Development of Cathode Materials and Electrolytes for High-energy Lithium-sulfur Batteries

Open Access
Author:
Chen, Shuru
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 03, 2015
Committee Members:
  • Donghai Wang, Dissertation Advisor
  • Chao Yang Wang, Committee Member
  • Md Amanul Haque, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • Lithium-Sulfur
  • Batteries
  • Electrolyte
  • Cathode
  • Dimethyl Disulfide
  • Silicon anode
Abstract:
Rechargeable lithium-sulfur (Li-S) batteries have attracted great attention because they promise an energy density 3-5 times higher than that of current state-of-the-art lithium ion batteries at lower cost. However, current Li-S frequently suffer from low practical energy density, poor cycle life, low efficiency, and high self-discharge. Those issues mainly stem from the poor conductivity of sulfur and its lithiated products, the dissolution and side-reactions of intermediate lithium polysulfides, and the unstable lithium-electrolyte interface. This dissertation focuses on development of high-sulfur-fraction carbon/sulfur composite cathode materials and efficient electrolyte systems for Li-S batteries, aiming to improve both their practical energy densities and electrochemical performance. In Chapter 3, hollow carbon (HC) spheres with extremely high specific volume (>10 cm3 g-1) are shown to accommodate ultrahigh sulfur fraction (~90 wt%) in their nano-scale pores. The obtained HC/S composites enable high areal sulfur loading of up to 6.9 mg cm-2 in the cathode electrode using industry-adopted coating techniques. In addition, a new hydrofluoroether-based electrolyte is shown to significantly mitigate polysulfide dissolution and also to facilitate the electrochemical reactions of sulfur cathodes. Combined with this new electrolyte, the high-sulfur-fraction and high-areal-loading HC/S composite cathode can achieve exceptional performance, which can significantly improve both the cyclability and the practical energy density of the Li-S batteries. In chapter 4, substituting soluble Li polysulfides for conventional Li salts in the commonly used Li-S electrolyte is found to not only contribute extra capacity but also significantly improve the cycling performance of Li-S cells. In chapter 5, a new functional electrolyte system using electrochemically active organosulfides (e.g., dimethyl disulfide) as co-solvents is shown to reduce the required electrolyte amount while at the same time increasing cell capacity. The organosulfides lead to a new reaction pathway for sulfur cathodes, which involves the chemical reactions between organosulfides and sulfur to new intermediate organopolysulfides, followed by their subsequent electrochemical reactions during cell cycling. Through this new mechanism, the functional organosulfide electrolyte not only contributes a significant amount of capacity, but also enables good cathode cyclability by way of an automatic discharge shutoff mechanism. This new functional electrolyte system thus promises high energy density for Li-S batteries. In the appendix, the development of silicon-carbon yolk-shell nanocomposite materials is discussed. These high-performance silicon anode materials can potentially be used to replace the Li anode, which in the long term can improve the cycle life and safety of Li-S batteries.