COF-Net on CNT-Net as a Molecularly Designed, Hierarchical Porous Chemical Trap for Polysulfides in Lithium–Sulfur Batteries

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• 作者：JongTae Yoo ; Sung-Ju Cho ; Gwan Yeong Jung ; Su Hwan Kim ; Keun-Ho Choi ; Jeong-Hoon Kim ; Chang Kee Lee ; Sang Kyu Kwak ; Sang-Young Lee
• 刊名：Nano Letters
• 出版年：2016
• 出版时间：May 11, 2016
• 年：2016
• 卷：16
• 期：5
• 页码：3292-3300
• 全文大小：751K
• 年卷期：0
• ISSN：1530-6992

文摘

The hierarchical porous structure has garnered considerable attention as a multiscale engineering strategy to bring unforeseen synergistic effects in a vast variety of functional materials. Here, we demonstrate a “microporous covalent organic framework (COF) net on mesoporous carbon nanotube (CNT) net” hybrid architecture as a new class of molecularly designed, hierarchical porous chemical trap for lithium polysulfides (Li₂S_x) in Li–S batteries. As a proof of concept for the hybrid architecture, self-standing COF-net on CNT-net interlayers (called “NN interlayers”) are fabricated through CNT-templated in situ COF synthesis and then inserted between sulfur cathodes and separators. Two COFs with different micropore sizes (COF-1 (0.7 nm) and COF-5 (2.7 nm)) are chosen as model systems. The effects of the pore size and (boron-mediated) chemical affinity of microporous COF nets on Li₂S_x adsorption phenomena are theoretically investigated through density functional theory calculations. Benefiting from the chemical/structural uniqueness, the NN interlayers effectively capture Li₂S_x without impairing their ion/electron conduction. Notably, the COF-1 NN interlayer, driven by the well-designed microporous structure, allows for the selective deposition/dissolution (i.e., facile solid–liquid conversion) of electrically inert Li₂S. As a consequence, the COF-1 NN interlayer provides a significant improvement in the electrochemical performance of Li–S cells (capacity retention after 300 cycles (at charge/discharge rate = 2.0 C/2.0 C) = 84% versus 15% for a control cell with no interlayer) that lies far beyond those accessible with conventional Li–S technologies.