In-Operando Raman Study on the Use of 2D and Suboxide Titanium Host Materials for Lithium-Sulfur Batteries

Abstract

While Lithium-Sulfur (Li-S) batteries have promised high capacities and low-cost material inputs, their potential has yet to be realized due to inherent issues with sulfur cathodes. In particular the polysulfide shuttle effect and sulfur’s intrinsic insulating properties stand in the way of a commercial battery, the demands of which include high sulfur loading and high cycling stability. Engineering the sulfur cathode, via the use of promising new materials has been an avenue of research pursued in the hopes of mitigating the shuttle effect via polysulfide entrapment and introducing more conductive materials.Of particular interest have been titanium oxide based materials which have shown polysulfide adsorption capabilities. However, the most common titanium oxide, anatase titanium (IV) oxide (TiO2) acts as an insulator, limiting its use in high sulfur loading batteries. Therefore, the use of more conductive titanium oxide materials is an attractive avenue of research. A previously reported freestanding titanium suboxide (TiO) carbon nanofiber cathode demonstrated excellent capacities (~790 mAh/g, ~2 mg/cm2). A rare lepidocrocite phase has also been observed via Raman spectroscopy in a newly discovered titanium carbide derived titanium oxide nanofilament (1D-NF). This material demonstrates properties that makes it attractive as a sulfur host material, having a high surface area of ~1700 m2/g, improved polysulfide reduction kinetics via the formation of polythionates, and polysulfide-cathode host interactions. The inclusion of these nanofilaments in sulfur cathodes yields capacities of ~800 mAh/g with ~1 mg/cm2 sulfur loading.The proposed mechanism via which these titanium oxide based materials function has never been investigated in-operando, and herein we conduct an in-operando Raman study to understand the behavior of these materials. Principally we will investigate the Eg band whose vibration can be moderated by the interaction of terminal sulfur atoms acting as Lewis bases and titanium atoms acting as Lewis acids, due to their vacant valance electrons. This interaction has been observed via postmortem XPS. The use of in-operando Raman allows us to uniquely observe transient behavior of the host material as well as the impact of crystal structure on polysulfide host interactions.