Solventless Reactive Extrusion of Modified Lignin-Based Solid Polymer Electrolytes for High-Performance Supercapacitors

Herein, modified lignosulfonate (mLigS)-based solid polymer electrolytes (SPEs) are investigated as sustainable and mechanically robust materials for high-performance supercapacitors (SCs). mLigS is synthesized via urethane linkage formation between the hydroxyl groups of lignosulfonate and an isocyanate group of difunctional isocyanate compound, leaving unreacted isocyanate sites available for further cross-linking. The incorporation of mLigS into PEO-LLZO-LiTFSI matrices via solventless thermomechanical reactive extrusion produces uniform, flexible SPE films with partial cross-linked structures. The optimized composition of PEO-LLZO-LiTFSI and mLigS exhibits a maximum ionic conductivity of ∼9.8 × 10^–5 S cm^–1 at 25 °C and 4.1 × 10^–4 S cm^–1 at 50 °C, which is attributed to enhanced ion transport via the segmental motion of PEO facilitated by crystallinity suppression induced by mLigS. The solventless reactive extrusion process suggests efficient, scalable, and environmentally benign fabrication of high-performance SPEs. A symmetric SC assembled with the SPE demonstrate an ideal electric double-layer capacitive behavior up to 3 V, retaining 98.3% capacitance and 99.5% Coulombic efficiency for at least 10,000 cycles. It delivers high specific energy and power of ∼10.37 Wh kg^–1 and 65.4 W kg^–1, which are comparable to or exceed reported SPE systems. Pouch-type SCs employing the SPE maintain stable CV under different bend radii and retain 95% capacitance after 1000 bending–straightening cycles, and pouch cell powering a green LED after charging confirms the applicability of the SPE in flexible, high-voltage energy storage applications. These results demonstrate the potential of SPEs as sustainable, high-performance electrolytes for next-generation flexible energy storage devices.