Synthesis And Electrochemical Performances Of Si-based Nanocomposites Prepared By Arc-discharge Plasma

Lithium ion batteries(LIBs)are widely used in portable electronics,electric vehicles,hybrid electric vehicles and secondary energy storage system due to its high energy/power densities and long cycle life.Graphite as the traditional anode materials,it is difficult to meet the increased requirements of LIBs with high energy density,high power density and high security performance in practical applications,because of its low theoretical specific capacity(372 mAh g-1),low Li+ions diffusion rate(10-9-10-10 cm2 s-1)and the similar lithiation potential(0.05 V vs.Li/Li+)with the deposition potential of metal lithium.Silicon(Si)as the most potential anode material with the high theoretical specific capacity(Li15Si4,3579 mAh g-1)and moderate potential(0.4 V vs.Li/Li+)has attracted the extensive attention and research interests.But the large volume changes up to 300%during lithiation/delithiation processes gives rise to a pulverization of the active phase,loss of electrical contact with surrounding conductive matrixes and even peeling off from the current collector,resulting in the rapid degradation of capacity.Additionally,the exposed new surfaces caused by the volume variation will constantly consume the electrolyte to form thicker SEI films and increase the internal resistance of batteries.Silicon is a kind of semiconductor materials with inferior electrical conductivity,the above shortcomings seriously hindered its practical application as anodes for LIBs.According to the aforementioned problems,designing a variety of Si structures including nanostructures or combining Si with the second phase composition can effectively improve the electrochemical performance of Si-based anode materials.In this thesis,Si or Si-based nanocomposites with particular morphologies and phase compositions were synthesized by controlling the compositions of raw materials in targets,synthetic atmosphere and gas content conditions using the direct current(DC)arc-discharge plasma as the evaporation heat source.Then reconstructed the structures and made physical/chemical modifications on Si composites via the spray drying and high temperature treatment techniques,further optimized their electrochemical performances,thus established the corresponding relationship between the condition parameters in preparation processes and the compositions,structures,morphologies as well as electrochemical performances of as-prepared composites,improved the energy density,power density and cycling stability of Si-based anode materials to a certain extent.The main research contents and conclusions of this thesis are summarized as follows:(1)Carbon-coated Si NSs(Si NSs@C)nanocomposites are synthesized via the high temperature pyrolysis technique,using two-dimensional Si nanosheets(NSs)prepared through the DC arc-discharge plasma method as the active units,phenolic resin as the carbon source.Si NSs with the average size of 31 nm and thickness of 2.4 nm are embedded into the phenolic resin-derived amorphous carbon to form a carbon coated structure.The content of phenolic resin precursor can optimize the carbon content and its coating thickness in nano structural products,tremendously influence the electrochemical performance of anode materials in LIBs.In Si NSs@C nanocomposites,Si NSs play a leading role to react with Li+ ions to contribute the capacity,while the amorphous carbon coating layer provides an electrical contact/connect among Si NSs active units,facilitates the diffusion and transport of Li+ions and electrons.It can also avoid the direct contact between Si NSs and the electrolyte,ensure the formation of stable SEI films in cycles.Testing results indicate that the Si NSs@C nanocomposite electrode with 66.4 wt.%carbon content possess the stable discharge specific capacity and good rate capability.The capacity can keep at 822 mAh g-1 at a current density of 100 mA g-1 after 200 cycles.Controlling the content of carbon coating layer plays a critical role for enhancement of the electrochemical performances.(2)Porous carbon(PC)substrate is prepared by high-temperature calcination of the mixture of glucose and calcium carbonate,followed by acid etching the products,then two-dimensional Si NSs are anchored onto the PC matrix via physical adhesion mechanism assisted with the ultrasonic blending.The electrochemical performances of Si NSs/PC composite electrode are optimized through changing the loaded content of Si NSs.This electrode material can facilitate the infiltration of electrolyte inside the active materials and improve the initial coulombic efficiency of electrode.It is found that when load the content of 30 wt.%Si NSs on the PC substrate,it exhibits the best electrochemical performances,typically a stable discharge specific capacity of 1252 mAh g’1 with the coulombic efficiency of 99.58%at the current density of 100 mA g-1 after 100 cycles,while retains the capacity of 850 mAh g-1 even at a high current density of 1 A g-1 after 800 cycles.In comparison with the entire Si NSs electrode,the best Si/PC composite anode shows two times higher in the capacity retention ability and the rate performance,due to the homogeneous dispersion of Si NSs on the PC substrate,good electrical conductivity,large specific surface and pore volume of the PC matrix can facilitate the infiltration of electrolyte and rapid transport/diffusion of electrons/Li+ions inside the electrodes.(3)Si@Cu3Si nanowires,Si@Cu3Si nanorods and Si@Cu3Si(Cu)nano capsules are synthesized.Polymorphic Si-Cu anisotropic nanostructures are formed through the instantaneous vapor-liquid-solid(VLS)growth mechanism within the high temperature plasma.Optical emission spectroscopy is adopted to reveal the energetic states of excited atoms and ions,thus the electrical temperature of plasma and the evaporation rate of elements can be evaluated.Monitoring the composition and energy state in the preparation process of Si-Cu nano powders is helpful to control and synthesize nanostructures.Si@Cu3Si nanorods with optimal composition and microstructure as anode for LIBs show excellent electrochemical behaviors,it can display a stable discharge capacity of 783 mAh g-1 with the coulombic efficiency of 98.51%at the current density of 100 mA g-1 after 100 cycles.Good performances are attributed to the proper mass ratio of Si and Cu,one-dimensional Si-Cu nanostructures,as well as favorable channels and pathways for Li+ions’ diffusion.Metallic Cu component released from Cu3Si precursor during cycling can enhance the conductivity of Si matrix,buffer the volume change of active Si units and facilitate the cycling stability of electrode.(4)Si/SiC/C nanocomposites with spherical Si nanoparticles and one-dimensional SiC@C nanorods as the main phase are separately synthesized by the DC arc-discharge plasma method via controlling the content of carbonaceous source(CH4).The introduce of SiC ceramic phase improves the rate capability and structural stability of Si-based electrode.Testing results indicate that the composite electrode with SiC@C nanorods as the main phase possesses excellent electrochemical performances.It can deliver a superior discharge specific capacity of 1065 mAh g-1 with a coulombic efficiency of 98.49%at a current density of 100 mA g-1 after 200 cycles,even reserve the capacity of 776 mAh g-1 at a higher current density of 2 A g-1.Good cycling stability and rate capability are attributed to the excellent conductivity of graphite-like layers,strong bonds in the SiC crystals and the interconnected network microstructures inside the composites.The lithiation mechanism of SiC crystal unit is explored by the first-principles calculations.It confirms the possible insertion of 1～2 Li+ ions into one SiC unit cell at the potential of～0.76 V.The experimental results further verify the theoretic prediction,prove the potential feasibility of the SiC nanocrystals as anode materials for LIBs.(5)Isochronous nitrification mechanism is applied into the DC arc-discharge plasma,core-shell structural Si@Si3N4 nanoparticles(NPs)are in-situ synthesized via this method.Then Si@Si3N4 NPs are uniformly dispersed in the graphene matrix to prepare(Si@Si3N4)/Graphene nanocomposites(NCs)through the spray drying and heat treatment techniques.The existence of Si3N4 shell is beneficial to form Li+ion conductive Li3N phase in the SEI film,facilitating the transport of Li+ions and buffering the fracture and pulverization of Si core caused by the volume variation during cycling.The introduction of graphene matrix can improve the electrical conductivity of the(Si@Si3N4)/Graphene NCs electrode,thus it behaves superior electrochemical activity,cycling performance and rate capability.Testing results indicate that the electrode can display a stable discharge specific capacity of 1444 mAh g-1 with a coulombic efficiency of 99.77%at a current density of 100 mA g-1 after 100 cycles,even reserve the capacity of 827 mAh g-1 at a higher current density of 2 A g-1 after 450 cycles.Furthermore,the diffusion coefficient of Li+ions in(Si@Si3N4)/Graphene NCs electrode is 4.19×10-16 cm2 s-1,two orders higher than that of Si@Si3N4 NPs electrode(3.48×10-18 cm2 s-1),also three orders higher than that of pure Si nanowires electrode(1.13×10-19 cm2 s-1).The(Si@Si3N4)/Graphene NCs behave great potential as the high-performance anode material for LIBs.