Effect of cation side-chain structure on the physicochemical properties of pyrrolidinium-based electrolytes upon mixing with sodium salt

In recent years, the development of next-generation secondary batteries employing resource-abundant metals such as Na has garnered significant attention. However, the high reactivity of Na raises safety concerns, necessitating the development of safer devices. To address this, ionic liquids (ILs) and organic ionic plastic crystals (OIPCs) have emerged as promising novel electrolytes. Despite their potential, studies investigating the influence of cation structures on various properties remain scarce, particularly in composites where Na salts are introduced into OIPCs. This study focuses on the effects of cation species and Na-salt concentration in OIPCs, specifically in <i>N,N</i>-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C₂epyr][FSA]) and <i>N</i>-ethyl-<i>N</i>-isopropylpyrrolidinium bis(fluorosulfonyl)amide ([C_i3epyr][FSA]), with the addition of sodium bis(fluorosulfonyl)amide (NaFSA). The phase transition behavior, dissociation state of Na salts, and electrochemical properties exhibited significant differences based on the cationic structure of the OIPCs. The combination of each OIPC with Na salt resulted in liquid mixtures, and the ionic conductivity increased significantly as the Na salt concentration increased. High ionic conductivities were achieved with [C₂epyr][FSA]/NaFSA (20 mol%) and [C_i3epyr][FSA]/NaFSA (10 mol%), showing values of 2.7 × 10^-3 and 2.2 × 10^-3 S cm^-1 at 25°C, respectively. Linear sweep voltammetry results indicated superior oxidative stability in the [C_i3epyr][FSA] system. Solvation numbers of Na⁺, influenced by differences in cationic side-chain structures, were determined to be 2.7 for the [C₂epyr]⁺ system and 2.9 for the [C_i3epyr]⁺ system. The results suggest that controlling solvation numbers is a critical factor in the molecular design of high-performance ionic conductors.