Year

2019

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Lithium-sulfur batteries (Li-S) are regarded as a promising candidate for next-generation energy storage systems due to their high specific capacity (1675 mA h g-1) and energy density (2600 Wh kg-1), as well as the abundance, safety, and low cost of their sulfur materials. Nevertheless, many disadvantages still exist that are hindering the further development of Li-S batteries, such as the insulating nature of the active materials, the dissolution of intermediate products, large volume expansion, and safety concerns related to the lithium metal anode. Therefore, tremendous efforts need to be made to overcome these drawbacks, including designing suitable conductive matrices to improve the utilization of active materials, modifying the separator to suppress the dissolution of intermediates, and synthesizing Li2S cathode paired with lithium-free anode to avoid the safety concerns. In this thesis, flexible polypyrrole (PPy) film, S-coated PPy nanofibers cathode, PPy modified separator, uniform PPy-coated S/graphene aerogel, Li2S-PPy composites, and Li2S coated nitrogen-doped carbon nanofibers have been synthesized and applied in Li-S batteries to improve their capacity and cycling stability.

A free-standing sulfur-polypyrrole cathode and a polypyrrole coated separator were designed for flexible Li-S batteries. The free-standing sulfur-polypyrrole cathode was prepared by directly pasting sulfur coated polypyrrole (S@PPy) nanofiber composite on a flexible and conductive polypyrrole (PPy) film. Compared with carbonaceous matrices, PPy has a strong interaction with polysulfides to mitigate their dissolution, due to its unique chain structure and the lone pair electrons in nitrogen atoms in PPy. In addition, the as-prepared PPy film not only shows excellent mechanical elasticity, but also possesses a rough surface, which can accommodate the volume expansion, enhance the adhesion of active materials, and further trap the dissolved polysulfides. Due to the synergistic effects provided by PPy flim, the free-standing sulfur-polypyrrole cathode shows better electrochemical performance than the traditional cathode with S@PPy composite coated on Al foil. In order to further improve the cycling stability of Li-S batteries, a PPy coated separator was prepared, which acts as a fishing net to capture polysulfides and alleviate the shuttle effect, leading to a stable cycling performance. Moreover, the PPy layer coated on commercial separator is much lighter than many other free-standing interlayers reported previously. Considering the flexibility of the free-standing sulfur cathode and the PPy coated separator, a soft-packaged flexible Li-S battery based on them has been designed and fabricated to power a device consisting of 24 light emitting diode (LED) light. After repeated bending, the Li-S battery can still maintain good performance, indicating its excellent mechanical flexibility.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.