2 W kg −1 of batteries and 10 2 -10 6 W kg −1 of electrochemical capacitors, among the energy storage devices. [6,7] Polymers represented by biaxially oriented polypropylene (BOPP) are preferred dielectrics for high-energydensity capacitors because of their high breakdown strength (>700 MV m −1 ), low energy loss (0.02% at 25 °C), great reliability, and facile processability. [8] One of the critical challenges for technological implementation of polymer dielectrics is the largely deteriorated capacitive performance with increasing operation temperature. [9][10][11] While BOPP exhibits excellent charge-discharge efficiencies (η) at room temperature, its η decreases steeply with increasing temperature, e.g., from 96.2% at 25 °C to 68.5% at 120 °C at 400 MV m −1 , which limits the operation of BOPP at temperatures below 105 °C under the applied fields. [12,13] At temperatures above 85 °C, the operating voltage of BOPP film capacitors must be derated. On the other hand, the rising trend of transportation electrification and the growing demand for electronics used in harsh environment applications, such as those found in aerospace and underground oil and gas exploration systems, require polymer dielectrics to operate efficiently at high temperatures. [9,[14][15][16][17] For instance, in electric vehicles, BOPP film capacitors in the power converters are located near engines where the temperature is around 140-150 °C. [18] To reach the full potential of polymer dielectrics in advanced electronics and electrified transportation, it calls for efficient operation of high-energy-density dielectric polymers under high voltages over a wide temperature range. Here, the polymer composites consisting of the boron nitride nanosheet/polyetherimide and TiO 2 nanorod arrays/polyetherimide layers are reported. The layered composite exhibits a much higher dielectric constant than the current high-temperature dielectric polymers and composites, while simultaneously retaining low dielectric loss at elevated temperatures and high applied fields. Consequently, the layered polymer composite presents much improved capacitive performance than the current dielectric polymers and composites over a temperature range of 25-150 °C. Moreover, the excellent capacitive performance of the layered composite is achieved at an applied field that is about 40% lower than the typical field strength of the current polymer composites with the discharged energy densities of >3 J cm −3 at 150 °C. Remarkable cyclability and dielectric stability are established in the layered polymer nanocomposites. This work addresses the current challenge in the enhancement of the energy densities of high-temperature dielectric polymers and demonstrates an efficient route to dielectric polymeric materials with high energy densities and low loss over a broad temperature range.

The 0.94(BNT–BST)–0.06KNN ceramic possesses an excellent stored energy storage density (Ws = ∼3.13 J cm−3), a recoverable energy storage density (Wr = ∼2.65 J cm−3), and maintains a relatively high efficiency (η ∼ 84.6%).

The 0.65(NBT-BKT)–0.35SBT ceramic possesses an ultra-high recoverable energy storage density (Wrec ∼ 4.06 J cm−3) and maintains a relatively high efficiency (η = ∼87.3%).

Dielectric capacitors have become a key enabling technology for electronics and electrical systems. Although great strides have been made in the development of ferroelectric ceramic and thin films for capacitors, much less attention has been given to preventing polarization fatigue, while improving the energy density, of ferroelectrics. Here superior capacitive properties and outstanding stability are reported over 107 charge/discharge cycles and a wide temperature range of −60 to 200 °C of ferroelectric Aurivillius phase Bi3.25La0.75Ti3O12‐BiFeO3 (BLT‐BFO), which represents one of the best capacitive performances recorded for the ferroelectric materials. The modification of BLT thin films with BFO overcomes the constraints of ferroelectric Aurivillius compounds and presents an unprecedented combination of the ideal features including improved polarization, reduced ferroelectric hysteresis, and lowered leakage current for high‐energy‐density capacitors. Given the lead‐free and fatigue‐free nature of this Aurivillius phase ferroelectric, this work unveils a new approach towards high‐performance eco‐friendly ferroelectric materials for electrical energy storage applications.

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