Superlattice Engineering in Relaxor Ferroelectric-based Capacitors for Ultra-Highly Efficient Recoverable Energy Storage Density

In this work, we show that superlattice engineering, through the combination of relaxor ferroelectric 0.5Ba(Zr 0.2 Ti 0.8 )O 3 −0.5(Ba 0.7 Ca 0.3 )TiO 3 and paraelectric SrTiO 3 layers, is a powerful approach to simultaneously optimize energy density and efficiency of recoverable energy storage at low applied electric fields, outperforming both single layer 0.5Ba(Zr 0.2 Ti 0.8 )O 3 −0.5(Ba 0.7 Ca 0.3 )TiO 3 thin film and other lead-free ferroelectric and relaxor ferroelectric thin film capacitors. The mechanisms responsible are a) decreasing the size of polar nanoregions which promote relaxor behavior by superlattice confinement and b) formation of a strain gradient in the superlattice which plays a significant role in modification of the built-in electric field, to enhance imprint, allowing for further enhancement of the energy storage performance. Overall, our interface-engineered Pb-free 0.5Ba(Zr 0.2 Ti 0.8 )O 3 −0.5(Ba 0.7 Ca 0.3 )TiO 3 /SrTiO 3 superlattices show record dielectric energy storage efficiency close to 99% at 920 kV/cm, with high recoverable energy storage density of 12.6 J/cm 3 , significantly outperforming other lead-free ferroelectric and relaxor ferroelectric thin film capacitors. Our thin film system meets the performance requirements for environmentally friendly energy storage devices for future self-powered electronics.