Enhanced Li₁₊ Al Ge_2- (PO₄)₃ Anode-Protecting Membranes for Hybrid Lithium-Air Batteries by Spark Plasma Sintering

NASICON-type Li₁₊ Al Ge_2- (PO₄)₃ (LAGP) is a promising electrolyte with high ionic conductivity (>10^-4 S cm^-1), excellent oxidation stability, and moderate sintering temperature. However, preparing dense LAGP pellets with high ionic conductivity is still challenging because of the hazards of dopant loss and partial decomposition on conventional sintering. Here, spark plasma sintering (SPS) of LAGP membranes is explored as a promising ultrarapid manufacturing technique, yielding dense electrolyte membranes. Optimizing the SPS temperature is important to achieve desirable density and hence ionic conductance. Our results show that LAGP samples spark plasma-sintered at 750 °C exhibit the highest total ionic conductivity of 3.9 × 10^-4 S cm^-1 with a compactness of 97% and nearly single-crystalline particles. Our solid-state NMR results, X-ray diffraction studies, and scanning electron microscopy micrographs confirm that the achievable ionic conductivity is controlled by the retention of the Al dopant within the LAGP phase, necking between particles, and the minimization of grain boundaries between crystallites within a particle. To benchmark the performance of our spark plasma-sintered solid electrolyte membranes over conventionally prepared LAGP, we demonstrate their favorable performance in hybrid Li-air batteries. The highest energy efficiency is achieved for the fastest ion-conducting membrane sintered at 750 °C.