We carry out DFT calculations to understand the Ag + doping mechanism. The defect formation energy (𝐸 𝑓 ) of ion exchanging Li + with Ag + in the pristine LLZO was computed as per the methods section. In the ion exchange reaction 𝐿𝑖 𝐿𝑖 + 𝐴𝑔 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 → 𝐴𝑔 𝐿𝑖 × + 𝐿𝑖 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 (Kröger-Vink notation) where metallic Ag is the reactant and metallic Li as the product, the 𝐸 𝑓 is ~ 3.7 eV, which is prohibitively high. Yet, the presence of trace impurities can lower the defect formation energy significantly, specifically oxygen. In an oxidizing environment, the ion exchange reaction becomes 𝐿𝑖 𝐿𝑖 + 𝐴𝑔 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 1 we compute an 𝐸 𝑓 of ~ 0.92 eV (with respect to O2 molecule). This significantly lowered 𝐸 𝑓 is due to the formation of stable Li2O rather than metallic Li, and indicates that the Ag-Li ion exchange is possible, especially at elevated temperatures. The computationally predicted Ag-Li ion exchange is consistent with experimental observations that lithium in some form segregates to the LLZO surface after Ag + doping (Figure ) and precipitate as a second phase (Figure ).