In this work, we present an ab-initio investigation of point defects in solid electrolyte gamma-Li3PO4 and in negative electrode-electrolyte interface (Li/gamma-Li3PO4). Our results on Li defects on gamma-Li3PO4 exhibit that Li interstitial defects dominate over vacancy defects, and that Li vacancy-interstitial pair defect formation energy in-the-interface is comparable to the sum of-Li vacancy defect in the electrode and Li ion interstitial defects in the electrolyte region. Our study reveals that the high Li ion defect formation energy is the determining factor for the low ionic conductivity across Li metal/electrolyte interface. Moreover, in a realistic interface, the mechanical strain at the interface increases with the concentration of the impurities produced as a result of the cycling of the battery or due to surface impurities, also affecting the electrostatic potential and charge distribution. Thus, the study of the Li metal/electrolyte interface provides information on the defect formation and mechanical stability and, hence, it helps to understand the realistic modeling of the interface-as a way to-improve the ionic conductivity and stability of future solid state Li-ion batteries. (C) 2014 The Electrochemical Society. All rights reserved.