Sustainable Cellulose Biopolymers with Tunable Ionic Conductivity and Adhesion for High‐Performance Solid‐State Batteries

ABSTRACT Solid‐state electrolytes (SSEs) offer a promising pathway for replacing flammable liquid electrolytes in lithium metal batteries. However, their widespread application remains limited by low ionic conductivity, interfacial incompatibility, and high production costs. Here we introduce a cellulose‐based SSE featuring sulfonyl‐functionalized polymer chains that simultaneously enhance ionic transport and interfacial adhesion. The rich microdomain gradient‐concentration sulfonated cellulose (RMGC‐SC) achieves optimal ionic conductivity (1.11 × 10 −3 S cm −1 ) and intimate interface contact, and supports 1000 h of uninterrupted plating/stripping in Li||Li symmetric cells at 0.2 mA cm −2 (0.2 mAh cm −2 ). Moreover, Li||LiFePO 4 coin cells deliver outstanding cycling performance with a minimal capacity fade of only 0.023% per cycle, and a 5 × 5 cm 2 pouch cell with an areal loading of 11.2 mg cm −2 still retains 94.9% of its initial capacity, highlighting the electrolyte's practical robustness. Furthermore, a non‐solvent‐induced phase separation strategy enables efficient recovery and reuse of the cellulose matrix and lithium salt, establishing a closed‐loop recycling approach that reduces costs and environmental impact. This work demonstrates the potential of polymer molecular functionalization in precisely tuning SSE properties, offering a sustainable and scalable pathway toward high‐performance solid‐state battery technologies.