The fabrication of polymer-based solid-state electrolytes (SSEs) is often limited by solution-based processing that requires organic solvents and restricts the choice of polymer matrices. Here, we report a solvent-free dry-processing strategy mediated by supercritical CO₂. Leveraging its gas-like diffusivity and liquid-like solvation capability, supercritical CO₂ enables kinetically favored selective diffusion of lithium salts within poly(ethylene oxide) (PEO), initially into amorphous regions followed by gradual permeation into crystalline domains during treatment. This diffusion-driven microstructural evolution leads to a PEO electrolyte with an ionic conductivity of 4.9 × 10⁻⁴ S·cm⁻¹, nearly four times higher than its solution-cast counterpart. These findings highlight the role of supercritical CO₂ as a mediator of polymer–salt interactions and provide a solvent-free pathway for fabricating polymer electrolytes with tailored crystallinity.