Design And Preparation Of Silicon-based Anode Materials And Electrochemical Properties For Li-ion Batteries

Lithium ion batteries are widely used in various electronic devices and electric vehicles due to many advantages,including high energy density,long cycle life,high safety and no memory effect.At present,the commercial anode material is graphite.However,the theoretical capacity of graphite is too low to meet the demand of high specific energy lithium ion batteries.Silicon has a theoretical capacity of 4200 mA h g^-1 and is extremely abundant in the earth.However,the huge volume change of silicon in the process of alloy and dealloying,the information of unstable SEI film on the electrode-eletrolyte interface and the low conductivity hinders its application.To slove the above problems,in this thesis,we prepare silicon-based materials with different structures,and investigate the effect of these structures on the electrochemical properties of silicon anode.Then,in order to reduce the cost of silicon production,silicon based anode materials are prepared by using rice husk and excellent electrochemical properties are obtained.Firstly,porous silicon is prepared by sintering and HF etching using SiO as raw material.XRD and Raman tests show that the synthesized material is silicon.The structure of porous silicon is studied by means of SEM,TEM and specific surface and pore size analysis techniques.It is found that porous silicon contains a large number of mesoporous and is a secondary micron particle composed of many nano Si particles.The porous silicon has a specific capacity of 1379 mA h g^-1 after 150 cycles at a current density of 100 mA g^-1 and an areal specific capacity of 1.82 mA h cm^-2after 100 cycles at a high mass loading of 1.76 mg cm^-2.Furthermore,the porous silicon has an excellent rate performance,and at the current density of 6 A g^-1,the porous silicon can obtain a specific capacity of 370 mA h g^-1.It is found that porous silicon has a smaller charge-transfer resistance than C-SiO and D-SiO through EIS measurement.The experimental results show that the excellent electrochemical performance of porous silicon is attributed to its special structure.Then,TiN,which is high electron conductivity,is used as the coating layer for Si@TiN composite with york-shell structure.The Si@Ti N composite is prepared by template method.The phase,morphology and structure of the Si@TiN are confirmed by XRD,SEM and TEM.Electrochemical performance test shows that Si@TiN has better cyclic stability than Si nanoparticles.At the current density of 1 A g^-1,Si@TiN has a specific capacity of 2047 mA h g^-1 after 180 cycles,the specific capacity is maintained 1429 mA h g^-11 after 500 cycles.Furthermore,the rate performance test shows Si@TiN has a specific capacity of 850 mA h g^-1 at the current density of 12 A g^-1.Electrochemical impedance spectroscopy?EIS?shows the structure of Si@TiN can protect the generated SEI film from destruction,and ensure that the electrode material will not be pulverized or even delaminates from the current collector,thus realizing the rapid transmission of electrons and lithium ions.It is found that Si@TiN can reduce the electrode swelling after cycles,which ensures the close contact between the electrode and the collector,and obtain a stable electrode structureAfter exploring the effect of the special structure above on improving the electrochemical properties of silicon anode,we have selected the biomass waste rice husk as the raw material for the preparation of silicon-based anode materials,which is expected to realize the large-scale production of silicon by reduce the production cost of silicon.Firstly,SiO_x/C composite is synthesized by two-step sintering process,which has the advantages of low production cost,no use of toxic,flammable and explosive materials and simple preparation process.XRD and Raman tests show that SiO_x in SiO_x/C composite is amorphous,and the graphitization degree of carbon in SiO_x/C composite is improved.XPS shows that part of the Si⁴⁺is reduced to Si³⁺and Si²⁺.SEM and TEM images show the SiO_x particles in composite is tightly coated by carbon.Due to the unique structure,SiO_x/C composite has a specific capacity of nearly 600 mA h g^-1 after 100 cycles at the current density of 100 mA g^-1.At the rate of 1 A g^-1,SiO_x/C composite has a discharge specific capacity of nearly 300 mA h g^-1.Although SiO_x/C has stable cycling performance,the carbon contained in the material greatly reduces its electrochemical capacity.In order to meet the requirement of the future market for further increasing battery energy density,we still use rice husk as raw material to prepare nano Si by aluminothermic reduction at low temperature.This method can not only save energy consumption,but also avoid danger caused by high temperature process.SEM and TEM measurements show that nano Si has interconnected porous structure and the size of the silicon particles is between 10 nm and 50 nm.The nano Si shows good electrochemical performance.At the current density of 500 mA g^-1,the nano Si has a discharge specific capacity of2264 mA h g^-1 after 100 cycles.In this thesis,different structures of silicon materials are designed,and their structure and morphology are characterized.Then,the effects of these structures on the electrochemical properties of Si-based anode materials are studied.On this basis,in order to reduce the production cost,SiO_x/C composite and nano Si are synthesized by using rice husk as raw materials.Then,we study their electrochemical properities,respectively.This thesis has some reference value and guiding significance for the high efficiency and low cost preparation of Si-based anode materials.