Lithium/sulfur (Li/S) batteries have attracted much attention as one of the most promising next-generation energy storage systems due to the high theoretical specific capacity (1,675 mAh g⁻¹), low cost, natural abundance and environmentally benign nature of sulfur.¹ However, the poor electrical conductivity of elemental S (and its reaction product Li₂S) often results in low utilization and capacity of S, thus insufficient specific energy of Li/S batteries.² Selenium (Se), as a congener of S, has been studied as an alternative to S, and exhibited much improved electrode kinetics due in part to higher conductivity of Se than S.^{3, 4} However, the high cost, toxic nature and lower gravimetric capacity of Se hinder the adoption of Li/Se batteries. Thus, it is crucial to rationally combine the complementary properties of Se and S for practically viable energy storage systems.

Selenium disulfide (SeS₂), with a theoretical specific capacity of 1342 mAh g⁻¹, is a promising novel cathode material because of its attractive merits beyond individual S and Se. In this report, we first designed and investigated hierarchical network architectures of carbon as effective hosts for SeS₂. By a facile two-step heat treatment method, we synthesized SeS₂@KetjenBlack 600 (KB600) nanocomposites in which amorphous SeS₂ were uniformly infused/distributed inside porous carbon while creating the carbon-rich surface layer. This unique hierarchical structure with well-interconnected carbon network can effectively confine dissolved SeS₂ as well as provide excellent transportation pathways of both Li-ions and electrons. To further enhance the rate capability as well as protect lithium metal anode in Li/SeS₂ batteries, we employed LiNO₃ and ionic liquids (N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, Py₁₄TFSI) as additives in ether-based electrolytes. It was observed that LiNO₃ contributed to higher discharge capacity at a given discharge/charge rate while Py₁₄TFSI was beneficial in suppressing the dissolution of polysulfides/polyselenides of Li/SeS₂ batteries. Under optimized composition of electrolytes, hierarchical SeS₂@KB600 electrodes showed good initial discharge capacity of 873 mAh g⁻¹ at 250 mA g⁻¹ and high initial Coulombic Efficiency (CE) of 98.3 %. The effects of LiNO₃ and Py₁₄TFSI on the electrochemical performance of Li/SeS₂ batteries were further correlated with the changes in chemical composition of Li metal surface after cycling by combined electrochemical and microstructural analysis. Symmetrical Li/Li cells were also assembled in order to investigate the chemical/morphological stability of Li metal with various electrolytes in this work.

Reference:

1. Song, M. K.; Zhang, Y.; Cairns, E. J. Nano Lett 2013, 13, (12), 5891-9.
2. Fang, R.; Zhao, S.; Sun, Z.; Wang, D. W.; Cheng, H. M.; Li, F. Adv Mater 2017.
3. Eftekhari, A. Sustainable Energy Fuels 2017, 1, (1), 14-29.
4. Cui, Y.; Abouimrane, A.; Lu, J.; Bolin, T.; Ren, Y.; Weng, W.; Sun, C.; Maroni, V. A.; Heald, S. M.; Amine, K. J Am Chem Soc 2013, 135, (21), 8047-56.