Spinel oxides in lithium-sulfur: catalysis and interaction with lithium nitrate

Abstract

Due to the increasing demand for improved energy storage devices to enable renewable energy sources, there has been great demand for improved batteries. One possible avenue for improved batteries with performance beyond that of conventional lithium-ion batteries is the lithium-sulfur system. This battery chemistry allows for significantly greater specific capacity using more abundant materials than those in conventional lithium batteries, but they face several issues. These issues include poor conductivity, volume expansion, the polysulfide shuttle effect, and poor rate capability. To overcome these issues, one possible approach is the use of electrocatalysts and polysulfide adsorbers in the cathode to alleviate polysulfide flooding and accelerate reaction kinetics. This is often combined with the ubiquitous additive lithium nitrate in the electrolyte as an anti-shuttle agent.

However, despite significant attention from the research community, the mechanism behind many of these catalysts remains poorly understood. Furthermore, the possibility of adverse or beneficial interactions of such catalysts with electrolyte additives remains unexplored. This thesis, therefore, aims to examine the relationship between the physical and electronic properties of spinel metal oxides, a highly flexible class of material, with their catalytic and polysulfide adsorbing properties in typical lithium-sulfur cells. Towards this end, a polysulfide adsorber, magnesium ferrite, was tested in combination with the common electrolyte additive lithium nitrate. The two beneficial components interfered with each other, with greatest effect at high charge/discharge rate and high lithium nitrate concentrations. Therefore, lithium nitrate concentrations were kept low for the next study examining various spinel ferrites as lithium-sulfur catalysts. The results of this study suggest that their catalytic performance may be described using their metal-oxygen bond covalency. Greater covalency was found to be beneficial up to a limit, beyond which more covalent character was detrimental. This was followed by an examination of the effects of metal coordination site on catalytic performance. Composition and annealing temperature could be used to tune the site occupation of iron and cobalt between tetrahedral and octahedral sites in zinc-substituted magnetite and cobalt aluminate. The results indicated that tetrahedral site occupation is beneficial for catalytic performance. These established relationships will be useful in the future design of more effective lithium-sulfur catalysts and polysulfide adsorbers.