Are lead-free relaxor ferroelectric materials the most promising candidates for energy storage capacitors?

Dielectric capacitors offer high-power density and ultrafast discharging times as compared to electrochemical capacitors and batteries, making them potential candidates for pulsed power technologies (PPT). However, low energy density in different dielectric materials such as linear dielectrics (LDs), ferroelectrics (FEs), and anti-ferroelectric (AFEs) owing to their low polarization, large hysteresis loss and low breakdown strength, respectively, limits their real time applications. Thus, achieving a material with high dielectric constant, large dielectric breakdown strength and slim hysteresis is imperative to obtain superior energy performance. In this context, relaxor ferroelectrics (RFEs) emerged as the most promising solution for energy storage capacitors. This review starts with a brief introduction of different energy storage devices and current advances of dielectric capacitors in PPT. The latest developments on lead-free RFEs including bismuth alkali titanate based, barium titanate based, alkaline niobite based perovskites both in ceramics and thin films are comprehensively discussed. Further, we highlight the different strategies used to enhance their energy storage performance to meet the requirements of the energy storage world. We also provide future guidelines in this field and therefore, this article opens a window for the current advancement in the energy storage properties of RFEs in a systematic way.