Abstract
We investigate theoretically the site occupancy of Al3+ in the fast-ion-conducting cubic-garnet Li7–3xAl3+xLa3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured 27Al NMR chemical shifts an analysis shows that Al3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad 27Al NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al3+ site has not been observed experimentally.
Highlights
The fast Li-ion conductor with the nominal composition Li7La3Zr2O12 (LLZO) is receiving much scientific attention since its discovery in 2007.1 It has a garnet-based structure, and it occurs in at least two structural modifications.[2,3] At room temperature, LLZO is tetragonal (I41/acd) while the cubic modification (Ia-3d) is stable above approximately 150 °C.4Geiger et al argued that the better conducting cubic phase can be stabilized at room temperature (RT) through the incorporation of small amounts of Al3+.4 The stabilizing effect of Al3+ has been confirmed by a number of subsequent investigations.[5−19] The exact role Al3+ plays in cubic Al-bearingLLZO is important because LLZO shows a high ionic conductivity of about 10−4 S/cm at RT
In order to understand the computational models used in this investigation, we provide a short description of the crystal structure of cubic LLZO garnet (Figure 1)
Our calculations show that Al3+ at 96h is 4-fold coordinated, and distorted from tetrahedral coordination, with Al−O bond lengths varying from 1.76 to 1.92 Å, because of the displacement of Al3+ towards a vacant 24d site
Summary
The fast Li-ion conductor with the nominal composition Li7La3Zr2O12 (LLZO) is receiving much scientific attention since its discovery in 2007.1 It has a garnet-based structure, and it occurs in at least two structural modifications.[2,3] At room temperature, LLZO is tetragonal (I41/acd) while the cubic modification (Ia-3d) is stable above approximately 150 °C.4Geiger et al argued that the better conducting cubic phase can be stabilized at room temperature (RT) through the incorporation of small amounts of Al3+.4 The stabilizing effect of Al3+ has been confirmed by a number of subsequent investigations.[5−19] The exact role Al3+ plays in cubic Al-bearingLLZO is important because LLZO shows a high ionic conductivity of about 10−4 S/cm at RT. The fast Li-ion conductor with the nominal composition Li7La3Zr2O12 (LLZO) is receiving much scientific attention since its discovery in 2007.1 It has a garnet-based structure, and it occurs in at least two structural modifications.[2,3] At room temperature, LLZO is tetragonal (I41/acd) while the cubic modification (Ia-3d) is stable above approximately 150 °C.4. Geiger et al argued that the better conducting cubic phase can be stabilized at room temperature (RT) through the incorporation of small amounts of Al3+.4. LLZO is important because LLZO shows a high ionic conductivity of about 10−4 S/cm at RT. LLZO has good chemical and thermal stability, as well as a wide energy potential window making it an excellent candidate for use as an electrolyte in an all-solid-state lithium-ion battery.[1,20]
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