Realization of superior ionic conductivity by manipulating the atomic rearrangement in Al-doped Li7La3Zr2O12

Abstract

High ionic conductivity is usually observed in the cubic phase of garnet Li7La3Zr2O12 (LLZO), which can be stabilized by dopants including Ca2+, Fe3+, Al3+, Ga3+, Ti4+, and Si4+. However, the ionic conductivity shows a strong dependence on both the lithium and the dopant atomic occupancy of the garnet LLZO crystal structure (space group Ia 3‾ d), i.e., the 24d (LiO4) or 96 h (LiO6) sites. Dopants occupying the 24d site can induce a strong local electric field which blocks the Li+ diffusion path. The sparking plasma sintering (SPS) process, which limits the diffusion and relaxation time of the material, introduces a metastable state of the cubic LLZO with strong lattice distortion and inadequate ion distribution. Post-annealing promotes ion rearrangement, especially the Al3+ migration from the 24d to 96 h sites, which is driven by the mitigation of the metastable state as identified by structure and composition analysis. Ionic conductivity as high as 3.3 × 10−4 S cm−1 is achieved along with low electronic conductivity of 5.17 × 10−7 S cm−1 by post-annealing at 500 °C in an oxygen environment after the SPS process, meanwhile, the activation energy of is SPS500 as low as 0.36 eV, representing the low energy barriers for ion transportation. This work demonstrated a strategy to fabricate LLZO solid-state electrolyte with high ionic conductivity by manipulating its metastable state and reveal atomic occupation is a vital factor in influencing ionic conductivity besides doping content and bulk density.