Crystal Structure of Garnet-Related Li-Ion Conductor Li7-3x Ga x La3Zr2O12: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

Accelerating the Development of LLZO in Solid-State Batteries Toward Commercialization: A Comprehensive Review

Solid-state batteries (SSBs) are under development as high-priority technologies for safe and energy-dense next-generation electrochemical energy storage systems operating over a wide temperature range. Solid-state electrolytes (SSEs) exhibit high thermal stability and, in some cases, the ability to prevent dendrite growth through a physical barrier, and compatibility with the "holy grail" metallic lithium. These unique advantages of SSEs have spurred significant research interests during the last decade. Garnet-type SSEs, that is, Li7La3Zr2O12 (LLZO), are intensively investigated due to their high Li-ion conductivity and exceptional chemical and electrochemical stability against lithium metal anodes. However, poor interfacial contact with cathode materials, undesirable lithium plating along grain boundaries, and moisture-induced chemical degradation greatly hinder the practical implementation of LLZO-based SSEs for SSBs. In this review, the recent advances in synthesis methods, modification strategies, corresponding mechanisms, and applications of garnet-based SSEs in SSBs are critically summarized. Furthermore, a comprehensive evaluation of the challenges and development trends of LLZO-based electrolytes in practical applications is presented to accelerate their development for high-performance SSBs.

A Workflow for Identifying Viable Crystal Structures with Partially Occupied Sites Applied to the Solid Electrolyte Cubic Li7La3Zr2O12

To date, experimental and theoretical works have been unable to uncover the ground-state configuration of the solid electrolyte cubic Li7La3Zr2O12 (c-LLZO). Computational studies rely on an initial low-energy structure as a reference point. Here, we present a methodology for identifying energetically favorable configurations of c-LLZO for a crystallographically predicted structure. We begin by eliminating structures that involve overlapping Li atoms based on nearest neighbor counts. We further reduce the configuration space by eliminating symmetry images from all remaining structures. Then, we perform a machine learning-based energetic ordering of all remaining structures. By considering the geometrical constraints that emerge from this methodology, we determine that a large portion of previously reported structures may not be feasible or stable. The method developed here could be extended to other ion conductors. We provide a database containing all of the generated structures with the aim of improving accuracy and reproducibility in future c-LLZO research.

Extreme lithium-metal cycling enabled by a mixed ion- and electron-conducting garnet three-dimensional architecture

The development of solid-state Li-metal batteries has been limited by the Li-metal plating and stripping rates and the tendency for dendrite shorts to form at commercially relevant current densities. To address this, we developed a single-phase mixed ion- and electron-conducting (MIEC) garnet with comparable Li-ion and electronic conductivities. We demonstrate that in a trilayer architecture with a porous MIEC framework supporting a thin, dense, garnet electrolyte, the critical current density can be increased to a previously unheard of 100 mA cm-2, with no dendrite-shorting. Additionally, we demonstrate that symmetric Li cells can be continuously cycled at a current density of 60 mA cm-2 with a maximum per-cycle Li plating and stripping capacity of 30 mAh cm-2, which is 6× the capacity of state-of-the-art cathodes. Moreover, a cumulative Li plating capacity of 18.5 Ah cm-2 was achieved with the MIEC/electrolyte/MIEC architecture, which if paired with a state-of-the-art cathode areal capacity of 5 mAh cm-2 would yield a projected 3,700 cycles, significantly surpassing requirements for commercial electric vehicle battery lifetimes.

Review of the Developments and Difficulties in Inorganic Solid-State Electrolytes

All-solid-state lithium-ion batteries (ASSLIBs), with their exceptional attributes, have captured the attention of researchers. They offer a viable solution to the inherent flaws of traditional lithium-ion batteries. The crux of an ASSLB lies in its solid-state electrolyte (SSE) which shows higher stability and safety compared to liquid electrolyte. Additionally, it holds the promise of being compatible with Li metal anode, thereby realizing higher capacity. Inorganic SSEs have undergone tremendous developments in the last few decades; however, their practical applications still face difficulties such as the electrode-electrolyte interface, air stability, and so on. The structural composition of inorganic electrolytes is inherently linked to the advantages and difficulties they present. This article provides a comprehensive explanation of the development, structure, and Li-ion transport mechanism of representative inorganic SSEs. Moreover, corresponding difficulties such as interface issues and air stability as well as possible solutions are also discussed.

Excellent Stability of Ga-Doped Garnet Electrolyte against Li Metal Anode via Eliminating LiGaO2 Precipitates for Advanced All-Solid-State Batteries

Ga-doped garnet-type Li₇La₃Zr₂O₁₂ (Ga-LLZO) ceramics have long been recognized as ideal electrolyte candidates for all-solid-state lithium batteries (ASSLBs). However, in this study, it is shown that Ga-LLZO easily and promptly cracks in contact with molten lithium during the ASSLB assembly. This can be mainly ascribed to two aspects: (i) lithium captures O atoms and reduces Ga ions of the Ga-LLZO matrix, leading to a band-gap closure from >5 to <2 eV and a structural collapse from cubic to tetrahedral; and (ii) the in situ-formed LiGaO₂ impurity phase has severe side reactions with lithium, resulting in huge stress release along the grain boundaries. It is also revealed that, while the former process consumes hours to take effect, the latter one is immediate and accounts for the crack propagation of Ga-LLZO electrolytes. A minute SiO₂ is preadded during the synthesis of Ga-LLZO and found effective in eliminating the LiGaO₂ impurity phase. The SiO₂-modified Ga-LLZO solid electrolytes display excellent thermomechanical and electrochemical stabilities against lithium metals and well-reserved ionic conductivities, which was further confirmed by half-cells and full batteries. This study contributes to the understanding of the stability of garnet electrolytes and promotes their potential commercial applications in ASSLBs.

Unlocking the hidden chemical space in cubic-phase garnet solid electrolyte for efficient quasi-all-solid-state lithium batteries

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolytes (SE) demonstrates appealing ionic conductivity properties for all-solid-state lithium metal battery applications. However, LLZO (electro)chemical stability in contact with the lithium metal electrode is not satisfactory for developing practical batteries. To circumvent this issue, we report the preparation of various doped cubic-phase LLZO SEs without vacancy formation (i.e., Li = 7.0 such as Li₇La₃Zr_0.5Hf_0.5Sc_0.5Nb_0.5O₁₂ and Li₇La₃Zr_0.4Hf_0.4Sn_0.4Sc_0.4Ta_0.4O₁₂). The entropy-driven synthetic approach allows access to hidden chemical space in cubic-phase garnet and enables lower solid-state synthesis temperature as the cubic-phase nucleation decreases from 750 to 400 °C. We demonstrate that the SEs with Li = 7.0 show better reduction stability against lithium metal compared to SE with low lithium contents and identical atomic species (i.e., Li = 6.6 such as Li_6.6La₃Zr_0.4Hf_0.4Sn_0.4Sc_0.2Ta_0.6O₁₂). Moreover, when a Li₇La₃Zr_0.4Hf_0.4Sn_0.4Sc_0.4Ta_0.4O₁₂ pellet is tested at 60 °C in coin cell configuration with a Li metal negative electrode, a LiNi_1/3Co_1/3Mn_1/3O₂-based positive electrode and an ionic liquid-based electrolyte at the cathode|SE interface, discharge capacity retention of about 92% is delivered after 700 cycles at 0.8 mA/cm² and 60 °C.

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.

Polymorphism of garnet solid electrolytes and its implications for grain-level chemo-mechanics

Understanding and mitigating filament formation, short-circuit and solid electrolyte fracture is necessary for advanced all-solid-state batteries. Here, we employ a coupled far-field high-energy diffraction microscopy and tomography approach for assessing the chemo-mechanical behaviour for dense, polycrystalline garnet (Li₇La₃Zr₂O₁₂) solid electrolytes with grain-level resolution. In situ monitoring of grain-level stress responses reveals that the failure mechanism is stochastic and affected by local microstructural heterogeneity. Coupling high-energy X-ray diffraction and far-field high-energy diffraction microscopy measurements reveals the presence of phase heterogeneity that can alter local chemo-mechanics within the bulk solid electrolyte. These local regions are proposed to be regions with the presence of a cubic polymorph of LLZO, potentially arising from local dopant concentration variation. The coupled tomography and FF-HEDM experiments are combined with transport and mechanics modelling to illustrate the degradation of polycrystalline garnet solid electrolytes. The results showcase the pathways for processing high-performing solid-state batteries.

Probing the Phase Transition during the Formation of Lithium Lanthanum Zirconium Oxide Solid Electrolyte

Lithium lanthanum zirconium oxide (LLZO) has long been considered as a promising solid electrolyte for all-solid-state lithium (Li) metal batteries because of its interfacial stability when coupled with a Li metal anode. However, the cubic phase of LLZO (c-LLZO) with a higher Li-ion conductivity has a complex atomic structure and is subject to complicated phase transition during its processing and working conditions, which remain largely elusive. Here, we reveal the phase transition process during the formation of c-LLZO nanotubes through detailed microscopic characterization by scanning and transmission electron microscopy as well as X-ray diffraction. We find four typical stages during the formation of c-LLZO along with several intermediate phases including lanthanum (La)-rich cubic lanthanum zirconium oxide (La-rich c-LZO), c-LZO, and La-rich c-LLZO. We also reveal the role of m-Li₂CO₃ and h-Li₂O₂ as the "phase mediator".

Energy landscape for Li-ion diffusion in the garnet-type solid electrolyte Li6.5La3Zr1.5Nb0.5O12 (LLZO-Nb)

A comprehensive understanding of the nexus of diffusion mechanisms on the atomic scale as well as structural influences on the ionic motion in solid electrolytes is key for further development of high-performing all-solid-state batteries. Therefore, current research not only focuses on the search for innovative materials, but also on the study of diffusion pathways and ion dynamics in ionic conductors. In this context, we report on the extended characterization of the ionic electrolyte Li6.5La3Zr1.5Nb0.5O12 (LLZO-Nb). The commercially available material is analyzed by a combination of powder X-ray (either lab- or synchrotron-based) and neutron diffraction. Details of lithium disorder were obtained from high-resolution neutron diffraction data, from which the ionic transport of Li ions was determined by applying the maximum entropy method in combination with the one-particle potential formalism.

Li-stuffed garnet electrolytes: structure, ionic conductivity, chemical stability, interface, and applications

Current lithium-ion batteries have been widely used in portable electronic devices, electric vehicles, and peak power demand. However, the organic liquid electrolytes used in the lithium-ion battery are flammable and not stable in contact with elemental lithium and at a higher voltage. To eliminate the safety and instability issues, solid-state (ceramic) electrolytes have attracted enormous interest worldwide, owing to their thermal and high voltage stability. Among all the solid-state electrolytes known today, the Li-stuffed garnet is one of the most promising electrolytes due to its physical and chemical properties such as high total Li-ion conductivity at room temperature, chemical stability with elemental lithium and high voltage lithium cathodes, and high electrochemical stability window (6 V vs. Li+/Li). In this short review, we provide an overview of Li-stuffed garnet electrolytes with a focus on their structure, ionic conductivity, transport mechanism, chemical stability, and battery applications.

Simultaneously Improved Cubic Phase Stability and Li-Ion Conductivity in Garnet-Type Solid Electrolytes Enabled by Controlling the Al Occupation Sites

Here, we, for the first time, report on the simultaneous enhancement in cubic phase stability and Li-ion conductivity of garnet-type solid electrolytes (SEs) by adding excess Li/Al. The excess Al/Li creates very large grains of up to 170 μm via the segregation of Al at the grain boundaries and enables preferential Al occupation at 96h sites over 24d sites, a behavior contrary to previous observations. The resulting SE shows improved Li-ion conductivity due to the large grain size and less blocking Li pathway caused by different preferential Al occupation. Surprisingly, it is observed that the cubic phase of the garnet-type SE is transformed to the tetragonal phase on the surface and in the bulk under the applied voltage, and the preferential Al occupation enables its cubic phase stability. Under battery operating conditions, the LLZO SE with excess Li/Al can maintain high ionic conductivity due to the cubic phase stability and large grain size. We clearly demonstrate that the cubic phase stability and ionic conductivity of LLZO can be simultaneously improved by excess Li/Al without any post-treatments. The findings and understanding will provide new insights into practical use of the garnet-type SEs for advanced all solid-state batteries.

Compositional and structural control in LLZO solid electrolytes

Garnet-based solid-state electrolytes (SSEs) represent a promising class of materials for next-generation batteries with improved safety and performance. However, lack of control over the composition and crystal structure of the well-known Li₇La₃Zr₂O₁₂ (LLZO) garnet material has led to poor reproducibility with a wide range of ionic conductivities reported in the literature. In this study, the role of precursor homogeneity in controlling the compositional and structural evolution of Al-doped LLZO is explored. A novel solution-based synthesis approach is employed to demonstrate enhanced atomic-scale mixing of the starting materials in comparison to conventional solid-state preparation methods. Through this technique, it is shown that the stability and formation temperature of the highly conductive cubic phase is directly impacted by the spatial distribution of the doping element and reactant species in the precursor mixture. Precursor homogeneity was also an important factor in mitigating the formation of unwanted secondary impurities. These findings can be used to guide the synthesis of SSEs with reproducible material characteristics and enhanced electrolytic performance.

Enhanced precursor homogeneity is achieved through a novel solution-based synthesis approach to enable precise control over material structure and composition.

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.

Assessing the Importance of Cation Size in the Tetragonal-Cubic Phase Transition in Lithium-Garnet Electrolytes

Lithium garnets are promising solid-state electrolytes for next-generation lithium-ion batteries. These materials have high ionic conductivity, a wide electrochemical window and stability with Li metal. However, lithium garnets have a maximum limit of seven lithium atoms per formula unit (e.g., La₃ Zr₂ Li₇ O₁₂ ), before the system transitions from a cubic to a tetragonal phase with poor ionic mobility. This arises from full occupation of the Li sites. Hence, the most conductive lithium garnets have Li between 6-6.55 Li per formula unit, which maintains the cubic symmetry and the disordered Li sub-lattice. The tetragonal phase, however, forms the highly conducting cubic phase at higher temperatures, thought to arise from increased cell volume and entropic stabilisation permitting Li disorder. However, little work has been undertaken in understanding the controlling factors of this phase transition, which could enable enhanced dopant strategies to maintain room temperature cubic garnet at higher Li contents. Here, a series of nine tetragonal garnets were synthesised and analysed by variable temperature XRD to understand the dependence of site substitution on the phase transition temperature. Interestingly the octahedral site cation radius was identified as the key parameter for the transition temperature with larger or smaller dopants altering the transition temperature noticeably. A site substitution was, however, found to make little difference irrespective of significant changes to cell volume.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that [Formula: see text] ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides [Formula: see text], Li-bearing garnets ([Formula: see text]) and thiophosphates such as [Formula: see text]. In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in [Formula: see text] ([Formula: see text]) cation mixing influences F anion dynamics, in [Formula: see text] ([Formula: see text]) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor [Formula: see text] cation-cation repulsions immediately lead to a boost in [Formula: see text] diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured [Formula: see text] or [Formula: see text]. Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Structural and Chemical Compatibilities of Li1- x Ni0.5 Co0.2 Mn0.3 O2 Cathode Material with Garnet-Type Solid Electrolyte for All-Solid-State Batteries

All-solid-state batteries (ASSBs) based on ceramic materials are considered a key technology for automobiles and energy storage systems owing to their high safety and stability. However, contact issues between the electrode and solid-electrolyte materials and undesired chemical reaction occurring at interfaces have hindered their development. Herein, the chemical compatibility and structural stability of composite mixtures of the layered cathode materials Li_{1- x} Ni_0.5 Co_0.2 Mn_0.3 O₂ (NCM523) with the garnet-type solid electrolyte Li_6.25 Ga_0.25 La₃ Zr₂ O₁₂ (LLZO-Ga) during high-temperature co-sintering under various gas flowing conditions are investigated. In situ high-temperature X-ray diffraction analysis of the composite materials reveals that Li diffusion from LLZO-Ga to NCM523 occurs at high temperature under synthetic air atmosphere, resulting in the decomposition of LLZO-Ga into La₂ Zr₂ O₇ and the recovery of charged NCM523 to the as-prepared state. The structural stability of the composite mixture at high temperature is further investigated under N₂ atmosphere, revealing that Li diffuses toward the opposite direction and involves the phase transition of LLZO-Ga from a cubic to tetragonal structure and the reduction of the NCM523 cathode to Ni metal. These findings provide insight into the structural stability of layered cathode and garnet-type solid-electrolyte composite materials and the design of stable interfaces between them via co-sintering for ASSBs.

Evaluation of Ga0.2Li6.4Nd3Zr2O12 garnets: exploiting dopant instability to create a mixed conductive interface to reduce interfacial resistance for all solid state batteries

The next major leap in energy storage is thought to arise from a practical implementation of all solid-state batteries, which remain largely confined to the small scale due to issues in manufacturing and mechanical stability. Lithium batteries are amongst the most sought after, for the high expected energy density and improved safety characteristics, however the challenge of finding a suitable solid-state electrolyte remains. Lithium rich garnets are prime contenders as electrolytes, owing to their high ionic conductivity (>0.1 mS cm^-1), wide electrochemical window (0-6 V) and stability with Li metal. However, the high Young's modulus of these materials, poor wetting of Li metal and rapid formation of Li₂CO₃ passivating layers tends to give a detrimentally large resistance at the solid-solid interface, limiting their application in solid state batteries. Most studies have focused on La based systems, with very little work on other lanthanides. Here we report a study of the Nd based garnet Ga_0.2Li_6.4Nd₃Zr₂O₁₂, illustrating substantial differences in the interfacial behaviour. This garnet shows very low interfacial resistance attributed to dopant exsolution which, when combined with moderate heating (175 °C, 1 h) with Li metal, we suggest forms Ga-Li eutectics, which significantly reduces the resistance at the Li/garnet interface to as low as 67 Ω cm² (much lower than equivalent La based systems). The material also shows intrinsically high density (93%) and good conductivity (≥0.2 mS cm^-1) via conventional furnaces in air. It is suggested these garnets are particularly well suited to provide a mixed conductive interface (in combination with other garnets) which could enable future solid-state batteries.

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.

Crystal Structure and Preparation of Li7La3Zr2O12 (LLZO) Solid-State Electrolyte and Doping Impacts on the Conductivity: An Overview

As an essential part of solid-state lithium-ion batteries, solid electrolytes are receiving increasing interest. Among all solid electrolytes, garnet-type Li7La3Zr2O12 (LLZO) has proven to be one of the most promising electrolytes because of its high ionic conductivity at room temperature, low activation energy, good chemical and electrochemical stability, and wide potential window. Since the first report of LLZO, extensive research has been done in both experimental investigations and theoretical simulations aiming to improve its performance and make LLZO a feasible solid electrolyte. These include developing different methods for the synthesis of LLZO, using different crucibles and different sintering temperatures to stabilize the crystal structure, and adopting different methods of cation doping to achieve more stable LLZO with a higher ionic conductivity and lower activation energy. It also includes intensive efforts made to reveal the mechanism of Li ion movement and understand its determination of the ionic conductivity of the material through molecular dynamic simulations. Nonetheless, more insightful study is expected in order to obtain LLZO with a higher ionic conductivity at room temperature and further improve chemical and electrochemical stability, while optimal multiple doping is thought to be a feasible and promising route. This review summarizes recent progress in the investigations of crystal structure and preparation of LLZO, and the impacts of doping on the lithium ionic conductivity of LLZO.

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.

Spray Flame Synthesis (SFS) of Lithium Lanthanum Zirconate (LLZO) Solid Electrolyte

A spray-flame reaction step followed by a short 1-h sintering step under O₂ atmosphere was used to synthesize nanocrystalline cubic Al-doped Li₇La₃Zr₂O₁₂ (LLZO). The as-synthesized nanoparticles from spray-flame synthesis consisted of the crystalline La₂Zr₂O₇ (LZO) pyrochlore phase while Li was present on the nanoparticles’ surface as amorphous carbonate. However, a short annealing step was sufficient to obtain phase pure cubic LLZO. To investigate whether the initial mixing of all cations is mandatory for synthesizing nanoparticulate cubic LLZO, we also synthesized Li free LZO and subsequently added different solid Li precursors before the annealing step. The resulting materials were all tetragonal LLZO (I4₁/acd) instead of the intended cubic phase, suggesting that an intimate intermixing of the Li precursor during the spray-flame synthesis is mandatory to form a nanoscale product. Based on these results, we propose a model to describe the spray-flame based synthesis process, considering the precipitation of LZO and the subsequent condensation of lithium carbonate on the particles’ surface.

Gallium-Doping Effects on Structure, Lithium-Conduction, and Thermochemical Stability of Li7-3x Gax La3 Zr2 O12 Garnet-Type Electrolytes

One of the most promising electrolytes for all-solid-state lithium batteries is Li7 La3 Zr2 O12 . Previously, their thermodynamic stability, Li-ion conductivity, and structural features induced by Ga-doping have not been empirically determined or correlated. Here, their interplay was examined for Li7-3x Gax La3 Zr2 O12 with target xGa=0, 0.25, 0.50, 0.75, and 1.00 atoms per formula unit (apfu). Formation enthalpies, obtained with calorimetry and found to be exothermic at all compositions, linearly decreased in stability with increased xGa. At dilute xGa substitution, the formation enthalpy curve shifted stepwise endothermically, and the conductivity increased to a maximum, coinciding with 0.529 Ga apfu. This correlated with percolation threshold analysis (0.558 Ga apfu). Further substitution (0.787 Ga apfu) produced a large decrease in the stability and conductivity due to a large increase in point defects and blocked Li-migration pathways. At xGa=1.140 apfu, a small exothermic shift was related to defect cluster organization extending the Li hopping distance and decreased Li-ion conductivity.

Water based synthesis of highly conductive GaxLi7-3xLa3Hf2O12 garnets with comparable critical current density to analogous GaxLi7-3xLa3Zr2O12 systems

Next generation lithium ion batteries are envisaged as those which feature an all solid-state architecture. This will enable the higher energy density storage required to meet the demands of modern society, especially for the growing electric vehicle market. Solid state batteries have, however, proved troublesome to implement commercially due to the lack of a suitable solid-state electrolyte, which needs to be highly conductive, have a low interfacial resistance and a suitably wide electrochemical stability window. Garnet materials are potential contenders for these batteries, demonstrating many of the desired properties, although there remain challenges to overcome. Here we report a facile synthesis of Li₇La₃Hf₂O₁₂ and Ga/Al_xLi_7-3xLa₃Hf₂O₁₂ garnets, with the synthesis of Ga_0.2Li_6.4La₃Hf₂O₁₂ requiring only dissolution of precursors in water and heating to 700 °C. Ga_0.2Li_6.4La₃Hf₂O₁₂ was shown to display a high room temperature conductivity (0.373 mS cm^-1 at 28 °C). Moreover, in Li|garnet|Li cells, we observed a comparable critical current density compared to Ga_0.2Lai_6.4La₃Zr₂O₁₂, despite a lower density and higher area specific resistance compared to literature values, suggesting Hf systems may be further engineered to deliver additional improvements for use in future solid state 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.

Reduced Energy Barrier for Li+ Transport Across Grain Boundaries with Amorphous Domains in LLZO Thin Films

The high-resistive grain boundaries are the bottleneck for Li+ transport in Li7La3Zr2O12 (LLZO) solid electrolytes. Herein, high-conductive LLZO thin films with cubic phase and amorphous domains between crystalline grains are prepared, via annealing the repetitive LLZO/Li2CO3/Ga2O3 multi-nanolayers at 600 °C for 2 h. The amorphous domains may provide additional vacant sites for Li+, and thus relax the accumulation of Li+ at grain boundaries. The significantly improved ionic conductivity across grain boundaries demonstrates that the high energy barrier for Li+ migration caused by space charge layer is effectively reduced. Benefiting from the Li+ transport paths with low energy barriers, the presented LLZO thin film exhibits a cutting-edge value of ionic conductivity as high as 6.36 × 10-4 S/cm, which is promising for applications in thin film lithium batteries.

Research Progresses of Garnet-Type Solid Electrolytes for Developing All-Solid-State Li Batteries

All-Solid-State Batteries (ASSBs) that use oxide-based solid electrolytes (SEs) have been considered as a promising energy-storage platform to meet an increasing demand for Li-ion batteries (LIBs) with improved energy density and superior safety. However, high interfacial resistance between particles in the composite electrode and between electrodes and the use of Li metal in the ASBS hinder their practical utilization. Here, we review recent research progress on oxide-based SEs for the ASSBs with respect to the use of Li metal. We especially focus on research progress on garnet-type solid electrolytes (Li₇La₃Zr₂O₁₂) because they have high ionic conductivity, good chemical stability with Li metal, and a wide electrochemical potential window. This review will also discuss Li dendritic behavior in the oxide-based SEs and its relationship with critical current density (CCD). We close with remarks on prospects of ASSB.

NMR Investigations of Crystalline and Glassy Solid Electrolytes for Lithium Batteries: A Brief Review

The widespread use of energy storage for commercial products and services have led to great advancements in the field of lithium-based battery research. In particular, solid state lithium batteries show great promise for future commercial use, as solid electrolytes safely allow for the use of lithium-metal anodes, which can significantly increase the total energy density. Of the solid electrolytes, inorganic glass-ceramics and Li-based garnet electrolytes have received much attention in the past few years due to the high ionic conductivity achieved compared to polymer-based electrolytes. This review covers recent work on novel glassy and crystalline electrolyte materials, with a particular focus on the use of solid-state nuclear magnetic resonance spectroscopy for structural characterization and transport measurements.

Versatile Strategy for Realizing Flexible Room-Temperature All-Solid-State Battery through a Synergistic Combination of Salt Affluent PEO and Li6.75La3Zr1.75Ta0.25O12 Nanofibers

All-solid-state lithium metal batteries are highly attractive because of their high energy density and inherent safety. However, it is still a great challenge to design the solid electrolytes with high ionic conductivity at room temperature and good electrode/electrolyte interfacial compatibility simultaneously in a facile and scalable way. In this work, for the first time, the combination of salt affluent Poly(ethylene oxide) with Li_6.75La₃Zr_1.75Ta_0.25O₁₂ nanofibers was designed and intensively evaluated. The synergistic effect of each component in the electrolyte enhances the ionic conductivity to 2.13 × 10^-4 S cm^-1 at 25 °C and exhibits a high transference number of 0.57. The composite electrolyte possesses superior interfacial stability against Li metal for over 680 h in Li symmetric cells even at a relatively high current density of 2 mA cm^-2. The all-solid-state batteries employing the solid electrolytes exhibit excellent cycling stability at room temperature and superior safety performance. This work proposes a brand-new strategy to design and fabricate solid electrolytes in a versatile way for room-temperature all-solid-state batteries.

Al/Ga-Doped Li7La3Zr2O12 Garnets as Li-Ion Solid-State Battery Electrolytes: Atomistic Insights into Local Coordination Environments and Their Influence on 17O, 27Al, and 71Ga NMR Spectra

Li₇La₃Zr₂O₁₂ (LLZO) garnets are among the most promising solid electrolytes for next-generation all-solid-state Li-ion battery applications due to their high stabilities and ionic conductivities. To help determine the influence of different supervalent dopants on the crystal structure and site preferences, we combine solid-state ¹⁷O, ²⁷Al, and ⁷¹Ga magic angle spinning (MAS) NMR spectroscopy and density-functional theory (DFT) calculations. DFT-based defect configuration analysis for the undoped and Al and/or Ga-doped LLZO variants uncovers an interplay between the local network of atoms and the observed NMR signals. Specifically, the two characteristic features observed in both ²⁷Al and ⁷¹Ga NMR spectra result from both the deviations in the polyhedral coordination/site-symmetry within the 4-fold coordinated Li1/24d sites (rather than the doping of the other Li2/96h or La sites) and with the number of occupied adjacent Li2 sites that share oxygen atoms with these dopant sites. The sharp ²⁷Al and ⁷¹Ga resonances arise from dopants located at a highly symmetric tetrahedral 24d site with four corner-sharing LiO₄ neighbors, whereas the broader features originate from highly distorted dopant sites with fewer or no immediate LiO₄ neighbors. A correlation between the size of the ²⁷Al/⁷¹Ga quadrupolar coupling and the distortion of the doping sites (viz. XO₄/XO₅/XO₆ with X = {Al/Ga}) is established. ¹⁷O MAS NMR spectra for these systems provide insights into the oxygen connectivity network: ¹⁷O signals originating from the dopant-coordinating oxygens are resolved and used for further characterization of the microenvironments at the dopant and other sites.

[(Ba19Cl4)(Ga6Si12O42S8)]: a Two-Dimensional Wide-Band-Gap Layered Oxysulfide with Mixed-Anion Chemical Bonding and Photocurrent Response

Mixed-anion compounds play an essential part in modern structural chemistry. In this Communication, an unprecedented hexanary oxysulfide, [(Ba₁₉Cl₄)(Ga₆Si₁₂O₄₂S₈)] (FJ-1), was synthesized at 1073 K by a standard solid-state method, which is a new phase in the AE/M^III/M^IV/O/Q/X (AE = alkaline-earth metal; M^III = group 13 metal; M^IV = group 14 metal; Q = chalcogen; X = halogen) system. FJ-1 adopts a new structure type and crystallizes in the orthorhombic system with space group Cmcm. In the structure, unique two-dimensional [Ga₆Si₁₂O₄₂S₈]^34- layers formed by the familiar [SiO₄] species and unusual heteroligand [GaO₂S₂] and [GaO₃S] tetrahedra extend the intralayer linking. Significantly, a photoelectrochemical test revealed that FJ-1 is photoresponsive under ultraviolet illumination. Moreover, density functional theory calculations were employed to gain insight into the relationship between the electronic structure and optical properties. Such work will be conducive to the structural diversity of gallium coordination chemistry by exploration of the new mixed-anion functional chalcohalides.

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.

Atomistic Insight into Ion Transport and Conductivity in Ga/Al-Substituted Li7La3Zr2O12 Solid Electrolytes

Garnet-structured Li₇La₃Zr₂O₁₂ is a promising solid electrolyte for next-generation solid-state Li batteries. However, sufficiently fast Li-ion mobility required for battery applications only emerges at high temperatures, upon a phase transition to cubic structure. A well-known strategy to stabilize the cubic phase at room temperature relies on aliovalent substitution; in particular, the substitution of Li⁺ by Al³⁺ and Ga³⁺ ions. Yet, despite having the same formal charge, Ga³⁺ substitution yields higher conductivities (10^-3 S/cm) than Al³⁺ (10^-4 S/cm). The reason of such difference in ionic conductivity remains a mystery. Here, we use molecular dynamic simulations and advanced sampling techniques to precisely unveil the atomistic origin of this phenomenon. Our results show that Li⁺ vacancies generated by Al³⁺ and Ga³⁺ substitution remain adjacent to Ga³⁺ and Al³⁺ ions, without contributing to the promotion of Li⁺ mobility. However, while Ga³⁺ ions tend to allow limited Li⁺ diffusion within their immediate surroundings, the less repulsive interactions associated with Al³⁺ ions lead to a complete blockage of neighboring Li⁺ diffusion paths. This effect is magnified at lower temperatures and explains the higher conductivities observed for Ga-substituted systems. Overall, this study provides a valuable insight into the fundamental ion transport mechanism in the bulk of Ga/Al-substituted Li₇La₃Zr₂O₁₂ and paves the way for rationalizing aliovalent substitution design strategies for enhancing ionic transport in these materials.

Lithium-ion conducting oxide single crystal as solid electrolyte for advanced lithium battery application

Today, all-solid-state secondary lithium-ion batteries have attracted attention in research and development all over the world as a next-generation energy storage device. A key material for the all-solid-state lithium batteries is inorganic solid electrolyte, including oxide and sulfide materials. Among the oxide electrolytes, garnet-type oxide exhibits the highest lithium-ion conductivity and a wide electrochemical potential window. However, they have major problems for practical realization. One of the major problems is an internal short-circuit in charging and discharging. In the polycrystalline garnet-type oxide electrolyte, dendrites of lithium metal easily grow through the void or impurity in grain boundaries of the sintered body, which causes serious internal short-circuits in the battery system. To solve these problems, we present an all-solid-state battery system using a single-crystal oxide electrolyte. We are the first to successfully grow centimeter-sized single crystals of garnet-type by the floating zone method. The single-crystal solid electrolyte exhibits an extremely high lithium-ion conductivity of 10^-3 S cm^-1 at 298 K. The garnet-type single-crystal electrolyte has an advantageous bulk nature to realize the bulk conductivity without grain boundaries such as in a sintered polycrystalline body, and will be a game-changing technology for achieving highly safe advanced battery systems.

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.

Gallium-Doped Li7La3Zr2O12 Garnet-Type Electrolytes with High Lithium-Ion Conductivity

Owing to their high conductivity, crystalline Li_7-3xGa_xLa₃Zr₂O₁₂ garnets are promising electrolytes for all-solid-state lithium-ion batteries. Herein, the influence of Ga doping on the phase, lithium-ion distribution, and conductivity of Li_7-3xGa_xLa₃Zr₂O₁₂ garnets is investigated, with the determined concentration and mobility of lithium ions shedding light on the origin of the high conductivity of Li_7-3xGa_xLa₃Zr₂O₁₂. When the Ga concentration exceeds 0.20 Ga per formula unit, the garnet-type material is found to assume a cubic structure, but lower Ga concentrations result in the coexistence of cubic and tetragonal phases. Most lithium within Li_7-3xGa_xLa₃Zr₂O₁₂ is found to reside at the octahedral 96h site, away from the central octahedral 48g site, while the remaining lithium resides at the tetrahedral 24d site. Such kind of lithium distribution leads to high lithium-ion mobility, which is the origin of the high conductivity; the highest lithium-ion conductivity of 1.46 mS/cm at 25 °C is found to be achieved for Li_7-3xGa_xLa₃Zr₂O₁₂ at x = 0.25. Additionally, there are two lithium-ion migration pathways in the Li_7-3xGa_xLa₃Zr₂O₁₂ garnets: 96h-96h and 24d-96h-24d, but the lithium ions transporting through the 96h-96h pathway determine the overall conductivity.

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.

Investigating the all-solid-state batteries based on lithium garnets and a high potential cathode - LiMn1.5Ni0.5O4

All-solid-state Li-ion batteries based on lithium garnets give new prospects for safer battery operations avoiding liquids, and could enable the integration of high energy density electrode materials. Herein, we critically investigate the structural and chemical stability of the high voltage cathode material, LiMn_1.5Ni_0.5O₄, based on the solid lithium garnet electrolyte LLZO (c-Li_6.4Ga_0.2La₃Zr₂O₁₂) for all-solid Li-ion batteries. We manufacture battery cells based on nano-grained synthesized LLZO and composite cathodes (LiMn_1.5Ni_0.5O₄/LLZO/C) fabricated via direct slurry casting of the cathode material and additives on sintered LLZO pellets against metallic Li anodes. The galvanostatic tests of such all-solid-state batteries up to 4.9 V at 95 °C reveal the incompatibility of the solid electrolyte and the cathode material under given conditions. Post-mortem analyses of the all-solid-state batteries demonstrate the formation of new inactive phases at the LLZO/LiMn_1.5Ni_0.5O₄ interfacial region through an irreversible reaction starting at ∼3.8 V during charging. The discovered limited chemical stability under the investigated conditions raises the question if LLZO could be a promising solid-electrolyte for future all-solid-state Li-ion batteries especially at higher operation potentials and demanding operation conditions.

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.