Si-based Nanocomposites With Multi-Buffering Mechanism As Anode Materials For Li-ion Batteries

Developing rechargeable batteries with high energy density,long cycle life and low costs is a key technology for constructing a new-energy society for human beings.In this technology pursuit,silicon(Si)has attracted considerable attention as a promising anode for next-generation lithium ion batteries(LIBs)due to its ultra-high theoretical specific capacity,which is about ten times higher than that of the currently used graphite anode.However,the huge volume changes of Si anodes during lithiation/de-lithiation can easily cause the pulverization of active particles,the collapse of electrode structure and repeated growth of surface solid electrolyte interface film(SEI),leading to poor cyclability and low Coulombic efficiency at cycling.These problems seriously hinder the battery application of Si anodes and the development of high-energy-density LIBs.In this thesis,we prepared two cycling stable Si-based anode materials,FeSi/C nanofiber and double core-shelled Si@SiOx@C composite both with multi-buffering mechanisms.The main contents and results are as follows:1.Preparation and characterization of FeSi/C nanofiber composite.FeSi/C nanofiber composite was prepared by electrospinning of the mixed slurry of FeSi alloy nanoparticles and polyacrylonitrile(PAN)and subsequent high-temperature treatment of FeSi/PAN nanofiber to make PAN carbonized,and its preparation conditions were also optimized.The experiment results showed that the FeSi/C/C nanofiber composite,which is prepared by chemical vapor deposition(CVD)of carbon layer on the pyrolysis product of FeSi/PAN fibers with a weight ratio of FeSi:PAN ? 1:0.5,demonstrates optimum electrochemical performances.The results from the electrochemical measurements showed that the as-prepared FeSi/C/C nanofiber composite electrode exhibits a high initial specific capacity of 953 mAh g-1,a good cycling stability with a capacity retention of 60%after 200 cycles,and a high Coulombic efficiency of?99.5%at cycling.Even at a high current density of 1000 mA g-1,the composite electrode can still deliver a capacity of ? 700 mAh g-1.The good cycling performance of FeSi/C/C composite benefits from its multi-buffering mechanisms,including inert FeSi2 phase in FeSi alloy,the carbon fiber matrix and the CVD-deposited carbon layer,which effectively depress the volume expansion of active Si core,and maintain the structure stability of the surface SEI film.2.Preparation and characterization of Si@SiOx@C composite.Depending on the hydrogen chemical bond between carbanyl group of poly(acrylic acid)(PAA)and hydroxyl group on Si surface,a dual core-shell structured Si@SiOx@C nanocomposite was prepared by direct pyrolysis of PAA on the surface of Si nanoparticles.Since PAA polymer can thermally decomposed in the pyrolysis process with their alkyl group converted to carbon and the residue oxygen recombining with Si to form SiOx,the dual core-shell structure can be conveniently formed in a one-step procedure.Benefiting from the strong buffering effect of the SiOx interlayer and the efficient blocking action of dense outer carbon layer in preventing electrolyte permeation,the obtained nanocomposite demonstrates a high capacity of 1110 mAh g-1,a stable cycling performance with a remained capacity of>664 mAh g-1 over 150 cycles,exhibiting a great promise for practical uses.