Garnet‐type Li7La3Zr2O12 (LLZ) and its analogs are considered potential candidates as solid electrolytes for all‐solid‐state Li metal batteries because their fast Li‐ion conductivity and chemical stability against Li metal anodes result in both safe and large energy densities. To date, several computational and experimental studies have been performed to obtain a more detailed understanding of the nonreactivity of garnet‐type LLZ with Li metal and related phenomena. Herein, first‐principles calculations based on the density functional theory approach are performed for Li metal/LLZ interfaces to elucidate the electronic and atomistic level aspects. It is confirmed that the valence band maximum and the conduction band minimum for the LLZ phase do not cross the Fermi level corresponding to the Li 2s band. In addition, it is found that the defect formation energies associated with both Li vacancy formation and interstitial Li formation are largely reduced. In particular, the formation energy associated with interstitial Li sites, i.e., Li+ insertion into the LLZ phase, is negative, indicating that spontaneous Li insertion is likely to proceed in the vicinity of the Li/LLZ interface. This new suggestion may help further elucidation of the reported interfacial behavior.

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