Achieving Superior Energy Storage Performance in KNN‐Based Ceramics Under Moderate Electric Fields via Tailoring K/Na Ratio

ABSTRACT K 0.5 Na 0.5 NbO 3 (KNN)‐based ceramics are promising lead‐free candidates for energy storage applications, yet improving their energy storage performance remains a significant challenge. Here, we propose tailoring K/Na ratio to achieve a synergistic integration of relaxor, grain, and band engineering in the 0.94K x Na 1‐ x NbO 3 ‐0.06Sr 0.7 La 0.2 ZrO 3 ceramics. The tailored K/Na ratio of x = 0.7 induces dynamic polar nanoregions (PNRs), suppresses grain growth, and widens the band gap. These characteristics collectively enhance the breakdown strength ( E b ) and polarization behavior, yielding an ultrahigh recoverable energy density ( W rec ) of 8.0 J·cm −3 , a value that is competitive among the reported KNN‐based energy storage ceramics, along with a high energy storage efficiency ( η ) of 84% under a moderate electric field of 500 kV·cm −1 . Additionally, the ceramic exhibits excellent stability in frequency, cycling, and temperature. This work demonstrates that tailoring K/Na ratio is an effective approach for achieving superior energy storage performance in KNN‐based ceramics under moderate electric fields, which is expected to be generalizable for developing high‐performance dielectric materials.