The synergistic effects of Al and Te on the structure and Li+-mobility of garnet-type solid electrolytes

Abstract

The cubic garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ with aliovalent doping exhibits a high ionic conductivity. However, the synergistic effects of aliovalent co-doping on the ionic conductivity of garnet-type electrolytes have rarely been examined. In this work, the synergistic effects of co-dopants Al and Te on the ionic conductivity of garnets were investigated using X-ray diffraction (XRD), ²⁷Al/⁶Li Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR), Energy Dispersive X-ray Spectroscopy (EDS), Neutron Powder Diffraction (NPD) and Alternating Current (AC) impedance measurements. It was shown that co-dopants Al and Te stabilized the cubic lattice of Li_7−2x−3yAl_yLa₃Zr_2−xTe_xO₁₂ with specific Al/Te ratios, where additional Al had to be included in the structure if the amount of doped Te content x was below 0.5. In the Al and Te co-doped crystal structure, Al was incorporated into the tetrahedral 24d sites of lithium and Te occupied 16a sites of Zr. It was revealed that the occupancy of the latter could suppress the insertion of Al. High-resolution ⁶Li MAS NMR was able to differentiate the two lithium sites of interest in the garnet structure. Furthermore, it was shown that the mobility of Li ions at 24d sites mainly determined the bulk conductivities of garnet-type electrolytes.