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 electrolyte for all-solid-state batteries due to its excellent air stability, acceptable electrochemical stability window, and cost-effective precursor materials. However, further improvement in the ionic conductivity performance of oxide solid-state electrolytes is hindered by the presence of grain boundaries and their associated morphologies and composition. These key factors thus represent a major obstacle to the improved design of modern oxide based solid-state electrolytes. This study establishes a correlation between the influence of the grain boundary phases, their 3D morphology, and compositions formed under different sintering conditions on the overall LAGP ionic conductivity. Spark plasma sintering has been employed to sinter oxide solid electrolyte material at different temperatures with high compacity values, whereas a combined potentiostatic electrochemical impedance spectroscopy, 3D FIB-SEM tomography, XRD, and solid-state NMR/materials modeling approach provides an in-depth analysis of the influence of the morphology, structure, and composition of the grain boundary phases that impact the total ionic conductivity. This work establishes the first 3D FIB-SEM tomography analysis of the LAGP morphology and the secondary phases formed in the grain boundaries at the nanoscale level, whereas the associated 31P and 27Al MAS NMR study coupled with materials modeling reveals that the grain boundary material is composed of Li4P2O7 and disordered Li9Al3(P2O7)3(PO4)2 phases. Quantitative 31P MAS NMR measurements demonstrate that optimal ionic conductivity for the LAGP system is achieved for the 680 °C SPS preparation when the disordered Li9Al3(P2O7)3(PO4)2 phase dominates the grain boundary composition with reduced contributions from the highly ordered Li4P2O7 phases, whereas the 27Al MAS NMR data reveal that minimal structural change is experienced by each phase throughout this suite of sintering temperatures.