Symmetrical-Branched Pyrrolidinium Ionic Plastic Crystal Electrolytes: Synthesis and Sodium-Ion Battery Potential

Ionic plastic crystals (IPCs), akin to ionic liquids (ILs) but possessing unique properties, are of interest to remarkably enhance battery safety. However, most IPCs or ILs are commonly synthesized via a conventional method – a substitution reaction with an N-alkyl pyrrolidone and alkyl halide which commonly requires a long reaction time and large volume of organic solvents. In this study, we attempted a greener and facile method to synthesize the symmetrical cation - N,N- diisopropylpyrrolidinium bromide ([C_i3i3pyr][Br]) via a microwave-assisted synthesis procedure followed by a metathesis reaction to obtain [C_i3i3pyr][FSA]. The microwave method provides a viable alternative with potential benefits in reaction speed and environmental impact compared to conventional techniques. Among the three salts tested for anion exchange, KFSA emerged as the most effective in the conversion from Br⁻ to FSA⁻. Comprehensive spectroscopic and thermal analyses were conducted to validate the physicochemical and electrochemical properties of [C_i3i3pyr][FSA]. The material displayed three solid-solid transitions (T_s-s) at −27, 13, and 68°C. The ionic conductivity of [C_i3i3pyr][FSA] at ambient temperature is approximately 10⁻⁷ S cm⁻¹. Investigation into the temperature dependence of ionic conductivity revealed a significant increase from ∼10⁻¹⁰ to ∼10⁻⁸ S cm⁻¹ over the temperature range of −30 to 10°C, followed by a notable jump in conductivity from 60 to 70°C, corresponding to the T_s-s observed at 13 and 68°C. Additionally, the potential for the performance of this material in sodium-ion batteries was evaluated by evaluating electrochemical properties with varying sodium salt concentrations, demonstrating significant improvement in conductivity and redox behavior. These findings highlight the potential of [C_i3i3pyr][FSA] as a promising electrolyte for safer, more efficient sodium-ion batteries, particularly due to its facile and eco-friendly microwave synthesis method.