Fast‐Ion‐Conductor Multiscale Nanoconfinement Overcomes Ion‐Transport Limitations in All‐Solid‐State Sodium Batteries

ABSTRACT Composite polymer electrolytes (CPEs) hold significant potential for high‐performance all‐solid‐state sodium batteries, yet their development remains hindered by compromised ionic transport kinetics arising from limited conduction pathways and strong Na + coordination. Here, we report a fast‐ion‐conductor multiscale nanoconfinement strategy that enables continuous high‐throughput Na + migration in CPEs by embedding polyethylene glycol (PEG)‐confined boron‐rich covalent organic framework (BCOF) nanotubes into a poly(ethylene oxide) (PEO) matrix. Size‐compatible PEG oligomers as fast‐ion‐conductors are effectively confined within the well‐defined nanopores/tunnels of BCOF nanotube via Lewis acid‐base interactions, creating interconnected Na + migration pathways. Simultaneously, the intermolecular interactions between Lewis‐acidic boron sites in BCOF and oxygen atoms in PEO/PEG weaken Na + ─O coordination strength, further boosting Na + transport kinetics. This pioneering design allows the constructed CPEs to achieve exceptional ionic conductivity of up to 1.99 mS cm −1 at 60°C and 0.36 mS cm −1 at 30°C, with a high Na + transference number of 0.89. As such, the Na/Na symmetric cell delivers stable Na plating/stripping over 3200 h at 0.1 mA cm −2 . High‐loading all‐solid‐state pouch cells exhibit exceptional cycling stability, maintaining 90.7 % capacity retention over 800 cycles at 1 C and near‐ambient conditions. This study emphasizes the significant impact of multiscale nanoconfinement chemistry on the advancement of all‐solid‐state batteries.