  Oxide solid electrolytes (OSEs) have the potential to achieve improved safety
and energy density for lithium-ion batteries, but their high grain-boundary
(GB) resistance is a general bottleneck. In the most well studied perovskite
OSE, Li3xLa2/3-xTiO3 (LLTO), the ionic conductivity of GBs is about three
orders of magnitude lower than that of the bulk. In contrast, the related
Li0.375Sr0.4375Ta0.75Zr0.25O3 (LSTZ0.75) perovskite exhibits low GB resistance
for reasons yet unknown. Here, we used aberration-corrected scanning
transmission electron microscopy and spectroscopy, along with an active
learning moment tensor potential, to reveal the atomic scale structure and
composition of LSTZ0.75 GBs. Vibrational electron energy loss spectroscopy is
applied for the first time to characterize the otherwise unmeasurable Li
distribution in GBs of LSTZ0.75. We found that Li depletion, which is a major
reason for the low GB ionic conductivity of LLTO, is absent for the GBs of
LSTZ0.75. Instead, the low GB resistivity of LSTZ0.75 is attributed to the
formation of a unique defective cubic perovskite interfacial structure that
contained abundant vacancies. Our study provides insights into the atomic scale
mechanisms of low GB resistivity and sheds light on possible paths for
designing OSEs with high total ionic conductivity.
