Effect of Simultaneous Substitution of Y and Ta on the Stabilization of Cubic Phase, Microstructure, and Li⁺ Conductivity of Li₇La₃Zr₂O₁₂ Lithium Garnet

Abstract

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.

Electrochemical performance of a garnet solid electrolyte based lithium metal battery with interface modification

“…Additional information regarding the phase composition and structure of LS-LLZA was obtained using Raman spectroscopy and the recorded Raman spectra of LS-LLZA and bare LLZA are both shown inFigure S3. The Raman modes of the collected spectrum (LS-LLZA) matches well with the reported Raman spectrum of cubic garnet phase34 which confirms that LS-LLZA after heat treatment at 300 °C have the single phase of cubic garnet structures.The effect of Li2SiO3 as a buffer layer between NMC and LLZA was qualitatively assessed by simple peel-off experiment (adherence test) and from cross-sectional SEM images. For the peeloff experiment, adhesive tape was stuck with NMC and then peeled from the surface of NMC coated over pristine LLZA and LS-LLZA.…”

“…Earlier Raman studies on lithium garnets suggested that the Raman bands observed between 100 and 150 cm -1 corresponds to La cation, the broad and fairly overlapping Raman band observed between 200 and 500 cm -1 corresponds to the internal modes of LiO6 and LiO4 and is due to the dynamic disorder of highly mobile lithium ion. [34][35][36] The impedance data was fitted with the equivalent circuit shown as inset in Figure 1c. From the fitting result the semicircle at high-frequency regime seems to be mainly related to bulk response and the grain boundary contribution appears to be negligible.…”

High Capacity All-Solid-State Lithium Battery Using Cathodes with Three-Dimensional Li+Conductive Network

“…[5][6][7][8] Recent reports reveal that doping with elements such as Ga 3+ and Ta 5+ can improve its room temperature conductivity to 0.4∼1.0 × 10 −3 S cm −1 .…”

Phase evolution, structure, and up‐/down‐conversion luminescence of Li6CaLa2Nb2O12:Yb3+/RE3+ phosphors (RE = Ho, Er, Tm)

“…41 This verified the structure model of Li 6 CaLa 2 Nb 2 O 12 in this work. The two bands at ~ 121 and 158 cm −1 may reveal important information for the dodecahedral site, since they are arising from LaO 8 and CaO 8 vibrations, 42 respectively. Noteworthy is that the two modes occur at lower frequencies than those of YO 8 in YAG, which implies that Li 6 CaLa 2 Nb 2 O 12 garnet would have a lower probability of multiphonon relaxation and is better suited for efficient luminescence of rare-earth activators.…”