Covalent organic frameworks with lithiophilic and sulfiphilic dual linkages for cooperative affinity to polysulfides in lithium-sulfur batteries

Confining lithium polysulfides (PSs) within the cathode materials is one of effective solutions to improve cycling stability of lithium-sulfur batteries. The lithiophilicity and sulfiphilicity are two mainstreaming chemical interactions to stabilize lithium PSs, but they have been demonstrated independently in most of cathode materials with well-defined structures and chemical compositions. It is a great challenge for designing new materials to simultaneously elucidate such two types of chemical interactions at molecular level. Here, we develop a self-condensation approach of single building block to construct a covalent organic framework (COF) containing triazine and boroxine units (TB-COF). TB-COF was generated from 4-cyanophenylboronic acid through simultaneous formation of two types of reversible covalent bonds. The triazine unit chemically absorbs lithium ions through lithiophilic interaction, while boroxine unit traps PSs through sulfiphilic interaction. The amphiphilic chemisorption of lithium PSs has been elucidated by spectroscopic studies and theoretical calculations. The TB-COF/S electrode delivers a reversible capacity of 663 mA h g(-1) after 800 cycles at 1 C with the average capacity decay per cycle as low as 0.023%, which outperforms those in the reported COF-based sulfur cathodes.