Note: Descriptions are shown in the official language in which they were submitted.
201 59 ~1 u8 ~ebrua~ 1990
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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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SUBSTITUTE SHEE~ , PCT In~ernatlf,~.~l Appllcatlon
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tJ8 Febnu~ IY90
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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
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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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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
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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 ~`
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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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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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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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SUBSTiTUTE SI~EEF PCT Internatio~el
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U8 F~bru~ I990
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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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