Rechargeable lithium-sulfur batteries (LSBs) have been extensively studied as one of the most promising next-generation energy storage devices due to their high theoretical capacity of 1675 mAh/g and large theoretical energy density of 2567 Wh/kg. Other advantages include natural abundance, low cost and environmental benignity, making sulfur even more attractive as the cathode for rechargeable batteries. Nevertheless, the commercialization of LSBs has been limited by some critical issues, such as the inherently low conductivities of sulfur and reaction products, Li₂S and Li₂S₂. Apart from the large volume expansion of ~80% generating from elemental sulfur to Li₂S during the discharge process, another critical challenge is the dissolution of polysulfide intermediates, Li₂S_x (x=4...[ Read more ]

Rechargeable lithium-sulfur batteries (LSBs) have been extensively studied as one of the most promising next-generation energy storage devices due to their high theoretical capacity of 1675 mAh/g and large theoretical energy density of 2567 Wh/kg. Other advantages include natural abundance, low cost and environmental benignity, making sulfur even more attractive as the cathode for rechargeable batteries. Nevertheless, the commercialization of LSBs has been limited by some critical issues, such as the inherently low conductivities of sulfur and reaction products, Li₂S and Li₂S₂. Apart from the large volume expansion of ~80% generating from elemental sulfur to Li₂S during the discharge process, another critical challenge is the dissolution of polysulfide intermediates, Li₂S_x (x=4~8), in liquid organic electrolyte, causing the so-called “shuttle effect” with an associated rapid capacity loss and a low Coulombic efficiency.

In pursuit of the superior electrochemical performance of LSBs, this thesis is dedicated to addressing the aforementioned issues by designing and fabricating novel materials for cathodes, interlayers and separators.

Cathodes: three-dimensional graphene aerogel/TiO₂/sulfur (GA/TiO₂/S) composites are synthesized through hydrothermal route as the cathode for LSBs. With a sulfur content of 75.1 wt %, the composite electrode delivers a high discharge capacity of 512 mAh/g after 250 cycles at 1 C (1 C = 1675 mA/g) with a low capacity decay of 0.128% per cycle. The excellent capacities and cyclic stability arise from several unique functional features of the cathode. (i) The conductive graphene aerogel framework ameliorates ion/electron transfer while accommodating the volume expansion, and (ii) TiO₂ nanoparticles play an important role in restricting the dissolution of polysulfides by chemical bonds with sulfur. Another graphene-based composite, graphene/RuO₂, is also prepared as sulfur substrate for high battery performance. Apart from entrapping polysulfides via chemical interactions, conductive RuO₂ nanoparticles enhance the redox reaction kinetics. Benefiting from these characteristics, the optimized graphene/RuO₂/S electrode with a high sulfur content of 79.0 wt% maintains remarkable capacities of 508 and 330 mAh/g after 200 cycles at high current rates of 1 and 2 C, respectively.

Interlayers: two novel interlayers made from Fe₃C/carbon nanofibers (Fe₃C/CNFs) and graphene oxide/carbon nanotube hybrids (GO/CNTs) are synthesized using electrospinning and vacuum filtration, respectively. The interlayer placed between the separator and the sulfur cathode plays many synergistic roles, offering (i) the high conductivity to provide fast pathways for electron transfer; (ii) the porous structure to facilitate ion transport and electrolyte penetration; and (iii) the functional groups to entrap active materials so as to enhance re-utilization. As a result, the LSB with an Fe₃C/CNF interlayer delivers an excellent discharge capacity of 893 mAh/g after 100 cycles, maintaining 76% of its initial capacity of 1177 mAh/g, while the cell containing an optimal GO/CNT interlayer achieves a reversible capacity of 671 mAh/g after 300 cycles at 0.2 C with a low degradation rate of 0.043% per cycle.

Separators: carbon nanotubes (aCNTs) are chemically activated to serve as both the substrate for sulfur cathode and the carbon layer on the separator. The LSB consisting of an aCNT/S cathode and an aCNT modified separator with an optimal composition presents an excellent capacity of 621 mAh/g after 500 cycles at 0.5 C with a low capacity decay of 0.043% per cycle. The encouraging performance benefits from the ameliorating functional characteristics of aCNTs in both the two battery components. The highly porous and conductive aCNT network facilitates fast electron/ion transport and easy electrolyte penetration into the cathode, enhancing full utilization of active materials. The aCNT layer on the separator effectively constrains the polysulfide shuttling by physically blocking their migration to the anode while immobilizing polysulfides by strong adsorption onto the micropores of aCNT surface, as revealed by molecular dynamic simulations.