Energy is essential in modern society, however ensuring that it is both affordable and environmentally benign presents a major challenge. Electrical energy storage is important in overcoming these issues; grid energy storage and electric cars being some of the primary uses. Lithium-sulfur (Li-S) batteries are one of the most promising candidates in next generation electrical energy storage, possessing a theoretical specific energy of 2600 W h kg^-1.¹ Several other advantages of using sulfur as the positive electrode include its abundance, low cost and non-toxicity. The complete reduction of sulfur (S₈) to lithium sulfide (Li₂S) occurs via a multistep reaction pathway, which has been the subject of much debate. Unlike the lithium-ion (Li-ion) battery, the discharge/charge reactions do not occur in the solid state but rather electron transfer occurs between the carbon positive electrode interface and the various sulfur species dissolved in the electrolyte. Due to the high solubility of the intermediate polysulfides species, several technical challenges exist that have hindered Li-S battery commercialization, including the widely reported shuttle mechanism.² In addition to their high solubility, the intermediate polysulfide species are only stable in solution, sensitive to moisture and are known to disproportionate/comproportionate to a variety of polysulfides that change further on dilution.³ In order to overcome these issues, a better understanding of the polysulfide composition in the electrolyte, their concentration and the number of phases present is required.

We have developed two techniques, one which is able to determine the concentration of dissolved polysulfides in terms of the total sulfur content [S] and the other is able to determine the average oxidation state of the sulfur species, between S⁰ (as S₈) and S^2- (as Li₂S). To determine the concentration of sulfur species present in the electrolyte, gravimetric analysis has been performed exploiting the low solubility of barium sulfate (BaSO₄). To determine the average oxidation state of sulfur species, a redox titration has been developed. Using these methods and the information obtained, it is possible to develop a ternary phase diagram between S, Li₂S and the electrolyte. This can be used to predict phase boundaries and determine the appearance of Li-S discharge profiles. The figure shows the proposed ternary phase diagram with the phases and degrees of freedom stated as determined using Gibbs’ phase rule: Freedom = Components – Phases, at fixed temperature and pressure.

[1] P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J. M. Tarascon, Nat. Mater., 2012, 11, 19-29.

[2] S. M. Al-Mahmoud, J. W. Dibden, J. R. Owen, G. Denuault and N. Garcia-Araez, J. Power Sources, 2016, 306, 323-328.

[3] D. Zheng, X. R. Zhang, C. Li, M. E. McKinnon, R. G. Sadok, D. Y. Qu, X. Q. Yu, H. S. Lee, X. Q. Yang and D. Y. Qu, J. Electrochem. Soc., 2015, 162, A203-A206.