We also analyzed the elemental distribution at the Ag and LLZO interface. Cryo-scanning transmission electron microscopy electron energy loss spectroscopy (Cryo-STEM EELS) mapping suggests silver diffusion into LLZO by approximately 10 nm (Figure ). To more robustly quantify the extent of silver diffusion in LLZO, we employed NanoSIMS on the 3-nm metallic Ag coated LLZO, offering elemental sensitivity to the ppm level as well as lateral spatial resolution up to 50 nm (Figure ). After post-annealing, Ag is detected at a depth between 20 and 50 nm in LLZO with an average peak position at a depth of 28 nm. The Ag depth profiling varies substantially as a function of lateral position, likely due to grain-to-grain variation of silver diffusion. Additionally, a surface Li-enriched layer is observed on the annealed sample (Figure ). In contrast, Ag is detected only at depths less than 20 nm on the non-annealed sample (corresponding to the metallic Ag film), with all samples yielding silver peak (maximum Ag counts) splitting within a tight 1 nm window (additional NanoSIMS data are shown in Figure ). The profile broadening observed on the non-annealed sample could be attributed to the finite probe size and atomic mixing resulting from primary ion bombardment. We hypothesize that the large profile broadening (with respect to lateral position) on the annealed sample is due to a variation of silver diffusion rates in LLZO grains of different crystallographic orientations, possibly due to diffusion along the grain boundary (Figure ). Indeed, we observe silver segregation at grain boundaries to an average depth of 33 nm (Figure ). We note that the actual depth of silver diffusion may exceed this, as silver segregation to grain boundaries at greater depths could be beyond the detection limit of NanoSIMS. Together, optical microscopy, XPS and NanoSIMS depth profiling confirm the diffusion of Ag + in LLZO after post-annealing metallic Ag thin films. Hereafter, we will use the term "Ag + -LLZO" to denote Ag + doped LLZO, obtained by annealing 3-nm metallic Ag on LLZO at 300 °C, and "Ag 0 |LLZO" to denote LLZO coated with metallic Ag without annealing.