Influence of AlPO 4 Impurity on the Electrochemical Properties of NASICON‐Type Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 Solid Electrolyte

Densification of ceramic electrolytes is a key enabler in producing electrolyte pellets for solid‐state batteries. This requires understanding the correlation between the starting grain size of electrolytes, chemical phase evolution and degree of compaction which determine ion conductivity and chemical stability of solid electrolytes. In our work we were able to optimize the densification process of LATP at 700 °C with a high total ionic conductivity of 3.45×10 −4 S cm −1 after hot pressing, balancing pristine LATP crystallite size and AlPO 4 impurity formation. By combining several techniques such as in situ heating X‐ray diffraction (XRD), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR), we explored the formation mechanism of AlPO 4 during the synthesis process of NASICON‐type Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) electrolyte and we showed the effects of the annealing temperature on the crystal size of the material. Density functional theory (DFT) calculations on the chemical stability of the electrolyte imply a metastable behaviour of LATP furtherly enhanced by particle nano‐structuring at high temperature. Our results point to facile manufacturing of ceramic electrolytes towards energy dense and safe solid‐state battery technology.