Transparent, bio-based dielectric films with ultrahigh breakdown strength fabricated from hydrogen-bonded cellulose acetate and tannic acid for energy storage.

Dielectric capacitors are essential for advanced electronics due to their high power density and fast charge-discharge capability. However, achieving high breakdown strength and energy density while maintaining optical transparency remains a major challenge, as conventional strategies often rely on inorganic fillers that compromise transparency. Here, fully organic, transparent, and flexible dielectric films were prepared by incorporating a small amount of tannic acid (TA) into a cellulose acetate (CA) matrix. TA acts as a multifunctional organic additive that establishes localized hydrogen-bonding interactions with CA chains and may create possible carrier-interaction sites. The optimized CA/0.1%TA film maintains a high transmittance of 91% at 550 nm, while exhibiting significantly improved mechanical properties. It achieves a discharged energy density of 11.32 J cm-3 with an efficiency of 79% at 700 MV m-1. The enhanced performance is attributed to improved local chain packing, enhanced intermolecular cohesion, and suppressed charge transport. These effects are associated with localized intermolecular hydrogen-bonding interactions between CA and TA, which may reduce leakage current and enhance breakdown strength. This work demonstrates that low-content organic molecules can regulate local molecular packing and electrical behavior in cellulose-based dielectrics, providing a simple and effective route to develop transparent, flexible, and high-performance energy storage materials.