Morpholinium-Based Organic Ionic Plastic Crystals: Effects of Alkyl Chain Length on Phase Behavior and Ionic Conductivity

Organic ionic plastic crystals (OIPCs) are a promising class of solid electrolytes due to their plasticity, high electrochemical stability, and ionic conductivity. Ether-functionalized OIPCs are of particular interest due to the presence of a Lewis base─the oxygen atom─which can promote additional dissociation of lithium salts. However, this family of plastic crystals has not been extensively studied, and no clear relationship between their chemical structure and physicochemical properties has been established to date. This study reports on the effect of the alkyl substituent length on the thermal, structural, and electrochemical properties of N-methyl-N-alkylmorpholinium [Cnmmor][BF4] (n = 1–4). As the length of the alkyl substituent increased, both the phase transition temperature to the disordered phase (II → I) and the melting temperature decreased. In the case of compounds with short alkyl substituents, [C1mmor][BF4] and [C2mmor][BF4], wide ranges of plastic phase existence (108–209 and 85–164 °C, respectively) and extremely low melting entropies (11.0 and 8.3 J × mol–1 × K–1) were observed. At temperatures below the II → I phase transition, increasing the alkyl chain length led to an increase in conductivity from 7.4 × 10–8 S/cm for [C1mmor][BF4] to 5.5 × 10–7 S/cm for [C4mmor][BF4] at 65 °C. At elevated temperatures, compounds with short alkyl substituents exhibited superionic conductivity, with maximum values of 1.3 × 10–3 S/cm at 180 °C for [C1mmor][BF4] and 2.7 × 10–4 S/cm at 145 °C for [C2mmor][BF4], which can be attributed to significant disordering of the cation and anion sublattices, as revealed by solid-state NMR spectroscopy. Compositions of [C2mmor][BF4] with 10 mol % and 20 mol % LiBF4 demonstrated lower melting points (85 and 59 °C, respectively) compared to the pure salt and relatively high conductivity values (2.0 × 10–6 S/cm and 1.2 × 10–5 S/cm at 30 °C). Thus, morpholinium-based plastic crystals show potential for use as solid electrolytes whose properties can be tuned by varying the length of the alkyl substituent.