Use of electrolytes with low lithium polysulfide (Li₂S_n) solubility limits has attracted considerable interest in recent literature.¹ This “sparingly solvating” strategy has been adopted by some as a means for significantly reducing redox shuttling of Li₂S_n between the cathode and anode. Interestingly, some sparingly solvating electrolyte compositions, employing significantly high concentrations of LiTFSI, have not only shown promising cycling performance, but have also shown a unique discharge voltage profile—lacking the typically observed two plateau response produced by sequential soluble long chain Li₂S_n and short chain Li₂S_n formation. This unique voltage profile is thought to be the result of significantly suppressed concentrations of longer chain Li₂S_n (n > 4) leading to initial sulfur reduction through a solid state pathway followed by Li₂S_nreduction mediated by a low solubility intermediate.

Predicting Li₂S_n Solubility and Reaction Pathway:

What remains to be elucidated is a more quantitative assessment for the solubility regimes in which the alternative sulfur reduction pathway occurs. Furthermore, a deeper understanding of the electrolyte characteristics that dictate Li₂S_n solubility is lacking. In this talk, a unique ²³Na NMR-derived donor number (DN) scale is presented, similar to that described by Schmeisser et al.² Evaluation of donicity in this manner allows for a direct comparison between electrolytes utilizing different solvating species; see Figure 1 for THF and acetonitrile (AcN)-based electrolytes. It will be demonstrated that the ²³Na-DN assignments are predictive indicators for solubility of Li₂S_n (Figure 2). Finally, it will be shown that solubility can be used to predict the thermodynamic redox potentials for Li-S cells as a function of discharge capacity. Figure 3 shows representative galvanostatic intermittent titration technique (GITT) experiments performed on two different AcN_xLiTFSI electrolytes, with drastically different Li₂S_n solubility limits; the black traces follow the open circuit potentials at given capacities. Clearly, AcN₆LiTFSI shows a classic two plateau response, in which the second plateau (later discharge) is at a lower potential than the early discharge plateau—consistent with higher Li₂S_n solubility. Alternatively, AcN₂LiTFSI shows a mid-discharge plateau higher than that of the potential at early discharge—consistent with a mechanism driven by a lack of solution phase Li₂S_n (n > 4) species. Further details describing how these voltage profiles change as a function of Li₂S_nsolubility will be provided.

References:

1. Cheng, L.; Curtiss, L. A.; Zavadil, K. R.; Gewirth, A. A.; Shao, Y.; Gallagher, K. G. ACS Energy Lett. 2016, 1, 503−509.

1. Schmeisser, M.; Illner, P.; Puchta, R.; Zahl, A.; Eldik, R. Chem. Eur. J. 2012, 18, 10969–10982.­­