Phase formation of a garnet-type lithium-ion conductor Li7−3Al La3Zr2O12 (original) (raw)
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High conductivity of mixed phase Al-substituted Li7La3Zr2O12
Journal of Electroceramics, 2015
Al-substituted Li 7 La 3 Zr 2 O 12 (LLZ:Al) was synthesized via conventional solid state reaction. Different dwell times at sintering temperature of 1200°C led to a varying Li content in LLZ:Al which significantly affected the Li-ion conductivity. Electrochemical impedance spectroscopy and X-ray diffraction were used to characterize the sintered pellets which showed a maximum total ionic conductivity of~3 × 10 −4 S cm −1 at room temperature although the samples were composed of cubic and tetragonal LLZ:Al, with the tetragonal phase as its major phase. Inductively coupled plasma optical emission spectroscopy revealed that the Li content steadily decreased from 7.5 to 6.5 Li per formula unit with increasing sintering time. The highest conductivity was observed from the sample with the lowest Li concentration at 6.5 per formula unit. Scanning electron microscopy images revealed the formation of large grains, about 500 μm in diameter, which additionally could be the reason for achieving high total Li-ion conductivity. Electrochemical tests showed that mixed phase LLZ:Al is stable against metallic Li up to 8 V.
Fast Lithium Ion Conduction in Garnet-Type Li7La3Zr2O12
Angewandte Chemie International Edition, 2007
Rechargeable (secondary) all-solid-state lithium batteries are considered to be the next-generation high-performance power sources and are believed to have remarkable advantages over already commercialized lithium ion batteries utilizing aprotic-solution, gel, or polymeric electrolytes with regard to battery miniaturization, high-temperature stability, energy density, and battery safety. Solid electrolytes with high Li ion conductivity but negligible electronic conductivity, with stability against chemical reactions with elemental Li (or Limetal alloys) as the negative electrode (anode) and Co-, Ni-, or Mn-containing oxides as the positive electrode (cathode), and with decomposition voltages higher than 5.5 V against elemental Li are especially useful to achieve high energy and power densities as well as long-term stability. Lithium ion conduction has been reported for a wide range of crystalline metal oxides and halides with different types of structures. [1, 2] In general, oxide materials are believed to be superior to non-oxide materials for reasons of handling and mechanical, chemical, and electrochemical stability. [1] So far, most of the discovered inorganic lithium ion conductors have had either high ionic conductivity or high electrochemical stability, but not both. Some oxides are excellent lithium ion conductors; for example, Li 3x La (2/3)Àx & (1/3)À2x TiO 3 (0 < x < 0.16; "LLT"; & represents a vacancy) exhibits a bulk conductivity of 10 À3 S cm À1 and a total (bulk + grain-boundary) conductivity of 7 10 À5 S cm À1 at 27 8C and x % 0.1. However, this compound becomes predominantly electronically conducting within the lithium activity range given by the two electrodes. [3] It has been attempted to replace the transition metal Ti in LLT with Zr, which is fixed-valent and more stable (against chemical reaction with elemental lithium); however, this attempt was unsuccessful owing to the ready formation of the pyrochlore phase La 2 Zr 2 O 7. [4] Although a large number of possible lithium electrolytes have been reported for the Li 2 O-ZrO 2 system, none of them
Li Ion Dynamics in Al-Doped Garnet-Type Li 7 La 3 Zr 2 O 12 Crystallizing with Cubic Symmetry
Zeitschrift für Physikalische Chemie, 2012
Lithium-ion dynamics in the garnet-type solid electrolyte "Li 7 La 3 Zr 2 O 12 " (LLZ) crystallizing with cubic symmetry was probed by means of variable-temperature 7 Li NMR spectroscopy and ac impedance measurements. Li jump rates of an Al-containing sample follow Arrhenius behavior being characterized by a relatively high activation energy of 0.54(3) eV and a pre-exponential factor of 2.2(5) × 10 13 s −1. The results resemble those which were quite recently obtained for an Al-free LLZ sample crystallizing, however, with tetragonal symmetry. Hence, most likely, the significantly higher Li conductivity previously reported for a cubic LLZ sample cannot be ascribed solely to the slight structural distortions accompanying the change of the crystal symmetry. Here, even Al impurities, acting as stabilizer for the cubic polymorph at room temperature, do not lead to the high ion conductivity reported previously.
Chemistry of Materials, 2016
Several "Beyond Li-Ion Battery" concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li 7 La 3 Zr 2 O 12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅ d, No. 230) to "non-garnet" (I4̅ 3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10 −4 S cm −1 to 1.2 × 10 −3 S cm −1 , which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm 2 , the lowest reported value for LLZO so far. These results illustrate that understanding the structure−properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.
Chemistry of materials : a publication of the American Chemical Society, 2016
Li-oxide garnets such as Li7La3Zr2O12 (LLZO) are among the most promising candidates for solid-state electrolytes to be used in next-generation Li-ion batteries. The garnet-structured cubic modification of LLZO, showing space group Ia-3d, has to be stabilized with supervalent cations. LLZO stabilized with Ga(3+) shows superior properties compared to LLZO stabilized with similar cations; however, the reason for this behavior is still unknown. In this study, a comprehensive structural characterization of Ga-stabilized LLZO is performed by means of single-crystal X-ray diffraction. Coarse-grained samples with crystal sizes of several hundred micrometers are obtained by solid-state reaction. Single-crystal X-ray diffraction results show that Li7-3x Ga x La3Zr2O12 with x > 0.07 crystallizes in the acentric cubic space group I-43d. This is the first definite record of this cubic modification for LLZO materials and might explain the superior electrochemical performance of Ga-stabilized ...
Structure and dynamics of the fast lithium ion conductor “Li7La3Zr2O12”
Physical Chemistry Chemical Physics, 2011
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 longrange 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 Â 10 À4 Scm À1 to 4 Â 10 À4 Scm À1 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 Â 10 À6 Scm À1 , 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 eÀ is of the order of 10 À7. Thus, cubic LLZO is an almost exclusive lithium ion conductor at ambient temperature.
Novel Fast Lithium Ion Conduction in Garnet-Type Li 5 La 3 M 2 O 12 (M = Nb, Ta)
Journal of the American Ceramic Society, 2003
Lithium metal oxides with the nominal composition Li 5 La 3 M 2 O 12 (M ؍ Nb, Ta), possessing a garnetlike structure, have been investigated with regard to their electrical properties. These compounds form a new class of solid-state lithium ion conductors with a different crystal structure compared with all those known so far. The materials are prepared by solid-state reaction and characterized by powder XRD and ac impedance to determine their lithium ionic conductivity. Both the niobium and tantalum members exhibit the same order of magnitude of bulk conductivity (ϳ10 ؊6 S/cm at 25°C). The activation energies for ionic conductivity (<300°C) are 0.43 and 0.56 eV for Li 5 La 3 Nb 2 O 12 and Li 5 La 3 Ta 2 O 12 , respectively, which are comparable to those of other solid lithium conductors, such as Lisicon, Li 14 ZnGe 4 O 16. Among the investigated materials, the tantalum compound Li 5 La 3 Ta 2 O 12 is stable against reaction with molten lithium. Further tailoring of the compositions by appropriate chemical substitutions and improved synthesizing methods, especially with regard to minimizing grain-boundary resistance, are important issues in view of the potential use of the new class of compounds as electrolytes in practical lithium ion batteries.
Crystal Chemistry and Stability of “Li 7 La 3 Zr 2 O 12 ” Garnet: A Fast Lithium-Ion Conductor
Inorganic Chemistry, 2011
Recent research has shown that certain Li-oxide garnets with high mechanical, thermal, chemical, and electrochemical stability are excellent fast Li-ion conductors. However, the detailed crystal chemistry of Li-oxide garnets is not well understood, nor is the relationship between crystal chemistry and conduction behavior. An investigation was undertaken to understand the crystal chemical and structural properties, as well as the stability relations, of Li 7 La 3 Zr 2 O 12 garnet, which is the best conducting Li-oxide garnet discovered to date. Two different sintering methods produced Li-oxide garnet but with slightly different compositions and different grain sizes. The first sintering method, involving ceramic crucibles in initial synthesis steps and later sealed Pt capsules, produced single crystals up to roughly 100 μm in size. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry (ICP-MS) measurements show small amounts of Al in the garnet, probably originating from the crucibles. The crystal structure of this phase was determined using X-ray single-crystal diffraction every 100 K from 100 K up to 500 K. The crystals are cubic with space group Ia3d at all temperatures. The atomic displacement parameters and Li-site occupancies were measured. Li atoms could be located on at least two structural sites that are partially occupied, while other Li atoms in the structure appear to be delocalized. 27 Al NMR spectra show two main resonances that are interpreted as indicating that minor Al occurs on the two different Li sites. Li NMR spectra show a single narrow resonance at 1.2-1.3 ppm indicating fast Li-ion diffusion at room temperature. The chemical shift value indicates that the Li atoms spend most of their time at the tetrahedrally coordinated C (24d) site. The second synthesis method, using solely Pt crucibles during sintering, produced fine-grained Li 7 La 3 Zr 2 O 12 crystals. This material was studied by X-ray powder diffraction at different temperatures between 25 and 200°C. This phase is tetragonal at room temperature and undergoes a phase transition to a cubic phase between 100 and 150°C. Cubic "Li 7 La 3 Zr 2 O 12 " may be stabilized at ambient conditions relative to its slightly less conducting tetragonal modification via small amounts of Al 3þ . Several crystal chemical properties appear to promote the high Li-ion conductivity in cubic Al-containing Li 7 La 3 Zr 2 O 12 . They are (i) isotropic three-dimensional Li-diffusion pathways, (ii) closely spaced Li sites and Li delocalization that allow for easy and fast Li diffusion, and (iii) low occupancies at the Li sites, which may also be enhanced by the heterovalent substitution Al 3þ S 3Li. Figure 7. (a, b) Crystal structure model for tetragonal Li 7 La 3 Zr 2 O 12 (ref 20) projected on (001) and (100), respectively. Note that possible Li diffusion pathways are different for the two orientations. (41) Armbruster, T.; Basler, R.; Mikhail, P.; Hulliger, J. J. Solid State Chem. 1999, 145, 309-316.
In this review work it has been tried to briefly summarize solid state electrolytes conductivity status. As the very essential component for battery efficiency and performance, electrolytes need be given due attention as safety problems could also emanate from it as well. The oxide solid state electrolytes are very promising electrolytes for allsolid-state batteries for large applications. The garnet-structured Li 7 La 3 Zr 2 O 12 has shown high ionic conductivity that is comparable to the liquid electrolytes with large potential windows. At lower temperature Li 7 La 3 Zr 2 O 12 will have high Li-ordered and forms the tetragonal structure which is less ionic conductor as compared to the less Li-ordered cubic structure. A total ionic conductivity of the order of 10 -3 Scm -1 has been achieved by the cubic structures of Li 7 La 3 Zr 2 O 12 which will let it to be applicable in practice.
Tantalum-doped garnet (Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 , LLZTO) is a promising candidate to act as a solid electrolyte in all-solid-state batteries owing to both its high Li + conductivity and its relatively high robustness against the Li metal. Synthesizing LLZTO using conventional solid-state reaction (SSR) requires, however, high calcination temperature (>1000°C) and long milling steps, thereby increasing the processing time. Here, we report on a facile synthesis route to prepare LLZTO using a molten salt method (MSS) at lower reaction temperatures and shorter durations (900°C, 5 h). Additionally, a thorough analysis on the properties, i.e., morphology, phase purity, and particle size distribution of the LLZTO powders, is presented. LLZTO pellets, either prepared by the MSS or the SSR method, that were sintered in a Pt crucible showed Li + ion conductivities of up to 0.6 and 0.5 mS cm −1 , respectively. The corresponding activation energy values are 0.37 and 0.38 eV, respectively. The relative densities of the samples reached values of approximately 96%. For comparison, LLZTO pellets sintered in alumina crucibles or with γ-Al 2 O 3 as sintering aid revealed lower ionic conductivities and relative densities with abnormal grain growth. We attribute these observations to the formation of Al-rich phases near the grain boundary regions and to a lower Li content in the final garnet phase. The MSS method seems to be a highly attractive and an alternative synthetic approach to SSR route for the preparation of highly conducting LLZTO-type ceramics.