Subtle Local Structural Details Influence Ion Transport in Glassy Li⁺ Thiophosphate Solid Electrolytes

Many of the promising, high-performing solid electrolytes for lithium-ion batteries are amorphous or contain an amorphous component, particularly in the Li thiophosphate Li₂S-P₂S₅ (LPS) compositional series. An explicit study of the local structure in four samples of ostensibly identically prepared 70Li₂S-30P₂S₅ glass reveals substantial variation in the ratio between the two main local structural units in this system: PS₄3^- tetrahedra and P₂S₇^4- corner-sharing tetrahedral pairs. Local structural and compositional probes including Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray pair distribution function analysis are employed here to arrive at a consistent description of the relative amounts of isolated tetrahedral units, which vary by 13% across the samples measured. This local structure variation translates to differences in the activation energies measured by electrochemical impedance spectroscopy in these samples, such that the higher concentration of isolated tetrahedra corresponds to a lower activation energy. The measured temperature-dependent ionic conductivity data are compared to conductivity results across the literature reported on the same compositions, highlighting the variation in the measured activation energy for nominally identical samples. These findings have implications for the critical need to play close attention to the local structure in solid electrolytes, particularly in systems that are glasses or glass ceramics, or those that comprise any amorphous contribution.