2, University of Michigan–Ann Arbor, Ann Arbor, Michigan, United States
The cubic garnet phase of Li7La3Zr2O12 (LLZO) is a promising solid electrolyte with a Li-ion conductivity approaching 10-3 mS/cm at room temperature. Its interfacial stability with Li metal remains an issue, however, and its study is made challenging by rapid oxidization in air. We have developed a UHV system in which lithium can be sputtered onto LLZO and its interface immediately characterized by X-ray photoelectron spectroscopy (XPS). Here, we exploit this sytem to investigate the stability of Al, Ta, and Nb-doped LLZO against metallic lithium. We find that the redox reaction at the interface is strongly dependent on the particular dopant. For Nb-doped LLZO, Nb is reduced from 5+ to 4+ by lithium; however, for the Ta and Al-doped electrolytes, Zr is reduced from 4+ to 3+. Considering the molar ratio of Nb doping element, the total charge transfer between Li and the LLZO is approximately the same for all three samples. Despite the relatively large change in valence, we find that the change in binding energy is less than 0.5 eV (toward lower binding energies) in the La-core level spectra for all samples. Among them, the Nb doped sample has the largest energy shift. The energy shifts are likely associating with the in-built interface potential; therefore, the Nb-doped LLZO is expected to show more upward band bending at the interface. Furthermore, to investigate how sensitive this interfacial redox reaction is to the oxidation layer at LLZO surface, we compared the results with those from LLZO surfaces that had been treated differently before lithium deposition. To completely remove the Li2CO3 species at the surface, some samples were heated in UHV or sputtered with Ar ions. Others were lightly sanded in a glove box, with a small portion of the oxidation layer left on surface.