Decoupling enhancements of breakdown strength and dielectric constant in PMIA-based composite films for high-temperature capacitive energy storage

Polymer-based dielectric films are increasingly demanded for capacitive energy storage. However, the negative coupling between dielectric constant (ɛ_r) and breakdown strength (E_b) presents a significant challenge to further enhancements, especially at high temperatures. Here, we propose dielectric composite films employing poly(m-phenylene isophthalamide) (PMIA) as the matrix, with nanodiamond (ND) particles modified by polydopamine (PDA) serving as reinforcing fillers. At 150 °C, the 1.0 wt% film demonstrates an ultrahigh discharge energy density (U_e) of 5.15 J/cm³ at a charge-discharge efficiency (η) exceeding 90 %. Even the temperature increases to 200 °C, the film maintains a desirable U_e of 2.36 J/cm³ with η > 90 %, achieving a record energy storage performance that outperforms numerous previous works. In addition to the inherent hydrogen bonds among PMIA molecular chains, ND@PDA fillers, enriched with hydroxyl groups, facilitate the formation of additional hydrogen bonds with PMIA, generating a hydrogen bonding network. This network provides additional dipoles for overall polarization, enhances Young's modulus for electromechanical resistance, and suppresses dielectric loss upon temperature increase, thereby reducing conduction loss. Both experimental and simulation results indicate that this hydrogen bonding network is extremely stable at high temperatures, effectively promoting the decoupling enhancements of ɛ_r and E_b for high-temperature energy storage applications.