Interface modification in solid-state lithium batteries based on garnet-type electrolytes with high ionic conductivity

Robust and Manufacturable Lithium Lanthanum Titanate-Based Solid-State Electrolyte Thin Films Deposited in Open Air

State-of-the-art solid-state electrolytes (SSEs) are limited in their energy density and processability based on thick, brittle pellets, which are generally hot pressed in vacuum over the course of several hours. We report on a high-throughput, open-air process for printable thin-film ceramic SSEs in a remarkable one-minute time frame using a lithium lanthanum titanium oxide (LLTO)-based SSE that we refer to as robust LLTO (R-LLTO). Powder XRD analysis revealed that the main phase of R-LLTO is polycrystalline LLTO, accompanied by selectively retained crystalline precursor phases. R-LLTO is highly dense and closely matched to the stoichiometry of LLTO with some heterogeneity throughout the film. A minimal presence of lithium carbonate is identified despite processing fully in ambient conditions. The LLTO films exhibit remarkable mechanical properties, demonstrating both flexibility with a low modulus of ∼35 GPa and a high fracture toughness of >2.0 . We attribute this mechanical robustness to several factors, including grain boundary strengthening, the presence of precursor crystalline phases, and a decrease in crystallinity or ordering caused by ultrafast processing. The creation of R-LLTO-a ceramic material with elastic properties that are closer to polymers with higher fracture toughness-enables new possibilities for the design of robust solid-state batteries.

Bonding Lithium Metal with Garnet Electrolyte by Interfacial Lithiophobicity/Lithiophilicity Transition Mechanism over 380 °C

Garnet electrolytes, possessing high ionic conductivity (10^-4 -10^-3 S cm^-1 at room temperature) and excellent chemical/electrochemical compatibility with lithium metal, are expected to be used in solid-state lithium metal batteries. However, the poor solid-solid interfacial contact between lithium and garnet leads to high interfacial resistance, reducing the battery power capability and cyclability. Garnet electrolytes are commonly believed to be intrinsically lithiophilic, and lithiophobic Li₂ CO₃ on the garnet surface accounted for the poor interfacial contact. Here, it is proposed that the interfacial lithiophobicity/lithiophilicity of garnets (LLZO, LLZTO) can be transformed above a temperature of ≈380 °C. This transition mechanism is also suitable for other materials such as Li₂ CO₃ , Li₂ O, stainless steel, and Al₂ O₃ . By using this transition mechanism, uniform and even lithium can be strongly bonded no-surface-treated garnet electrolytes with various shapes. The Li-LLZTO interfacial resistance can be reduced to ≈3.6 Ω cm² and sustainably withstood lithium extraction and insertion for up to 2000 h at 100 µA cm^-2 . This high-temperature lithiophobicity/lithiophilicity transition mechanism can help improve the understanding of lithium-garnet interfaces and build practical lithium-garnet solid-solid interfaces.