Grain Boundary-Driven Lattice Dynamics in a Solid-State Li-Ion Conductor

Grain boundaries (GBs) are ubiquitous in polycrystalline solid electrolytes and play pertinent roles in determining their ionic conductivity, electronic properties and stability. Despite their importance, the impact of GBs on lattice dynamics in solid electrolytes has not been elucidated. In this work, first-principles phonon calculations are undertaken on the GBs of a model solid electrolyte, namely, anti-perovskite Li₃OCl, to explore their influence on lattice dynamics and the potential consequences for Li-ion transport. Our results indicate that Li-ion vibrational hardening exists within the GBs, which suggests an increased barrier for Li-ion transport, in agreement with both previous experimental and computational studies. Conversely, the anion sublattice in the GBs undergoes vibrational softening, which has been related to an increase in motion and potential for detrimental side reactions, leading to structural degradation and potentially device failure. Furthermore, consideration of the alignment between the Li-ion migration pathways and eigenvectors of Li-ion vibrations indicates a poorer alignment on average in the GBs compared to the bulk. This study provides a proof of concept for the simulation of lattice dynamics at solid electrolyte GBs and links the connections made between bulk materials and phonon properties with the phenomenon of GB resistance for the first time.