  Li1.5Al0.5Ge1.5(PO4)3 (LAGP) is a promising oxide solid electrolyte for
all-solid-state batteries due to its excellent air stability, wide
electrochemical stability window and cost-effective precursor materials.
However, further improvement in their ionic conductivity performance is
hindered by the presence of inter-grain phases leading to a major obstacle to
the advanced design of oxide based solid-state electrolytes. This study
establishes and quantifies the influence of inter-grain phases, their 3D
morphology, and formed compositions on the overall ion conductivity properties
of LAGP pellets fabricated under different Spark plasma sintering conditions.
Based on complementary techniques, such as PEIS, XRD, 3D FIB-SEM tomography and
solid-state MAS NMR coupled with DFT modelling, a deep insight into the
inter-grain phase microstructures is obtained revealing that the inter-grain
region is comprised of Li4P2O7 and a disordered Li9Al3(P2O7)3(PO4)2 phase. We
demonstrate that optimal ionic conductivity for the LAGP system is achieved for
the 680 {\deg}C SPS preparation when the disordered Li9Al3(P2O7)3(PO4)2 phase
dominates the inter-grain region composition with reduced contributions from
the highly ordered Li4P2O7 phases.
