Tetragonal LLZO Research Articles

Li sublattice and diffusive response to uniaxial loads in the solid electrolyte, Li[formula omitted]La[formula omitted]Zr[formula omitted]O12 (LLZO)

The polymorphism of the garnet solid electrolyte, Li7La3Zr2O12 (LLZO), is a critical facet of the material’s performance and viability as an electrolyte candidate. Indeed, the cubic polymorph has an ionic conductivity several orders of magnitude higher than the tetragonal polymorph. While the transition from tetragonal to cubic LLZO has been reported over a range of temperatures, the response of the material to asymmetrical mechanical loads is not fully understood. To fill the knowledge gap, this paper conducted classical molecular dynamics (MD) simulations to investigate the diffusive response of LLZO under uniaxial stress conditions at 1000 K. It is found that the transition from cubic to tetragonal LLZO occurs around 500 MPa in compression and 750 MPa in tension. Near these threshold stresses, the Li sublattice undergoes a smooth transition from a cubic-like arrangement of interstitial occupancies to a tetragonal-like arrangement, with the occupancies of 24d and 96h interstitial sites decreasing and increasing respectively as the stress increases. The activation energy for diffusion increases with both tension and compression, increasing at a rate of 39 × 10−6 and 7 × 10−6 eV/MPa in compression and tension respectively.

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.

Addition of Calcined Na2B4O7 on the Synthesis of Li7La3Zr2O12

Li7La3Zr2O12 (LLZO) is a garnet-type electrolyte for all-solid-state lithium-ion batteries (ASSB). It has good chemical and electrochemical stability against lithium and a relatively high ionic conductivity. However, the ionic conductivity needs to be further increased to provide a high specific capacity of the ASSB. Element doping into LLZO is an effort to increase molecular defect, known to enhance the conductivity. This research studied the effect of the Na2B4O7 addition on the LLZO synthesis, producing LLZBO(A). The investigation aims to understand whether the sodium ions dope into the LLZO structure during synthesis, or it is only B ions to enter into the structure. Therefore, another synthesis with B2O3 of B precursor was conducted for comparison (LLZBO(B)). The precursors were mixed stoichiometrically by following the formula of Li7-xLa3-xZr2-xBxNaxO12 (LLZBO, x= 0.15; 0.20; 0.30). XRD analysis equipped with Le Bail refinement found that LLZBO(A) and LLZBO(B) mainly consist of cubic and tetragonal LLZO with a %mol of 69.06 – 69.84 %, and the main secondary phase is La2Zr2O7. The surface morphology of LLZBO(A) and LLZBO(B) is almost similar to the irregular form of large aggregates. The particles become more dispersed when 0.3 %mol dopant was submitted. Impedance analysis found a high ionic conductivity of LLZBAO(A)0.3 1.042x10-3 Scm-1.

High-purity and high-density cubic phase of Li7La3Zr2O12 solid electrolytes by controlling surface/volume ratio and sintering pressure

• Phase composition of LLZO dependent with surface/volume ratio and pressure were studied. • The optimal surface/volume ratio to obtain high-purity cubic phase is 0.23 ~ 0.32 cm −1 . • High-purity cubic phase LLZO with high-density of 99.8% is obtained. The Li + conductivity of Li 7 La 3 Zr 2 O 12 (LLZO) solid electrolyte mainly depends on the bulk density and the purity of cubic phase. However, current studies show that high-density and high-purity cubic phase is extremely difficult to be obtained due to the easy lithium loss. Herein, the evolutions of phase composition and microstructure of LLZO are systematically studied by precisely controlling the surface/volume ratio and sintering pressure. Results show that the surface/volume ratio affects the phase composition by regulating the lithium content per unit volume. With too large surface/volume ratio of > 0.32 mm −1 , lithium element in unit volume is too poor to synthesize cubic phase LLZO. With too small surface/volume ratio of < 0.23 mm −1 , lithium element per unit volume is too rich which lead to the formation of tetragonal phase LLZO. The optimal surface/volume ratio for obtaining high-purity cubic phase is 0.23 ~ 0.32 cm −1 . Besides, pure cubic phase can only be obtained at 0 ~ 10 MPa, higher pressure of 15 ~ 20 MPa will accelerates lithium loss which result in the formation of La 2 Zr 2 O 7 and La 2 O 3 phases. The density of pure cubic phase increases with the sintering pressure, bulks sintered with 10 MPa have a high relative density of 99.8%. In conclusion, both the surface/volume ratio and sintering pressure are critical for obtaining high-density and high-purity cubic phase.

Exploring Li-ion conductivity in cubic, tetragonal and mixed-phase Al-substituted Li7La3Zr2O12 using atomistic simulations and effective medium theory

Garnet Li7La3Zr2O12 (LLZO) is a promising solid electrolyte candidate for solid-state Li-ion batteries, but at room temperature it crystallizes in a poorly Li-ion conductive tetragonal phase. To this end, partial substitution of Li+ by Al3+ ions is an effective way to stabilize the highly conductive cubic phase at room temperature. Yet, fundamental aspects regarding this aliovalent substitution remain poorly understood. In this work, we use molecular dynamics and advanced hybrid Monte Carlo methods for systematic study of the room temperature Li-ion diffusion in tetragonal and cubic LLZO to shed light on important open questions. We find that Al substitution in tetrahedral sites of the tetragonal LLZO allows previously inaccessible sites to become available, which enhances Li-ion conductivity. In contrast, in the cubic phase Li-ion diffusion paths become blocked in the vicinity of Al ions, resulting in a decrease of Li-ion conductivity. Moreover, combining the conductivities of individual phases through an effective medium approximation allowed us to estimate the conductivities of cubic/tetragonal phase mixtures that are in good agreement with those reported in several experimental works. This suggests that phase coexistence (due to phase equilibrium or gradients in Al content within a sample) could have a significant impact on the conductivity of Al-substituted LLZO, particularly at low contents of Al3+. Overall, by making a thorough comparison with reported experimental data, the theoretical study and simulations of this work advance our current understanding of Li-ion mobility in Al-substituted LLZO garnets and might guide future in-depth characterization experiments of this relevant energy storage material.

Exploring Li-Ion Conductivity in Cubic, Tetragonal and Mixed-Phase Al-Substituted Li &lt;sub&gt;7&lt;/sub&gt;La &lt;sub&gt;3&lt;/sub&gt;Zr2O &lt;sub&gt;12&lt;/sub&gt; Using Atomistic Simulations and Effective Medium Theory

Garnet Li7La3Zr2O12(LLZO) is a promising solid electrolyte candidate for solid-state Li-ion batteries, but at room temperature it crystallizes in a poorly Li-ion conductive tetragonal phase. To this end, partial substitution of Li+ by Al3+ ions is an effective way to stabilize the highly conductive cubic phase at room temperature. Yet, fundamental aspects regarding this aliovalent substitution remain poorly understood. In this work, we use molecular dynamics and advanced hybrid Monte Carlo methods for systematic study of the room temperature Li-ion diffusion in tetragonal and cubic LLZO to shed light on important open questions. We find that Al substitution in tetrahedral sites of the tetragonal LLZO allows previously inaccessible sites to become available, which enhances Li-ion conductivity. In contrast, in the cubic phase Li-ion diffusion paths become blocked in the vicinity of Al ions, resulting in a decrease of Li-ion conductivity. Moreover, combining the conductivities of individual phases through an effective medium approximation allowed us to estimate the conductivities of cubic/tetragonal phase mixtures that are in good agreement with those reported in several experimental works. This suggests that phase coexistence (due to phase equilibrium or gradients in Al content within a sample) could have a significant impact on the conductivity of Al-substituted LLZO, particularly at low contents of Al3+. Overall, by making a thorough comparison with reported experimental data, the theoretical study and simulations of this work advance our current understanding of Li-ion mobility in Al-substituted LLZO garnets and might guide future in-depth characterization experiments of this relevant energy storage material.

Structural stability of the Li-ion conductor Li7La3Zr2O12 investigated by high-pressure in-situ X-ray diffraction and Raman spectroscopy

The high-pressure behavior of Li7La3Zr2O12 (LLZO) with a garnet-related structure has been investigated for the first time by high pressure in situ X-ray diffraction measurements and Raman spectroscopy using a diamond anvil cell. It was found that the tetragonal LLZO transforms to the high-pressure cubic phase (HP-c-LLZO) at 71.8 GPa in a N2 medium and at 25.8 GPa in the absence of pressure mediums. Uniaxial compression in the absence of pressure mediums induced the phase transition at much lower pressures than in the hydrostatic case when N2 was used as the pressure medium. High pressure in situ Raman spectroscopy suggested that the Li coordination in HP-c-LLZO was different from that reported in cubic LLZO, which had been stabilized by doping Al at ambient pressure and room temperature.

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.

Origin of the Phase Transition in Lithium Garnets

Molecular dynamic and density functional theory based simulations were performed to obtain a better understanding of the origin of phase transition in garnet-type Li7La3Zr2O12 solid electrolyte. The phase transition coincided with the lithium redistribution among all sites. With the investigation of lithium distribution and dynamics, we found one temperature-dependent lithium migration pathway in lithium garnets. Lithium ions exhibited uniformly 3-dimensional diffusion in cubic LLZO, while the lithium diffusion in tetragonal LLZO was mainly in the a and b direction. The constrained diffusion in the c direction in tetragonal LLZO could be ascribed to the blocking effect of 16f sites which were found to be thermodynamically more stable than tetragonal 8a and 32g sites through density functional theory based calculations. Besides, the stabilizing effect of supervalent doping on cubic phase was also studied through Ta-doped LLZO. Further site occupancy investigations indicated that supervalent doping introduc...

Li-Metal/Solid Electrolyte Interfacial Stability Elucidated Via in Situ Electron Microscopy

Despite their different chemistries, novel energy-storage systems, e.g. Li-air, Li-S, all-solid-state Li batteries, etc., face one critical challenge of forming a conductive and stable interface between Li metal and a solid electrolyte. An accurate understanding of the formation mechanism and the exact structure/chemistry of the rarely existing benign interfaces, such as the Li/cubic-Li7-3x Al x La3Zr2O12 (c-LLZO) interface, is crucial towards enabling the use of Li metal anodes. Due to spatial confinement and structural/chemical complications, current investigations are largely limited to theoretical calculations. Here, through an in situ formation of Li/c-LLZO interfaces inside an aberration corrected scanning transmission electron microscope, we successfully reveal the interfacial chemical and structural progression. Upon contact with Li metal, the surface of the c-LLZO is slightly reduced, and simultaneously receives Li+ from the Li metal, thereby maintaining the charge balance. Instead of triggering decomposition reactions, a localized phase transition occurs leading to the formation of a 6 nm thick interfacial tetragonal LLZO phase that is stable over time (Figure 1). Although this interphase is ultra-thin and can be easily overlooked, it may effectively prevent further interfacial reactions without compromising the ionic conductivity. Although the cubic-to-tetragonal transition is typically undesired during LLZO synthesis, the similar structural change was found to be the likely key to the observed benign interface. These insights provide a new viewpoint for designing Li/solid electrolyte interfaces that can enable the use of Li metal anodes in next-generation batteries. Figure 1

Recent Developments in Garnet-Type Solid State Electrolyte Thin-Films Grown By CO2-Laser Assisted Chemical Vapor Deposition

The research on solid state electrolytes (SSE) is motivated by their excellent safety properties, their potential for battery miniaturization, and their possibility to enhance the energy density of the battery. However, their often low Li-ion conductivity compared to liquid electrolytes is a drawback. In this respect garnet-type Li7La3Zr2O12 (LLZO) attracted much attention, since it combines features such as a very good electrochemical stability over a wide potential range as well as against lithium metal and a reasonably high Li-ion conductivity of around 10-6 S·cm-1 in tetragonal and 10-4 S·cm-1 in cubic modification.[1] Regarding fabrication, chemical vapor deposition (CVD) is a suitable method to grow functional thin-films for Li-ion batteries, since it allows for a homogeneous growth over large areas with high deposition rates and a very high purity.[2] Despite these advantages, reports on CVD grown thin-films for Li-ion battery application are rare. One reason may be that with conventional CVD precursor delivery systems it is often difficult to achieve the correct composition of complex, multicomponent materials such as typically used for Li-ion batteries. Recently, our group established a novel CVD precursor delivery system applying CO2-laser flash evaporation of solid precursors (LA-CVD) and demonstrated its applicability for Li-ion battery research.[3] In this contribution we report on the latest progress in LLZO thin-film deposition using LA-CVD. The detailed characterization includes results on microstructure, phase composition and electrochemical performance. Thin-films that are dense, free of cracks and mainly consist of tetragonal LLZO show a Li-ion conductivity of about 10-6 S·cm-1, probed with impedance spectroscopy. This compares with lithium phosphorus oxynitride (LiPON), which is a glassy solid electrolyte typically used in current all-solid-state cells. To further improve the Li-ion conductivity of the LLZO thin-films two approaches are followed up: one is to increase the bulk conductivity by stabilization of the cubic LLZO phase via doping. The second is to reduce the grain boundary contribution by tuning the morphology of the thin-films.

Structure and ionic conductivity of cubic Li7La3Zr2O12 solid electrolyte prepared by chemical co-precipitation method

Solid electrolyte Li7La3Zr2O12 (LLZO) powder was successfully synthesized by co-precipitation method. The LLZO powder calcined at 850°C with a tetragonal phase was confirmed by X-ray diffraction (XRD). Scanning electron microscopy (SEM) showed that the particles sizes of LLZO precursor powder were in the range of 0.5~1μm. LLZO pellets were sintered at different temperatures from 1150 to 1200°C in air atmosphere. When the temperature was above 1150°C, tetragonal LLZO would completely transform into cubic phase. The total ionic conductivity of LLZO pellet sintered at 1180°C was 2.0×10−4Scm−1 at 30°C, and the activation energy was 0.25eV. This result indicates that high quality LLZO solid electrolytes can be obtained by chemical co-precipitation method.

Nebulized spray pyrolysis of Al-doped Li7La3Zr2O12 solid electrolyte for battery applications

Nebulized spray pyrolysis followed by consolidation and sintering was used for the first time to prepare Al-doped garnet-based Li7−3xLa3Zr2AlxO12 (x = 0–0.25) (LLZO) ultra-fine grained ceramics. The structural changes from the tetragonal (x = 0), via a mixture of the cubic and the tetragonal (x = 0.07, 0.10) to the cubic modification (x = 0.15–0.25) were observed. 27Al NMR study showed that aluminum occupy the tetrahedral 24d lithium sites. Despite their low relative density (47–56%), preliminary ionic conductivities of the LLZO ceramics, were found to be 1.2 · 10− 6 S cm− 1 and 4.4 · 10− 6 S cm− 1 for tetragonal and cubic LLZO at room temperature, with activation energies of 0.55 eV and 0.49 eV, respectively.

Solid-State Electrolytes: Revealing the Mechanisms of Li-Ion Conduction in Tetragonal and Cubic LLZO by First-Principles Calculations

In this study, we applied first-principles-based calculations, such as ab-initio molecular dynamics simulation, metadynamics, and nudged-elastic band calculations, to successfully identify the mechanisms responsible for the considerable difference in ionic conductivity between the tetragonal and the cubic phases of LLZO (Li7La3Zr2O12)—a promising candidate for use as a highly Li-ion conductive solid-state electrolyte in Li-based batteries. Whereas in tetragonal LLZO the motion of Li ions is of fully collective nature or synchronous, we identified an asynchronous mechanism dominated by single-ion jumps and induced collective motion in cubic LLZO. The latter mechanism is possible at considerably lower energetic cost. The calculated energetic barriers that represent the two distinct mechanisms show good agreement with experimental values. Moreover, we were able to map the different mechanisms to the structural features of the particular polymorphs.

High conductivity of dense tetragonal Li7La3Zr2O12

Hot-pressing at 1050°C lead to near theoretical density (∼98% relative density) tetragonal LLZO. The total conductivity value for dense tetragonal LLZO is ∼2.3×10−5Scm−1. This is the highest reported value for tetragonal LLZO. This vast improvement in total conductivity is a result of the higher density achieved as a result of hot-pressing compared to conventional solid-state sintering. The value of the Li-ion lattice conductivity for dense tetragonal LLZO is 1.1×10−4Scm−1. The microstructure of dense tetragonal LLZO consist of twins within the grains. It is suggested that the presence of twin boundaries adds a significant contribution to the total resistance.

Structure and dynamics of the fast lithium ion conductor “Li7La3Zr2O12”

The solid lithium-ion electrolyte "Li(7)La(3)Zr(2)O(12)" (LLZO) with a garnet-type structure has been prepared in the cubic and tetragonal modification following conventional ceramic syntheses routes. Without aluminium doping tetragonal LLZO was obtained, which shows a two orders of magnitude lower room temperature conductivity than the cubic modification. Small concentrations of Al in the order of 1 wt% were sufficient to stabilize the cubic phase, which is known as a fast lithium-ion conductor. The structure and ion dynamics of Al-doped cubic LLZO were studied by impedance spectroscopy, dc conductivity measurements, (6)Li and (7)Li NMR, XRD, neutron powder diffraction, and TEM precession electron diffraction. From the results we conclude that aluminium is incorporated in the garnet lattice on the tetrahedral 24d Li site, thus stabilizing the cubic LLZO modification. Simulations based on diffraction data show that even at the low temperature of 4 K the Li ions are blurred over various crystallographic sites. This strong Li ion disorder in cubic Al-stabilized LLZO contributes to the high conductivity observed. The Li jump rates and the activation energy probed by NMR are in very good agreement with the transport parameters obtained from electrical conductivity measurements. The activation energy E(a) characterizing long-range ion transport in the Al-stabilized cubic LLZO amounts to 0.34 eV. Total electric conductivities determined by ac impedance and a four point dc technique also agree very well and range from 1 × 10(-4) Scm(-1) to 4 × 10(-4) Scm(-1) depending on the Al content of the samples. The room temperature conductivity of Al-free tetragonal LLZO is about two orders of magnitude lower (2 × 10(-6) Scm(-1), E(a) = 0.49 eV activation energy). The electronic partial conductivity of cubic LLZO was measured using the Hebb-Wagner polarization technique. The electronic transference number t(e-) is of the order of 10(-7). Thus, cubic LLZO is an almost exclusive lithium ion conductor at ambient temperature.