Lithium-Sulfur Batteries with a Vertical Co9S8 Hollow Nanowall Arrays-Modified Celgard Separator
Authors: He Jiarui , Manthiram Arumugam 
Published: February 2020

Advanced energy-storage technologies are urgently needed to satisfy the energy demands of the society. Sulfur is an appealing candidate for high energy-density batteries, owing to its high theoretical capacity (1,675 mA h g-1), natural abundance, and low cost.1-3 However, the rapid capacity degradation, low Coulombic efficiency, and short cycle life originating from polysulfide dissolution and migration remain challenging for the practical application of lithium-sulfur (Li-S) batteries.4, 5

Our previous reports have demonstrated that configuring interlayers between the separator and the sulfur cathode is an effective and convenient strategy to alleviate the shuttle effect.6 However, most of those interlayers are fabricated by vacuum-filtration method, which makes those polar materials easily stack together and thus form a very thick interlayer. Therefore, on the one hand, the transport of lithium ions will be limited by the thick polar interlayers, which is not desirable for fast insertion/de-insertion of Li ions and high rate capacity. On the other hand, the stacked thick interlayers, as an inactive material, will decrease the overall cell energy density.
To address such issues, we present here a novel Co9S8 nanowall array with vertical hollow naoarchitecture as an efficient barrier for lithium polysulfides (LiPS) in Li-S batteries.7 We present well-aligned, hollow Co9S8 arrays in-situ grown on a Celgard (Co9S8-Celgard) separator as an efficient polysulfide barrier for high-performance Li-S cells without any significant increase in the weight and volume (Fig. 1a). This novel concept/strategy of designing a multifunctional separator via in-situ grown polar and conductive materials (Co9S8 hollow arrays) on a commercial separator dramatically suppresses the shuttle effect of LiPSs and significantly improves the electrochemical performance of Li-S cells. Due to its well-designed structure, in-situ growth/transformation, and the polarity and high conductivity of Co9S8, the Li-S cell with the Co9S8-Celgard separator not only effectively blocks the LiPSs even with pure sulfur cathodes with a high sulfur loading (5.6 mg cm-2), but also delivers excellent specific capacity, outstanding rate capability, and remarkable cycling stability for an impressive number of 1,000 cycles (Fig. 1b - d). In essence, the novel design and in-situ growth of MOF-derived multifunctional Co9S8 layers are crucial to suppress the severe polysulfide diffusion and alleviate the shuttle effect of LiPSs. We believe that this approach would promote greatly the development of modified separators, particularly the design and synthesis of multifunctional separators.

Fig. 1 (a) Schematic illustration of the synthesis process of Co9S8-Celgard. (b) Rate performances at various cycling rates with the Celgard, MOF-Celgard, and Co9S8-Celgard separators. (c) Cycling performances of Li-S cells with high sulfur-loading cathodes with Co9S8-Celgard separators. (d) Long-term cycling performances of the Li-S cells with the Co9S8-Celgard separators at 1C rate for 1,000 cycles.

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6 Y. Su, A. Manthiram, Nat. Commun. 2012, 3, 1166.
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Figure 1