Thermal regimes of Li-ion conductivity in beta-eucryptite

TL;DR

This study investigates the temperature-dependent ionic conduction mechanisms in beta-eucryptite using electrochemical impedance spectroscopy and molecular calculations, revealing three conduction regimes with distinct activation energies.

Contribution

It provides a detailed analysis of the different conduction regimes and activation energies in beta-eucryptite, clarifying the nature of Li-ion motion across temperature ranges.

Findings

Three conduction regimes identified with distinct activation energies.

Conduction shifts from correlated to uncorrelated and back to correlated with temperature.

Activation energies align with molecular static calculations.

Abstract

While it is well established that ionic conduction in lithium aluminosilicates proceeds via hopping of Li ions, the nature of the various hoping-based mechanisms in different temperature regimes has not been fully elucidated. The difficulties associated with investigating the conduction have to do with the presence of grains and grain boundaries of different orientations in these usually polycrystalline materials. Herein, we use electrochemical impedance spectroscopy (EIS) to investigate the ion conduction mechanisms in -eucryptite, which is a prototypical lithium aluminosilicate. In the absence of significant structural transitions in grain boundaries, we have found that there are three conduction regimes for the one-dimensional ionic motion along the c axis channels in the grains, and determined the activation energies for each of these temperature regimes. Activation energies computed from molecular statics calculations of the potential energy landscape encountered by Li ions suggest that at temperatures below 440 {\deg}C conduction proceeds via cooperative or correlated motion, in agreement with established literature. Between 440 {\deg}C and 500{\deg}C, the activation barriers extracted from EIS measurements are large and consistent with those from atomistic calculations for uncorrelated Li ion hopping. Above 500 {\deg}C the activation barriers decrease significantly, which indicates that after the transition to the Li-disordered phase of -eucryptite, the Li ion motion largely regains the correlated character.