Sodium vs. Lithium Batteries.
Eongyu Yi, Eleni Temeche, Richard M. Laine*

Dept. of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. talsdad@umich.edu

β’’-Al2O3 is used commercially as a Na+ conducting ceramic electrolyte for its high ionic conductivity (0.2-0.3 S cm-1 at 300°C) and low materials’ costs. However, for batteries using β'’-Al2O3, about half of the cell resistance arises from the electrolyte itself because traditionally processed electrolyte materials are 1-2 mm thick. One can anticipate dramatic drops in cell resistance at thicknesses <100 µm. However, the traditional high sintering temperatures of 1600 °C/0-4 h cause rapid and excessive Na2O loss driving formation of β- rather than β'’-Al2O3 lowering ionic conductivities limiting final properties.

Known methods of β'’-Al2O3 sintering commonly involve covering the sample with the same powder to reduce Na2O loss. The quality (phase, particle size, particle morphology, etc.) of the starting powder has been shown to have a strong effect on β’’-Al2O3 sintering behavior.

In line with our latest success using flame made nanopowders (NPs) to minimize the external energy input for sintering Li7La3Zr2O12, the Li+ conducting ceramic electrolyte known for its difficulty in sintering, we show that the same approach can be used to process Na+ β’’-Al2O3. In this study, β’’-Al2O3, TiO2 and ZrO2 NPs were produced by liquid-feed flame spray pyrolysis (LF-FSP). The NPs were then processed to green films (80 µm) by tape casting generating β'’-Al2O3 thin films on sintering.

As expected, superior densification of β'’-Al2O3 films occurs with increasing TiO2 wt. %. Near full densities are reached at ≥ 1360 °C/2 h. However β’’-Al2O3 content reaches only ~65 wt.% and the orientation of the c-axis is perpendicular to the film surface per XRD. The preferred conduction plane is perpendicular to the c-axis reducing net conductivity. Also, the large grain sizes seen in the 2 and 3 wt. % TiO2 samples suggest liquid phase sintering. Thus further efforts were explored to perturb grain reorientation during liquid phase sintering, and also pin grain boundaries to reduce grain growth that also reduced Na2O loss rate by reducing surface exposure of β’’-Al2O3. These efforts will be described as well as methods to sinter β'’-Al2O3 at 1320 °C, the lowest ever reported providing superionic sodium ion conductivities at room temperature. Symmetrical Na/ β’’-Al2O3/Na cells offer room temperature performance pointing the way to all solid state, thin-film Na batteries.

Work supported on NSF subcontract from Na4B to University of Michigan and by a gift from Mercedes-Daimler.