| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 103 === Nowadays, lithium-ion batteries (LIBs) are extensively applied in numerous portable devices such as smart-phones and laptops. However, current LIBs based on the conventional intercalation mechanism cannot meet the requirements of the electronics industry and electric vehicles. Therefore, it is extremely urgent to seek for systems with a significant reduction in cost and increase in capacity and energy density. Among various promising candidates, lithium–sulfur (Li–S) batteries with a high theoretical capacity are very attractive. This study aims at significantly raising the sulfur content of active material, decorating the electrode material and influence of polymer film in Li-S battery.
Firstly, in order to increase the sulfur content of the active material, the synthesis of the sulfur carbon nanocomposite material was introduced by two different methods which were vacuum heating and anti-solvent heating methods. Except for the advantages that less sulfur particles left outside the pores of carbon, vacuum heating method encountered a limitation of sulfur content in the S-C nanocomposite material, due to the calcination temperature and carbon pore volume. Therefore, high sulfur content nanocomposite material could be synthesized by another method, the anti-solvent heating method. Anti-solvent processes are largely used in the industry, which were based on the polarity of two solvents immiscible to each other. Furthermore, to enhance the cycling stability and rate capability, the surface modified Al foil was applied as the current collector, especially for the long-term cycling at high current density. From the electrochemical performance, particularly the c-rate performance, the obvious differences of the initial reversible capacity and polarization between using the graphite coated Al foil and without coating could be observed. The favorable performance obtained by using the conductive material coated on Al foil demonstrated that graphite was a promising material for enhancing the electrochemical performance at higher current density. Hence, the combination of anti-solvent heating method and graphite coated Al foil was a feasible approach to test the higher sulfur content Li-S batteries at high current density for a long-term cycles.
Based on the previous work, the use of a Nafion-ionomer film in Li-S battery could efficiently confine the polysulfides. Therefore, a novel separator coating with a Nafion polymer film was prepared by dipping that was used in high sulfur content (75 wt.%) Li-S batteries. The S-C nanocomposite of 75wt% sulfur content featuring a Nafion coated separator exhibited an initial capacity of 1060 mAh g-1 at 0.2 C, and the discharge capacity declined slowly, to 650 mAh g-1, after 100 cycles. Even at high c-rate of 1 C, the cell with Nafion-coated separator presented a reversible capacity of 590 mAh g-1 after 200 cycles which was superior than that without Nafion-coated separator. The Nafion-coated separator also improved Coulombic efficiency of high sulfur content Li-S cells at various current densities. The Nafion polymer coated separator displayed a structure of few small and uniform pores that allowed penetration of lithium-ions transmission, meanwhile, it could effectively prevented polysulfide anions transporting in the electrolyte, as well. It is demonstrated from the electrochemical performance that the Nafion-coated separator was quite effective in reducing shuttle effect, enhancing the stability and the reversibility of the electrode.
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