Polymorphic Heterogeneous Polar Structure Enabled Superior Capacitive Energy Storage in Lead-Free Relaxor Ferroelectrics at Low Electric Field.

High-performance energy storage dielectrics capable of low/moderate field operation are vital in advanced electrical and electronic systems. However, in contrast to achievements in enhancing recoverable energy density (W), the active realization of superior W and energy efficiency (η) with giant energy-storage coefficient (W/E) in low/moderate electric field (E) regions is much more challenging for dielectric materials. Herein, lead-free relaxor ferroelectrics are reported with giant W/E designed with polymorphic heterogeneous polar structure. Following the guidance of Landau phenomenological theory and rational composition construction, the conceived (BiNa)TiO-based ternary solid solution that delivers giant W/E of ≈0.0168 µC cm, high W of ≈4.71 J cm and high η of ≈93% under low E of 280 kV cm, accompanied by great stabilities against temperature/frequency/cycling number and excellent charging-discharging properties, which is ahead of most currently reported lead-free energy storage bulk ceramics measured at same E range. Atomistic observations reveal that the correlated coexisting local rhombohedral-tetragonal polar nanoregions embedded in the cubic matrix are constructed, which enables high polarization, minimized hysteresis, and significantly delayed polarization saturation concurrently, endowing giant W/E along with high W and η. These findings advance the superiority and feasibility of polymorphic nanodomains in designing highly efficient capacitors for low/moderate field-region practical applications.