Li7La3Zr2O12 (LLZO) garnet is
one kind of solid electrolyte drawing extensive attention due to its
good ionic conductivity, safety, and stability toward lithium metal
anodes. However, the stability problem during synthesis and storage
results in high interfacial resistance and prevents it from practical
applications. We synthesized air-stable dual-doped Li6.05La3Ga0.3Zr1.95Nb0.05O12 ((Ga, Nb)-LLZO) cubic-phase garnets with ionic conductivity
of 9.28 × 10–3 S cm–1. The
impurity-phase species formation on the garnet pellets after air exposure
was investigated. LiOH and Li2CO3 can be observed
on the garnet pellets by Raman spectroscopy, X-ray diffraction (XRD),
and X-ray photoelectron spectroscopy (XPS) once the garnets are exposed
to humid air or come in contact with water. The (Ga, Nb)-LLZO garnet
is found to form less LiOH and Li2CO3, which
can be further reduced or removed after drying treatment. To confirm
the stability of the garnet, an electrochemical test of the Li//Li
symmetric cell was also performed in comparison with previously reported
garnets (Li7La2.75Ca0.25Zr1.75Nb0.25O12, (Ca, Nb)-LLZO). The dual-doped (Ga,
Nb)-LLZO showed less polarized and stable plating/stripping behavior
than (Ca, Nb)-LLZO. Through Rietveld refinement of XRD patterns of
prepared materials, dopant Ga was found to preferably occupy the Li
site and Nb takes the Zr site, while dopant Ca mainly substituted
La in the reference sample. The inherited properties of the dopants
in (Ga, Nb)-LLZO and their structural synergy explain the greatly
improved air stability and reduced interfacial resistance. This may
open a new direction to realize garnet-based solid electrolytes with
lower interfacial resistance and superior air stability.
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