Improved Electrochemical Performances of Mesoporous Carbon-Sulfur Composites with Lithium Polysulfides Added Electrolyte

Lithium-Sulfur batteries have been intensively studied as a power source of the electric transportation and energy storage system for renewable energy due to their high theoretical capacity of 1675 mAh g^-1 and relatively inexpensive raw materials than commercial lithium ion batteries.¹ To introduce lithium sulfur batteries to commercial market, there are some drawbacks to be solved. The major problems of lithium sulfur batteries are low active material utilization, severe capacity decay and low coulombic efficiency. Several successive approaches have been adopted to develop practical lithium sulfur batteries either by selecting electrolyte 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 On sulfur cathode  side, modification of morphology of active materials have been studied by mixing conductive carbons with sulfur particles, encapsulating sulfur into porous carbon matrix and  conductive polymer coating on the surface of active materials. These approaches have led to the development of lithium–sulfur battery with improved cycle life and rate capability.

However these progresses of lithium sulfur batteries have not satisfied requirements of commercialization. These lithium sulfur batteries are still suffered from the problem of the solubility of the polysulfides. Recentely, Tarascon and co-workers⁸ and Zhang and Read reported noticeable achievement by addition soluble lithium polysulfide into the electrolyte.⁷

In this work, we studied the electrochemical and physical effect of the Li₂S₈ polysulfide additive diminishing the dissolution of the sulfur cathode and small participation in redox reaction as a dissolved cathode.^{7, 8} We demonstrated intentionally dissolved Li₂S₈ polysulfides in the electrolyte suppress polysulfide dissolution from the sulfur cathode by controlling its concentration according to Le Chatelier’s principle. Li₂S₈ polysulfide additives was deposited on the cathode as sulfur on charging to compensate eventual capacity losses that result from partial cathode dissolution during discharge.^2–8

Fig. 1 shows the cycling performance and efficiency of a mesoporous carbon-sulfur electrode in a lithium cell using the TEGDME–LiCF₃SO₃ electrolyte with addition of 5 wt% Li₂S₈. The cell cycled at 20 ℃ shows a small initial capacity decay, that is, from 1650 to 1500 mAh g^-1s, followed by a stable trend with an acceptable, minor capacity decay, that is, from 1500 to about 1200 mAh g^-1_Sover 50 cycles, with a coulombic efficiency approaching 95%.

References

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