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 display similar properties same as well known Ionic liquids (ILs), such as non-flammability and non-volatility, but carry the advantage of being solids at room temperature. They show a long-range ordered crystalline lattice, but short-range disorder that typically comes from the ions' translational and rotational motions. Beneficial properties such as negligible volatility, 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-(iso)-propyl-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. The thermal and transport properties were measured by Differential Scanning Calorimetry (DSC) and Electrochemical Impedance Spectroscopy (EIS) respectively. Solid-state NMR have also been carried out to investigate the ion dynamics in the materials. Of the new solid salts, the FSI-based OIPCs shows higher conductivity values compared to the TFSI-based OIPCs. [C_(i3)mmor][FSI] has the highest conductivity of 1 × 10^-6 S cm^-1 at 30 °C. Furthermore the [C₂mmor][FSI] OIPC shows the widest temperature range of the most conductive phase (‘phase I’) ranging from 11°C - 130 °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).