In this study, we applied first-principles-based calculations, such as ab-initio molecular dynamics simulation, metadynamics, and nudged-elastic band calculations, to successfully identify the mechanisms responsible for the considerable difference in ionic conductivity between the tetragonal and the cubic phases of LLZO (Li7La3Zr2O 12)-a promising candidate for use as a highly Li-ion conductive solid-state electrolyte in Li-based batteries. Whereas in tetragonal LLZO the motion of Li ions is of fully collective nature or synchronous, we identified an asynchronous mechanism dominated by single-ion jumps and induced collective motion in cubic LLZO. The latter mechanism is possible at considerably lower energetic cost. The calculated energetic barriers that represent the two distinct mechanisms show good agreement with experimental values. Moreover, we were able to map the different mechanisms to the structural features of the particular polymorphs. © 2014 American Chemical Society.