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GB2114114A - Beta-alumina electrolyte material - Google Patents

Beta-alumina electrolyte material Download PDF

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Publication number
GB2114114A
GB2114114A GB08202028A GB8202028A GB2114114A GB 2114114 A GB2114114 A GB 2114114A GB 08202028 A GB08202028 A GB 08202028A GB 8202028 A GB8202028 A GB 8202028A GB 2114114 A GB2114114 A GB 2114114A
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GB
United Kingdom
Prior art keywords
sodium
alumina
beta
layer
green shape
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB08202028A
Inventor
John Wing-Keung Lau
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Chloride Silent Power Ltd
Original Assignee
Chloride Silent Power Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Chloride Silent Power Ltd filed Critical Chloride Silent Power Ltd
Priority to GB08202028A priority Critical patent/GB2114114A/en
Publication of GB2114114A publication Critical patent/GB2114114A/en
Withdrawn legal-status Critical Current

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Classifications

    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00Secondary cells; Manufacture thereof
    • H01M10/36Accumulators not provided for in groups H01M10/05-H01M10/34
    • H01M10/39Accumulators not provided for in groups H01M10/05-H01M10/34 working at high temperature
    • H01M10/3909Sodium-sulfur cells
    • H01M10/3918Sodium-sulfur cells characterised by the electrolyte
    • H01M10/3927Several layers of electrolyte or coatings containing electrolyte
    • CCHEMISTRY; METALLURGY
    • C04CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04BLIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/01Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
    • C04B35/10Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on aluminium oxide
    • C04B35/111Fine ceramics
    • C04B35/113Fine ceramics based on beta-aluminium oxide
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10Energy storage using batteries

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Ceramic Engineering (AREA)
  • Materials Engineering (AREA)
  • Structural Engineering (AREA)
  • Organic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Secondary Cells (AREA)

Abstract

A beta alumna electrolyte element formed by sintering a green shape of finely powdered beta alumina or a mixed precursor material of the required composition, is, before sintering, coated over at least part of its surface with magnesium oxide and/or lithium oxide. The green shape may be calcined before or after coating. The coating forms a magnesium and/or lithium alumina spinel layer on the surface of the substrate by depleting the alumina content of the underlying surface layer whereby this underlying surface layer has a higher sodium oxide content than the main body of the substrate.

Description

SPECIFICATION Solid electrolyte material This invention relates to beta alumina solid electrolyte material.
Beta alumina solid electrolyte material is used in electrochemical cells such as sodium sulphur cells and other energy conversion devices employing liquid sodium. The thermochemical analysis carried out by N. Choudhury has suggested the need to attain thermodynamic equilibrium between the electrolyte and the electrodes in order to avoid adverse reactions resulting in deterioration of the electrolyte. To achieve such equilibrium, in a sodium sulphur cell, the electrolyte on the sodium side has to have a high sodium oxide activity or content. On the sulphur side, a low sodium oxide activity or content is required.
The present invention is directed to beta alumina electrolyte elements and their manufacture and is directed more particularly to the provision of a surface layer on a beta alumina element, the surface layer having a high sodium oxide content or activity (compared with the substrate). As will be apparent from the following description however the invention furthermore provides a technique enabling such a surface layer to be formed as an intermediate layer between the substrate and an outer porous layer, which is advantageous for certain purposes.
According to one aspect of the present invention a method of making a beta alumina electrolyte element comprises the steps of forming a green shape of finely powdered beta alumina or of a mixed precursor material of the required composition to form a green shape, coating at least part of the surface of the green shape with magnesium oxide or lithium oxide or a mixture of magnesium oxide and lithium oxide, and then sintering the coated green shape.
It is well-known that a solid beta alumina element may be formed by sintering a compact of powdered beta alumina or a mixture of the required oxides or other precursor material in the required proportions. It is often the practice to calcine the compact after the green shape is formed but before sintering to convert the mixed material into beta alumina or to remove volatile materials, e.g. to decompose carbonates.
The coating of the green shape may be effected before or after calcining, if the material is calcined before sintering.
The magnesium oxide or lithium oxide or mixed magnesium and lithium oxide coating, on sintering, causing a magnesium or lithium alumina spinel layer to form over the surface of the substrate.
The effect of this is to deplete the alumina content of the immediately underlying surface layer of the substrate and, as a result, this underlying surface layer forms a beta alumina with a higher sodium oxide content than the main body of the substrate. In magnesium doped beta aluminas, it is generally accepted that a high percentage of beta" alumina (the "three block" variant of beta alumina) is associated with a ceramic having a high soda content and a high magnesium dopant level, typically near the 4% by weight magnesium oxide, 9% by weight sodium oxide composition, the balance being aluminium oxide. Reference may be made to Production of#-Al2O3 Electrolyte by I. Wynn Jones and L. J. Miles in Proc. British Ceramic Society, No. 19 March 1971, pages 161-1 78 for a further explanation of the structure of the material.In lithium doped beta" aluminas, the soda level required for high beta" contents is similar but the LiO2 content required is only about 0.7% by weight. Generally speaking, minimum bulk resistivity is obtained when the phase composition is close to 100% of the beta" variant of beta alumina but materials of this composition have greater susceptibility to mechanical degradation than materials of or containing a substantial proportion of the "twoblock beta alumina variant. In this specification, the term beta alumina is used generically to include materials of the two-block or of the threeblock variants or containing both variants.
In forming a beta alumina, the material of the green shape may typically have a composition of between 5 and 10% by weight Na2O, up to 4% by weight MgO and/or Li2O, the balance being Awl203.
In the manufacture of solid electrolyte elements for electrochemical cells such as sodium sulphur cells the composition has to be chosen to meet a number of different requirements and it is the common practice to use somewhat lower magnesium and soda contents than the 4% magnesia, 9% sodium mentioned above (referred to as a 4/9 material). Typically the weight percentage of magnesia and of sodium oxide might be about 2% magnesia, 8% sodium oxide (referred to as a 2/8 material). Using such a 2/8 material and coating the green shape with magnesia before sintering as described above, it has been found possible to obtain a spinel coated beta alumina in which a surface layer of the order of 50-60 microns thickness had a beta" content in excess of 90% compared with the 45~50% beta" found in a normal sintered 2/8 material.It will be seen therefore that this technique enables a higher soda content to be obtained near one surface of a beta alumina substrate by coating that surface on the green shape with magnesia or lithia or mixed magnesia and lithia and then sintering. In particular, if the substrate is of beta alumina with less than 100% beta" alumina, this technique enables the proportion of beta" alumina in a surface layer to be increased.
If the outer spinel layer, which is porous, is not required it can be removed. Many techniques are possible for this. For tubular electrolyte material in which the spinel layer is on the outside of the tube, conveniently such a spinel layer could be removed by rumbling. For many applications of beta alumina, for example in a sodium sulphur cell, however, it may be preferred to have such a porous layer over the surface. It is a well-known practice in sodium sulphur cells to provide capillary means or a wicking media to cause the sodium to cover the surface of the electrolyte material in a thin layer, the sodium being drawn up from a sodium reservoir. The porous spinel layer provides such a wicking media over the surface of the electrolyte.
The material produced in the above way has been found to have another significant advantage.
The coated electrolyte is inhomogeneous and has large beta grains aligned perpendicularly to the surface. These grains therefore have their conduction planes aligned perpendicular to the surface so giving a higher than normal conductivity in this direction. It will be appreciated that, in electrochemical cells, a high ionic conductivity through the material is required.
The magnesia or lithia or the magnesia/lithia mixture may conveniently be applied by spraying or painting a slurry on the substrate, the slurry being formed using a readily evaporable carrier, for example an alcohol such as propanol. Using such a carrier, the body can be left to dry in air at room temperature for a few minutes before carrying out the sintering operation.
If the green shape is formed from mixed oxide materials or precursors, it is preferably prefired, that is to say calcined, to remove volatiles before the magnesium oxide or lithium oxide or magnesium oxide/lithium oxide is applied to the surface.
The sintering operation after applying the magnesium oxide or lithium oxide or mixed magnesium oxide/lithium oxide, may be carried out in the known way and may be a slow sintering operation in a closed furnace or may be a rapid sintering operation using a pass-through furnace as described for example in Specification No.
1297373 or Specification No. 1375167 or Specification No. 1458221.
As has previously been explained, the abovedescribed treatment enables one surface of a beta alumina electrolyte element to be made with a surface layer higher in sodium oxide content or activity compared with the substrate. It may be desired that the opposite surface should be formed with a layer depleted in soda. This may be achieved by a heat treatment in air after the element has been sintered. For this purpose the element may be subjected to a heat treatment in air with the surface having the enhanced soda layer buffered, for example with beta alumina chips of the same composition as the substrate, whilst leaving the other surface open to the atmosphere.
The invention furthermore includes within its scope a beta alumina electrolyte element made by the above-described method.
Considered from another aspect, the invention includes a beta alumina electrolyte element of magnesium-doped beta alumina having on one surface a porous coating formed of a magnesia alumina spinel or a lithium alumina spinel or a magnesia and lithia alumina spinel with an intermediate layer of alumina depleted beta alumina underneath the spinel.
The invention still further includes within its scope a sodium sulphur cell having an electrolyte element as described above separating liquid sodium constituting an anodic reactant from sulphur/sodium polysulphides constituting a cathodic reactant The electrolyte element finds particular application in a sodium sulphur cell in which the sodium lies in a capillary region between the electrolyte and a current collector or housing with the spinel layer forming a wicking means for drawing sodium from a sodium reservoir gmucover a surface of the electrolyte element.
The following is a description of one example of the production of a solid electrolyte tube for use in a sodium sulphur cell, reference being made to the accompanying drawing which is a transverse section through a cell.
In this example a green shape for sintering to form a beta alumina electrolyte tube was made of mixed powdered materials to produce a beta alumina having a composition of approximately 8% by weight Na2O, 2% by weight MgO, the balance Awl203. The starting materials were sodium aluminate, magnesium oxide and finely ground high activity alumina. The materials were mixed in the correct proportions to give the required final composition as powders which were dry milled together. The powder was then formed into the required tubular green shape by isostatic pressing using a polyurethane pressure tube and an isostatic pressing pressure of typically 40,000 p.s.i. This green shape was prefired at a calcining temperature, typically 7000 C, for a period of 5 hours.
In this particular embodiment, the tube was for use in a sodium sulphur cell of the central sulphur type that is to say having the sulphur/sodium polysulphides inside the tube and the sodium outside the electrolyte tube. It was required therefore, in carrying out the present invention, to increase the sodium content on the outer surface of the green shape. The outer surface was therefore sprayed with a magnesia slurry prepared by wet milling "Analar" magnesia in propanol with approximately 200 gms of solid magnesia per litre propanol, the wet milling continuing for 24 hours. The coating could alternatively be applied to the tube with a soft brush. After the coating had dried, in approximately 1 minute, the green body was ready for sintering. In this particular case the tube was zone sintered in a sloping zone sintering furnace as described in Specifications Nos.
1458221 and 1458222 with peak temperature of 1 7000C. The tube was then annealed in a conventional kiln at 1 3000 for a period of 10 hours.
The above-described process left the tube with an outer porous spinel layer of about 5 microns thickness on the densely sintered beta alumina substrate. In the substrate, the surface of about 50 or 60 microns had a beta" content in excess of 90% compared with the 45~50% of beta" alumina in the bulk of the substrate.
Referring to the accompanying drawing which is a section through a sodium sulphur cell, this electrolyte tube 10 havingua porous spinel layer 11 on its outer surface with the sodium enriched layer 12 beneath the spinel was assembled in a cylindrical housing 13 to form a sodium sulphur cell. The cell was of the central sulphur type with sulphur/sodium polysulphides in an annular carbon fibre matrix 14 between the inside surface of the electrolyte tube 10 and a central axially extending current collector 15. The sodium was in a narrow annular region 16 between the housing 13 and the electrolyte tube. The porous spinel layer 11 formed a wicking medium for drawing liquid sodium up from a sodium reservoir located beyond one end of the electrolyte tube and serves to keep the surface of the layer 12 wetted with sodium. In the known way, the amount of sodium in the annular region 16 may be reduced by packing this region with a particulate material, for example zircon sand.

Claims (17)

Claims
1. A method of making a beta alumina electrolyte element comprising the steps of forming a green shape of finely powdered beta alumina, or of a mixed precursor material of the required composition, to form a green shape, coating at least part of the surface of the green shape with magnesium oxide or lithium oxide or a mixture of magnesium oxide and lithium oxide, and then sintering the coated green shape.
2. A method as claimed in claim 1 wherein the green shape is calcined before coating.
3. A method as claimed in claim 1 wherein the green shape is calcined after coating but before sintering.
4. A method as claimed in any of the preceding claims wherein the material forming the green shape has a composition of between 5 and 10% by weight sodium oxide, up to 4% by weight of magnesium oxide and/or lithium oxide, the balance being alumina.
5. A method as claimed in any of claims 1 to 3 wherein the material forming the green shape has a composition, by weight, of about 2% magnesia, 8% sodium oxide, the balance being alumina.
6. A method as claimed in any of the preceding claims wherein the composition and the coating are such as to deplete the soda content of a surface layer in the green shape so as to provide, in the sintered body, a surface layer of the order of 50-60 microns thickness with a beta content in excess of 90%.
7. A method as claimed in any of the preceding claims wherein the outer spinel layer on the sintered element is removed.
8. A method as claimed in any of the preceding claims wherein the magnesia or lithia or the magnesia/lithia mixture is applied by spraying or painting a slurry on the substrate, the slurry being formed using a readily evaporable carrier.
9. A method as claimed in any of the preceding claims wherein the coating is applied to one surface of a green shape to make one surface of the beta alumina electrolyte element with a surface layer higher in sodium oxide content or activity compared with the substrate and wherein the opposite surface is formed with a layer depleted in soda by a heat treatment in air after the element has been sintered.
10. A method as claimed in claim 9 wherein said heat treatment in air is effected with the surface having the enhanced soda layer buffered whilst leaving the other surface open to the atmosphere.
11. A method of making a beta alumina electrolyte element substantially as hereinbefore described.
12. A beta alumina electrolyte element made by the method of any of claims 1 to 11.
13. A beta alumina electrolyte element of magnesium-doped beta alumina having on one surface a porous coating formed of a magnesia alumina spinel or a lithia alumina spinel or a magnesia and lithia alumina spinel with an intermediate layer of alumina depleted beta alumina underneath the spinel.
14. A sodium sulphur cell having an electrolyte element as claimed in claim 12 or claim 13 separating liquid sodium constituting an anodic reactant from sulphur/sodium polysulphides constituting a cathodic reactant.
15. A sodium sulphur cell as claimed in claim 13 and having an electrolyte element with a porous spinel layer on one surface and in which sodium lies in a capillary region between the electrolyte and a current collector or housing with the spinel layer forming a wicking means for drawing sodium from a sodium reservoir to cover a surface of the electrolyte element.
16. A beta alumina electrolyte element substantially as hereinbefore described.
17. A sodium sulphur cell substantially as hereinbefore described.
GB08202028A 1982-01-25 1982-01-25 Beta-alumina electrolyte material Withdrawn GB2114114A (en)

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GB08202028A GB2114114A (en) 1982-01-25 1982-01-25 Beta-alumina electrolyte material

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Application Number Priority Date Filing Date Title
GB08202028A GB2114114A (en) 1982-01-25 1982-01-25 Beta-alumina electrolyte material

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2144559A (en) * 1983-08-05 1985-03-06 Bicc Plc Optical cables
WO1991003080A1 (en) * 1989-08-16 1991-03-07 Licentia Patent-Verwaltungs-Gmbh High-energy secondary battery
FR2662306A1 (en) * 1990-05-16 1991-11-22 Programme 3 Patent Holdings SOLID ELECTROLYTE SEPARATOR FOR ELECTROCHEMICAL ACCUMULATOR ELEMENT AND PROCESS FOR MANUFACTURING SAME.
US5763117A (en) * 1995-06-26 1998-06-09 Electro Chemical Holdings Societe Anonyme Electrochemical cell
US6007943A (en) * 1997-02-06 1999-12-28 Electro Chemical Holdings Societe Anonyme High temperature electrochemical cell with molten alkali metal anode

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2144559A (en) * 1983-08-05 1985-03-06 Bicc Plc Optical cables
WO1991003080A1 (en) * 1989-08-16 1991-03-07 Licentia Patent-Verwaltungs-Gmbh High-energy secondary battery
FR2662306A1 (en) * 1990-05-16 1991-11-22 Programme 3 Patent Holdings SOLID ELECTROLYTE SEPARATOR FOR ELECTROCHEMICAL ACCUMULATOR ELEMENT AND PROCESS FOR MANUFACTURING SAME.
GB2244850A (en) * 1990-05-16 1991-12-11 Programme 3 Patent Holdings Solid electrolyte separator for electrochemical cell
US5219682A (en) * 1990-05-16 1993-06-15 Programme 3 Patent Holdings Method of making a solid electrolyte separator
US5763117A (en) * 1995-06-26 1998-06-09 Electro Chemical Holdings Societe Anonyme Electrochemical cell
US6007943A (en) * 1997-02-06 1999-12-28 Electro Chemical Holdings Societe Anonyme High temperature electrochemical cell with molten alkali metal anode

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