Organic ionic plastic crystals composed of tetrahydrothiophenium cation with high conductivity

Abstract

Organic ionic plastic crystals (OIPCs) are soft crystalline materials that exhibit plasticity and ionic conductivity, making them promising candidates for use as solid electrolytes. Previously, IPCs based on pyrrolidinium cations derived from the heterocyclic five-membered ring pyrrolidine have been synthesized, and their ionic conductivities have been reported. However, their performance has not yet achieved the required standard. In this study, we focused on tetrahydrothiophene, another five-membered heterocyclic compound, as a novel cationic structure. A series of novel IPCs was synthesized using tetrahydrothiophenium cations in combination with five different anions, yielding 15 compounds. Thermal analysis was conducted to determine the decomposition and phase-transition temperatures. Six of the synthesized compounds were identified as IPCs, and five were classified as ionic liquids. Among them, the compound 1-ethyltetrahydrothiphenium trifluoro(trifluoromethyl)borate ([C₂tht][CF₃BF₃]), consisting of ethyl-substituted tetrahydrothiophenium cation and CF₃BF₃ anion, exhibited an ionic conductivity of 7.19 × 10⁻⁴ S cm⁻¹ at 25 °C. Notably, [C₂tht][CF₃BF₃] demonstrated an approximately one order of magnitude higher ionic conductivity at room temperature than conventional pyrrolidinium-based IPCs.