Li-oxide
garnets such as Li
7
La
3
Zr
2
O
12
(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
-3
d
, has to be stabilized with supervalent
cations. LLZO stabilized with Ga
3+
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 Li
7–3
x
Ga
x
La
3
Zr
2
O
12
with
x
> 0.07 crystallizes in the acentric cubic space group
I
-43
d
. 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 Ga
3+
.
7
Li NMR spectroscopy
indicates an additional Li-ion diffusion process for LLZO with space
group
I
-43
d
compared to space group
Ia
-3
d
. Despite all efforts undertaken to
reveal structure–property relationships for this class of materials,
this study highlights the potential for new discoveries.
The interface stability versus Li represents a major challenge in the development of next-generation all-solid-state batteries (ASSB), which take advantage of the inherently safe ceramic ...electrolytes. Cubic Li7La3Zr2O12 garnets represent the most promising electrolytes for this technology. The high interfacial impedance versus Li is, however, still a bottleneck toward future devices. Herein, we studied the electrochemical performance of Fe3+-stabilized Li7La3Zr2O12 (LLZO:Fe) versus Li metal and found a very high total conductivity of 1.1 mS cm–1 at room temperature but a very high area specific resistance of ∼1 kΩ cm2. After removing the Li metal electrode we observe a black surface coloration at the interface, which clearly indicates interfacial degradation. Raman- and nanosecond laser-induced breakdown spectroscopy reveals, thereafter, the formation of a 130 μm thick tetragonal LLZO interlayer and a significant Li deficiency of about 1–2 formula units toward the interface. This shows that cubic LLZO:Fe is not stable versus Li metal by forming a thick tetragonal LLZO interlayer causing high interfacial impedance.
Garnet-type oxides are considered to belong to the most attractive solid Li+ electrolytes. This is due to their wide electrochemical stability window as well as their superior ionic conductivity, ...with a Li-ion transference number of almost one. Usually ionic conductivities are studied via impedance spectroscopy on a macroscopic length scale. Time-domain NMR methods, however, have been used much less extensively to shed light on the elementary hopping processes in highly conducting oxide garnets. Here, we used NMR relaxometry and stimulated echo NMR to study Li+ self-diffusion in Li6.5La3Zr1.75Mo0.25O12 (LLZMO), which served as a model compound to collect information on the 7Li spin dynamics. It turned out that NMR spin–lattice relaxation (SLR) recorded in both the laboratory and rotating frame of reference shows features that seem to be a universal fingerprint for fast conducting garnets that have been stabilized in their cubic modification. In contrast to Al-doped garnet-type Li7La3Zr2O12 that modifies the Li sublattice, in LLZMO the Li sublattice remains intact, offering the possibility to get to the bottom of Li-ion dynamics in LLZO-based garnets. Most importantly, whereas NMR SLR rates measured at 194.3 MHz reflect an almost universal behavior of local hoppings being thermally activated by only 0.151(3) eV, the spin-lock technique (33.3̅ kHz) gives evidence of two separate, overlapping rate peaks with activation energies on the order of 0.29 eV for the elementary steps of Li-ion hopping. This points to a less pronounced distribution of Li+ jump rates on the kilohertz time scale than has been observed for the Al-stabilized LLZO samples. The NMR results obtained also entail information on both the Li+ diffusion coefficients and the shape of the underlying motional correlation functions. The latter has been provided by 7Li NMR spin-alignment echo correlation spectroscopy that also shows the involvement of 24d and 96h sites in Li+ diffusion.
Li oxide garnets are among the most promising candidates for solid-state electrolytes in novel Li ion and Li metal based battery concepts. Cubic Li7La3Zr2O12 stabilized by a partial substitution of ...Zr4+ by Bi5+ has not been the focus of research yet, despite the fact that Bi5+ would be a cost-effective alternative to other stabilizing cations such as Nb5+ and Ta5+. In this study, Li7-xLa3Zr2-xBixO12 (x = 0.10, 0.20, ..., 1.00) was prepared by a low-temperature solid-state synthesis route. The samples have been characterized by a rich portfolio of techniques, including scanning electron microscopy, X-ray powder diffraction, neutron powder diffraction, Raman spectroscopy, and 7Li NMR spectroscopy. Pure-phase cubic garnet samples were obtained for x ≥ 0.20. The introduction of Bi5+ leads to an increase in the unit-cell parameters. Samples are sensitive to air, which causes the formation of LiOH and Li2CO3 and the protonation of the garnet phase, leading to a further increase in the unit-cell parameters. The incorporation of Bi5+ on the octahedral 16a site was confirmed by Raman spectroscopy. 7Li NMR spectroscopy shows that fast Li ion dynamics are only observed for samples with high Bi5+ contents.
In the present work, the oxidation and spin state of Fe and the local structure around Fe in the supercapacitor birnessite with different contents of the Fe dopant were investigated using Mössbauer ...spectroscopy. It was found that Fe ions were exclusively present as high spin Fe
3+
in octahedral coordination with about 70% iron occupying the Mn
3+
positions and about 30% iron occupying the Mn
4+
positions in the MnO
6
octahedra for all Fe-doped birnessite samples. Based on these new findings, the trend of typical cell parameters, selected bond lengths of the Fe-doped birnessites and their corresponding quadrupole splittings in the Mössbauer spectra were well explained by considering both the weakened Jahn–Teller effect during the replacement of Mn
3+
by Fe
3+
and the expansion of octahedra during the replacement of Mn
4+
by Fe
3+
. The present work offers some new insights into the understanding of the mechanism of the heterogeneous atomic doping on the crystal structure of birnessite, with importance for both mineralogy and material science.
Lorandite from Allchar as geochemical detector for pp-solar neutrinos Pavićević, Miodrag K.; Amthauer, Georg; Cvetković, Vladica ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
07/2018, Letnik:
895, Številka:
C
Journal Article
Recenzirano
Odprti dostop
LOREX (LORandite EXperiment) is a geochemical project addressing the solar proton–proton neutrino flux for the period of 4.31(2) Ma from the reaction 205Tl + νe→ 205Pb + e− with a very low threshold ...(52 keV) for solar pp-neutrino capture. A decisive step for this purpose is to obtain the precise, background-corrected ratio of 205Pb/205Tl in the mineral lorandite (TlAsS2) as geochemical detector occurring in the ore deposit of Allchar in Macedonia. This study presents a report on the excavation of lorandite bearing ore from adit P-21 of the ore body Crven Dol as well as on the separation of pure lorandite from the raw ore. A detailed mineralogical and chemical investigation of the separated lorandite is performed with special regard to the question of its use as detector for solar pp-neutrinos.
A total of 30 synthetic samples of the Ca2Fe2-xAlxO5, 0.00 ≤ x ≤ 1.34 solid solution series have been investigated by single crystal X-ray diffraction at 25°C. Pure Ca2Fe2O5 and samples up to x = ...0.56 have space group Pnma, Z = 4, whereas samples with x > 0.56 show I2mb symmetry, Z = 4. The substitution of Fe3+ by the smaller Al3+ cation decreases unit-cell parameters and average octahedral and tetrahedral bond lengths and induces distinct changes in the O-atom coordination of the interstitial Ca atom. Discontinuities in the structural parameters vs. the Al3+tot content and changes in slope of these quantities are associated with the phase transition. The essential difference between the two modifications is the cation-O atom-cation angle within the planes of corner sharing octahedra, which is close to 180° in I2mb, but ≈184° in the Pnma phase, and the existence of two different orientations of the tetrahedral chains in Pnma as opposed to one in I2mb. At low overall Al3+ concentrations Al3+ preferentially enters the tetrahedral site until ≈2/3 of it is filled. Additional Al3+ cations, substituted for Fe3+, are equally distributed over octahedral and tetrahedral sites. At high temperature pure Ca2Fe2O5 transforms to a body-centered structure at 724(4) °C. Substituting Al3+ for Fe3+ linearly decreases the transition temperature by 15°C per 0.1 Al3+ down to 623(5)°C for x = 0.65.
We investigate theoretically the site occupancy of Al3+ in the fast-ion-conducting cubic-garnet Li7-3xAl3+xLa3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured 27Al ...NMR chemical shifts an analysis shows that Al3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad 27Al NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al3+ site has not been observed experimentally.
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