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

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.