Excellent energy storage performance in cross-linked polyimide dielectrics with positively charged cross-linking points

Abstract

In PIs, the conjugation effects of benzene rings in the main chain, secondary relaxation of the polymer caused by the movement of local chain segments, leakage current density, and dielectric loss (tan δ) increase at high temperatures, resulting in a decrease in the discharge energy density (U_d) and charge–discharge efficiency (η). In this study, in order to attain stable operation of polymer dielectric capacitors and enhance their energy storage performance in high-temperature environments, a cross-linked PI (MPI) has been prepared using melamine (MA) as the cross-linker. The U_d of MPI can reach 2.89 J cm⁻³ at 150 °C and 471.53 kV mm⁻¹ when the MA content is 1.5 wt%. The cross-linked structure not only limits losses from the polarization of polymer molecular chains but also restricts the thermal movement of molecular chains at elevated temperatures, which improves η. Additionally, the positively charged cross-linked points act as traps to capture carriers, suppressing carrier conduction at both normal and high temperatures, thereby significantly increasing the breakdown strength (E_b) and U_d at high temperatures. This study provides new insights for enhancing the high-temperature stability of polymer dielectric capacitors.