CA2147121A1

CA2147121A1 - Electrode structure for nickel metal hydride cells

Info

Publication number: CA2147121A1
Application number: CA002147121A
Authority: CA
Inventors: Douglas Blake Frye; Saverio F. Pensabene; Vincent J. Puglisi
Original assignee: Individual
Current assignee: Edgewell Personal Care Brands LLC
Priority date: 1993-08-27
Filing date: 1994-08-26
Publication date: 1995-03-02
Also published as: EP0667040A1; CN1122168A; BR9405559A; US5478594A; JPH08502856A; KR950704825A; WO1995006333A1; AU7637494A; SG49023A1

Abstract

An electrode is disclosed for use in a wound nickel metal hydrido electrochemical cell. The electrode has an electrochemically active material carried on the substrate. The electrode includes the improvement of an effective amount of an elastic binder coating the outer surface of the active material to enhance the integrity of the electrode and to substantially inhibit infantile shorting during operation thereof.

Description

PCT/US94/09571 (-- 21';7121 ~npro~ed Electrode Structure for Nickel Metal ~ydride Cc115 Bacl~r~L~nd o~ the Invention 1. Field of the ~rention The present u~ention relates to unproved electrodes for ~ able ele~lluchG~cal cells, and more particularl~, to improvet elcctn~dcs for n~ckel metal hydnde cells. Specifically, the present invention relates to an impro~ed electrode design for niclcel metal hydride cells which ic~ntly inhibits the in~ntile shorting of the wount cells.
. Descr;~t;GIl of the Prior Art Generally, the rn~nu~-hlre ant use of t:lectrodes for electroche~ic~l cells, inr.~ ine nickel metal hydrite cel~s, is Icno~r to those skiUed in the art. Such electrodes usu~lly inclute a porous or p~ur~t~ ~dl ctive meta~ substrate onto which is deposit~A a layer of electro~ m;~-slly active material T~e electro~"i~;rLIly active material may be applied to the substrate as a pressed or pasted m~ttri~l wl,~.~. an organ~c binder provides m~ iç~l integ~ty. The pasted substrate is then subjected to mild heat and pressure ~n order to form a co~ a~,t layer of active rnaterial on the substrate A~tematively, both positive and negati~e electrodes rmay be of a sincered desi~n. For exarnple, a perforated or wire mesh steel substrate rnay be sintered ~4ith a carbonyl niG~cel po~rder layer or l~yers to folm a porous electrode plate.
The re~1t~nt porous plate is then conven~io~lly i~ Jr~,n~red w~th a solution of the electroch~ic~11y acti~e msterial. Such t~l ni.lues of electrode fc~ tion are well hlown in the art.
Once ~he elestrodes are forrned, they mulst then be m~nllf~7~ed into a cell. ln one method of m~m~f~ct~l~ng aylindncsl cel~s, a ne~gative electrode plate and a positive electrode plate are altelT~ed ~rith two pieces of separ~l~,r m5lt~ri~1, such as ~yl~n. This A~SP~hly is then wound into a spiral such that the s~rfaces of the positive and negative electrode plates are Ju~ctaposed throu~h~ut the cell T~ese cell components are generally wo~nd around a removal arbor which is positioned within a nest having a di~m~ter approxir~ately that of the container of the electrorl~ic~l cell. U.S. Patent No 4,8l47242; ~o. 4,765,799; and ~o. 4,929,519 all disclose Ni-Cd e~ ~ical ~ells using latex binder materials of sorne sort tO bind the active PCT/US94/09571 2 14~ t 21 ~aterial together. Hourever, some are directed to Ni metal hydride cells nor the problem discu~s~ below.
One of the tlifficvl1ie~ with the development of the ruckel tnetal hydride cells, inr.]~rling the AB2 and ~BS ~lloys, is that they have had a tendenc~ to be somewhst brittle and infl~hle Tllus, when the electrodes are bent when being wound, there is a t~nr~e~ry ~or the electror11emi~lly acti~e ~ ;al to spall and flake. This t~n~en~y can at times cause penetrahon of the separator rn~t~ l, and shorting of the cell. This is particularly true during the ir~itial or infantile stzrt-up use of the cells, with up to 10% of cells ~requentl~r bein~ lost throuE~h intemal shorting. Thus, there remains a need for developing niekel metal hydnde electrodes, both positive and negal:i~e, which are much more flexlblS ha~e greater int~ and do not spall or flake du~ wind~ng ~thout ~ ;..g the cycle life ofthe ceil.

S~m~ ot`the In~ention Accordingly, it is one object of the present in~ention to provide an improvet electrode design ~hiCh inhibit~ in~r~tile shorting.
It is another object of the present invention to prov~de a nickel metal hydride electrode whic~ is flexible for purposes of ~nding ant slitting so as to prevent flaking of the electroch~mic.~l ~ctive material.
Yet another object of the present invenhon is to pro~de a sintered negative metal h$dride electrode which has retuced ele~tncal shor~ng te~dçnri.os yet enhanced cycle life.
To ach~eve the foregoing and other objects and ~n accordance with a purpose of the present irlvention as embodied and broadly des~ribed hereln, an clectrode is disclosed for use in a ~rould nickel metal hydride electrochemical cell. The electrode also includes an effective amount of an elastic binder Goating which coats the outer surface of the acti~e mAt~rj~l to enhance the integrity of the electrode and to subst~ntiAlly inhibit infantile shorting during the operation thereof L ~ l / U 7 1 U Y ~ 1 4 ~ 1 2 1 Brief Dacription of the D~aw~ngs The ~G~TTlpor~ g tra~g which is iJ~co~lJo~atet in and forms a part of the specifi~t;o~
il~ustrates a y~ r~d ~mho~liment of the present invention and, toget~er with a de~cliytion.
serfes to e~plain the p~ ir~s o~the invenkio~L II2 the d~ ~iAgs:
~ ig. 1 is a cros~s~iono.1 vie~r of one ernhot~ime~ of a wound electrode o~cs~ y of the pre~ent in~ention as in~t7O11ed w~thin the cor ~inPr of a sea1ed electn cl ~ .irol de~ce.

Detailed Drsr il,lion of the Preferred Embodiment As lisc~ssed abo~re, the present inve~tion relates to coa~ed electrodes for rccL&r~,eable nickel metal hytride de~ orl.~ r~ I cells ~ i~rte the same. R~ to Fig. 1, a sealed electro~ n.ical cell lO in~ludes a c~oJ~inPr melT-her 12 ha~rin~ an ~nner surface 14 and a wound electrode ~csp~hly 20. The elec~rode ~sPlnhly 20 is dimensioned and configured to be dispo ed within the cQ~t~inP~ 12 tightly agaulst the surface 14. The c~ nt~ r 12 may be constructed ~om any kI-O~II rn~t~n~l in the art suitable for its purpose, such as n~ckel pla~ed steel.
Referr~ng with more paIticularity to the electrode assembly 20, a negative electrode plate 22 is illustrated and inr,ludPs a substrate memhPr 24 and an electrorhrn~ lly active m~t~ l 26.
As illustrated, the electro~ G~Ily a~ve matenal is secured to both faces of the substrate 24 and may be securet to the substrate 24 in any rnanner kno~rl in ~e art such as by pressing suitable paste, Yhich is a rI~xture pnmarily of active rnatenal and binder, onto and through the pc.rolalions of the porous substrate 24. Alte~natively, the active matenal my be preferably bondet to the substrate 24 by mans of cim~lt~eous e~trusion ~om a cross-head die in an organic slurTy co~ e the active mater~al in suspension onto both sides of the substrate7 thereby perrneating the substrate and rO~ ,g ~ layer on ea~h side thereo As previously indie~ted, the electrocherr~cally active matenal may also be 5intered to the substrate plate 24.
Likew~se, a positive plate 28 is illustrated and includes a substrate matenal 30, and an electrochem~c~ly active material 32 secured to both faces of the substrate 30 Ag~in, the electrochemically active mate~ial 32 may be secured to the plate 30 in anS/ known manner as discussed aboYe.

~ v ~ V J ~ ~ ' Egl-n22A ~147121 The selection of materials for the substrate and the electro~he~ie~lly active ~natenal may be any ~s desired and known in the nickel metal hyd~de ~t depending on the cell desired In ~dtlitinn the positive pl~te 28 and the negative plate 22 ~e sepat~l~,d by a pair of separator sheets 34, 36. The se~ator sheets 34, 36 are aga~n of standard desi~n in the art and enable flow t~ough of electroch~mirq11y active material ~ithout direct electrical contact ~,L~ n tlle positive and negati~e plates. In the ill~st~ted çmbo-limPnt the nega~ive plate 22 is coated with an elastic binter la,~er 38 in accordance with ehe present invention. The positive plate 28 may also be co~ted as tescnbed belo~ thouEh t~is coating is ~ot illustrated in Fig. I
for cimrliafy and cl~ztion purpo~es.
As both t~e ne,;~L~. and positive pl~tes 22, 2B a~e wound to fonII the cell 10, there has pre~iousl~ b~n a ter~dency for the elect~och~m~lly active m~tçn~l 26, 32 to splinter, flake and spall. Unfortunately, this sQrn~imeo7 resulted in the sepal2~tol~ 347 36 being permeated thereby causing shorts in the cell 10, pamcularly dunng initial start-up opera~ion. Such i~utial shortulg of the cells is termed inf~ntile shorting ~nd t~pic~lly has resulted in at lea~7t 10% and fiequently more of the new cells having to be discardet res"llin~ from un~rc~Ftable cell performance due to sueh infantile shorting. Urhile pnor t~ickel cadmi-~m rechargeable cells expenenced a certain ~nount such infi~ntile shorting, this problem became acute with nickel metal hydride cells due to the more brittle nature of the AB2 and ABS alloys utilized in the electrodes o~ such cells7 and in pa~icular the negati~e electrode It ~as disco~ered that if the electrode surface ~as coated ~rith a layer of an elas7tiG binder matenal of sufficient $1~ rn~s~, this tendency for the electrochemically active matenal to flake and splinter was substantially reduced. When this tendency wa~7 red~l~ed, it w2s discovered that the ~rnount of infantile shorting being expenenced was also subst~nti~lly reduced. It was tetermined that a prima~y cause for this redu~tion was the fact that the elastic binder material, when present in a~ effective amount, atded subst~nti~l flexibility to the electrode thereby off-setting to some degree the brittle nature of the electroG~ lly active materlal. In this manner, the electrodes can be woulld In stal-dard fashion without the electrochemically acti~e matenal being spalled and flaked thereby reducing loss resulting from cell shorting.

PCT/U594/09S71 2 ~ 4 7121 E91 `22A

~ hile any elastic binder material suitable for the above described fimction may be util~d with tbe present invenhon, i~ is prefc,l~d that this m~tçn*l be a hytrophobic polymer having strong binding properties as welI as high elasticity. The binding pr~pe,ties help confine the surface of the clc~lro~.F . ;r~lly ac~e mste~ial ~ight to the substrate l~ost prefe~bly, the binder coating is s~ler,fPd froJn styrene ethylene~butylcne-styrene block copolymers m~n~f~rtllret Imder the trad~n~me KRAYI O~ by the Shell Chemical G)i-,pa~.
The KRAYTON coating not o~Iy coats the electrotes to reduce flalcing, b-tt it also improves the ,.ltegrit~ of the electrode structure. The E~RAYTON coating is present ~n ~n eec~ve amollnt to achieve these filnctions, ant that effective amount is preferably no greater than app,o~ ately .002 inches thi~esc Therefore, only a light coating ~s n~ s~ to accomplish the above. If the ~ir~C ~e;Q--~s too great, the l~ pho~.c nature of KRAYION may cause problen~s turing disc~rge in~cm~lch as the ele~ d~3 then becomP rate sensitive Ho~rever, it has been discoveret that the KR~TON a~so reduces the chance of tile electro~ emt~ y acti~e alloy to oxidize durirlg cycle life thereby pr~ cr~i~ the cycle life of the cell if the KRA~TON is not present in too great of an a~ount. Sin~e oxi~atiQn ofelectroch~mic~lly active alloys is a prima~y cause of cycle life failllre of nickel metal hydride cells, this unexpected S~nction is an added advantage to the pr~r~ d KRA~TON coating Moreover, in the ABS electrochemical cell st~ucture, the ABS alloy ~ends to ~row and expand ~hen cycled thereby losing its cornracted nature. ~his tendency previously migrated has the electror-lemi~pl active material into the separator during suGh expansion, and also durin~ ini~ial construction as a result fiom ultrasonic welding. The KRAYTON coating pro~des atditional integnty to the electrode thereby preven~g such expansion into the separator.
~ he bin~er coating may be applied to the positive and negative clec~lc,des utili7ing any number of available tec~niques In one instance, the electrodes may be s~mply dipped into liquid KRAYTO~ or other liquid binder, dep~nrlinE upon the selection. For purposes of discussion herea~er, the preferred binder coatmg K~AYIO~ ~ill be rer~ll~ to although it is to be under~tood that any suitable elastic binder materi~l as dcscnbed above may be u~ilized. Another manner of applyin~g the KR~YTON coating is to dip the electrodes into liq~lid KRA~TON, and then ~pe or sq-leP!~ it off as to elimin~t~ y excess rnaterial. The KR~YTON coating may also be simply wiped or pamted on to the electrode surfaces, or, in tl~e alte~native, it may be sprayed on in l~yers. ~ any event, the liquid KRAYTO~ is appliet to t~e sur~ace of the elec~de in ap~rup,iate t~ s~c and then allowed to try.
It is belie~ed t~at the KR~YTON will tend to penetrate any pores in the electrode stlucture thereby prov~ding the enh~nr~ ~nteg~ity pre~riously ~lis~lsse~l Moreo~er, certam electrode stmctures include grooves or other dimples in the substrate surface in order to ~ssist in ad~enng the el..,lro~ G~Tly ac~e rnaterial to the substrate. Such structural fo~rns in the substrate may also be s~lecte~ in the over-aII electrode forrn after the electrocll~mic~lly acti~e rr~terial has been d~os~ onto the substrate. It has been discovered tha~ the RRAYTO~
coating ~ill tend to fill the dips ant gaps in such grooves or timples there~by ~ 5ishrl~ in the prevention of flaking and spalling by physicslly holding the splinter or flalce in place, and not allowing it to spall off as the electrode is wount.
Other ad~rantages to the KE~AYI ON coati~g have been dis~o~eret. One such ad~ance is in redu~cing the arnount o dust generation dunng the winding process resulting from dust particles being discharged ~om the electrochernically material as the electrode is being wound.
Moreover, it has been discovered that the K;RA~ON coatin~ assists in cutting or slicing the electrodes dusing m~n.~ re, since the electrode plates are generally made in large sheets, and then cut to size depending upon the p~ticular electrocllPInic~l cell size into which the electrodes are to be placed. Pre~ious to utili7in~ the KRA~TO~ coating rli~ol~se~ abo~e, such slicing or cutting operations tended to spall some of the electrochemical material from the substrate at the edges of the slice as well as the generated dust during the slicing operation. The ~RAYTO~
coating assists in decreasing the amount of d~st ~ne- ~tion, and spalling dur~ng slicing operations.
As previously discussed, it had been indicated that at least 10% of niclcel metal hydride cells were being lost due to infantile shorting. In certain instances, the scrap rate has been as high as 20-50% pTior to utilizing the KR~TON coating as described abo~re. As a result of applying the KRA~TON costing in accordance with the invention descnbed above, the windin~

~CTI'~S94/09571 21 4 7 1~1 E91- ~2P~

losses ha~e been shown to be reduced by a factor of 5-10%. Mo~eover, the reduction of infantile shortage losses as cc~ ared to controls ha~, been upward to 50%. Thus, the present invention has signi~ntly reduced the scrap rate of n~ckel metal hydïide cells resulting from the aforem~ntio~A problems.

EXA~IPLE I
In one e~ ent, four types of po~ e sLt~ d niclcel eiectrodes were produced for tes~, in I~ickel metal hydride cells. These elect~otes were as follows:
Type A: inrlllde~ a siIltered plaque which ~;vas very pliable, soft, auld had notend~ncy for the surface to b~eak into sharp ~ut~u~,;ons. Thig was placed on theelectrode substr~te.
Type B: a standard negative electrode plate ~as m~ntlhctured, ~nd then coated v~th a KRAYTO~ binder as descnbed above.
Type C: a standard plate ide~tir~l that of Type B waF, rrl~m~f~red, and then onl~
coated at the leading edge w~th the KRAYTON binder, which leadi~3 edge is utilized in star~ing the winding dunn~ roll ~s~bly.
Type D: t~e standard plate ~as m~n~ çhlred, and utilized as in electrodes B and C, but had no surf~ce treatm~nt ~hatsoe~er.
Each positive electrode A-D was wound with a metal hydride AB2 negative plate utili~in~ a nylon separator, and the same winting rn~rhi-le. For each type A-D, 50 electrode assernblies were wound, and placed in ele~trochem~cal oell cans to foml electro~herniG~I cells.
These cells ~ere then placet into operation, and the shorts were deteGte~l. The results were as follows:
Type A. 0 infan~ile shorts Type B; O inf~n~ile shorts Type C: 4 infantile shorts Type D 4 inf~ntile shorts The results of this partiGular Example I indicated that the Type D, sintered r~ckel metal electrodes for nickel metal hydnde cells ~enerated 8% shorts overall. It was a~so clear that by pc /uss4/oss7l 2147121 simply coating the leading edge with KRA~ON as indiC~tçd by T~pe C pro~ided no additional change or re~ucti~n ~n shorting. Type B cells constru~ed i~ accordance with the present in~enhon clearly indi~ted that the present invention ~rould protect against infi~ntile shorts as described abo~e. The ~pe A cells also ;~di~led that pliable, and soflc sintered nickel electrode surfaoes which res~st brea1cage and de~relopment of sha~p protrusion also will reduce the number of inf~nble shorts, thereby further 5UppCS. Ii~g the theo~r of inf~ntil~ shorts as tescnbed aboYe is cor~ect. Afi:er testing, the abo~e cells were taken ~pa~t and analyæt, and this analysis clearly indic~t~d that the above shorts for Type C and D cells were the result of shasp prottusions ~assing through the separators.

E~AMPLE ;~
In ~his eY~mE~lç~ nickel metal hydride 4/3A cells were made uti~i7ing ABS comrarte~
neg~hve electrodes coated with about a .002 inch KRAYTO~ layer. The control electrodes had no KRA~TOl~, and the test cells were divided into two categories - one and ~vo passes (applications) of KRAYTON. 'rhe res~llts are as follo~s % Shorta~e/Fa~lures Control 1~ 4 1 la~lrer 7 3 2 laYer 7.1 As can be seen, this text clearly illustrated that the KRAYTON coat~ng significantly reduced or inhibited infantile shortages of these cells.

EXAMPLE m Yet another test of the present invention was p.. ~~ ed wherein shorta~es due to rnetal particle ~bration through the separator resulting frorn ultrasound welding of cells w~s e~raluated In this irlst~nrp~ the same electrodes and cell types of EXA~I,E I~ were utilized in this test.

~PCT/US94/09571 æl47 121 The result are as follow:
No. Tested % Shortage/Failure Control 12 ~0 I pair l9 16 2 pair l9 5 Again, the benefit~ of the present invention a~e de-arly illustrated by the results obtained, and show that sho~ages due to mPn~1f~ch1~ng pro~ssir~g can also be subsl~h~ ly inhil3it~d b~
the presellt ~nvention.
~ s c~ be seen fiom the above7 nickel metal hyd~ide cells have experienced an unacceptable high loss rate due to electrical shortage at start up r~su~ from flaking and spalling oft~e electrochem~cally acti~e m~tP~ l ant sholt~ng out tbe cells. It has been discovered t ~at L~p~ards to about 90% of ~his shortage loss results from flalcing of the negative electrode, which the balance beir~g attributed to n~g of the positi~re electrode. The present invention is a s~mple yet highly effective te~-~ ri~ e fol siErlific~nt7y re~luei~ this electr~cal shortage loss and br~n~g the loss to ~ithin acceptable limits. The presen~ in~ention, moreover, enb~nres t~e integlity ofthe electrodes as we71 as reduces environ~nent~1 problems dunn~
m~nl~f~c.~lre ofthe electrodes and ~inding ofthe cells. Consequentl~, the present invention not only proi~des significant eCorl~nic advantages due to reduced losses but also Pnh~nced m~n~ ring procedures.
The foregoir~ descnption and the illustrative clnho~1ime~ts of the present ~nvention have been descnbed in detail in varying mo~ific~t~o~t and alternate embo~imPnts. It should be understood, ho~ever, ~hat the foregoin~ des~ription of the in~ention is exemplary only, and that the scope of the invention is to be lirni~ed only to the c1aims as interpreted ~n ~riew of the pnor art.

Claims

WE CLAIM:

1. In an electrode for use in a wound nickel metal hydride electrochemical cell, said electrode having an electrochemically active material carried on a substrate, the improvement comprising an effective amount of an elastic binder coating the out surface of said active material to enhance the integrity of said electrode and to substantially inhibit infantile shorting during operation thereof.

2. The improvement in accordance with claim 1, wherein said binder coating is present in an effective amount to substantial reduce flaking and dust generation during winding of said electrode without limiting cycle life.

3. The improvement in accordance with claim 2, wherein said effective amount comprises up to approximately .002 inch in thickness.

4. The improvement in accordance with claim 1, wherein said binder coating comprises a hydrophobic polymer material having strong binding properties and high elasticity.

5. The improvement in accordance with claim 4, wherein said binder coating comprises a stryrene-ethylene / butylene-styrene block copolymer.

6 The improvement in accordance with claim 1, wherein said electrode comprises a nickel metal hydride negative electrode.

7. The improvement in accordance with claim 6, wherein said electrode comprises a sintered nickel electrode.

8. The improvement in accordance with claim 6, wherein said electrode comprises a pasted nickel electrode.

9 The improvement in accordance with claim 1, wherein said electrode comprises a nickel metal hydride positive electrode.

10. A sintered nickel metal hydride electrode having enhanced electrical and cycle life properties comprising a substrate, electrochemically active material sintered to said substrate, and an outer layer covering said sintered active material and including an effective amount of an elastic hydrophobic binder to enhance the integrity of said electrode and to substantially inhibit infantile electrical shorting during use thereof.

11. The electrode as claimed in claim 10, wherein said effective amount comprises no greater than approximately .002 inches thickness.

12. The electrode as claimed in claim 10, wherein said binders are present in an amount of effective to reduce flaking upon winding of said electrode and to enhance the slitting capability thereof.

13. The electrode as claimed in claim 10, wherein said binder comprises a hydrophobic polymer material having strong binding and high elasticity properties.

14. The electrode as claimed in claim 13, wherein said binder comprises a styrene-ethylene/butylene-styrene block copolymer.

15. A process for producing an electrode for use in a nickel metal hydride cell having reduced infantile electrical shorting comprising:
forming an electrochemically active material;
applying said active material to a substrate; and coating said substrate and electrochemically active material with an effective amount with an elastic binder to enhance the integrity of said electrode and to substantially inhibit infantile shorting during use thereof.

16. The process as claimed in claim 15, wherein said substrate and said active material are coated by dipping said substrate with said active material thereon into said elastic binder and then drying the same.

17. The process as claimed in claim 15, wherein said substrate and said active material are coated by spraying said elastic binder onto the outer surfaces of said active material.

18. The process as claimed in claim 15, wherein said substrate and said active material are coated by painting the surfaces thereof utilizing brush means for applying said binder to the outer surfaces of said active material.

19. The process as claimed in claim 15, wherein said elastic binder comprises a hydrophobic polymer material having strong binding properties and a high elasticity with an effective amount thereof comprising approximately .002 inches in thickness or less.

20. The process as claimed in claim 19 wherein said elastic binder comprises a styrene-ethylene/butylene-stryrene block copolymer.