Design and Synthesis of Hierarchical SeS2/C Nanocomposite Cathodes for High Performance Lithium Batteries with Ether-Based Electrolytes
Authors: Dong Panpan , Lee Jung-In , Cha Younghwan , Zhang Xiahui , Song Min-Kyu 
Published: February 2020

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
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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.

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