The garnet-type Li7La3Zr2O12 (LLZO) solid electrolyte is of particular interest because of its good chemical stability under atmospheric condition, suitable for practical all-solid-state batteries (ASSBs). However, recent works observed electrochemical instability at the LLZO/Li interfaces. Herein, we have revealed the origin of the instability by performing a comprehensive first-principles investigation with a high-throughput interface structure search scheme, based on the density functional theory framework. Based on the constructed phase diagrams of low-index surfaces, we found that the coordinatively unsaturated (i.e. coordination number < 6) Zr sites exist widely on the low-energy LLZO surfaces. These undercoordinated Zr sites are reduced once the LLZO surface is in contact with the Li metal, leading to chemical instability of the LLZO/Li interface. Besides, the calculated formation and adhesion energies of interfaces suggest that the Li wettability on the LLZO surface is dependent on the termination structure. The employment of the approaches such as by controlling the synthesis atmosphere are needed for preventing the reduction of LLZO against the Li metal. The present analysis with comprehensive first-principles calculations provides a novel perspective for the rational optimization of the interface between LLZO electrolyte and Li metal anode in the ASSB.
Keywords: all-solid-state battery; first-principles calculations; interface stability; lithium metal anode; solid electrolyte; surface morphology.