Strategy and Issue for Li-S Batteries with High Energy Density

As an inexpensive, abundantly available, and environmental-friendly material, sulfur has become one of the most promising positive electrode active materials because of its high theoretical specific capacity (1675 mAh g -1 ). However, lithium sulfur batteries (Li-S Battery) still have some issues for practical application: the electronically insulating nature of sulfur and its discharge products (Li 2 S/Li 2 S 2 ); the dissolution and shuttle effect of lithium polysulfides (LiPSs) in the conventional dioxolane/dimethoxyethane (DOL/DME) electrolyte. In addition, excess amounts of electrolyte that overcompensate for its minimum requirement are necessary for achieving a high specific capacity of sulfur, which severely reduces the energy density of the battery. However, only poor charge/discharge behavior could be obtained when the E/S ratios lower than 5 with the conventional DOL/DME electrolyte. 1 In order to overcome the above problems, we have proposed sparingly solvating electrolytes made with lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and sulfolane (SL), in which LiPSs have limited solubility to inhibit the LiPS shuttling effect. Moreover, to realize high energy density Li-S batteries, it is essential to reduce the E/S ratio (E/S [μL-electrolyte/mg-sulfur]). Here, we fabricated a pouch cell using the sparingly solvating electrolytes and the cyclability with various E/S ratios was studied. The sparingly solvating electrolytes are [Li(SL) 2 ][TFSA] (Li[TFSA] : SL = 1 : 2) in which a poorly solvating diluent, 1,1,2,2,-tetrafluoroethyl-2,2,3,3,-tetrafluoropropyl ether (HFE), was added at different molar ratios. Sulfur cathodes consisted of sulfur and Ketjen black (KB) at a ratio of 3: 1, which are coated on a carbon-coated aluminum foil with carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) binders. Pouch cells, which have laminated structures, were prepared using the sulfur cathode, Li metal anode, and Celgard3501 separator. Constant current charge/discharge tests were carried out at cut-off voltage 1.0-3.3 V and at 30 ℃. There were non-negligible differences in electrochemical performances of Li-S batteries between coin cells and pouch cells. Especially, under lean electrolyte conditions (low E/S), only pouch cells gave reasonable results. A high energy density Li-S cell having higher energy density than 300 Wh/kg was successfully demonstrated. Therefore, fabrication of pouch cell is a necessary step to probe the stability of the e