Advanced Electrodes and Electrolytes For Long-Lived and High-Energy-Density Lithium-Sulfur Batteries

Abstract

The increasing demand on renewable but intermittent energy and the need for electrified transportation place great emphasis on energy storage. Lithium-sulfur (Li-S) batteries are promising systems due to the high theoretical energy density and natural abundance of sulfur. This thesis presents a thorough investigation on strategies to confine the polysulfides and to build long-lived and high-energy-density Li-S batteries. A series of sulfur host materials and a class of sparingly solvating electrolyte are presented. Chapter 3 presents an approach to confine polysulfides within a cobalt sulfide material, which exhibits both metallic conductivity and high polysulfide adsorptivity. First-principles calculations and X-ray photoelectron spectroscopy studies consistently demonstrate the coupled interaction between the ionic components of cobalt sulfide and lithium polysulfides. The interconnected nanosheets form 3D networks and enable high sulfur loading electrodes with stable cycling. Chapter 4 presents a novel dual-doping strategy on porous carbon for effective binding of polysulfides. The N and S heteroatoms respectively bond with the Li cations and S anions in the polysulfides. The synthesis is based on liquid-crystal driven self-assembly of bio-sustainable cellulose nanocrystals. Chapter 5 reports a light-weight graphitic carbon nitride material that incorporates high concentration of active N-doping sites for polysulfide binding. Excellent long-term cycling performance of the sulfur electrode is achieved with only 0.04% capacity fading per cycle over 1500 cycles. Chapter 6 further reports a comprehensive strategy on coupling a hybrid sulfur host with an in-situ cross-linked binder in order to construct high loading electrodes while using a low electrolyte volume. Alternative stacking of graphitic carbon nitride and graphene offers both Li-N based adsorption for polysulfide and high electronic conductivity. Benefiting from the high elasticity of the cross-linked binder, crack-free high loading electrodes are fabricated at an electrolyte/sulfur ratio of 3.5:1 (µl:mg). Chapter 7 presents a comprehensive study on the ACN2-LiTFSI-TTE electrolyte with sparing solubility for polysulfides at elevated temperature. A quasi-solid state reaction is demonstrated by the distinct Li-S voltage profiles and sulfur/lithiu sulfide phase evolution as probed by operando XRD measurements. This discovery will inspire further studies into modifying the local structure of electrolytes to control the reaction pathways of dissolution-precipitation electrochemistry.