Invited Presentation: Ionic Liquid Based Electrolytes for Li-Sulfur Batteries

Li-Sulfur (LiS) technology has great potential for large-scale energy storage applications due to the very high theoretical capacity. However, the implementation of the concept has been limited due to low sulphur utilization, severely decreasing the practical energy density, and poor cyclability in terms of fading capacity and rate capability. One major cause of these problems is the relatively large solubility of polysulfides (Li₂S_n, n≥3) in the commonly used electrolyte systems. A direct consequence is the loss of active cathode material decreasing the capacity and at the same time increasing the resistance of the electrolyte. The dissolved polysulphides also cause degradation of the anode through shuttle reactions when they migrate in the electrolyte, forming insoluble layers on the anode surface.

The materials problems in the LiS battery can be addressed by new concepts for both the electrolyte and the cathode. For development of the electrolyte promising approaches include moving away from traditional organic solvents, improving both performance and safety. In addition the use of polymer gel-polymer, or solid-polymer, electrolytes or ceramic electrolytes has also been proposed to prevent, in particular, the shuttle reactions.

Ionic liquids have been highlighted as base for new and safe electrolytes [1]. Ionic liquids are low melting salts with intrinsic properties such as high ionic conductivity, low vapour pressure, and high thermal, chemical and electrochemical stability. Thus, they have the potential to both improve the performance and the safety of Li-ion and LiS-batteries. Ionic liquids can be used both to fully replace organic solvents as electrolyte solution or as co-solvents/additives improving both safety and performance. An ionic liquid-based electrolyte solution can also readily be incorporated in gel-type polymer membranes [2].

In this contribution we present new results on the use of ionic liquids as base for electrolytes in LiS-batteries. We have investigated systems based purely on ionic liquids, mixed electrolytes, where ionic liquids partially replace an organic solvent, and the recently proposed “solvated ionic liquids” [3]. In particular we focus on the dissolution and speciation of polysulfides in the electrolyte solution. One key issue is the role of polysulfides present in the electrolyte for the formation and stability of surface films on Li-anodes [4]. We also report results of the compatibility of the electrolytes with new nanostructured and functionalized sulphur carbon composite electrodes.

[1] A. Matic and B. Scrosati, MRS Bulletin38, 533 (2013)

[2] J. Pitawala, M. Assunta Navarra, B. Scrosati, P.  Jacobsson and A. Matic

Journal of Power Sources, 245, 830-835 (2013)

[3] K. Ueno, K. Yoshida, M. Tsuchiya, N. Tachikawa, K. Dokko, and M. Watanabe, J. Phys. Chem. B 116, 11323 (2012)

[4] S. Xiong, K. Xie, E. Blomberg, P. Jacobsson, and Aleksandar Matic

Journal of Power Sources, 252, 150 (2014)