Suppressing Conduction Losses of Polymer Composites Dielectrics for High-Temperature Capacitive Energy Storage

Electrostatic capacitor that capitalizes on the rapid electric field-induced polarization and depolarization effects in dielectric materials to store and release electrical energy is one of the most important passive components in modern electronic devices and electrical power systems. The energy storage performance of electrostatic capacitors is mainly determined by the dielectric inside. Compared with inorganic dielectrics, polymer dielectrics from synthetic resins exhibit stable dielectric performance over a wide range of frequencies, superior high voltage endurance, excellent flexibility and processing properties. High-temperature resistance ability of polymer dielectrics is extremely important for the harsh operating environment with high temperature requirements. Nevertheless, most polymer dielectrics are more vulnerable to high temperatures due to their low glass transition temperature (<i>T_g</i>). The capacitive performance of polymer dielectrics degrades rapidly at high temperatures and electric fields, which is mainly attributed to the exponential increase of conduction loss. The increased leakage current further leads to an undesirable temperature rise inside the polymer dielectrics and consequently accelerates the thermal runaway of capacitors. Therefore, inhibiting the conduction loss of polymer dielectrics is extremely important for high-temperature capacitive performance.<br/><br/>The bulk-limited conduction and electrode-limited conduction are the two main mechanisms inside the polymer dielectrics. With increasing the electric field and temperature, more trapped charge carriers are thermally activated to overcome the potential barrier of traps, even though the carrier energy is lower than the maximum energy of the potential barrier, and participate in the electrical conduction. Therefore, the conduction loss inside polymer dielectrics displays an exponential growth with temperature under high electric field. Introducing more deep charge traps is beneficial to suppress the bulk-limited conduction. Incorporating a small amount of wide bandgap inorganic fillers or small organic molecules with strong electronegativity into the high-temperature resistant polymer dielectrics is an effective method to increase the density of deep traps and trap depth. These fillers can act as trap centers to immobilize the charge carriers and constrain their mobility inside polymer composites, accordingly leading to the reduction in conduction loss. As for the electrode-limited conduction loss, charge carriers from the electrode can gain sufficient energy provided by thermal activation to overcome the energy barrier at metal-dielectric interface and increase the conduction loss. increasing the barrier height at the electrode/polymer interface is helpful for the suppression of electrode-limited conduction loss. Inserting a charge-blocking layer such as highly insulated inorganic and organic layers between the electrode and polymer dielectric can significantly increase the barrier height for the charge injection from the electrode.<br/><br/>From this, we unify the bulk and electrode-limited conduction losses and simultaneously suppress these two types of conduction loss through the surface engineering of polymer composites dielectrics. The leakage current at high temperatures and electric fields is significantly inhibited and the breakdown strength is greatly improved compared with un-modified polymer dielectrics. Accordingly, the polymer composites dielectrics exhibit superior capacitive performance with high discharged energy density and discharge-charge efficiency at high temperatures.