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

Abstract 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.…