Correlating Transport and Structural Properties in Li_{1+ x}Al _xGe_{2- x}(PO₄)₃ (LAGP) Prepared from Aqueous Solution

Li_{1+ x}Al _xGe_{2- x}(PO₄)₃ (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe₂(PO₄)₃ and AlPO₄. The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol-gel process requiring costly organic germanium precursors. In this work, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li⁺ incorporation in the material and the actual Li⁺ content in the sample increases with the nominal Li⁺ content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing carrier density, the bulk activation energy increases. This correlation suggests that changes in the transport mechanism and correlated motion may be at play in the Li_{1+ x}Al _xGe_{2- x}(PO₄)₃ solid solution.