Confining Polymer Electrolyte in MOF for Safe and High‐Performance All‐Solid‐State Sodium Metal Batteries

Abstract Nanoconfined polymer molecules exhibit profound transformations in their properties and behaviors. Here, we present the synthesis of a polymer‐in‐MOF single ion conducting solid polymer electrolyte, where polymer segments are partially confined within nanopores ZIF‐8 particles through Lewis acid‐base interactions for solid‐state sodium‐metal batteries (SSMBs). The unique nanoconfinement effectively weakens Na ion coordination with the anions, facilitating the Na ion dissociation from salt. Simultaneously, the well‐defined nanopores within ZIF‐8 particles provide oriented and ordered migration channels for Na migration. As a result, this pioneering design allows the solid polymer electrolyte to achieve a Na ion transference number of 0.87, Na ion conductivity of 4.01×10 −4 S cm −1 , and an extended electrochemical voltage window up to 4.89 V vs. Na/Na + . The assembled SSMBs (with Na 3 V 2 (PO 4 ) 3 as the cathode) exhibit dendrite‐free Na‐metal deposition, promising rate capability, and stable cycling performance with 96 % capacity retention over 300 cycles. This innovative polymer‐in‐MOF design offers a compelling strategy for advancing high‐performance and safe solid‐state metal battery technologies.