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

The electrochemical performance of lithium-sulfur(Li-S) batteries is strongly hampered by the shuttle effect and slow redox kinetics of lithium polysulfides(Li PSs). 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 Li PSs 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 Li PSs, attributing to the optimal interaction with Co nanoparticles and Li PSs. Therefore, 1-phenylpyrrole modified Co-CN interlayer enables the best electrochemical performance for the Li-S batteries, delivering a specific capacity of 562 m Ah g-1at 5 C and a capacity of 538, 526, and 449 m Ah g-1after 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 m Ah g-1after 500 cycles at 1 C. This work reveals the interaction mechanism among Li PSs, Co nanoparticles and the molecular modifiers in improving the electrochemical performance of Li-S batteries.