Imaging the diffusion pathway of Al ³⁺ ion in NASICON-type (Al _0.2 Zr _0.8 ) _20/19 Nb(PO ₄ ) 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 (Al0.2Zr0.8)20/19Nb(PO4)3 oxide is identified by temperature-dependent neutron powder diffraction and aberration-corrected scanning transmission electron microscopy experiments. (Al0.2Zr0.8)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.