Progress and outlook of dielectric energy storage characteristics of Lead–free (K,Na)NbO3 based bulk ceramics/composite films for capacitors

Abstract

The energy storage characteristics of lead-free (K,Na)NbO₃-based ceramics and composite films has advanced substantially through systematic tuning of composition and processing routes, leading to substantial improvements in energy density up to ∼20 J/cm³. Despite achieving high energy densities, the simultaneous optimization of energy storage efficiency remains a critical challenge for real-time device applications. Many strategies have been developed by the researchers over the last decade to obtain simultaneous high energy density along with high efficiency in KNN-based relaxors. This review opens with a concise introduction to capacitor fundamental and then focuses on KNN-based bulk ceramics & polymer composites for energy storage applications. Specifically, it highlights strategies such as grain size reduction, enhanced microstructural homogeneity, multiple phase co-existence, high entropy, polymer composites and optimized sintering, which play a crucial role in improving dielectric polarization and breakdown strength, both of which are essential for achieving high energy density and efficiency. Moreover, we discuss the advanced characterization techniques to study the dynamic Polar Nano Regions (PNRs) and microstructure-property correlations. This review opens a new gateway for systematic synthesis of KNN-based energy storage ceramics and polymer composites to deal with the growing energy demands.