The Evolving Search for Positive Electrode Host Materials for the Li-S Battery
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High energy density storage systems are constantly being researched to support the demand for electric vehicles and use for renewable energy systems. One such energy storage system of interest is lithium-sulfur batteries which utilize inexpensive, earth-abundant sulfur as the active material. In order to fully realize the potential of Li-S batteries being commercialized, many problems must still be overcome. Among them are: 1) low conductivity of sulfur species, 2) polysulfide redox shuttle in the electrolyte and self-discharge, 3) volume expansion of active material upon cycling, and 4) lithium metal dendrite formation. The work in this thesis will first focus on a diagnostic method that probes ideal characteristics of sulfur host materials, and then focus on a new material that will address the first three problems. The exhaustive search for a material that has a high electronic conductivity to facilitate charge transfer, substantial surface area and pore volume to allow high sulfur loading, and well-suited physical/chemical surface properties to inhibit Sn2- diffusion into the electrolyte is still ongoing. It is vital to find a material with these characteristics, so as to bring the Li-S battery one step closer to commercialization. In the first section of this thesis, a versatile, cost-effective electrochemical analysis strategy is described that determines the specific Sn2- adsorptivity of materials. This analytical method for screening sulfur host materials is based on metrics: a quantitative electro-oxidation reaction provides the value for Sn2- adsorptivity – which coupled with surface area – is correlated to the extent of self-discharge at an intermediate state of discharge during a 5-day cycling protocol. Measurement of nine different materials with varying surface area, and hydrophobicity using the analytical method determined optimum properties for capacity stabilization. In fact, materials (such as MnO2) that have a high surface area and the ability to chemically interact with intermediate polysulfides realized improved long-term cycling performance and mitigated self-discharge. In the second part of this thesis, a positive electrode material is proposed as a sulfur host. Surface thiosulfate groups are known to drastically mitigate polysulfide diffusion through the formation of a polythionate complex which acts as an internal mediator catenating long-chain polysulfides. The presence of the polythionate complex through an extrinsic additive was explored for low-cost carbon materials. In this study, carbon materials which contained functional groups able to form the polythionate complex saw up to 60% reduction in the irreversible capacity loss as a result of self-discharge. With 15 wt% additive, polysulfide adsorptivity increased and excellent long-term cycling was realized with a capacity fade of 0.085% per cycle over 200 cycles. By combining a polysulfide adsorber with a carbon matrix, long-term cycling is realized with a sulfur loading of ~1.5 mg/cm2.