Imaging the diffusion pathway of Al³⁺ ion in NASICON-type (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ as electrolyte for rechargeable solid-state Al batteries

Among all-solid-state batteries, rechargeable Al-ion batteries have attracted most attention because they involve three-electron-redox reactions with high theoretic specific capacity. However, the solid Al-ion conductor electrolytes are less studied. Here, the microscopic path of Al³⁺-ion conduction of NASICON-type (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ oxide is identified by temperature-dependent neutron powder diffraction and aberration-corrected scanning transmission electron microscopy experiments. (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ shows a rhombohedral structure consisting of a framework of (Zr,Nb)O₆ octahedra sharing corners with (PO₄) tetrahedra; the Al occupy trigonal antiprisms exhibiting extremely large displacement factors. This suggests a strong displacement of Al ions along the c axis of the unit cell as they diffuse across the structure by a vacancy mechanism. Negative thermal expansion behavior is also identified along a and b axes, due to folding of the framework as temperature increases.