CA2015961A1 - Alkali metal energy conversion device and method of construction - Google Patents

Abstract

ABSTRACT

The cathode region of a sodium sulphur cell has hitherto sometimes been sealed by a thin metal member welded at its outer periphery to the metal casing and at its inner periphery to the alpha alumina lid of the electrolyte. The welding operation damages the essential protective coating of the thin metal member. This invention discloses sealing the cathode region with a thin metal member secured to a concentric outer thicker metal member, the thin member being sealed to the lid and the thicker member being sealed to the casing. The thickness of the outer member eliminates the need for a protective coating so that the combined seal has greater integrity than previous seals.

Description

201 59 ~1 u8 ~ebrua~ 1990 , 8-2 ALXALI i`~ET~L ENERG'~ CONVE~SION DEVICE
~ND r~E~HOD OF CONSTRUCTION

This invention relates to alkali metal energy conversion devices, such as for example alkali metal cells and particularly sodium sulphur cells and methods of ccnstructing such devices. Such c:ells typically employ a solid elect~olyte element se.parating cathodic and anodic reactants -ihich are liquicl at the cell opera~lng temperature. ~, -lS
.~ known constr ction of device comprises an exter... al casing, a solid electrolyte element dividing the .... terior ~r the casing~ into two electrode regions, ;;~
an electrically insulatin~ element joined to ~he ~o elec_~lyte element, and at least one metal member sealed to the insulatlng element. This structure -,p~ollly for-s part oi' the sealing arrangement for the device, sealing o~f he two electrode regions both fro~
each orher and from the ambient environment. For _ exam~le, the e~ternal casing of the device r.av be of ~e~al, so that any sealing of an electrode region ~equires a seal to be made between the metal of the casing~ and the elect~olyte element. ~owever the metal or ~.e casing mus~ be electrically insulated ,~rom the 'o elec~_olyte element ~nd the insulation is provided by -he n~errening elec~ically insulating element.
An example o~ such an arrangement applied to a sod _- sulphur cell s shown in GB-~-2102622 ~hich has n ai~ha alumina lid closing a tubular electrolyte ~ ;
~5 el2ment. A centrall~; located curren~ col}ector is .~oun ed in an aperture through the alpha alumina lid ..
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089/~068 PCT/GB88~00909 and ;nsulated by t~.e lid frc~ the electrolyre element.
The ou~er elec~roàe region, on the out5lde o~ the -elec~_olyte element, is sealed ~ Deans of a thin ~e~al closure mem~er welded about its periphery to a metal S casing for the cell, and sealed about an inner perlphery to the alpha alumina lid. This latter seal , between the thin metal closure member and the alpha alu~ina lid has sometimes been made by compression bond~g usin~ an intPrmedia~e layer, between the thin mecal closure element a~d the ceramic lid, of a soft ma~erial, e.g. alu..inium, to provide the necessary bonc ng.
Suc;. kon~ing ~s car~ied out after the insulatir.~ ;
ele~en~, i.e. the alpha alu3ina ceramic lic, as '_ a~:~c.~ed tc ~he elec~~olyte eleme~ since this a;-~c~~ient s e~ ec-ed by glazing at eleva~ed ;empera~ures which ~ould destroy seals manu'accured b-~.~e-..ocompression bcnding using an intermediate layer ~el~een the ;~i n me~al ciosure elemen~ and _he ceramlc ~lter.at el~, ~P-~-0166605 d~s~loses ~he -ec-.iques c dlrec~1y t~er~..ocompressi~n bcnding the ~ ,etal .,e~ber ~_ the insula~in~ cera~ic id .o -rc-_de a seal bel~een the t~o which is not adversel~
_ ~r_ec~ed ~y subsequen~ temperature c~cling such as ~a~
ke e~Dloyed _~ ~he i~sulat~ng element ls subsequently ,oi~ied to the elec.~~lyte elemen~ by glazin~. The a il~ o _~r~ suc.. a seal ~e~ween the thi~ me~al -emke- and _`~e insulating element ke~ore t~.e 'a~ter s ed t~ t.~e elec~-~ly~e element grea~li facili~ates -.~e -.akin~ c~ ~his seal. For instance, a s,ack o' -,e~z_ ~embers and i~.sulating elements may ~e neous_~ seales .o or,e another ~o crc~l~e 2 ~ i _; c seale~ -~irs.
_ ~' ;i'_ ~e apF-eclated ~hat ~.e seal~ o~ sodl~.
,ui~ c2i' a an~ c~~.er alkai_ ~etal enero~ nvers1o.~

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devices is of c itical importance in the manu~acture ~f the cell to ensure good performance and safet~ and is a part cularly difficult problem because of the high operating temperatures of these cells, typically _ 350C. Bonding techniques using cements have not pro~ed practical.
In particular, the seal to the auter electrode region effected by the welding o~ the thin me~al closure member about ts periphery to the melal casing '0 is de~endent upon the integrity of a thin fil~, generally aluminium oxide with the materials commonly used, that is rormed on the thin metal member during thermocompresSion. This thin film protects the metal member fro~ attack by corrosive substances in the ~_ outer, i.e. sul2hur electrode, region, these in part cular being polysulphides formed therein.
:~ithert~, the Lhin metal member has been directly ~eLded to the metal casing after thermocompression seali.~g of the ~etal ~ember to the alpha alumina lid.
~0 ~his ~rocedure, however, results in the disr~ption o~
'he protective aluminium oxide film in the vicinity of the ~eld due LO the heat associated with welding. The ~enera~ion or polysulphides when the cell is in use .~ill result in the corrosion of the thin metal member -_ ~nd, ~itima~el~, the ailure of the seal.

.~ccordi~g to the present invention, a .e~hod af .
cons~_~cting ~n al~;ali ~etal anergy conversicn device :,avina an e.Y~ernal casing, a solid elect-oly~ element ~ividina the _nterior of the casing into two electrode re~ions, an electrically insulating element joined to ~he electroIyte elemenl, ~at least one first ~e~al S 3~mber sealed ~o the insulating element,is characterised by steps of securing the Elrst metal member to a , ! .
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_ii~ : ' : ' SUBSTITUTE SHEE~ , PCT In~ernatlf,~.~l Appllcatlon 2~L5~
tJ8 Febnu~ IY90 8~2-lqq~

subs~antially thicker further metal ~ember and sealing the ~lrst metal member to the insulating element by ther~ocompression bonding, and securing the furth~r metal member to the casing.
Arranging for a substantially thicker ~etal member to seal to the metal casing results i.n the prctective film on the thin metal member remaini.ng undamaged-and thus obviates the problems associatecl with a vulnerable `.
thin protective film coating the thin metal member used }o hitherto. The further metal member is, by virtue of its greater thickness, resistan~ to the corrosive polysulphides and conseauently does not require a protective thin film. Furthermore, retaining a thin ~:~
metal member to seal to the insulating element satisries the requirement for flexibility in this latter seal to cope ~.~ith differential heat expansion duri~.g thermocompression bonding and subsequent opera~icns o~ glazing and cell testing.

Also according -o the invention there is ?rovi~ed an al.~ali metal energ~ conversion device having an e.Yte~nal casing, a soiid electrolyte element in the -asing to di~ide the nterior into two electrode regions, an elactrically insulating element joined to the ?lec-_olyte element, and a first metal member secured bv -hermocompression bonding to the insulating element -narac~erised by a substantially thicker further metal ~ember bonded to said first metal member and ~oined to ~he exlernal casing to seal off one of said electrode ~- e a icns.
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~Patent Officël SUBSTITUTE SHEE~M ~-LAPPllcation ¦
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~ _ The invention T.~i}l now be descri~ea by ~av of exam~le T~ith reference -o the drawings in whic;~:-?lgure ' is a diagrammatic longitudinal cross-section through a known sodium sulphur cell; and ~igures 2 and 3 are each diagra~natic longit~dinal cross-sec ons through an arrangemen~ or a portion of a sodium sulphur cell in accordance with the invention.

_~ ~eferring ~o r'igure } o~ the drawings, a sodium sul~u_ cell or the central sodium type is illustrated ~, 50mp, ' sing a c~.~lindrical beta alumina electrol~lte tubular elemen- 14 which is integrally closed at one end as shown at 2~ and has its other end closed bv an ~- al~ha alumina end pla~e 15. The end plate 15 ~s sealed, by glazing to one end of the electrolyte elemenl l~ and provides electrical insulation as well as a -ecAanical seal. ~ithin the sealed assembly there ~ay ~e either an iron -oil elemen~ (not shown) or a ~esh alement (no~ shown) closely adjacent the inner -yLi..~.ical sur~ace o~ the electrolyte tube 1~ to leave a capillary -egion aàjacent ~ha~ surface constituting a ~ic.~. rhe in~erior of the assembly is filled .~ith sodi~ 0 ~Jhich is liquid at the operating te~perature _~ or _.~e cell: 'he capillary ~aintains a layer o~ liquid ~ -sodi~ over th.e inner surface of the electrolyte tube ,',:
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SUBSTITUTE SHEE~ ~ ~cr ~ ~L

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- `~ 089/~068 PcTlGB88~oo~o9 1~. A cur-ent collector rc~ 21 extends in~^ this .
sodiu~, passing t~rcuoh an aperture 22 ~n t~.e alpha alu.~inia element 1_. Arourd the outside o. he cyli~drical portion of the elec~rolyte element 14 is a _ cathode struc~ure of annular form constituted by three thi~d-cylindrical elements 23 and a further cup shaped base element (nat shown) of carbon iibre material impre~nated with sulphur. These ele~ments lie between the elec~rolyte tube 14 and an outer metal case 1, the cath-de elemenrs 23 being in contact both with the beta alu~ina elec_~olyte tube 1~ and the case 1. These cath_ae elemen~s may be fo_...ed in the known ~ay ky cc~ ession cf _he ibre ma~erial wAich is npregnatea rb~it~. hot sul?hur, the sulph~- then being cceled so as :_ _o ~e solid~C~ed and thereb~.~ to hold the eleme!nt ~n cor.p~esslon :_ fac~ ate assembly of the ceil. When ~he cell is aised __ the operatin~ tempera~ure, ~ -311y 35~, the sulphu- melts and the resilience c_ _.. e ribre ma~er_al causes the eiements ~3 to maXe ~-~^ aooc con~ac~ h the case ' and the elec_~_lvte 1;. . ;
~.e case 1 i~ made ~~eferab_- of Inconel 600 or:~
c-r^-.ssed mild s~eel and, c~ its internal su-'^ace, is coa~ed with an an-i-corrosive elec~ronically conduc~_ve coa~:.g 2~ ._ p.ovice an elec~ronically cond~c~ive pa~b ._ ~e~-been the case ar.d _he ca-_on fibre matersal.
The alpha alu-.lna pla~e 15 is for~ed as a disc ~ . a central aper~-are 22. This disc is sealed t~ the case _ by means of an annular metal member G5 for~ed of ~nc~el 6Q0 cr Fecralloy A hAich is secureà -y welding Ae peripherf of .he housing and by :he~ compression bonding t_ ~he disc 15 ~ ~n annular _egl-~ arqund .he cen~ral aDert~re ~. The cent~al c_~pa- men~ e^ ~he c_ll s c~sed by means _ a cu--en-c_l:ac~ assir~ ~hr-uc.. -.~e aDer~ure ~_ ana secu_ed ~- 3n _-re_ -.etal ele~en~ ~ also bc..-ed -c ~.~e ~l~h~ alu.~lr. 3rour:- ~he 2~e-_ure ~ hy reans c~

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~089/~068 PCT/GB88~0~909 , .

thermocompression bonding. The element 9 ls spaced radially inwardly f~om the annular metal member 25 so that they are elect~_cally insulated from one another by the alpha alumina disc.
In the manufacture of the celll, the metal members 25 and 9 are bonded to the alpha alumina end plate 15 before further assembly of the eel~. This bondincJ is .;
effected by compression at an elevated temperature and under vacuum conditions or in an inert atmosphere. :
Typical bonding conditions for 0.075mm (3 thou) thic}:
Fecralloy metal me~bers are:-~aximum Bonding Temperature = 1000C
Time at Maximur., Temperature = 20 mins .
Bonding Pressure - 24.5 Nm~2 Bonding Chamber Vacuum = 4 x 10-2 torr ~`
.
The inner metal member 9 is of relatively small radial extent and the seal is effected by app~ying ~ .
pressure through a backing washer 10 to seal the outer peripheral edge of inner metal member 9. The material of the washer 10 is such as to become bonded to the member. The outer annular metal member 25 is sealed to the alpha alumina lid over a small annular re~ion around the inner member but slightly spaced therefrom by applying pressure through a further backing washer 11 .
A strengthening washer 8 is also thermocompression bonded to the upper surface of the inner metal member, annular sheet 9. The strengthening washer 8 has a thickness greater than the thickness-of the member 9 ;:
and serYes to keep the inner peripheral portion of the sheet 9 substantiall~ rigid. The outer dia~eter of the washer 8 is substantially less ~han the diameter of the backing washe- 10, c~~~esponding to the position of the seal between 'he member 9 and the alpha alumlna lid 1~

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- W089/~068 PCT/GB88/00909 The cen~ral current collector 21 extending through the aperture 22 has an annular shoulder 7 whioh seals against the inner edge of the stren~thening washer ~
and is welded thereto to provide the necessary her~etic seal.
Because the annular sheet 9 is bonded to the aplha alumina lid 15 only about the outex periphery of the sheet 9, some flexibility is proYidled between the seal with the central current collector 21 and the seal to lo lid 15. The ma~erial of the sheet 9 is made .
sufficiently thin to permit some distortion in the region indicated qenerally at 26 between the backing washer ~0 and the strengthening washer 8.
using this technique, a plurality of sub-assemblies comprising the end plates 12 with the~.r metal members can be stacked and produced in a single operation. ~
The present invention is more particularly concerned with the sealing of the outer of the electrode regions, i.e. that lying between the - -electrolyte tube 14 and the outer metal case 1.
Turning now to Figure 2 in which reference -numerals used hitherto refer to like integers~ a firs~
metal member 5 is shown secured to a substantially thicker further metal member 4.
The first metal member 5, which may be essentiall~
annular, is secured to the alpha alumina ceramic lid 15 at a narrow re~ion at it~ inner periphery, bonding pressure being applied through an annular metal washer 6 o~ a material which becomes bonded to said firs~
metal member 5. An interface washer 2 is provided to sandwich the first metal member 5 in batween said interface washer and the further metal member 4, an elec~ron beam weld~securing the ~irst metal member in ;~
position. Generall;, Fecralloy is used for t~e ~irst m~tal member 5, Inconel for the fur~her me~al member ~ ~

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u 8 Febn~ 0 8-2- l~Clo and the interface washer 2 and Nilo K for the backing washer. Preferably, the first metal mem~er 5 is welded to the further metal me~ber to provide a sub-assembly which is subsequently secured to the ceramic lid 15 by thermocompression bonding. Finally, the further metal member 4 is electron beam welded to the metal case 1 to provide a seal to the outer electrode region.
The electron beam weldin~ referred to herei~ may be replaced, in practise, by any other low energy welding technique such as laser welding or micro-pulsed TIG.
The geometry of this seal is such that th~
abutting portions provide a joint which is essentially -self-jigging. Consequently, the electron beam welding process is facilitated, allowing a high welding yield.
Re~errin~ now to Figure 3, the substantially thicker further member 4 is formed as an annular member. Preferably, the first metal me~ber 5 is sandwiched, at its ou~er periphery, between a further annular member 12 and the said substantially thicker ~ur~her member 4, and is secured thereto by electron beam welding. Subsequently, the first metal member is secured to the ceramic lid 15 by the earlier described thermocompression bonding process. The annular member ~ is finally secured to the metal casing 1 by electron ;~
beam welding to provide a seal to the outer electrode region. Conveniently, the annular members 4 and 12 may be rings slit ~rom a tube, thus allowing efficient ~aterial usage.
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United Kingd~m P~tent Offic~
SUBSTiTUTE SI~EEF PCT Internatio~el 2 ~ r U8 F~bru~ I990 - 10 - ~ - Z~

Conveniently, it may be arranged for any of the embodiments previously described that a plurality of the first metal members are bonded to respective insulating ceramic elements simultaneously by stacking said first metal members and said insulating elements - and applying bonding pressure and heàt to the stack.

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SUBSTITUTE SHEE~ c~erna~io~

Claims (24)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of constructing an alkali metal energy conversion device having an external casing (1), a solid electrolyte element (14) dividing the interior of the casing into two electrode regions, an electrically insulating element (15) joined to the electrolyte element, at least one first metal member (5) sealed to the insulating element (15), characterised by the steps of securing the first metal member (5) to a substantially thicker further metal member (4) and sealing the first metal member (5) to the insulating element (15) by thermocompression bonding, and securing the further metal member (4) to the casing (1).

2. A method as claimed in Claim 1, characterised in that a second metal member (2, 12) is secured to the first metal member (5), a portion of said first metal member (5) being between said second metal member (2, 12) and said substantially thicker metal member (4).

3. A method as claimed in Claim 1 or Claim 2, characterised in that the first metal member (5) is secured directly to the insulating element (15) by thermocompression bonding before joining the insulating element (15) to the electrolyte element (14).

4. A method as claimed in any preceding claim, characterized in that a plurality of first metal members (5) are bonded to respective insulating elements (15) simultaneously by stacking said first metal members (5) and said insulating elements (15) and applying bonding pressure and heat to the stack.

A method as claimed in any preceding claim, characterised in that the first metal member (5) is annular having a central opening through which extends, insulatingly spaced from the first metal member (5), a current collector (21).

6. A method as claimed in Claim 5, characterised in that the first annular metal member (5) is secured by thermocompression bonding to the insulating element (15) over a narrow region at the inner periphery of the first annular metal member (5).

7. A method as claimed in Claim 5 or Claim 6, characterised in that the pressure to produce bonding is applied through an annular metal washer of a material which becomes bonded to said first annular metal member.

8. A method as claimed in any one of Claims 5 to 7, characterised in that after the first annular metal member (5) has been secured to the insulating element (15) and the electrolyte element (14) has subsequently been joined to the insulating element (15), the external casing (1) is welded around its periphery at one end to seal the outer of said two electrode regions.

9. A method as claimed in any one of Claims 5 to 8, characterised in that the insulating element (15) is a disc shaped ceramic lid for a tubular electrolyte element (14) and an inner metal element (9) is secured to the insulating element (15) by thermocompression bonding to provide a metal element for sealing around a current collector (21) which is subsequently inserted through the inner metal element (9) and through an aperture (22) in the insulating element (15).

10. A method as claimed in Claim 9, characterised in that the inner metal element (9) is bonded to the insulating element (15) at the same time as the first metal member (5).

11. A method as claimed in Claim 9 or Claim 10, characterised in that the pressure to produce bonding of said inner metal element (9) to the insulating element (15) is supplied through an annular metal backing element (10) of a material which becomes bonded to said inner metal element (9).

12. A method as claimed in any one of Claims 9 to 11, characterised in that said inner metal element (9) is formed as a flat annular sheet having an outer diameter which is less than the inner diameter of said first metal member (5), the inner metal element (9) being thermocompression bonded to the insulating member (15) about its outer periphery only.

13. A method as claimed in Claim 12, characterised in that a metal strengthening washer (8) is bonded to the inner metal element (9) adjacent its inner periphery, the outer diameter of the washer (8) being less than the diameter at which the inner metal element (9) is sealed to the insulating member (15), the current collector (21) being sealed in electrical connection to the washer (8).

14. A method as claimed in any preceding claim, characterised in that the insulating element (15) is formed or alpha alumina and the electrolyte element (14) is formed of beta alumina.

15. An alkali metal energy conversion device having an external casing (1), a solid electrolyte element (14) in the casing to divide the interior into two electrode regions, an electrically insulating element (15) joined to the electrolyte element (14), and a first metal member (5) secured by thermocompression bonding to the insulating element (15) characterised by a substantially thicker further metal member (4) bonded to said first metal member (5) and joined to the external casing (1) to seal off one of said electrode regions.

16. A device as claimed in Claim 15, characterised by a second metal member (2, 12) secured to the first metal member (5), a portion of said first metal member (5) being between said second metal member (2, 12) and said substantially thicker metal member (4).

17. A device as claimed in Claim 15 or Claim 16, characterised in that the external casing (1) is cylindrical, the electrolyte element (14) is tubular and the first metal member (5) is annular having a central opening through which extends, insulatingly spaced from the first metal member (5), a current collector (21).

18. A device as claimed in Claim 17, characterised in that the first metal member (5) is secured by thermocompression bonding to the insulating element (15) in a narrow region at the inner periphery of the first metal member (S).

19. A device as claimed in Claim 18, characterised in that the first metal member (5) is sandwiched between a metal backing washer (6) and the insulating element (15), being thermocompression bonded to both in an annular region.
.

20. A device as claimed in any one of Claims 17 to 19, characterised in that the insulating element (5) is a disc shaped ceramic lid for the tubular electrolyte element (14) and there is an inner metal element (9) secured to the insulating element (15) by thermocompression bonding to provide a metal element for sealing around a current collector (21).

21. A device as claimed in Claim 20, characterised in that the inner metal element (9) is sandwiched between an annular metal backing element (10) and the insulating member (15), being thermocompression bonded to both in an annular region.

22. A device as claimed in Claim 20 or Claim 21, characterised in that said inner metal element (9) is formed as a flat annular sheet having an outer diameter which is less than the inner diameter of the first metal member (5) and the inner metal element (9) is thermocompression bonded to the insulating member (15) about its outer periphery only.

23. A device as claimed in Claim 22, characterised by a metal strengthening washer (8) bonded to the inner metal element (9) adjacent its inner periphery, the outer diameter of the washer (8) being less than the diameter at which the inner metal element (9) is sealed to the insulating member (15), and the current collector (21) being sealed in electrical connection to the washer (8).

24. A device as claimed in any one of Claims 15 to 23, characterised in that the first metal member (5) and/or the further metal member (4) are formed of an iron based or nickel based alloy resistant to chemical attack by sodium and/or sulphur/sodium polysulphides.