Balancing ion transport and high-voltage stability in solvent-free UV-crosslinked PEO/PEC solid polymer electrolytes with succinonitrile

Lithium metal batteries coupled with high-voltage cathodes offer outstanding energy density, but the development of solid polymer electrolytes (SPEs) simultaneously stable against lithium metal and oxidative cathodes remains a major challenge. Herein, a fully solvent-free route combining melt extrusion and UV curing is proposed to fabricate poly(ethylene oxide)-poly(ethylene carbonate) (PEO-PEC) SPEs plasticized with succinonitrile (SN). By tuning the PEO/PEC ratio and introducing SN, the electrolyte properties are optimized through the combined control of crystallinity, segmental mobility, and interfacial stability. Among the investigated formulations, the PEO/PEC 7:3 SPE encompassing SN exhibits the best overall performance, delivering practical ionic conductivity at 40 °C and anodic stability up to ca. 4.8 V vs. Li+/Li. In symmetric Li||Li cells, the SPE enables continuous reversible lithium plating/stripping for more than 1400 h, maintaining moderate and stabilized polarization, and highlighting its enhanced compatibility with lithium metal. Its practical applicability is further demonstrated in truly all-solid-state Li-metal cells using both LiFePO4 and NMC622 cathodes, which deliver full practical specific capacities and stable cycling at 40 °C. Overall, this work demonstrates that solvent-free reactive processing of SN-plasticized PEO-PEC SPEs is an effective strategy to mitigate the limitations of conventional polymer electrolytes and to enable high-energy density solid-state Li-metal batteries.