Clarifying the Dopant Local Structure and Effect on Ionic Conductivity in Garnet Solid-State Electrolytes for...

Area of Science:

• Materials Science
• Solid-State Chemistry
• Electrochemistry

Background:

• Li-rich garnets (Li7La3Zr2O12) are promising solid electrolytes for solid-state batteries due to high ionic conductivity and electrochemical stability.
• Aluminum (Al) and gallium (Ga) doping are common strategies to stabilize the cubic phase of Li7La3Zr2O12 at room temperature.
• The precise local structure and dopant site occupancy in Al- and Ga-doped Li7La3Zr2O12 remain poorly understood, despite extensive research.

Purpose of the Study:

• To unambiguously determine the site occupancy of Al and Ga dopants in Li7La3Zr2O12.
• To investigate the origin of observed NMR resonances and their relationship to dopant configurations.
• To elucidate the role of synthesis conditions, particularly excess lithium, on dopant behavior and phase formation.

Main Methods:

• Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy to analyze local structure and chemical environments.
• Electrochemical Impedance Spectroscopy (EIS) to measure lithium-ion conductivity.
• Analysis of synthesis byproducts and their influence on the final material structure.

Main Results:

• High-frequency NMR resonances previously attributed to dopants on lattice sites are definitively identified as originating from γ-LiAlO2 and LiGaO2 side products.
• Both Al and Ga dopants exclusively occupy the 24d crystallographic site within the Li7La3Zr2O12 lattice.
• Excess lithium used during synthesis consumes Al/Ga dopants, leading to the formation of these side products and the stabilization of the tetragonal phase.

Conclusions:

• The presence of γ-LiAlO2 and LiGaO2 side products, formed by excess lithium consuming dopants, significantly affects the actual dopant concentration in the Li7La3Zr2O12 lattice.
• These side products persist even after sintering and influence the lithium-ion conductivity of the solid electrolyte.
• Accurate control over synthesis conditions is crucial to minimize side product formation and optimize the performance of Li7La3Zr2O12 solid electrolytes.