WO1999065642A1

WO1999065642A1 - Brazing composition and seal for electrochemical cell

Info

Publication number: WO1999065642A1
Application number: PCT/IB1999/001112
Authority: WO
Inventors: Bernd Wegner
Original assignee: Bi-Patent Holding S.A.; Van Der Walt, Louis, Stephanus
Priority date: 1998-06-15
Filing date: 1999-06-15
Publication date: 1999-12-23
Also published as: AU3952399A

Abstract

A method of making a seal for a rechargeable high temperature electrochemical cell is provided. The cell is of the type which includes a housing containing an anode and a cathode, the housing having an interior divided by a solid electrolyte separator into an anode compartment and a cathode compartment. The anode compartment and the cathode compartment contain respectively the anode and the cathode. The cell has a charged state in which the anode includes an alkali metal or alkali metal alloy, and an operating temperature at which the anode is molten. The separator comprises a conductor of alkali metal ions, and the cathode comprises, at said operating temperature and in said charged state, an electronically conductive porous electrolyte-permeable matrix having a porous interior impregnated with a molten salt electrolyte. The matrix contains, dispersed in its porous interior, active cathode material comprising at least one chloride selected from the group consisting of the chlorides of nickel, iron, cobalt chromium, copper, manganese and antimony. The housing is in the form of a metal canister. The separator is sealed to a ceramic insulating collar which in turn is sealed to the metal canister by an outer seal and to a metal collar by an inner seal spaced from the outer seal. The inner and outer seals each include a brazing alloy comprising the metals nickel, niobium and titanium, said alloy containing less than 10 % and at least 3 % by mass titanium, and containing niobium in a proportion of 10 % to 70 % by mass, relative to its combined nickel and niobium contents.

Description

BRAZING COMPOSITION AND SEAL FOR ELECTROCHEMICAL CELL

THIS INVENTION relates to electrochemical cells. More particularly, it relates to a method of making seals for electrochemical cells and of making seals for reference electrodes. Still more particularly, the invention relates to a method of making metal-ceramic seals suitable for rechargeable alkali metal anode high temperature electrochemical cells having alkali metal anodes, and precursors of such cells, also being suitable for alkali metal thermoelectric conversion devices, and also being suitable for alkali metal high-temperature reference electrodes. The invention also relates to seals for such cells, devices and electrodes.

The following description of the invention describes in particular rechargeable high temperature electrochemical cells employing solid electrolyte separators, but from this description the use of the invention in alkali metal thermoelectric conversion devices and high temperature alkali metal reference electrodes can easily be inferred.

According to one aspect of the invention, there is provided a method of making a seal for a rechargeable high temperature electrochemical cell which includes a housing containing an anode and a cathode, the housing having an interior divided by a solid electrolyte separator into an anode compartment and a cathode compartment, the anode compartment and the cathode compartment containing respectively the anode and the cathode, the cell having a charged state in which the anode includes an alkali metal or alkali metal alloy, and the cell having an operating temperature at which the anode is molten, the separator comprising, a conductor of alkali metal ions, and the cathode comprising, at said operating temperature and in said charged state, an electronically conductive porous electrolyte-permeable matrix having a porous interior impregnated with a molten salt electrolyte, the matrix containing, dispersed in its porous interior, active cathode material comprising at least one chloride selected from the group consisting of the chlorides of nickel, iron, cobalt chromium, copper, manganese and antimony, the housing being in the form of a metal canister, and the separator being sealed to a ceramic insulating collar which in turn is sealed to the metal canister by an outer seal and to a metal collar by an inner seal spaced from the outer seal, the inner and outer seals each including a brazing alloy comprising the metals nickel, niobium and titanium, said alloy containing less than 10% and at least 3% by mass titanium, and containing niobium in a proportion of 1 0% to 70% by mass, relative to its combined nickel and niobium contents.

According to another aspect of the invention, there is provided a brazing composition suitable for sealing ceramics in a high-temperature rechargeable electrochemical cell of the type which comprises a molten alkali metal or alkali metal alloy active anode material and a transition metal chloride active cathode material dispersed in a porous electrolyte-permeable electronically conductive matrix impregnated by a molten salt electrolyte and separated from the molten active anode material by an alkali metal ion-conductive solid electrolyte separator, the brazing composition being provided as a paste having metal particles with a median particle size of between 20/vm and 70μm dispersed in a binder medium, the binder medium being eliminated by brazing .

The metal particles of the brazing composition may be of an alloy containing less than 1 0% and at least 3% by mass titanium, and containing niobium in a proportion of 1 0% to 70% by mass, relative to its combined nickel and niobium contents. The alloy may also include iron. Both aspects also apply to sealing reference electrodes to be used in a high temperature environment, and to sealing alkali metal thermoelectrical conversion devices. The term high temperature is meant to comprise temperatures below the eutectic point of the brazing alloy.

The invention extends to a method of making a seal for a high temperature rechargeable alkali metal/transition metal chloride cell or for a precursor of such cell, the precursor containing, after loading, a cathode composition which corresponds to an overdischarged state of the cathode of the cell and the precursor containing no metallic alkali metal, the alkali metal being generated on charging the sealed cell precursor.

Cells of the type described above with molten sodium as the alkali metal anode and molten sodium aluminium chloride as the molten salt electrolyte have become known as ZEBRA cells. As with related sodium/sulphur cells, the solid electrolyte separator is usually a yS-alumina-type ceramic, typically in the shape of a tube closed off at one end, the other end being open and joined to a ceramic insulating collar, usually of α-alumina, to which in turn are joined both an outer metal collar and an inner metal collar. The artefact (σ-alumina insulating collar with the two metal collars) is often known as a header in the art and is referred to as such herein.

In accordance with the method of the invention for making a seal for a cell, both metal collars are welded after joining thereof to the insulating collar, the outer collar to a metal canister serving as a cell housing, and the inner metal collar to a current collector extending into the separator tube and in contact with the active electrode material contained therein, the inner collar being welded shut to close the cell after assembly.

The joint between the solid electrolyte and the ceramic insulating member of the header is usually made by glassing, i.e. glass welding, after making the header, suitable glass compositions being known in the art. However, the metal-ceramic seals required for joining the σ-alumina insulating member to either the metal canister of the housing or to the metal collars cannot be made reliably by glassing. Seals in high-temperature cells must withstand both thermal cycling and corrosive media, which in the case of ZEBRA-type cells are liquid sodium and the sodium aluminium chloride melt, together with additives which include sulphur or sulphur compounds. Therefore, thermocompression bonded seals have usually been used in those cells and in their precursors, eg as described in US 5 009 357.

Making metal/ceramic seals for such cells by brazing would appear advantageous from the point of view of cost and speed, but no braze has been known to withstand the environment referred to above, the braze requiring, in combination, at the operating temperature of the ZEBRA cells, resistance to molten alkali metals such as liquid sodium, resistance to molten salt electrolytes such as molten sodium aluminium chloride under electrochemical stress, and resistance to oxidation.

Further requirements include a suitably ductile behaviour of the braze, the exclusion of elements such as boron and silicon which react with titanium to form high-melting alloys, and the selection of brazing times and temperatures to achieve good fracture strength by avoiding unacceptable crystal growth in the metals to be joined .

The present invention provides such a braze, and with it, a method of sealing the above cells or their precursors, respectively.

Thus, according to still another aspect of the invention, there is provided a method of making a seal for a high temperature rechargeable alkali metal/transition metal chloride ceil or for a precursor of such a cell, the precursor containing, after loading, a cathode composition which corresponds to an overdischarged state of the cathode of the cell and the precursor containing no metallic alkali metal, the alkali metal being generated on charging the sealed cell precursor, the method including joining a solid electrolyte separator, in the form of a tube closed off at one end with the other end being open, at its open end to a ceramic insulating collar; joining an outer metal collar and an inner metal collar to the ceramic insulating collar by means of a brazing alloy; joining the outer metal collar to a metal canister serving as a cell housing; joining the inner metal collar to a current collector extending into the solid electrolyte separator tube; and closing off the inner metal collar after active electrode material has been loaded into the solid electrolyte separator tube.

The brazing alloy may contain less than 1 0% and at least 3% by mass titanium, and may contain niobium in a proportion of 10% to 70% by mass, relative to its combined nickel and niobium contents. The brazing alloy may also include iron.

Joining the outer metal collar to the metal canister may be effected by welding, joining the inner metal collar to the current collector may be effected by welding, closing off the inner metal collar may be effected by welding the inner metal collar shut, and joining the solid electrolyte separator to the ceramic insulating collar may be effected by glassing.

The sequence of joining operations outlined above for rechargeable cells, i.e. first making a header from a ceramic insulating collar and two metal collars and then glassing the header to the solid electrolyte separator, then welding the canister to the outer metal collar and welding the current collector to the inner metal collar, is only illustrative for the understanding of the joining and assembly operations and is not meant to limit the scope of the invention. Moreover, the term collar is meant to comprise metal or ceramic parts of circular or polygonal shape, including squares, and in general all shapes compatible with a particular cell design.

The invention may be used for brazing a variety of metals, including nickel and nickel/cobalt/iron alloys, to alumina-based ceramics. A guide for selection of a particular composition from the general compositions outlined above is to include the base metal of the metal part to be brazed in the composition of the braze. Thus, iron is included in the composition of the braze for brazing iron-based metal parts.

Also , alloys with coefficients of thermal expansion approaching as closely as possible that of the ceramic in the temperature range covered by the thermal cycling of the brazed join are preferred .

Among iron alloys well known in the art for metal/ceramic joints and which may be brazed in accordance with the invention to alumina-based ceramics are Co-Fe alloys with or without a nickel constituent such as Vacon and Vacodil , and Ni-Fe alloys with or without a chromium constituent such as Vacovit , Kovar™ and Dilver . Such alloys are often used in metal/ceramic joins because of their low coefficient of thermal expansion which is closer to that of the ceramic than that of iron or nickel, in the temperature range of operation and also in the temperature range of glass transformation of the glasses used in manufacturing glassed joins.

A preferred composition for brazing Dilver™ or Vacon is (in % by mass) Ni 41 .6%, Fe 30.9%, Nb 1 4.1 %, Co 9.4%, Ti 4,0%, melting between 1 1 81 and 1 1 90°C, with a preferred brazing temperature between 1 230 °C and 1 280°C. To achieve good flow and wetting of the braze the temperature is raised by about 20°C for a few minutes after melting of the braze has set in. Subsequently, rapid cooling of the workpiece is permitted . The brazing alloy may be employed as a metal foil or wire, or may be formulated into a paste containing water and/or organic fluids.

A brazing alloy precursor metal powder mixture may be prepared from commercial metal powders, including powders obtained by spraying molten metals into a vacuum or into an inert gas. The powder mixture may be prepared by a paste formulating process using a viscous binder, and the paste or dispersion thus obtained may be stored in an alternating electromagnetic field to counteract sedimentation of the metal in the paste.

Instead, the precursor metals or metal alloys may be melted to form homogeneous melts before either forming them into particles or shaping them into foils or wires. Forming them into particles may comprise spraying the alloy melts into a vacuum or into an inert gas to obtain a pre-alloyed powder of the braze, or may be by milling ingots or pellets of the alloy.

In both cases the titanium component essential for active brazing may be added as titanium powder or titanium hydride powder or as a water- based titanium hydride paste to a precursor powder mix or to a pre-alloyed powder prepared beforehand . Instead, the titanium content may be incorporated by including titanium in the alloying process of the precursor metals. In a two- step process, the titanium, which is essential for reactive bonding to the ceramic, may be applied first as a titanium hydride paste or dispersion to the ceramic, and after this primer has been caused to adhere, eg by drying the primer, the remainder of the brazing alloy, known as filler metal in the art, may be applied separately in either of the forms mentioned above, together with location thereof in place on the ceramic or on the metal part(s) to be joined, or thereafter.

In contrast with commercially available silver-based active brazes, the brazes of the present invention show a high solubility for titanium, and, consequently, the titanium can diffuse in the molten braze very quickly to the reaction zone on the ceramic surface. Painting of the ceramic surface with titanium hydride paste is thus not essential.

The precursor paste or braze paste may be formulated to allow automated application thereof to one or both of the surfaces to be joined by brazing, eg by a printing process or other dispensing process.

As indicated above, the brazing alloy material according to the invention may be made into a brazing foil or wire which in use is suitably positioned for bonding the surfaces to be joined at the brazing temperature.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings and Examples, in which:

Figure 1 shows a schematic sectional side elevation of a header used in trials performed to test the present invention; and

Figure 2 shows a schematic sectional side elevation of a sodium reference electrode half cell made in accordance with the method of the present invention.

Extensive trials have been performed by the applicant to test strength and the corrosion of seals made in accordance with the method of the present invention.

Thus, Figure 1 shows a header used in such trials, generally designated 1 0 and consisting of an α-alumina collar 1 2 having rebates 1 4 and 1 6 into which respectively an inner metal collar 1 8 and an outer metal collar 20 are joined, the collar 1 2 being joined to the collars 1 8 and 20 by brazing via brazing fillers 22 and 24 respectively, to form ceramic/metal joins of height H .

EXAMPLE 1

In a first example, the collars 1 8 and 20 were made of nickel having a thickness of 0.5 mm. There were 1 1 samples whose fracture strengths are listed below and these samples were exposed to different environments. The environments are described in Table 1 hereunder and the fracture strengths are set forth in Table 2 hereunder.

TABLE 1

TABLE 2

The standard fracture strength for the same design but employing thermocompression bonding of the collar 1 2 to the collars 1 8, 20 was 1 .5 - 3.0 kN, which is not superior to the values given in Table 2. EXAMPLE 2

Cyclovoltammetry was used to test the corrosion-stability of brazing alloys in the cathode compartment of a sodium/transition metal chloride cell. The experimental setup, operated in a dry box, comprised a working electrode formed from a brazed specimen, an aluminium metal counterelectrode, and a nickel reference electrode, immersed in a sodium aluminium chloride melt contained in a glass beaker. The working electrode and counterelectrode were separated from each other by a glass frit. The temperature of the melt was kept at 200°C. After recording oxidation and reduction cycles, a chronoamperometπc test at 3 V for 30 minutes, followed by 1 ,8 V for another 30 minutes, was conducted by recording the oxidation currents which resulted from the potentials in question The currents recorded after 10-mιnute potential holds were taken as relative measures of the corrosion-resistance of the specimen. After the chronoamperometπc test, two additional voltammetπc scans were performed to study changes of the voltammogram. The corrosion-stability of the active braze materials determined in this fashion was lower than that of nickel, but tolerable.

Satisfactory results were obtained for the braze compositions listed in the following table, Table 3.

TABLE 3

In these experiments, filler metal alloys were used comprising no titanium, to simulate the conditions of a two-step process in whicha titanium hydride paste is first applied to the ceramic. EXAMPLE 3

The outer metal collar 20 brazed to the σ-insulating collar 1 2 in this case was FeNι46 (Vacodil 46 ^M), the inner collar 1 8 was nickel and the brazing was carried out at 1 230 °C at about 1 0 millibars pressure with a dwell time of 1 0 minutes, with heating and cooling rates of 360 °C/mιnute respectively.

The tests described in Example 1 were repeated and yielded analogous satisfactory results.

EXAMPLE 4

The outer metal collar 20 in this case was FeNι42 (Imphy, Dϋsseldorf), the inner collar 1 8 was nickel, and the brazing was carried out at

1 280°C at about 1 0 millibars pressure with a dwell time of 5 minutes, with heating and cooling rates respectively of 800°C/mιnute.

EXAMPLE 5

Figure 2 illustrates application of the braze for sealing a sodium reference electrode half-cell generally designated 26 comprising an σ-alumma tube 28 of about 2 mm exterior and 1 mm interior diameter, and a length of about 200 mm, fitted with two metal sleeves 30, 32 of Dilver alloy which are joined to said σ-alumina tube 28 by two seals 34, 36, the brazed seal 34 sealing the tube 28 hermetically at one end . Seal 36 may also be braze seal but is preferably a glass seal The other end of the tube is sealed by a sodium lon- conductive /?-alumιna ceramic plug 38 which is glass welded to said alpha alumina tube by borate glass 40. A nickel wire 42 of 0.5 mm diameter extends between the braze seal 34 and the beta alumina plug 38 and is used as an electrode for filling sodium into the α-alumina tube cavity from a sodium salt melt by electrolysis, with sodium passing through the ion-conductive ?-alumina plug, the wire 42 being thereafter used as a reference electrode conductor. The other electrode is constituted by the Dilver alloy sleeve 32 which is brazed or welded to a metal part of the cell in which the reference electrode is to be used . Sleeve 32 may be fitted with a flange or collar for that purpose.

Claims

1 A method of making a seal for a rechargeable high temperature electrochemical cell which includes a housing containing an anode and a cathode, the housing having an interior divided by a solid electrolyte separator into an anode compartment and a cathode compartment, the anode compartment and the cathode compartment containing respectively the anode and the cathode, the cell having a charged state in which the anode includes an alkali metal or alkali metal alloy, and the cell having an operating temperature at which the anode is molten, the separator comprising, a conductor of alkali metal ions, and the cathode comprising, at said operating temperature and in said charged state, an electronically conductive porous electrolyte-permeable matrix having a porous interior impregnated with a molten salt electrolyte, the matrix containing, dispersed in its porous interior, active cathode material comprising at least one chloride selected from the group consisting of the chlorides of nickel, iron, cobalt chromium, copper, manganese and antimony, the housing being in the form of a metal canister, and the separator being sealed to a ceramic insulating collar which in turn is sealed to the metal canister by an outer seal and to a metal collar by an inner seal spaced from the outer seal, the inner and outer seals each including a brazing alloy comprising the metals nickel, niobium and titanium, said alloy containing less than 1 0% and at least 3% by mass titanium, and containing niobium in a proportion of 1 0% to 70% by mass, relative to its combined nickel and niobium contents

2 A method as claimed in claim 1 , in which the brazing alloy includes iron

3 A brazing composition suitable for sealing ceramics in a high-temperature rechargeable electrochemical cell of the type which comprises a molten alkali metal or alkali metal alloy active anode material and a transition metal chloride active cathode material dispersed in a porous electrolyte-permeable electronically conductive matrix impregnated by a molten salt electrolyte and separated from the molten active anode material by an alkali metal ion-conductive solid electrolyte separator, the brazing composition being provided as a paste having metal particles with a median particle size of between 20 ym and 70╬╝m dispersed in a binder medium, the binder medium being eliminated by brazing .

4. A brazing composition as claimed in claim 3, in which the metal particles are of an alloy containing less than 1 0% and at least 3% by mass titanium, and containing niobium in a proportion of 1 0% to 70% by mass, relative to its combined nickel and niobium contents.

5. A brazing composition as claimed in claim 4, in which the alloy includes iron.

6. A method of making a seal for a high temperature rechargeable alkali metal/transition metal chloride cell or for a precursor of such a cell, the precursor containing, after loading, a cathode composition which corresponds to an overdischarged state of the cathode of the cell and the precursor containing no metallic alkali metal, the alkali metal being generated on charging the sealed cell precursor, the method including joining a solid electrolyte separator, in the form of a tube closed off at one end with the other end being open, at its open end to a ceramic insulating collar; joining an outer metal collar and an inner metal collar to the ceramic insulating collar by means of a brazing alloy; joining the outer metal collar to a metal canister serving as a cell housing; joining the inner metal collar to a current collector extending into the solid electrolyte separator tube; and closing off the inner metal collar after active electrode material has been loaded into the solid electrolyte separator tube.

7. A method as claimed in claim 6, in which the brazing alloy contains less than 1 0% and at least 3% by mass titanium, and contains niobium in a proportion of 1 0% to 70% by mass, relative to its combined nickel and niobium contents.

8. A method as claimed in claim 7, in which the brazing alloy includes iron .

9. A method as claimed in any one of claims 6 to 8 inclusive, in which joining the outer metal collar to the metal canister is effected by welding, joining the inner metal collar to the current collector is effected by welding, closing off the inner metal collar is effected by welding the inner metal collar shut, and joining the solid electrolyte separator to the ceramic insulating collar is effected by glassing.

1 0. A method of making a seal as claimed in claim 1 or claim 6, substantially as herein described and illustrated.

1 1 . A brazing composition as claimed in claim 3, substantially as herein described and illustrated .

1 2. A new method of making a seal, or a new brazing composition, substantially as herein described .