Struture Design And Preparation Study Of Silicon Based Anode For Lithium Ion Batteries

The theoretical capacity of silicon anode material of lithium-ion battery isup to4200mAh/g. However, commercial application of silicon anodes ishindered by dramatic volume expansion during the lithium intercalation process,resulting in poor cycle performance, continual rupturing and reformation of theSEI, and many other issues. To finitely overcome the expansion of silicon, wemust design a micro-nano hybrid structure containing void space in the inner ofparticles to accommodate the expansion. To begin with, we investigated thepreparation of porous silicon by HF etching to lay the foundation for thepreparation of later materials. And then we designed and prepared a quasi-pomegranate structured silicon-based materials and a hollow core-shell structuredporous silicon-based materials. XRD, SEM, BET and other characterizationmethods were used to investigate the structural characteristics of the silicon-based materials. Galvanostatical charge-discharge test and electrochemicalimpedance were also implemented to evaluate their electrochemical performance.Porous silicon could be successfully prepared in the Ag/HF-H2O2system,but it’s cycle performance was still poor. To etch SiO in the Ag/HF-H2O2system,the better conditions were that the solvent was water/ethanol (1:1) and the HFconcentration was5mol/L. When etching silicon in the Cu/HF-H2O2system andthe Cu/HF-Fe3+systems, the surface of silicon only showed shallow pits.Disproportionation SiO could be etched by NaOH. The first charge capacity ofthe porous materials was793mAh/g and the capacity retention was72.9%after30cycles.We prepared a quasi-pomegranate structured silicon-based materials throughthe technology roadmap of disproportionation, Etching and carbon-coating. Theparameters of each steps and it’s impact on performance were investigated. It isshown that the minimal temperature was900℃to ensure SiO disproportionationand cross-linked porous structure could be obtained by HF etchingdisproportionation SiO. The impact of material loading, acetylene gas flowingrate, temperature and time on chemical vapor deposition process was alsoinvestigated.Polyaniline was used as carbon source to prepare hollow core-shellstructured porous materials and the structure parameters were further optimized. It is shown that increasing the void volume and thickness of carbon shell wasbeneficial to the cycle performance. When mixing HF-mSiO@C materials and20%graphite, the capacity retention increased to95.2%after100cycles. Byselecting sucrose as the carbon source, the preparation process could besimplified as ball milling, disproportionation+carbonization, and etching. Theprepared HF-mSiO@C material showed the initial reversible capacity of1067mAh/g, the initial coulombic efficiency of60.1%and the capacity retentionof86.4%after140cycles. The average coulombic efficiency was99.5%during140cycles. By Prelithiating30min, the first cycle Coulombic efficiency of HF-mSiO@C material was up to80.0%and the capacity retention was up to88.8%after150cycles. By a longer prelithiation, the first cycle coulombic efficiencycould exceed100%.By determining the cross-sectional image of electrode before and aftercycling, it is confirmed that hollow core-shell structured porous silicon-basedmaterial could effectively alleviate the volume expansion. However, the volumeexpansion could not be completely accommodated by void space of the material,that is the main reason for the failure of the electrode. The expanded Li–Sioccupied all the void spaces and then ruptured the carbon shell and caused coreand carbon shell complete separation.