Preparation And Electrochemical Research Of S@conductive Coating Composites For Li-S Cathodes With High Specific Capacity

With the development of society, energy shortage and environmental degradation becoming the two major challenges for human society, hence, research for sustainable energy shows its significance for solving such problems. Lithium-ion batteries as the new generation of secondary power sources became extremely important in contemporary society energy storage and conversion devices. Among all kinds of lithium-ion batteries, Lithium-Sulfur(Li-S) battery with its advantages such as high capacity, low cost, abundant source, environmentally friendly property and easy recovery, in terms of energy storage demonstrated attractive prospect, became the most potential for the next generation of cathode materials for Lithium ion batteries. But, poor conductivity of elemental S and the soluble(in organic solvents) discharge products, leading to the low active material utilization, poor circulation performance, slow charging/discharging rate and so on. Hence, this paper was focused on the use of sulfur and carbon-based conductive composites as cathode materials. The related work was mainly in the following studies:First, a one-pot preparation of sulfur @ polypyrrole(S@PPy) composites was researched. Fe Cl3 was not only the oxidant and polymerization of pyrrole, but also played a role as the catalyst for S. We prepared PPy spherical shell with good eletronical and ionic conductivity, together with a moderate thickness and adequate volume for loading S; Triton-100(TX-100) effectively maintained the size and dispersion of S particles, and it contributed to the PPy coating; H3PO4 is then added to improve the overall dynamic performance of the material. Characterization and test data showed that: The composite had better capacity retention rate and cycle dynamic performance, the initial capacity was 810 m Ah g-1, and after 50 cycles of testing, the capacity of this material still protected at about 600 m Ah g-1(74 % in retention).We secondly developed hydrogen iodide(HI) reduction of graphene oxide r GO and surfactant-assisted chemical reaction-deposition method to form hybrid material of S encapsulated in reduced r GO sheets for Li-S cathode. The surfactant-assisted chemical reaction deposition method strategy provided intimate contact between the S and r GO. The chemical reduced r GO with high conductivity as carbon coating layer prevented the dissolution of polysulfide ions and improved the electron transfer. This novel core-shell structured S@r GO composites with high S content showed high reversible capacity, good discharge capacity retention and enhanced rate capability used as cathodes in rechargeable Li/S cells. We demonstrated that such electrode prepared from S and r GO with upto 85 wt% S maintains a stable discharge capacity of about 980 m Ah g-1 after 200 cycles at 0.05 C and 570 m Ah g-1 at 1 C. These results emphasized the importance of r GO with high electrical conductivity after r GO coating on the surface of S, therefore, effectively alleviating the shuttle phenomenon.Thirdly in this paper, ultrathin microporous carbon(UMPC) for Li-S cathode with uniform pore width of approximately 0.6 nm and dozens nm in thickness was synthesized with graphene oxide(GO) as template by glucose hydrothermal carbonization and surfactant-assisted assembling method. The UMPC supplied desirable S pregnancy space and the intimate contact between UMPC and S, therefore improving the conductivity of S@UMPC composite for better dynamic performance. Smaller S molecules limited in UMPC thoroughly prevented the formation of electrolyte-soluble polysulfides, hence excellent cycling performance with 900 m Ah g-1 after 150 cycles was kept. Ultrathin three-dimensional carbon nanosheets were significant to fast electron transfer and Li+ diffusion contributing to excellent dynamic performance(710 m Ah g-1 at 3 C).The design strategy and physical characterization of S@PPy, S@r GO and S@UMPC were shown detailedly in this paper. Results from AC impedance, CV curve and charging/discharging were tested by the assembled cells. All these strategies above improved the performances, practicality and the prospect of Li-S cathodes markedly.