Several “Beyond Li-Ion Battery”
concepts such as
all solid-state batteries and hybrid liquid/solid systems envision
the use of a solid electrolyte to protect Li-metal anodes. These configurations
are very attractive due to the possibility of exceptionally high energy
densities and high (dis)charge rates, but they are far from being
realized practically due to a number of issues including high interfacial
resistance and difficulties associated with fabrication. One of the
most promising solid electrolyte systems for these applications is
Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability
against reduction by Li metal. Nevertheless, the fabrication of dense
LLZO membranes with high ionic conductivity and low interfacial resistances
remains challenging; it definitely requires a better understanding
of the structural and electrochemical properties. In this study, the
phase transition from garnet (Ia3̅d, No. 230) to “non-garnet” (I4̅3d, No. 220) space group as a function of composition and
the different sintering behavior of Ga and Al stabilized LLZO are
identified as important factors in determining the electrochemical
properties. The phase transition was located at an Al:Ga substitution
ratio of 0.05:0.15 and is accompanied by a significant lowering of
the activation energy for Li-ion transport to 0.26 eV. The phase transition
combined with microstructural changes concomitant with an increase
of the Ga/Al ratio continuously improves the Li-ion conductivity from
2.6 × 10–4 S cm–1 to 1.2
× 10–3 S cm–1, which is close
to the calculated maximum for garnet-type materials. The increase
in Ga content is also associated with better densification and smaller
grains and is accompanied by a change in the area specific resistance
(ASR) from 78 to 24 Ω cm2, the lowest reported value
for LLZO so far. These results illustrate that understanding the structure–properties
relationships in this class of materials allows practical obstacles
to its utilization to be readily overcome.
Li-oxide
garnets such as Li7La3Zr2O12 (LLZO) are among the most promising candidates for
solid-state electrolytes to be used in next-generation Li-ion batteries.
The garnet-structured cubic modification of LLZO, showing space group Ia-3d, has to be stabilized with supervalent
cations. LLZO stabilized with Ga3+ shows superior properties
compared to LLZO stabilized with similar cations; however, the reason
for this behavior is still unknown. In this study, a comprehensive
structural characterization of Ga-stabilized LLZO is performed by
means of single-crystal X-ray diffraction. Coarse-grained samples
with crystal sizes of several hundred micrometers are obtained by
solid-state reaction. Single-crystal X-ray diffraction results show
that Li7–3xGaxLa3Zr2O12 with x > 0.07 crystallizes in the acentric cubic space group I-43d. This is the first definite record
of this
cubic modification for LLZO materials and might explain the superior
electrochemical performance of Ga-stabilized LLZO compared to its
Al-stabilized counterpart. The phase transition seems to be caused
by the site preference of Ga3+. 7Li NMR spectroscopy
indicates an additional Li-ion diffusion process for LLZO with space
group I-43d compared to space group Ia-3d. Despite all efforts undertaken to
reveal structure–property relationships for this class of materials,
this study highlights the potential for new discoveries.
Fast-conducting
phase-pure cubic Ga-bearing Li7La3Zr2O12 was obtained using solid-state
synthesis methods with 0.08 to 0.52 Ga3+ pfu in the garnet.
An upper limit of 0.72 Ga3+ pfu in garnet was obtained,
but the synthesis was accompanied by small amounts of La2Zr2O12 and LiGaO3. The synthetic
products were characterized by X-ray powder diffraction, electron
microprobe and SEM analyses, ICP-OES measurements, and 71Ga MAS NMR spectroscopy. The unit-cell parameter, a0, of the various garnets does not vary significantly
as a function of Ga3+ content, with a value of about 12.984(4)
Å. Full chemical analyses for the solid solutions were obtained
giving: Li7.08Ga0.06La2.93Zr2.02O12, Li6.50Ga0.15La2.96Zr2.05O12, Li6.48Ga0.23La2.93Zr2.04O12, Li5.93Ga0.36La2.94Zr2.01O12, Li5.38Ga0.53La2.96Zr1.99O12, Li4.82Ga0.60La2.96Zr2.00O12, and Li4.53Ga0.72La2.94Zr1.98O12. The NMR
spectra are interpreted as indicating that Ga3+ mainly
occurs in a distorted 4-fold coordinated environment that probably
corresponds to the general 96h crystallographic site
of garnet.
Cubic Li7La3Zr2O12 (LLZO) garnets are exceptionally well suited to be used as solid electrolytes or protecting layers in "Beyond Li-ion Battery" concepts. Unfortunately, cubic LLZO is not stable at room temperature (RT) and has to be stabilized by supervalent dopants. In this study we demonstrate a new possibility to stabilize the cubic phase at RT via substitution of Zr(4+) by Mo(6+). A Mo(6+) content of 0.25 per formula unit (pfu) stabilizes the cubic LLZO phase, and the solubility limit is about 0.3 Mo(6+) pfu. Based on the results of neutron powder diffraction and Raman spectroscopy, Mo(6+) is located at the octahedrally coordinated 16a site of the cubic garnet structure (space group Ia-3d). Since Mo(6+) has a smaller ionic radius compared to Zr(4+) the lattice parameter a0 decreases almost linearly as a function of the Mo(6+) content. The highest bulk Li-ion conductivity is found for the 0.25 pfu composition, with a typical RT value of 3.4 × 10(-4) S cm(-1). An additional significant resistive contribution originating from the sample interior (most probably from grain boundaries) could be identified in impedance spectra. The latter strongly depends on the prehistory and increases significantly after annealing at 700 °C in ambient air. Cyclic voltammetry experiments on cells containing Mo(6+) substituted LLZO indicate that the material is stable up to 6 V.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.