Using ab-initio density-functional theory (DFT) methods, the atomic structure and electronic properties of one of the most promising family of solid electrolytes for Li-ion battery applications, lanthanum oxides with a garnet-type structure (c-Li7La3Zr2O12) are studied. The Li-ion (Li+) defects including Li/Li+ vacancies, interstitials, and vacancy-interstitial pair defect formation energy within the Li7La3Zr2O12 supercell are systematically investigated. This study is essential to understand the defect chemistry and the Li+ conductivity mechanisms. Our results indicate that the Li + vacancy defects are thermodynamically more favorable than interstitial Li + defects. This work will therefore be helpful to elucidate the atomic level mechanisms of Li defect formation in order to improve the ionic conductivity for future Li-ion battery applications. (C) 2014 Elsevier B.V. All rights reserved.