The Preparation And Modification Study Of Porous Silicon For Lithium Ion Batteries

With its high specific capacity of4200mAh/g, silicon material has becomea research hotspot, however, the serious volume effect leads to unsatisfactorycycling performance. One of the means to alleviate the volume change is todesign porous silicon particles, for the voids can be a buffer space so as toslowing down the structure damage during the lithium insertion/extraxtionprocess. The main content of this article is to explore the specific steps of themetal-assisted chemical etching methods to prepare porous silicon. For carboncoating of the porous silicon, acetylene gas phase deposition was employed toimprove the electrical conductivity. SEM, EDS, TG and other physicalcharacterization methods were used to study the morphology and composition ofthe porous silicon material,and the constant current charge-discharge test, cyclicvoltammograms and other means were used to evaluate the electrochemicalproperties of porous silicon.Silver mirror reaction was utilized to deposit metal Ag onto the surface of200mesh silicon powder and silicon powder after the ball milling, there is noporous morphology on silicon after etching, which is due to the Ag particles areattached onto the surface of the silicon substrate only depending on the physicalforce.As a result, the binding force of Ag and silicon is not so strong to etchingsilicon continually. With galvanic reaction to deposit Ag and etch200meshsilicon powder, we can prepare the porous silicon particles.At the current densityof100mA/g, for the first time the discharge capacity of porous silicon is2805mAh/g, and coulombic efficiency is73%. and the capacity decreased to722mAh/g after10cycles.We can find that the porous structure did not achieve thegoal of improving silicon cycling stability, the main reason is the silicon particlesis so large that the voids can not buffer the large stress caused by volume change.At the same time, the binder can not wrap up the big silicon particles effectively,so, porous silicon material still undergoes serious volume change and poorelectrochemical cycling performance.Galvanic reaction was carried out to etch small silicon particles after ballmilling, and there is no porous structure generated.The possible reason is thesmall silicon particles has a high reactivity with HF, masking the metal Agcatalytic effect. So, choosing an appropriate size of silicon particles is very important for porous silicon preparation.In order to enhance the electrical conductivity of the porous silicon furtherly,acetylene gas phase deposition was adopted to coat canbon onto poroussilicon.The porous silicon present a better performance when the acetylene gasflow is40ml/min, and the carbon content is about12%. At current density of100mA/g, the first discharge capacity is1710mAh/g, coulombic efficiencyreaches81%. After10cylces, the reversible capacity is926mAh/g, Theimproved electrochemical performance is mainly due to the increased electricalconductivity. However, owing to the unsatisfactory of the particle size, theaperture size, the hole depth, the material still experiences serious performanceattenuation in the subsequent cycles.In this article, porous silicon was ball milled and nanopore was destroyed toform nanoscale silicon particles. Thus the small size can play a buffer role for thevolume expansion of silicon. At the current density of200mA/g, the firstdischarge capacity is3124mAh/g, the coulombic efficiency is up to71%, after10cycles the reversible capacity is1210mAh/g, even after50cycles, thereversible capacity is674mAh/g. Compared with silicon material before ballmilling, the performance of silicon is improved greatly, which is mainly thanksto the decreasing size of silicon particles that buffering the volume change.