Variation of ionic conductivity in a plastic-crystalline mixture

TL;DR

This study explores how mixing different plastic crystals affects ionic conductivity, revealing that the coupling between ion movement and molecular reorientation remains consistent across compositions, unlike in succinonitrile mixtures.

Contribution

It provides a detailed analysis of phase behavior and charge transport mechanisms in cyclohexanol and cyclooctanol mixtures with lithium ions, expanding understanding beyond succinonitrile-based systems.

Findings

All mixtures exhibit plastic-crystalline phases with glass transitions.

Conductivity is dominated by the 'revolving-door' mechanism.

Coupling between ionic and molecular dynamics remains stable across compositions.

Abstract

Ionically-conducting plastic crystals are possible candidates for solid-state electrolytes in energy-storage devices. Interestingly, the admixture of larger molecules to the most prominent molecular PC electrolyte, succinonitrile, was shown to drastically enhance its ionic conductivity. Therefore, binary mixtures seem to be a promising way to tune the conductivity of such solid-state electrolytes. However, to elucidate the general mechanisms of ionic charge transport in plastic crystals and the influence of mixing, a much broader data base is needed. In the present work, we investigate mixtures of two well-known plastic-crystalline systems, cyclohexanol and cyclooctanol, to which 1 mol% of Li ions were added. Applying differential scanning calorimetry and dielectric spectroscopy, we present a thorough investigation of the phase behavior and the ionic and dipolar dynamics of this system. All mixtures reveal plastic-crystalline phases with corresponding orientational glass-transitions. Moreover, their conductivity seems to be dominated by the "revolving-door" mechanism, implying a close coupling between the ionic translational and the molecular reorientational dynamics of the surrounding plastic-crystalline matrix. In contrast to succinonitrile-based mixtures, there is no strong variation of this coupling with the mixing ratio.