Effect of Lithium Polysulfide As Electrolyte Additive in Lithium Sulfur Batteries

Recently, the lithium–sulfur battery is considered as a high-energy-density storage and power device for sustainable electric vehicles (EVs) due to its high theoretical energy density and natural abundanc.¹ However, there are some drawbacks to introduce this battery into commercial devices. Some progress has been achieved on the electrolyte side, either by selecting systems more adequate than conventional carbonate solutions, such as 1,3 dioxolane (DOL), dimethyl ether (DME), and tetraethylene glycol dimethyl ether (TEGDME), or by the addition of soluble polysulfides.^2–7 Recently, Tarascon and co-workers⁸demonstrated that the use of sulfur in contact with the lithium negative electrode may be beneficial for improving the life of lithium–sulfur cells. Progress has also been achieved on the electrode side, mainly by improving its morphology. These strategies have led to an optimization of the electrode morphology and to the development of lithium–sulfur battery prototypes with improved cycle life and rate capability.

However these progresses have not yet placed the lithium–sulfur battery at the level of real practical interest. In particular, the problem of the solubility of the polysulfides has not yet been completely solved. Herein, we show that this issue may be favorably addressed by following the approach originally reported by Tarascon and co-workers⁸ and Zhang and Read⁷; they achieved an improvement by replacing the additives Li₂S₅ and Li₂S₉, respectively, with the soluble Li₂S₈ polysulfide as well as by using a TEGDME₄–LiCF₃SO₃solution.

The aim of this work was principally to use the electrochemical and physical effect of the polysulfide in buffering the dissolution of the sulfur cathode and only marginally its action as dissolved cathode.^{7, 8} Hence, we demonstrate that the added Li₂S₈ polysulfide provides a mass buffering effect, which is controlled by its concentration, according to Le Chatelier’s principle; this reduces the cathode dissolution during the cycling of the lithium–sulfur cell. In addition, the dissolved Li₂S₈ polysulfide is deposited on the cathode as sulfur on charging to compensate eventual capacity losses that result from partial cathode dissolution during discharge; this is due to an “electrochemical” buffering effect resulting from the fact that the added Li₂S₈ partially acts as a dissolved cathode, that is, as a “catholyte”.^2–8

Fig. 1 illustrates the cyclic voltammograms of a carbon electrode in a lithium cell using the TEGDME–LiCF₃SO₃ electrolyte with addition of 5% w/w Li₂S₈. The CV response is typical of a medium in which a redox couple is dissolved [here, the Li₂S₈/Li₂S_x (1<x<8) couple]. The voltammogram shows a first cycle that is characterized by low peak intensity (red curve) due to the initial Li₂S₈deposition–dissolution process that facilitates and enhances the same process in the following cycles (blue curves).

Figure 1. First (red) and following (blue) cyclic voltammograms of a carbon electrode in a lithium cell using the TEGDME–LiCF₃SO₃ electrolyte with addition of 5% w/w Li₂S₈. Scan rate 0.2 mVs^-1, potential limits 1–3 V versus. Li.

References

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