Tailoring Li‐ion Storage and Transport in Two‐Dimensional Conjugated Metal‐Organic Frameworks via Precise Nitrogen Incorporation

ABSTRACT Two‐dimensional conjugated metal‐organic frameworks (2D c ‐MOFs) are advantageous of defined crystal structures, accessible porosity, intrinsic electrical conductivity, and abundant redox active sites, enabling them as promising electrode materials for electrochemical lithium‐ion storage. Nevertheless, the boosting of ion storage capacity in 2D c ‐MOFs via chemical functionalization has remained a great challenge, and the impact of structural engineering on ionic diffusion has not been well explored. Here, we report two nitrogen‐doped 2D c ‐MOF bulk crystals (namely Cu‐N x ‐OHBA, x = 2 or 4), highlighting a remarkable trade‐off effect between their ion storage and transport behaviors. The increase in skeletal nitrogen density introduces more electron‐rich active sites onto the π‐d conjugated network, thereby improving Li‐ion capacitance through enhanced electrical double‐layer charge adsorption and redox‐induced pseudocapacitance. Notably, Cu‐N 4 ‐OHBA displays a specific capacitance of 323 F g −1 , 66% higher than that of Cu‐N 2 ‐OHBA (194 F g −1 ). Conversely, despite its lower nitrogen density, Cu‐N 2 ‐OHBA unexpectedly exhibits a much higher Li‐diffusion coefficient (5.00 × 10 −9 cm 2 s −1 ) than that of Cu‐N 4 ‐OHBA (6.00 × 10 −11 cm 2 s −1 ) at room temperature. The reduced Li‐ion transport kinetics in nitrogen‐denser framework can be accounted for by an enhanced charge‐dipole interaction between Li‐ion and skeletal nitrogen moieties.