Design Principles for Grain Boundaries in Solid-State Lithium-Ion Conductors

Solid-state lithium-ion batteries promise high energy densities and excellent safety, but there are many challenges to overcome before these goals can be realised. Crucial to the operation of solid-state batteries is the solid electrolyte, which must be able to compete with more traditional liquid electrolytes. Solid electrolytes are typically produced through the sintering of powders, meaning that grain boundaries are highly prevalent and should be expected to have profound effects on the performance of a device across its lifetime. These effects might include a reduction in efficiency through decreased ionic conductivity, or even device failure through encouraging the formation of dendrites. Experimental studies on polycrystalline materials are notoriously difficult to perform and interpret, so computational techniques are invaluable for providing insight at the atomic scale.<br/><br/>We carry out first-principles calculations on grain boundaries in three representative materials in different classes of solid electrolytes, namely, oxide Li₃OCl, sulfide Li₃PS₄ and halide Li₃InCl₆. By<br/>performing ab-initio molecular dynamics on the bulk and grain boundaries of each material, we demonstrate the differing impacts that grain boundaries have on ionic mobility in each class of material. Even where grain boundaries do not significantly impact ionic conductivity, we consider to what degree perturbations to the electronic structure, such as narrowed band gaps, should be expected to contribute to undesirable electrical conductivity and to dendrite formation. These results highlight important aspects of grain boundaries that must be considered when engineering solid electrolyte materials.