Tailoring Ionic Transport in NASICON‐Type LAGP Solid Electrolytes via Solid‐State Synthesis for Safer Lithium Metal Batteries

NASICON‐type Li + ‐ion‐conducting solid electrolytes have emerged as promising materials for enabling safer and more stable solid‐state lithium metal batteries (SSLMBs), thereby effectively addressing key challenges associated with conventional batteries that use flammable and thermally unstable liquid electrolytes. Among them, LAGP offers a favorable balance of structural stability, high ionic conductivity, and a wide electrochemical stability window, validating its viability as a promising solid electrolyte for SSLMBs. In this study, LAGP solid electrolyte was synthesized through a solid‐state reaction at significantly lower temperatures (∼800 °C) than those employed in melt‐quenching approaches (>1300 °C). The effects of calcination and sintering temperatures on the phase formation, hardness, density, and ionic conductivity of LAGP were investigated. LAGP sintered at 775 °C exhibited the highest ionic conductivity of 2.6 × 10 −4 S cm −1 , attributed to improved phase purity and densification. LSV measurements revealed a wide electrochemical stability window of 5.8 V vs. Li/Li + . A liquid‐assisted solid‐state lithium metal cell with an LFP/LAGP/Li configuration was assembled, delivering a discharge‐specific capacity of 155 mA h g −1 at 0.1 C. Finally, XRD, XPS, and impedance measurements were performed after the rate studies to evaluate the stability of LAGP at the electrode interfaces.