PhD Defence Yinyu Xiang: "Porous carbon-based composites for lithium-sulfur batteries"

Promotors: Prof. Yutao Pei, Prof. Ajay Kottapalli

Abstract: In this dissertation, various porous carbons and their composites were developed and configurated in Li-S batteries. Considerable progresses including optimization of sulfur/carbon cathodes, configuration of carbon nanofibers (CNFs) interlayers, and stabilization of Li-metal anodes, have been developed to address the challenges of the Li-S cells and improve the performance. At first, aspect ratio of ordered mesoporous carbon-based sulfur host was tuned and the aspect ratio effect on electrochemical performance of S@OMCs cathodes was studied via systematic electrochemical measurements and characterizations. More impressive, the evolution of sulfur species in OMCs at different discharge state was explored. Next, to endow sulfur host with chemisorption toward polysulfides, cobalt/carbon spheres were introduced into CNTs. The hierarchical porous frameworks with large pore volume and high specific surface area alleviated the volume exchange and physically restricted the polysulfides. The partially remained cobalt nanoparticles were beneficial for the chemical adsorption and conversion of polysulfides. Further, the crystallinities of TiN coating on copper-embedded CNFs were tuned and the nanofibers were used as interlayers. Compared with highly-crystalline TiN, low-crystalline TiN not only strengthened chemical adsorption toward polysulfides, but also facilitated the redox conversions, contributing to impressive high-rate performance and remarkable cyclic stability. Finally, to achieve dendrite-free Li plating/stripping, nitrogen-doped CNFs with Cu/Cu3P decorations were developed. The lithiophilic nitrogen-doped carbon sites and the embedded functional Cu/Cu3P heterostructure endowed ultralow nucleation overpotential and small polarization. Thus, Cu/Cu3P-N-CNFs electrodes exhibited high Coulombic efficiency (~94%) after 500 cycles’ plating/stripping and the corresponding Li@ Cu/Cu3P-N-CNFs outperformed in symmetrical cells and Li-S full cells.