Terahertz Conductivity and Hindered Molecular Reorientation of Lithium Salt Doped Succinonitrile in its Plastic Crystal Phase

Abstract

The terahertz complex permittivity of the molecular plastic crystal succinonitrile (SN) or 1,2 dicyanoethane (N≡C−CH2−CH2−C≡N), doped with the lithium salts LiBF4, LiPF6, LiTFSI, and LiClO4 to form solid-state plastic crystal electrolytes, is measured and compared using temperature-dependent terahertz time-domain spectroscopy (THz-TDS). In contrast to the trends at low frequency, SN’s terahertz conductivity decreases slightly when doped with Li-salts. This indicates that at high frequencies the dielectric response is not dominated by ionic charge transport, but instead by relaxational processes which are hindered by the presence of the ionic dopants. Assuming a single Cole-Cole distribution of Debye-like processes dominates the measured spectra, the average relaxation times τ and Arrhenius activation energies Ea are extracted for each electrolyte and are shown to increase significantly relative to undoped SN’s τ and Ea, indicating the relaxational processes are hindered by the presence of the ionic dopants.