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Al–LLZ with 1 wt% of Li4SiO4 added and sintered at 1200 °C was found to be relatively dense which enhances the total (bulk + grain-boundary) Li+ conductivity by reducing the grain-boundary contribution.

Polycrystalline cubic Li 7 La 3 Zr 2 O 12 (LLZ) with garnet-related type structure has been synthesized at 700°C by modified sol-gel processes using citric acid as organic complexing agent and butan-1-ol or propan-2-ol as surface active agent. Thermal analysis (thermogravimetric/differential thermal analysis) indicated that the gel must be annealed at around 700°C to completely remove the organic solvent. X-ray powder diffraction, X-ray fluorescence, and scanning electron microscopic investigations revealed that Al may not be essential to form cubic-phase LLZ; however, the addition of Al 2 O 3 led to enhanced sintering of LLZ.

Garnet-type lithium stuffed oxide Li7La3Zr2O12 (LLZ) in the cubic phase has received significant attention because of its high Li(+) conductivity at room temperature and excellent stability against lithium metal anodes. In addition to the high Li(+) conductivity, the dense microstructure is also a critical issue for the successful application of LLZ as a solid electrolyte membrane in all-solid-state lithium and lithium-air batteries. The stabilization of LLZ in the cubic phase with dopants indicated a reduction in sintering temperature with La(3+) site doping and improved conductivity by doping the Zr(4+) site. However, there are only a few reports regarding the simultaneous substitution on the La(3+) and on the Zr(4+) site in LLZ. In the present study, systematic investigations have been carried out on Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0, 0.125, 0.25, and 0.5) to understand the effect of simultaneous substitution of Y(3+) for La(3+) and Ta(5+) for Zr(4+) in LLZ on the stabilization of the high conductive cubic phase, microstructure, and Li(+) conduction behavior. Powder X-ray diffraction (PXRD) revealed the stabilization of a cubic-like garnet structure for the entire selected compositional range of Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0, 0.125, 0.25, and 0.5) samples sintered at 750 °C. However, the Raman spectra revealed that the cubic phase stabilized at around 750 °C for the Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0, 0.125, 0.25, and 0.5) samples is different from the high Li(+) conductive cubic phase (Ia3̅d), and the transformation to the high Li(+) conductive cubic phase with a distorted lithium sublattice (Ia3̅d) is observed only for the samples sintered at elevated temperature. Preliminary thermogravimetric (TG), Raman, and Fourier transform infrared (FTIR) studies indicated that the observed low temperature cubic phase of the investigated samples sintered at 750 °C might result from insertion of water vapor from the humid atmosphere into the crystal lattice and subsequent replacement of the lithium ions by protons to form O-H bonds. The AC impedance analysis indicated that the optimal Y substitution in Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0.125 and 0.25) helps to reduce the grain boundary resistance in a major way and also helps to reduce the bulk resistance slightly. Among the investigated compositions, Li6.6La2.75Y0.25Zr1.6Ta0.4O12 sintered at 1200 °C exhibits a maximized room temperature total (bulk + grain boundary) Li(+) conductivity of 4.36 × 10(-4) S cm(-1) along with the improved ceramic density.

Lithium garnet Li7La3Zr2O12 (LLZ) sintered at 1230 °C has received considerable importance in recent times as result of its high total (bulk + grain boundary) ionic conductivity of 5 × 10−4 S cm−1 at room temperature. In this work we report Li+ transport process of Li7−2xLa3Zr2−xWxO12 (x = 0.3, 0.5) cubic lithium garnets. Among the investigated compounds, Li6.4La3Zr1.7W0.3O12 sintered relatively at lower temperature 1100 °C exhibits highest room temperature (30 °C) total (bulk + grain boundary) ionic conductivity of 7.89 × 10−4 S cm−1. The temperature dependencies of the bulk conductivity and relaxation frequency in the bulk are governed by the same activation energy. Scaling the conductivity spectra for both Li6.4La3Zr1.7W0.3O12 and Li6La3Zr1.5W0.5O12 sample at different temperatures merges on a single curve, which implies that the relaxation dynamics of charge carriers is independent of temperature. The shape of the imaginary part of the modulus spectra suggests that the relaxation processes are non-Debye in nature. The present studies supports the prediction of optimum Li+ concentration required for the highest room temperature Li+ conductivity in LixLa3M2O12 is around x = 6.4 ± 0.1

A facile combustion method was developed for the rapid synthesis of high conductive cubic phase Al-LLZ solid electrolyte with uniform particle sizes at the nanometer level for the fabrication of rechargeable all-solid-state Li and Li-air batteries.

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