Dielectric ceramic capacitors are promising for pulsed-power electronics owing to their high power density and rapid charge−discharge, yet their wider application is limited by a relatively low recoverable energy density (W_rec) and difficulty in simultaneously achieving high W_rec and high energy efficiency (η). Herein, a relaxor-to-superparaelectric crossover is engineered in NaNbO₃–(Bi_0.5K_0.5)TiO₃–BaZrO₃ multilayer ceramics, yielding an impressive W_rec of ~16.5 J·cm⁻³, a superior η of ~96.2% and a large W_rec/E merit value of 206.3 J·(kV)⁻¹·mm⁻². Multiscale structural analysis reveals that the introduced (Bi_0.5K_0.5)TiO₃ and BaZrO₃ stabilize the ferroelectric phase, disrupt long-range polar order, and shift the dielectric permittivity maximum close to room temperature, collectively creating a relaxor–superparaelectric transitional state composed of heterogeneous polar nanoregions (PNRs) with diverse symmetries and sizes. These PNRs exhibit highly dispersive reorientation dynamics under electric fields and thus enable high maximum polarization and simultaneously minimum hysteresis, accounting for the concurrent enhancement in both W_rec and η. Furthermore, the broad thermal stability range of this transitional state leads to excellent temperature-insensitive performance from 25 to 150 °C (W_rec = 8.5 J·cm ±3.2%, η = 96.1%±2.8%). This work demonstrates a viable material strategy for engineering relaxor–superparaelectric crossover to develop high-performance dielectric ceramics for advanced energy storage.

The development of high-performance lead-free energy storage capacitors is crucial for sustainable technologies, yet hindered in NaNbO₃-based antiferroelectric (AFE) ceramics because of significant polarization hysteresis from field-induced AFE-ferroelectric (FE) phase transitions. This hysteresis fundamentally limits the simultaneous optimization of recoverable energy density (W_rec) and efficiency (η). Herein, we demonstrate that lamellar nanodomain engineering via compositional design in a (0.87–x)NaNbO₃–0.13Bi_0.5Na_0.5TiO₃–xBi(Mg_0.5Ti_0.5)O₃ system effectively overcomes this bottleneck. The optimized composition (x = 0.05) delivers exceptional energy storage performance with a W_rec of ~8.2 J/cm³, a η of ~88.9%, and a power density of ~207 MW/cm³. Analysis on multiscale structure evolution reveals that this compositional tuning induces a phase transformation from AFE P to AFE R symmetry, accompanied by an enhanced local structural disorder. Critically, the formation of lamellar AFE R-phase nanodomains with width ranging from 2 nm to 6 nm drives a quasi-linear polarization response with minimal hysteresis. Concurrently, the refined grain size improves the ceramic resistivity, substantially enhancing dielectric breakdown strength. These synergistic effects collectively yield outstanding energy storage properties, demonstrating that engineering lamellar AFE R-phase nanodomains is an efficient strategy to optimize overall energy storage performance of NaNbO₃-based materials.