Investigation of Lithium Solvated Ionic Liquid for High Performance Li-S Batteries

1:1 equimolar complex electrolytes composed of low-molecular-weight ether (glyme) and Li salt have been reported to exhibit high thermal and electrochemical stability due to strong interactions between the oxygen in the ether and the Li cation (Li salt glyme solvate ionic liquids) [1].  Favorable performances were achieved using 3V-class [LiFePO₄ | Li metal] and 4V-class [LiNi_1/3Mn_1/3Co_1/3O₂ | Li metal] cells [2].  Moreover, Li salt glyme solvate ionic liquids have a weak coordinating ability, which allows for innovative Li-sulfur batteries with a long cycle life (over 400 cycles) by suppressing the dissolution of polysulfide and redox shuttle [3]. More specifically, it is becoming clear that Li salt glyme solvate ionic liquids have important performance benefits besides their safety.  In previous research, we used electrochemical quartz crystal microbalance (EQCM) measurements to show that an observed decrease in local viscosity near the electrode-electrolyte interface is attributable to the concentration distribution of Li salt and the transitional liberation of glyme molecules during the deposition of Li metal [4].  Therefore, to improve the electrochemical stability and polysulfide solubility of Li salt glyme solvate ionic liquids, it is necessary to form long-lived robust complexes between oxygen in the ether electrolyte and Li cations.  So, we propose Li salt glyme solvate ionic liquids that are non-equimolar and that are prepared by adding Li salts to achieve a higher Li salt concentration than that in the original 1:1 equimolar complex of glyme and Li salt.  Moreover, we investigated relationships between sulfur positive electrode (composite material of elemental sulfur and high surface area carbon) and Li-S battery performances.  In the presentation, we will report the relationships between Li-S battery performances and electrolyte / electrode materials.

This study was supported by ALCA project, from JST, Japan.

[1] K. Yoshida, M. Nakamura, Y. Kazue, N. Tachikawa, S. Tsuzuki, S. Seki, K. Dokko, M. Watanabe, J. Am. Chem. Soc., 133, 13121 (2001).

[2] S. Seki, N. Serizawa, K. Takei, K. Dokko, M. Watanabe, J. Power Sources, 243, 323 (2013).

[3] K. Dokko, N. Tachikawa, K. Yamauchi, M. Tsuchiya, A. Yamazaki, E. Takashima, J-W. Park, K. Ueno, S. Seki, N. Serizawa, M. Watanabe, J. Electrochem. Soc., 160, A1304 (2013).

[4] N. Serizawa, S. Seki, K. Takei, H. Miyashiro, K. Yoshida, K. Ueno, N. Tachikawa, K. Dokko, Y. Katayama, M. Watanabe, T. Miura, J. Electrochem. Soc., 160, A1529 (2013).