Lithium Ion‐Conducting Glass–Ceramics of Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 – x Li 2 O ( x =0.0–0.20) with Good Electrical and Electrochemical Properties

NASICON‐type structured Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 – x Li 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 Li 2 O 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 × 10 −3 and 7.25 × 10 −4 S/cm, respectively. In addition, the Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 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.