New Layered Triazine Framework/Exfoliated 2D Polymer with Superior Sodium‐Storage Properties

The efficient synthesis of 2D polymers (2DPs) with tailorable structures and properties is highly desired but remains a considerable challenge. Here, the first solution synthesis of millimeter‐size crystalline covalent triazine frameworks (CTFs) with a clear lamellar structure, which can be exfoliated into micrometer‐size few‐layer 2DP sheets via both micromechanical cleavage and liquid sonication, is reported. The obtained CTFs or 2DPs show a unique staggered AB stacking with a dominant pore size of ≈0.6 nm, which is different from the common eclipsed AA stacking in various covalent organic frameworks. The preference for AB stacking is due to the specific interaction of triflic acid with CTFs as revealed computationally. When explored as new polymeric anodes for sodium‐ion batteries, both crystalline bulk CTF and exfoliated 2DP exhibit very high capacities (225 and 262 mA h g^?1 at 0.1 A g^?1, respectively), impressive rate capabilities (67 and 119 mA h g^?1 at 5.0 A g^?1, respectively), and excellent cycling stability (95% capacity retention after 1200 cycles) due to their robust conjugated porous structure, outperforming most organic/polymeric sodium‐ion battery anodes ever reported.