Polarization‐Driven Energy Storage Enhancement in KNN‐Based Relaxor Ceramics under Moderate Electric Field

Abstract Conventional strategies focused on achieving ultrahigh breakdown electric fields ( E _b ) have been extensively studied to enhance the energy storage performance (ESP) of lead‐free dielectric ceramics, which is not conducive to the packaging and use of actual devices. Thus, exploring approaches that deliver high ESP under moderate electric fields is desirable. However, reports on attaining ESP values above 6 J cm ⁻³ under moderate E _b in K _0.5 Na _0.5 NbO ₃ ‐based ceramics remain scarce. In this study, a relaxor state is constructed near room temperature, as well as based on the theoretical relationship between dielectric permittivity ( ε _r ) and electric field, a design strategy is proposed by introducing Ca(Mg _1/3 Ta _2/3 )O ₃ into a high‐ ε _r matrix of K _0.44 La _0.02 Na _0.5 NbO ₃ to maintain ionic polarizability. Remarkably, a high saturation polarization of 61.1 µC cm ^{− 2} in x = 0.06 ceramic, along with the formation of dynamic polar nanoregions. High ESP of 6.4 J cm ⁻³ is achieved under 370 kV cm ⁻¹ . The optimal component exhibited a short discharge time of 50.8 ns, as well as improved transparency and mechanical properties. Phase‐field simulations further confirmed that the high density of grain boundaries per unit volume positively contributes to the breakdown strength. The findings offer a practical and innovative pathway for designing high‐performance energy storage capacitors operable under moderate electric fields.