Suppression of Shuttle Phenomenon of Li-S Batteries Using Cation Exchange Membrane Separator with Polysulfide-Containing Solution

Abstract

Introduction Sulfur is the most promising candidate of positive active material for next-generation batteries with high energy density because of high theoretical specific capacity (1675 mAh g-1). However, this material has serious problems of the capacity fading and low coulombic efficiency in case of using electrolyte solution. These are derived from the active mass loss and the polysulfide shuttle due to dissolution of polysulfide intermediate into the electrolyte solution and the migration of its polysulfide with redox reaction between positive and negative electrode. In order to solve these problems, methods to suppress the polysulfide dissolution by applying polysulfide-containing solution to compensate for active mass loss1 and the shuttle by using cation exchange membrane as a separator which has the potential to transfer Li+ ion and inhibit the migration2 have been reported. In this study, we developed Li-S battery by using both polysulfide-containing solution in only positive electrode and cation exchange membrane as a separator to suppress the active mass loss and shuttle. Experimental The cation exchange membrane as a separator was using Nafion membrane (Nafion® NRE-212, Dupont). The Li+ ion exchange for the membrane was carried out by immersing it in a solution of 1.0 M LiOH / H2O : ethanol = 50 : 50 (vol%) at 80oC for 12 h. After the procedure, the membrane was swollen by 1,3-Dioxolane (DOL) : 1,2-Dimethoxyethane (DME) = 50 : 50 (vol%) (DOL-DME). Polysulfide-containing solution of 0.5 M Li2S6 in DOL-DME solution was prepared by dissolving stoichiometric amounts of Li2S and elemental sulfur. The positive electrode was fabricated with sulfur-porous carbon composite (SPC3, sulfur : porous carbon = 70 : 30 (mass%)), acetylene black and PVdF binder in weight ratio of 85 : 10 : 5. The Li-S test cell was assembled by stacking in turn lithium metal as the negative electrode, Nafion separator and the positive electrode with 1.0 M lithium bis (trifluoro methane sulfonyl) imide (LiTFSI) in DOL-DME electrolyte solution or 0.5 M Li2S6 in DOL-DME solution. The charge-discharge tests were carried out at a constant current of 0.1 C (167.5 mA g-1-sulfur) in the voltage range of 1.0 to 3.0 V or capacity limitation of 1170 mAh g-1calculated from theoretical capacity of sulfur and its content in the composite. Results and discussion The charge-discharge cycle performances of Li-S test cells with Nafion separator and conventional polyethylene (PE) separator were shown in Fig. 1, respectively. Although the each degree of capacity fading for test cells was almost same level, coulombic efficiency of the cell with Nafion separator was maintaining approximately 100% and higher than that of PE separator. These results were derived from the fact that polysulfide dissolves into electrolyte solution during charge-discharge process resulting in active mass loss, but the polysulfide migration to the negative electrode was effectively suppressed only in case of Nafion separator. Furthermore, it turned out that the capacity at the first discharge of Li-S test cell with Nafion separator was lower than that of PE separator. This low capacity is caused by high Li+ ion transfer resistance at the interface between electrolyte solution and Nafion separator judging from the result of electrochemical impedance spectroscopy measurement that its resistance reduced by surface roughening treatment for Nafion separator due to increasing of specific surface area resulting in the high frequency of Li+ ion conduction. This high resistance was thought to bring the non-uniformizing current distribution in the positive electrode resulting in decrease of utilization of sulfur as a positive material. The charge-discharge cycle performances and profiles of Li-S test cell with both Nafion separator and polysulfide-containing solution were shown together with that of PE separator in Fig. 2 and 3, respectively. The test cell with Nafion separator found out to have high and more stable capacity corresponding capacity limited value of 1170 mAh g-1 without rapid voltage drop indicating Li2S formation at the end of discharge and show higher coulombic efficiency. This high and stable capacity was derived from that polysulfide-containing solution not only suppressed polysulfide dissolution from the positive electrode but also contributed to electrochemical reaction as an active material. Furthermore, the high efficiency was caused by inhibition of polysulfide migration by Nafion separator. References 1) Shuru Chen et al., The Royal Society of Chemistry, 3, 3540 (2013). 2) Zhaoqing Jin et al., Journal of Power Sources, 218, 163 (2012). 3) Heisuke Nishikawa et al., GS Yuasa Technical Report, 12 (1), 9 (2015). Figure 1