The Preparation And Application Of Graphene Based Supercapacitors And Li-S Batteries

Energy density of the commercial supecapacitor is relatively low compared with the batteries; the poor cycle stability of Lithium-Sulfur (Li-S) battery has hindered its commercial application. Therefore, it has important to improving the energy density of supercapacitor and the cycle stability of Li-S battery. Graphene, one atom thick and two-dimensional (2D) single layer, has high theoretical specific surface area, excellent electronic conductivity, mechanical flexibility, and chemical and thermal stability, which make it suitable for application in preparation of energy storage materials.In this paper, we focused on the preparation of high performance graphene based electrode materials of supercapacitors and Li-S batteries to improve the energy density of supercapacitors and enhance the cycle stability of Li-S batteries. In addition, we have prepared a high-performance GO-doped ion gel for the first time that may has great potential for applications in all-solid-state supercapacitors.1. The high performance graphene/porous-carbon composite materials were prepared via a simple and cost-effective hydrothermal carbonization and an existing industrial level KOH activation processes using the cross-linked PVA/GO hydrogel as precursor. A high specific surface area and excellent conductivity are achieved for these composite materials. Furthermore, using industry practice and method, supercapacitors using these materials as electrode materials were fabricated and measured, which demonstrate outstanding performance with high power characteristic and excellent cycle stability in1M TEABF4/AN electrolyte and high specific capacitance and energy density in EMIMBF4electrolyte. The high performance of these materials, combined with their easily scale-up ability and low cost make them a highly competitor to the currently active carbons used for supercapacitors.2. A high-performance GO-doped ion gel (P(VDF-HFP)-5EMIMBF4-1wt%GO) with high ionic conductivity, wide electrochemical window, and high thermal stability is prepared by exploiting copolymer (poly(vinylidene fluoride-hexafluoro propylene), P(VDF-HFP)) as a polymer matrix,5times of ionic liquid (1-Ethyl-3-methylimidazolium tetrafluoroborate, EMIMBF4) as a plasticizer and supporting electrolyte, and1wt%of GO as a novel additional material. With the incorporation of only a small amount of GO (1wt%) in ion gels, there have been a dramatic improvement in ionic conductivity of about260%compared with that of ion gels without GO addition. In addition, using the industry practice and method, the all-solid-state supercapacitors are fabricated and measured using the GO-doped ion gels as gel polymer electrolytes and separators, which demonstrate more superior electrochemical performance than the conventional supercapacitors using the neat ionic liquid electrolyte of EMIMBF4, in the aspect of higher capacitance performance, smaller internal resistance and more excellent cycle stability. Our demonstration could be an important basis for the polymer electrolyte design and development of flexible energy storage devices.In addition, we demonstrate a high-performance all-solid-state supercapacitor with the graphene/porous-carbon electrode material and a graphene oxide (GO)-doped ion gel ((P(VDF-HFP)-5EMIMBF4-lwt%GO) as a gel polymer electrolyte and separator. The all-solid-state supercapacitor demonstrates outstanding performance with high energy density and power density. In addition, the all-solid-state supercapacitor exhibits similar and excellent performance as does the compared conventional liquid supercapacitor in respect of specific capacitance, capacitance retention, internal resistance, and frequency response.3. We designed and synthesized a graphene-based layered porous carbon material, via a simple and cost-effective hydrothermal carbonization and KOH activation processes, with high electronic conductivity, high specific surface area with mesoporous distribution, and high pore volume. In addition, layered graphene/porous carbon-sulfur composites with68wt%sulfur loading were prepared using liquid-phase infiltration and melt diffusion method. In this composite, the sulfur existed in a highly dispersed amorphous state, which could result in the high utilization of sulfur in electrochemical reactions. Furthermore, Lithium-sulfur batteries using these materials were fabricated and measured, which demonstrate outstanding electrochemical performance with high specific capacity, excellent rate performance, and excellent cycle stability. Two important factors are thought to contribute to the excellent electrochemical performance and cycle stability:(1) the highly porous carbon covering the graphene, with high surface area and high pore volume, can make the high sulfur content exist in a highly dispersed amorphous state to improve the conductivity and electrochemical activity;(2) the pores in the porous carbon act as polysulphide reservoirs and prevent polysulfides from diffusing out of the cathode, which can diminish the shuttle effect and significantly improve the cycling stability.