Large polarization and record-high performance of energy storage induced by a phase change in organic molecular crystals

Abstract

Dielectrics that undergo electric-field-induced phase changes are promising for use as high-power electrical energy storage materials and transducers. We demonstrate the stepwise on/off switching of large polarization in a series of dielectrics by flipping their antipolar or canted electric dipoles via proton transfer and inducing simultaneous geometric changes in their π-conjugation system. Among antiferroelectric organic molecular crystals, the largest-magnitude polarization jump was obtained as 18 μC cm⁻² through revisited measurements of squaric acid (SQA) crystals with improved dielectric strength. The second-best polarization jump of 15.1 μC cm⁻² was achieved with a newly discovered antiferroelectric, furan-3,4-dicarboxylic acid. The field-induced dielectric phase changes show rich variations in their mechanisms. The quadruple polarization hysteresis loop observed for a 3-(4-chlorophenyl)propiolic acid crystal was caused by a two-step phase transition with moderate polarization jumps. The ferroelectric 2-phenylmalondialdehyde single crystal having canted dipoles behaved as an amphoteric dielectric, exhibiting a single or double polarization hysteresis loop depending on the direction of the external field. The magnitude of a series of observed polarizations was consistently reproduced within the simplest sublattice model by the density functional theory calculations of dipole moments flipping over a hydrogen-bonded chain or sheet (sublattice) irrespective of compounds. This finding guarantees a tool that will deepen our understanding of the microscopic phase-change mechanisms and accelerate the materials design and exploration for improving energy-storage performance. The excellent energy-storage performance of SQA was demonstrated by both a high recoverable energy-storage density W_r of 3.3 J cm⁻³ and a nearly ideal efficiency (90%). Because of the low crystal density, the corresponding energy density per mass (1.75 J g⁻¹) exceeded those derived from the highest W_r values (∼8–11 J cm⁻³) reported for several bulk antiferroelectric ceramics , without modification to relaxor forms.