Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures

TL;DR

This study investigates how mixing different nitrile plastic crystals with lithium ions enhances ionic conductivity and explores the molecular dynamics underlying this phenomenon, aiming to improve solid-state electrolyte performance.

Contribution

It provides new insights into the effects of mixing nitrile-based plastic crystals on ionic conductivity and molecular dynamics, expanding understanding of their potential in battery applications.

Findings

Mixing nitrile plastic crystals significantly increases ionic conductivity.

Molecular reorientational dynamics are coupled with ionic mobility.

The 'revolving-door' mechanism likely facilitates ion transport.

Abstract

Many plastic crystals, molecular solids with long-range, center-of-mass crystalline order but dynamic disorder of the molecular orientations, are known to exhibit exceptionally high ionic conductivity. This makes them promising candidates for applications as solid-state electrolytes, e.g., in batteries. Interestingly, it was found that the mixing of two different plastic-crystalline materials can considerably enhance the ionic dc conductivity, an important benchmark quantity for electrochemical applications. An example is the admixture of different nitriles to succinonitrile, the latter being one of the most prominent plastic-crystalline ionic conductors. However, until now only few such mixtures were studied. In the present work, we investigate succinonitrile mixed with malononitrile, adiponitrile, and pimelonitrile, to which 1 mol% of Li ions were added. Using differential scanning calorimetry and dielectric spectroscopy, we examine the phase behavior and the dipolar and ionic dynamics of these systems. We especially address the mixing-induced enhancement of the ionic conductivity and the coupling of the translational ionic mobility to the molecular reorientational dynamics, probably arising via a "revolving-door" mechanism.