Study On Microstructure And Electrochemical Properties Of Silicon-carbon Anode Materials For Lithium Ion Batteries

It is urgent to find a new anode material considering the low energy density of current commercial lithium-ion battery anode materials and the poor performance in charging/discharging processes under large current.Silicon has a theoretical specific capacity(4200 mAhg^-1)as a lithium ion battery anode material,it is ten times higher than the current carbon anode material(372 mAhg^-1),and its low operating voltage,no pollution and abundant reserves.Silicon material is considered to the next best anode material.However,the high volume effect?300%?of silicon materials during charging/discharging process,the poor conductivity of silicon particles,easy pulverization,and low cyclic stability,which hinder commercial application in industry production.In this paper,we add carbon and silicon for composite treatment to obtain silicon-carbon composite materials.In order to combine the excellent mechanical properties and electrical conductivity of carbon materials with silicon materials,which significantly improves the electrochemical properties of silicon anode materials.First,commercial carbon powders?C?and commercial standard silicon powder is milled mechanically together,to explores the ball milling crafts and the optimal compounding ratio of silicon and carbon.Secondly,with the optimized ball milling process crafts bio-artificial graphite?Biomass Graphite,BG?was added into commercial carbon powder?C?to improve the comprehensive electrochemical performance of silicon materials.Finally,the mechanism of BG to improve the performance of silicon anode materials was investigated,which help to mass production.The main results of this paper are as follows:?1?Under the protection of argon,Si@C composite material is prepared by high-energy mechanical ball milling?Si:C=5:1,2.5:1,1:1,0.5:1?.After observation carbon is covered on the surface of Si particles,which formed a"carbon layer"to alleviate the volume effect of the negative electrode.The Si@C composite electrode?Si:C=2.5:1?maintains a specific capacity of more than 1100 mAhg^-1 at a discharge current of 0.5 Ag^-1.This proportion?Si:C=2.5:1?can improve the high rate discharge ability of silicon anodes dramatically.However,silicon-carbon composite materials do not cause excessive loss of specific capacity due to the addition of carbon.?2?The residue of traditional Chinese medicine is used as a carbon source.After the carbonization at high temperature,biomass artificial graphite is obtained.Observing its microscopic morphology,the biomass artificial graphite has a porous,lamellar structure.Testing the electrochemical performance of Si@BG electrodes under 0.1C,0.2C,0.5C,1C,2C and 0.1C discharge currents,the capacity retention rate is above 88.5%,showing excellent high rate dischargability performance.Si@BG composite is obtained by high energy mechanical ball milling?Si:C=2.5:1?with argon protection.The discharge capacity of the Si@BG composite electrode is 3465 mAhg^-1 in the first charging/discharging cycle.The Si@BG composite electrode is circulated for 100 cycles at a discharge current of 0.5 Ag^-1,and finally its capacity remained above 995 mAhg^-1.The capacity retention rate of Si@BG?Si:BG=2.5:1?composite electrode is 119.6%higher than that of Si@C?Si:C=2.5:1?composite electrode.The Si@BG composite electrode has a discharge specific capacity of 995 mAhg^-1after 100 cycles at a discharge current of 1 Ag^-1.It shows that the Si@BG composite electrode has the high rate dischargability.It is that to say the high-energy mechanical ball milling composite of biomass artificial graphite and silicon can effectively improve the microstructure and electrochemical performance of silicon anode materials,and this material is expected to be applied to large-scale production of silicon anodes.