Electrochemical and Analytical Characterizations of Li-S Batteries
Authors: Mastouri Afef , Guery Claude , Morcrette Mathieu 
Published: February 2020

During the last decades more attention has been paid to developing new batteries based on the lithium-sulphur electrochemical system, having high theoretical specific energy (2600 Wh kg-1). However, lithium-sulphur batteries are still far from reaching the market place due to several limitations.  Among them are: (i) the use of a Li metal anode with liquid electrolyte, (ii) the insulating character of sulphur and the ending product Li2S and (iii) the soluble polysulphide species generated during the battery operation, which diffuses throughout the separator and deposits on the Li side, resulting in a loss of active material.

Despite numerous reports on the mechanism of polysulfide formation the direct information on the magnitude of polysulfide shuttle (diffusion coefficients, solubility) is still missing. Thus reliable quantification of dissolved S in the electrolyte is a necessity to investigate the role of soluble S species in the S redox reaction. This might of particular interest for theoretical help in the search for the optimized composition of the battery components. Thus within these studies we focused on the development of the new simple analytical techniques/procedures that could give such information.

Firstly, the electrochemistry of elemental sulphur S8 and synthesized polysulfides (Li2S4, Li2S6 and Li2S8) has been studied for the first time in the non-aqueous electrolyte 1M LiTFSIa in 1:1 (v/v) TEGDME/Dioxolaneb and in the ionic liquid electrolytes (ILs) such as 1M LiTFSI in DEME TFSIc, 1M LiTFSI  in EMIm TFSId  and 1M LiTFSI  in BMP TFSIe. The cyclic voltammetry of S8 in the electrolytes used and mentioned above is different to the behavior reported in some organic solvents (such as THF, DMF, DMSO, or CH3CN), with two reduction and one oxidation peaks obtained. The cyclic voltammograms (CVs) in different liquids were obtained at constant temperature (25 °C) using a PC controlled Autolab PGSTAT30 potentiostat/galvanostat and a three-electrode cell.

UV_Vis spectroscopy and spectro-electrochemical studies have been carried out to help the identification of the various sulphur species formed in the solutions. The main reduction products of S8 in the electrolytes used have been identified as S6
2- and S4
2- and plausible pathways for the formation of these species are proposed. The most important results of this study will be presented in more details. This study has allowed us to understand the solid sulphur-electrolyte interaction (S solubility in electrolyte, Kinetic of dissolution).

Secondly, we will present our last results using HPLC - UV Visible technique to show how we were able to determine the solubility limit of sulphur, Li2S and polysulfides in different non-aqueous electrolytes for Li-S batteries. By using high performance liquid chromatography with a UV detector, the solubility of S in the different pure solvents and in the different electrolytes mentioned above, was determined. In addition, the S content in the electrolyte recovered from a discharged Li-S battery was successfully determined by the proposed HPLC/UV method. Thus, the feasibility of the method to the online analysis for a Li-S battery is demonstrated. It’s worth to emphasize that the S was found super-saturated in the electrolyte recovered from a discharged Li-S cell.

Finally, we will present the results obtained for cycling using coin cells. In order to understand the influence of dissolved sulphur on cycling properties, we were interested in the study of the influence of the nature and the amount of electrolyte on the OCP self discharge. In all cases, the cycling of batteries was done in GCPL mode.

In conclusion, these results are crucial for applications since they demonstrate the important effect of the electrolyte versus sulphur ratio for the good functioning of Li/S batteries.

Abbreviations

a Bis(trifluoromethane)sulfonimide lithium

b Tetraethylene glycol dimethyl ether/1,3-Dioxalane

c N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium)

d Ethyl Méthyl Imidazolium Tétra fluorosulfonyl imide

e 1-Butyl-1-Methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide

Keywords: Li-S battery, Electrochemistry, S8, Polysulfides, Ionic liquid, HPLC-UV, Self discharge.