COMPOSITION FOR FORMING CERAMIC ELECTROLYTE, AND RESULTING ELECTROLYTE

Abstract

A composition is provided for forming a sodium-ion conducting electrolyte structure, comprising particles of a sodium-ion-conducting ceramic, combined with particles of at least one transition metal oxide, such as copper, titanium and niobium oxides, or iron oxide, or precursors for these oxides, so the metal oxides make up no more than 5% by weight of the weight of the particles. The sodium-ion-conducting ceramic may be of the types referred to as Nasicon, or ?-alumina. The metal oxides may constitute no more than 2% of the weight of the particles. The metal oxides act as a sintering aid, making it possible to achieve densification at a reduced sintering temperature, while having no significant detrimental effect on the electrical properties of the sintered ceramic. The invention also encompasses an electrode structure made by sintering this composition.

Claims

1. A composition for forming a sodium-ion conducting electrolyte structure, comprising particles of a Nasicon sodium-ion-conducting ceramic, and particles of at least one transition metal oxide, or at least one precursor for a transition metal oxide, so the transition metal oxide or oxides make up no more than 5% by weight of the weight of the particles, wherein the particles are of iron oxide, or the particles are of copper, titanium and niobium oxides, or the precursor is a precursor for iron oxide, or the precursors are precursors for copper, titanium and niobium oxides.

2. (canceled)

3. The composition of claim 1 wherein the metal oxides comprise oxides of copper, titanium and niobium, and the proportion of copper oxide is greater than that of titanium oxide, while the proportion of titanium oxide is greater than that of niobium oxide.

4. (canceled)

5. The composition of claim 3, wherein the proportions by weight of the oxides CuO:TiO.sub.2:Nb.sub.2O.sub.5 are in the ratios 4:2:1.

6. (canceled)

7. The composition of claim 1, wherein the metal oxide particles make up no more than 3% of the weight of the particles in the composition.

8. The composition as claimed in claim 7 wherein the metal oxide particles make up no more than 2% of the weight of the particles in the composition.

9. The composition of claim 1, wherein the metal oxide particles have a smaller median size than the particles of the sodium-ion-conducting ceramic, so they fit into voids between the sodium-ion-conducting ceramic particles during processing to form an electrolyte structure.

10. The composition of claim 1, wherein the particles of metal oxide are nanopowders, with a median size less than a tenth that of the particles of sodium-ion-conducting ceramic.

11. The composition of claim 10, wherein the metal oxide particles are made by thermal decomposition of precursor salts onto the surface of the particles of sodium-ion-conducting ceramic.

12. An electrolyte structure formed by sintering a composition as claimed in claim 1, to form a sodium-ion-conducting sintered ceramic.

13. (canceled)

14. (canceled)

15. The electrolyte structure of claim 12, also comprising a perforated metal sheet to support the sintered sodium-ion-conducting ceramic.

16. The electrolyte structure as claimed in claim 10, further comprising a porous layer formed on the perforated metal sheet, and an impermeable layer formed on the opposite face of the porous layer.

17. The electrolyte structure as claimed in claim 10, further comprising at least three ceramic layers formed on the perforated metal sheet, the ceramic layers having progressively lower levels of porosity, the last such layer being an impermeable layer.

Description

[0019] The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which:

[0020] FIG. 1 shows dilatometry data for pellets of various compositions sintered up to 1000? C.;

[0021] FIG. 2 shows dilatometry data for pellets of various compositions sintered up to 1050? C.;

[0022] FIG. 3 shows dilatometry data for pellets of various compositions sintered up to 1100? C.;

[0023] FIG. 4 shows images of fracture surfaces of Nasicon pellets sintered up to 1000? C. for 1 hour, with oxide sintering aid additions, at a magnification ?10,000;

[0024] FIG. 5 shows images of fracture surfaces of Nasicon pellets sintered up to 1100? C. for 1 hour, with oxide sintering aid additions, at a magnification ?10,000;

[0025] FIG. 6 shows images of fracture surfaces of Nasicon pellets sintered up to 1000? C. and 1100? C. for 1 hour without oxide sintering aid additions, at a magnification ?10,000;

[0026] FIG. 7 shows measurements of DC conductivity for Nasicon pellets of different compositions, at room temperature;

[0027] FIG. 8 shows measurements of DC conductivity for Nasicon pellets of different compositions, at 250? C.;

[0028] FIG. 9 shows graphically the electrical impedance data as a Nyquist plot, showing the variation of imaginary impedance with real impedance, for pellets of Nasicon produced with and without sintering aids; and

[0029] FIG. 10 shows a sectional view of a Nasicon electrolyte fabricated on FeCralloy steel using Nasicon with a 2% Fe (nitrate decomposition route) sintering aid.

EXAMPLE 1BASE Layer

[0030] Electrolyte layers based on ?-alumina were prepared by forming a slurry containing propan-2-ol and particles of ?-alumina of average size 1.3 ?m. This slurry is spread onto a suitable substrate, dried, consolidated, and then sintered. For experimental purposes, pellets of the same electrolyte material were made with the same composition, the composition being dried, formed into a pellet in a die, then compressed and then sintered in the same way as for the layer on a substrate. In one case a mixture of metal oxide nanoparticles, containing nanoparticles of copper oxide (CuO), of titanium dioxide (TiO.sub.2) and of niobium oxide (Nb.sub.2O.sub.5) in weight proportions 4:2:1 was included in the slurry, and constituted 5% of the solid matter by weight.

[0031] In each case the sintering took place at 1100? C. for one hour; this temperature is about 500? C. less than the usual sintering temperature for ?-alumina. After this sintering treatment it was found that the specimens that contained only ?-alumina showed no signs of densification, being only about 50% dense. In contrast, the specimens that contained both ?-alumina and the mixture of metal oxides had achieved about 72% densification. These densification measurements were made on the pellets as it is hard to do quantitatively on the layers. The effect on the layer is still clear in this case as the ceramic powder with no sinter aids is essentially still a loose powder that can be removed from the steel substrate easily, whilst the ceramic powder provided with sinter aids is well adhered and has strength.

EXAMPLE 2NASICON LAYER

[0032] Electrolyte layers based on Nasicon of the composition Na.sub.3Zr.sub.2(SiO.sub.4).sub.2(PO.sub.4) were prepared by forming a slurry containing propan-2-ol and particles of Nasicon. The Nasicon material had been milled to give a powder with a monomodal size distribution with a d50 less than 1 micron, in this case the d50 being 0.36 microns, as this enhances packing and sinterability.

[0033] For experimental purposes, pellets of the same electrolyte material were made in substantially the same way, with the same composition, except that the composition was dried, and then formed into a pellet in a die, before being compressed and then sintered in the same way as for the layer on the substrate. Sintering was assessed by use of a dilatometer which measures shrinkage of the pellet during the sintering process. In three cases a mixture of metal oxide nanoparticles, containing nanoparticles of copper oxide (CuO), of titanium dioxide (TiO.sub.2) and of niobium oxide (Nb.sub.2O.sub.5) in weight proportions 4:2:1 was included in the slurry; in one case this oxide mixture constituted 5% of the solid matter by weight, in another case the metal oxide particles constituted 2% by weight, and in the other case 1% by weight. These examples are referred to as CTN in Table 1 below and other captions.

[0034] In a further three cases iron oxide was added by a precursor decomposition route (0.05 M to 0.5 M iron nitrate in propan-2-ol subsequently decomposed to form the oxide at) <300? ? C.; in one case this iron oxide yielded a mixture of 5% of the solid matter by weight, in another case the iron oxide particles constituted 2% by weight, and in the other case 1% by weight. In a final two cases iron oxide was added as iron oxide nanoparticles of size about 20 nm; in one case this oxide mixture constituted 2% of the solid matter by weight and in the other case 1% by weight.

[0035] Sintering was performed by ramping the temperature up to a maximum, holding it at that maximum for one hour, and then ramping down again. This was carried out with maximum temperatures of 1000?, 1050? or 1100? ? C. to determine optimal temperatures; these temperatures are about 100? ? C. to 200? ? C. less than the usual sintering temperature required for full densification of Nasiconi.e. to achieve no connected porosity. The densification achieved with the pellets is shown in Table 1:

TABLE-US-00001 TABLE 1 % Densifica- % Densifica- % Densifica- % metal oxides tion at 1000? C. tion at 1050? C. tion at 1100? C. 0 71 84 94 1% CTN 89 100 2% CTN 95 100 5% CTN 98.5 1% Fe oxide 89 100 (via precursor decomposition) 2% Fe oxide 100 100 100 (via precursor decomposition) 5% Fe oxide 100 100 100 (via precursor decomposition) 1% Fe oxide 88 100 (nano powder) 2% Fe oxide 99 100 100 (nano powder)

[0036] In addition dilatometry data was obtained in each case to show how the percentage shrinkage varied with temperature during the sintering process. The dilatometry data showing the shrinkage with temperature for these pellets are shown for the three maximum sintering temperatures in FIGS. 1, 2 and 3. In FIGS. 1 and 3 a comparison graph is also included for a raw powder pellet, that is to say a pellet made of the same Nasicon powder as described above, but without any metal oxide sintering aid.

[0037] It will thus be appreciated that the provision of a small proportion of the metal oxide mixture improves the densification achieved at this lower temperature; and that best results are obtained at around 2% by mass addition. Closer inspection of the derivatives of the dilatometry curves indicates that the maximum rate of densification is not only increased by the use of transition metal oxide addition but also the temperature at which this occurs is reduced typically by at least 50? C. The metal oxide particles hence act as a sintering aid.

[0038] These different results are also evident in the scanning electron microscopy images of FIGS. 4 and 5, showing broken surfaces of Nasicon pellets made from compositions that included between 1% and 2% (wt) of the sintering aids, and FIGS. 6, showing a broken surface of a Nasicon pellet made without provision of a sintering aid. In FIG. 4 you can see that at 1000? C. (a very low sintering temperature) there are very few pores visible between particles of ceramic; in FIG. 5 after sintering up to 1100? C. there is negligible porosity between the particles of Nasicon. In contrast, in FIG. 6 you can see the material is very porous particularly in the case for the pellet sintered at 1000? C.

[0039] Impedance testing was then carried out at room temperature on Nasicon pellets made in this way. The data was obtained at room temperature (FIG. 7) and at 250? C. (FIG. 8) using an electrochemical impedance analyser working between 7 MHz and 100 Hz giving clear and repeatable impedance spectra. FIG. 7 also shows the data for a pellet made with raw powder, i.e. without any metal oxide sintering aid, and this is labelled as raw powder.

[0040] FIG. 7 shows the values of DC conductivity for Nasicon pellets with a range of different sintering aids, sintered at 1000? C. and 1100? C., measured at room temperature, while FIG. 8 shows the values of DC conductivity for such Nasicon pellets with different sintering aids, but measured at 250? C.

[0041] Measurements of real and imaginary impedance, i.e. resistance and reactance, were made at a range of different frequencies on a Nasicon pellet without sintering aid (11.68 mm diameter and 2.16 mm thick), and on a Nasicon pellet made with 2% (wt) of the CNT sintering aid (9.39 mm diameter and 1.93 mm thick), and the results are shown graphically in FIG. 9 as a Nyquist plot, the measurements shown by black circles being for the pellet without sintering aid, and the results shown by white circles being for the pellet made with the sintering aid.

[0042] These measurements are in line with literature values for Nasicon and suggest that there is no significant detriment to the sodium ion conductivity with the level of doping with the metal oxide sintering aids at 1 and 2% and only a small detrimental impact at 5%. In fact apparent impedance overall appears lower due to the enhanced densification but the shape of the Nyquist spectra produced (example given in FIG. 9) would suggest that there has been some slight increase in impedance (reduction in conductivity), if like for like densification were compared. It is anticipated that the slight reduction in conductivity could be because the lower sintering temperature produces a structure that has much finer grains. The sintering aids pin grain boundaries and significantly increase densification as compared to other sintering mechanisms, such as grain growth. The conductivity of a material is a combination of both the bulk and grain boundary components, it is thus anticipated that materials sintered at lower temperature have a larger proportion of grain boundary resistivity to the total resistivity. Therefore, the reduction in conductivity is not considered to be due to a material interaction with the sintering aid, but rather to having a fundamentally different microstructure. This is beneficial, as the sintering aid ensures all grains are similar in size (homogeneous) and are small, which makes the ceramic significantly tougher and more durable in operation. The potential for negative impact on the conductivity of the Nasicon is possibly mitigated by the fact that all the sintering aids (iron, copper, titanium and niobium oxides) are viable substitutions into the Nasicon structure to form phases with sodium ion conductivity (Journal of Power Sources 273 (2015) 1056-1064).

[0043] As mentioned above, an electrode may comprise a layer of sodium-ion-conducting electrolyte supported by or bonded to a perforated metal sheet. The metal of which the perforated sheet is formed must be inert in the sense that it does not react chemically with components of the cell with which it is in contact during use; it may for example be a metal such as nickel, or aluminium-bearing ferritic steel (such as the type known as Fecralloy?), or a steel that forms an electronically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air. The adhesion of the ceramic to the metal may be better when using a metal alloy such as Fecralloy that forms an oxide coating of alumina. The perforated sheet may be of thickness no more than 1.0 mm, or no more than 0.5 mm, for example 0.1 mm or 0.2 mm. The sheet is perforated so it has a very large number of through holes, and the perforations or holes may be of mean diameter less than 50 ?m, for example 30 ?m or less, or of mean diameter between 50 ?m and 300 ?m, and may for example be produced by a laser drilling process or by chemical etching. The through holes may have their centres spaced apart at between 100 ?m and 500 ?m, for example 150 ?m. Such layers fabricated on Fecralloy steel at a firing temperature of 1050? C. for 1 hour, using Nasicon with 2% iron oxide as sintering aid produced by in situ ferrous nitrate decomposition, have been found to be smooth and coherent and firmly bonded to the metal surface, and are densified sufficiently to give helium permeability readings in the range of 1?10.sup.?08 to 3?10.sup.?07 mbar L/s which is sufficiently leak tight for successful cell operation.

[0044] The perforated sheet may have a margin around its periphery that is not perforated; this margin may make it easier to seal the periphery of the perforated plate to adjacent components of the cell. This margin may be of width no more than 15 mm, for example 10 mm or 5 mm or 3 mm.

[0045] The electrode may comprise a layer of sodium-ion-conducting electrolyte bonded to a perforated metal sheet, with a porous ceramic layer between the metal sheet and the impermeable sodium-ion-conducting layer.

[0046] There may be more than one such porous layer, for example a first porous layer on the metal surface, covered by a less porous layer, and finally covered by an impermeable layer. The different degrees of porosity and permeability can be achieved by using ceramic particles of different sizes, and different amounts of transition metal sintering aids. The first porous layer may for example be deposited by screen printing, the mixture of particles also including binders and flow-aids; the binders and flow-aids would be burned out at the start of the sintering process during a binder burn out step typically 300? C., or below. The subsequent layer or layers may be deposited by spray coating, screen printing or electrophoretic deposition.

[0047] An example of such a layered structure is shown in FIG. 10, which is a scanning electron micrograph showing a cross-section. The electrolyte has three layers of Nasicon. The first, most porous layer was deposited by a screen printing process where the Nasicon particle size used in the ink had a D95 of 16.6 ?m. This layer was formed in a single print and drying process followed by a 1000? ? C. firing for 1 hour to give a strong but porous layer that was well adhered to the substrate Fecralloy steel. Onto this was spray coated a layer with an intermediate particle size with a D95 of 4.7 ?m to form a structure with finer interconnected porosity. This layer contained 2% wt iron oxide added via the precursor decomposition route; it was consolidated under an applied pressure and fired at 1000? C. for 1 hour to produce a strong and well adhered but still porous layer. Finally, onto this layer was spray coated a layer with a fine particle size with a 2% wt iron oxide addition added via the precursor decomposition route with the same specification as used in the pellet densification studies described above. This layer was then fired at 1050? C. for 1 hour and gave a dense and impermeable layer.