Phase transition and lithium conduction of Nb-doped Li7La3Zr2O12 garnet-type electrolytes

The lithium-stuffed garnet-type oxide Li7La3Zr2O12 (LLZO) is a promising candidate electrolyte material in all-solid-state Li-ion batteries. In order to further increase the ionic conductivity, we report a combined experimental and computational investigation of high-valence Nb5+ doping of the Zr4+-site in LLZO. In this paper, the nominal compositions of Li7-xLa3Zr2-xNbxO12 (x = 0, 0.25, 0.375, 0.5, and 0.75) electrolyte sheets were prepared by solid-state reaction. The effect of Nb5+ doping was investigated through the theoretical calculation and experimental characterization measurement. The super-valent doping strategy causes a reduction in the octahedral site (tetragonal 16f and 32g sites) occupancy, which can weaken the blocking influence of tetragonal 16f sites, promote the lithium redistribution, lower the temperature of tetragonal-cubic phase transition, and eventually enhance the ionic conductivity. The garnet-type Li6.5La3Zr1.5Nb0.5O12 ceramic sample calcined at 950 degrees C for 12 h and 1160 degrees C for 24 h shows the maximum ionic conductivity of 1.05 x 10-3 S cm-1 at room temperature and the minimum the activation energy of 0.297 eV, which can be correlated to the highest relative density of 95.7%, and good crystallinity of the grains. Further, the Li symmetric battery and Li-sulfur half batteries using garnet-type Li6.5La3Zr1.5Nb0.5O12 electrolyte exhibit good electrochemical performances. This work provides a simple and effective strategy on high-performance solid-state electrolytes.