Constructing superrelaxor critical state towards giant energy storage in lead-free dielectric ceramics

Lead-free relaxor ferroelectric ceramics are promising candidates for advanced pulsed power systems owing to their combination of exceptional power density and ultrafast charge-discharge capabilities. However, the simultaneous realization of ultrahigh recoverable energy density (W_rec) and high efficiency (η) remains a persistent challenge, as strategies to enhance polarization typically increase hysteresis losses. To address this issue, we propose a strategy actively constructing a superrelaxor critical state-a crossover from dynamic to static/frozen relaxor states-through targeted compositional tuning and polarization configuration control. Guided by phase-field simulations and first-principles calculations, we introduce BaHfO₃ into a Sr_0.5Bi_0.25Na_0.25TiO₃ relaxor matrix. This approach successfully shifted the dielectric maximum temperature to room temperature and enhanced the strength of relaxor behavior. Atom-scale structural characterization reveals that this structure weakens local domain interactions within 3 - 5 nm refined polar nanoregions yet preserving robust polar atomic displacements, effectively bridging the kinetic advantage of superparaelectrics with the dipole magnitude of classical relaxors. As a result, the superrelaxor critical state delivers a giant energy-storage capability, including W_rec of 16.2 J/cm³ with a high η of 92%, outperforming most reported lead-free ceramics. This work establishes a generalizable strategy for engineering critical polarization states in dielectric oxides toward next-generation capacitive energy storage.