The long-term reliability of Li / LiPON / LixCoO2 all-solid-state thin-film microbatteries is studied through a deepened electrical characterization protocol regularly performed during aging tests. Hence, from galvanostatic low rate discharges and impedance spectroscopy measurements, the key characteristics of active layers and solid interfaces, such as charge transfer resistances and exchange currents densities are extracted. After 1,500 electrochemical cycles performed in accelerated conditions, the intrinsic parameters representative of each active solid interface are found quite stable. Such result might be explained by the absence of Solid Electrolyte Interface (SEI) typical of conventional liquid electrolyte Li-ion systems, which cannot appear in all-solid-state microbatteries systems. Therefore, this study highlights the reliability performances of EnFilm microbattery technology, mainly due to the stability of interfacial phenomena. Furthermore, in the considered accelerated conditions, the aging level of microbatteries is studied through the analysis of relative capacity loss origin. Hence, the reliability intrinsic behavior could be mainly correlated with some structural changes in LiCoO2 electrode material. (C) 2015.The Electrochemical Society. All rights reserved.