The influence of 90 degrees domain boundaries in (La,Li)TiO3 (LLTO) on the Li conduction mechanism has been examined by a combination of state-of-the-art electron microscopy techniques and first-principles calculations. The atomistic structure of 90 degrees domain boundaries in LLTO was determined from aberration-corrected scanning transmission electron microscopy images. At 90 degrees domain boundaries, each perovskite unit of one domain is connected by an La-rich layer to units of the neighboring domain. First-principles calculations of a model domain boundary show that Li migration through the La layer has a very high activation energy, E-a, of 3.58 eV, indicating that La layers serve to block Li migration. However, if La vacancies are present within La layers, the migration energy decreases significantly to 0.58 eV, a value more in line with experimental observation. The results show that Li conduction in LLTO is strongly influenced by 90 degrees domain boundaries. The activation energy in a single crystal (E-a = 0.19 eV) is much lower, suggesting that if 90 degrees domain boundaries could be eliminated, an increase in conductivity of approximately three orders of magnitude compared with conventional LLTO could be achieved. (C) 2014 Elsevier B.V. All rights reserved.