Tailoring Li–Al–O Interphases in Garnet-Type Solid-State Electrolytes via Powder Atomic Layer Deposition

Garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) solid-state electrolyte (SSE) faces challenges such as high interfacial resistance and lithium dendrite propagation. Meanwhile, atomic layer deposition (ALD) offers precise control over surface chemistry and nanoscale interfacial structures, enabling critical advancements in SSE design. Here, we investigate the influence of Al₂O₃ ALD powder coatings on LLZTO, with emphasis on structural evolution, chemical interdiffusion, and electrochemical performance. ²⁷Al magic angle spinning NMR, XPS, and STEM measurements confirm lithium diffusion during ALD, forming a compositionally graded, nanocrystalline Li-Al-O interphase. Subsequently, this ALD layer forms a multiphase microstructure during high-temperature sintering comprising LiAlO₂, Li₂ZrO₃, and LaAlO₃ with their phase fractions and spatial distribution being directly controlled by ALD coating thickness, enabling tunable densification and ion transport characteristics. Thickness-dependent regimes of sintering are introduced, which, evaluated by electrochemical experiments, show that medium-thickness coatings of ∼6.8 nm (25 ALD cycles) yield optimal performance. With a room temperature ionic conductivity of 0.39 mS cm^-1 and a critical current density of 0.35 mA cm^-2, they outperform both thinner and thicker coatings, as the former suffer from insufficient densification, while the latter suffer from phase overgrowth. This work provides mechanistic insight into the ALD-guided modification of the chemical and morphological landscape of garnet-type SSEs. More broadly, it establishes design principles for engineering interphases with tailored transport properties, offering a scalable and tunable strategy for advancing the performance of solid-state lithium metal batteries.