Aiming at the bottleneck of low breakdown field strength and high polarization loss faced by potassium sodium niobate (KNN)-based lead-free piezoelectric ceramics in terms of energy storage performance, this paper puts forward a synergistic strategy for the regulation of the atom-nano-mesoscopic multi-scale interface. A 0.5% lattice strain gradient is induced by La3⁺/Sm3⁺ gradient doping, which, combined with a two-step sintering process, facilitates the formation of a 5–8 nm-thick BiFeO₃–Bi₂O₃ amorphous insulating layer at the grain boundaries. In addition, a local electric field-enhanced interface is formed by introducing periodically arranged Ag/TiO₂ quantum dots (15 ± 2 nm). Experiments show that this strategy enhances the breakdown field to 320 kV/cm (48% higher than that of conventional KNN), reduces the residual polarization to 1.2 μC/cm2, and achieves an ultra-high releasable energy storage density of 12.5 J/cm3 at 50 kV/mm field (η = 87.3% efficiency), with a change of dielectric constant Δεr < 5% in the temperature domain of 25–150 ℃. These results propose a new strategy for developing KNN through the introduction of periodically aligned Ag/TiO₂ quantum dots (15 ± 2 nm). Theoretical calculations and COMSOL simulations reveal that the 0.3 eV Schottky barrier at the Ag/TiO₂ interface effectively suppresses leakage current, and the electric field enhancement factor at the quantum dot tip reaches 3.8, providing a new paradigm for the cross-scale design of lead-free energy storage ceramics.