Thermal and Transport Properties of Ionic Electrolytes with Tailored Guanidinium and Pyrrolidinium-Based Cation Structures for Na-Ion Batteries

Organic ionic plastic crystals (OIPCs) are plastic solids composed fully of ionic species, analogous in structure to ionic liquids but solid at room temperature. OIPCs are a group of materials exhibiting advantageous properties as potential electrolytes for metal-ion batteries. However, the fundamental laws governing the relationship between the thermal and transport properties and their relationship to the ion structure and chemistry in the OIPCs have not yet been fully understood. Expanding this understanding aims to allow for the design of ions with tailored characteristics to create high-performing electrolytes. In this work, two novel OIPC materials were investigated for the first time as Na-ion electrolytes. The N-isopropyl-N-methylpyrrolidinium ([Ci3mpyr]+) and diethyl tetramethyl guanidinium ([DETMG]+) cations were combined with a commonly employed anion in electrolytes research, the bis(fluorosulfonyl)imide ([FSI]−) anion. To create battery electrolytes, the materials were mixed with NaFSI salt in a wide range of concentrations, and their thermal and transport properties were investigated by differential scanning calorimetry, EIS, linear sweep voltammetry, viscosity measurements, PFG-NMR, and 23Na NMR. The proposed phase diagram is distinct for the two different cations, although both systems showed the formation of a liquid phase and no crystallization at 30–43 mol % NaFSI compositions for [DETMG][FSI] and 30–50 mol % NaFSI for the [Ci3mpyr][FSI]. These liquid compositions were the primary focus of the subsequent study of the transport properties. The [Ci3mpyr][FSI]-based electrolytes showed higher conductivity values than the respective [DETMG][FSI] mixtures, e.g., with 30 mol % NaFSI: 2.14 and 1.24 mS cm–1 at 30 °C, respectively. Both electrolytes were compared between each other, as well as with their analogues, hexamethylguanidinium ([HMG]+) for [DEMTG]+, and N-ethyl-N-methylpyrrolidinium [C2mpyr]+ and N-propyl-N-methylpyrrolidinium [C3mpyr]+ for [Ci3mpyr]+. The organic cation structure was found to influence the dynamics of all ionic species in the mixtures, and the mechanisms behind it have been discussed.