Investigation of Structural and Electrochemically Active Frameworks for Li-S Cells

Abstract

With a theoretical capacity of 1672 mA h g-1, more than five times higher than any commercially available lithium-ion (Li-ion) cell systems, the lithium-sulfur (Li-S) cell is an attractive candidate for next generation energy storage. Despite this high theoretical capacity, Li-S cells generally suffer from poor capacity retention and working lifetimes that prevent them from mass commercialisation. This is mainly due to current limitations in managing the inherent Li-S redox reactions which involve diffusion and migration of electrochemically active polysulfides. One approach to prevent polysulfide migration is by rational design of the sulfur electrode framework.

The aim of this research is to investigate the electrochemical implications of using different frameworks for entrapment of redox active species, mainly designed for the Li-S cell system. The two types of frameworks investigated are: (1) mixed-morphology carbon feeds derived from waste sources wherein the intention is for the carbon to purely act as a structural framework to trap lithium polysulfides, and (2) sulfur-rich copolymers wherein redox active sulfur is covalently bound within the framework. More specifically, the goals involve determining: (1) whether carbon acts purely as a structural framework to trap redox active species during electrochemistry, and (2) whether sulfur-rich copolymers act purely as a sulfur feed.

Achieving these goals requires a thorough understanding of what properties in each framework are ideal for the Li-S cell. The main conclusion drawn from this work is that neither of the materials studied behaved as pure structural or covalent frameworks partaking in various side processes. Using specialised techniques such as X-ray powder diffraction, solid-state NMR, and X-ray absorption near-edge structure spectroscopy, the beneficial and parasitic side processes involved in each framework are able to be determined. Overall, a significantly enhanced understanding of the Li S cell chemistry when using these materials is presented in this work.