Two‐dimensional sp2‐carbon‐linked covalent organic framework for large‐capacity and long‐life Na metal batteries

Abstract Na metal batteries are regarded as an encouraging route for energy‐dense and low‐cost battery systems. However, the unstable and irreversible Na plating/stripping, caused by the uncontrolled dendritic Na growth, prevents their practical applications. Herein, a two‐dimensional sp2‐carbon‐linked covalent organic framework (cyano‐sp2c‐COF) is adopted as seeding/hosting coating layer for a highly stable interface with long cycling life, large capacity, and high Na utilization. Benefit from the features of a fully π‐conjugated structure and well‐defined cyano groups, cyano‐sp2c‐COF with superior sodiophilicity and small interface resistance can reduce the nucleation barrier, enable Na ion flux uniformity, and enhance interface stability. Ultimately, the system achieves a low nucleation overpotential of only 10 mV, a remarkable average Coulombic efficiency of 99.7% maintained over 500 cycles in half cells, and exceptional interfacial durability of 8500 h with a high accumulated capacity of 8.5 Ah cm−2 in symmetric cells. Furthermore, the symmetric cells also present a steady cycling, even increasing the depth of discharge up to 90%. As proof, full cells demonstrate a long lifespan enduring 2700 cycles with tiny capacity decay, providing valuable insights into the long‐life Na batteries.