Fast Li-Ion-Conducting Garnet-Related Li7-3x Fe x La3Zr2O12 with Uncommon I4̅3d Structure

Clarifying the Dopant Local Structure and Effect on Ionic Conductivity in Garnet Solid-State Electrolytes for Lithium-Ion Batteries

The high Li-ion conductivity and wide electrochemical stability of Li-rich garnets (Li7La3Zr2O12) make them one of the leading solid electrolyte candidates for solid-state batteries. Dopants such as Al and Ga are typically used to enable stabilization of the high Li+ ion-conductive cubic phase at room temperature. Although numerous studies exist that have characterized the electrochemical properties, structure, and lithium diffusion in Al- and Ga-LLZO, the local structure and site occupancy of dopants in these compounds are not well understood. Two broad 27Al or 69,71Ga resonances are often observed with chemical shifts consistent with tetrahedrally coordinated Al/Ga in the magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of both Al- and Ga-LLZO, which have been assigned to either Al and/or Ga occupying 24d and 96h/48g sites in the LLZO lattice or the different Al/Ga configurations that arise from different arrangements of Li around these dopants. In this work, we unambiguously show that the side products γ-LiAlO2 and LiGaO2 lead to the high frequency resonances observed by NMR spectroscopy and that both Al and Ga only occupy the 24d site in the LLZO lattice. Furthermore, it was observed that the excess Li often used during synthesis leads to the formation of these side products by consuming the Al/Ga dopants. In addition, the consumption of Al/Ga dopants leads to the tetragonal phase formation commonly observed in the literature, even after careful mixing of precursors. The side-products can exist even after sintering, thereby controlling the Al/Ga content in the LLZO lattice and substantially influencing the lithium-ion conductivity in LLZO, as measured here by electrochemical impedance spectroscopy.

Regulating Li-Ion Transport through Ultrathin Molecular Membrane to Enable High-Performance All-Solid-State-Battery

Solid-state lithium metal batteries with garnet-type electrolyte provide several advantages over conventional lithium-ion batteries, especially for safety and energy density. However, a few grand challenges such as the propagation of Li dendrites, poor interfacial contact between the solid electrolyte and the electrodes, and formation of lithium carbonate during ambient exposure over the solid-state electrolyte prevent the viability of such batteries. Herein, an ultrathin sub-nanometer porous carbon nanomembrane (CNM) is employed on the surface of solid-state electrolyte (SSE) that increases the adhesion of SSE with electrodes, prevents lithium carbonate formation over the surface, regulates the flow of Li-ions, and blocks any electronic leakage. The sub-nanometer scale pores in CNM allow rapid permeation of Li-ions across the electrode-electrolyte interface without the presence of any liquid medium. Additionally, CNM suppresses the propagation of Li dendrites by over sevenfold up to a current density of 0.7 mA cm-2 and enables the cycling of all-solid-state batteries at low stack pressure of 2 MPa using LiFePO4 cathode and Li metal anode. The CNM provides chemical stability to the solid electrolyte for over 4 weeks of ambient exposure with less than a 4% increase in surface impurities.

Dopant-induced modulation of lithium-ion conductivity in cubic garnet solid electrolytes: a first-principles study

Cubic garnet Li₇La₃Zr₂O₁₂ (c-LLZO) is a promising solid electrolyte for all-solid-state batteries, often doped with Ga, Al, and Fe to stabilize the structure and enhance Li-ion conductivity. Despite introducing the same amount of Li vacancies, these dopants with +3 classical charge yield different Li-ion conductivities by around an order of magnitude. In this study, we used density functional theory (DFT) calculations to investigate the impact of Ga, Fe, and Al dopants on Li chemical potential and Li-ion conductivity variations. We identified the energetically favorable dopant location in c-LLZO and determined the optimal U value of 7.5 eV for DFT+U calculations for dopant Fe in c-LLZO. Our calculations showed that Ga or Fe doping enhances the Li chemical potential by 0.05-0.08 eV, reducing Li-ion transfer barriers and increasing Li-ion conductivity, while Al doping lowers the Li chemical potential by 0.08 eV, reducing Li-ion conductivity. To determine the cause of Li chemical potential variations, we performed a combined analysis of the projected density of states, charge density, and Bader charge. The distinct charge distribution from dopant atoms to neighboring O atoms is critical for determining the Li-ion chemical potential. Ga and Fe dopants retain more electrons, which consequently makes the adjacent O atoms acquire a more positive charge that destabilizes Li ions by reducing the restraining force acting on them, thereby enhancing Li-ion conductivity. In contrast, Al doping transfers more electrons to neighboring O atoms, resulting in greater attraction forces to Li ions and reducing Li-ion conductivity. Additionally, Fe-doped LLZO exhibits extra states in the bandgap, potentially causing Fe reduction, as observed in experiments. Our findings provide valuable insights into the design of solid electrolytes and highlight the importance of the local charge distribution around the dopant and Li atoms in determining Li-ion conductivity. This insight could serve as a guiding principle for future materials design and optimization in solid-state electrolyte systems.

Li+/H+ exchange of Li7La3Zr2O12 single and polycrystals investigated by quantitative LIBS depth profiling

Li₇La₃Zr₂O₁₂ (LLZO) garnets are highly attractive to be used as solid electrolyte in solid-state Li batteries. However, LLZO suffers from chemical interaction with air and humidity, causing Li⁺/H⁺ exchange with detrimental implication on its performance, processing and scalability. To better understand the kinetics of the detrimental Li⁺/H⁺ exchange and its dependence on microstructural features, accelerated Li⁺/H⁺ exchange experiments were performed on single crystalline and polycrystalline LLZO, exposed for 80 minutes to 80 °C hot water. The resulting chemical changes were quantified by analytical methods, i.e. inductively coupled plasma optical emission spectroscopy (ICP-OES) and laser induced breakdown spectroscopy (LIBS). From the time dependence of the Li⁺ enrichment in the water, measured by ICP-OES, a bulk interdiffusion coefficient of Li⁺/H⁺ could be determined (7 × 10^-17 m² s^-1 at 80 °C). Depth dependent concentrations were obtained from the LIBS data for both ions after establishing a calibration method enabling not only Li⁺ but also H⁺ quantification in the solid electrolyte. Short interdiffusion lengths in the 1 μm range are found for the single crystalline Ga:LLZO, in accordance with the measured bulk diffusion coefficient. In polycrystalline Ta:LLZO, however, very long diffusion tails in the 20 μm range and ion exchange fractions up to about 70% are observed. Those are attributed to fast ion interdiffusion along grain boundaries. The severe compositional changes also strongly affect the electrical properties measured by impedance spectroscopy. This study highlights that microstructural effects may be decisive for the Li⁺/H⁺ ion exchange kinetics of LLZO.

Deep hydration of an Li7-3xLa3Zr2MIIIxO12 solid-state electrolyte material: a case study on Al- and Ga-stabilized LLZO

Single crystals of an Li-stuffed, Al- and Ga-stabilized garnet-type solid-state electrolyte material, Li7La3Zr2O12 (LLZO), have been analysed using single-crystal X-ray diffraction to determine the pristine structural state immediately after synthesis via ceramic sintering techniques. Hydrothermal treatment at 150 °C for 28 d induces a phase transition in the Al-stabilized compound from the commonly observed cubic Ia-3d structure to the acentric I-43d subtype. LiI ions at the interstitial octahedrally (4 + 2-fold) coordinated 48e site are most easily extracted and AlIII ions order onto the tetrahedral 12a site. Deep hydration induces a distinct depletion of LiI at this site, while the second tetrahedral site, 12b, suffers only minor LiI loss. Charge balance is maintained by the incorporation of HI, which is bonded to an O atom. Hydration of Ga-stabilized LLZO induces similar effects, with complete depletion of LiI at the 48e site. The LiI/HI exchange not only leads to a distinct increase in the unit-cell size, but also alters some bonding topology, which is discussed here.

Garnet to hydrogarnet: effect of post synthesis treatment on cation substituted LLZO solid electrolyte and its effect on Li ion conductivity

We investigated why commercial Li₇La₃Zr₂O₁₂ (LLZO) with Nb- and Ta substitution shows very low mobility on a local scale, as observed with temperature-dependent NMR techniques, compared to Al and W substituted samples, although impedance spectroscopy on sintered pellets suggests something else: conductivity values do not show a strong dependence on the type of substituting cation. We observed that mechanical treatment of these materials causes a symmetry reduction from garnet to hydrogarnet structure. To understand the impact of this lower symmetric structure in detail and its effect on the Li ion conductivity, neutron powder diffraction and ⁶Li NMR were utilized. Despite the finding that, in some materials, disorder can be beneficial with respect to ionic conductivity, pulsed-field gradient NMR measurements of the long-range transport indicate a higher Li⁺ diffusion barrier in the lower symmetric hydrogarnet structure. The symmetry reduction can be reversed back to the higher symmetric garnet structure by annealing at 1100 °C. This unintended phase transition and thus a reduction in conductivity is crucial for the processing of LLZO materials in the fabrication of all-solid state batteries.

Investigation of commercial Li₇La₃Zr₂O₁₂ (LLZO) with various substituents. Although impedance spectroscopy suggests something else: the ion conductivity does not show a strong dependence on the substituting cation, but rather on the sample treatment.

Aging Behavior of Al- and Ga- Stabilized Li7La3Zr2O12 Garnet-Type, Solid-State Electrolyte Based on Powder and Single Crystal X-ray Diffraction

Li7La3Zr2O12 garnet (LLZO) belongs to the most promising solid electrolytes for the development of solid-state Li batteries. The stability of LLZO upon exposure to air is still a matter of discussion. Therefore, we performed a comprehensive study on the aging behavior of Al-stabilized LLZO (space group (SG) Ia3¯d) and Ga-stabilized LLZO (SG I4¯3d) involving 98 powder and 51 single-crystal X-ray diffraction measurements. A Li+/H+ exchange starts immediately on exposure to air, whereby the exchange is more pronounced in samples with smaller particle/single-crystal diameter. A slight displacement of Li from interstitial Li2 (96h) toward the regular tetrahedral Li1 (24d) sites occurs in Al-stabilized LLZO. In addition, site occupancy at the 96h site decreases as Li+ is exchanged by H+. More extensive hydration during a mild hydrothermal treatment of samples at 90 °C induces a structural phase transition in Al-LLZO to SG I4¯3d with a splitting of the 24d site into two independent tetrahedral sites (i.e., 12a and 12b), whereby Al3+ solely occupies the 12a site. Li+ is preferably removed from the interstitial 48e site (equivalent to 96h). Analogous effects are observed in Ga-stabilized LLZO, which has SG I4¯3d in the pristine state.

Improving the Ionic Conductivity of the LLZO–LZO Thin Film through Indium Doping

A solid-state electrolyte with an ionic conductivity comparable to that of a liquid electrolyte is demanded of all-solid-state lithium-ion batteries. Li7La3Zr2O12 (LLZO) is considered to be a promising candidate due to its good thermal stability, high ionic conductivity, and wide electrochemical window. However, the synthesis of a stable cubic-phase LLZO thin film with enhanced densification at a relatively low thermal treatment temperature is yet to be developed. Indium is predicted to be a possible dopant to stabilize the cubic-phase LLZO (c-LLZO). Herein, via a nanolayer stacking process, a LLZO–Li2CO3–In2O3 multilayer solid electrolyte precursor was obtained. After thermal annealing at different temperatures, the effects of indium doping on the formation of c-LLZO and the ionic conductivities of the prepared LLZO–LZO thin film were systematically investigated. The highest ionic conductivity of 9.6 × 10−6 S·cm–1 was obtained at an annealing temperature of 800 °C because the incorporation of indium promoted the formation of c-LLZO and the highly conductive LLZO–LZO interfaces. At the end, a model of LLZO–LZO interface-enhancing ionic conductivity was proposed. This work provides a new approach for the development of low-temperature LLZO-based, solid-state thin-film batteries.

Wet-Environment-Induced Structural Alterations in Single- and Polycrystalline LLZTO Solid Electrolytes Studied by Diffraction Techniques

Li₇La₃Zr₂O₁₂ (LLZO) is one of the potential candidates for Li metal-based solid-state batteries owing to its high Li⁺ conductivity (≈10^-3 S cm^-1) at room temperature and large electrochemical stability window. However, LLZO undergoes protonation under the influence of moisture-forming Li₂CO₃ layers, thereby affecting its structural and transport properties. Therefore, a detailed understanding on the impact of the exchange of H⁺ on Li⁺ sites on structural alteration and kinetics under the influence of wet environments is of great importance. The present study focuses on the Li⁺/H⁺ exchange in single-crystal and polycrystal Li₆La₃ZrTaO₁₂ (LLZTO) garnets prepared using the Czochralski method and solid-state reactions subjected to weathering in air, aqueous solutions at room temperature, and in aqueous solution at 363 K using X-ray diffraction (XRD) and neutron diffraction (ND) techniques. Based on 36 single-crystal diffraction and 88 powder diffraction measurements, we found that LLZTO crystallizes with space group (SG) Ia3̅d with Li located in 96h (Li(2)) and 24d (Li(1)) sites, whereas the latter one is displaced toward the general position 96h forming shorter Li(1)-Li(2) jump distances. The degradation in air, wet air, water, and acetic acid leads to a Li⁺/H⁺ exchange that preferably takes place at the 24d site, which is in contrast to previous reports. Higher Li⁺/H⁺ was observed for LLZTO aged in water at 363 K that reduced the symmetry to SG I4̅3d from SG Ia3̅d. This symmetry reduction was found to be related to the site occupation behavior of Li at the tetrahedral 12a site in SG I4̅3d. Moreover, Li⁺ is exchanged by H⁺ preferably at the 48e site (equivalent to 96h site). We also found that the equilibrium H⁺ concentrations in all media tested remains very similar, which is related to the H⁺ diffusion in the LLZTO-controlled exchange process. Only the increase in temperature led to a significant increase in the exchange capacity as well as in the Li⁺/H⁺ exchange rate. Overall, we found that the exchange rate, exchange capacity, site occupation behavior of Li⁺ and H⁺, as well as the structural stability of LLZTO, strongly depend on the composition. These findings suggest that measurements on a single LLZTO variant sample do not lead to a general conclusion for all garnets to guide the field toward better materials. In contrast, each composition has to be analyzed exclusively to understand the interplay of composition, structure, and exchange kinetic properties.

Insights into Crystal Structure and Diffusion of Biphasic Na2Zn2TeO6

The layered oxide Na₂Zn₂TeO₆ is a fast Na⁺ ion conductor and a suitable candidate for application as a solid-state electrolyte. We present a detailed study on how synthesis temperature and Na-content affect the crystal structure and thus the Na⁺ ion conductivity of Na₂Zn₂TeO₆. Furthermore, we report for the first time an O'3-type phase for Na₂Zn₂TeO₆. At a synthesis temperature of 900 °C, we obtain a pure P2-type phase, providing peak performance in Na⁺ ion conductivity. Synthesis temperatures lower than 900 °C produce a series of mixed P2 and O'3-type phases. The O'3 structure can only be obtained as a pure phase by substituting Li on the Zn-sites to increase the Na-content. Thorough analysis of synchrotron data combined with computational modeling indicates that Li enters the Zn sites and, consequently, the amount of Na in the structure increases to balance the charge according to the formula Na_2+xZn_2-xLi_xTeO₆ (x = 0.2-0.5). Impedance spectroscopy and computational modeling confirm that reducing the amount of the O'3-type phase enhances the Na⁺ ion mobility.

The interaction of defects in a mayenite structure

A relatively small variation in the oxygen partial pressure at high temperature leads to the appearance of differences in Ca₁₂Al₁₄O₃₃ structure.

Effect of Gallium Substitution on Lithium-Ion Conductivity and Phase Evolution in Sputtered Li7-3 xGa xLa3Zr2O12 Thin Films

Replacing the liquid electrolyte in conventional lithium-ion batteries with thin-film solid-state lithium-ion conductors is a promising approach for increasing energy density, lifetime, and safety. In particular, Li₇La₃Zr₂O₁₂ is appealing due to its high lithium-ion conductivity and wide electrochemical stability window. Further insights into thin-film processing of this material are required for its successful integration into solid-state batteries. In this work, we investigate the phase evolution of Li_{7-3 x}Ga _xLa₃Zr₂O₁₂ in thin films with various amounts of Li and Ga for stabilizing the cubic phase. Through this work, we gain valuable insights into the crystallization processes unique to thin films and are able to form dense Li_{7-3 x}Ga _xLa₃Zr₂O₁₂ layers stabilized in the cubic phase with high in-plane lithium-ion conductivities of up to 1.6 × 10^-5 S cm^-1 at 30 °C. We also note the formation of cubic Li₇La₃Zr₂O₁₂ at the relatively low temperature of 500 °C.

Interface Instability of Fe-Stabilized Li7La3Zr2O12 versus Li Metal

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.

Materials space of solid-state electrolytes: unraveling chemical composition–structure–ionic conductivity relationships in garnet-type metal oxides using cheminformatics virtual screening approaches

Several candidate garnet-related compounds have been recommended for synthesis as potential materials for solid-state electrolytes.

Synthesis, Crystal Structure, and Stability of Cubic Li7-xLa3Zr2-xBixO12

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–xLa₃Zr_2–xBi_xO₁₂ (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.

The cubic modification of Li₇La₃Zr₂O₁₂ can be stabilized by a by a partial substitution of Zr⁴⁺ by Bi⁵⁺. The incorporation of Bi⁵⁺ leads to an increase in the unit-cell parameters. Samples prepared by a low-temperature preparation route are sensitive to CO₂ and H₂O from air, causing a protonation of the garnet phase. ⁷Li NMR spectroscopy shows that fast translational Li ion dynamics are only observed for samples with high Bi⁵⁺ contents.