Ion-Conduction in Solid-State Electrolytes

Abstract

Recent studies on various solid-state electrolytes showed that while improvements to the ionic conductivity are progressing swiftly, the understanding of the fundamental conduction mechanism is still lagging behind. We attempt to improve this understanding by providing a more complete overview of how different static (structural) and dynamic (lattice-ion interaction) properties relate to the ion diffusion mechanism, by investigating the differences between the Na3PnS4 isostructural compounds (Pn = P, As, Sb). The static bottleneck descriptors previously used in literature, based on the S-atoms coordinating the ion migration pathway, are found to not predict the ionic conductivities accurately. On the dynamic influences, we find that based on the melting points, Born Effective Charges, vibrational frequencies and dissociation energies, it seems that of the Pn-S bonds the P-S bonds are significantly stiffer than the As-S and Sb-S bonds and that based on the differences in electronegativities, Bader Charges and Electron Localization Functions the bonds are least polar for As-S, followed by P-S and Sb-S. The changes in the bond polarity were found to correlate more closely with the observed differences in ionic conductivity than the bond stiffness, and closer inspection of the differences in the bond polarity suggest that the Pn substitution in the PnS4 anions (following the order As  P  Sb) causes a decrease in the Na-S bonding strength through electron transfer from the Na-ions to the S-ions. We quantified the conduction process further by determining the activation barrier with the Nudged Elastic Band method, with which we find that both the ionic conductivities and thus polarity correlate well with the activation barriers. Finally, we find that while the statis bottleneck descriptors are not great predictors of the overall conductivity, they do correlate with the activation volume, indicating an important role for these structural descriptors in studying pressure effects on conductivity.