Crystal engineering for solid-state electrolytes: exploring order-to-disorder phase transition dynamics

This doctoral research addresses the critical need for advanced solid-state electrolytes to overcome current energy storage and harvesting device limitations by using crystal engineering tools. The study explores two distinct material classes that undergo order-to-disorder phase transitions. The first investigation focuses on R-(+)-(3)-hydroxyquinuclidinium salts [QH]X, with X representing SO42−, BPh4−, BF4−, or PF6−. By employing diverse anions, we can gain a more all-encompassing comprehension of plastic phase transitions, which expose distinctive characteristics determined by anion size, shape, and charge. Solid solutions of [QH](PF6)x(BF4)1−x demonstrate a novel phenomenon called "reordering frustration," that expands the plastic phase range. In parallel, a set of methanesulfonate salts (MS) is characterized, featuring diverse globular cations, including the achiral 3-quinuclidonium [QHco]+, the racemic (3)-hydroxyquinuclidinium [QHrac]+, and the enantiopure (3)-hydroxyquinuclidinium R-[QH]+. Despite their high compositional similarity, only the enantiopure salt R-[QH]MS exhibits a reversible plastic transition at elevated temperatures. The second aspect explores supramolecular complexes of 18-crown-6 ether with solid acids containing hydrogen sulfates and selenates of alkali metals (K+, Rb+, Cs+). Analyzing diverse interactions sheds light on superprotonic phase transitions. When 18-crown-6 forms supramolecular complexes with solid acids KHSO4 and RbHSO4, it leads to solid-solid transitions resulting in superprotonic phases. These crystalline solids exhibit enhanced proton conductivity, which is confirmed by impedance spectroscopic measurements. A comprehensive study that combines structural and spectroscopic analyses shows that the conduction characteristics are closely related to the initiation of dynamic motions within anhydrous crystalline materials, specifically 18-crown-6∙KHSO4 and 18-crown-6∙RbHSO4. Incorporating 18-crown-6 ether into the hydrogen selenate salt structures significantly reduces the superprotonic phase transition temperatures compared to pure solid acids. Among the 18-crown-6 ether complexes, the phase transition temperature decreases notably from 18-crown-6∙KHSeO4 to 18-crown-6∙CsHSeO4. However, the Rb-complex, 18-crown-6∙RbHSeO4, exhibits a second-order phase transition at higher temperatures compared to the other two compounds in the series.