Pressureless sintering of Al-free Ta-doped lithium garnets Li7-xLa3Zr2-xTaxO12 and the degradation mechanism in humid air

In this work, pressureless sintering (PLS) method was used to prepare Li7La3Zr2O12 based Al-free garnet electrolytes with the substitution of tantalum (Ta). A series of Ta-doped lithium garnets Li7-xLa3Zr2-xTaxO12 (LLZTO, x = 0, 0.25, 0.5, 0.75 and 1.0) were prepared, and the effect of Ta on the structure, densification and ionic conductivity was investigated. Both relative density and ionic conductivity of the LLZTO pellets are dependent on the content of Ta substitution, with the maximum values being reached at x = 0.5. We also studied the stability and degradation mechanism of lithium garnets (Li6.5La3Zr1.5Ta0.5O12) in humid air with relative humidity of above 70%. It is revealed that after exposure in the humid air for 4 weeks, a layer of LiOH center dot H2O was observed on the grain boundaries, which lowered its ionic conductivity considerably. In comparison, the pellet stored in a desiccator had clear surface with negligible change in the ionic conductivity.