Relaxor ferroelectrics (RFEs) are being actively investigated for energy-storage applications due to their large electric-field-induced polarization with slim hysteresis and fast energy charging-discharging capability. Here, a novel nanograin engineering approach based upon high kinetic energy deposition is reported, for mechanically inducing the RFE behavior in a normal ferroelectric Pb(Zr<sub>0.52</sub> Ti<sub>0.48</sub> )O<sub>3</sub> (PZT), which results in simultaneous enhancement in the dielectric breakdown strength (E<sub>DBS</sub> ) and polarization. Mechanically transformed relaxor thick films with 4 µm thickness exhibit an exceptional E<sub>DBS</sub> of 540 MV m<sup>-1</sup> and reduced hysteresis with large unsaturated polarization (103.6 µC cm<sup>-2</sup> ), resulting in a record high energy-storage density of 124.1 J cm<sup>-3</sup> and a power density of 64.5 MW cm<sup>-3</sup> . This fundamental advancement is correlated with the generalized nanostructure design that comprises nanocrystalline phases embedded within the amorphous matrix. Microstructure-tailored ferroelectric behavior overcomes the limitations imposed by traditional compositional design methods and provides a feasible pathway for realization of high-performance energy-storage materials.