Origin of High Li+ Conduction in Doped Li7La3Zr2O12 Garnets

Substituting a native ion in the crystals with a foreign ion that has a difference in valence, termed as aliovalent doping, has been widely attempted to upgrade solid-state ionic conductors for various charge carriers including O^2–, H⁺, Li⁺, Na⁺, and so forth.(1-4) The doping aids to promote the high-conductive framework and dredge the tunnel for fast ion transport. The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a fast Li⁺ solid conductor, which received vast attention as an electrolyte candidate for all-solid-state lithium ion batteries, showing great potential to offer high energy density and minimize battery safety concerns to meet extensive applications in large energy storage systems such as electric vehicles and aerospace.(5-8) In the Li-stuffed garnet framework of LLZO, the 3D pathway formed through the incompletely occupied tetrahedral sites bridged by a single octahedron enables the superior Li⁺ conductivity.(9, 10) For the purpose of optimal performance, many efforts of aliovalent doping have been made throughout metal elements (Al³⁺, Ta⁵⁺) and metalloid elements (Ga³⁺, Te⁶⁺) in the periodic table with various valences(11-14) to stabilize the high-conductive phase and increase the Li vacancy concentration.(7, 10, 15)