The Study On The Synthesis Of Graphene Or Carbon Fibers Based Composites And Their Properties For Lithium Ion Storage

Since the2000, many countries paid much attention to the energy safety. Fossil fuels may be exhausted in the nearly future. Wind and solar energy as renewable energy systems are unstable based on current technology. So it is urgent to develop stable and high efficiency energy devices. Besides, the demand of increasing portable devices on the portable energy devices became higher. Lithium ion batteries with high theoretical properties may be one of the possible strategies to solve above issues. Now, the common anodes for lithium ion batteries are carbon, whose theoretical capacity only is372mA h/g. Those lithium ion batteries cannot meet the demands of the humans for lithium ion batteries with high capacities and high power density. Transition metal oxides (including FexOy, CoOx) and tin-based compounds are anodes with high theoretical capacity, and may be the candidates for carbon. However, their stabilities are relative poor and their power densities are relative low. According to the literature, preparing the composites based on above materials and carbon materials (such as graphene and carbon fibers) are promising route to improve the properties of anodes. Whereas, the traditional methods for the synthesis of graphene-FexOy and graphene-tin-based compounds are tedious, environmental pollution and low efficiency. Besides, the synthesis mechanism, power density and mechanical stability of carbon fibers-CoO composites need further investigation.The present thesis will investigate the lithium ion batteries, which include the science of material physics and chemistry, inorganic chemistry and electrochemistry. The graphene-based and carbon fibers based transition metal oxides and tin compounds will be investigated in detail from the angles of synthesis mechanism, microstructure modification and their electrochemical properties. This study will focus on the following aspects to achieve the goals.(1) Taking into account of microwave absorbability of graphene oxide, graphene sheets were prepared in30min by microwave irradiation using ascorbic acid as reductants. Thermo gravimetric analysis results discovered that the temperature corresponding to the thermal stability of the graphene sheets prepared by microwave irradiation was50℃higher than that of graphene sheets prepared by other heat methods. The enhancement could be attributed to the lower densities of defect sites and higher graphitization degree of graphene sheets synthesized by microwave irradiation. (2) Graphene/Fe3O4composite anodes were synthesized by employing microwave irradiation as heat sources, using nontoxic ascorbic acid as reductants to in situ deposit Fe3+on the surface of graphene oxide, and following heat treatment to partial reduce Fe3+to Fe2+. The Fe3O4nanoparticles were separated by graphene sheets. Graphene/Fe3O4composites displayed enhanced cyclic stability (showing a reversible capacity of690mA h/g after50cycles) and excellent rete capacity (350mA h/g at a rate of5C).(3) Taking into account of the relative high reducibility of Sn2+in basic condition, graphene/SnO2composites with high properties were prepared by microwave irradiation and the theory of "Sn2+reducing graphene oxide to graphene" was proposed. As anodes for lithium ion batteries, graphene/SnO2composites showed good cyclic properties (a reversible capacity of550mA h/g after100cycles) and excellent rate capacities (460mA h/g at a rate of5C).(4) Based on the theory of "Sn+reducing graphene oxide to graphene" and the reducibility of Fe2+in basic condition, graphene/Fe2O3composites with excellent properties were synthesis by microwave irradiation. This method was of high efficiency to prepare the composites at large scale. A theory of "the metal ions with variable valence reducing graphene oxide to graphene" was proposed based the experimental results. As anodes for lithium ion batteries, graphene/Fe2O3composites showed enhanced properties compared with mechanical mixture of graphene-Fe2O3, bare Fe2O3, whose reversible capacity was about800mA h/g after100cycles.(5) Based on the theory of "the metal ions with variable valence reducing graphene oxide to graphene", graphene/tin sulfide composites with excellent properties were prepared using graphene oxide to oxidize Sn2+to Sn4+. With the help of graphene oxide, the tin sulfide nanoparticles in graphene/tin sulfide with diameters of5nm were dispersed homogenously on graphene sheets. Since the special structure, the Li2S arising from the decomposition of tin sulfide may be reversibly decomposed at a relative low potential. Based on this mechanism, graphene/tin sulfide composites could deliver a discharge capacity of860mA h/g at a rate of0.2C, higher than that of bare SnS. Their rate capacities also were higher than those of bare SnS.(6) Because the valence of cobalt in carbon fiber/cobalt composites was controversial and the demands for anodes with high power density were increasing, this study investigated the synthesis of carbon/CoO nanofiber networks by electrospinning and following heat treatment. As binder-free anodes for lithium ion batteries, the carbon/CoO nanofiber networks synthesized at650℃could deliver a discharge capacity of633mA h/g after52cycles at a current density of0.1A/g. This value was higher than those networks obtained at550and600℃, and pure carbon fiber networks synthesized at600and650℃. In addition, the rate capacities of the carbon/CoO nanofiber networks (650℃) also were higher than those of carbon/CoO networks (600℃) and pure carbon fiber networks (650℃). The excellent electrochemical properties of carbon/CoO nanofiber networks could be ascribed to the enhanced conductivity, fast diffusion rate, and the stability of CoO by carbon fibers.(7) Since the brittleness of carbon/CoO as binder-free anodes and the excellent mechanical and electronic properties of graphene, the binder-free flexible mats of carbon-graphene-CoO were prepared by electrospinning and following heat treatment. The structure characterization results showed that the graphene oxide could control the particle sizes of CoO. As binder-free anodes for lithium ion batteries, the carbon-graphene-CoO binder-free flexible mats displayed enhanced cyclic stability (690mA h/g after352cycles at a current density of0.5A/g) and excellent rate capacity (400mA h/g at a current density of2A/g) compared with CoO-carbon, graphene-carbon, and pure carbon fiber mats. The improvement could be attributed to the flexible mats to make sure the fast diffusion of Li+and to improve the mechanical strength and conductance. Besides, the defect sites will be improved by the introduction of CoO.