The ionic conduction plays an important role in the charging/discharging kinetics in rechargeable batteries, mainly including the steps of diffusion in the electrodes, transport in the electrolytes, and permeation through the electrode/electrolyte interphases. [8][9][10] Generally, the ionic transport is a critical control step in the electrochemical reactions, because it is much slower kinetics than electron transport. [8,11] Thus, regulating the ion-transport behavior is very vital to achieve fast-charging secondary batteries. [6,12] Mechanism exploration and novel material design become two important strategies to tackle the sluggish ionconduction kinetics. [13][14][15][16][17] In the secondary battery, the environment of ionic conduction can be roughly divided into liquid electrolytes and solid conductors. The ionic diffusion in the liquid electrolytes can be very fast. However, the flammability of organic solvents and the instability of salts (such as LiPF 6 ) at elevated temperature limit its commercial application. [18,19] The solid conductor is a safer choice to replace the current liquid electrolyte, but still suffers a serious impediment of low ionic conductivity. [20,21] Recently, covalent organic frameworks (COFs), an emerging type of crystalline porous materials, are considered to be a potential solid conductor. [22,23] Depending on the topological establishment of building blocks, COFs are classified into 2D COFs and 3D COFs. Unless otherwise noted, the COFs in this review specifically refer to 2D COFs. 2D COFs typically crystallize as stacked sheets through the π-π interaction between adjacent monolayers, meanwhile the organic units are connected by the covalent bond to form reticular framework with periodic channel structure. [24][25][26][27] Based on the nature of synthetic reactions, COFs have been synthesized with boroxine, boronate ester, borosilicate, triazine, imine, hydrazone, borazine, imide, spiroborate, amide linkages, etc. [24,25] In addition, based on the symmetry of the monomers, COFs can be designed in various topologies, such as tetragonal, hexagonal, rhombic, and trigonal. [24,25] Over the recent decades, a variety of synthetic approaches have been developed to prepare COFs, such as solvothermal synthesis, microwave synthesis, ionothermal synthesis, mechanochemical synthesis, and interfacial synthesis. [24,25] Among them, the solvothermal synthesis under a sealed vessel with a suitable temperature can produce highly crystalline COFs, but this method always needs a long Ionic conduction plays a critical role in the process of electrode reactions and the charge transfer kinetics in a rechargeable battery. Covalent organic frameworks (COFs) have emerged as an exciting new class of ionic conductors, and have made great progress in terms of their application in rechargeable batteries. The unique features of COFs, such as well-defined directional channels, functional diversity, and structural robustness, endow COF-based conductors with a low ionic diffusion energy barrier and excellent t...
