Boosting Selective Na⁺ Migration Kinetics in Structuring Composite Polymer Electrolyte Realizes Ultrastable All‐Solid‐State Sodium Batteries

Composite polymer electrolytes demonstrate the predictable potential for achieving high‐performance all‐solid‐state sodium metal batteries (ASSMBs). However, the insufficient ionic conductivity resulting from the sluggish Na + transport kinetics and the inferior interfacial stability caused by simultaneous Na + and anion transport have hindered practical applications. Herein, a rational structural design strategy is proposed to construct an anion‐trapping boron‐contained covalent organic framework (B‐COF) network in the polymer matrix to facilitate selective Na + migration and interfacial compatibility for ASSMBs. The abundant Lewis‐acid sites on the B‐COF network can promote the dissociation of sodium salt and simultaneously constrain the migration of TFSI − anions through the strong anion‐capturing effect. Moreover, the well‐defined ion‐conducting channel formed by the in situ generation of intimately packed B‐COF combined with the above synergistic effects can afford continuous and accessible pathways for selectively rapid Na + transport, which significantly elevates the ionic conductivity and Na + transference number, respectively. Surprisingly, the Na plating/stripping with small polarization is retained under 0.1 mA cm −2 for more than 365 d (>8800 h), representing a record‐high cycling stability for ASSMBs. As proof of applied studies, the ASSMBs exhibit a high capacity retention (≈81.2%) after 1200 cycles at 1 C, signifying promising application in all‐solid‐state electrochemical energy storage systems.