Solution-based Fabrication Of Graphene/sulfur Hybrid Materials And Their Applications In Lithium-sulfur Batteries

Carbon materials are key components in energy storage and conversion devices due to their excellent conductivity, high surface area, exceptional chemical stability and diverse structure. Graphene, as a one-atom thick sp2 hybridized carbon film, is believed to be promising in electronics, energy storage and biology due to its fantastic properties. More importantly, graphene is the basic building block of other carbon architectures. Thus, starting from graphene, the design and construction of novel carbon materials with multi-functions are of great importance to accelerate the future development of energy storage devices. In this dissertation, starting with the mass-production of graphene-based materials, we proposed the significance and mechanism of low-temperature preparation of graphene for their large-scale production. A series of solution-based fabrication of graphene-based materials for lithium-sulfur(Li-S) batteries are developed: H2 S was employed as a novel reducing agent to realize the effective reduction of graphene oxide(GO) under mild condition, and the products were highlighted as promising cathode materials in Li-S batteries. By tuning the reduction process of GO, a high-density graphene/sulfur(HDGS) monolith was constructed with excellent volumetric performance towards Li-S batteries. For solving the bottlenecks in the fabrication of carbon/sulfur cathode materials for Li-S batteries, highly dispersible sulfur nanocrystals were developed via the reaction between sulfurous gas, highlighting the significance and feasibility of solution-based fabrication of cathode materials for Li-S batteries.Specifically, we illustrated the relationship between the surface chemistry of graphene and its electrochemical performance. We firstly gave a deep insight into the low-temperature vacuum-promoted fabrication of graphene, and enough pressure difference inside and outside graphene layers is the key for successful exfoliation. The influence of different graphite precursors on the structure and properties of graphite oxide and graphene was investigated, and the low temperature exfoliation is effective enough to eliminate the difference from the graphite. Furthermore, post-annealing can effectively tune the surface chemistry of graphene, and the electrochemical capacitance decreased dramatically with the increasing temperature, demonstrating the importance of surface chemistry to the electrochemical performance of graphene. The versatile mechanism was also proposed with some future comments.Based on the surface chemistry of GO, we innovatively proposed the solution-based fabrication of graphene/sulfur hybrid starting from H2 S and GO. H2 S, a reducing gas with high sulfur content up to 94% was employed to reduce GO, and the in-situ homogeneous deposition of sulfur on reduced GO was realized simultaneously. The as-prepared products possessed unique curly structure for a better confinement of sulfur, showing excellent cycle and rate performance. This strategy demonstrates a novel but effective approach for the construction of high performance cathode materials for Li-S batteries.Following the above strategy, we successfully designed and constructed a HDGS cathode material with promising volumetric capacity. By adjusting the surface chemistry and reduction degree of GO, the self-assembly of three-dimensional graphene/sulfur hydrogel was realized in solution. The microstructure and pore size can be readily tuned by the controllable removal of water, and the high density(1.53 g cm-3) and unique bottle-neck pore structure brought excellent conductivity and strong confinement of sulfur, further improving the volumetric capacity as cathode material towards Li-S batteries with high volumetric energy density.Finally, to further improve the sulfur content in graphene/sulfur hybrid, we extended the applications of solution-based fabrication of sulfur cathode materials. Highly dispersible sulfur(DS) nanocrystals were innovatively fabricated by the simple reaction between H2 S and SO2 in aqueous solution. The particle size and concentration can be readily tuned and the sulfur nanocrystals showed excellent electrochemical performance. DS was further used to realize the solution-based preparation of graphene/DS and CNT/DS cathode materials, which demonstrated the uniqueness and significance for the future application of Li-S batteries.