A thin-film solid-state battery was prepared with a vanadium pentoxide cathode and a lithium phosphate electrolyte and studied in situ by ultrahigh vacuum scanning tunneling microscope/atomic force microscopy ͑STM/AFM͒. Orientation of the ͑001͒ plane of V 2 O 5 parallel to the substrate was detected via observation of the periodicity of 11.7 Ϯ 0.5 Å, which is consistent with the unit cell spacing in the ͗100͘ direction. Conductance of the battery was studied locally with the probe tip of the STM/AFM in the regime of mechanical contact with a constant repulsive force. Lateral variation of contact conductance from 0.4 to 2.2 nA was detected as a function of position of the tip in contact with the cathode. The device revealed an extremely high current density of 1 A/cm 2 due to the low thickness of the electrolyte and the cathode and the concentration of electric field under the scanning probe microscope tip. Transformation of cathode structure due to Li ion intercalation was observed in real time.Thin-film solid-state microbatteries have been studied with great interest in recent years because of their potential applications in microelectronics, communications, smart cards, and medical devices. 1-4 Li-ion conducting devices have been the focus of this research because of their light weight and high energy density. Typically, a microbattery consists of a lithium-containing anode, a separating film of some lithium ion conducting electrolyte, and a cathode material capable of intercalation of the lithium into its own structure. 3 The entire film structure is usually coated with a final layer to serve as an electrical contact and protective layer. Much of the effort in the development of these batteries has focused on the properties of the individual films 3,5-9 and new materials are constantly being evaluated. 7, [10][11][12] Clearly, the performance of these devices must depend not only on the bulk properties of the individual film materials, but also on interface behavior. However, while there have been studies of the interface between liquid electrolytes and electrodes, 13 there have been far fewer studies of interfaces in solid-state devices because of the difficulty of accessing buried interfaces. There has been interest in the possibility of microheterogeneity in the conductance of a thin film device, but no direct analysis of such heterogeneity has been presented.In the work presented in this paper, we have used in situ deposition and ultrahigh vacuum scanning probe microscopy ͑UHV-SPM͒, which includes UHV scanning tunneling microscopy ͑STM͒ and UHV atomic force microscopy ͑AFM͒ to address interface structure and heterogeneity in solid-state oxide Li-ion conducting thin-film devices. The electronic effects can be very local in STM due to the effect of the concentration of the electric field at the probe tip and the tunneling effect, allowing manipulation of a molecule 14,15 or ion, 16,17 including mechanical transfer and chemical reduction with electron injection. By placing the UHV-SPM probe tip in mechanical ...