Engineering Pb-free relaxor ferroelectric thin films for low voltage energy storage applications

Pulsed power technologies demand dielectric capacitors that possess a high energy storage density and efficiency at low applied electric fields/voltages. In this work, we engineered the morphology of lead-free 0.85[0.6Ba(Zr_0.2Ti_0.8)O₃-0.4(Ba_0.7Ca_0.3)TiO₃]-0.15SrTiO₃ (BZCT-STO) epitaxial thin films, fabricated using the pulsed laser deposition technique. Through control of the annealing time, we observed both grain shape and size changes, which induced a change in the relaxor behaviour of the BZCT-STO films. The enhanced relaxor behaviour, assigned to the formation of polar nanoregions, was achieved in the film with uniform smaller spherical grains, which is relevant for improved energy storage performance at low electric fields. The dependence of the electric field on the ferroelectric and energy storage properties of the BZCT-STO thin films was investigated. It is found that the LSMO/BZCT-STO/Au capacitor with enhanced relaxor behaviour shows the optimum energy storage performance, attributable to a moderate maximum polarization and remanent polarization difference, and the highest electric breakdown field. An energy storage density of 9.24 J cm^-3 with an efficiency of 86.4% at an applied electric field of 1500 kV cm^-1 was obtained. The increased energy storage density and efficiency in these BZCT-STO thin film capacitors at a low electric field make them one of the most promising systems reported in the literature for energy storage applications. The results reported here clearly evidence the significant impact of the film morphology on the dielectric, ferroelectric and energy storage properties.