NASICON-type structured Li1.5Al0.5Ge1.5(PO4)(3)-xLi(2)O Li-ion-conducting glass-ceramics were successfully prepared from as-prepared glasses. The differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, and field emission scanning electron microscope results reveal that the excess Li2O is not only incorporated into the crystal lattice of the NASICON-type structure but also exists as a secondary phase and acts as a nucleating agent to considerably promote the crystallization of the as-prepared glasses during heat treatment, leading to an improvement in the connection between the glass-ceramic grains and hence a dense microstructure with a uniform grain size. These beneficial effects enhance both the bulk and total ionic conductivities at room temperature, which reach 1.18 x 10(-3) and 7.25 x 10(-4) S/cm, respectively. In addition, the Li1.5Al0.5Ge1.5(PO4)(3)-0.05Li(2)O glass-ceramics display favorable electrochemical stability against lithium metal with an electrochemical window of about 6 V. The high ionic conductivity, good electrochemical stability, and wide electrochemical window of LAGP-0.05LO glass-ceramics suggest that they are promising solid-state electrolytes for all solid-state lithium batteries with high power density.