Rotation‐Coupled Guest Anions Stabilize Polymer Solid‐State Sodium‐Metal Batteries

ABSTRACT Polymer solid‐state sodium‐metal batteries are constrained by sluggish Na + migration and unstable sodium–metal interfaces, both of which are closely associated with poorly regulated solvation chemistry. Here we report a rotation‐coupled guest‐anion strategy in a poly(vinylidene fluoride‐co‐hexafluoropropylene)‐based solid‐state electrolyte (PHSE) using Mg(OTf) 2 . Mg 2+ preferentially coordinates DMF and TFSI − , weakening Na + –TFSI − association and limiting solvent‐derived reactions at the sodium metal anode. OTf − enters the Na + solvation shell as a mobile guest anion. Its C─S rotational axis and equivalent oxygen coordination sites allow conformational rotation to synchronize with Na + hopping along polymer chains. This anchor–rotator chemistry reorganizes Na + coordination, lowers the migration barrier, and promotes an inorganic‐rich, Mg─O‐containing interphase. The optimized electrolyte enables Na 3 V 2 (PO 4 ) 3 ||Na cells to retain 94.2% capacity after 1100 cycles at 2C and supports a 94 mAh pouch cell with stable cycling. It also enables high‐voltage Na 3 V 2 (PO 4 ) 2 O 2 F||Na cells to retain 93.6% capacity after 1000 cycles at 0.5C over 2.5–4.2 V. Comparative evaluation across a total of 20 polymer electrolyte systems with varied polymer matrices and anion structures identifies anion symmetry, rotational periodicity, and finite rotational barriers as molecular criteria for designing stable polymer solid‐state sodium‐metal batteries.