Azra SOURJAH1, Luke O'DELL1, Jennifer PRINGLE1
1Deakin University, Melbourne Australia, Burwood, Australia
Organic ionic plastic crystals (OIPCs) are emerging candidates as solid-state ion conductors for various applications, especially batteries.1 They can also be potential candidates for developing new gas separation membranes.2 OIPCs are disordered solids at room temperature, which are made entirely of ions. They show a long-range ordered crystalline lattice, but short-range disorder that typically comes from the ions' translational and rotational motions. OIPCs show beneficial properties such as negligible volatility, which makes them suitable for long term device use, while the high thermal and electrochemical stability delivers the primary necessity to be used as solid-state electrolytes for many device applications.3 Thus, this has encouraged many researchers to explore different cation and anion combinations to develop new OIPCs with good properties.
Here we report the synthesis and characterization of new OIPCs utilizing morpholinium cations. The morpholinium ring is substituted with linear ethyl and branched isopropyl substituents to form 4-ethyl-4-methyl morpholinium [C2mmor]+ and 4-isopropyl-4-methyl morpholinium [C(i3)mmor]+ cations respectively. These cations were combined with the charge diffuse bis(fluorosulfonyl)imide [FSI]- or bis(trifluoromethanesulfonyl)imide [TFSI]- anions to produce four new OIPC salts. A fundamental study of the thermal behaviour and transport properties were determined by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). Solid-state NMR have also been carried out to investigate the ion dynamics in the materials. The electrochemical stability window was studied through cyclic voltammetry (CV). ). Of the salts reported [C2mmor][FSI] exhibit a wider phase I range from 11 to 129 °C which is advantageous for plasticity. [C(i3)mmor][FSI] displayed the highest conductivity of 1 × 10-6 S cm-1 at 30 °C. These results are promising for further investigation of these materials, for example as electrolytes for lithium or sodium batteries.
References
1. Zhu, H., MacFarlane, D. R., Pringle, J. M. & Forsyth, M. "Organic Ionic Plastic Crystals as Solid-State Electrolytes" Trends in Chemistry vol. 1 (2019).
2. Ramos, F., Forsyth, M. & Pringle, J. M. "Organic Ionic Plastic Crystal-Based Composite Membranes for Light Gas Separation: The Impact of Varying Ion Type and Casting Method" ChemSusChem 13, 5740–5748 (2020).
3. Jin, L. et al. "An organic ionic plastic crystal electrolyte for rate capability and stability of ambient temperature lithium batteries" Energy Environ. Sci. 7, 3352–3361 (2014).