Cross-Linked Polyphosphazene Solid Polymer Electrolytes with Partially Immobilized Anions for Stable Lithium Metal Batteries

Polyphosphazenes pair an inorganic, flame-retardant −P═N– backbone with unusually high chain mobility, yet their softness has limited their use as solid polymer electrolytes. Here, we convert that flexibility into a free-standing solid polymer electrolyte by in situ radical cross-linking between polyphosphazenes bearing oligo(ethylene glycol) (EG repeating unit, n = 1–4) and allyloxy side chains (PEGnAP) and a polymerizable single-ion conducting lithium salt (LiMTFSI), thereby simultaneously introducing network-anchored anionic moieties and building a free-standing network. Varying the oligo(ethylene glycol) side-chain length modulated both the ionic conductivity and lithium-ion transference number (tLi+), with an intermediate side-chain length (n = 2, P2SPE0.05) providing the best overall balance, reaching a tLi+ of 0.62 and an ionic conductivity of 1.90 × 10–4 S cm–1 at 60 °C. Partial anion immobilization may contributes to more regulated Li+ flux and promote stable solid electrolyte interphase (SEI) formation, yielding stable lithium stripping/plating over 1000 h with low polarization. Post-mortem XPS and SEM analyses revealed the formation of an inorganic-dominant interphase accompanied by a uniform lithium surface morphology. Furthermore, LFP/P2SPE0.05/Li cells exhibited improved cycling stability compared to cells employing the P2DEG0.05 mobile-anion control electrolyte. These findings demonstrate that cross-linked polyphosphazenes incorporating single-ion conducting moieties provide an effective platform for regulating interfacial lithium-ion transport and enhancing the stability of lithium metal batteries.