Structure and dynamics in an organic ionic plastic crystal, N-ethyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl) amide, mixed with a sodium salt

Abstract

We present for the first time, the solid state phase behaviour of the organic ionic plastic crystal (OIPC) N-methyl-N-ethyl-pyrrolidinium bis(trifluoromethanesulfonyl)amide, [C₂mpyr][NTf₂], upon mixing with the sodium salt, Na[NTf₂]. Whereas the behaviour of OIPCs mixed with lithium salts has been well established, the influence of adding a sodium salt has not previously been reported. The phase diagram is presented for Na[NTf₂] compositions between 1 and 50 mol%, and shows a eutectic composition at 15 mol% with a eutectic temperature at 63 °C. In contrast to the lithium doping of the same OIPC, the conductivity does not increase significantly at, or below room temperature, however the material containing 40 mol% Na[NTf₂] has an ionic conductivity of 10⁻⁴ S cm⁻¹ in the solid state at 60 °C, which is more than 3 orders of magnitude higher than the pure OIPC. Synchrotron XRD, solid-state NMR and SEM all indicate the presence of two distinct phases across all of the compositions studied. One phase is identical to that of the pure [C₂mpyr][NTf₂] and the other phase is a mixed cation compound, quite distinct from the pure Na[NTf₂] material. The higher eutectic temperature (63 °C) in the sodium based system compared to lithium (30 °C) leads to purely solid state conductivity over the entire composition range up to 63 °C.