Exploring Li-ion conductivity in cubic, tetragonal and mixed-phase Al-substituted Li7LaZr2O12using atomistic simulations and effective medium theory (original) (raw)
Related papers
ACS Applied Materials & Interfaces
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 Al 3+ and Ga 3+ ions. Yet, despite having the same formal charge, Ga 3+ substitution yields higher conductivities (10 −3 S/cm) than Al 3+ (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 Al 3+ and Ga 3+ substitution remain adjacent to Ga 3+ and Al 3+ ions, without contributing to the promotion of Li + mobility. However, while Ga 3+ ions tend to allow limited Li + diffusion within their immediate surroundings, the less repulsive interactions associated with Al 3+ 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 Li 7 La 3 Zr 2 O 12 and paves the way for rationalizing aliovalent substitution design strategies for enhancing ionic transport in these materials.
Garnet-type solid electrolytes are a class of materials that could potentially revolutionize Li-ion battery technology. In this work, ab-initio-based MD simulations have been performed to investigate the ion dynamics in pure garnet-type cubic Li 5 La 3 Ta 2 O 12 (LLTaO) over the temperature range from 873 to 1773 K. A strong tendency for disorder in the Li sublattice was verified for LLTaO that explains the relative ease of stabilizing the reported cubic phase for this material. The Li + conduction mechanism was determined to be facilitated by a cooperative hopping process characterized by long, multiple-site successive hops with a very small time scale for fluctuations at intermediate positions. A comparative study is also carried out between LLTaO and garnet-type Li 7 La 3 Zr 2 O 12 (LLZrO), another candidate solid electrolyte.
Enhanced lithium ion transport in garnet-type solid state electrolytes
Journal of Electroceramics, 2017
We have replaced the original with a corrected Figure 4, showing proper units. I suggest to mention the room T ionic conducitivity values explicitely on p. 10 and to compare them with literature data. I have the impression that the Ar sample is in line with several data sets found in literature and the air sample is worse than average literature data? We have added several lines on p. 10 giving the room temperature ionic conductivities of the two different samples, along with some extra literature references for Al-substituted LLZO (all highlighted). Reported conductivities vary a great deal depending on processing details, with values ranging from about less than 0.1-0.5 mS/cm 2. The values we report here for LLZO_air are similar to what we reported before for similarly made samples (see reference 25). Response to Reviewer Comments
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 ...
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
Self-diffusion in garnet-type Li7La3Zr2O12 solid electrolytes
Scientific Reports
Tetragonal garnet-type Li7La3Zr2O12 is an important candidate solid electrolyte for all-solid-state lithium ion batteries because of its high ionic conductivity and large electrochemical potential window. Here we employ atomistic simulation methods to show that the most favourable disorder process in Li7La3Zr2O12 involves loss of Li2O resulting in lithium and oxygen vacancies, which promote vacancy mediated self-diffusion. The activation energy for lithium migration (0.45 eV) is much lower than that for oxygen (1.65 eV). Furthermore, the oxygen migration activation energy reveals that the oxygen diffusion in this material can be facilitated at higher temperatures once oxygen vacancies form.
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.