Mechanism of High Li-Ion Conductivity in Li-Excess Garnet Li_7+xLa_3-xSr_xZr₂O₁₂

Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for all-solid-state batteries, and various dopants have been investigated to improve its conductivity. However, the dopants have mainly been studied to increase the conductivity by reducing the amount of Li⁺, and dopants that increase the amount of Li⁺ have yet to be studied. Therefore, the structure of LLZO with excess Li⁺ and its function as an electrolyte need to be clarified. Here, we report on the correlation between the conductivity and the structure of LLZO with excess Li⁺ by partial substitution of Sr at the La sites.<br/>Sr-doped LLZO sintered pellets were prepared by solid-state reaction method. The sintered pellets exhibit high conductivity of up to 5.7×10^-4 S/cm at room temperature, indicating a promising material as a solid electrolyte.<br/>Sr <i>K</i>-edge XANES spectra confirmed that Sr was located at the La sites of the LLZO. The crystal structure of Sr-substituted LLZO was measured by X-ray diffraction and neutron diffraction, and the results showed that the crystal structure of LLZO continuously changes from tetragonal to cubic with increasing the amount of Sr substitutions. At the same time, the Li⁺ arrangement was also changed, with Li⁺ occupying some tetrahedral sites that are vacant in the unsubstituted LLZO.<br/>The local structure of Sr-substituted LLZO was discussed from Zr <i>K</i>-edge and La <i>K</i>-edge XANES. La <i>K</i>-edge XANES spectra did not change with Sr substitution, while Zr <i>K</i>-edge XANES spectra showed a spectral change corresponding to the amount of Sr substitution. This result suggests that the crystal structure change of Sr-substituted LLZO is not the result of the homogeneous deformation of the entire structure but is mainly due to the deformation of the Zr sites, which are unsubstituted.<br/>The above results suggest that the high conductivity of Sr-substituted LLZO is due to multiple factors: the change in the Li⁺ arrangement and the change in the Li⁺ conduction pathway due to the local structure deformation. Our results contribute to a better understanding of the correlation between the structure and properties of LLZOs in Li excess compositions and the development of higher-performance electrolytes.