A Fundamental Understanding of Sulphur Speciation in Ionic Liquids By in-Situ UV-Vis Method
Authors: Pal Urbi , Pozo Gonzalo Cristina , MacFarlane Douglas R , Howlett Patrick C , Forsyth Maria 
Published: February 2020

Ionic liquids (ILs) are liquids at room temperature which consist of a large variety of cation and anion. The dipolar and van der waals interactions between the cationic and anionic species govern the physical properties of ILs and their applications. Since last few decades, the potential application of ILs in lithium based battery has increased rapidly as some of the ILs offer wide electrochemical stability window, lithium redox compatibility, fast ionic diffusivity and low flammability [1]. Among other Li based battery systems, lithium sulphur (Li-S) battery is one of the most promising candidates for the next generation high energy density battery with high theoretical capacity of 1675 mAh/g [2]. However, the electrochemical redox reaction in Li-S battery is a complex phenomenon. The reduction of elemental sulphur (S8) proceeds via the formation of various intermediate (Sn
2- , n>2) polysulphides before reaching the final product S2-. The stability of these polysulphides are highly dependent on the electrolyte medium [3]. Hence the selection of appropriate electrolyte components play an important role in the overall performance of Li-S battery. The electrochemistry of sulphur-polysulphide system has been well studied in organic solvents, however very less literature is reported on the sulphur reduction mechanism in ILs. In this study, we have investigated the sulphur reduction and speciation in three ionic liquids: N-methyl-N-propylpyrrolidinium bis(fluorosulphonyl)imide (Pyr13 FSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide (Pyr13 TFSI) , trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P111i4 FSI). The electrochemical redox reactions of sulphur in these ILs have been studied through cyclic voltammetry (CV) and the polysulphide intermediates are simultaneously investigated by in-situ UV Vis method. These studies showed two reductions and one oxidation peak and the plausible pathways have also been identified. The redox behaviour of sulphur speciation in P111i4 FSI is observed to be very similar to Pyr13 TFSI where glassy carbon is used as working electrode in a three electrode electrochemical cell. On the other hand, the redox process of Pyr13 FSI is found to be distinctively different than Pyr13 TFSI. From the in operando UV-vis study, it is found that Pyr13 FSI electrolyte is responsible for the formation of S3.- radical anion, whereas the Pyr13 TFSI and P111i4FSI electrolyte do not assist the radical formation during the reduction process. Additionally it can also be stated that, in case of P111i4 FSI, the polysulphides are stabilised by small phosphonium cation which protects the FSI anion from the nucleophilic attack of polysulphides, resulting in the stable sulphur redox cycling in P111i4FSI. Thus, based on molecular reactivity, CV and in-situ UV-vis, we propose that a novel phosphonium cation based ionic liquid (P111i4 FSI) is capable of using in Li-S battery. Therefore, this study brings forward a new concept that both of the cation and anion of an IL cumulatively affect the redox behaviour and sulphur speciation in Li-S battery systems.
[1] Hallett, J. P.; Welton, T., Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chemical Reviews 2011, 111 (5), 3508-3576.
[2] Ji, X.; Nazar, L. F., Advances in Li-S batteries. Journal of Materials Chemistry 2010, 20 (44), 9821-9826.
[3] Zhang, S. S., Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions. Journal of Power Sources 2013, 231 (Supplement C), 153-162.