Smith, Taylor William (2015-12). Effects of Electrolyte Composition and Polysulfide Species on the Reactivity of Lithium Anodes in Lithium-Sulfur Batteries. Master's Thesis. Thesis uri icon

abstract

  • In attempting to develop energy storage systems possessing superior properties to traditional lithium ion batteries (LIBs), numerous alternative chemistries have undergone study and development. Of these, the lithium-sulfur battery seems one of the more promising contenders for replacing LIBs, particularly for applications like electric vehicles. Nevertheless, a variety of limitations have prevented lithium-sulfur battery introduction to the marketplace, in spite of almost fifty years of research, including polysulfide shuttle, reactivity of the electrolyte, and anodic microstructure evolution. This thesis will explore the use of first-principles computational techniques in understanding the impact of electrolyte composition, polysulfide molecules, and lithium crystal structure on the reactions taking place near the lithium anode in order to better address the problems facing lithium-sulfur batteries. Using ab initio molecular dynamics simulations (AIMD), in conjunction with static density functional theory (DFT) optimizations, Bader charge analysis, and additional analytical techniques, the interactions and impacts of the different components of the typical lithium-sulfur battery can be examined on a molecular basis. It is the author's hope that a better theoretical understanding of how these species behave will enable design and implementation of real-world lithium-sulfur systems capable of meeting and overcoming the difficulties facing their commercialization. In order to test the effects of lithium crystal structure on electrolyte stability and surface morphology evolution, both a (100) and (110) lithium metal surface were created and tested using AIMD simulations. There was a minimal difference in the results for each structure, in both the surface morphology and ratio of solvent molecules reduced by the lithium. In testing the effects and stability of various solvents, it was found that ethylene carbonate reduced readily, while dioxolane, dimethoxyethane, and fluorinated ether molecules were quite stable in the presence of the anode. AIMD simulations of polysulfide molecules in the vicinity of the lithium surface show high reactivity, as seen experimentally, and subsequent DFT calculations indicate the reduction of long-chain polysulfide molecules in the presence of Li atoms is a thermodynamically favorable reaction pathway. Finally, it was observed that high molarity salt systems have properties capable of improving cell performance.
  • In attempting to develop energy storage systems possessing superior properties to traditional lithium ion batteries (LIBs), numerous alternative chemistries have undergone study and development. Of these, the lithium-sulfur battery seems one of the more promising contenders for replacing LIBs, particularly for applications like electric vehicles. Nevertheless, a variety of limitations have prevented lithium-sulfur battery introduction to the marketplace, in spite of almost fifty years of research, including polysulfide shuttle, reactivity of the electrolyte, and anodic microstructure evolution. This thesis will explore the use of first-principles computational techniques in understanding the impact of electrolyte composition, polysulfide molecules, and lithium crystal structure on the reactions taking place near the lithium anode in order to better address the problems facing lithium-sulfur batteries. Using ab initio molecular dynamics simulations (AIMD), in conjunction with static density functional theory (DFT) optimizations, Bader charge analysis, and additional analytical techniques, the interactions and impacts of the different components of the typical lithium-sulfur battery can be examined on a molecular basis. It is the author's hope that a better theoretical understanding of how these species behave will enable design and implementation of real-world lithium-sulfur systems capable of meeting and overcoming the difficulties facing their commercialization.

    In order to test the effects of lithium crystal structure on electrolyte stability and surface morphology evolution, both a (100) and (110) lithium metal surface were created and tested using AIMD simulations. There was a minimal difference in the results for each structure, in both the surface morphology and ratio of solvent molecules reduced by the lithium. In testing the effects and stability of various solvents, it was found that ethylene carbonate reduced readily, while dioxolane, dimethoxyethane, and fluorinated ether molecules were quite stable in the presence of the anode. AIMD simulations of polysulfide molecules in the vicinity of the lithium surface show high reactivity, as seen experimentally, and subsequent DFT calculations indicate the reduction of long-chain polysulfide molecules in the presence of Li atoms is a thermodynamically favorable reaction pathway. Finally, it was observed that high molarity salt systems have properties capable of improving cell performance.

publication date

  • December 2015