Operando X-ray Photoelectron Spectroscopy Reveals Oxide Solid Electrolyte Stability against Electrochemically Plated Lithium

Abstract X-ray photoelectron spectroscopy (XPS) has revealed many possible Li-induced interfacial reactions in solid-state electrolytes (SSEs), but most in situ studies form Li|electrolyte interfaces through non-electrochemical Li deposition, including evaporation, sputtering, or electron-beam-induced deposition. These routes may introduce additional energy that activates reaction pathways not accessed during electrochemical Li plating. Here, we use lab-based operando XPS to directly evaluate oxide solid-state electrolyte stability during electrochemical Li formation. Using this approach, we find sharply different responses for two representative oxide SSEs: NASICON-type Li1+xAlxTi2–x(PO4)3 (LATP) is reduced upon Li plating, forming a Li+/Ti3+-rich mixed-conducting interphase that partially relaxes through coupled ionic–electronic redistribution, whereas garnet-type Li7La3Zr2O12 (LLZO) shows no spectroscopically resolvable Zr or La reduction. This work represents the first direct operando XPS evaluation of SSE stability against electrochemically plated Li metal.