Lithium Garnet Oxide Research Articles

Using neutron diffraction to explore lithium displacement within cubic phase stabilised Ga-doped Li6.75La3Zr1.75Ta0.25O12 lithium garnet oxides

Typical Li6.75La3Zr1.75Ta0.25O12 exists as a mixture of tetragonal and cubic arrangements, but adding small amounts of Ga3+ (Li6.75–3xGaxLa3Zr1.75Ta0.25O12 x ≥ 0.1) resulted in a single cubic (I a-3d) phase lithium garnet oxide. Following the stabilisation of the cubic phase, the effects on lithium distributions were explored with neutron powder diffraction concerning Ga3+ content and temperature. Increasing the amount of Ga3+ reduced the amount of lithium within the structure, directly decreasing the Li 96h site occupancy and showing a minimal effect on the Li 24d site occupancy. High-temperature neutron diffraction studies revealed the migration of lithium from the Li 24d site to the Li 96h with increasing temperature. The inclusion of Ga3+ improved the total ionic conductivity over the gallium-free system. However, with increasing gallium content (x > 0.1), a negative correlation between the garnet's gallium content and total lithium ionic conductivity is observed, showing how the total amount of free lithium ions impact the system's total ionic conductivity. Though the electrolytes explored here show some limitations, the lithium-ion displacement trends with doping and temperature give us further insight into how these lithium garnet systems respond to chemical and physical change.

Achieving enhanced densification and superior ionic conductivity of garnet electrolytes via a co-doping strategy coupled with pressureless sintering

Lithium garnet oxides with 6.5 mol Li, such as Li6.5La3Zr1.5(Ta/Nb)0.5O12, typically crystallise in cubic structure and exhibit excellent room-temperature ionic conductivity close to 1 mS cm−1. However, it is challenging to densify garnet oxides. In this work, we investigated how the co-doping of tantalum (Ta) and niobium (Nb) affects the densification of pressureless sintered garnet electrolytes with compositions of Li6.5La3Zr1.5Ta(0.5−x)NbxO12, where x = 0–0.5. The highest densification (94.5% of relative density) was achieved in Li6.5La3Zr1.5Ta0.1Nb0.4O12 (TN-LLZO) when it was sintered at 1150 °C for 6 h. This TN-LLZO garnet electrolyte delivers an ionic conductivity of 1.04 × 10−3 S cm−1 (at 22 °C) with a low activation energy of 0.41 eV. Our findings demonstrate that the content of dopants (Ta and Nb) plays a critical role in enhancing the sintering performance of garnet ceramics at ambient pressure.

Transport in Lithium Garnet Oxides as Revealed by Atomistic Simulations

Lithium garnet oxides are a family of fast-ion conductors with appreciable lithium ionic conductivity in the solid state, making them promising candidates as electrolytes for all-solid-state batteries. In their structures, lithium is partially (along with vacancy) distributed among more than one crystallographically distinct sites, just as with other fast-ion conductors. This disorder has a great influence on lithium's transport properties such as diffusivity and ionic conductivity. We review atomistic simulation studies in conjunction with complementary experimental investigations, which offer atomic-scale visualization of and insight into lithium transport phenomena in lithium garnet oxides.

Study of diffusion and conduction in lithium garnet oxides LixLa3Zrx-5Ta7-xO12 by machine learning interatomic potentials.

Lithium garnet oxides are an attractive family of solid-state electrolytes due to their high Li-ion conductivity and good chemical stability against Li metal. Experimental study of these materials is often troubled by chemical contamination (e.g. Al) or lithium loss, while computational study, theoretically with controlled composition, is often limited either by accuracy (e.g. conventional interatomic potential) or efficiency (e.g. density-function theory or DFT). In this work, we report the study of diffusion and conduction of lithium garnets by a machine learning interatomic potential (MLIP) that is approaching DFT accuracy but orders of magnitude faster. We found that this MLIP is more accurate than other commonly applied models to study lithium garnets and is able to predict diffusion and conduction properties that are consistent with experiments. Computational studies enabled by this MLIP, combined with experimental data, suggest that ionic conduction is non-Arrhenius and maximum conductivity occurs around x = 6.6 to 6.8 in LixLa3Zrx-5Ta7-xO12.

Electrochemical properties of Li6+yLa3−yBayNbZrO12 lithium garnet oxide solid-state electrolytes with co-doping barium and zirconium

In this work, we co-doped barium (Ba) and zirconium (Zr) in a lithium garnet oxide, to improve its electrochemical properties. In the doping range of y = 0–1.0, the Li6+yLa3−yBayNbZrO12 (LLBNZO) oxides maintain a cubic crystal structure. A secondary phase of BaZrO3 emerges when y ≥ 0.75. The ionic conductivity of the developed garnet oxides first increases with Ba-doping in the low doping range and then decreases in the high doping range. It reaches the maximum value in Li6.5La2.5Ba0.5NbZrO12 when y = 0.5. Cyclic voltammetry shows that the garnet oxides have a wide electrochemical voltage window up to 6 V. The constant-current polarization results on the symmetric cells assembled from Li6.5La2.5Ba0.5NbZrO12 reveal that this garnet oxide is chemically stable against metallic lithium and has stable cycling performance.

Microstructural engineering in lithium garnets by hot isostatic press to cordon lithium dendrite growth and negate interfacial resistance for all solid state battery applications

Hot isostatic press (HIP) post sintering technique was utilized on the garnet structured oxides, prepared by solid state reaction method, to enhance density, Li+ conductivity and barricade lithium dendrite growth. Simultaneous application of heat and isostatic pressure is an effective technique to enhance the grain boundary integrity and minimize the pores, which in turn, increase the relative density of the lithium garnet oxide pellets. The advantage of HIP is well echoed by the enhanced total (grain + grain boundary) Li+ conductivity and transparency of the Li6.16Al0.28Zr2La3O12 (Al-LLZ) pellet. The room temperature (25 °C) total lithium ion conductivity of Al-LLZ and 1 wt% of Li4SiO4 (LSO) added Al-LLZ, were found to be 0.13 mS/cm and 0.3 mS/cm, respectively. This is further enhanced to 0.31 mS/cm and 0.41 mS/cm, respectively, along with improved relative density of around 98% through the HIP treatment. The HIP treated transparent Al-LLZ pellet successfully barricades lithium dendrite growth even at higher current density of 250 μA/cm2 and also reduces the metallic lithium and Al-LLZ interfacial resistance by increasing the lithium wettability.

Investigation of Li Ion Dynamics in Garnet Oxide Li6.5La3Zr1.5Ta0.5O12 Using Molecular Dynamics Simulation

To overcome the safety issue of liquid electrolytes in conventional lithium-ion batteries, various solid-state inorganic electrolytes have been studied. Among them, lithium garnet oxides have been attracting research interests due to their high ionic conductivity, Li7-xLa3Zr2-xTaxO12 (LLZTO) with have a at room temperature with a maximum value around x = 0.51,2. There are two different structures of lithium garnet oxides, a cubic structure with space group Iad and a tetragonal phase with space group I41/acd. The cubic structure has two types of Li sites, i.e., tetrahedral (Td) 24d sites and octahedral (Oh) 48g sites. The tetragonal structure has four types of Li sites, i.e., Td-8a, Td-16e, Oh-16f, and Oh-32g sites. The fully empty 16e sites and fully occupied the other sites in tetragonal structure makes its conductivity about 2 orders lower than the cubic phase. However, the complexity of structure hindered the understanding of the mechanism of lithium ion conduction. In this paper, we performed molecular dynamics (MD) simulation on Li6.5La3ZrTaO12 based on classic MD simulation from both the interatomic potential and DFTB methods to study its structure and properties including lattice parameters, diffusion parameters, ionic conductivities, dielectric constants, and elastic moduli. The results are extracted through correlation function that was used for simple liquids3. The consistency of our results with the reported experimental ones helped us to understand the Li diffusion mechanism. References (1) Y. X. Wang and W. Lai "High Ionic Conductivity Lithium Garnet Oxides of Li7-xLa3Zr2-xTaxO12 Compositions," Electrochem. Solid State Lett. 2012, 15, A68-A71. http://dx.doi.org/10.1149/2.024205esl(2) Y. T. Li, J. T. Han, C. A. Wang, H. Xie, and J. B. Goodenough "Optimizing Li+ conductivity in a garnet framework," J. Mater. Chem. 2012, 22, 15357-15361. http://dx.doi.org/10.1039/c2jm31413d(3) In Theory of Simple Liquids (Fourth Edition); J.-P. Hansen and I. R. McDonald, Eds.; Academic Press: Oxford, 2013, p i.

Lithium garnet based free-standing solid polymer composite membrane for rechargeable lithium battery

Electrolytes with high lithium-ion conductivity, better mechanical strength and large electrochemical window are essential for the realization of high-energy density lithium batteries. Polymer electrolytes are gaining interest due to their inherent flexibility and nonflammability over conventional liquid electrolytes. In this work, lithium garnet composite polymer electrolyte membrane (GCPEM) consisting of large molecular weight (Wavg ~ 5 × 106) polyethylene oxide (PEO) complexed with lithium perchlorate (LiClO4) and lithium garnet oxide Li6.28Al0.24La3Zr2O12 (Al-LLZO) is prepared by solution-casting method. Significant improvement in Li+ conductivity for Al-LLZO containing GCPEM is observed compared with the Al-LLZO free polymer membrane. Maximized room temperature (30 °C) Li+ conductivity of 4.40 × 10−4 S cm−1 and wide electrochemical window (4.5 V) is observed for PEO8/LiClO4 + 20 wt% Al-LLZO (GCPEM-20) membrane. The fabricated cell with LiCoO2 as cathode, metallic lithium as anode and GCPEM-20 as electrolyte membrane delivers an initial charge/discharge capacity of 146 mAh g−1/142 mAh g−1 at 25 °C with 0.06 C-rate.

Synthesis of lithium garnet oxides of the compositions series Li7-xLa3Zr2-xTaxO12

In order to obtain a safe, reliable, long-lived battery system without use of flammable, volatile, and relatively unstable organic liquid-based electrolytes, lithium garnet oxides with formulas Li7-xLa3Zr2-xTa x O12 (x=0.2-1) were synthesized by the solid state reaction method. Single cubic phases were observed in the composition x range between 0.2 and 1. The lattice parameters decreased with the addition of Ta due to the smaller ionic radius of Ta5+ compared with that of Zr4+, following the Vegard’s law. The total conductivity of the x = 0.3 composition is 6.03×10-5 S·cm-1 at room temperature with an activation energy of 0.30 eV. These lithium garnet oxides exhibit lithium ionic transport that is relevant to lithium battery application.

Lithium self-diffusion in a model lithium garnet oxide Li5La3Ta2O12: A combined quasi-elastic neutron scattering and molecular dynamics study

In this work lithium self-diffusion in the model lithium garnet oxide Li5La3Ta2O12 was investigated by combining quasi-elastic neutron scattering experiments and molecular dynamics simulation. The Q-dependence of the quasi-elastic broadening measured experimentally and of the mean relaxation rate of lithium calculated by molecular dynamics simulations were both well described by the Singwi-Sjölander diffusion model. This model describes lithium nuclei that undergo diffusion via a jump-type mechanism consisting of a distribution of jumps with a mean square jump distance and residence time. The extracted mean jump length is consistent with a jump between tetrahedral (24d) to octahedral (48g and 96h) sites within the Ia3¯d symmetry structure and the residence time of Li at these sites obeys an Arrhenius relation from ~ps timescales at 1100K to the ~ns range at 400K. This result supports a lithium diffusion mechanism in which jumps are more frequent (smaller residence time) and shorter at higher temperature. The self-diffusivities of Li from both experiment and calculation were in good agreement, but deviations from those previously measured using nuclear magnetic resonance and muon spin relaxation were observed and discussed. Analysis of stretching parameter describing the relaxation of lithium calculated by molecular dynamics indicated that Li motion is more cooperative at shorter length scales, below ~4.4Å, which corresponds to the distance between octahedral sites across the tetrahedral site.

Structure, Ionic Conductivity, and Dielectric Properties of Li-Rich Garnet-type Li5+2xLa3Ta2-xSmxO12 (0 ≤ x ≤ 0.55) and Their Chemical Stability.

Lithium garnet oxides are considered as very promising solid electrolyte candidates for all-solid-state lithium ion batteries (SSLiBs). In this work, we present a cubic garnet-type Li5+2xLa3Ta2-xSmxO12 (0 ≤ x ≤ 0.55) system as a potential electrolyte for SSLiBs. Powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) were employed to investigate the structural stability of Li5+2xLa3Ta2-xSmxO12. The results from PXRD and SEM suggested structural and morphological transformation as a function of dopant concentration. In addition to Li-ion transport in Li5+2xLa3Ta2-xSmxO12, the dielectric properties were also investigated in the light of electron energy loss functions, which showed some surface energy loss and negligible volume energy loss for the studied garnets. Surface and volume energy loss functions of a mixed conducting LiCoO2 was studied for comparison. The long-term chemical stability of one of members, Li5.3La3Ta1.85Sm0.15O12, was performed on aged sample using PXRD, SEM, and thermogravimetric analysis.

Finite-size effects on the molecular dynamics simulation of fast-ion conductors: A case study of lithium garnet oxide Li7La3Zr2O12

A useful tool to study ionic conduction mechanisms in fast-ion conductors is the molecular dynamics (MD) simulation performed on finite simulation cells with periodic boundary conditions. While there have been many studies of the effect of cell size on the thermodynamics and kinetics of simple liquids, the finite-size effect in fast-ion conductors remains elusive. This work presents a case study to investigate the finite-size effect on the phase transition, self-diffusivity, ionic conductivity, Haven ratio, thermodynamic factor, and Fickian diffusivity using lithium garnet oxide Li7La3Zr2O12 as a model material. It was found that cell sizes influence extracted thermodynamic and kinetic characteristics in different ways with magnitude ranging from weak to strong. For Li7La3Zr2O12, reliable properties can be obtained with a 3×3×3 (5184 atoms) cell.

Local structure and dynamics of lithium garnet ionic conductors: tetragonal and cubic Li7La3Zr2O7.

To better understand the ionic conduction in lithium garnet oxides, we employed molecular dynamics simulation to investigate the local structure and dynamics of a model material Li7La3Zr2O12 and origin of its tetragonal to cubic phase transition. Our simulations were able to produce lattice parameter, neutron scattering, and conductivity data close to those gathered using experimental techniques, which allows us to study atomic-scale details of this complex material. First, it was found that lithium atoms primarily perform oscillation and "structured diffusion" dynamics in the tetragonal and cubic phase, respectively. Second, we believe that the tetragonal to cubic phase transition is an entropy-driven one that involves redistribution of lithium atoms among all tetrahedral sites. The transition is likely to initiate on the tetrahedral 8a site but needs the cooperation of neighboring octahedral 32g Li as relay atoms. Finally, it was found that a few types of lithium clusters dominate in both phases of Li7La3Zr2O12, which leads to highly correlated motion of lithium atoms. The local symmetry of these clusters dictates a "center-pass" mechanism as lithium goes through the bottleneck.

Phase transition in lithium garnet oxide ionic conductors Li7La3Zr2O12: The role of Ta substitution and H2O/CO2 exposure

High Li-content lithium garnet oxides are promising solid electrolyte materials for lithium batteries. Being the highest Li-content lithium garnet oxides, Li7La3Zr2O12 has been reported to crystallize in either the tetragonal or cubic phase with no consensus on the exact conditions under which these two phases are formed, which may be due to unintentional Al contamination and air exposure. In this work, the effects of Ta substitution and H2O/CO2 exposure have been studied under Al-contamination free conditions with minimal air exposure. We showed that 1) the Ta-substitution induced phase transition occurred through a two-phase mechanism and a minimum 0.6 mol of Ta substitution to Zr is needed to stabilize the cubic phase; 2) H2O and CO2 can individually induce the tetragonal-cubic phase transition in Li7La3Zr2O12 through proton exchange and Li extraction, respectively, which can have great influence on the transport properties of Li7La3Zr2O12.

Understanding the Li Disorder and Migration in Cubic Garnet Ionic Conductors through Diffraction Techniques and Computer Modeling

Solid-state lithium ionic conductors have received much attention recently as the issues associated with liquid electrolyte materials were increasingly recognized. Lithium garnet ionic conductors emerged as the most promising candidate in the oxide system owing to their high conductivity and good chemical and thermal stability [1, 2]. Majority of the research done on this family of compounds focused on the characterization of transport properties on a wide range of compositions with different doping strategy [3, 4], leaving a void of knowledge on the conduction mechanism. The limited number of structural studies [5, 6] typically employed a single technique. The lack of cross-check with other techniques left the findings vulnerable to challenges on the ground of limitations of individual technique.To understand local Li disorder and conduction pathway, we applied various diffraction and computational techniques to the lithium garnet oxides. The model material Li5La3Ta2O12 was picked because of its relative simplicity. Diffraction based techniques employed in this study include Rietveld refinement using time-of-flight neutron powder diffraction data, pair distribution analysis using total scattering data and maximum entropy analysis using constant wavelength neutron diffraction data. Static and dynamic structures were modeled with energy minimization method and molecular dynamics using conventional interatomic potentials. Rietveld refinement results revealed large anisotropic displacement factors on both tetrahedral and octahedral Li sites, suggesting strong positional disorder. Nuclear density map as calculated from maximum entropy analysis added more details to the lithium distribution. We adopted the iterative algorithm to minimize the structural biases. At low levels, the densities at the tetrahedral and octahedral sites connected, indicating the conduction pathway. This was confirmed by the modeled static structure where low energy state positions were represented by nuclear density maps obtained using a statistical approach. Pair distribution function analysis using reverse Monte Carlo modeling provided experimental evidence of the local structures and Li disorders. The positional disorders of both Li sites were further confirmed and the occupancies were in good agreement with Rietveld refinement results. Note that this is the first PDF analysis on garnet compounds so far and we proved that useful information can be extracted with this method despite the challenges as posed by the structural complexity and small scattering contribution from Li. Finally, the MD modeling simulated the conduction pathway of Li which is consistent with the indication of other techniques. To sum up, all the techniques tell one consistent story: positional disorders are so strong that the Li ions in Li-stuffed garnet compounds can be consider as composing an amorphous phase (i.e., having no symmetry at all) within a high symmetry framework; both tetrahedral and octahedral sites are involved in the conduction and the conduction pathway can be represented by connecting neighboring tetrahedral and octahedral Li sites. Figure 1. Nuclear density maps generated by (a) Rietveld refinement, (b) maximum entropy method, (c) energy minimization, (d) molecular dynamics; (e) experimental and fitted pair distribution functions.AcknowledgementThis work is supported by the Ceramics Program of National Science Foundation (DMR-1206356). We are grateful to Spallation Neutron Sourse at ORNL, Lujan Neutron Scattering Center at LANL and NIST Center for Neutron Research for making all facilities available. We wish to acknowledge the Michigan State University High Performance Computing Center and the Institute for Cyber-Enabled Research for access to their computing resources.

High Ionic Conductivity Lithium Garnet Oxides of Li7−xLa3Zr2−xTaxO12 Compositions

Lithium garnet oxides of the composition series Li7−xLa3Zr2−xTaxO12 (x = 0−2) were synthesized by the solid state reaction and characterized by the powder X-ray diffraction and the impedance spectroscopy. Single cubic phases were obtained between x = 0.2 and 2, while the end-member Li7La3Zr2O12 exhibited a tetragonal phase. The lattice parameters of the cubic series followed the Vegard’s law. The maximum bulk (9.6 × 10−4 S/cm) and total (6.9 × 10−4 S/cm) conductivities were achieved at x = 0.3 and x = 0.2, respectively at room temperature. Electrochemical tests with a hybrid solid|liquid electrolyte configuration were performed to evaluate the electrochemical performance of the solid electrolytes. © 2012 The Electrochemical Society. [DOI: 10.1149/2.024205esl] All rights reserved.