Processing–Microstructure–Interface Relation in Garnet‐Type LLZO: From Powder Synthesis to Polymer–Ceramic Architectures

ABSTRACT Garnet‐type Li 7 La 3 Zr 2 O 12 (LLZO) is a leading oxide electrolyte for solid‐state lithium batteries because cubic LLZO offers millisiemens‐per‐centimeter Li + conductivity, thermal stability, and kinetic compatibility with lithium metal. However, practical performance is limited by lithium loss during heating, secondary phases, porosity, resistive grain boundaries, surface contamination, and uneven Li|LLZO contact, which can promote lithium filaments. This review links these problems to powder synthesis, phase stabilization, and densification. Solid‐state, sol–gel/Pechini, co‐precipitation, combustion, aerosol, and molten‐salt routes are compared by cation uniformity, lithium retention, particle morphology, and sinterability. Pressureless, pressure‐assisted, field‐assisted, ultrafast, and liquid‐phase sintering are evaluated for density, grain‐boundary resistance, phase purity, and lithium compatibility. The discussion also covers LLZO/polymer membranes, where filler shape, connectivity, interphase chemistry, thickness, and compliance control ion transport. Overall, bulk conductivity alone is insufficient; meaningful comparison requires total conductivity, density, microstructure, surface condition, interfacial resistance, and critical‐current data under clearly defined testing conditions.