Thermal and Transport\nProperties of Ionic Electrolytes\nwith Tailored Guanidinium and Pyrrolidinium-Based Cation Structures\nfor Na-Ion Batteries

Organic ionic plastic crystals (OIPCs) are plastic solids\ncomposed\nfully of ionic species, analogous in structure to ionic liquids but\nsolid at room temperature. OIPCs are a group of materials exhibiting\nadvantageous properties as potential electrolytes for metal-ion batteries.\nHowever, the fundamental laws governing the relationship between the\nthermal and transport properties and their relationship to the ion\nstructure and chemistry in the OIPCs have not yet been fully understood.\nExpanding this understanding aims to allow for the design of ions\nwith tailored characteristics to create high-performing electrolytes.\nIn this work, two novel OIPC materials were investigated for the first\ntime as Na-ion electrolytes. The <i>N</i>-isopropyl-<i>N</i>-methylpyrrolidinium ([C<sub>i3</sub>mpyr]<sup>+</sup>)\nand diethyl tetramethyl guanidinium ([DETMG]<sup>+</sup>) cations\nwere combined with a commonly employed anion in electrolytes research,\nthe bis(fluorosulfonyl)imide ([FSI]<sup>−</sup>) anion. To\ncreate battery electrolytes, the materials were mixed with NaFSI salt\nin a wide range of concentrations, and their thermal and transport\nproperties were investigated by differential scanning calorimetry,\nEIS, linear sweep voltammetry, viscosity measurements, PFG-NMR, and <sup>23</sup>Na NMR. The proposed phase diagram is distinct for the two\ndifferent cations, although both systems showed the formation of a\nliquid phase and no crystallization at 30–43 mol % NaFSI compositions\nfor [DETMG][FSI] and 30–50 mol % NaFSI for the [C<sub>i3</sub>mpyr][FSI]. These liquid compositions were the primary focus of the\nsubsequent study of the transport properties. The [C<sub>i3</sub>mpyr][FSI]-based\nelectrolytes showed higher conductivity values than the respective\n[DETMG][FSI] mixtures, e.g., with 30 mol % NaFSI: 2.14 and 1.24 mS\ncm<sup>–1</sup> at 30 °C, respectively. Both electrolytes\nwere compared between each other, as well as with their analogues,\nhexamethylguanidinium ([HMG]<sup>+</sup>) for [DEMTG]<sup>+</sup>,\nand <i>N</i>-ethyl-<i>N</i>-methylpyrrolidinium\n[C<sub>2</sub>mpyr]<sup>+</sup> and <i>N</i>-propyl-<i>N</i>-methylpyrrolidinium [C<sub>3</sub>mpyr]<sup>+</sup> for\n[C<sub>i3</sub>mpyr]<sup>+</sup>. The organic cation structure was\nfound to influence the dynamics of all ionic species in the mixtures,\nand the mechanisms behind it have been discussed.