Preparation of Li-S Polymer Battery Utilizing High Compatibility of Carbonaceous Materials with Highly Concentrated Electrolytes

Abstract

Li-S batteries have received much attention as the next-generation secondary batteries because of their extremely high theoretical energy density. A roll-to-roll fabrication method based on continuous laminating and winding of the sheet-like electrodes and electrolyte without an electrolyte injecting process would enable a more efficient manufacturing process for mass production of Li-S batteries. In order to realize such batteries, it is necessary to prepare the sheet-type sulfur cathode electrode in which an electrolyte is incorporated. We recently reported that a less volatile, highly concentrated lithium salt electrolyte could serve as a good dispersing media of carbon nanotube (CNT).[1] In this study, the same method was applied not only to CNT but also to other carbonaceous materials to obtain sheet sulfur cathode electrode containing highly concentrated electrolytes that serve as the dispersing media. As a result, we fabricated a Li-S polymer battery by combining the sheet sulfur cathode and the polymer gel electrolyte membrane.[2] This is a trail to realize polymer Li-S battery fabricated by the roll-to-roll process.The cathode slurry was prepared by adding a dispersion solvent, NMP (N-methyl-2-pyrrolidone), to the mixture (sulfur, Ketjen black (KB), polymer, and highly concentrated electrolyte: [Li(SL)2][TFSA] (SL : sulfolane, [TFSA] = [N(SO2CF3)2])[3]). This slurry was coated to an Al foil current collector and dried to obtain the gel cathode. The gel electrolyte was prepared from PVDF-HFP (poly(vinylidene fluoride-co-hexafluoropropylene)) and [Li(SL)2][TFSA] by a solvent casting method. The Li-S polymer battery was then prepared using the gel cathode, electrolyte and Li-anode. Figure 1(a) shows a conceptual scheme of roll-to-roll making process for the Li-S polymer battery. By using the sheet-like cathode and gel electrolyte, a battery can be manufactured simply via laminating them with Li-anode. It is worth mentioning that the subsequent electrolyte injecting step can be omitted here because the gel cathode and electrolyte already contain uniformly distributed electrolytes. Figure 1(b) shows the charge-discharge curves of the Li-S polymer battery. The battery delivered the initial discharge capacity of 850 mAh g−1. After 20 cycles, nearly 700 mAh g−1 of capacity could be retained, which indicated reasonable cycling stability. In our previous research,[1] CNT was used as the carbonaceous material for the S gel cathode. However, in this study, we found the amounts of electrolyte could be greatly reduced by using KB. It is considered that CNT requires more electrolyte to disperse the aggregated bundles and to unbundle them by cation-π interaction. Hence, our results would indicate a great potential on designing the sulfur gel cathode by a very simple method with lower cost materials. The detailed electrochemical behavior of the Li-S polymer batteries will be discussed in the presentation. Further, studies on the effects of composition and thickness of the S gel cathode and gel electrolyte on the electrochemical behavior will be presented.