Ion Dynamics in Solid Electrolytes: NMR Reveals the Elementary Steps of Li⁺ Hopping in the Garnet Li_6.5La₃Zr_1.75Mo_0.25O₁₂

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 Li_6.5La₃Zr_1.75Mo_0.25O₁₂ (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

Crystal Structure of Garnet-Related Li-Ion Conductor Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

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 <i>Ia</i>-3<i>d</i>, 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 Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂ with <i>x</i> > 0.07 crystallizes in the acentric cubic space group <i>I</i>-43<i>d</i>. 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>I</i>-43<i>d</i> compared to space group <i>Ia</i>-3<i>d</i>. Despite all efforts undertaken to reveal structure–property relationships for this class of materials, this study highlights the potential for new discoveries

Crystal Structure of Garnet-Related Li-Ion Conductor Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

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 <i>Ia</i>-3<i>d</i>, 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 Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂ with <i>x</i> > 0.07 crystallizes in the acentric cubic space group <i>I</i>-43<i>d</i>. 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>I</i>-43<i>d</i> compared to space group <i>Ia</i>-3<i>d</i>. Despite all efforts undertaken to reveal structure–property relationships for this class of materials, this study highlights the potential for new discoveries

Site Occupation of Ga and Al in Stabilized Cubic Li_{7–3(<i>x</i>+<i>y</i>)}Ga_<i>x</i>Al_<i>y</i>La₃Zr₂O₁₂ Garnets As Deduced from ²⁷Al and ⁷¹Ga MAS NMR at Ultrahigh Magnetic Fields

Li-containing garnets, which are stabilized in their cubic modification by doping with Al or Ga, show very high Li-ion conductivities. This property qualifies them to be used as solid electrolytes in advanced all-solid-state batteries. The relation between local structures and dynamic properties, however, is still not fully understood. Here, cubic mixed-doped Li_{7–3(<i>x</i>+<i>y</i>)}Ga_<i>x</i>Al_<i>y</i>La₃Zr₂O₁₂ garnet solid solutions with different portions of Al and Ga were synthesized. It turned out that the solubility of Ga is higher than that of Al; the evaluation of 42 different doping compositions indicated an increase of the lattice parameter <i>a</i>₀ with increasing Ga content. ⁷¹Ga MAS NMR spectra recorded at 21.1 T revealed two ⁷¹Ga NMR resonances, corresponding to Ga occupying both the 24<i>d</i> (243 ppm) and 96<i>h</i> sites (193 ppm). This behavior, which has been observed for the first time in this study, is very similar to that of Al. The ⁷¹Ga NMR line at 193 ppm observed here remained invisible in previous NMR studies that were carried out at lower magnetic fields. The invisibility at lower field is because of large second-order quadrupolar broadening that has a lower effect on the ⁷¹Ga NMR spectra at higher magnetic field. Most importantly, the similarity in site preference of Al and Ga found here inevitably raises a question about the significance of a blocking effect on long-range Li-ion transport. It weakens the assumption that the site preference of dopants is responsible for the higher Li diffusivity of Ga-doped samples compared to the Al-doped analogues. Concerning Li-ion dynamics, our ⁷Li NMR line shape measurements indicate that the change in lattice constant <i>a</i>₀ with increasing doping level seems to have a larger influence on Li-ion dynamics than the Al:Ga ratio

Synthesis, Crystal Chemistry, and Electrochemical Properties of Li_{7–2<i>x</i>}La₃Zr_2–<i>x</i>Mo_<i>x</i>O₁₂ (<i>x</i> = 0.1–0.4): Stabilization of the Cubic Garnet Polymorph via Substitution of Zr⁴⁺ by Mo⁶⁺

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 Zr⁴⁺ by Mo⁶⁺. A Mo⁶⁺ content of 0.25 per formula unit (pfu) stabilizes the cubic LLZO phase, and the solubility limit is about 0.3 Mo⁶⁺ pfu. Based on the results of neutron powder diffraction and Raman spectroscopy, Mo⁶⁺ is located at the octahedrally coordinated 16<i>a</i> site of the cubic garnet structure (space group <i>Ia</i>-3<i>d</i>). Since Mo⁶⁺ has a smaller ionic radius compared to Zr⁴⁺ the lattice parameter <i>a</i>₀ decreases almost linearly as a function of the Mo⁶⁺ 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 Mo⁶⁺ substituted LLZO indicate that the material is stable up to 6 V

Fast Li-Ion-Conducting Garnet-Related Li_{7–3<i>x</i>}Fe_<i>x</i>La₃Zr₂O₁₂ with Uncommon <i>I</i>4̅3<i>d</i> Structure

Fast Li-ion-conducting Li oxide garnets receive a great deal of attention as they are suitable candidates for solid-state Li electrolytes. It was recently shown that Ga-stabilized Li₇La₃Zr₂O₁₂ crystallizes in the acentric cubic space group <i>I</i>4̅3<i>d</i>. This structure can be derived by a symmetry reduction of the garnet-type <i>Ia</i>3̅<i>d</i> structure, which is the most commonly found space group of Li oxide garnets and garnets in general. In this study, single-crystal X-ray diffraction confirms the presence of space group <i>I</i>4̅3<i>d</i> also for Li_{7–3<i>x</i>}Fe_<i>x</i>La₃Zr₂O₁₂. The crystal structure was characterized by X-ray powder diffraction, single-crystal X-ray diffraction, neutron powder diffraction, and Mößbauer spectroscopy. The crystal–chemical behavior of Fe³⁺ in Li₇La₃Zr₂O₁₂ is very similar to that of Ga³⁺. The symmetry reduction seems to be initiated by the ordering of Fe³⁺ onto the tetrahedral Li1 (12<i>a</i>) site of space group <i>I</i>4̅3<i>d</i>. Electrochemical impedance spectroscopy measurements showed a Li-ion bulk conductivity of up to 1.38 × 10^–3 S cm^–1 at room temperature, which is among the highest values reported for this group of materials

Synthesis, Crystal Structure, and Stability of Cubic Li_7–<i>x</i>La₃Zr_2–<i>x</i>Bi_<i>x</i>O₁₂

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, Li_7–<i>x</i>La₃Zr_2–<i>x</i>Bi_<i>x</i>O₁₂ (<i>x</i> = 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 <i>x</i> ≥ 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 16<i>a</i> site was confirmed by Raman spectroscopy. ⁷Li NMR spectroscopy shows that fast Li ion dynamics are only observed for samples with high Bi⁵⁺ contents