Abstract
Fast lithium-ion conductors are key materials for the development of next generation energy storage devices such as all-solid-state lithium secondary batteries and rechargeable lithium-air batterie[1,2]. Among the fast lithium-ion conductors reported, garnet-like Li7La3Zr2O12 (LLZ) and its derivatives are one of the most promising materials because of their excellent chemical and electrochemical stability with lithium metal[2,3]. The substitution of Ta5+ in LLZ results in the highest ionic conductivities among the garnet-like compounds[3,4]. However, the contamination of Al3+ from Al2O3 crucible occurs during the synthesis. The difficulty in the determination of the Li+ and Al3+contents has impeded understanding of the conduction properties of these compounds.
Therefore, we have investigated the phase relation in the Li7-x-3yAlyLa3Zr2-xTaxO12 system. We began by investigation of the phase relations in the pseudo-binary Li5La3Ta2O12–Li7La3Zr2O12 system, and then extended the investigation to the Al-doped system. Al-free Li7-xLa3Zr2-xTaxO12 pellets were fired in a magnesia crucible to avoid contamination with Al. To prevent the formation of the low-temperature cubic phase, which is easily formed by the Li+/H+exchange reaction with moisture in air below 400 °C, the samples obtained were annealed in an inert atmosphere prior to measurements. The structure and conduction properties of these compounds were then investigated using neutron diffraction, and AC impedance and nuclear magnetic resonance (NMR) spectroscopies. The relationship between the structure and ionic conductivity is discussed based on the results obtained.
Polycrystalline Li7-x-3yAlyLa3Zr2-xTaxO12 (x = 0–2.0, y = 0–0.14) was synthesized by solid-state reaction. Phase identification was conducted using XRD (Rigaku RINT 2500 and Bulker D8) with Cu Kα radiation. Diffraction data were collected at each 0.02° step width in the 2θ range from 10 to 80°. Neutron diffraction data were corrected using time-of-flight (TOF) diffractometers: iMATERIA (BL20). Structural parameters were refined using the Rietveld refinement programs Z-Rietveld and Topas Ver. 4 for neutron diffraction and XRD data, respectively. The Li, La, Zr, Ta, Mg and Al compositions were analyzed using inductivity coupled plasma-optical emission spectroscopy (ICP-OES; Agilent Technologies ICP-OES 710). The ionic conductivity was measured by the AC impedance method using a frequency response analyzer (Solartron 1260) in the frequency range of 0.1 Hz to 1 MHz and in the temperature range from -20 to 100 °C in an inert gas atmosphere. Lithium diffusion in the bulk was determined from pulsed-gradient spin-echo (PGSE) NMR measurements.
The tetragonal phase formed at x = 0–0.375 and the cubic phase appeared with x = 0.4–2.0 in the Li7-xLa3Zr2-xTaxO12 system. In the Al-doped system of Li7-x-3yAlyLa3Zr2-xTaxO12, the tetragonal phase was formed at x + 3y < 0.4. The border between the tetragonal and the cubic phases exists at Li6.6-z/2Alz/2La3Zr1.6+zTa0.4-zO12. The tetragonal/cubic structure change corresponds to the order/disorder of lithium ions and is dependent on the cation content at the lithium sites. The ionic conductivity of the cubic compounds has a positive tendency with respect to the lithium content, whereas that of the tetragonal compounds is opposite. A high total ionic conductivity exceeding 5.0 × 10-4 S cm-1 at 25 °C was observed for Al-doped Li6.6-z/2Alz/2La3Zr1.6+zTa0.4-zO12. The highest total conductivity of 1.03 × 10-3 S cm-1 at 25 °C with an activation energy of 0.35 eV was obtained at z = 0.275. Nuclear magnetic resonance spectroscopy revealed that Al3+ substitution decreases the diffusion of lithium ions in the structure. The high total conductivity of Al-doped Li6.6-z/2Alz/2La3Zr1.6+zTa0.4-zO12may be due to the enhancement of lithium diffusion at the grain boundaries.
This work was partly supported by the Japan Science and Technology Agency (JST) under the project "Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA Spring)".
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