Synthesis, Crystal Chemistry, and Electrochemical Properties of Li7–2xLa3Zr2–xMoxO12 (x = 0.1–0.4): Stabilization of the Cubic Garnet Polymorph via Substitution of Zr4+ by Mo6+

Interfacial Modification of Ga-Substituted Li7La3Zr2O12 against Li Metal via a Simple Doping Method

Garnet Li7La3Zr2O12 (LLZO) is considered a promising solid electrolyte for all-solid-state lithium-ion batteries due to its outstanding performance in which Ga-doped LLZO particularly exhibits excellent ionic conductivity. However, the application of Ga-doped LLZO is limited by the interfacial instability between Ga-doped LLZO and Li metal. In this study, Ga3+- and Sb5+-codoped LLZO is prepared using a conventional solid-state reaction method, and the effects of dual-doping on the crystal structure, microstructure, conductivity of LLZO, and battery cycle stability are investigated. The results demonstrate that the introduction of an appropriate amount of Sb5+ into Ga3+-stabilized cubic-phase LLZO promotes grain contact and enhances the total ionic conductivity. The optimized Li6.3Ga0.2La3Zr1.9Sb0.1O12 solid electrolyte exhibits the highest total ionic conductivity of 4.65 × 10-4 S cm-1 at room temperature. Additionally, the introduction of Sb5+ suppresses the formation of the LiGaO2 impurity phase, thereby improving the interface stability between Ga-doped LLZO and the Li metal. The assembled Li||Ga,Sb0.1-LLZO||Li symmetric cell demonstrates stable cycling for 500 h at room temperature under a current density of 0.13 mA cm-2. The Li||Ga,Sb0.1-LLZO||LiFePO4 full cell delivers a reversible capacity of about 140 mA h g-1, exhibiting negligible decay after 50 cycles. These findings suggest that the application of Ga-doped LLZO in all-solid-state lithium-ion batteries holds great promise by simply doping Zr sites with high-valence ions.

Influence of Microstructure on the Material Properties of LLZO Ceramics Derived by Impedance Spectroscopy and Brick Layer Model Analysis

Variants of garnet-type Li7La3Zr2O12 are being intensively studied as separator materials in solid-state battery research. The material-specific transport properties, such as bulk and grain boundary conductivity, are of prime interest and are mostly investigated by impedance spectroscopy. Data evaluation is usually based on the one-dimensional (1D) brick layer model, which assumes a homogeneous microstructure of identical grains. Real samples show microstructural inhomogeneities in grain size and porosity due to the complex behavior of grain growth in garnets that is very sensitive to the sintering protocol. However, the true microstructure is often omitted in impedance data analysis, hindering the interlaboratory reproducibility and comparability of results reported in the literature. Here, we use a combinatorial approach of structural analysis and three-dimensional (3D) transport modeling to explore the effects of microstructure on the derived material-specific properties of garnet-type ceramics. For this purpose, Al-doped Li7La3Zr2O12 pellets with different microstructures are fabricated and electrochemically characterized. A machine learning-assisted image segmentation approach is used for statistical analysis and quantification of the microstructural changes during sintering. A detailed analysis of transport through statistically modeled twin microstructures demonstrates that the transport parameters derived from a 1D brick layer model approach show uncertainties up to 150%, only due to variations in grain size. These uncertainties can be even larger in the presence of porosity. This study helps to better understand the role of the microstructure of polycrystalline electroceramics and its influence on experimental results.

Recent Strategies for Lithium-Ion Conductivity Improvement in Li7La3Zr2O12 Solid Electrolytes

The development of solid electrolytes with high conductivity is one of the key factors in the creation of new power-generation sources. Lithium-ion solid electrolytes based on Li7La3Zr2O12 (LLZ) with a garnet structure are in great demand for all-solid-state battery production. Li7La3Zr2O12 has two structural modifications: tetragonal (I41/acd) and cubic (Ia3d). A doping strategy is proposed for the stabilization of highly conductive cubic Li7La3Zr2O12. The structure features, density, and microstructure of the ceramic membrane are caused by the doping strategy and synthesis method of the solid electrolyte. The influence of different dopants on the stabilization of the cubic phase and conductivity improvement of solid electrolytes based on Li7La3Zr2O12 is discussed in the presented review. For mono-doping, the highest values of lithium-ion conductivity (~10-3 S/cm at room temperature) are achieved for solid electrolytes with the partial substitution of Li+ by Ga3+, and Zr4+ by Te6+. Moreover, the positive effect of double elements doping on the Zr site in Li7La3Zr2O12 is established. There is an increase in the popularity of dual- and multi-doping on several Li7La3Zr2O12 sublattices. Such a strategy leads not only to lithium-ion conductivity improvement but also to the reduction of annealing temperature and the amount of some high-cost dopant. Al and Ga proved to be effective co-doping elements for the simultaneous substitution in Li/Zr and Li/La sublattices of Li7La3Zr2O12 for improving the lithium-ion conductivity of solid electrolytes.

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.

3D Impedance Modeling of Metal Anodes in Solid-State Batteries-Incompatibility of Pore Formation and Constriction Effect in Physical-Based 1D Circuit Models

A non-ideal contact at the electrode/solid electrolyte interface of a solid-state battery arising due to pores (voids) or inclusions results in a geometric constriction effect that severely deteriorates the electric transport properties of the battery cell. The lack of understanding of this phenomenon hinders the optimization process of novel components, such as reversible and high-rate metal anodes. Deeper insight into the constriction phenomenon is necessary to correctly monitor interface degradation and to accelerate the successful use of metal anodes in solid-state batteries. Here, we use a 3D electric network model to study the fundamentals of the constriction effect. Our findings suggest that dynamic constriction as a non-local effect cannot be captured by conventional 1D equivalent circuit models and that its electric behavior is not ad hoc predictable. It strongly depends on the interplay of the geometry of the interface causing the constriction and the microscopic transport processes in the adjacent phases. In the presence of constriction, the contribution from the non-ideal electrode/solid electrolyte interface to the impedance spectrum may exhibit two signals that cannot be explained when the porous interface is described by a physical-based (effective medium theory) 1D equivalent circuit model. In consequence, the widespread assumption of a single interface contribution to the experimental impedance spectrum may be entirely misleading and can cause serious misinterpretation.

Interplay of Dynamic Constriction and Interface Morphology between Reversible Metal Anode and Solid Electrolyte in Solid State Batteries

In an all-solid-state battery, the electrical contact between its individual components is of key relevance in addition to the electrochemical stability of its interfaces. Impedance spectroscopy is particularly suited for the non-destructive investigation of interfaces and of their stability under load. Establishing a valid correlation between microscopic processes and the macroscopic impedance signal, however, is challenging and prone to errors. Here, we use a 3D electric network model to systematically investigate the effect of various electrode/sample interface morphologies on the impedance spectrum. It is demonstrated that the interface impedance generally results from a charge transfer step and a geometric constriction contribution. The weights of both signals depend strongly on the material parameters as well as on the interface morphology. Dynamic constriction results from a non-ideal local contact, e.g., from pores or voids, which reduce the electrochemical active surface area only in a certain frequency range. Constriction effects dominate the interface behavior for systems with small charge transfer resistance like garnet-type solid electrolytes in contact with a lithium metal electrode. An in-depth analysis of the origin and the characteristics of the constriction phenomenon and their dependence on the interface morphology is conducted. The discussion of the constriction effect provides further insight into the processes at the microscopic level, which are, e.g., relevant in the case of reversible metal anodes.

Anomalies in Bulk Ion Transport in the Solid Solutions of Li7La3M2O12 (M = Hf, Sn) and Li5La3Ta2O12

Cubic Li₇La₃Zr₂O₁₂(LLZO), stabilized by supervalent cations, is one of the most promising oxide electrolyte to realize inherently safe all-solid-state batteries. It is of great interest to evaluate the strategy of supervalent stabilization in similar compounds and to describe its effect on ionic bulk conductivity σ'_bulk. Here, we synthesized solid solutions of Li_7-x La₃M_2-x Ta _x O₁₂ with M = Hf, Sn over the full compositional range (x = 0, 0.25...2). It turned out that Ta contents at x of 0.25 (M = Hf, LLHTO) and 0.5 (M = Sn, LLSTO) are necessary to yield phase pure cubic Li_7-x La₃M_2-x Ta _x O₁₂. The maximum in total conductivity for LLHTO (2 × 10^-4 S cm^-1) is achieved for x = 1.0; the associated activation energy is 0.46 eV. At x = 0.5 and x = 1.0, we observe two conductivity anomalies that are qualitatively in agreement with the rule of Meyer and Neldel. For LLSTO, at x = 0.75 the conductivity σ'_bulk turned out to be 7.94 × 10^-5 S cm^-1 (0.46 eV); the almost monotonic decrease of ion bulk conductivity from x = 0.75 to x = 2 in this series is in line with Meyer-Neldel's compensation behavior showing that a decrease in E _a is accompanied by a decrease of the Arrhenius prefactor. Altogether, the system might serve as an attractive alternative to Al-stabilized (or Ga-stabilized) Li₇La₃Zr₂O₁₂ as LLHTO is also anticipated to be highly stable against Li metal.

Building Better Batteries in the Solid State: A Review

Most of the current commercialized lithium batteries employ liquid electrolytes, despite their vulnerability to battery fire hazards, because they avoid the formation of dendrites on the anode side, which is commonly encountered in solid-state batteries. In a review two years ago, we focused on the challenges and issues facing lithium metal for solid-state rechargeable batteries, pointed to the progress made in addressing this drawback, and concluded that a situation could be envisioned where solid-state batteries would again win over liquid batteries for different applications in the near future. However, an additional drawback of solid-state batteries is the lower ionic conductivity of the electrolyte. Therefore, extensive research efforts have been invested in the last few years to overcome this problem, the reward of which has been significant progress. It is the purpose of this review to report these recent works and the state of the art on solid electrolytes. In addition to solid electrolytes stricto sensu, there are other electrolytes that are mainly solids, but with some added liquid. In some cases, the amount of liquid added is only on the microliter scale; the addition of liquid is aimed at only improving the contact between a solid-state electrolyte and an electrode, for instance. In some other cases, the amount of liquid is larger, as in the case of gel polymers. It is also an acceptable solution if the amount of liquid is small enough to maintain the safety of the cell; such cases are also considered in this review. Different chemistries are examined, including not only Li-air, Li-O2, and Li-S, but also sodium-ion batteries, which are also subject to intensive research. The challenges toward commercialization are also considered.

Perfect Self-Assembling of One-Dimensional Lead Iodides with Tetrahedral Cu4I6S4 Clusters: A High-Symmetry Cubic Packing

Hybrid perovskites are attractive for their applications in photovoltaic devices. We synthesized a novel 1-D hybrid lead iodide, (tu)₂Cu₂PbI₄, in which 1-D PbI₃ chains are tetrahedrally orientated to form a crystal lattice with high-symmetry cubic space group Ia3̅ d (No. 230). Optoelectronic and fluorescence properties are studied.

Effects of Fluorine Doping on Structural and Electrochemical Properties of Li6.25Ga0.25La3Zr2O12 as Electrolytes for Solid-State Lithium Batteries

Solid-state lithium batteries (SSLBs) are promising technologies with great potential in improving safety and energy density, compared with the traditional liquid based lithium ion batteries. However, the bottleneck of SSLBs lies in the issues of poor interface contact and low electrolyte conductivity. In this work, the crystal structure of garnet-type Li_6.25Ga_0.25La₃Zr₂O₁₂ (LGLZO) was engineered more rigidly with subdued atoms displacement by fluorine doping and thus smoother and faster lithium ion diffusion paths are formed. The ionic conductivity of garnet-type electrolyte is significantly increased from 5.43 × 10^-4 S/cm to 1.28 × 10^-3 S/cm at 25 °C and the activation energy is reduced from 0.33 to 0.28 eV. The solid-state symmetric cell consisting of F doped Li_6.25Ga_0.25La₃Zr₂O₁₂ electrolyte and lithium metal has lower resistance and displays stable lithium plating/stripping for over 650 h with smaller overpotentials than those on LGLZO electrolyte. Moreover, the all solid-state lithium battery with F-LGLZO electrolyte and LiFePO₄ composite electrode exhibits an improved rate capability, which can still keep 132.9 mAh/g at 1 C. Fluorine substitution in garnet-type electrolyte opens new avenues to design new solid-state electrolytes for practical applications of SSLBs.

Dendrite nucleation in lithium-conductive ceramics

A chemomechanical analysis suggests that bulk lithium plating in polycrystalline LLZO becomes energetically favourable above a critical current. This grain-coating mechanism rationalizes dendrite nucleation without making reference to surface cracks.

Space-Charge Layers in All-Solid-State Batteries; Important or Negligible?

All-solid state batteries have the promise to increase the safety of Li-ion batteries. A prerequisite for high-performance all-solid-state batteries is a high Li-ion conductivity through the solid electrolyte. In recent decades, several solid electrolytes have been developed which have an ionic conductivity comparable to that of common liquid electrolytes. However, fast charging and discharging of all-solid-state batteries remains challenging. This is generally attributed to poor kinetics over the electrode-solid electrolyte interface because of poorly conducting decomposition products, small contact areas, or space-charge layers. To understand and quantify the role of space-charge layers in all-solid-state batteries a simple model is presented which allows to asses the interface capacitance and resistance caused by the space-charge layer. The model is applied to LCO (LiCoO₂) and graphite electrodes in contact with an LLZO (Li₇La₃Zr₂O₁₂) and LATP (Li_1.2Al_0.2Ti_1.8(PO₄)₃) solid electrolyte at several voltages. The predictions demonstrate that the space-charge layer for typical electrode-electrolyte combinations is about a nanometer in thickness, and the consequential resistance for Li-ion transport through the space-charge layer is negligible, except when layers completely depleted of Li-ions are formed in the solid electrolyte. This suggests that space-charge layers have a negligible impact on the performance of all-solid-state batteries.

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.

Lattice-geometry effects in garnet solid electrolytes: a lattice-gas Monte Carlo simulation study

Ionic transport in solid electrolytes can often be approximated as ions performing a sequence of hops between distinct lattice sites. If these hops are uncorrelated, quantitative relationships can be derived that connect microscopic hopping rates to macroscopic transport coefficients; i.e. tracer diffusion coefficients and ionic conductivities. In real materials, hops are uncorrelated only in the dilute limit. At non-dilute concentrations, the relationships between hopping frequency, diffusion coefficient and ionic conductivity deviate from the random walk case, with this deviation quantified by single-particle and collective correlation factors, f and f_I, respectively. These factors vary between materials, and depend on the concentration of mobile particles, the nature of the interactions, and the host lattice geometry. Here, we study these correlation effects for the garnet lattice using lattice-gas Monte Carlo simulations. We find that, for non-interacting particles (volume exclusion only), single-particle correlation effects are more significant than for any previously studied three-dimensional lattice. This is attributed to the presence of two-coordinate lattice sites, which causes correlation effects intermediate between typical three-dimensional and one-dimensional lattices. Including nearest-neighbour repulsion and on-site energies produces more complex single-particle correlations and introduces collective correlations. We predict particularly strong correlation effects at x_Li=3 (from site energies) and x_Li=6 (from nearest-neighbour repulsion), where x_Li=9 corresponds to a fully occupied lithium sublattice. Both effects are consequences of ordering of the mobile particles. Using these simulation data, we consider tuning the mobile-ion stoichiometry to maximize the ionic conductivity, and show that the 'optimal' composition is highly sensitive to the precise nature and strength of the microscopic interactions. Finally, we discuss the practical implications of these results in the context of lithium garnets and other solid electrolytes.

Lattice-geometry effects in garnet solid electrolytes: a lattice-gas Monte Carlo simulation study

Ionic transport in solid electrolytes can often be approximated as ions performing a sequence of hops between distinct lattice sites. If these hops are uncorrelated, quantitative relationships can be derived that connect microscopic hopping rates to macroscopic transport coefficients; i.e. tracer diffusion coefficients and ionic conductivities. In real materials, hops are uncorrelated only in the dilute limit. At non-dilute concentrations, the relationships between hopping frequency, diffusion coefficient and ionic conductivity deviate from the random walk case, with this deviation quantified by single-particle and collective correlation factors, f and f_I, respectively. These factors vary between materials, and depend on the concentration of mobile particles, the nature of the interactions, and the host lattice geometry. Here, we study these correlation effects for the garnet lattice using lattice-gas Monte Carlo simulations. We find that, for non-interacting particles (volume exclusion only), single-particle correlation effects are more significant than for any previously studied three-dimensional lattice. This is attributed to the presence of two-coordinate lattice sites, which causes correlation effects intermediate between typical three-dimensional and one-dimensional lattices. Including nearest-neighbour repulsion and on-site energies produces more complex single-particle correlations and introduces collective correlations. We predict particularly strong correlation effects at x_Li=3 (from site energies) and x_Li=6 (from nearest-neighbour repulsion), where x_Li=9 corresponds to a fully occupied lithium sublattice. Both effects are consequences of ordering of the mobile particles. Using these simulation data, we consider tuning the mobile-ion stoichiometry to maximize the ionic conductivity, and show that the 'optimal' composition is highly sensitive to the precise nature and strength of the microscopic interactions. Finally, we discuss the practical implications of these results in the context of lithium garnets and other solid electrolytes.

Composite Polymer Electrolytes with Li7La3Zr2O12 Garnet-Type Nanowires as Ceramic Fillers: Mechanism of Conductivity Enhancement and Role of Doping and Morphology

Composite polymer solid electrolytes (CPEs) containing ceramic fillers embedded inside a polymer-salt matrix show great improvements in Li⁺ ionic conductivity compared to the polymer electrolyte alone. Lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZO) with a garnet-type crystal structure is a promising solid Li⁺ conductor. We show that by incorporating only 5 wt % of the ceramic filler comprising undoped, cubic-phase LLZO nanowires prepared by electrospinning, the room temperature ionic conductivity of a polyacrylonitrile-LiClO₄-based composite is increased 3 orders of magnitude to 1.31 × 10^-4 S/cm. Al-doped and Ta-doped LLZO nanowires are also synthesized and utilized as fillers, but the conductivity enhancement is similar as for the undoped LLZO nanowires. Solid-state nuclear magnetic resonance (NMR) studies show that LLZO NWs partially modify the PAN polymer matrix and create preferential pathways for Li⁺ conduction through the modified polymer regions. CPEs with LLZO nanoparticles and Al₂O₃ nanowire fillers are also studied to elucidate the role of filler type (active vs passive), LLZO composition (undoped vs doped), and morphology (nanowire vs nanoparticle) on the CPE conductivity. It is demonstrated that both intrinsic Li⁺ conductivity and nanowire morphology are needed for optimal performance when using 5 wt % of the ceramic filler in the CPE.

Solid-State Lithium-Sulfur Batteries Operated at 37 °C with Composites of Nanostructured Li7La3Zr2O12/Carbon Foam and Polymer

An all solid-state lithium-ion battery with high energy density and high safety is a promising solution for a next-generation energy storage system. High interface resistance of the electrodes and poor ion conductivity of solid-state electrolytes are two main challenges for solid-state batteries, which require operation at elevated temperatures of 60-90 °C. Herein, we report the facile synthesis of Al³⁺/Nb⁵⁺ codoped cubic Li₇La₃Zr₂O₁₂ (LLZO) nanoparticles and LLZO nanoparticle-decorated porous carbon foam (LLZO@C) by the one-step Pechini sol-gel method. The LLZO nanoparticle-filled poly(ethylene oxide) electrolyte shows improved conductivity compared with filler-free samples. The sulfur composite cathode based on LLZO@C can deliver an attractive specific capacity of >900 mAh g^-1 at the human body temperature 37 °C and a high capacity of 1210 and 1556 mAh g^-1 at 50 and 70 °C, respectively. In addition, the solid-state Li-S batteries exhibit high Coulombic efficiency and show remarkably stable cycling performance.

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.

Fast Li-Ion-Conducting Garnet-Related Li7-3x Fe x La3Zr2O12 with Uncommon I4̅3d 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 I4̅3d. This structure can be derived by a symmetry reduction of the garnet-type Ia3̅d 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 I4̅3d also for Li_7-3x Fe _x 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 (12a) site of space group I4̅3d. 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.

Structural and Electrochemical Consequences of Al and Ga Cosubstitution in Li7La3Zr2O12 Solid Electrolytes

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.

Crystal Structure of Garnet-Related Li-Ion Conductor Li7-3x Ga x 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 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 Li_7-3x Ga _x La₃Zr₂O₁₂ 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.