Engineering Freestanding, Binder-free Electrospun Nanofiber Cathodes and Use of In-Situ Infrared Spectroelectrochemistry for Lithium-Sulfur Batteries

As energy demands increase, development of energy storage and conversion systems becomes critical to meet the needs of society. The performance and lifetime of energy devices are highly dependent on the material chemistry, processing, and nanostructure. 1D nanomaterials show great promise for advanced energy materials owing to their high aspect ratios, mechanical flexibility, among other unique properties. Electrospun nanofibers are particularly advantageous owing to the versatility, in which control of the nanoarchitecture and composition is easily achievable. Electrospinning is a relatively simple fiber formation technique that uses a strong electric field to rapidly stretch and elongate a polymer-based solution or melt jet, creating ultrathin fibers with diameters on the order of 50 – 500 nm. The nanofibers form continuous into a non-woven fiber mat that is inherently free-standing which allows for direct usage without additional processing with insulating, inactive binder materials. My dissertation will focus on the synthesis and characterization of electrospun nanofibers materials and will establish key process-structure-property relationships for creating simple materials for organic photovoltaics and lithium-sulfur batteries. The nanomaterials for these systems have very different requirements, thus, these studies focused on the challenges associated with each application such that the nanofiber synthesis and types of characterization techniques were tailored to each application. The lithium-sulfur (Li-S) rechargeable battery is an emerging, close-to-market chemistry with several key advantages over lithium-ion: high energy density ~5 times greater than lithium-ion, high theoretical capacity of 1,675 mA h g-1 (vs < 300 mA h g-1 of li-ion), and sulfur is inexpensive, non-toxic, and environmentally-benign. However, several challenges must be addressed to develop high performance Li-S batteries. The electronically insolating nature of sulfur and the dissolution and shuttling of soluble reaction intermediates (Li2Sn, 4 ≤ n ≤ 8) into the electrolyte significantly reduce performance, such as rapid capacity fade. Thus, research intensely focuses on sulfur cathodes with conductive components that confine sulfur to hinder polysulfide intermediate shuttling. Complex conductive nanoarchitectures to encapsulate or trap sulfur to the cathode require length methods of sulfur diffusion (~8-10 hours) and require binders and heavy current collectors to integrate the composites in a final electrode. We developed a rapid (5-second) sulfur deposition technique is demonstrated on electrospun carbon nanofibers to fabricate binder-free, free-standing cathodes for lithium-sulfur batteries. The 5-second procedure melts sulfur into carbon nanofiber mats, which play a significant role as a built-in conductive matrix without the hindrance of conventional insulating binding agents. Meanwhile, the large inter-fiber spacing facilitates electrolyte diffusion. The cathodes thus obtained deliver 550 mA h g-1, effectively an effective discharge…