In this paper we report an X-ray diffraction (XRD), Raman spectroscopy and electrochemical study of the Li10GeP2S12 lithium ion conducting solid electrolyte. The XRD results confirm the structure of the electrolyte, the Raman spectroscopy evidences the composite nature of the solid solution showing some spectral features typical of the starting Li2S, GeS2, and P2S5 materials, whereas a band peaked at about 495 cm−1 is identified as the specific fingerprint of the Li10GeP2S12 compound. The electrochemical studies, involving impedance spectroscopy, scan voltammetry and chrono-amperometry, demonstrate an ionic conductivity of the order of 10−3 S cm−1 over a wide temperature range with activation energy of approximately 0.1 eV, lithium transference of 0.99 and a stability window extending from 0 V to 6 V vs. Li. Further tests yield preliminary results obtained by Potentiodynamic Cycling with Galvanostatic Acceleration (PGCA) evaluation, which were carried out on a lithium metal anode as well as on lithium iron phosphate LiFePO4 and lithium nickel manganese oxide LiNi0.5Mn1.5O4 cathodes in cells using Li10GeP2S12 as electrolyte. The material properties described above in conjunction with these tests identify Li10GeP2S12 as a very promising electrolyte for the development of advanced solid-state batteries.