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

Here, we convert that flexibility into a free-standing solid polymer electrolyte by <i>in situ</i> radical cross-linking between polyphosphazenes bearing oligo(ethylene glycol) (EG repeating unit, <i>n</i> = 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 (<i>t</i>_Li⁺), with an intermediate side-chain length (<i>n</i> = 2, P2SPE0.05) providing the best overall balance, reaching a <i>t</i>_Li⁺ 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.