Abstract
An upgrade of the scalable fabrication of high-performance sulfur-carbon cathodes is essential for the widespread commercialization of this technology. Herein we present a simple, cost-effective and scalable approach for the fabrication of cathodes comprising sulfur and high-surface area, N,S-codoped carbons. The method involves the use of a sulfur salt, i.e. sodium thiosulfate, as activating agent, sulfur precursor and S-dopant, and polypyrrole as carbon precursor and N-dopant. In this way, the production of the porous host and the incorporation of sulfur are combined in the same procedure. The porous hosts thus produced have BET surface areas in excess of 2000 m2 g−1, a micro-mesoporous structure, as well as sulfur and nitrogen contents of 5–6 wt% and ~2 wt%, respectively. The elemental sulfur content in the composites can be precisely modulated in the range of 24 to ca. 90 wt% by controlling the amount of sodium thiosulfate used. Remarkably, these porous carbons are able to accommodate up to 80 wt% sulfur exclusively within their porosity. When analyzed in lithium-sulfur batteries, these sulfur-carbon composites show high specific capacities of 1100 mAh g−1 at a low C-rate of 0.1 C and above 500 mAh g−1 at a high rate of 2 C for sulfur contents in the range of 50–80 wt%. Remarkably, the composites with 51–65 wt% S can still provide above 400 mAh g−1 at an ultra-fast rate of 4 C (where a charge and discharge cycle takes only ten minutes). The good rate capability and sulfur utilization was additionally assessed for cathodes with a high sulfur content (65–74%) and a high sulfur loading (>5 mg cm−2). In addition, cathodes of 4 mg cm−2 successfully cycled for 260 cycles at 0.2 C showed only a low loss of 0.12%/cycle.
Highlights
In the quest for high energy/high power density energy storage systems capable of powering electric vehicles, Lithium-Sulfur batteries have emerged as one of the most promising alternatives
Li et al demonstrated the in situ synthesis of sulfur nanoparticles in 3D porous carbon by the thermal treatment of a mixture of glucose as carbon precursor, NaCl as template and Na2S as template and sulfur precursor[23]
Sodium thiosulfate is used as both activating agent and sulfur precursor, which makes it possible to reduce the number of steps involved in the synthesis procedure, and decrease the amount of chemical products consumed and quantity of waste produced
Summary
In the quest for high energy/high power density energy storage systems capable of powering electric vehicles, Lithium-Sulfur batteries have emerged as one of the most promising alternatives. Both S and the end-product Li2S are poor electronic/ionic conductors; a high-volume expansion/ shrinking takes place during the lithiation and delithiation processes (i.e., 80%) and active material is continuously lost during cycling due to the dissolution and migration of the intermediate lithium polysulfides, LiPSs (Li2Sx, 4 ≤ x ≤ 8) (commonly known as the “shuttle effect”)[1,3,4] All these phenomena lead to an inefficient use of the sulfur that limits the practical specific capacity, and to low coulombic efficiencies and premature failure of the batteries. Their long-term stability is demonstrated by a capacity retention of 70% after 260 cycles at 0.2 C (a loss of 0.12%/cycle)
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