Li-Sulfur (LiS) batteries have 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 a severely decreased practical energy density and poor cyclability. A major problem is the relatively large solubility of polysulfides in the standard electrolytes, which results in loss of active material and capacity fading. In addition, the dissolved polysulphides cause degradation of the anode through shuttle reactions when they migrate through the electrolyte, forming insoluble layers on the anode surface.

Ionic liquids have been highlighted as a candidate for new and safe electrolytes for high capacity battery technologies [1]. The attractiveness arises from the intrinsic properties such as high ionic conductivity, low vapour pressure, and high thermal, chemical and electrochemical stability. For LiS-technology there is also an additional interest to explore the interplay between polysulfides and ionic liquids, with the possibility to tune the dissolution and side reactions.

In this contribution we present results on the use of ionic liquids as base for electrolytes in LiS-batteries and relating their physical properties to the functionality in the battery application. We have investigated electrolytes based on neat ionic liquids [2] and mixed electrolytes where organic solvents are added to the ionic liquid electrolyte [3,4]. A key feature of ionic liquids when considering them for LiS-batteries is the, in general, low solubility of sulfur and polysulfide species. This limits the dissolution of active material from the cathode and results in stable cycling, i.e. limited capacity fading [2]. There is also a second beneficial effect from the fact that a stable solid electrolyte interphase, with a relatively low interface resistance, is formed on Li-metal by the combined presence of the ionic liquid electrolyte and polysulphides. This stable interphase passivates the Li-metal anode and is a second reason for stable cycling of LiS-cells with Li-metal anodes.

Even though stable cycling of LiS-cells is a positive feature the application of ionic liquid electrolytes has a serious drawback in a limited capacity compared to cells with conventional electrolytes based on organic solvents. One of the reasons behind this is the relatively high viscosity and low Li-ion conductivity of ionic liquid electrolytes. A viable strategy to mitigate this shortcoming is to add a small amount of organic solvent to the ionic liquid electrolyte. We show that the addition of 10% of organic solvent can more than double the capacity of a LiS-cell, compared to the capacity of a cell utilising a neat ionic liquid electrolyte, while retaining stable cycling [3]. This improvement in performance can be related to an overall decrease in viscosity of the mixed electrolyte, but also to a change of the local environment, i.e. the coordination, of the Li-ions [3]. There is a strong tendency for a preferred coordination of the Li-ions to organic solvent molecules rather than to the ion ionic liquids anion in mixed electrolytes [4]. In addition, the mixed electrolytes present a strong enhancement of the low temperature ionic conductivity due to the inhibition of crystallization, an effect which most often limits the application of conventional electrolytes, i.e. based on organic solvents, as well as for neat ionic liquid electrolytes. We assign this effect to the presence of several different local coordinating structures in the mixed electrolytes [4].

References

[1] A. Matic and B. Scrosati, MRS Bulletin 2103, 38, 533

[2] S. Xiong, K. Xie, E. Blomberg, P. Jacobsson, and A. Matic, J. Pow. Souc. 252, 150 (2014)

[3] S. Xiong, J. Scheers, L. Aguilera, D-H. Lim, P. Jacobsson, and A. Matic, RSC Advances 2015, 5, 2122

[4] L. Aguilera, J. Scheers, and A. Matic, Phys. Chem. Chem. Phys. 18, 25458 (2016)