Preparation And Electrochemical Performance Of Silicon-based Anode Materials

As the most competitive anode materials for the next generation of lithium-ion batteries（LIBs）,silicon-based anode materials have become a research focus in recent years.However,silicon-based anode materials usually suffer from large volume change during the charge and discharge process,leading to subsequent pulverization of silicon and poor cycle life.To solve these problems,two main strategies involving nanostructures and composites have been put forward.Under the guidance of physical and chemical synthesis methods,silicon-based anode materials with different morphologies were designed and synthesized,including zero-dimensional nanoparticles,three-dimensional porous structure,direct core-shell structure,yolk-shell structure,and complex hierarchical structure,and the cyclic performance of silicon-based anode materials can be significantly improved.However,commercial applications of silicon-based anode materials still face many challenges.This requires interdisciplinary integration to construct a hierarchical structure that can synergize the mechanical properties and the transport of lithium ions and electrons.This is also the focus and difficulty of current research on silicon-based anode materials.In chapter 1.A brief introduction about the development,working mechanisms,and common anodes materials of LIBs.After that,we review the research process,electrochemical reaction mechanism and preparation strategy of silicon-based anode materials in detail.In addition,we emphasized the influence of carbon spatial distribution and heteroatom doping on the electrochemical properties of silicon/carbon composite anode materials.Although a lot of research work has been done so far,the research on silicon-based anode materials still faces many challenges.From the perspective of material design,how to realize the recombination of silicon and carbon at the molecular scale and how to construct a hierarchical structure that can synergize the mechanical properties and the transport of lithium ions and electrons are important issues that deserve our attention.In this paper,some new solutions and ideas are proposed from the aspects of carbon distribution and heteroatom doping.In chapter 2,1,4-Bis（triethoxysilyl）benzene（BTEB）was selected as a precursor.Through a sol-gel and subsequent calcination process,a silicon-based composite consisting of homogenous atomic scale distribution（ASD-SiOC）can be prepared.As the precursor features molecularly organic-inorganic hybrid compositions where the organic R moiety homogeneously distributes within the whole framework,the carbon will distribute at atomic scale within the Si-O-Si network in the ASD-SiOC.Benefiting from the unique structure,the optimized nanocomposite exhibits comprehensively high performance for LIBs in terms of superior cycling stability and outstanding structure integrity.Specifically,at a current density of 0.2 A g^-1,the ASD-SiOC demonstrates a higher CE of 95.4%in the 2^nd cycle and an average post-cycling CE of 99.3%from the 2^th to the200^th cycles.Furthermore,the ASD-SiOC anode presents an ultrahigh average post-cycling CE of99.8%from the 11^th to the 500^th cycles at a current density of 5 A g^-1.This novel design brings about multiple fascinating merits:（a）Uniform composition between active SiO_x and carbon matrix can be achieved at atomic scale which effectively stabilizes the lithiation process;（b）The electronic conductivity of SiO_x can be significantly enhanced by three-dimensional carbon network;（c）This porous ASD-SiOC nanocomposite will translate into a more stable nanocomposite during first cycle which contribute to the high post-cycling CE.The results demonstrate that carbon distribution plays an important role in maintaining the structural and performance stability of the composite anode materials,and also provides a new idea for the design of the silicon/carbon composite anode materials.In chapter 3,firstly,several porous silicon oxides/carbon（SiO_x/C）nanospheres with different carbon distribution were synthesized,and then several Si/SiO_x/C composites with different morphology were prepared by molten salt reduction.The results show that the densification of the three-dimensional carbon network affects the kinetics of reduction process,and the appropriate carbon network is helpful to form hollow sphere structure.When the carbon network in the precursor is too dense,it will hinder the internal diffusion and form Pitaya-like structure;when the carbon network is too sparse or without carbon,the reduction products are easy to break up to form large aggregates.The electrochemical performance test shows that the Si/SiO_x/C composite with hollow sphere structure exhibits the best cycling and rate performance.In the chapter 4,we report a novel boron doping-induced interconnection-assembly（BDIIA）approach for fabricating an unprecedented assembly of mesoporous silicon oxides/carbon（SiOC）nanospheres which are derived from periodic mesoporous organosilicas（PMOs）.The as-prepared architecture is composed of interconnected,strongly coupled nanospheres with coarse surface.Significantly,through delicate analysis of the as-formed boron doped species,a novel melt-etching and nucleation-growth mechanism is proposed,which offers a new horizon for the developing interconnected assembling technique.Furthermore,such unique strategy shows precise controllability and versatility,endowing the architecture with tunable interconnection size,surface roughness,and switchable primary nanoparticles.Impressively,this interconnected assembly along with tunable surface roughness enable its intrinsically dual（both structural and interfacial）stable characteristics.As the lithium-ion battery anodes,the micrometer-sized B-SiOC assembly exhibits superior cyclability with 0.03%capacity decay per cycle after 1000 cycles at high current density of 2 A g-1,indicating ultra-stable interfacial stability of this unique assembly.In the chapter 5,the chemical structure,thermal stability and graphitization degree of boron doped carbon nanotubes（B-CNT）,activated carbon nanotubes（aCNT）and commercial carbon nanotubes（CNT）have been systematically studied.The electrochemical performance shows that the B-CNT possesses excellent rate performance and long cycling stability at large current desity(an average CE of 99.8%can be achieved at a current density of 10 A g^-1).Through the study of capacity contribution mechanism and electrochemical kinetics,we found that the high capacity of boron doped carbon nanotubes under high current mainly comes from the contribution of pseudocapacitance,and has higher lithium ion diffusion coefficient and lowest charge transfer resistance,which shows that boron doping can further affect the electrochemical performance by adjusting the electronic structure of carbon layer.As a verification,the composite electrode with b-cnt as additive and silicon as additive also shows high rate performance.Therefore,these basic researches will be helpful to design carbon nanotube composite electrode materials more reasonably.At last,both the innovation and shortcomings of the above work are summarized.