| In this word, with the decreasing of fossil resources and increasing of environmental pollution, developing sustainable clean energy and advanced energy storage technology are the great challenges. At present, Lithium sulfur batteries have become the focus in the high energy density electrochemical energy storage system, while supercapacitors have become the focus in the high power density electrochemical energy storage system. Carbon materials are used for Lithium batteries and supercapacitors. By far, carbon materials is the most ideal material for energy storage as the electrode material for supercapacitors or a conductive base material of elemental sulfur in lithium-sulfur batteries, due to its high conductivity, excellent mechanical properties, adjustable pore structure, large specific surface area and low price. Porous carbon materials are used in supercapacitors and lithium-sulfur batteries, including mesoporous carbon, carbon nanotubes, carbon fiber, graphene, et al. In this paper, from the practical point of view, using a simple and easy preparation method, we synthesized a kind of disordered mesoporous carbon material, a kind of boron- doped disordered mesoporous carbon material and a kind of disordered carbon having a microporous/mesoporous structure.We used these carbon materials as the positive electrode of loading the element sulfur in lithium sulfur battery to prepare the sulfur/carbon cathode composite and completed electrochemical performance testing and characterization. In addition, using the commodity graphene as raw material, through further reduction at different high temperature, the nitrogen-doped graphene materials of thermal stability were obtained. We used these nitrogen-doped graphene as the electrode material of supercapacitors, then investigated by electrochemical performance test. The influence of the carbon materials’ functional groups on electrochemical properties are discussed in the two aspects work. The main research work includes the follows:1.Using phenolic resin prepolymer as the carbon source, tetraethyl orthosilicate as the pore forming agent, a kind of disordered mesoporous carbon DMC with a stable double pore structure was prepared, the preparation was easy and repeatable. This disordered mesoporous carbon material was used as the host materials for lithium-sulfur battery by melting method. The different carbon/sulfur composites with different sulfur content were prepared. The structure and electrochemical properties of carbon/sulfur composites were investigated and the influence of of loading amount of sulfur in disordered mesoporous carbon on the electrochemical properties was probed. The results showed that, with the disordered mesoporous carbon DMC as the carrier of sulfur, using simple hot-melt method, when the loading amount of sulfur was less(<66.7wt%), the element sulfur could enter into the smaller nanoporous position of mesoporous carbon and highly dispersed. When the loading amount of sulfur was large(75wt%), a small amount of sulfur was attached on the surface of the carbon materials. Trace O atoms in the carbon/sulfur composites internal structure had some chemical adsorption with the element sulfur. With the increase of sulfur content of carbon/sulfur composites, the discharge specific capacity and active material utilization rate declined. To the carbon/sulfur composites with 66.7 wt%, the capacity retention rate was higher at 1C rate, the coulomb efficiency approached about 100%. This suggested that, the carbon structure in the carbon/sulfur composites was stable, it′s mechanical strength was high, it had the good strain capacity with the change of current density. It would have very important significance with the material used for the actual lithium sulfur batteries.2.Using boric acid as the boron source, tetraethyl orthosilicate as the pore forming agent, a kind of boron-doped disordered mesoporous carbon material BDMC was prepared, this disordered mesoporous carbon material was used as the cathode materials for lithium-sulfur battery by melting method. The boron-doped mesoporous carbon/sulfur composite material structure and electrochemical properties were investigated, the advantages of boron-doped mesoporous carbon/sulfur composites of electrochemical properties was compared. Through the experiment of XPS, RS tests, a small amount of B atomics had confirmed to present in the BDMC materials; BDMC and DMC materials were not so much difference in pore structure, specific surface area, pore volume. XPS tests showed that B atoms of BDMC produced slight positive polarization phenomenon, so the carbon and sulfur on the interface of BDMC materials had a greater adsorption which could produced some chemical adsorption of polysulfide anions in the charge/discharge process and inhibited the dissolved of polysulfide anions into the electrolyte. Therefore, the electrochemical performance at different discharge rate of boron-doped carbon/sulfur composites were superior to the non boron-doped carbon/sulfur composites.3.Using phenolic resin prepolymer as the carbon source, associating KOH and Zn Cl2 as the pore forming agent, a kind of disordered carbon MC with a larger surface area and pore volume was prepared. The disordered carbon material had microporous/mesoporous structure and were used as the cathode materials for lithium-sulfur battery. The electrochemical tests of the carbon/sulfur(MC:S=1:2) composites by melting method show that, the specific capacity and cycle performance of MC:S=1:2 composites were superior to DMC:S=1:2 composites. The reasons were a larger surface area and pore volume of MC and the microporous/mesoporous structure. The microporous structure could adsorb the polysulfide anions, the larger surface area and pore volume could adsorb the sulfur and the polysulfide anions, alleviate sulfur cathode volume expansion in the charge/discharge process. In addition, coating a layer of PANI conductive with the MC:S =1:2 composite surface could further improve the capacity of the batteries and the utilization of active material sulfur.4.Using a commercial reducing graphene oxide C-r GO product as the raw material, the nitrogen and hydrogen mixed gas as reducing agent, the thermal stability reducing TS-r GO of different temperatures treatment was prepared. These graphene materials were used as the electrode materials for supercapacitors, and the electrochemical performance was compared and the reasons were analysed and discussed. The experimental results showed: there had little effect to the micromorphology, lattice structure of the nitrogen-doped graphene material at different heat treatment temperature. The TS-r GO materials were still nitrogen-doped graphene materials; With the increase of heat treatment temperature, the decomposition and decrease of O-/N-based functional groups in the processes of the heating treatment and reduction of the interplanar distance of the crystal plane,at 800℃, part of the graphene sheet size increases;Although the values of specific capacitance, specific energy and power densities are negatively correlated to the heating temperature, the cycle performance at high rate is good; the cycle performance of C-r GO was lower, the capacitance retention at the initial 1000 th cycle seriously declined(declining 10.1%), but the thermal stability reducing TS-r GO of different temperatures treatment showed better cycle stability electrochemical properties in terms of cycle life as well as high-rate performance. The capacitance retention of TS-r GO(700) material declined 0.3%at the initial 1000 th cycle, The capacitance retention of TS-r GO(700) is up to 99.7% in the10000 th cycle. It is found that the optimization of the functional groups of doped nitrogen atoms may contribute to the improvement of cycle life and decrease of leakage current density, and the enhanced rate performance can be attributed to the increase of electrical conductivity. |
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