Abstract
With growing global energy demand and environmental concerns, developing efficient energy storage systems is urgently needed. Although lithium-ion (Li-ion) batteries have dominated the energy storage market in recent decades, they have reached capacity limitations due to their low capacity, which prevents them from satisfying the requirements of electric vehicles and grid-scale energy storage systems. Growing attention has been turned to lithium–sulfur (Li–S) batteries that have superior theoretical capacity and lifespan. However, Li–S batteries encounter several roadblocks in practical applications, such as capacity loss, safety concerns, and contamination issues. To tackle these challenges, various nanostructured materials (nanocarbons, metal-organic frameworks, metal oxides etc.) have been extensively developed for sulfur hosting, separator modification, and lithium metal anode protection. Sustainable and cost-effective nanocarbons, which possess excellent electrochemical and mechanical properties, have emerged as promising materials for constructing Li–S batteries.
This dissertation aims to develop advanced carbon nanomaterials and simulation methods to facilitate the commercial applications of Li–S batteries. Experimental and computational methods have been utilized to explore how nanocarbons mitigate the shuttle effect, accommodate sulfur volumetric expansion, and suppress lithium dendrite growth. To further confirm the advantages of nanocarbons in Li–S batteries, spent cathode powders have been recycled to promote decarbonization, which prevents contamination and extends the life of end-of-life Li–S batteries.
Specifically, in Chapter 2, cotton textiles were successfully converted to multiwalled carbon nanotubes (MWCNTs) via a low-cost approach. The combination growth process of vapor−liquid−solid (VLS) and solid−liquid−solid (SLS) growth mechanism were uncovered through both experiments and molecular dynamics (MD) simulations. The obtained Fe/Fe3C-encapculated multiwalled carbon nanotubes (Fe/Fe3C-MWCNT) were utilized for constructing cathodes and interlayer in Li–S batteries, which exhibited a superlative cycling stability and a remarkable specific capacity (1273 mAh g−1 at 0.1 C). In Chapter 3, coupled MD and finite element analysis (FEA) simulations were used to reveal the chemo-mechanics of rate-dependent sulfur anomalous volumetric changes, unveiling that partial lithiation of sulfur at a high cycling rate was found to buffer the expansion by 48.64 %. Furthermore, the nanocarbon hosts minimize sulfur expansion by restricting the lithiation process through blocking the flow of lithium ions. In Chapter 4, MWCNTs and graphene were massively produced by cotton textiles using simple steps. The obtained cotton-derived Fe3C-encapsulated multiwalled carbon nanotubes (Fe3C-MWCNTs) and graphene were used to construct cathodes and interlayers in Li–S batteries, effectively suppressing lithium dendrite growth. Both experimental observations and MD simulations jointly unveiled a new polysulfide-induced mechanism for lithium dendrite formation, which demonstrated the advantages of Fe3C-MWCNTs and graphene in Li–S batteries. In Chapter 5, cathode powders from spent Li–S batteries were recycled to reduce CO2 emission. Coupled MD simulations and digital image correlation (DIC) analysis unlocked the exfoliation of CNT walls to improve the specific surface area and CO2 adsorption capacity. During cycling, the detachment of polysulfides transformed their kinetic energy into strain energy within the walls of CNTs, facilitating their peeling off.
These findings demonstrate the successful conversion of low-cost biomass into valuable CNTs and graphene using environmentally friendly and straightforward techniques, greatly improving sustainability. Additionally, this dissertation offers valuable strategies for addressing challenges in energy storage systems and provides promising decarbonization applications for spent Li–S batteries, guiding the way to industrial applications of Li–S batteries.
