Compatibility of Polymer Binders with Solvate Ionic Liquid Electrolytes in Lithium-Sulfur Batteries
Authors: Dokko Kaoru , Nakazawa Toshitada , Ikoma Ai , Kido Ryosuke , Kitazawa Yuzo , Ueno Kazuhide , Watanabe Masayoshi 
Published: February 2020

The molten complex of tetraglyme (G4) and Li[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide), [Li(G4)][TFSA], is a solvate ionic liquid (SIL) composed of complex [Li(G4)]+ cation and [TFSA] anion.1 We reported that stable operation of Li-S batteries can be achieved using [Li(G4)][TFSA] electrolyte.2 The solubility of lithium polysulfides (Li2S
m
, 2 ≤ m≤ 8), which are reaction intermediates of the S cathode, is suppressed in the SIL, and the redox shuttle effect in the Li-S cell is prohibited, leading to the high coulombic efficiency of discharge/charge and long cycle life of the cell. In this study, we focus on the effects of the compatibility of polymer binders with SIL electrolytes on the electrochemical reactions of the S cathode. The S cathode is composed of sulfur, a conductive agent, and a polymer binder, and the composite cathode has a porous structure. In the porous composite cathode, the surface of active material is partially covered with the binder. Here we report that the electrochemical reaction mechanism, discharge capacity, and rate capability of S cathode change depending on the compatibility of polymer binder with the SIL electrolyte.
[Li(G4)][TFSA] electrolyte was prepared by mixing Li[TFSA] and G4 in 1:1 molar ratio in an Ar-filled glove box. The ionic conductivity of [Li(G4)][TFSA] is 1.6 mS/cm at 30 °C. The S composite cathode was composed of elemental sulfur, carbon powder (Ketjenblack, specific surface area of 1270 m2/g), and a polymer. Poly(vinyl alcohol) (PVA-x) with different degrees of saponification (x%) were used as polymer binders.The mass ratio of sulfur, carbon, and polymer in the composite cathode was 60:30:10.
The electrolyte uptake of PVA-x film was estimated. PVA-100 was negligibly swollen with [Li(G4)][TFSA] electrolyte. The electrolyte uptake of PVA-x increased on decreasing the degree of saponification. The weights of PVA-80 and PVA-66 films increased by 8 and 17%, respectively, owing to electrolyte uptake. [Li(G4)][TFSA] electrolyte was incorporated into the acetate moiety of the PVA-x polymer chain, and the PVA-x film was partially swollen with the electrolyte.3

Figure 1 shows the dependence of discharge capacity of Li-S cells on current density. As decreasing the saponification degree of PVA, the discharge capacities increased. The compatibility of binder also affects the charge-discharge rate capability of Li-S cells. The partial swelling of binder in the electrolyte facilitated Li+ ion conduction in the S composite cathode. This results in the high utilization of active material of cathode and the higher rate capability of the cell. 3

Acknowledgements

This study was supported in part by Specially Promoted Research for Innovative Next Generation Batteries (SPRING) of the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST), and JSPS KAKENHI (No. 15H03874 and No. 15K13815) from the Japan Society for the Promotion of Science (JSPS).

References

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K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013).

T. Nakazawa, J. Power Sources, 307, 746 (2016).

Figure 1