It is widely recognized that Al plays a dual role in the fabrication of garnet-type Li ion conductors, i.e., a dopant that stabilizes the cubic structure and a sintering aid that facilitates the densification. However, so far, the sintering effect of Al₂O₃ has not been well-understood due to the common practice of ‘unintentionally’ introducing Al from crucibles during the sintering process. In this study, the sintering effect of Al on the phase composition, microstructure and ionic conductivity of Li_6.5La₃Zr_1.5Ta_0.5O₁₂ was investigated by using an Al-free crucible and intentionally addition of various amounts of γ-Al₂O₃.

XRD analysis showed that all sample have pure cubic phase regardless of the quantity of γ-Al₂O₃ addition up to 0.3 mol per formula unit of LLZT0.5. While the sample with no sintering aid cannot be successfully densified at 1100 °C, 0.05 mol of Al sintering aid can effectively enhance the densification. The morphology is characterized by small densely-packed polygonal grains. Further addition of Al sintering aids leads to rapid grain growth and increased secondary phases which are detrimental to the densification process. The drastically different morphology of samples with different Al contents are explained by the liquid phase sintering mechanism based on the Li₂O-Al₂O₃ eutectic system. Our EDS analysis showed that Al prefers to stay at the grain boundary rather than enter the garnet lattice. Uneven Al distribution in high Al-content samples is observed as Al predominately exists in small, concentrated area as secondary phases. Transport properties characterized by impedance spectroscopy are largely consistent with the morphology results that the sample with 0.05 mol of Al showed the best densification and a maximum total ionic conductivity of 0.5 mS/cm was achieved. Therefore, a 0.05 mol of Al in the form of γ-Al₂O₃ is the optimal quantity of sintering aids for this system. This study clarifies the role of Al as sintering aids and will be useful for harnessing the sintering power of Al in a controllable manner.

Acknowledgment

This work was supported by the U.S. Department of Energy’s (DOE’s) Office of Electricity Delivery & Energy Reliability (OE), U.S. Department of Energy Wind and Water Power Technologies Office and the U.S. Army Corps of Engineers, Portland District. We appreciate the useful discussions with Dr. I. Gyuk of the DOE-OE Grid Storage Program.