Investigation On Preparation,lithium Storage Performance And Prelithiation Of Silicon Carbon Composites As Anode Materials

The capacity of commercial graphite anode is close to its theoretical limit.In order to meet the development requirements of high-energy-density lithium-ion batteries,it is urgent to develop new-type anode materials with high capacity to replace graphite anode.Silicon-based materials are most expected to be applied to the next generation of high-energy-density lithium-ion battery anode materials due to their high theoretical capacity,abundant mineral reserves and low discharge platform.However,the huge volume expansion effect,poor electronic conductivity and low initial coulombic efficiency(ICE)of silicon anode materials seriously hinder their commercial application.In respect of the issues above,this thesis takes SiO_x,nano silicon and porous silicon which have commercial prospects as the research object,and based on the new carbon materials such as graphene oxide nanoribbons(GONRs)and vertical graphene(VG)with high electronic conductivity and stable structure,by physical mixing,electrostatic attraction,plasma enhanced chemical vapor deposition(PECVD)technical method to design and construct a series of silicon carbon composites in order to improve the lithium storage performance of silicon-based anodes.In addition,in order to reduce the irreversible capacity loss in the first cycle and improve the ICE,the silicon carbon composites are prelithiated,and the effect of prelithiation technology on ICE is systematically studied.The main research contents are as follows:1.In view of the poor cyclic stability and rate performance of commercial SiO_x,GONRs and nitrogen-doped carbon coated SiO_x composites(SiO_x@NGONRs)are successfully synthesized by physical mixing method.The electrochemical lithium storage performance of SiO_x,GONRs wrapped SiO_x composites(SiO_x/GONRs)and SiO_x@NGONRs is compared.The double wrapping of GONRs and nitrogen-doped carbon not only increases the electronic conductivity of SiO_x,but also effectively suppresses the volume expansion of SiO_x,thus guarantees the structure stability of SiO_x@NGONRs during the cycling.The SiO_x@NGONRs electrode has a capacity of 919m A h g^-1 after 200 cycles at 0.5Ag^-1;its reversible capacity is 419mAhg^-1 at a current density of 5Ag^-1,which achieves excellent charge-discharge performance.In addition,in order to reduce the irreversible consumption of lithium ions in the first cycle,the ICE of SiO_x@NGONRs is improved by physical contact static pressure method,and the effect of static pressure time on ICE is systematically studied.2.In order to solve the disadvantages of huge volume expansion and poor electronic conductivity of nano silicon,GONRs tightly entwining nano silicon composites(Si/GONRs)are prepared by electrostatic attraction self-assembly method.The strong interfacial interaction between nano silicon and GONRs as well as the 3D network structure of GONRs can simultaneously improve the electronic conductivity of nano silicon,prevent the aggregation of nano silicon and restrain the volume expansion of silicon.The effect of different addition amounts of GONRs on the electrochemical properties of the composites is studied.The optimal ratio of the Si/GONRs-0.4 composite realizes excellent cycling stability(the capacity is 1516mAhg^-1 after 300 cycles at 0.5A g^-1)and rate performance(the capacity is 730mAhg^-1 at 6Ag^-1).The effect of different prelithiation time gradients on ICE of the Si/GONRs-0.4 composite is studied.The effects of prelithiation on the microstructure,elemental composition and cycling performance of the electrode are analyzed.3.With low cost,high-performance silicon carbon anode materials as the research target,nitrogen-doped GONRs wrapping porous silicon composites(npSi@NGONRs)are successfully synthesized under the interaction of strong electrostatic attraction induced by chitosan.By comparing GONRs and porous silicon composites(npSi/GONRs)prepared by simple physical mixing,the effect of electrostatic interaction on lithium storage performance and its mechanism are further studied and clarified.The tight wrapping of nitrogen-doped GONRs not only increases the electronic conductivity of porous silicon,but also buffers the volume expansion of silicon,while avoiding direct contact between porous silicon and electrolyte.Furthermore,the SEI layer formed in the process of prelithiation is analyzed and discussed in terms of microstructure and elemental composition,and the mechanism of prelithiation reducing the irreversible capacity loss of the composite in the first cycle and improving the ICE are explained.4.The porous silicon wrapped by nitrogen-doped GONRs shows excellent cycling stability,whereas its specific capacity needs to be improved.Hereto,a new-type structure of porous silicon@vertical graphene composites is designed by PECVD(PSi@VG).The effect of different CVD growth time on the microstructure of PSi@VG is compared,and the growth mechanism of vertical graphene is discussed.The abundant pore structure in the interior of PSi@VG can effectively provide a buffer space for the volume expansion of silicon.At the same time,the vertical graphene with high graphitization degree enhances the electronic conductivity of the composite and promotes the fast lithium ion diffusion and electron transport.In addition,as a protective shell,vertical graphene has a strong binding force with porous silicon,which ensures the structural stability of PSi@VG particles,thus endows PSi@VG excellent lithium storage performance(the capacity is 1744mAhg^-1 after 100 cycles at 0.2Ag^-1,the capacity is maintained at 819m A h g^-1 at high current density of 10Ag^-1).Furthermore,in order to improve the ICE of PSi@VG,PSi@VG is prelithiated by liquid phase chemical prelithiation.The effects of different prelithiation reagents,reagent concentration and prelithiation time on ICE of the composite are systematically studied.