All-Covalent Organic Framework Nanofilms Assembled Lithium-Ion Capacitor to Solve the Imbalanced Charge Storage Kinetics.

Free-standing covalent organic framework (COFs) nanofilms exhibit a remarkable ability to rapidly intercalate/de-intercalate Li in lithium-ion batteries, while simultaneously exposing affluent active sites in supercapacitors. The development of these nanofilms offers a promising solution to address the persistent challenge of imbalanced charge storage kinetics between battery-type anode and capacitor-type cathode in lithium-ion capacitors (LICs). Herein, for the first time, custom-made COF and COF nanofilms are synthesized as the anode and cathode, respectively, for an all-COF nanofilm-structured LIC. The COF nanofilm with strong electronegative-CF groups enables tuning the partial electron cloud density for Li migration to ensure the rapid anode kinetic process. The thickness-regulated cathodic COF nanofilm can fit the anodic COF nanofilm in the capacity. Due to the aligned 1D channel, 2D aromatic skeleton and accessible active sites of COF nanofilms, the whole COF//COF LIC demonstrates a high energy density of 318 mWh cm at a high-power density of 6 W cm, excellent rate capability, good cycle stability with the capacity retention rate of 77% after 5000-cycle. The COF//COF LIC represents a new benchmark for currently reported film-type LICs and even film-type supercapacitors. After being comprehensively explored via ex situ XPS, Li solid-state NMR analyses, and DFT calculation, it is found that the COF nanofilm facilitates the reversible conversion of semi-ionic to ionic C-F bonds during lithium storage. COF exhibits a strong interaction with Li due to the C-F, C=O, and C-N bonds, facilitating Li desolation and absorption from the electrolyte. This work addresses the challenge of imbalanced charge storage kinetics and capacity between the anode and cathode and also pave the way for future miniaturized and wearable LIC devices.