The effect of water vapor on structure and electrochemical performance of an aluminum-free niobium-doped garnet electrolyte

In this work, we investigated how the substitution of Nb in the well-established Li7La3Zr2O12 (LLZO) affects its structure and electrochemical performance. The Nb-doped solid-state electrolytes Li7-xLa3Zr2-xNbxO12 (x = 0, 0.25, 0.5, 0.75 and 1.0) were synthesized in zirconia crucibles instead of alumina, in order to eliminate the adverse influence of alumina. The stabilization of the cubic structure of the lithium garnet has been confirmed. The sintered density and ionic conductivity of the Nb-doped LLZO garnets (designated LLZNO) first increase until the Nb content reaches x = 0.5, beyond which these values decrease. We also found that exposing the synthesized garnets in humid air deteriorates ionic conductivity. The degrading mechanism relating to the exposure of the garnets is also discussed. In addition, the cycling stability of the LLZNO (x = 0.5) was assessed by cyclic voltammetry (CV) and charge/discharge cycling testing.