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High energy density and intrinsic safety are the central pursuits in developing rechargeable Zinc‐ion batteries (ZIBs). The capacity and stability of nickel cobalt oxide (NCO) cathode are unsatisfactory because of its semiconductor character. Herein, we propose a built‐in electric field (BEF) approach by synergizing cationic vacancies and ferroelectric spontaneous polarization on cathode side to facilitate electron adsorption and suppress zinc dendrite growth on the anode side. Concretely, NCO with cationic vacancies was constructed to expand lattice spacing for enhanced zinc‐ion storage. Heterojunction with BEF leads to the Heterojunction//Zn cell exhibiting a capacity of 170.3 mAh g−1 at 400 mA g−1 and delivering a competitive capacity retention of 83.3 % over 3000 cycles at 2 A g−1. We conclude the role of spontaneous polarization in suppressing zinc dendrite growth dynamics, which is conducive to developing high‐capacity and high‐safety batteries via tailoring defective materials with ferroelectric polarization on the cathode. [ABSTRACT FROM AUTHOR]

Supercapacitors with the advantages of high power density and fast discharging rate have full applications in energy storage. However, the low energy density restricts their development. Conventional methods for improving energy density are mainly confined to doping atoms and hybridizing with other active materials. Herein, a Co3O4/g‐C3N4 p–n junction with excellent capacity is developed and its application in an all‐solid‐state flexible device is demonstrated, whose capacity and energy density are considerably enhanced by simulated solar light irradiation. Under photoirradiation, the capacity is increased by 70.6% at the maximum current density of 26.6 mA cm−2 and a power density of 16.0 kW kg−1. The energy density is enhanced from 7.5 to 12.9 Wh kg−1 with photoirradiation. The maximum energy density reaches 16.4 Wh kg−1 at a power density of 6.4 kW kg−1. It is uncovered that the lattice distortion of Co3O4, reduces defects of g‐C3N4, and the facilitated photo‐generated charge separation by the Co3O4/g‐C3N4 p–n junction all make contributions to the promoted electrochemical storage performance. This work may provide a new strategy to enhance the energy density of supercapacitors and expand the application range of photocatalytic materials. [ABSTRACT FROM AUTHOR]