All-solid-state lithium-ion batteries is considered a front runner among next-generation battery technologies. Solid-electrolytes which allows for Li-ion only mobility and practically inflammable nature, it solves safety and cross migration issues. Additionally, all solid-sate batteries promise significantly high energy as well as power densities. [1], [2] Among the many challenges pertaining to all-solid-state batteries, the electrochemical stability of solid-electrolytes as well as the interfaces remains a key challenge. Recent computational as well as experimental studies have shown that many of these lithium-ion solid electrolytes has poor electrochemical stability at high voltage. [3]–[5] Lithium-stuffed garnet-type solid-electrolyte is one the most widely studied material for all-solid-state batteries for its’ high ionic conductivity and kinetic stability against lithium metal anode. [6], [7] The computational works for Li₇La₃Zr₂O₁₂ (LLZO) garnets have predicted poor electrochemical stability, with oxidation onsetting as low as 2.9 V vs. Li/Li+. [8], [9] Indeed, poor cycling performance has been noted in many experimental studies employing LLZO-type garnets solid-electrolytes, [10]–[12]  which among other factors, possibly, implicate as predicted low oxidation potential. Earlier studies based on utilizing electrochemical techniques have suggested oxidative decomposition at potential as low as 3.7 V vs. Li/Li⁺. [9], [13], [14]. Nonetheless, there remains a gap in the literature for local structural changes in these solid-electrolytes in response to relevant high electrochemical potential.

In this study, we studied local structural and impedance change of representative Al-doped LLZO garnet solid electrolyte as a result of electrochemical reactivity. The experimental techniques utilized the Zr K-edge extended x-ray absorption fine structure (EXAFS) to characterize the local structure and O K-edge soft x-ray absorption spectroscopy (XAS) to deconvolute degradation products. The EXAFS spectra were collected at Advanced Photon Source (APS) beamline 10-ID-B, Argonne National Laboratory. The O K-edge XAS was collected at beamline 23-ID-2 at NSLS-II, Brookhaven National Laboratory.The results shows that Zr coordination shells in LLZO underwent major shrinkage with added, seemingly, loss of coordination number. These results are in agreement with electrochemical induced loss of oxygen from the lattice, a mechanism reported earlier. [9], [15] The O K-edge XAS which further confirms the degradation of LLZO by showing suppression of its’ signature feature as a result of electrochemical oxidation. The growth of impedance after polarization suggest Li-mobility is significantly reduced as a result of electrochemical reactivity at higher potential. In summary, our results show significant local structural change in garnet-type solid-electrolytes under electrochemical potentials at 4.3 V vs. Li/Li⁺. These local changes in structure directly impedes Li-ion mobility. Cell degradation at these potentials could result from oxidation instability of the garnet electrolyte.