Is the single-ion conductor cubic Li₇La₃Zr₂O₁₂ a binary ionic electrolyte?

Area of Science:

• Solid-state electrochemistry
• Materials science
• Lithium-ion battery technology

Background:

• Garnet-type cubic Li$_{7}$La$_{3}$Zr$_{2}$O$_{12}$ (c-LLZO) is recognized as a promising solid electrolyte for lithium batteries.
• Despite its designation as a single-ion conductor, lithium dendrite formation during plating suggests complex ionic transport mechanisms.

Purpose of the Study:

• To analyze the charge carriers present in c-LLZO under electrochemical conditions.
• To elucidate the mechanisms behind lithium dendrite initiation in c-LLZO symmetric cells.
• To reconcile the observed dendrite behavior with fundamental electrochemical principles.

Main Methods:

• Electrochemical analysis of Li|c-LLZO|Li symmetric cells during one-way lithium plating.
• Theoretical analysis of coexisting species and charge carriers within c-LLZO.
• Investigation of concentration polarization phenomena.

Main Results:

• Lithium dendrite initiation in c-LLZO mirrors mechanisms observed in binary liquid electrolytes.
• Four distinct species coexist in c-LLZO during electrochemical reactions.
• Significant concentration polarization occurs before dendrite onset in c-LLZO.

Conclusions:

• Under Faradaic reaction conditions, c-LLZO functions as a binary ionic electrolyte, not a single-ion conductor.
• Electrically generated lithium vacancies significantly influence long-range ion transport in c-LLZO.
• The transformation of c-LLZO into a binary electrolyte explains observed dendrite initiation phenomena.