Fabrication And Electrochemical Properties Of Nanofibrous Silicon-based Composite Materials

In recent years,with the progress of nano science and technology,the performances of lithium-ion batteries have been greatly improved.However,there are still many bottlenecks,especially in the design and construction of new high-performance electrode materials.As one kind of the most promising anode materials for lithium-ion batteries,silicon-based materials have attracted much attention due to their high theoretical specific capacities,which are much higher than that of the traditional commercial graphite anode.But how to overcome the drawbacks of silicon-based anode materials,such as the low conductivity and the huge volume changes in the lithiation/de-lithiation processes,is the key problem to further improve their electrochemical performances.Bio-mimetic synthetic strategy based on the self-assembly technique,that is,the combination of specific synthetic chemical processes and biological substances,provides a short-cut pathway for the design and fabrication of new functional materials with tailored stuctures and properties.As being one of the most abundant natural polymers,natural cellulose substance possesses unique structural hierarchies from macro-to molecular scales,as well as porous network structures at nanometer levels;hence it has application potential in fabrication of new nanostructured electrode materials.In this study,nanofibrous silicon-based nanocomposite materials are fabricated by employing natural cellulose substance(ordinary laboratory quantitative filter paper)as structural template or carbon scaffold.The nanocomposites show improved electrochemical performance when used as anode materials for lithium-ion batteries by enhancing their mechanical properties and conductivity,and provide some guidance on the application of energy storage devices.The main research contents are as follows:1.Nanofibrous silica/carbon composites are fabricated by employing a natural cellulose substance(ordinary laboratory filter paper)as both scaffold and carbon source.Thin silica-gel layer is first deposited uniformly on the surface of the cellulose nanofiber by a sample sol-gel method.Thereafter the resulted silica-gel/cellulose composite is calcined and carbonized in argon atmosphere to produce the silica/carbon composite.In the nanocomposite,thin silica layer is uniformly and conformally coated on the surface of the carbon nanofiber.Owing to the high specific surface area,the unique porous network structure and the carbon nanofiber matrix possessed by it facilitate electrolytes diffusion and electron transport,and efficiently alleviate the huge volume changes of the active materials upon cycling.Hence,the nanocomposite exhibits enhanced high specific capacity,improved cycling and rate performances when used as an anode material for lithium-ion batteries.Further coating an amorphous carbon layer or loading silver nanoparticles on the silica/carbon nanofiber surface result in improved lithium storage performances.2.In order to further improve the electrochemical properties of the nanofibrous silica/carbon composite,the silica component of the nanocomposite is reduced through the magnesiothermic reduction method to yield the nanofibrous silicon/carbon composite.The resulted nanocomposite process a porous network structure that was inherited from the initial cellulose substance,and it is composed of thin silicon layer uniformly coated on the carbon nanofiber surface.For such a silicon/carbon nanocomposite with 25.7 wt%of silicon content and silicon layer with the thickness of about 40 nm,it exhibits a good cycling stability with a specific capacity of 750.6 mAh g-1 after 150 cycles at 100 mA g-1.Further coating an amorphous carbon layer or depositing silver nanoparticles onto the silicon/carbon nanofibers result in much more improved elelctrochemical performances,and specific capacities of 775.3 and 1018.7 mAh g-1 are delivered by the corresponding composites obtained after 150 cylces at 100 mA g-1.3.A nanofibrous titania/silicon composite is synthesized by employing natural cellulose substance(filter paper)as template.It is achieved by coating of the cellulose nanofibers with silica gel layer and titania gel film,respectively,followed by calcination in air to give the nanofibrous titania/silica composite.This resulted titania/silica composite is reduced by magnesiothermic reduction method to produce the nanofibrous titania/silicon composite.In the nanocomposite,the anatase titania nanoparticles anchor as a thin coating layer uniformly coated on the silicon nanofiber surface.When used as anode materials for lithium-ion batteries,the composite with 54.3 wt%titania content exhibits a specific capacity of 498.9 mAh g-1 after 200 discharge/charge cycles at 200 mA g-1.The improved electrochemical performances of the composite are attributed to its unique porous network structure,as well as the buffering effect of the titania coating layer on the huge volume change of the silicon active material during the repeated discharge/charge processes.4.A nanofibrous TiOx/silicon/carbon composite is fabricated using cellulose substance as both the carbon source and the structural scaffold by combination of self-assembly technique and magnesiothermic reduction method.It holds the network structure of the initial cellulose substance,and is composed of thin silicon layer sandwiched between the carbon nanofiber and the outer titania coating layer consisting of anatase titania nanoparticles with oxygen defects.The thickness of the silicon layer is about 50 nm,and the diameter of titania nanoparticle is ca.5-10 nm with an ultrathin amorphous carbon layer coated on its surface.Owing to the synergetic effect of the unique network structured scaffold,thin silicon layer and outer TiOx coating layer,this nanocomposite shows significantly enhanced specific capability and cycling stability.For the composite with 24.1 wt%of titania,It delivers a stable reversible capacity of ca.792.6 mAhg-1 at a current density of 100 mA g-1 after 160 discharge/charge cycles,which is much higher than those of the counter materials such as the nanofibrous silicon matter and the nanofibrous silicon-carbon counter material.5.A nanofibrous tin-oxide/silicon composite with tin oxide nanoparticles anchored on the silicon nanofiber surface is fabricated employing a natural cellulose substance(filter paper)as structural template.Tin oxide gel is deposited uniformly onto the surface of the silicon nanofibers that prepared by magnesiothermic reduction of the silica replica of the filter paper,and thereafter,the as-prepared tin-oxide-gel/silicon nanocomposite is calcined in argon atmosphere to yield the tin-oxide/silicon nanocomposite.The resulted tin-oxide/silicon nanocomposite possesses a network structure that inherited from the initial cellulose substance.When the nanocomposite is evaluated as an anode material in lithium-ion batteries,it exhibits superior lithium storage properties compared with the pure silicon nanofiber and tin oxide powder matters.For such a nanocomposite with 58.8 wt%tin oxide content delivers a specific reversible capacity of 547.8 mAh g-1 after 150 discharge/charge cycles at 100 mA g-1.The improved lithium storage performances are attributed to the unique porous network structure of the nanocomposite,high dispersed fine tin oxide nanoparticles on the silicon nanofiber surface,as well as the synergistic effects between the silicon nanofibers and the tin oxide nanoparticles on the lithium storage.Silicon-based materials,especially those with specific nanoscale structures,have been the focus of the research of lithium-ion batteries.In the present thesis,a series of nanofibrous silicon-based composite materials derived from the natural cellulose substance(filter paper)are designed and prepared,and their electrochemical properties are studied.Owing to the unique porous network structure and the high specific surface area of them provide enough local empty space to accommodate the serious volume change of the anodes upon cycling,and facilitate the electrode-electrolyte contact.Hence,enhanced structural stability of the anodes and therefore improved cycling and rate performances are achieved by these silicon based composite when they were employed as anode materials for lithium-ion batteries.This thesis demonstrates that bio-mimetic chemical synthesis would hold considerable potentials for the design and development of energy-related materials.