Ion transport mechanism in anhydrous lithium thiocyanate LiSCN part II: frequency dependence and slow jump relaxation

Abstract

Specific aspects of the Li⁺ cation conductivity of anhydrous Li(SCN) are investigated, in particular the high migration enthalpy of lithium vacancies. Close inspection of impedance spectra and conductivity data reveals two bulk relaxation processes, with comparatively fast ion transport at high frequencies and slow ion migration at low frequencies. The impedance results are supported by solid state nuclear magnetic resonance (ssNMR), and pair distribution function (PDF) analysis. This behavior reflects a frequency dependent conductivity, which is related to the extremely slow thiocyanate (SCN)⁻ anion lattice relaxation that occurs when a Li⁺ cation jumps to the next available site. Two possible migration models are proposed: the first model considers an asymmetric energy landscape for Li⁺ cation hopping, while the second model is connected to the jump relaxation model and allows for 180° rotational disorder of the (SCN)⁻ anion. A complete kinetic analysis for the hopping of Li⁺ cations is presented, which reveals new fundamental insights into the ion transport mechanism of materials with complex anions.