Technical Study On Structure Modification Of Silicon Anode Materials For Lithium Ion Batteries

Energy crisis and environmental problems that people are facing should be solved.Lithium ion battery, a new type of clean and safe energy, has attracted much attention,and people are working on developing lithium ion battery with large capacity, long cycle life and good safety.Silicon as anode material has a large theoretical capacity compared with other electrode materials(4200mAhg-1),which has become a hotspot. Since this material generates huge volume expansion in the process of lithium intercalation, which leads to structure pulverization, and thus hinder electron transfer, the electrodes have huge loss of the capacity, and short cycle life. Additionally the conductivity of silicon material is poor,which affects the cycle performance of the battery as well. Many effective efforts have been made to solve these problems, which includes reducing the particle size and designing porous structure. Specific surface area can increase with the decrease of particle size, which can reduce the insertion depth of lithium ions, shorten the diffusion path of ions, and weaken the polarization of the electrode during charge and discharge process. The pore structure can provide a buffer space for the volume change to decrease structural fracture. So in order to prepare small size and porous silicon anode materials,in this thesis, the particle size is tried to be reduced by ball milling method, based on which porous silicon have been prepared as anode materials for lithium ion batteries. The composition and morphology of the materials have been studied. The principle of charge discharge and cycle performance have been investigated. The detailed contents are as follows:1) Reduce the size of silicon particles. Silicon particles are brittle, and necessarily ground by manual operation and mechanical ball milling. Mechanical milling time of,12,20 and 28 h was taken. The sample granule of ball-milling with 28 h is about 1-2 um.Compared with 200 mesh silica powder it is found that the capacity loss for the ball-milled sample is reduced and the cycle performance is improved. This is because the particle size decreases, which shortens the ion diffusion path for good ion transportation and lithium storage performance improvement.2) Prepare porous silicon structure by using the metal etching method. Ag ion of AgN03/HF corrosive solution plays an important role in the etching process and the concentration of the AgN03 influences the morphology. The sample etched by the AgN03 with 0.10molL-1 has the best cycle performance and minimum capacity loss.This is because the porous structure increases the contacting area with the electrolyte,and the pore provides the buffer space for volume change, which reduces structural damage and capacity loss, and is better for lithium storage.