Ultrahigh Energy Storage Density in Superparaelectric-Like Hf 0.2 Zr 0.8 O 2 Electrostatic Supercapacitors.

Electrostatic capacitors attract great interest in energy storage fields due to their advantages of high power-density, fast charge/discharge speed, and great reliability. Intensive efforts have been placed on the development of high-energy-density of capacitors. Herein, a novel supercapacitor with Hf 0.2 Zr 0.8 O 2 /xAl 2 O 3 /Hf 0.2 Zr 0.8 O 2 (HAHx) is designed to improve the breakdown strength (E b ) through optimizing Al 2 O 3 (AO) film thickness. Low-temperature annealing is first proposed to enhance the polarization difference (P m -P r ) due to the formation of dispersed polar nanoregions, which is called "superparaelectric-like" similar to previous super-paraelectric behavior of perovskite structures. As results, both large E b and P m -P r values are obtained, leading to an ultrahigh energy storage density of 87.66 J cm -3 with a high efficiency of 68.6%, as well as a reliable endurance of 10 7 cycles. This work provides a feasible pathway to improve both the polarization difference and breakdown strength of HfO 2 -based films by the combination of insulation insertion layer and low-temperature annealing. The proposed strategy can contribute to the realization of high-performance electrostatic supercapacitors with excellent microsystem compatibility.