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.

Non-nucleophilic electrolytes are promising next-generation highly stable electrolytes for magnesium-ion batteries (MIBs). However, a passivation layer on Mg metal anode usually blocks Mg²⁺ diffusion, leading to poor reaction kinetics and low Coulombic efficiency of the Mg plating/stripping in these electrolytes. Here we explore the utilization of phenyl disulfide (PDF) as a film-forming additive for non-nucleophilic electrolytes to regulate the interfacial chemistry on Mg metal anode. Phenyl-thiolate generated from the PDF additive was found to suppress the unfavorable surface blocking layer, resulted in a high Coulombic efficiency of up to 99.5% for the Mg plating/stripping process as well as a remarkably decreased overpotential. The full battery consisting of Mg metal anode and Mo₆S₇Se cathode remained stable in the PDF additive-containing electrolyte at 0.1 C over 150 cycles at room temperature.