Aromatic heterocyclic anion based ionic liquids and electrolytes

Abstract

Five new ionic materials comprising fluorine-free aromatic heterocyclic anions based on pyridine and pyrazine combined with a common n-tetrabutylphosphonium cation, (P₄₄₄₄)⁺, result in two room temperature ionic liquids (RTILs), one semi-solid, and two organic ionic plastic crystals (OIPCs) with melting points >20 °C. The OIPCs showed a plastic crystalline phase, multiple solid–solid transitions, and plastic crystalline and melt phases. For both the neat RTILs and the Li⁺ conducting electrolytes, the nature and strength of the ion–ion interactions mainly depend on the position of the nitrogen atom with respect to the carboxylate group in the anions. Furthermore, for the RTILs the ionic conductivity is effected by the electronic structure and flexibility of the ions and the anions diffuse faster than the (P₄₄₄₄)⁺ cation, but are slowed down in the electrolytes due to the strong electrostatic interactions between the carboxylate group of the anions and the Li⁺, as shown both experimentally and computationally. Overall, this study describes the effect of structural tuning of aromatic anions on the ion–ion interactions and introduces new ionic materials with promising properties to be used as solid and liquid electrolytes in energy storage devices.