)as the research object,using metal-assisted chemical etching(MACE)technology combined with traditional hydrometallurgy purification technology,finally realizes the reinforcement to remove impurities from MG-Si,the porous structure regulation(ease volume expansion)and metal nanoparticles controlled introduction(increase the conductivity),aims to solve step cheap MG-Si source used in high-performance lithium electricity silicon anode facing problems.The specific research contents and conclusions are as follows:(1)Metallic impurities in MG-Si generally do not exist alone but mainly exist in the grain boundaries in the alloy phases form,Fe and Al are the most important impurities in MG-Si and will coexist with other impurities.According to the study on the composition of the impurity phase,the impurity Fe mainly exists in the form of Si₃Al₃Fe₂ and Fe Si₂,and the impurity Al mainly exists in the form of Si₃Al₃Fe₂.(2)The proposed successive MACE methods optimize the complex operation of the two-step MACE process.After a large number of MACE experiments,the introduction of porous structures and impurity removal from MG-Si have been studied.The suitable experimental conditions for Ag ACE are as follows:2h leaching time,10m M Ag NO₃,0.5M H₂O₂,4.6M HF,silicon powder particle size 106?150?m,temperature 25?;Suitable Cu ACE experimental conditions:The etching time is 2h,the concentration of Cu(NO₃)₂ is 40m M,the concentration of H₂O₂ is 0.5M,the concentration of HF is 3.45M,the particle size of silicon powder is150?180?m,and the temperature is 25?.(3)Cu ACE method could effectively remove the insoluble metallic impurities Ca,but the removal effect of the non-metallic impurities B and P is relatively limited.Nevertheless,the Ag ACE method effectively strengthen the removal of B and P from MG-Si.Based on the Cracking Shrinking Model(CSM),Ag ACE leaching kinetics of B and P are controlled by both of interfacial transfer and diffusion through the product layer process,and the diffusion through product layer process is the most important.(4)RTP and ultrasonic field treatment further strengthen the removal of impurities in the porous silicon,which has a significant effect on the insoluble non-metallic impurity B,the removal rate of B can be significantly increased to 76.07%.In addition,combining ultrasonic field,RTP and acid treatment,high purity porous silicon powder with a purity of 99.995%can be successfully obtained.(5)The calculation results by the quantum chemistry software Materials Studio(MS)firstly revealed the interactions between ions and the main impurities Fe,Al,B,and P of MG-Si in the MACE system.It provides a theoretical basis for the MACE method to enhance the removal of impurities from silicon.(6)The optimized Cu ACE method has a special chemical crushing effect on MG-Si,and obtained the Si nanoparticles with a particle size of 50-300 nm.Furthermore,by introducing a porous structure and selectively retaining nucleated nano silver particles in the obtained Si nanoparticles by the Ag ACE method,PSi/Nano-Ag composite material can be successfully prepared.In the PSi/Nano-Ag composite,the formed nano-silver particles have good crystallinity and mainly exist in the form of embedded in the porous structure,and their sizes are mainly distributed in 5?10 nm.The main impurities in the raw MG-Si materials are effectively removed,the concentration of Fe,Ni,Co,Zn,Cr,and Cu have reached the national standard GB/T33827—2020 for lithium battery anode materials.(7)The electrochemical performance test of the PSi/Nano-Ag composite achieved the highest level of Si-based anode materials from MG-Si.The results showed a high first discharge capacity of 3095 m A hg^-1 at high current density of 1A g^-1,and the first coulombic efficiency can be reached to 89.09%.It can still maintain a high discharge capacity of 1930 m A hg-1 after 50cycles.The PSi/Nano-Ag composite material also exhibits excellent rate performance,when it is fully charged and discharged and then switched back to 0.1 A g-1,its reversible capacity quickly returned to 2660 m A hg^-1.After 100 cycles of charging and discharging at 1A g^-1,the PSi/Nano-Ag composite electrode still maintained a good structure.The research results show that the optimized MACE method for strengthening the deep removal of impurities from MG-Si and preparing porous silicon is simple,and practical,which is expected to provide an effective way and promising strategy for the development of a new generation of low-cost and high-performance silicon materials for renewable energy applications.