ELEMENT CONDUCTING SODIUM IONS FOR USE IN ELECTROCHEMICAL CELLS AND METHOD FOR PRODUCING IT

Abstract

The invention relates to sodium-ion-conducting elements for use in electrochemical cells, more particularly as solid electrolyte/separator in high-temperature batteries. In these elements, a surface of a porous substrate bears a coating which is obtained by sintering at a temperature of not more than 1100 C. and which is formed with the system Na.sub.2OSiO.sub.2R.sub.2O.sub.5R1.sub.2O.sub.3, in which R1=Sc, Y, La and/or B and R2=P, Sb, Bi, Sn, Te, Zn and/or Ge.

Claims

1. A sodium-ion-conducting element for use in electrochemical cells, more particularly as solid electrolyte/separator in high-temperature batteries, wherein a surface of a porous substrate bears a coating which is obtained by sintering at a temperature of not more than 1100 C. and which is formed with the system Na.sub.2OSiO.sub.2R.sub.2O.sub.5R1.sub.2O.sub.3, in which R1=Sc, Y, La and/or B and R2=P, Sb, Bi, Sn, Te, Zn and/or Ge.

2. The element as claimed in claim 1, characterized in that the coating has a thickness in the 3 m to 750 m range.

3. The element as claimed in claim 1, characterized in that the material of the coating has a coefficient of thermal expansion which is lower, preferably lower by not more than 1.5 ppm/K, than the coefficient of thermal expansion of the substrate material.

4. The element as claimed in claim 1, characterized in that the substrate is an element in the shape of a plate, honeycomb, or tube which is open at one side.

5. The element as claimed in claim 1, characterized in that the substrate is formed of Al.sub.2O.sub.3, mullite, spinel, fosterite, ZrO.sub.2, a silicatic ceramic material, an electrically conducting ceramic material, or a metal or a metal alloy.

6. The element as claimed in claim 1, characterized in that electrically conducting ceramic substrate material consists of Nb-doped TiO.sub.2, Ca.sub.1-xLa.sub.xTiO.sub.3 or Sr.sub.1-xY.sub.xTiO.sub.3 or of a mixture of these components with Al.sub.2O.sub.3, mullite, spinel, fosterite, ZrO.sub.2 or a silicatic ceramic material.

7. The element as claimed in claim 1, characterized in that the substrate has a porosity in the 30% to 80% range.

8. The element as claimed in claim 1, characterized in that the coating has a density which is above 95% of the theoretical density after sintering.

9. A method for producing an element as claimed in claim 1, characterized in that a powder formed with SiO.sub.2 at a fraction of 47 mol % to 63 mol %, Na.sub.2O at a fraction of 33 mol % to 43 mol %, R1.sub.2O.sub.3 at a fraction of 3 mol % to 14 mol %, and R2.sub.2O.sub.5 at a fraction of 0.1 mol % to 10 mol %, in the form of a paste or suspension comprising this powder, to a surface of the porous substrate, which as a layer is applied and then, to form a coating on the surface of the substrate, sintering is carried out at a temperature of not more than 1100 C., where R1 is scandium, yttrium, lanthanum and/or boron and R2 is phosphorus, antimony, bismuth, tin, tellurium, zinc and/or germanium.

10. The method as claimed in claim 9, characterized in that the layer is formed by knife coating, spraying, dipping, plasma spraying or by vacuum slip casting on the surface of the substrate prior to sintering.

Description

EXAMPLE 1

[0040] Melting of a mixture having the composition 53 mol % SiO.sub.2, 38 mol % Na.sub.2O, 6 mol % Y.sub.2O.sub.3, and 4 mol % P.sub.2O.sub.5 in a platinum crucible at 1500 C. for 2 h with subsequent fritting of the melt in water and subsequent drying at 150 C. for 12 h. [0041] Preliminary grinding of the frit in an oscillatory disk mill provided with hard-metal lining, to an average particle diameter of about 125 m after sieving. [0042] Fine grinding of the resulting powder in an Attritor with ethanol as grinding medium to an average particle size of d.sub.50=2 m. [0043] Preparation of this powder to form a paste having a solids content of 65 mass % using ethylcellulose as binder and terpineol as solvent. [0044] Manual application of the paste as a layer with a plastic knife to a surface of a porous Al.sub.2O.sub.3 tube as substrate (porosity>40% pore diameter>1 m). [0045] Drying of the paste at 120 C. for 1 h in air. [0046] Firing of the layer in air according to the following profile: RT-2 K/min-->450 C./1 h-2 K/min-->900 C./1 h-2 K/min-->RT. [0047] After firing had been carried out, layer thicknesses between 150 m and 250 m were found via microscopic analyses on ground sections. [0048] Measurements of the ion conductivity on a high-temperature measuring facility (in-house construction), using a salt melt (NaNO.sub.3/NaNO.sub.2) as liquid electrolyte, found the following specific conductivities.

TABLE-US-00001 Temperature [ C.] Conductivity [S cm.sup.1] 230 0.051 250 0.059 275 0.071 300 0.085 310 0.091 320 0.095 330 0.11

EXAMPLE 2

[0049] Melting of a mixture having the composition 53 mol % SiO.sub.2, 38 mol % Na.sub.2O, 6 mol % Y.sub.2O.sub.3, and 4 mol % P.sub.2O.sub.5 in a platinum crucible at 1500 C. for 2 h with subsequent fritting of the glass melt in water and drying at 150 C. for 12 h. [0050] Preliminary grinding of the resulting frit in an oscillatory disk mill provided with hard-metal lining, to an average diameter of about 125 m after sieving. [0051] Fine grinding of the powder in an Attritor with ethanol as grinding medium to an average particle size of d.sub.50=2 m. [0052] Preparation of this powder to form an aqueous slip (suspension) having a solids content of 70 mass % using commercial adjuvants as stabilizers and dispersants. [0053] A porous Al.sub.2O.sub.3 tube as substrate closed at one end (porosity>40%, pore diameter>1 m) is dipped into the slip, so that the open end of the substrate protrudes from the slip and the outer surface of the tube is coated with the slip. [0054] A vacuum with a reduced pressure of 1 mbar-10 mbar with respect to atmospheric pressure is then applied to the open end of the tube and maintained for 10 minutes. As a result, the suspension is drawn under suction into the interior of the tube, and particles are deposited as a layer on the outer surface. [0055] Drying of the Al.sub.2O.sub.3 tube coated in this way on the outward-facing surface at 150 C. in air for 15 h. [0056] Sintering of the layer in air according to the following profile: [0057] RT-2 K/min-->450 C./1 h-2 K/min-->900 C./1 h-2 K/min-->RT. [0058] After firing had been carried out, layer thicknesses between 400 m and 700 m were found via microscopic analyses on ground sections. [0059] Measurements of the ion conductivity on a high-temperature measuring facility (in-house construction), using a salt melt (NaNO.sub.3/NaNO.sub.2) as liquid electrolyte, found the following specific conductivities.

TABLE-US-00002 Temperature [ C.] Conductivity [S cm.sup.1] 230 0.049 250 0.058 275 0.069 300 0.084 310 0.089 320 0.093 330 0.10