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

Abstract

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.

Li Ion Dynamics in Al-Doped Garnet-Type Li7La3Zr2O12 Crystallizing with Cubic Symmetry

“…Quite recently, Geiger et al assumed that traces of Al ions incorporated during the hightemperature synthesis of cubic LLZ play an important role in the enhancement of the conductivity observed [12]. Note that an Al-containing sample of cubic LLZ, which was complementarily investigated by some of us using impedance measurements and NMR spectroscopy [13], indeed showed the extremely high lithium-ion conductivity mentioned above. Doping with Al might create additional vacancies at the regularly occupied Li sites, for example.…”

Advanced analytical electron microscopy for lithium-ion batteries

“…With abundant mobile ions and a small electronic conductivity, the solid electrolytes undergo much more severe electron beam irradiation than the cathode materials. 42,46,47 As a result, the atomic-resolution (S)TEM studies on solid electrolyte materials are quite rare. 44,[47][48][49][50][51][52][53][54][55] Regardless, the limited number of papers have provided unique insight into their ionic conduction behaviors and paved the way for the design and discovery of high-performance solid electrolytes.…”

“…The high spatial resolution and sensitivity to minor structural differences make AEM particularly suitable for studying these features. Using precession electron diffraction, which has the advantage of minimizing the distraction from double diffractions, Buschmann et al 47 successfully distinguished between the tetragonal and cubic phases of the garnet, Li 7 La 3 Zr 2 O 12 (LLZO). The disordered Li distribution in the cubic polymorph is essential for explaining its higher ionic conductivity.…”

Insights into the Lithium-Ion Conduction Mechanism of Garnet-Type Cubic Li5La3Ta2O12 by ab-Initio Calculations

“…29 Bernstein et al has revealed by DFT and variable cell shape MD simulations that the strong dependence of the tetragonal phase stability on the simultaneous ordering of the Li + ions on the Li sublattice and a volume-preserving tetragonal distortion that relieves internal structural strain. 30 36 and Buschmann et al 37 concluded, based on their 27 Al MAS NMR spectroscopy results, that Al sits mostly in the 24d tetrahedral site. Geiger et al 38 added that, according to their 27 Al MAS NMR spectral analysis, Al 3+ may also occupy a distorted site with 5-fold coordination which is presumably at the octahedral site; one of the two main resonances they found, which is assigned to an octahedral environment for Al 3+ , was attributed to the LaAlO 3 impurity phase.…”