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

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. Mg²⁺ 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 Na₃V₂(PO₄)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 Na₃V₂(PO₄)₂O₂F||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.