Pockets, jumps and filaments: classifying ionic motion and determining the role of structure in electrochemical properties of 2Li2S-GeS2 superionic glasses

Abstract

The structural properties of a typical solid electrolyte system (2Li2S-GeS2) is investigated from First principles molecular dynamics simulations. Results reveal that depolymerization of the base GeS2 network by alkali additives takes place but appears reduced with respect to the corresponding sodium analog glass. Experimental structure functions are reproduced and reveal that the network is dominated by GeS 4/2 tetrahedra that are connected by edges (four-membered rings) and corners and disrupted by the addition of lithium, albeit a non-negligible fraction of connecting tetrahedra (Q 2 units) are still present in the glass structure. Dynamic and electric properties are also studied and emphasize that the size of the migrating cation (Li) is essential for ensuring a good level of ionic conductivity as it displays increased values with respect to the parent Na-bearing system. On the atomic (picosecond) timescale, different typical Li trajectories are identified and their distribution calculated: reduced cage-like motion in pockets constrained by the surrounding (Ge,S) network, back and forth jump motions with short transition states and long-range filamentary motion.