Relaxor-Like Ferroelectric Behavior in Epitaxial NaNbO₃ Films for Capacitive Energy Storage

Electrostatic energy storage based on dielectric capacitors has garnered significant interest due to its fast charge-discharge speeds and high power density relative to electrochemical energy storage, but applications have been limited due to large leakage currents, relatively low energy storage density, and the presence of lead in materials^[1,2]. NaNbO₃ is a lead-free alternative which has received much research attention in the last decade but work has primarily focused on its bulk form, with NaNbO₃-based systems displaying both antiferroelectric and relaxor ferroelectric behaviors at room temperature with promising energy storage properties^[2,3]. Using pulsed laser deposition and selective etching, we have synthesized metal-insulator-metal heterostructures of La_0.7Sr_0.3MnO₃ and NaNbO₃ with high crystalline quality and coherent epitaxial strain across a range of thicknesses. Both strain and thickness have previously been noted to greatly affect the properties of NaNbO₃ due to the close proximity of its antiferroelectric and ferroelectric ground states^[4,5]. Through careful optimization of the growth conditions, we achieve minimal leakage current even under large external electric fields and can observe relaxor-like ferroelectric behavior in both our thinnest samples (below 50 nm of NaNbO₃) and our thickest samples (above 150 nm). This allows us to achieve improvements in both recoverable energy-storage density <i>W_rec</i> and energy efficiency <i>η</i> compared to previous NaNbO₃ thin films^[6,7]. This work demonstrates the potential of NaNbO₃ for dielectric energy storage and the advantages of epitaxial, single-crystal films for electrical characterization.<br/><br/>[1] H. Pan et al., Science 374, 100-104 (2021).<br/>[2] M.-H. Zhang et al., Nat. Comm. 14, 1525 (2023).<br/>[3] N. Luo et al., Nat. Comm. 14, 1776 (2023).<br/>[4] T. Schneider et al., ACS Omega 8, 23587-23595 (2023).<br/>[5] R. Xu et al., Adv. Mater. 35, 2210562 (2023).<br/>[6] T. Shiraishi et al., J. Appl. Phys. 128, 044102 (2020).<br/>[7] H. Dong et al., Ceram. Inter. 48, 16215-16220 (2022).