Research Of Silicon-based Anode Materials For High-energy- Density Lithium Ion Batteries

At present,the common anode material for commercial lithium ion batteries is graphite.In recent years,with the rapid development of society and economy,people have put forward higher requirements on the energy density of lithium ion batteries.The mass specific capacity of high-end graphite has reached 360-365 mAh/g,which is very close to its theoretical mass specific capacity of 372 mAh/g.It is difficult to have more room for improvement.Therefore,silicon materials have become a potential nextgeneration high-energy density anode material due to their higher theoretical mass specific capacity(3580 mAh/g at room temperature).After years of development,the currently commercialized silicon-based anode materials are mainly SiO@C materials and nano-Si/C materials.In this paper,the effects of different carbon coating with various contents,different particle sizes and different conductive additives on the electrochemical properties of SiO@C/graphite composite anode materials are investigated.Four SiO@C samples with different carbon contents(5wt%,10wt%,15wt%,35wt%)are separately mixed with graphite to obtain a SiO@C/graphite composite material with a mass specific capacity of about 600 mAh/g.It is found that the composite material corresponding to 15wt% has the best electrochemical performance.The analysis shows that the ionic conductivity of the composite material corresponding to 15wt% is smaller,and thus the polarization and impedance of the corresponding battery are smaller,thereby improving the cycle stability of the battery.While the battery is disassembled,it is found that the SEI on the surface of the electrode corresponding to 15wt% is less and more flat,which we considered to be one of the microscopic causes of the small impedance of the battery.Three SiO@C samples of different particle sizes(2?m,5?m and 10?m)are mixed with graphite to obtain a SiO@C/graphite with a mass specific capacity of about 600 mAh/g.It is found that the electrochemical properties of the composites corresponding to 2?m are optimal.It can be determined that the smaller electronic conductivity of the composite material corresponding to 2?m is one of the reasons for the lower impedance and polarization of the corresponding battery,but the influence of ionic conductivity is still uncertain.The analysis results when the batteries are disassembled show that in addition to the advantages of SEI growth on the surface of the electrode,the expansion of the electrode corresponding to 2?m is also smaller,indicating that the small particles can better release the stress generated by the volume change of silicon during the process of intercalating and deintercalating of lithium,which can protect SEI from repeatedly rupture and grow,and improving the cycle stability of the electrode.Several conductive additives commonly available on the market are added to the SiO@C/graphite composite material to prepare the anode electrode of the half-coincells.The differences in electrochemical performance of the corresponding battery are discussed,and the polarization,impedance,electrode surface SEI and electrode expansion are discussed to explain the reasons for the differences.We have also discussed some of the deeper reasons such as the morphology,dispersion,conductivity,carbon crystallinity,surface functional groups of conductive additives,but there is still much room for further research.If the conductive additives are divided into three types according to their mophology features: point,line and surface,the order of electrochemical cycling performance of the corresponding half-cells is Ketchen Black>Acetylene Black>Super-P,SWCNT>MWCNT>VGCF,graphene > KS-6.As for the industrialization of silicon-based anode materials,we have developed SiO@C materials and nano-Si/C materials that can be mass-produced and sold,and their comprehensive performance has certain advantages compared with most competitors.