The Impact of Intergrain Phases on the Ionic Conductivity of the LAGP Solid Electrolyte Material Prepared by Spark Plasma Sintering

Li1.5Al0.5Ge1.5(PO4)(3) (LAGP) is a promising oxide solid electrolytefor all-solid-statebatteries due to its excellent air stability, acceptable electrochemicalstability window, and cost-effective precursor materials. However,further improvement in the ionic conductivity performance of oxidesolid-state electrolytes is hindered by the presence of grain boundariesand their associated morphologies and composition. These key factorsthus represent a major obstacle to the improved design of modern oxidebased solid-state electrolytes. This study establishes a correlationbetween the influence of the grain boundary phases, their 3D morphology,and compositions formed under different sintering conditions on theoverall LAGP ionic conductivity. Spark plasma sintering has been employedto sinter oxide solid electrolyte material at different temperatureswith high compacity values, whereas a combined potentiostatic electrochemicalimpedance spectroscopy, 3D FIB-SEM tomography, XRD, and solid-stateNMR/materials modeling approach provides an in-depth analysis of theinfluence of the morphology, structure, and composition of the grainboundary phases that impact the total ionic conductivity. This workestablishes the first 3D FIB-SEM tomography analysis of the LAGP morphologyand the secondary phases formed in the grain boundaries at the nanoscalelevel, whereas the associated P-31 and Al-27 MASNMR study coupled with materials modeling reveals that the grain boundarymaterial is composed of Li4P2O7 anddisordered Li9Al3(P2O7)(3)(PO4)(2) phases. Quantitative P-31 MAS NMR measurements demonstrate that optimal ionic conductivityfor the LAGP system is achieved for the 680 & DEG;C SPS preparationwhen the disordered Li9Al3(P2O7)(3)(PO4)(2) phase dominatesthe grain boundary composition with reduced contributions from thehighly ordered Li4P2O7 phases, whereasthe Al-27 MAS NMR data reveal that minimal structural changeis experienced by each phase throughout this suite of sintering temperatures.