Preparation Of Silicon-based Nanomaterials As Anodes In Lithium Ion Batteries

It is well known that the morphology and structure of nano-materials will have important effects on their physical and chemical properties.As an important member of nano family,silicon-based nanomaterials have aroused much attention in recent years.Especially in the energy storage such as lithium ion batteries(LIBs),silicon has been regarded as one of the most promising candidate anodes.Unfortunately,despite Si owns the high theoretical specific capacity of 4200 mA h g’1(Li44;Si,about 1 folds than that of graphite),in the charge-discharge process,the alloying of silicon and lithium will lead to the huge volume changes of anodic host material,which will make the electrode pulverization and the formed SEI film un-stability.To tackle the problem,reducing the size of bulk silicon to micro or nano scale is a promising approach to improve the cycling stability of silicon anodes.which is ascribed to the high specific surface area of nanomaterials.Meanwhile,the void between particles will provide a buffer to alleviate the huge volume expansion and contraction.Up to now,a variety of Si nanostructures such as nanoparticles,nanowires,nanotubes and nanopores have been fabricated,and indeed,these novel structures exhibited good lithium-storage properties and improved cycling stabilities.Current strategies for synthesizing Si nanomaterials such as CVD method features with high cost and low product yield,which seriously hinder the practical,large scale applications of Si nano-materials.Hence,developing novel and facile methods to prepare Si nanomaterials has always been one of key efforts for inorganic scientists.This thesis aims to develop novel preparation methods for obtaining Si-based nanomaterials with different morphologies and structures;the electrochemical properties of fabricated Si-based materials that served as anodes in LIBs will be emphasized.Improved and modified preparative methods such as magnesiothermic reduction,and solvethermal reaction as well as a novel self-assembly approach are developed to prepare Si-based nanomaterials such as Si nanoparticles,nanospheres,ultra-thin nanosheets,mesoporous Si with controlled high specific surface area and pomegranate-like Si-mesoporous TiO2(pomgSi-mTiO2)which show outstanding electrochemical properties as LIB anodes.The thesis is divided into six chapters.Chapter 1,Introduction.A brief introduction of the development and research perspective of Si,including the classification,synthesis and application of Si nanomaterials in lithium ion secondary batteries was given.Chapter 2,Preparation of uniform Si nanoparticles from natural attapulgite for high-performance lithium ion battery anodes.From the starting mineral attapulgite.SiO2 nanorods were obtained and purified via acid washing and calcinations.Afterwards,Si nanoparticles with uniform size of around 10 nm were prepared via magnesiothermic reduction of SiO2 nanorods.After coated with polypyrrole(ppy),the as-synthesized ppy@Si nanocomposites showed excellent lithium-storage properties when served as anodes in LIBs.After charging-discharging for 200 cycles at a current density of 0.6 A g-1.the specific capacity was maintained at 954 mA h g-1.As Attapulgite distributed widely in the nature,this research provides a new means of utilization of abundant attapulgite.Chapter 3,Two-dimensional ultra-thin SiOx(0<x<2)nanosheets with long-term cycling stability as lithium ion battery anodes.Compared to Si nanoparticles,the synthesis of two-dimentional free-standing ultra-thin Si nanosheets has been scarcely reported.Strong acid(e.g.concentrated HCI)was previously exploited to exfoliate CaSi2 at low temperature(-30 C),which produced H-terminated Si6H6 ultra-thin layers after 7～10 days.Unlike this,a facile solvethermal method assisted with ultrasonic treatment was developed in this work to fabricate ultra-thin SiOx(0<x<2)nanosheets with thickness of around 4 nm.The obtained nanosheets are made up of stacked partly-oxidized Si atomic layers.After carbon coating,the SiOx@C nanosheets exhibited excellent cycling stability for high-capacity lithium ion battery applications.The specific capacity could be maintained as high as 760 mA h g-1 with almost no capacity decay after 400 cycles at a current density of 0.5 A g-1.Chapter 4,Solution synthesis of mesoporous silicon for high-performance lithium ion battery anodes.As one of the most important multifunctional semiconductors,mesoporous silicon(mPSi)has attracted great interest in many fields such as drug delivery,photothermal therapy,energy storage,optoelectronics and sensors in past decades.Conventional preparation method such as etching or magnesiothermic reduction usually leads to vast mass loss or unscalability.In this chapter,crystalline mesoporous Si with high BET specific surface area of 190 m2g-2 and pore size of 3.8 nm was facily prepared via the reaction of Mg2Si and HSiCl3 or Si2Cl6 in the presence of amine as catalysts at room temperature.PSi with different specific surface area could be obtained via adding cationic surfactants with different carbon chain length,and the surface area of PSi increased with the carbon chain length enlarging.Moreover,the different heat treatment time also forced significant impact on the BET specific surface area of porous Si.After coated with carbon(pyrolysis of dopamine precursors),the obtained c-mPSi-40 min@C nanocomposite showed excellent electrochemical properties,with a reversible specific capacity as high as 2800 mA h g-1(at 0.1 A g-1),good rate performance(over 1100 mA h g-1 at 2 A g-1),and capacity retention of 90%(992 A g-1 after 320 cycles at 2 A g-1).The successful synthesis of PSi via solution chemistry provides a convenient approach for large-scale preparation of porous Si nanomaterials as practical anode materials for high-performance Li-ion batteries.Chapter 5,Preparation of pomegranate-like Si-mesoporous TiO2(pomgSi-mTiO2)composite nanospheres and their application as anodes in lithium ion batteries.Based on solution synthesis and magnesiothermic reduction,pomgSi-mTiO2 were prepared via a step-by-step approach using polystyrene(PS)as templates.Layers of SiO2 and TiO2 were subsequently coated onto the surface of PS spheres with a diameter of around 120 nm.After removal of PS template and organic surfactants via calcination,pomgSi-mTiO2 nanospheres were obtained via magnesiothermic reduction.The pomgSi-mTiO2 composite nanospheres were shown excellent electrochemical properties when served as anodes in LIBs.The reversible specific capacity could reach as high as 1700 mA h g-1,and the specific capacity retention could be maintained at around 700 mAh g-1 after cycling at the current density of 1 A g-1 for 200 times,with almost no capacity decay observed.This work provided new insights on the fabrication of novel high-performance lithium ion battery anodes.Chapter 6,Conclusions.A brief conclusion on the thesis and an outlook for future research were presented.