Effect of Sintering on Structural Modification and Phase Transition of Al-Substituted LLZO Electrolytes for Solid State Battery Applications

Owing to high lithium ion conductivity and good stability with lithium metal, Li7La3Zr2O12 (LLZO-a solid electrolyte) has emerged as a viable candidate for solid-state battery applications. In the current study, Al-substituted LLZO (Al-LLZO) powder is synthesized using a typical solid-state reaction. The pellets are made with the synthesized powder and are subjected to annealing for different durations and its effect on the structural properties of the Al-LLZO is investigated in detail. Reitveld refinement of the powder X-ray diffraction pattern reveals that the sintered Al-LLZO belong to the cubic system with the Ia-3d space group at room temperature. Morphology and microstructural properties of sintered powder are analyzed using field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM)/selected area electron diffraction (SAED), respectively. The FESEM image of LLZO pellets shows well-structured cubic grains spread evenly over on the surface after sintering. The chemical compositions of the sample are identified using energy dispersive X-ray analysis (EDAX). The surface chemistry of the prepared samples is examined by X-ray photoelectron spectroscopy (XPS), which states that the observed photoelectron signals from O 1s at about 531 eV and Li1s at 54.52 eV correspond to the Li-O bond in Al-LLZO. Raman spectra have been analyzed and the observed Raman peaks appearing at 299 cm(-1), 393 cm(-1), 492 cm(-1), and 514 cm(-1) were assigned to E-g, F-2g, A(1g), and F-2g, respectively. Phase transformation from C-LLZO to the pyrochore LZO phase is noticed when the sample is sintered for 12 h at 1100 degrees C. The impedance analysis is carried out to measure the conductivity of the Al-LLZO pellet and is found to be 0.3 x 10(-5) S cm(-1), which is suitable for solid electrolyte applications in lithium ion batteries.