Study On Preparation And Mndification Of Carbon-Coated Silicon Materials As The Anode Of Lithium-Ion Battery

Silicon（Si）material has become one of the most potential anode materials for lithium-ion batteries because of its highest theoretical specific capacity（4200 m Ah/g）and low voltage platform in the field.However,the volume effect（>300%）of Si during alloying/dealloying with lithium lead to the pulverized of active material and destroyed electrical contact with the current collector,which causes extremely poor electrochemistry of Si-based materials.On the other hand,the low electronic conductivity of Si also limits its practical applications in related fields.In order to address these issues,we use pure silicon obtained by pickling industrial waste silicon as raw material to prepare carbon-coated Si（Si/C）in this thesis.We also developed new strategies to modify the Si/C materials,which resulted in largely enhanced electrochemical performance due to improved electronic conductivity and constrained volume expansion.In details,our work can be divided into the following parts:（1）The silicon powder purified by acid washing are simply ball-milled with polyvinyl butyral（PVB）and heat-treated at a high temperature of 750°C in a tube furnace under an argon flow to obtain Si/C composite material.Appropriate amount of the surface carbon after polymer pyrolysis can not only limit the volume expansion of silicon,but also provide more electron channels.The results show that the Si/C prepared with a mass ratio of 20%for the PVB exhibited the best electrochemcial performance.For instance,a reversible charging specific capacity of 1148.5 m Ah/g after 50 cycles was achieved.In addition,the initail coulombic efficiency（ICE）of the obtained samples has also increased from 52.6%to 72.3%after carbon-coating.（2）Although the carbon coating layer can alleviate the volume expansion of silicon materials,there is still large space to improve the electrochemical performance of the Si/C material.In this chapter,we use pitch as the carbon source and boric acid as the dopant.They were ball-milled with the pure Si,and then calcined at 700°C to obtain the B-doped Si/C materials.The incorporation of B atoms into the carbon layer can bring extra defects and gaps,acting as active sites to accelerate the transfer rate of lithium ions and electrons,which further improves the conductivity of Si/C materials.Based on the electrochemcial performance of the obtained samples,we concluded that the optimum amount of boric acid is 0.6%.In this condition,the ICE of prepared B doped Si/C materials displayed ICE as high as 85.8%and a specific capacity of 1650m Ah/g after 50 cycles,which is much higher than that of undoped Si/C materials（530.9 m Ah/g）.It is worth mentioning that when the carbon source is changed from asphalt to PVB,unexpected bad electrochemical properties were obtained.（3）In this chapter,using PVB as carbon source and phosphoric acid as the additive,P-doped Si/C samples were obtained by similar technique described above.By comparing the X-ray photoelectron spectra of Si/C material before and after doping,we found that the P was successfully doped into crystal structure of the Si to form n-type Si,which effectively improved the intrinsic conductivity of Si.The addition of phosphoric acid also favored the formation of Si O₂on the Si surface.It can react with the Li ions to form the reversible Li₂Si₂O₅,which also acted as a buffer effect to constrained the volume expansion of Si.Therefore,it can benefit the formation of a stable SEI film,reducing the side reaction with the electrolyte,that normall led to the capacity degradation.We investigated the effects of different ratio of phosphoric acid on the electrochemical performance of the synthesized Si/Si O₂/C materials.It revealed that the the addition of 7%H₃PO₄resulted in the best electrochemical performance of the obtained Si/C material.In details,the ICE increased from 72.3%（undoped one）to 86.7%.After 150 cycles,it still maintains a charge capacity of 1389.9 m Ah/g at a current density of 200 m A/g,which is nearly 3times higher than that of the undoped Si/C material（449.2 m Ah/g）.These results indicate the good cyclic stability of the prepared Si/Si O₂/C.The effect of different sintering temperatures on the electrochemical properties of the synthesized Si/Si O₂/C materials is also discussed in this chapter.The results show that the ICE of the synthesized Si/C materials can be improved with increasing the calcination temperature.