Tailoring Microphase Separation in All‐Polymer Dielectric Films for Capacitive Energy Storage at 200°C

ABSTRACT Polymers serve as key dielectric materials for energy storage capacitors, yet their performance is constrained by an inherent trade‐off between the dielectric constant ( ε r ) and breakdown strength ( E b ). Meanwhile, high‐temperature leakage current always induces severe energy loss during discharging. Here, we present all‐polymer dielectric films featuring microphase‐separated structures, achieving a discharged energy density ( U d ) of 8.09 J·cm −3 with an efficiency ( η ) of 84.90% at 200°C, while also sustaining a high U d of 5.79 J·cm −3 at η > 90%. During co‐blending polyetherimide (PEI) and poly sulfonated aramid (PSA) at appropriate ratios, microphase separation spontaneously occurs, driven by thermodynamic immiscibility. Isolated PEI‐rich phases initially emerge, encapsulated by the PSA‐PEI cross‐linking phase. Interphase dipoles and interfacial traps are generated between these two distinct phases. Containing 60 wt.% PEI, the film achieves the largest interfacial polarization gain, with a ε r of 4.30 at 200°C. Moreover, it attains the highest E b of 648.24 MV·m −1 , while leakage current is reduced by two orders of magnitude compared to the PEI film. The PSA/PEI films exhibit superior self‐healing capability that is inherited from PSA. Moreover, their cycling stability under harsh conditions and the practical viability of multi‐layer polymer capacitor devices further highlight the promising potential for high‐temperature capacitive energy storage.