Electrospun Nanofiber-Based Cathodes for Lithium-Sulfur Batteries

Abstract

Lithium-Sulfur is a next generation battery chemistry that offers a theoretical capacity of 1,675 mAh/g, an order of magnitude higher than that of the currently used Li-ion battery cathodes. However, a serious challenge that has plagued the development of this technology is the dissolution and shuttle of intermediate polysulfides (Li2Sx, x=4-8) in the electrolyte resulting in loss of active material and rapid capacity fade during cycling. I will present two projects from my group both centred around Li-S batteries. In the first part of my talk, I will discuss our work on stabilization of titanium monoxide (TiO) nanoparticles in carbon nanofibers (CNF) through electrospinning and carbothermal processes and their unique bi-functionality – high conductivity and ability to bind polysulfides via Lewis acid-base interactions–as sulfur hosts. The developed 3-D TiO/CNF architecture with the inherent inter-fiber macropores of nanofiber mats provides a much higher surface area (~427 m2 g-1) and overcomes the challenges associated with the use of highly dense powdered Ti-based suboxides/monoxide materials, thereby allowing for high active sulfur loading among other benefits. The developed TiO/CNF-S cathodes exhibit high initial discharge capacities of ~1080 mAh g-1, ~975 mAh g-1, and ~791 mAh g-1 at 0.1C, 0.2C, and 0.5C rates, respectively with long-term cycling. Furthermore, free-standing TiO/CNF-S cathodes developed with rapid sulfur melt infiltration (~5 sec) eradicate the need of inactive elements viz. binders, additional current collectors (Al-foil) and additives. Using postmortem XPS and Raman analysis, this study reveals the presence of strong Lewis acid-base interaction between TiO (3d2) and Sx 2- through the coordinate covalent Ti-S bond formation. In the second part, I will discuss our work on in-situ infrared spectroelectrochemistry to understand redox mechanisms – a critical step for rational design of cathode systems for Li-S batteries. Specifically, I will discuss the use of sulfur-rich copolymers as active cathode materials and demonstrate in-situ FTIR with attenuated total reflection (ATR) to monitor polysulfide (PS) speciation (S x 2-, 2 ≤ x ≤ 8) and polymeric signature evolution while simultaneously discharging/charging a full battery coin cell.