Highly Conductive Li Garnets by a Multielement Doping Strategy

Highly conductive Li7La3Zr2O12 (LLZ) garnet-type solid electrolytes were further optimized to improve Li-ion conduction by La(3+)-sites substitution with Ba(2+) and Zr(4+)-sites substitution with Ta(5+) and Nb(5+). Garnet-structured metal oxides of the nominal chemical compositions Li6.65La2.75Ba0.25Zr1.4Ta0.5Nb0.1O12, Li6.4La3Zr1.4Ta0.6-xNbxO12 (x = 0, 0.1, 0.2, and 0.3), and the parent LLZ, as a reference, were prepared via conventional solid-state reaction to investigate the effect of multielement doping on ionic conductivity. The phase formation, morphology, and Li ion conductivity were characterized using powder X-ray diffraction (PXRD), scanning electron microscopy, and alternating current impedance spectroscopy methods, respectively. In addition, solid-state (27)Al and (7)Li magic-angle spinning (MAS) NMR was used to study the effect of "Al doping" on the investigated multielement doped Li-stuffed garnet metal oxides. All the prepared samples obtained the cubic garnet-type structure (space group: Ia3̅d; No. 230) at 1150 °C, similar to that of cubic LLZ. Except for Li6.4La3Zr1.4Ta0.6O12, all the members show Al content by Al MAS NMR. However, it was not possible to detect Al-based impurity phases using PXRD in any of the investigated garnets. Among the samples investigated in this work, "Al-free" Li6.4La3Zr1.4Ta0.6O12 demonstrated a bulk Li ion conductivity of 0.72 mS cm(-1) at 25 °C, with apparent activation energy of 0.26 eV, significantly higher than the parent LLZ.