Giant energy density with ultrahigh efficiency achieved in NaNbO3-based lead-free ceramics with polymorphic antiferrodistortive polar nanodomains

•Polymorphic antiferrodistortive polar nanodomains were realized in NaNbO3-based ceramics.•A giant Wrec of ∼7.5 J/cm3 with an ultrahigh η of ∼94 % was achieved in the studied ceramic.•The ceramic also exhibits superior temperature-insensitive charge–discharge performance.•Electric field-induced domain morphology evolution was investigated by ex-/in-situ PFM and TEM. The development of dielectric ceramics with simultaneously high energy-storage density (Wrec) and efficiency (η) for capacitive energy storage poses a significant challenge. Herein, an effective strategy to achieve ultrahigh comprehensive energy-storage performance via designing polymorphic antiferrodistortive polar nanodomains is proposed, successfully realizing a giant Wrec of ∼7.5 J/cm3 with an ultrahigh η of ∼94 % in (0.9-x)NaNbO3-0.1BaZrO3-xBi(Mg0.5Ti0.5)O3 ((0.9-x)NN-0.1BZ-xBMT) lead-free ceramics at x = 0.08. The studied ceramic also exhibits excellent temperature-insensitive charge–discharge performances of high power density ∼204 ± 3 % MW/cm3, high discharge energy density ∼3.7 ± 3 % J/cm3 and fast discharge rate