Study On The Preparation And Electrochemical Properties Of A Novel Silicon-Carbon Composite Anode Material

Silicon-based materials have a higher theoretical capacity than traditional graphite anode materials.Therefore,they are considered to be the most promising new anode materials for lithium-ion batteries.However,the serious volume expansion of such materials during lithium intercalation and deintercalation can lead to the pulverization of active particles.As a result,the original structure is destructed and the capacity is rapidly decayed as well as other problems.All these issues can limit the commercial application of this kind of material.This thesis aims at the problems generated by the volume expansion in the application of silicon-based anode materials.It improves its cycle performance by disproportionate etching and carbon nanofiber coating on the material.Furthermore,by analyzing the material capacity decay mechanism in depth as well as using chemical vapor deposition?CVD?and high temperature,it prepared a silicon-carbon composite negative electrode material.Moreover,the microstructure and surface shape of the material were analyzed by X-ray Diffraction?XRD?,Scanning Electron Microscope?SEM?,Transmission Electron Microscope?TEM?,Thermogravimetric Analysis?TGA?and so on.Therefore,the appearance was analyzed,and the electrochemical performance was tested by constant current Discharge and Charge?D-C?,Cyclic Voltammetry?CV?and Electrochemical Impedance Spectroscopy?EIS?.This thesis mainly includes the following contents:1.The graphite was used as raw material,and a series of graphite/carbon nanofiber composite materials were prepared by chemical vapor deposition.The cycle performance of graphite/carbon nanofiber composites was improved compared to pure graphite materials and the cycle capacity was increased,the Coulomb efficiency was reduced for the first time.Moreover,the graphite/carbon nanofiber composite material containing 20%carbon nanofibers showed a specific discharge capacity of 649 mAh/g for the first time,and the capacity remained 406 mAh/g after 50 times of charge and discharge.The rate performance of graphite/carbon nanofiber composites was also improved compared to pure graphite materials.It was proved that the specific discharge capacities of graphite/carbon nanofiber composites at current densities of 0,1,0.2,0.4,0.6,0.8,and 1C are 441 and 408.1,respectively,366.4,335.2,297 and 251.1 mAh/g,and the reversible capacity can still reach 436.6 mAh/g even if the current density returns to 0.1C.2.It was explored the types of silicon sources?SiO,Si?,the particle size of silicon?100nm,500nm,5?m?,the type of binder?PVDF,CMC/SBR,LA133?and vapor growth system?Ni-C2H4,Fe-CO?and their effect on the electrochemical performance of the material.The first discharge capacity of SiO material was lower than that of Si material,but at the same time showed better cycle stability.It also showed that the smaller the particle size of pure silicon material,the better the cycle stability will be.What's more,the silicon carbon composite material prepared by Ni-C2H4 system electrochemical had better performance than Fe-CO system.And the cycle stability of silicon-based anode materials prepared with CMC/SBR,LA133 and S A as binders was better than that of silicon-based anode materials prepared with PVDF as binders.3.Multi-component silicon pSi?Si,SiO,SiO₂?was obtained by pretreatment of silicon oxide,and then a composite material of pSi and carbon nanofiber?CNF?with core-shell structure was designed by chemical vapor deposition?CVD??pSi-CNF?.In multi-component silicon,the Si and SiO can provide electrochemical reversible capacity and SiO₂ can suppress the volume expansion of silicon,and the shell structure formed by nano-carbon fiber coating can effectively improve the conductivity of the composite material.At the same time,it can further suppress the volume expansion of silicon to maintain the core-shell Structural integrity.Electrochemical performance tests showed that at a current density of 0.2 A/g,the reversible capacity was 1411 mAh/g after 100 cycles,and the capacity retention rate was 74%.It demonstrated good cycle stability and high reversible capacity.Furthermore,given the current density of 1 A/g,the reversible capacity was 735 mAh/g after 300 cycles and the capacity retention rate was 86%,which means it display good rate performance.