Development of high-energy cathode materials for lithium-sulfur batteries

Rechargeable lithium-sulfur batteries are energy storage devices based on the reversible electrochemical reaction between lithium metal and elemental sulfur. When fully utilized, its energy density could reach 3-5 times that of currently marketed lithium ion batteries. This thesis describes the work focused on the development of cathode materials for high-energy lithium-sulfur batteries. More specifically, the design, synthesis, and characterization of cathode materials based on porous carbon-sulfur (C-S) composites are discussed.;The work described in Chapter 2 focused on tuning the pore structure of porous carbon in order to improve electrochemical performance of the C-S composite cathode material. It was found the pore volume of the porous carbon directly affect the practical energy output of the composite, as it limits the useful sulfur content in the composite. Meanwhile, the pore sizes of the porous carbon need to be restrained for desirable sizes of sulfur particles in the composites for satisfactory battery performance. These two parameters are competing and need to be balanced. The proposed porous carbon, HPC, affords a high pore volume with pore sizes restrained under 20nm. This carbon material can embed up to 80wt.% sulfur in its pore structure and still lead to exceptional battery performance.;The work described in Chapter 3 focused on forming secondary structure of C-S composite in order to improve its energy density (both gravimetric and volumetric). Besides the nano-structure, the exterior morphology of C-S composite particles is also critical for its battery performance as the cathode material. Syntheses focused on the nano-structuring without proper control of particle morphology normally result in particles too small for battery applications, plaguing battery fabrication and failing to reach the full potential of the electrode materials. This work took on the challenge to control multi-scale features in C-S composite. The proposed PSC porous carbon spheres not only have a high volume of nano-sized pores, but also exhibit desirable particle-size distribution for battery applications. The resultant C-S composite showed enhanced battery performance on various aspects, including tap density, areal capacity, and projected gravimetric and volumetric energy density of the whole battery. The proposed synthesis method is also highly scalable and versatile to incorporate extra building blocks into the C-S composite.;The work described in chapter 4 focused on functionalizing porous carbon to stabilize battery performance. Lithium-sulfur batteries severely suffer from performance decay because sulfur has diffusion loss in forms of lithium polysulfides. It was observed that nitrogen doping in carbon could significantly enhance carbon's adsorption of polysulfides, consequently mitigate the diffusion loss of polysulfides and performance decay. This work details the synthesis and characterization of nitrogen-doped porous carbon, the study of its enhanced interaction with sulfur and polysulfides, and improved battery performance from its sulfur composite.