With depth, the valence state of silver shifts from a metallic Ag 0 (368.3 eV binding energy) to an oxidized Ag + (366.0 eV) (Figure ). In contrast, the Zr 3d and La 3d peak position remain constant (with shifts < 0.1 eV) (see Methods and Figure ). To confirm that the silver peak shift is not an artifact of Ar sputtering, we performed XPS depth profiling on a 1-µm-thick metallic silver film on silicon substrate; binding energy did not shift with depth (Figure ). Additionally, XPS on a thin metallic silver coating (3 nm) shows no metallic silver on the LLZO surface after post-annealing at 300 ºC (Figure ), confirming a complete diffusion into LLZO. Finally, the XPS revealed no detectable impurities in the sputtered metallic silver film (Figure ).