Regulation of covalent organic framework pore structure for boosting electrochemical performances of lithium-sulfur...

Three covalent organic frameworks (COFs), TMB-PPD, TMB-Bz, and TMB-DAT, with similar ring structures but various ring sizes from 11.4 to 18.9 Å, are designed and synthesized through conventional Schiff base reactions. When these COFs are used as sulfur host materials in lithium-sulfur batteries, the TMB-PPD, TMB-Bz, and TMB-DAT electrodes present the discharge capacities of 416, 522, 341 mAh g^-1 over 500 cycles at 1C with degradation rates of 0.126 %, 0.119 % and 0.179 % per cycle, respectively, demonstrating that the TMB-Bz COF, with the moderate surface polarity and the intermediate pore size of 1.42 nm has the best electrochemical performance in three COFs, which might benefit from the synergistic effects of pore surface polarity and nano-confinement. Two novel in-situ methods, electrochemical impedance spectroscopy (EIS)-galvanostatic intermittent titration technique and distribution of relaxation times analysis of EIS, are applied to discuss the electrochemical reaction steps of active sulfur conversion during the charge and discharge process, which clearly reveal that the pore-wall polarity and the ring dimension can not only significantly influence the ion transports, but also have much greater impacts on reaction activation energies for all conversion steps. This research not only exhibits that the three COFs with different pore sizes are successfully designed and fabricated as sulfur hosts, but also provides an effective strategy to improve the electrochemical behavior of lithium-sulfur cell via adjusting COF-ring sizes.