Discovering the Influence of Lithium Loss on Garnet Li7La3Zr2O12 Electrolyte Phase Stability

Garnet-type lithium lanthanum zirconate (Li7La3Zr2O12, LLZO)-based ceramic electrolyte has potential for further development of all-solid-state energy storage technologies including Li metal batteries as well as Li-S and Li-O-2 chemistries. The essential prerequisites such as LLZO's compactness, stability, and ionic conductivity for this development are nearly achievable via the solid-state reaction route (SSR) at high temperatures, but it involves a trade-off between LLZO's caveats because of Li loss via volatilization. For example, SSR between lithium carbonate, lanthanum oxide, and zirconium oxide is typically supplemented by dopants (e.g., gallium or aluminum) to yield the stabilized cubic phase (c-LLZO) that is characterized by ionic conductivity an order of magnitude higher than the other polymorphs of LLZO. While the addition of dopants as phase stabilizing agent and supplying extra Li precursor for compensating Li loss at high temperatures become common practice in the solid-state process of LLZO, the exact role of dopants and stabilization pathway is still poorly understood, which leads to several manufacturing issues. By following LLZO's chemical phase evolution in relation to Li loss at high temperatures, we here show that stabilized c-LLZO can directly be achieved by an in situ control of lithium loss during SSR and without needing dopants. In light of this, we demonstrate that dopants in the conventional SSR route also play a similar role, i.e., making more accessible Li to the formation and phase stabilization of c-LLZO, as revealed by our in situ X-ray diffraction analysis. Further microscopic (STEM, EDXS, and EELS) analysis of the samples obtained under various SSR conditions provides insights into LLZO phase behavior. Our study can contribute to the development of more reliable solid-state manufacturing routes to Garnet-type ceramic electrolytes in preferred polymorphs exhibiting high ionic conductivity and stability for all-solid-state energy storage.