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

K0.5Na0.5NbO3 (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 0.94KxNa1- xNbO3-0.06Sr0.7La0.2ZrO3 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 (Eb) and polarization behavior, yielding an ultrahigh recoverable energy density (Wrec) 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.