Deep Eutectic Solvent Gel Electrolytes Reinforced with Cellulose Nanofibers for High‐Performance Flexible Solid‐State Supercapacitors

Abstract Flexible solid‐state supercapacitors (SCs) are crucial for wearable electronics, offering high safety and outstanding cycling stability. However, conventional hydrogel electrolytes suffer from poor mechanical strength, weak electrode adhesion, low voltage windows, and instability under extreme conditions, limiting their applications. Herein, this study presents sustainable and eco‐friendly cellulose nanofibers (CNF)‐reinforced polyacrylic acid/deep eutectic solvents/CNF (PAA/DES/CNF) ion‐gel electrolytes with all‐round properties. The PAA/DES/CNF electrolyte exhibits exceptional ionic conductivity (45.5 mS cm −1 ), and a broad electrochemical stability window (0–1.6 V) over a wide temperature range (−20–60 °C). These enhanced properties are attributed to the dynamic ion transport pathways facilitated by the interactions between CNF, PAA, and DES. The abundant dynamic interaction sites on the CNF impart the ion‐gel electrolytes with superior mechanical strength, robust adhesion, and excellent self‐healing capabilities. Based on these characteristics, the assembled supercapacitor exhibits a high specific capacitance with 94.4 F g −1 at a current density of 1 A g −1 , and maintains over 93% capacity stability under mechanical deformation and extreme temperatures, alongside high capacitance retention (up to 90.1%) over 5000 cycles. This work provides valuable insights into the design and development of sustainable, environmentally adaptable, and highly flexible energy storage devices.