Ultrahigh Energy Storage in Supramolecular Biomass‐Based Dielectrics

Biomass‐based polymer dielectrics exhibit considerable potential for sustainable capacitive energy storage applications. However, they are limited by a relatively small discharged energy density (<20 J cm −3 ) with low efficiency compared to their state‐of‐the‐art petroleum‐derived counterparts. Herein, a green supramolecular architecture design is implemented by introducing poly (arylene ether urea) (PEEU) into a cyanoethyl cellulose (CEC) matrix. Hydrogen‐bond‐mediated PEEU supramolecular association is integrated with micrometer‐scale PEEU‐rich microdomains and abundant CEC/PEEU interfacial regions, collectively promoting structural densification and suppressing charge migration. Consequently, our supramolecular dielectric composites exhibit a remarkable breakdown strength of 758 MV m −1 and a superior discharged energy density of 36.4 J cm −3 with an efficiency of 76.9%, well surpassing currently existing biomass‐based counterparts. This work provides a rational supramolecular assembly framework to fabricate environmentally sustainable dielectrics featuring outstanding room‐temperature energy‐storage performance and self‐cealing capability for next‐generation capacitors.