Study of fluorinated ether-based electrolyte for high-voltage lithium ion batteries and functional porous sulfur cathode for lithium sulfur batteries

Restricted (Penn State Only)
Shi, Pei
Graduate Program:
Chemical Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 27, 2020
Committee Members:
  • Donghai Wang, Thesis Advisor/Co-Advisor
  • Phillip E Savage, Program Head/Chair
  • Seong Han Kim, Committee Member
  • Costas D Maranas, Committee Member
  • Lithium ion battery
  • electrolyte
  • Lithium sulfur battery
  • Porous cathode
  • High voltage electrolyte
Electrochemical energy storage devices with a high energy density are an important technology in modern society, especially for electric vehicles. And the demand for lithium batteries with high energy density is more urgent than ever before. The energy density of lithium batteries is determined mainly by the specific capacities and operating voltages of the cathode and the anode (capacity × voltage), to date, substantial research efforts have focused on the design and optimization of novel positive electrode materials with high operating voltage (e.g., ≥ 4 V vs. Li/Li+) and/or a large capacity (e.g., ≥ 200 mAh g-1). However, for the high operating voltage positive electrode, such as LiCoPO4 and LiNi0.5Mn1.5O4, the battery performance fades rapidly at such potentials (≥ 4.5 V vs. Li/Li+) due to limited oxidation stability of the conventional carbonate electrolytes and the parasitic reaction on the electrode surface. Thus, an electrolyte with high oxidation stability is required. For the high specific capacity positive electrode, sulfur is one of the most promising active materials because of its high theoretical capacity (1,675 mAh g−1), while achieving high energy density and long cycling life of lithium-sulfur (Li-S) batteries need a careful design of advanced sulfur electrodes to resolve self-discharge issue while keeping high sulfur mass loading and low electrolyte/sulfur (E/S) ratio. In this thesis, electrolyte with high oxidation stability and a functional sulfur cathode are studied separately to achieve the high energy density lithium batteries. In chapter 2, a new safe electrolyte was proposed for high voltage cathode by mixing tetramethylene sulfone (TMS) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE). The addition of F-EPE endows this new electrolyte with a high lash point (214 oC) and good wettability to separator and electrodes. And the new electrolyte endows the lithium ion battery with a much better cycling performance than the conventional electrolyte. In chapter 3, we develop a functional porous sponge sulfur (FSS) electrode using cross-linked polyethylenimine (PEI) as three-dimensional (3D) porous framework to allow high sulfur mass loading and low E/S ratio. Strong Li-ion affinity not only endows the FSS electrodes with Li polysulfides (LiPS) chemisorption, but also generates electroosmosis in porous electrodes to dynamically suppress the LiPS dissolution, providing a property of self-curing the self-discharge. Li-S batteries using FSS electrodes as cathodes deliver a high energy density with a stable cyclability at the conditions of high S mass loading and low E/S ratio. Finally, we concluded this thesis work in Chapter 4 and briefly discussed the possible future work.