Study On Surface Hard-carbon Coating Technology Of Metal-modified Silicon Anode Materials

With the rapid development of the times,people’s demand for wearable devices,smartphones and new energy vehicles is gaining.These industries have evolved rapidly,and lithium-ion batteries have likewise made great progress.At present,the anode material in commercial lithium-ion battery materials is mainly graphite.However,the theoretical specific capacity of graphite anode material is only 372 m Ah g^-1,which can not meet people’s demand for high energy density and also limits the development of the industry.Seeking a high energy density anode material to replace graphite is an important link in solving the current problem.At present,among the many known anode materials,the theoretical specific capacity of silicon is 4200 m Ah g^-1,and it has the advantages of a low lithium-deintercalation platform,a wide range of sources,and high safety performance.Therefore,silicon anode materials are considered to have the most potential to become high energy density materials.However,two most serious problems limit the commercialization of silicon anode materials.First of all,during the charging and discharging process of lithium-ion batteries,the silicon particles will have a volume expansion effect,which will cause the electrode material to pulverize and fall off on the current collector,thereby greatly reducing the cycle performance.Second,silicon is a semiconductor and has poor electrical conductivity,which makes it extremely poor in performance at high rates and cannot meet people’s fast charging needs.In response to the above problems,many researchers have proposed different solutions,such as silicon nanoization,porous silicon,and core-shell structure,etc.In this paper,by coating hard carbon on the surface of silicon,the volume expansion of silicon is effectively suppressed,thereby improving the cycle performance of silicon materials.At the same time,from the perspective of enhancing the conductivity of silicon,silver particles（Ag）are deposited on the surface of silicon by metal-assisted deposition to improve the conductivity of silicon,and on this basis,the surface is coated with hard carbon.Through electrochemical performance testing,material characterization and other research methods,the material properties were analyzed,and the following conclusions were obtained:（1）Using phenolic resin as the hard carbon source,the phenolic resin and silicon powder are mixed in a mass ratio of 1:10,and are mixed in a liquid-phase coating method,and the phenolic resin is evenly distributed on the surface of the silicon.By high-temperature pyrolysis,the phenolic resin was carbonized at 900°C for 5 h to obtain a hard carbon-coated silicon anode material（Si@HC）.Consequently,the cycle performance and rate performance have been greatly enhanced.After 200cycles at a current density of 0.5 C(1 C=4.2 A g^-1),the specific capacity is still690.58 m Ah g^-1,which is 280 m Ah g^-1higher than that of silicon raw material（Si）.At different rates of 0.1 C,0.2 C,0.5 C,1 C and 2 C,each rates is better than that of silicon raw materials.In particular,at the rate of 2 C,the capacity of Si@HC remains at 553.66 m Ah g^-1,and the capacity of Si is almost zero.The hard carbon coating not only inhibits the volume expansion of silicon and improves the cycle performance,but also improves the rate performance to a certain extent.（2）The silicon/silver composite（Si/Ag）was obtained by depositing silver particles on the surface of silicon by metal-assisted deposition method.Then,using the phenolic resin as the hard carbon source,the phenolic resin and the Si/Ag powder were mixed according to the mass ratio of 1:15,and ethanol was used to volatilize at high temperature.The above method was used to obtain a hard carbon-coated silicon-silver material（Si/Ag@HC）.After electrochemical tests,after 200 cycles at a current density of 0.5 C,the capacity is still 997.51 m Ah g^-1,which is 350.48 m Ah g^-1higher than that of Si/Ag,which is more than 2 times that of Si and also high on Si@HC.Due to the addition of Ag,Si/Ag@HC has excellent rate performance under different rate test conditions of 0.1 C,0.2 C,0.5 C,1 C,and 2 C.The discharge specific capacity remains at 639.16 m Ah g^-1when the current density is 2 C.Because the surface silver modification and hard carbon coating work together,it not only effectively suppresses the volume expansion,but also greatly improves the electrical conductivity of silicon and thus enhanes the electrochemical performance of silicon.