of 140 °C or above. [4,5] While dielectric ceramics are traditional materials for high-temperature capacitors, [6] they are severely limited by scalability, weight, fracture toughness, and breakdown strength in comparison to their polymer counterparts. [7][8][9][10][11][12][13][14][15][16][17] Biaxially oriented polypropylene film (BOPP), the state-of-the-art commercially available polymer dielectric, however, shows largely degraded high-field dielectric properties when operating at temperatures above 100 °C. [18] To address these imperative needs, a variety of well-established engineering polymers, including polycarbonate, polyimide (PI), polyetherimides, and poly(ether ether ketone), have been exploited as hightemperature dielectric materials. [19][20][21][22][23][24][25] As these aromatic polymers have high glass transition temperatures (T g ) and excellent thermal stability, it is anticipated that the engineering polymers would retain electromechanical properties and thus dielectric stability at high temperatures. However, when subjected to high applied fields, the engineering polymers exhibit limited working temperatures that are much lower than their T g s. [19,20] More recently, inorganic fillers represented by boron nitride nanosheets (BNNSs) have been incorporated into crosslinked divinyltetramethyldisiloxane-bis(benzocyclobutene) (c-BCB) to yield the dielectric polymer composites capable of operating efficiently at high temperatures, e.g. 150 °C. [26][27][28][29] Herein, we describe the hightemperature dielectric properties and capacitive performance of the PI-based polymer nanocomposites prepared via in situ polycondensation. Compared with c-BCB, PI possesses the inherent advantages including much better processability, considerably lower cost, and greater mechanical strength and flexibility, which potentially offers a scalable route toward robust hightemperature dielectric materials. [30,31] The investigation of the polymer composites containing the inorganic nanofillers with systematically varied dielectric constants (K) and bandgap (ΔE), including aluminium oxide (Al 2 O 3 ) with a K of 9.5 and a ΔE of 8.6 eV, hafnium dioxide (HfO 2 ) with a K of 25 and a ΔE of 5.8 eV, titanium dioxide (TiO 2 ) with a K of 110 and a ΔE of 3.5 eV, and BNNS with a K of 4 and a ΔE of 5.97 eV, [26,[32][33][34] would provide experimental guidelines for the design of highperformance high-temperature dielectric polymer composites. Modern electronics and electrical systems demand efficient operation of dielectric polymer-based capacitors at high electric fields and elevated temperatures. Here, polyimide (PI) dielectric composites prepared from in situ polymerization in the presence of inorganic nanofillers are reported. The systematic manipulation of the dielectric constant and bandgap of the inorganic fillers, including Al 2 O 3 , HfO 2 , TiO 2 , and boron nitride nanosheets, reveals the dominant role of the bandgap of the fillers in determining and improving the high-temperature capacitive performance of the polymer compo...