Jianhua Yan

Date of Graduation

2015

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Co-Chair

Ismail Celik

Committee Member

Bingyun Li

Committee Member

Xiaodong Shi

Committee Member

Xueyan Song

Abstract

Energy is one of the biggest challenges that mankind has been faced for many years. The ever-growing demand for energy is coupled with environmental concerns associated with the excessive use of fossil fuels such as coal, oil and gas. The development of new energy storage device with high energy density has unparalleled advantages in terms of easing the energy crisis and decreasing environmental pollution, and fueling the various energy applications. As one of the prospective energy storage systems, rechargeable lithium-sulfur (Li-S) batteries offer possibilities of low cost and high energy density since sulfur offers a high theoretical capacity of 1672 mA h g-1 and a high energy density of 2600 Wh kg-1 with relatively low-cost. However, despite 20 years of intensive efforts and advancements achieved in performance improvements, commercialization of the current liquid type Li-S batteries are still challenged by insufficient cycle life with rapid capacity fades and low practical energy/power densities.;In this dissertation, sulfur reaction mechanisms, challenges and recent developments related to Li-S batteries were first summarized. A major challenge of Li-S chemistry is the multi-step sulfur reaction from elemental sulfur (S8) to Li2S (S8→Li2S 8→Li2S6→Li2S4→Li 2S2→Li2S). During these processes, the long-chain polysulfide species (Li2Sx, 4 ≤ x ≤ 8) can easily dissolve into liquid electrolyte and can migrate back and forth between electrodes. This shuttle effect results in a series of problems like self-discharge and low practical capacity. More severely, these species can react with the Li-anode and make the anode unstable. Another challenge is associated with the insoluble and insulating nature of S8 and Li2S/Li2S 2, which cause high internal resistance and pose major issues for cycle life and power capacity for Li-S batteries. To address these obstacles, low sulfur-loading are always employed in sulfur electrodes, which result in a low practical energy density and inhibit the commercialization of Li-S batteries.;These challenges could be met to a large degree by adopting novel carbon structures with micrometer pores. Sulfurized carbon nanotubes (SCNT) and sulfurized polyanilines (SPANI) with large size pores were successfully synthesized and demonstrated improved performance when used as electrode materials in Li-S batteries. The synthesis method for SCNT involved surface treatment, solvent exchange, and low-temperature treatment. The SCNT composite had a high sulfur content (68 wt.%). The synthesis method for SPANI involved in situ chlorinated substitution and vulcanization reactions, which resulted in a high sulfur content of 65 wt.%. Both SCNT and SPANI could chemically trap polysulfide species from dissolving into liquid electrolytes by forming strong sulfur chemical bonds on SCNT and SPANI surfaces, thereby extended the cycle lifetime of Li-S batteries.;A binder-free porous multilayered sulfur cathode structure containing alternately arranged SCNT and SPANI layers demonstrated synergistic effects of both materials. The multilayered structure with multiple micrometer pores served as high efficiency binders, conductive agents, and 3-D mechanical scaffolds for efficient use of sulfur. In addition, the layered structures formed physical and chemical C-S bond barriers that retarded shuttle effects. A significant improvement not only in the active material utilization but also in capacity retention was observed. To move a step closer for commercialization of Li-S batteries, which required a much higher sulfur loadings and low cost, a cost-effective carbon nanofiber paper (CNP) electrode structure that could offer high sulfur loading (6-10 mg cm-2), and excellent cycling stabilities and rate capabilities was developed. This electrode had a hierarchical structure with micrometer pores conducive to fast mass transportation, and offered a high degree of interconnectivity of the millimeter length CNF to ensure electron conductivity across the entire film. Such a structure not only provided physical barrier to trap polysulfide anions, but also functioned as a current collector for efficient transportation of electrons and Li-ions. This electrode had presented good mass diffusion and electron transportation properties for high energy density Li-S batteries and is ready to be applied in real applications, and this simple and cost-effective procedure is feasible for mass production.;To elucidate the underlying mechanism of capacity fade, a comprehensive study of a binder-free sulfur-multiwalled CNT composite electrode with a commercialized CNF current collector in different quantity of electrolytes was conducted. By examining electrochemical performance, sulfur reaction kinetics, and EIS, the major reason for the rapid capacity fade was related to the formation of thick layers of solid and nonconductive Li2S2/Li 2S films.;Last, a summary was given, and future research recommendations for my research were also discussed.

Download

Share

COinS