Balancing Polarization and Breakdown for High Capacitive Energy Storage by Microstructure Design

The compromise of contradictive parameters, polarization, and breakdown strength, is necessary to achieve a high energy storage performance. The two can be tuned, regardless of material types, by controlling microstructures: amorphous states possess higher breakdown strength, while crystalline states have larger polarization. However, how to achieve a balance of amorphous and crystalline phases requires systematic and quantitative investigations. Herein, the trade-off between polarization and breakdown field is comprehensively evaluated with the evolution of microstructure, i.e., grain size and crystallinity, by phase-field simulations. The results indicate small grain size (≈10-35 nm) with moderate crystallinity (≈60-80%) is more beneficial to maintain relatively high polarization and breakdown field simultaneously, consequently contributing to a high overall energy storage performance. Experimentally, therefore an ultrahigh energy density of 131 J cm^-3 is achieved with a high efficiency of 81.6% in the microcrystal-amorphous dual-phase Bi₃NdTi₄O₁₂ films. This work provides a guidance to substantially enhance dielectric energy storage by a simple and effective microstructure design.