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Study On Carbon And Iron Oxide As Negative Electrode Of Lithium Ion Batteries

Posted on:2017-04-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:S P ZhuFull Text:PDF
GTID:1101330488994554Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
The theoretical capacity of commercialized anode-graphite of Li-ion batteries is just only 372 mA h g-1, and the practical capacity is nearing theoretical capacity. Carbon materials have been widely studied and applied in lithium ion batteries. The energy density of pure carbon materials can be further improved by modification. Preparation porous, hollow and nitrogen-doped carbon nanofiber by electrospinning is one of the research focus. In this paper, graphitized carbon nanofibers perepared by electrospinning and calcining were modified and graphitized porous carbon nanofibers were obtained. The discharge capacity has a great enhancement. In addition, iron oxides in transition metal oxides (MO) (transitional metal oxides, M include:Co, Ni, Cu and Fe etc) have the characteristics of abound raw materials, high theoretical capacity, low cost and environmentally friendly etc. But pure iron oxides as anode materials for lithium-ion batteries often have poor cycling performance owing to the great volume changes during charge-discharge cycles. The process will generate great volume stress and lead to polymerization of material and collapse of lattice structures. In this thesis, iron oxides were combined with carbonaceous materials to form advanced nanocomposites, the lithium storage capacities were further improved. On the one hand, carbon materials can make up for the poor conductivity of iron oxides and promote the electron transport. On the other hand, carbon materials as elastic buffer layers/supports can confine the position of iron oxides, prevent the agglomeration and cracking of crystal structures and maintain the integrity of the crystal structures which can enhance the cycle stability of electrode. What’s more, nanostructured electrode materials have some special characteristics, such as large proportion of surface atoms and small size etc which lead to higher electrode/electrolyte contact area, shorter path lengths for Li+ transport and higher charge/discharge rates.The main research works in this dissertation are divided into the following five parts:1. Graphitized porous carbon nanofibers prepared by electrospinning as anode materials for lithium ion batteriesGraphitized porous carbon nanofibers (GPCNFs) were prepared by electrospinning and subsequent continuous unique three calcining process:pre-oxidation at 250℃ in air for 2 h, graphitization at 1000℃ in Ar for 2 h and pore-forming at 400℃ in air for different calcining time. The physical properties of carbon nanofibers (CNFs) were characterized by X-radiation diffraction, thermogravimetric analysis, scan electric microscope, transmission electron microscopy, high-resolution transmission electron microscopy, Raman spectrum, four-probe measurements, Brunauer-Emmett-Teller method and Fourier transform infrared spectra etc. The electrochemical properties were observed by cyclic voltammetry and charge-discharge test of coin-type cells versus metallic lithium. The results showed GPCNFs exhibited good capacity retention and higher capacities than graphitized carbon nanofibers. The capacity of GPCNFs were increased with the extension of calcining time at 400 "C from 3 to 18 h. While calcining 18 h the capacity of GPCNFs achieved 473.5 mA h g-1 after 100 cycles at 0.5 Ag-12. A novel high conductive ferroferric oxide/porous carbon nanofibers composites prepared by electrospinning as anode materials for high performance Li-ion batteriesFerroferric oxide (Fe3O4) nanoparticles/porous carbon nanofibers (Fe3O4/PCNFs) composites were successfully fabricated by elctrospinning and subsequent calcinations. The composites were characterized by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy etc to analyze the structure, composition and morphology. The electrochemical performance was evaluated by coin-type cells versus metallic lithium. The results indicated that Fe3O4/PCNFs composites exhibited high reversible capacity and good capacity retention. The discharge capacity maintained 717.2 mA h g-1 at 0.5 A g-1 after 100 cycles. The excellent performances of Fe3O4/PCNFs composites are attributed to good crystallinity and uniformly dispersive Fe3O4 nanoparticles, and porous carbon shell with high conductivity. The carbon coating buffered the tremendous volumetric changes between Fe3O4 nanoparticles and Fe atoms in the charge/discharge processes and kept the structure integrity of Fe3O4 nanoparticles. Porous carbon nanofibers prepared by unique calcination process improved the conductivity of composites and provided free space for migration of lithium ions. The method is expected to be applied to prepare PCNFs coating other transition metal oxides as superior anode materials for lithium-ion batteries.3. Microwave assisted synthesis Fe2O3/reduced graphene oxide nanocomposites as anode materials for high performance lithium ion batteriesIron trioxide (Fe2O3)/reduced graphene oxide (rGO) (Fe2O3/rGO) nanocomposites were synthesized by a rapid and simple microwave method. Fe(OH)3 sol was used as the precursor of Fe2O3. Under the microwave heating, graphene oxide (GO) was reduced to rGO using hydrazine hydrate as a reductant and Fe(OH)3 sol transformed to a-Fe2O3 particles attached uniformly onto rGO surfaces at the same time. The structure, morphology and composition of Fe2O3/rGO nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, thermogravimetric analysis and Raman spectrum etc. The electrochemical characteristics of assembled coin-type cells versus metallic lithium were valuated by cyclic voltammetry and galvanostatic charge-discharge. The prepared Fe2O3/rGO nanocomposites exhibited high reversible specific capacity of 650 mA h g-1 after 50 cycles at a current density of 1.0 A g-1, showing more superior rate capability than both of a-Fe2O3 nanoparticles and rGO sheets themselves. At the larger current density of 10.0 A g-1, the capacity of Fe2O3/rGO nanocomposites still remained 400 mA h g-1. The significant improvements in the electrochemical properties of Fe2O3/rGO nanocomposite could be attributed to the uniform small Fe2O3 nanoparticles (30-50 nm) on the rGO substrate which provided high electrical conductivity, confined the position and buffered the volume changes of Fe2O3 nanoparticles.4. Microwave and hydrothermal preparation of FexOy/reduced graphene oxide nanocomposites as anode materials for lithium ion batteriesFerric chloride (FeCl3) was used as precursor of ferroferric oxide (Fe3O4). FeCl3 was attached to the surface of GO at the role of electrostatic force and changed into ferric hydroxide (Fe(OH)3) by the reaction with sodium acetate (NaAc). The Fe3O4/reduced graphene oxide (rGO) (Fe3O4/rGO) nanocomposites of ultra small Fe3O4 anchored onto the surfaces of rGO were prepared using ethylene glycol and hydrazine as mixed reductants by a simple and fast microwave method. Mean while, Fe3O4/rGO nanocomposites and Fe2O3/rGO nanocomposites were also synthesized by hydrothermal method at 120℃ for 12 h using ethylene glycol and hydrazine as mixed reductants and hydrazine as reductant, respectively. The structure, morphology and composition of synthesized products were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and thermogravimetric analysis etc. The coin cells were assembled versus metallic lithium and the electrochemical properties of the materials were evaluated by galvanostatic charge-discharge cycles and cyclic voltammetry. nanocomposites prepared using ethylene glycol and water with a mass ratio of 3 as solvent showed good electrochemical properties. The discharge capacity reached up to 561.3 mA h g-1 after 100 cycles at a current density of 0.5 A g-1. The Fe2O3/rGO nanocomposites preparing by hydrothermal method obtained the highest discharge capacity, which reached up to 609.6 mA h g-1 after 100 cycles at a current density of 0.5 A g-1.5 Hydrothermal synthesis of Fe3O4/ reduced graphene oxide as anode materials for lithium ion batteriesFe3O4/rGO nanocomposites were prepared by hydrothermal method at 180℃ for 12 h using 1 mmol Fe(OH)3 as precursor of iron oxides, GO as precursor of rGO,1.25 mL hydrazine and 1 mmol trisodium citrate as mixed reducing agent. Fe2O3/rGO nanocomposites were obtained only using 1.25 mL hydrazine as reducing agent. The morphologies, structures and compositions of the products were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction and thermogravimetric analysis etc. The electrochemical characteristics of assembled coin-type cells versus metallic lithium were valuated by cyclic voltammetry and galvanostatic charge-discharge. The results showed that Fe3O4/rGO nanocomposites kept higher discharge capacity than Fe2O3/rGO nanocomposites. The prepared Fe3O4/rGO nanocomposites and Fe2O3/rGO nanocomposites exhibited high reversible specific capacity of 728.5 mA h g-1 and 614.8 mA h g-1 after 100 cycles at a current density of 0.5 A g-1, respectively. Both of the Fe3O4/rGO nanocomposites and Fe2O3/rGO nanocomposites have uniform morphology. The higher reductive level of rGO and smaller Fe3O4 nanoparticles of Fe3O4/rGO nanocompostes can be responsible for this phenomenon.
Keywords/Search Tags:Anodes of Li-ion batteries, Electrospinning, Carbon nanofibers, Graphene, Iron oxides
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