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      Structural and Electrochemical Consequences of Al and Ga Cosubstitution in Li 7La 3Zr 2O 12 Solid Electrolytes

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          Abstract

          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 7La 3Zr 2O 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 ( Iad, No. 230) to “non-garnet” ( I4̅3 d, 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.

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          Efficient iterative schemes forab initiototal-energy calculations using a plane-wave basis set

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            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.
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              Optimizing Li+ conductivity in a garnet framework

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                Author and article information

                Journal
                Chem Mater
                Chem Mater
                cm
                cmatex
                Chemistry of Materials
                American Chemical Society
                0897-4756
                04 March 2016
                12 April 2016
                : 28
                : 7
                : 2384-2392
                Affiliations
                [§ ]Department of Chemistry and Physics of Materials, University of Salzburg , 5020, Salzburg, Austria
                [# ]Christian Doppler Laboratory for Lithium Batteries, Institute for Chemistry and Technology of Materials, DFG Research Unit 1277 molife, Graz University of Technology (NAWI Graz) , 8010, Graz, Austria
                []Lawrence Berkeley National Laboratory, Energy Storage and Distributed Resources Division, University of California , Berkeley, California 94720, United States
                []Department of Materials Science and Engineering, University of California , Berkeley, 94720, United States
                []Samsung Advanced Institute of Technology , 255 Main Street, Cambridge, Massachusetts 02140, United States
                []Institute for Chemical Technologies and Analytics, Vienna University of Technology , 1060 Vienna, Austria
                []Diffraction group, Institute Laue-Langevin (ILL) , 71 avenue des Martyrs, 38000 Grenoble, France
                Author notes
                Article
                10.1021/acs.chemmater.6b00579
                4836877
                27110064
                8c7ca85b-51f0-42e4-afb3-b4225f2ea08f
                Copyright © 2016 American Chemical Society

                This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.

                History
                : 08 February 2016
                : 04 March 2016
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                Custom metadata
                cm6b00579
                cm-2016-00579m

                Materials science
                Materials science

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