Spatial Ion-Gradient Solid Polymer Electrolytes for Stable Solid-State Sodium Batteries

A long-standing challenge in developing solid-state sodium batteries (NaSSBs) lies in reconciling three critical yet often conflicting electrolyte properties: high ionic conductivity, mechanical robustness, and a stable Na metal interface. Here, we propose a surface-bulk decoupling strategy via establishing an ion-concentrated surface layer within a polyvinylidene fluoride-based polymer electrolyte. Differential electrochemical mass spectrometry confirms that the layer effectively suppresses parasitic reactions while ensuring bulk advantages. The electrolyte exhibits a high room-temperature ionic conductivity of 4.52 × 10-4 S cm-1 and yields a dense and robust solid electrolyte interphase, enabling smooth Na deposition as characterized by operando 23Na NMR spectroscopy.. Consequently, Na||Na symmetric cells achieve stable cycling for over 360 h at 1 mA cm-2 (1 mAh cm-2). When paired with a Na4Fe3(PO4)2P2O7 cathode, the electrolyte enables a remarkable capacity retention of 90.9% after 5000 cycles at 1 C (2.1 mg cm-2) with an average Coulombic efficiency of 99.98% and maintains stable operation for over 500 cycles at 0.5 C under a high cathode loading of 11.0 mg cm-2. These results demonstrate a practical and effective approach to achieve a stable, kinetically favorable interface for advanced solid-state sodium batteries.