Investigation On Fabrication And Properties Of Composite Carbon Anode Materials For Lithium Ion Batteries

Advanced rechargeable lithium ion batteries are attractive for use in consumer electronic and electric vehicle(EV)application because of a favorable combination of voltage,energy density,cycling performance, self-discharge and environmental protection.The development of lithium ion batteries and carbon anode materials are reviewed in details,much attention is attached to the further improvement on performances of lithium ion batteries and reduction in the cost.Carbon anode materials are the key in the lithium ion batteries technologies.The short development, lots of species,different properties and indefinite reaction mechanism leave great promotion probability in performance.By creatively adding sodium salts and adopting dynamic melt-carbonization,the graphite and SnO_2^-graphite composites are covered amorphous carbon to gain a better performance.The main contents in this research as follows:The structures and characteristics of several graphite samples are measured by means of power X-ray diffraction(XRD),scanning electron microscope(SEM),Brunauer-Emmer-Teller(BET)surface area measurement,inductively coupled plasma(ICP)spectroscopy,particle size analysis and electrochemical measurements.The effects of origin, structures,impurity,particle size,specific surface area of carbon materials on the electrochemical characteristics are studied.The common natural graphite and the spherical natural graphite with abundant resources,low cost and favorable performance are determined as the raw materials for modification of graphite.The reversible capacity of natural graphite reaches 364.8mAh.g^-1by sodium salts modification.Three kinds of sodium salts are doped into graphite and exist in the form of NaCl,NaF,Na₂CO₃ respectively.The structure of the graphite is not changed and the electrochemical characteristics of graphite anode doped with sodium salts are enhanced. The performances of natural graphite are improved by the modification of sodium salts.It is believed that the improvement is attributed to the merged effects of the sodium/negative ions on the SEI(Solid Electrolyte Inter-face).The polarization of lithium diffusion at the interface is reduced and an order of magnitude of diffusion coefficient is gained.The sample modified with 1%NaCl had the best electrochemical performances with a reversible capacity of 364.8mAh·g^-1,an irreversible capacity of 47.4mAh·g^-1,and an initial coulombic efficiency of 88.5%.The cycling stability of the Li/C cell with modified graphite as anode was improved.The capacity retention ratio at the 30th cycle was up to 91.97%.Compared with the button lithium ion batteries using untreated graphite as anode material,those using modified graphite as anode materials have larger capacity,higher discharge voltage and better cycling capability.For the first time,the dynamic melt-carbonization is adopted to cover amorphous carbon on graphite.Disordered carbon materials are obtained by pyrolysis of pitch at high temperature.As the temperature of pyrolysis and the soak time increasing,the carbon materials become more stacked,the specific surface area reduces,both reversible capacity and irreversible capacity decrease,the initial coulombic efficiency increases, and the hysteresis in the voltage profile between charge and discharge is cut down.The graphite is covered with a thin film of disordered carbon according to the measurements of XRD,SEM and particle size analysis, pitch is used as the precursor of the shell-carbon materials by dynamic melt-carbonization,also the composite carbon materials are heat-treated at high temperature.Structure and performance of the composite structure carbon are studied in detail.The samples of solid-mixed and liquid-mixed for 5%pitch by dynamic melt-carbonization at 400℃for 3 h and heat-treated at 850℃for 2h have the best electrochemical performances with a reversible capacity of 358.2mAh·g^-1,362.0mAh·g^-1,and an initial coulombic efficiency of 89.3%,92.0%.The capacity retention ratios at the 30th cycle are up to 95.20%and 96.55%.Electrochemical measurements of button lithium ion batteries show that composite carbon coated with appropriate amount of pitch have improved charge/discharge and cycling performance.A mesocarbon-graphite composite is prepared by depositing mesocarbon from the coal tar pitch on the surface of global natural graphite though polymerization,separation,carbonization.The effects of the weight of spherical natural graphite,the temperature and time of heat-treatment on the structure,specific surface area and electrochemistry properties of composite carbon are studied in detail.The sample heat-treated at 700℃for 2h has the best electrochemical performances with a reversible capacity of 378mAh·g^-1and an initial coulombic efficiency of 91.3%.The cycling stability of the Li/C cells with carbon composite as anodes is improved,its capacity retention ratio at the 50th cycle is 96%.Electrochemical measurements of button lithium ion batteries show that composite carbon coated with appropriate amount of mesocarbon have improved charge/discharge and cycling performance more than spherical natural graphite.SnO₂-graphite composites are prepared by homogeneous precipitation method and for the first time it is modified by dynamic melt-carbonization of the pitch.The structure,morphology and electrochemical properties are studied.The results suggest that SnO₂ is uniformly distributed on the surface of graphite,and the SnO₂-graphite is covered with a thin film of disordered carbon.The composites of C-SnO₂-graphite(16%SnO₂)heat-treated at 650℃for 2h and modified by pitch(10%)have a reversible capacity of 465.4mAh·g^-1,an irreversible capacity of 103.2mAh·g^-1,and an initial coulombic efficiency of 81.85%.A mechanism of Li storage in C-SnO₂-graphite composite was proposed.Since the carbon and graphite acts as the framwork of the composite,restraining the volume change of SnO₂ and the reunition of the Sn particles in charge/discharge,the cycling performance is improved obviously.The kinetics behaviors of carbon anodes are studied by means of linear sweep voltammetry and chronoamperometry measurements.It is found that the exchange current(i₀)and diffusion coefficient of lithium (D_Li)increase with Li intercalation into carbon anodes,SnO₂ has larger i₀ (53.22mA/g)and smaller D_Li(9.48×10^-9cm²/s)than other materials.The natural graphite has smaller i₀(20.70 mA/g)and D_Li(2.38×10^-8cm²Â·s^-1). The i₀ and D_Lifor modified graphite and composite carbon materials improve an order of magnitude compared with natural graphite with the best value reaching 1.45×10^-7cm²/s for pitch covered carbon composites. The discharge capacities of carbon electrodes decrease with the increase of current rate,the composite carbon of modified by pitch has the best fast discharge capability among the untreated graphite,modified graphite by sodium salts,other composite carbon materials,its discharge capacity at 1C is up to 99.14%of that at 0.1C.