Labile Semi‐Cage Solvation Structures Enable Efficient Ion Transport in Solid Polymer Electrolytes

ABSTRACT Solid polymer electrolytes (SPEs) offer a promising pathway toward safer, higher‐energy‐density lithium metal batteries, yet their room‐temperature ionic conductivity remains limited by the strong coupling between Li + transport and polymer segmental motion. Here, we reveal the intrinsic solvation chemistry in poly(ethylene oxide) (PEO)‐based electrolytes using enhanced metadynamics sampling combined with 7 Li nuclear magnetic resonance spectroscopy. Rigid cage‐like solvation structures impose a key thermodynamic constraint on Li + mobility. By contrast, targeted salt and additive engineering enables the construction of a labile semi‐cage solvation structure that markedly enhances ion transport. At a Li: ether oxygen (EO) ratio of 0.10, Li + transitions from tightly bound PEO cages to semi‐cage structures by overcoming a moderate activation barrier of 19.88 kJ mol −1 . As Li + ‐EO coordination is weakened through competitive coordination with anions and additives, the ionic conductivity increases from 9.57 × 10 −3 to 2.91 × 10 −2 mS cm −1 at 30°C. Furthermore, tuning the donor number of solvent additives reveals that intermediate donor strength promotes rapid ligand exchange within semi‐cage structures, thereby accelerating local segmental relaxation through structural diffusion. This work establishes labile semi‐cage solvation as a rational design principle beyond conventional plasticization strategies for optimizing the transport properties of next‐generation SPEs.