Excellent Stability of Ga-Doped Garnet Electrolyte against Li Metal Anode via Eliminating LiGaO₂ Precipitates for Advanced All-Solid-State Batteries

Ga-doped garnet-type Li₇La₃Zr₂O₁₂ (Ga-LLZO) ceramics have long been recognized as ideal electrolyte candidates for all-solid-state lithium batteries (ASSLBs). However, in this study, it is shown that Ga-LLZO easily and promptly cracks in contact with molten lithium during the ASSLB assembly. This can be mainly ascribed to two aspects: (i) lithium captures O atoms and reduces Ga ions of the Ga-LLZO matrix, leading to a band-gap closure from >5 to <2 eV and a structural collapse from cubic to tetrahedral; and (ii) the in situ-formed LiGaO₂ impurity phase has severe side reactions with lithium, resulting in huge stress release along the grain boundaries. It is also revealed that, while the former process consumes hours to take effect, the latter one is immediate and accounts for the crack propagation of Ga-LLZO electrolytes. A minute SiO₂ is preadded during the synthesis of Ga-LLZO and found effective in eliminating the LiGaO₂ impurity phase. The SiO₂-modified Ga-LLZO solid electrolytes display excellent thermomechanical and electrochemical stabilities against lithium metals and well-reserved ionic conductivities, which was further confirmed by half-cells and full batteries. This study contributes to the understanding of the stability of garnet electrolytes and promotes their potential commercial applications in ASSLBs.