Polymer nanocomposites reinforced with Al₂O₃ nanoplates for high-temperature capacitive energy storage application

Polymer dielectrics with superior high-temperature capacitive energy storage capabilities have become increasingly crucial for modern power systems and high-voltage electrical devices. Current polymer dielectrics, however, suffer from high conduction losses when exposed to high temperatures and electric fields, which significantly impede their ability to achieve high discharge energy density (<i>U</i>_e) and efficiency (<i>η</i>). Herein, fluorene polyester (FPE) nanocomposites incorporating ultra-low content Al₂O₃ nanoplates (NPLs) are presented. At 150 °C, the FPE-based nanocomposites with 1 wt% Al₂O₃ NPLs exhibit a remarkable <i>U</i>_e of 4.96 J cm^-3 and an <i>η</i> of 81% at 520 MV m^-1, outperforming most polymer dielectrics. The improvement in <i>U</i>_e is attributed to three key factors: (I) the introduction of a wide bandgap of Al₂O₃ NPLs enables them to bear high local electric fields and delay the formation of breakdown pathways, leading to an enhanced breakdown strength (<i>E</i>_b) of 520 MV m^-1; (II) a minimal loading of Al₂O₃ NPLs as a deep trap can effectively capture free charges and inhibit the migration of charge carriers in the polymers, thus reducing conductive losses and remnant polarization (<i>P</i>_r); and (III) the small-size and ultra-low content of Al₂O₃ NPLs can create a high specific surface area that facilitates high Maxwell-Wagner-Sillars interfacial polarization, leading to increased maximum polarization (<i>P</i>_max). The enhanced <i>E</i>_b and the increased polarization difference (Δ<i>P</i> = <i>P</i>_max - <i>P</i>_r) together contribute to the substantial improvements in both <i>U</i>_e and <i>η</i>.