Clearly, increased polyanion reorientation at the GBs in Li 3 PS 4 is not increasing diffusivity beyond what would be seen in the bulk. The radial distribution functions (RDF) provides some insight. The GBs in Li 3 PS 4 are less crystalline than the bulk, evidenced by the much broader and less-defined peaks in the P-P peaks (Supplementary Information), but despite this, the Li-S and Li-Li RDFs are strikingly similar to the bulk. This suggests that the increased reorientation is not enabling greater concerted motion or diffusivity, but that the reorientation is acting to maintain a similar coordination environment for Li, which help to flatten the potential energy surface that the Li travels along. Experimentally, samples of Li 3 PS 4 prior to calcining and sintering to produce a highly-crystalline material, exhibit significant amorphous regions without significant loss of conductivity. We propose that the good conductivity in low-crystallinity regions -whether that is in amorphous regions or in the vicinity of GBs -may be attributable to fast reorientation allowing Li to maintain a favourable coordination environment even in a disordered structure. In comparison, the RDFs for the Li 3 InCl 6 GBs diverge more significantly from the bulk. The changes are primarily reflected in a lower degree of Li-Li coordination enabled by the more disordered structure; lowered Li-Li interactions is likely part of the reason that diffusion is more facile in these boundaries.