Semiphase Separated Polymer Electrolyte with Continuous Li ⁺ Transport and High Mechanical Robustness for Lithium Metal Batteries

Quasi solid-state polymer electrolytes struggle with a mechanical-ionic conductivity trade-off: rigid polymers enhance mechanical stability but hinder ionic transport, while liquid-like plasticizers improve ionic conductivity but lack mechanical robustness. This work addresses this dilemma by designing a nanostructured semiphase separated electrolyte via in situ copolymerization of solvophobic monomer and solvophilic monomer within a deep eutectic electrolyte. Fluorinated solvophobic monomers create cross-linked networks by excluding Li⁺-polymer interactions and enhance chemical stability, while solvophilic monomer containing polyether side chains improves ionic conductivity by connecting isolated Li⁺-rich phases at the nanoscale. Therefore, the optimized polymer electrolyte exhibits high tensile strain (570%), superior ionic conductivity (1.69 mS cm^-1 at 30 °C), high Li⁺ transference number (0.655), and excellent oxidation stability (4.95 V). Based on a mechanochemical synergistic interface stabilization mechanism derived from the carefully designed molecular structure, it also enables ultrastable lithium plating/stripping (3000 h) and a 4.1 mA cm^-2 critical current density, and supports 4.5 V NCM811 coin cells and a 307 Wh kg^-1 Li metal pouch cell. These advancements deepen the understanding of bridging the inherent mechanical-ionic conductivity trade-off, thereby accelerating the development of high-safety and high-energy-density lithium metal batteries.