Giant recoverable energy storage density for lead-free SrTiO₃-based dielectric films coated by the amorphous Al₂O₃ insulating layers

SrTiO₃ paraelectric materials, characterized by their linear polarization behavior, low polarization loss, and lead-free composition, are promising candidates for environmentally friendly energy storage dielectrics. However, their energy storage capabilities are limited by low breakdown field strength and polarization intensity. This work proposes a design strategy involving insulator‒paraelectric heterojunction multilayers by incorporating amorphous Al₂O₃ (AO) insulating layers into SrTi_0.99Mn_0.01O₃ (STM) thin films. By adjusting the number and spatial placement of the Al₂O₃ insulating layers, the effects of the Al₂O₃ layers on the microstructure and energy storage performance of the STM films were revealed. The results demonstrate that the AO/STM/AO top‒bottom configuration exploits the synergistic effects of electrode interface barriers and the built-in electric field (E_in) within the heterojunction to create a full-path charge transport barrier. This arrangement significantly enhances the breakdown strength and reduces conduction losses. The optimized composite film exhibited a breakdown field strength of 6467 kV/cm and a very large recoverable energy storage density of 73.33 J/cm³, with an energy storage efficiency of 80.17%, outperforming existing SrTiO₃-based energy storage materials. Furthermore, the composite film demonstrated excellent stability across a wide temperature range (20–250 °C), a broad frequency band (0.2–10 kHz), and high-frequency switch cycles (10⁷ cycles).