Thermal, structural and dynamic properties of ionic liquids and organic ionic plastic crystals with a small ether-functionalised cation

Abstract

Organic ionic plastic crystals (OIPCs) and ionic liquids (ILs) are promising safer electrolytes for energy storage applications. While ether-functionality in ILs is well known to impart beneficial properties such as low viscosity and high conductivity, cations containing short alkoxy chains are seldom investigated and this strategy is important for creating unique alkoxy-based OIPCs. Here, we report the synthesis and characterisation of the 1-methoxymethyl-1,1,1-trimethyl ammonium ([N_111,1O1]⁺) cation paired with six different anions. Solid salts were obtained with bis(fluorosulfonyl)imide ([FSI]⁻), hexafluorophosphate ([PF₆]⁻) and triflate ([OTf]⁻) anions, the dicyanamide ([DCA]⁻) and tetrafluoroborate ([BF₄]⁻) salts formed low melting point solids and the bis(trifluoromethanesulfonyl)imide ([TFSI]⁻) anion formed an IL. A fundamental investigation of the thermal, structural and dynamic behaviour of these materials through differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), single crystal X-ray diffraction (XRD), nuclear magnetic resonance (NMR) spectroscopy, and positron annihilation lifetime spectroscopy (PALS) is reported. Of the new solid salts, [N_111,1O1][FSI] displayed the highest conductivity of 2.1 × 10⁻⁵ S cm⁻¹ at 30 °C, consistent with the exceptionally large vacancy volume of 139 ų. The insights into the new materials and their structure/property relationships will be valuable for developing new electrolytes for energy storage applications.