Strategy and Issue for Li-S Batteries with High Energy Density
Authors: Nomura Nao , Li Shanglin , Ugata Yosuke , Tatara Ryoichi , Liu Jiali , Kaneko Tomoaki , Sodeyama Keitaro , Mandai Toshihiko , Ueno Kazuhide , Dokko Kaoru , Watanabe Masayoshi 
Published: December 2020

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 (Li2S/Li2S2); 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 electrode and to evaluate the electrochemical performances under different low E/S ratios. Fig. 1 shows the capacity retention of Li-S cells with various E/S ratios. Although the initial discharge capacity was almost the same, being independent of E/S ratios. However, the cycle stability significantly deteriorated with decreasing E/S. An internal short circuit due to the dendrite formation also occurred relatively early when E/S < 5. On the other hand, when E/S> 5, a high capacity could be maintained, and an internal short circuit hardly occurred even at the relatively high current density. In pouch cells, the electrode area is generally much larger than that in coin cells. Therefore, the absolute magnitude of current is much higher. As a result, if side reactions between electrolyte and Li anode occur and current density in the cell becomes inhomogeneous, the current concentration in the cell will leads to the dendrite growth. Considering the reduction of E/S is urgent for practical high energy density of Li-S battery, it is necessary to suppress the side reactions and dendrite growth on the lithium anode. By using ab initio molecular dynamics simulations and operando electrochemical mass spectroscopy together with cryo-STEM-EELS techniques, the side reactions between the electrolyte and Li anode were explored. How to reduce the side reactions will be discussed in detail in the presentation.

Acknowledgements

This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). We also appreciate Dr. Keisuke Shinoda for his kind support for Cryo-STEM-EELS measurement at National Institute for Materials Science (NIMS) Battery Research Platform.

References

1. M. Hagen et al., J.
Power Sources, 2014, 264, 30-34.

Figure 1