Development of Nanostructured Cathode Materials for Lithium-Sulfur Battery Applications

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Over the last decade demand for renewable energy technologies has been one of the primary issues of concern across the globe. It is in this context, lithium-sulfur battery based on sulfur cathode have drawn the particular interest owing to the high specific capacity, high energy density and low cost of eco-friendly sulfur. Nonetheless, there are still formidable challenges hindering the successful application of lithium-sulfur battery. Those challenges can be categorized as, poor electrical conductivity of elemental sulfur, lithium polysulfide intermediate dissolution / shuttling. In my doctoral work, I focused mainly on the cathodes, such as developing a new class of sulfur material and optimizing the cathode structure to improve the electrochemical performance of lithium-sulfur batteries. The first part of the thesis, we report a novel sulfur rich copolymer@ 3D graphene-carbon nanotubes (G-CNT) network cathode for high performance lithium-sulfur batteries. Unlike elemental sulfur as cathode, this squalene-derived copolymer can greatly suppress the dissolution of sulfur and polysulfides due to the chemical confinement from the crosslinking of polysulfur chains with the squalene molecules. While in the SP@G-CNT composite electrode, the interlinked Sp² G-CNT network not only enhance the polysulfide entrapment capability, but also provide the composite with an 3D electrically conductive path as well as an eminent mechanical resilience towards the huge volume change of sulfur. The as-developed cathode can deliver a high specific capacity, excellent rate performance and cycling stability. In the second part, a nitrogen-doped micro/mesoporous carbon is derived from an amine-functionalized metal oxide framework (UIO-66-NH₂ abbreviated as NH₂-MOF) to host sulfur. Moreover, a freestanding permselective membrane was fabricated by the layer-by-layer (LBL) assembly of NH₂-MOF and graphene oxides nanosheets and implicated as an interlayer. Such, multifunctional interlayer can block the shuttling of polysulfides in both physical and chemical ways without compromising the ion conductivity. The optimized lithium-sulfur cells realized high reversible capacity, extended cycling stability at high rate and much improved rate performance. In the third part, a well-designed bilayer cathode structure is proposed to increase the active material loading and improve the areal capacity. The support layer contains carbon nanofiber / nickel nanoparticles decorated nitrogen-doped graphene (Ni-NG) and the top layer composed of Ni-NG nanosheets. The porous and highly conductive bilayer host not only ameliorates high sulfur loading and increase active material utilization but also accelerates the rapid conversion of polysulfides. With Li₂S₆ catholyte, bilayer Ni-NG@CNF cathodes demonstrates low voltage polarization, superior cycling stability and excellent rate performance.
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