Date of Award

2-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Shengnian Wang

Abstract

Lithium-Sulfur batteries have a high energy storage capacity while their sulfur cathode suffers large volume change, polysulfides dissolution and shuttle effect, and capacity fading during long-term cycling. To help lock sulfur and mitigate these problems, we introduced halloysite, a natural clay material with a nanotube format, to disperse and confine sulfur nanoparticles as well as to suppress the dissolution and migration of polysulfides. Halloysite was made conductive by covering it with a glucose-derived carbon skin. Sulfur nanoparticles were then trapped in both the lumen and outside surface of individual nanotubes with a loading dosage up to 80 %. In this new halloysite/sulfur composites cathode, the hollow nanostructure of halloysite provides space to allow dimension changes of encapsulated sulfur nanoparticles during repeated lithiation while limiting their size up to the diameter of nanotube lumen (i.e., 25 nm or less). The stacked halloysite clusters further create many nanoscale voids to divide the sulfur-electrolyte interface into isolated domains and increase the migration tortuosity in electrolytes to suppress the dissolution and shuttle effect of polysulfides. These features together contribute to improved cycling stability, retaining nearly ~84% of the starting capacity over 250 cycles, though the diffusion of lithium ions going in and out of nanotubes show some differences.

In project 2, we worked on the anode development for LIBs. Silicon-rich (e.g., >30 wt.%) anodes are desired to leverage the current capacity of lithium-ion batteries (LIBs) towards commercial cell performance requirements in critical markets, such as the transportation sector. A new type of nanofiber-in-microfiber silicon/carbon composite electrode was prepared and tested as a potential silicon-rich anode candidate. A co-axial electrospinning setup was used to produce a unique hybrid composite fiber configuration, in which silicon nanoparticles were suspended in a polymer solution to serve as the middle stream while the sheath stream was comprised of another polymer solution. Polyvinyl alcohol (PVA) was chosen as the silicon dispersion fluid because of its limited viscosity increase even at a very high solid allowance, which after carbonization held those nanoparticles together as short, branched nanofibers. Polyacrylonitrile (PAN) sheath fluid helped wrap the formed short, silicon-rich nanofiber bundles to form a nonwoven, ductile microfiber mat. After being carbonized into composite anodes, the silicon-rich nanofibers were used to host the majority of lithium ions while their thin carbon skin, originating from carbonized PVA, promotes conductivity and charge transfer. The nanofibrous morphology and the mesoscale space in between help accommodate the notorious volume expansion issues in silicon anodes during lithiation/delithiation processes. The outside PAN-derived microfibers provide structural support for the encapsulated silicon-rich nanofibers and simultaneously serve as the three-dimensional current collector. The new composite anodes were confirmed on their unique fibrous configuration and improved electrochemical performance. With 40 wt% Si, such silicon-rich, nanofiber-in-microfiber anodes achieve ~900 mAhg-1 reversible capacity and ~90% capacity retention over 250 cycles by effectively balancing challenges on silicon-rich fibrous anode and electrode pulverization.

Beside battery research, we also worked on supercapacitors with high power density in project 3. Despite the great benefits plastics have brought to our modern lives, a large volume of plastic wastes increasingly threatens our environment and human health. Through a hydrothermal carbonization and crystallization process involving nitric acid and ethanol, drinking bottles made of polyethylene terephthalate were successfully converted into carbon quantum dots (CQDs) and thin carbon sheets simultaneously, with the former well dispersed and intercalated in the latter as a ball-sheet carbon structure (BSCs). The formed unique, connected, and conductive carbon network allows rapid transport of ions and electrons besides their large surface area and numerous ion hosting sites. The electrodes made of such a plastic ball-sheet carbon structure (PBSCs) therefore exhibit pseudocapacitance behavior with the specific capacity reaching 237 F/g at the charge rate of 1 A/g. Superior cycling stability on the energy storage was also found. Our method offers a new avenue to upcycle some plastic wastes as valuable energy storage systems, to help boost the recycling of plastic waste, and move forwards to the sustainable deployment of various clean energy resources.

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