Construction And Electrochemical Performance Of Nano Silicon-Based Anode Composite Materials

With the widespread application of lithium-ion batteries,high-capacity electrode materials have become a research focus.The theoretical lithium insertion capacity of silicon can reach4200 mAh g^-1,which is the highest among currently known anode materials,with the characteristics of low lithium insertion voltage and rich reserves.It is very promising to replace the current graphite electrode as the next generation anode material.However,due to silicon's huge lithium-insertion capacity,it also causes severe volume expansion(300%),wihch lead to the destruction of the material structure.At the same time,the electroconductivity of silicon is poor,which is prone to occur polarization during electrode reactions and causes more side reactions.These unfavorable factors make silicon as an electrode material often shows poor cycle performance,insufficient rate and low initial coulombic efficiency.Using silicon oxide or nano-silicon to build silicon-based composite materials can alleviate the above problems to a certain degree.But there are still shortcomings such as complicated preparation process,high cost,and mismatch of comprehensive electrochemical performance.In particular,the essential reason for the initial low coulombic efficiency of silicon-based composite anode materials is not very clear,and the lack of technical ways to effectively improve their initial coulombic efficiency.In view of the above problems,this paper intends to prepare a long-cycle,high-capacity,high-rate,and high-efficiency anode material by constructing a variety of structures of nano-silicon-based composite materials and systematically studying the material's initial coulombic efficiency,which provides guidance for the application of silicon-based composite materials.The main research contents and results are as follows:(1)A yolk structure of C/SiO₂/C and a hollow structure of C/SiO₂ composites with porous feature are synthesized by one-step method and two steps method,respectively.When evaluate as an anode material for lithium-ion batteries(LIBs),the C/SiO₂/C and C/SO₂ deliver an impressive cycle performance and exhibit an excellent rate capacity.More importantly,attributing to the one-step synthesis method of C/SiO₂/C,it possesses interconnected micropore and higher special surface area,which provide a more efficient ionic transportation path and facilitate the diffusion of Li⁺between the electrolyte and C/SiO₂/C,thus the activation time of SiO₂ can be effectively shorten.The capacity achieves the steady value of 1135 mAh g^-1(based on the weight of SiO₂ in the electrode material)only by 60 cycles at 50 mA g^-1 which is much faster than that of C/SiO₂ and delivers a high-rate capacity of 390 mAh g^-1 at 1000 mA g^-1.(2)A double core-shell structure of Si@PANI@Ti O₂ nanocomposite is synthesized by a simple in-situ growth method.The two shells of polyaniline(PANi)and Ti O₂,hand in hand,play a key role to improve the electrochemical performance:First,the flexible properties of polyaniline(PANi)effectively accommodate the volume change of Si during the cycling.Second,the good mechanical feature of TiO₂ can maintain the structural integrity and attenuate the volume expansion of Si cores.Finally,both of polyaniline and the lithiated TiO₂ enhance the conductivity of Si,which promotes the electrons transport.Resulting in the Si@PANI@TiO₂ double core-shell nanocomposite exhibits remarkable synergy in large,reversible lithium storage,delivering a reversible capacity as high as 1027 mAh g^-1 after 500cycles and a superior rate capacity of 640 mAh g^-1,at a current of 500 and 4000 mA g^-1,respectively.(3)A simple and efficient hydrothermal approach to fabricate the ternary and microsphere structure of SiO_x@SnO₂@C composite.Since the lithiated SnO₂ can significantly enhance electrical conductivity of Si,SnO₂ coating is constructed by getting the SnO₂,derived from hydrolyzed Na₂SnO₃,in-situ grow on the surface of nano silicon.Meanwhile,in the hydrolysis process of Na₂SnO₃,high amounts of OH^-generate,etching part of the Si in SiO₃^2-,which ultimately transforms to SiO_x after adding citric acid.Compared to Si,the change in volume of SiO_x during lithiation/delithiation process is smaller.After simple hydrothermal treatment and subsequent carbonization,both the SnO₂ and SiO_xnanoparticles are assembled into microspheres through Ostwald ripening.Thereby,the ternary and microsphere structure of SiO_x@SnO₂@C is formed,and Si cores have been uniformly embedded in the SiO_x,SnO₂ with carbon layer.Such unique architecture combining SiO_x,SnO₂ and amorphous carbon together possesses greater electrochemical properties.As a consequence,the SiO_x@SnO₂@C composite exhibits a reversible capacity of 796 mAh g^-1 at a current density of 1 A g^-1 for 300^th deep cycles,as well as a rate capacity of 515 mAh g^-1 at a high current density of 4 A g^-1.(4)Constructing the architectural stable silicon composite is significantly critical to enhance the Si electrode cycle life of lithium-ion batteries,in which the inevitable volume expansion exerts huge mechanical stress within the Si anode,then bringing about the destruction of the silicon structure and unsatisfactory cyclic performance.In this work,we report an integrated and 3D network structure of Si NWs/CNTs@MOFs composite,prepared by a facile in-situ growth method.The metal-organic frameworks(MOFs)derived porous coating and the three-dimensional(3D)conducting network structure of Si NWs/CNTs@C precursor,hand in hand,construct a structurally stable composite,with the Si NWs cores are fully covered by the MOFs coating.Attributing to MOFs derived porous coating,high conductivity of CNTs as well as the stable three-dimensional network structure,not only the transport of ions and electrons facilitates but also the stability of the structure during the electrochemical process maintains.The resulting integrated Si NWs/CNTs@MOFs composite enhances the electrode durability and presents a reversible capacity of 1223 mAh g^-1 at the current density of 100 mA g^-1 for 100 cycles,and a rate capacity of 765 mAh g^-1 at the high current density of 5 A g^-1.(5)The contents of the above chapters can effectively improve the cycle and rate performance of silicon,but the initial coulombic efficiency of silicon composite materials has not been effectively improved,and the mechanism that affects it is not clear.In order to explore the mechanism that affects the initial coulombic efficiency of silicon-carbon composites,a simple silicon/graphite model was used to prepare composite materials with different specific surface areas,charge transfer impedances,and silicon-carbon bonding methods.The effects of these factors on the initial coulombic efficiency were investigated.The results show that the first effect of carbon-silicon composite materials is not traditionally determined by the specific surface area of the material,but is affected by the conductivity of the material,the interface condition,and the silicon-carbon bonding mode.