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

Abstract

Garnet-type Li₇La₃Zr₂O₁₂ (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.