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

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 ⁷Li 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.