Dual‐Phase In Situ Symbiosis Structures for Ultrahigh Capacitive Energy Storage

Dielectric capacitors, known for their high‐power density and reliability, are key components in advanced electronics and electrical systems. However, the trade‐off between polarization and breakdown strength in dielectric materials has significantly limited the improvement of energy storage density. In this work, a dual‐phase structure is engineered in situ through phase separation to tackle this issue. Our results demonstrated that by introducing Hf into BaBi 4 Ti 4 O 15 ferroelectrics, a linear‐like pyrochlore phase with high breakdown strength and low hysteresis loss is induced. Moreover, the phase ratio between the BaBi 4 Ti 4 O 15 and pyrochlore phases can be effectively controlled by adjusting the Hf content. As a result, this dual‐phase in situ symbiosis structures exhibits simultaneously high polarization, low hysteresis loss, and high breakdown strength. Consequently, an ultrahigh energy density of 132.4 J cm −3 with an efficiency of 76% at a relatively low electric field of 4.9 MV cm −1 is achieved. This study highlights that the in situ construction of a dual‐phase structure via Hf doping provides a simple and effective approach to optimizing the energy storage properties of dielectric capacitors.