A duplex β″-Al2O3 solid electrolyte consisting of a thin dense layer and a porous substrate

• As a method to lower the ASR, the concept of duplex BASEs is presented.
• A duplex BASEs consists of a thin dense β″-alumina layer and a mechanically strong porous support.
• Processing factors including conversion, porosity, composition, and homogeneity influence the strength of a duplex BASE.
• The minimum ASR value of 0.31 Ω cm2 was achieved at 350 °C with decent strength of 260 MPa.

To improve the performance of Na-beta batteries at intermediate temperatures (≤ 200 °C) where much improved cyclability and reduced degradation can be achieved, there is a need to lower the resistance/polarization originated from β″-Al2O3 solid electrolyte (BASE) while maintaining its good strength. In this paper, the concept of a duplex BASE consisting of a thin dense electrolyte and a porous support was proposed as a solution to achieve low area-specific resistance (ASR) with good mechanical strength supported by the porous substrate. The effects of various factors including porosity, composition, and homogeneity of ingredients on the flexural strength of duplex BASEs were examined. In summary, lower porosity, higher YSZ content in the structure, and attrition milling of raw powders resulted in improved strength. The ASR measurement exhibited that the resistance of duplex BASEs was mainly originated from a dense layer. Overall, the maximum strength of 260 MPa and the ASR value of 0.31 Ω cm2 (at 350 °C) was achieved from a duplex BASE consisting of a 50 μm thick dense layer (Al2O3:YSZ = 7:3 in volume) and a 500 μm thick porous support (Al2O3:YSZ = 4:6 in volume with 19% open porosity). The effects of various factors on the properties of duplex BASEs will be discussed in detail.