Study of solvent relaxation of pristine succinonitrile and succinonitrile–salt mixtures using quasielastic neutron scattering

Abstract

Succinonitrile (SN) has been demonstrated as an excellent organic plastic crystalline “solid-like” solvent for dissolution of large variety of salts at ambient temperatures. The SN–salt electrolytes are highly conducting only in the plastic phase of SN. We discuss here the study of solvent dynamics of SN in the plastic crystalline state both in the absence and presence of salts using high resolution quasielastic neutron scattering (QENS, IN16 spectrometer at ILL-Grenoble). Elastic scan versus temperature for SN, 1 M SN–LiClO₄ (SN–lithium perchlorate), 1 M SN–(LiClO₄)_0.95(LiTFSI)_0.05 (SN–lithium perchlorate–lithium bis(trifluoromethane-sulfonyl)imide) revealed a sharp order–disorder transition at 233 K. Significant quasielastic broadening was observed above 233 K. The obtained QENS data (T = 245 K) displayed relaxations ≈ 100 ps which is localized and can be accounted on the basis of a one-molecule mathematical model proposed by Bée et al. (assuming all possible molecular conformations in its unit cell). The SN rotational dynamics which determines ionic conductivity is observed to be faster in the SN–salt mixtures than the pristine SN. The high resolution QENS data also revealed the localized diffusion of the whole molecule (which includes the coordinated Li-ion and the counter anion(s)) around its nearest neighbors. Assuming that this molecular diffusion is confined under a potential of spherical symmetry, the estimated value of the radius of the domain for whole molecule diffusion and the corresponding diffusion coefficients are extracted. While the time scale for whole molecular diffusion for pristine SN and 1 M SN–LiClO₄ is found to be very similar, the whole molecular diffusion for SN-LiClO₄)_0.95(LiTFSI)_0.05 is much slower, probably due to increased mass.