Microstructure of the Li-Al-O Second Phases in Garnet Solid Electrolytes

Area of Science:

• Materials Science
• Solid-State Chemistry
• Battery Technology

Background:

• The microstructure of lithium lanthanum zirconium oxide (LLZO) is crucial for all-solid-state lithium-ion battery performance.
• Grain boundary second phases formed during sintering significantly affect LLZO properties, but their structures and impacts are not fully understood.

Purpose of the Study:

• To investigate the crystal structures of second phases in LLZO pellets.
• To explore the structure-property relationships of these second phases.
• To determine how sintering aids influence LLZO microstructure and ionic conductivity.

Main Methods:

• Transmission electron microscopy (TEM) was employed to study the microstructure of LLZO pellets.
• Low-dose, high-resolution imaging was used to obtain atomic-scale lattice information of electron-beam-sensitive Li-Al-O second phases.
• Analysis of crystal structures of identified γ-LiAlO2, α-Li5AlO4, and β-Li5AlO4 phases.

Main Results:

• Three distinct Li-Al-O second phases (γ-LiAlO2, α-Li5AlO4, and β-Li5AlO4) were identified at the grain boundaries of LLZO.
• Atomic-scale structural data was obtained for these sensitive phases using advanced TEM techniques.
• A correlation was found between the Li/Al ratio of sintering aids and the formation of Li-rich second phases.

Conclusions:

• Sintering aids with a higher Li/Al ratio promote the formation of Li-rich second phases.
• These Li-rich second phases are associated with enhanced ionic conductivity in LLZO.
• This research provides critical insights into tailoring LLZO microstructure for improved solid-state battery performance.