Pyrrole derivatives as interlayer modifier of Li-S batteries: Modulation of electrochemical performance by molecular perturbation

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The electrochemical performance of lithium-sulfur (Li-S) batteries is strongly hampered by the shuttle effect and slow redox kinetics of lithium polysulfides (LiPSs). Surface modified interlayer of a separator of Li-S batteries is demonstrated to be an effective strategy to overcome this problem. Herein, cobalt nanoparticles confined in nitrogen co-doped porous carbon framework (Co-CN) were developed from pyrolysis of ZIF-67 and used as interlayer of PP separator for Li-S batteries, and were functionalized by four pyrrole derivatives, 1-phenylpyrrole, 1-methyl pyrrole, 1-(p-toluenesulfonyl) pyrrole, and 1-pyrrole, respectively, which were screened in terms of the electron-withdrawing/donating ability of the substituent groups on the pyrrolic nitrogen. The impact of the molecular structure of pyrrole derivatives on the interaction with LiPSs and the electrochemical performance of Li-S batteries were explored by nuclear magnetic resonance and theoretical calculation. It is uncovered that 1-phenylpyrrole shows the highest enhancement of redox kinetics of LiPSs, attributing to the optimal interaction with Co nanoparticles and LiPSs. Therefore, 1-phenylpyrrole modified Co-CN interlayer enables the best electrochemical performance for the Li-S batteries, delivering a specific capacity of 562 mAh g−1 at 5 C and a capacity of 538, 526, and 449 mAh g−1 after 500 cycles at 1, 2, and 3 C, respectively. At a high sulfur loading of 5.5 mg cm−2, it achieves a capacity of 440 mAh g−1 after 500 cycles at 1 C. This work reveals the interaction mechanism among LiPSs, Co nanoparticles and the molecular modifiers in improving the electrochemical performance of Li-S batteries.