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Aluminates for sodium based batteries

Thursday (08.10.2020)
15:50 - 16:10

Ceramic, Na+-ion-conducting Na-β’’-aluminate (SBA) electrolytes are established in commercial high temperature Na/NiCl2- and Na/S-batteries. In avoidance of mechanically induced cracks, the electrolyte thickness is typically about 2 mm. This thickness results in a high ohmic resistance. The presented project aims at a shortening the effective migration distance of Na+-ions through the solid electrolyte and therefore a bilayer system is introduced: a porous support layer provides mechanical strength and is infiltrated with a highly conductive liquid, while a dense layer prevents a short cut.

SBA can be prepared either by a solid-state reaction1 or sol-gel-chemistry2. The materials are mainly prepared in a powder form, which is pressed into a green body and sintered. The preparation of non-porous materials containing SBA is mainly described in the literature1,2. However, sintered porous SBA is also generally possible (e.g. pore volume of 0.2 cm3 g-1 with 630 nm pores)3.

A established sol-gel route for the preparation of porous α-Al2O3 monoliths was hence modified to integrate SBA. For the synthesis, polyethylene oxide, tenside and AlCl3 were dissolved in an ethanol-water mixture. This solution was cooled down and propylene oxide was added under stirring. After 10 min stirring, the gelation and ageing occurs at 40 °C for 24 h. Followed by an ethanol exchange, the gel was calcinated at 1200 °C. Two approaches were employed to integrate SBA into the porous α-Al2O3: Addition of sodium ions into the reaction mixture on the one hand and impregnation of the prepared Al2O3-gels with different sodium salt solutions on the other. The thus produced materials were characterized by mercury intrusion, SEM, XRD and XRF. As an example, direct integration of sodium ions into the sol-gel-synthesis yielded up to 10 wt.-% SBA in a bimodal porous α-Al2O3 matrix (60 and 174 nm, 0.39 cm3 g-1).

The materials were used to prepare porous discs (∅ = 2 cm), which were coated with SBA particles via spin-coating. The shrinkage during sintering of the deposited dense layer was adjusted to the shrinkage of the support by doping. After sintering, the electrolytes were used to assemble Na/NiCl2-cells with NaAlCl4 as infiltrating electrolyte. The cells were tested in comparison to cells with regular bulk SBA.

1) C. Zhu, J. Xue and G. Ji, Mat Sci Semicon Proc 31, 2015, 487-492.2) N. Li et al., J. Alloy Compd 479, 2009, 648-651.3) K. Fukui and Y. Suzuki, J Mater Sci 53, 2018, 1005-1013.

Cornelius Dirksen
Fraunhofer Institute for Ceramic Technologies and Systems IKTS
Additional Authors:
  • Antonia Hoppe (Speaker)
    Universität Leipzig
  • Prof. Dr. Dirk Enke
    Universität Leipzig
  • karl Skadell
    Fraunhofer IKTS