CA2117748C

CA2117748C - Metal hydride cells having improved cycle life and charge retention

Info

Publication number: CA2117748C
Application number: CA002117748A
Authority: CA
Inventors: Michael A. Fetcenko; Stanford R. Ovshinsky
Original assignee: Ovonic Battery Co Inc
Current assignee: Ovonic Battery Co Inc
Priority date: 1992-05-07
Filing date: 1993-05-05
Publication date: 1997-03-25
Anticipated expiration: 2013-05-05
Also published as: KR100262778B1; DK0639295T3; DE639295T1; NO944216L; NO944216D0; CN1048588C; EP0639295A1; CN1080783A; EP0639295B1; WO1993022801A1; PH30142A; DE69329906D1; RU94046043A; DE69329906T2; IL105597A0; RU2121198C1; KR950701457A; US5330861A; EP0639295A4; IL105597A

Abstract

A rechargeable hydrogen storage cell comprising a negative electrode having the composition: (Ovonic Base Alloy)aMb where Ovonic Base Alloy represents an Ovonic alloy that contains 0.1 to 60 atomic percent Ti, 0.1 to 25 atomic percent Zr, 0.1 to 60 atomic percent V, 0.1 to 57 atomic percent Ni, and 0.1 to 56 atomic percent Cr, as described above; a is at least 70 atomic per cent; M represents at least one modifier chosen from the group consisting of Co, Mn, Al, Fe, W, La, Mo, Cu, Mg, Ca, Nb, Si, and Hf; b is 0 to 30 atomic percent; b > 0; and a + b = 100 atomic percent; a positive electrode; and a separator of electrolyte ret-entive nylon or wettable polypropylene.

Description

93/22801 2 1 1 7 7 4 8 PCr/US93~042~3 ~o METAL HYDRIDE CELLS HAVING IMPROVED
CYCLE LIFE AND CHARGE RETENTION
FIELD OF THE INVENTION
S The present invention relates generally to improved metal hydride eells having improved cyele life and eharge retention. More ~ icul~uly~ the invention relates to the of the negative eleetrode alloys and the separator in order to improve eycle life and charge retendon.
BACKGROUND OF TrlE INVENTION
In Icl,ll~u~,~,~lc ~ ee ls, weight and portability are important c~ It is also a lv ~UIt~"~A~U 7 for lc~ u"-,~lc cells to have long operating lives without the neeessity of periodie Re~ u~ablc eells may be used as direet rll5 ~ for primary AA, C, and D eells in numerous eonsumer deviees such as c:~lrlllot~7~, portable radios, and flashlights. They are often configured into a sealed power pack that is designed as an integral part of a speeific device. R~ lr clc.,L,ucll~..lli.,l eells ean also be eonfigured as larger ee ls that ean be used, for example, in industrial, aerospace and eleetric vehicle ..~
The best lc~ eell is one that can operate as an "install and forget" power source. With the exception of periodie charging, a lc~.ll~"~ eell should performwithout attention and should not become a limiting factor in the life of the device it powers.
There are two basic types of niekel metal hydride IC~.Il Ul;~.alJIC hydrogen storage materials ("Ni-MH materials") the AB2 type and the AB5 type. These types of material are diseussed in detail in U.S. Patent No. 5,096,667 to Fetcenko, et. al, the eontents of whieh are r I herein by reference. The term "Ovonic alloy" is frequently used to rcfer to a l AB2 type materials in deference to their dc ~ . from amorphous thin film materials diseovered by Stanford R. Ovshinsky. Ovonie alloys are described in U.S.
Patent No. 4,551,400 for Hydrogen Storage Materials and Methods of Sizing and Preparing the Sarne for Ele~/ . ' ' Ar~7~i~n~7~7ns (hereinafter the '400 patent) to Sapru, Hong, Feteenko and Venkatesan, the eontents of whieh are illl,UllJI ' ' herein by reference.
As used herein, the term "Ovonic Base Alloy" refers to an AB2a7l10y having a base alloy or grain phase (as this term is described in the '400 patent) containing 0.1 to 60 Wo 93/22801 ~ ~ 17~ ~ 8 PCI/US93/04253 atomic percent Ti, 0.1 to 25 atomic percent Zr, 0.1 to 60 atomic percent V, 0.1 to 57 atomic percent Ni, and 0.1 to 56 atomic percent Cr.
In general, Ni-MH hydrogen storage cells or batteries (referred to collectively as "Ni-MH cells") utilize a negative dectrode that is capable of the reversible 5 cl~ r 1 ~ 1 storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electt~des are spaced apart in an aDcaline electrolyoe.
Upon application of an electrical potential across a Ni-MH cell, the Ni-MH
material of the nogative electrode is charged by the rl rl .r ~ absorption of 10hydrogen and the ~l~hu~,h~ l;~l evolution of a hydroxyl ion:
cb~e M+H20+e~ t > M-H+OH
~' The negative electrode reactions ate reversible. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron.
The reactions that take place at the positive electtode of a secondary cell are also reversible. For example, the reactions at a nickel hydroxide positive dectrode in a Ni-MH
ce11 are:
d~e Ni(OH)2 + OE < > NiOOH +H2O + e ~0 A suitable separator is usually positioned between the electtodes of Ni-MH cells.
The electrolyte is generally an alkaline electrolyte, for example, 20 to 45 weight percent potassium hydroxide. Lithium hydroxide may also be present in limited quantity.
A Ni-MH cell has an important advantage over Cul~ ,~le, cells and batteries: Ni-MH cells have sig.lill~ .lly higher specific charge capacities (both in terms of ampere hours per unit mass and ampere hours per Imit volume) than do cells with lead WO 93/22801 l 3 Z I 1 7 7 4 8 PCl/US93/0~253 or cadmium nGgative electrodes. As a result, a higher energy density (in terms of watt hours per unit mass or watt hours per unit volume) is possible with Ni-MH cells than with I,u~ Liulldl systems, making Ni-MH cells ~ suitable for many ~ mm~
R~l.o~ cells are generally either vented cells or sealed cells. During normal operation, a vented cell typically permits venting of gas to relieve excess pressure as part of the normal operatmg behavior. In contrast, a sealed cell generally does not permit venting on a regular basis. As a result of this difference, the vent assemblies and the amounts of dectrolyte in the cell container relative to the electrode geometry both differ Ci~ y Vented cells operate im a "flooded condition." The term "flooded condition" means that the electrodes are completely immersed in, covered by, and wetted by the electrolyte.
Thus, such cells are sometimes referred to as "flooded cells." A vented cell is typically designed for normal operating pressures of about 25 pounds per square inch after which 15 excess pressures are relieved by a vent m~r~ic~
A variation of the vented, cylindrical, .~ cells of the prior art are the "one time only" venting cells where, for example, a rupturable diaphragm and blade apparatus is employed. As irlternal cell pressure increases, the blade is forced against the ,lior~ ,m As the pressure increases further, the blade punctures the diaphragm, allowing excess gases to escape. This destructive type of venting m~r~o_;cm is both ll~rrAir~hlr from batch to batch and from cell to cell within a batch. Moreover, destructive venting is good for only one excessive pressure situation. After the diaphragm is punctured, it cannot even sustain normal cell operating pressures.
In contrast, sealed cells are designed to operate in a "starved" electrolyte cnnfi~lr~hr~n that is with a minimum amount of electrolyte. The enclosure for a sealed cell is normally metallic amd designed for operation of up to about lûO p.s.i. absolute or higher. Because they are sealed, such cells do not require periodic Typically, a sealed le.,l.~j~.~lc cell uses a cylindrical nickel-plated steel case as 5 the negative terminal and the cell cover as the positive terminal. An insulator separates the positive cover from the negative cell can. The electrodes are wound to form a compact "jelly roll" with the electrodes of opposite polarity isolated f~om each other by a porous, woven or non-woven separator of nylon or ~.ol~,lu~,~L,.~c, for example. A tab exunds from each electrode to create a smgle current path through which current is 10 distributed to the entire elecrrode area during charging and d;..~,l.~;i..g. The tab on each electrode is electrically connected to its respective terminal.
In sealed cells, the discharge capacity of a nickel based posirive electrode is limited by the amount of electrolyte, the amount of active material, and the charging ,rr;. 1 ... :~ c The charge capacity of a Ni-MH negative dectrode is limited by the amouM
15 of active material used, since its charge efficiency is nearly 100 percent, nearly a full state of charge is reached. To maintain the optimum capacity for a Ni-MH electrode, must be taken to avoid oxygen ' or hydrogen evolution before full charge is redched. This is generally ~ by providing an excess of negative dectrode material. However, I~ ~ A . ~ '"' 'S must be taken m the design and fabrication of 0 sealed cells to avoid the effects of over-~ ,.. associated with overcharge at IU~ high charge rates. Sealed cells are the preferred type of IC.,Il.U~,~,dlJlC, Ni-MH
h.. ;. ;11 cells where a particular applicaion requires a relatively - free power source.
The operaional life span, that is, the available number of charge amd discharge ~wo 93/22801 2 1 1 7 ~ 4 8 PCr/US93/04253 cycles of a sealed cell, typically determimes the kinds Of ~rF1in~*nnc for which a cell will be useful. Cells that are capable of undergoing more cycles have more potential Thus, longer life span cells are more desirable.
The life span of a sealed cell is directly related to the life span of its individual 5 ,.,,,1l... .,1~ The negative electrode materials are the most unique and have long been considered to be the cnmrnnr nt limiting cell life span. Therefore, to achieve longer life spans, . r ~ r ~ .i their efforts on producing an electrode tlloy material capable of withct~nr~ repeated charge and discharge cycles without breakdown. See, for example, U.S. Patent No. 4,728,586 for EnAranced Charge Retention E~e~hu~ "..cal Hydrogen Storage Alloys and an En~.ranced Charge Retent~on Ele~v~.r~,. cal CeL
However, Ovonic alloy negative electrode materials have been developed to the point where they are no longer the only component of the IG,I.~.able, cell that limit the life span of the entire cell.
The present inventors have found that in sealed Ni-MH cells using some of the Ovonic Base Alloys described herein, cell failure is sometimes the result of problems related to the separator, such as the depletion of electrolyte from the separator and separator r~r gr~ri~*r~n Thus, given the advances in Ovonic Base Alloy negative electrode materials described herein, the separator appears to be another factor im t-h-e cycle life and charge retention of the Ovonic Base Alloy cells of the present invention.
Nylon separators have been described in a variety of cnnfi~nr~*nnc and in a variety of cells. For example, U.S. Patent No. 3,147,1~0, to Carl et al. of Yardney, describes a battery usimg a separator made from a nylon film coated on fabric or resinous fiber. In addition, U.S. Patent No. 4,699,858, to Masaki of rlGud~.ll;..~;, describes a rechargeable aLcaline batte.y having an ion ttansporti.,g aLcaline electrolyte and a ~ `
r-~

21177~
Wo 93/22801 ~ PCrIUS93/04253 nonwoven polyamide separator made from continuous fiber of 3 to l0 microns treated with non-ionic surface active agents.
Generally, nylon separators of dhe type found in NiCd ~ batteries have been used as dhe standard separator in all types of Ni-MH cells. Such nylon separators 5 were specifically designed to prevent dhe short circuits that occur in NiCd cells due to the formation of dendrites between dhe electrodes as they change from a metal to a metal hydroxide in dhe .,;la~ /L~.,ll~K., cycle. These dendrites short circuit the NiCd negative terminal to the positive terminal if left unchecked. The nylon separator acts as a barrier laye} to prevent such dendrite formation.
Aldhough dendrite formation is not a concern in Ni-MH cells, nylon separators are frequendy used in Ni-MH cells because during dhe early ~v,' . of Ni-MH cells, nylon separators were readily available and appeared to function adequately widh ABs alloy cells, as well as with Ovonic alloy cells, of dhe prior art. However, the present invention recognizes that in the Ovonic Base Alloy cells of the present invention, prior 15 art nylon separators do not function adequately. The prior art nylon separators when used in Ovonic alloy cells are, for example, prone to breal~down and loss of electrolyte (drying out) after repeated cycling. Further, prior art nylon separators eventually react widh the electrolyte in Ovonic alloy cells fonning ~' , products which may adversely effect cell I ' As a result, nylon separators when used in Ovonic 20 alloy cells appear to be a significant factor in limiting the potential cycle life of such cells.
rUI~.u~,Jl~,..G separator materials have been used in lead acid batteries because of their resistance to sulfuric acid. For example, U.S. Patent No. 3,870,567, to Palmer et al. of W.R Grace, describes separators made from nonwoven mats that are ~

Wo 93/22801 PCI/IJS93104253 ~ 21~7748 to yidd small pores of high porosity. These mats are formed from l~yd.u~lluhil, polymedc matedals. The fibers ûf the mat are made wettable by mixing the polymedc resin with a wetting agent pdor to extrusion. This reference specifically teaches that polyolefins such as pol~,~.u~ are useful in lead acid battedes, and that nylon is the preferred 5 matedal for use in allcaline battedes.
The following art suggests poly~"ul!yh~ separators as an alternative to nylon because polyl,,u~ e is strong and resistant to all~aline electrolyte, al~hough its h.~Lu~ vb;c properties when compared to nylon require treatment with a wetting agent or its c ..~ ;..,. with amother matedal:
U.S. Patent No. 3,907,604, to Prentice of Exxon Research, descdbes a nonwoven pol~.u~Jyl~,.c mat that has been fuse-bonded using a press to increase its tensile strength.
U.S. Patent No. 3,947,537 to Butin et al. of Exxon Research, descdbes a process for making battery separators from nonwoven mats where the formed nonwoven mat is treated with a wetting agent, dried, heated, and c~ .1 to increase fiber-to-fiber bonding.
U.S. Patent No. 4,190,707 to Doi et al. of Asahi, descdbes a separator made of a porous polyolefm film having low electdcal resistance and high alkaline resistance.
U.S. Patent No. 4,414,090, to D'Agostino ûf RAI Research, descdbes a separator for a redox cell compdsing a polyolefin base film grafted to a vinyl substituted monomer with gamma radiation.
U.S. Patent No. 4,430,398, to Kujas ûf RCA, descdbes a pvl~,u~yh,l.~ separator for NiCd cells prepared from knitted, woven, or W093/22801 ~ 7~B PCr/[1593/04253 nonwoven yulyl~luL~h,llc that is treated with a corona discharge and then ' with phenylglycine or ~ ' y~hu~y~l~.,.lylglycine. The corona dischzrge functions to increase the wettability of the separator, and glycine derivative acts to prevent the penetration of the separator sheet by the S aLcaline electrolyte and metallic parLicles from the electrodes.
U.S. Patent No. 5,077,149, to llcoma of r~ ' describes a misch metal negative electrode, a nickel hydroxide positive electrode, and a sulfonated, non-woven ~oly~lulJ~L,.~G separator. The negative dectrode, the positive electrode, and the separator aU contain a zinc compound, such as zinc oxide, so that the electrolyte is retained in the negative electrode and the separator and does not migrate to the positive electrode, thus reducing the expansion of the positive electrode. In addition, the separator is treated with a hy,~ r resin. This patent states that expansion of the positive electrode causes a change in the electrolyte ~ , and an imcrease in internal resistance which makes nickeUhydrogen cells have an inferior cycle life compared to NiCd cells.
The plethora of references describing different l~inds of separators contains noindication that any one type of separator would be superior for any particular application or superior with any particular alloy or elGctrolyte. While all types of batteries have similar component parts, the extreme differences in chemistry make applying teachings from one type of battery to another I , ~idi.L~I~. "Swapping" Of ~- r ' does occur, but the present inventors are unaware of any instance where any research in the ~.,lu~ ,,.L of Ovonic alloys has been hastened as a result. For example, the use of NiCd nylon separators in Ni-MH cells as discussed above, proved adequa~e for all Ni-~093/22801 2~t PCIIUS93/042S3 MH alloys initially, howeve} these same separators now appear to be a limiting factor in realizing the full potential of the Ovonic Base Alloys of the present invention.
ullr~ , the prior art contains no theory or suggestion regarding a separator that might overcome this problem.
The It . ~ of the prior art is illustrated in U.S. Patent No. 5,077,149 ("the '149 patent"), to ~oma of rr ' discussed above. Read in its entirety, the '149 patent focuses on controlling the swelling of the positive electrode with zinc and fails to disclose any teaching relevant to the Ovonic Base Alloys of the present invention. The '149 patent teaches using misch met;al, an ABs Ni-MH material that those of skill in the art know has an rl-- l.~ 1.. ~1 behavior different than Ovonic alloys. A number of ~ ulur~u~c.~ became interested in AB~ alloys because these alloys appeared to be drop im ~ for NiCd negative electrodes. However, as discussed in detail in U.S.
Patent No. 5,096,667, AB5 materials represent a different class of materials from Ovonic alloys. This is ~!alLi~.~uly true of the Ovonic Base Alloys of the present invention.
It should be noted that prior art Ovonic alloys have yielded adequate ~, r~
when used as a drop in ~ negative dectrode for NiCd cells. However, the 1) r. ., . - ~ of Ovonic Base Alloys of the present invention can be SiE;Il~lca~ improved by using an optimized separator material.
In general, the references discussed abr~ve, contain no teaching or suggestion that 20 some Ovonic Base Alloys cdls will have an improved cycle life and reduced self-discharge when used with an ~~ chosen separator.

wo 93~22801 2 ~ 1 7 7 4 8 PCIJUS93~04253 .

SUMMARY OF THE INVENTION
The present invention describes a r~rh~r~ hl~ Ni-MH hydrogen storage cell having improved cycle life; improved charge retention; and both improved cycle life and improved charge retenion.
S One aspect of the present invention is a .~I.o~ ,~lc hydrogen storage cell rnmrl~cin~ a negative electrode having the following (Ovonic Base Alloy),Mb where Ovonic Base Alloy represents an Ovonic alloy that contains 0.1 to 60 atomic percent Ti, 0.1 to 25 atomic percent Zr, 0.1 to 60 atomic percent V, 0.1 to 57 atomic percent Ni, and 0.1 to 56 atomic percent Cr, as described above; a is at least 70 atomic percent; M represents at least one modifier chosen from the gn~up consisting of Co, Mn, Al, Fe, W, La, Mo, Cu, Mg, Ca, Nb, Si, and Hf; b is 0 to 30 atomic percent; b > 0;
and a + b = 100 atomic percent; a positive electrode; and a separator that is anelectrolyte retentive nylon separator, or a wettable polypropylene separator resist~nt to reaction with Hl gas and alkaline electrolyte. As used herein, "electrolyte retenive" is specifically defined to mean capable of retaining sufficient electrolyte as a result of wettability amd pore size rii~lTihlltinn to reduce the effects of separator dryout commonly known as electrolyte Ic~ , and "wettable" is specifically defined to mean that the polyL,.~ ,n~ fibers have been treated to make them effectively absorb and retain electrolyte using means such as etching, radiation, or treatmenl with a chemical surfactant, where the chosen means does not produce by-products that adversely effect, i.e. "poison", the charge retention properties of the Ovonic Base Alloys of the present invention.
Anoth_r aspect of the present in~reltion is a .~I.~ bl~ hydro~en storage cell 93/22801 PCr/US93104253 cr~mrr1Ci~ a negative electrode having the following ~
(Ovonic Base Alloy) ,CobMn,Md where Ovonic Base Alloy is the same as described above; a is at least 70 atomic percent;
b is 0 to 7, preferably 4 to 7 atomic percent; c is 0.1 to 8, preferably 6 to 8 atomic 5 percent; M represents at least one modifier chosen from the group consisting of 0.1 to 2.5, preferably 1 to 2.5 atomic percent Al, 0.1 to 6, preferably 1 to 2 or 5 to 6 atomic percent Fe, and 0.1 to 6, preferably 5.5 to 6 atomic percent Mo; d is 0 to 8, preferably 4 to 6 atomic percent; b + c + d > 0; and a + b + c + d = 100 atomic percent; a positive electrode; and a stable nylon separator or wettable PO~ U~ IU.I~ separator as 10 defined above.

BRIEF DESCRlEYrlON OF THE DRAWINGS
Figure 1 is a scanning electron lI~i~,lU~OI);l of a p~ lu~ , separator in which the hydrophilic coating is .l.~ ; . and has a variety of loose particles and barbs.
Pigure 2 is a scanning dectron u~ . of a wettable pol.~l,lu~ , separator, as defined above, in which the coating is continuous and ~ ' ' DETAILPD DESCRIPTION OF THE INVENTION
The Ovonic Base Alloys of the present invention can be formed into negative 20 electrodes for metal hydride cdls that exhibit significant I v~, i..l~)lU.~ ~ in cycle life and charge retention compared to prior art cdls. Specific ' ' of these alloys are given in Table 1.

WO 93/22801 , PCr/l~S93/04253

2~774~ ~

1. V22Til6Zr,6Ni32Cr7Co7 2. V2O 6Til5Zrl5Ni30Cr6 6Co6.6Mn3.6A~7

3. V22Til6Zr,6Ni39Fe7

4. V22Til6Zr,aNi3~C7Fe6 -

5. V2lTil5Zrl5Ni3lCr6C06Fe6

6. Vl5Ti,5Zr2,Ni3,cr6cO6Fe6

7. V,8Til5zrl8Ni3lcr6co6Fe6

8. V22TillZr2lNi39Fe7

9. V,8Ti,5Zrl8Ni29Cr5C07Mn8

10. V,5Ti,5Zr2lNi3,Co6Fe6Mn6

11. V,5Ti,5Zr20Ni28Cr5.3Co5 3Fe5 3Mn6

12. VlhTi,5Zr20Ni3lcr6Fe6Mn6

13. V,8Ti,5Zrl8Ni29Cr5C6FelMn8

14. V,8Til5Zr,8Ni29Cr4CO6Fe2Mn8

15. Vl5Til5Zr2~Ni29Cr5Co7Mn8

16. V,5Ti,5Zr2lNi29Cr5Co6FelMn8

17. Vl5Til5Zr2lNi29Cr4CO6Fe2Mn8 20 18. V,8Ti,5Zr,8Nid~'ri~sMnd~ c ~, ~ . , The Ovonic Base Alloys of the present invention can be further classifled as having a l.~ ,ou~, disordered ~ resulting from changes in the , of the elements of the alloy, wherein hydrogen in a particular phase is not easily discharged 25 even through low surface area, or through an oxide having limited poros;ity or catalytic properties.
The addition of 6 to 8 atomic percent Mn results in increased storage capacity as well as low cell pressure and high cycle life.

~0 93122801 2 1 17 7 ~8 PCIIUS93104253 Though not wishing to be bound by theory, it is believed that in the alloys of the present invention, Mn alters the ~ ,-u~L u~.~c in such a way that the ~ f phases is inhibited thus leaving hydrogen bond strengths within the range of ~lr~ h....,...1 usefulness. One way in which Mn appears to ~ ', s this is by 5 increasing the mutual solubiliy of the other elements during ~..li.l;l;. -1l...l In addition, Mn functions at the Pl~.,L~ ,rlly active surface oxide as a catalyst. The multiple oxidation states of Mn are believed to catalyze the Plr ~ l discbarge reaction by increasing the porosity, ~,u~ .Liv;Ly, andlor the surface area of the active surface oxide film.
In the Ovonic Base Alloys of the present invention, Mn can replace Fe. Though not wisbing to be bound by thcory, it is believed that when Mn is present without Fe, Mn assists the r.l~. Ilu h ...;~.~1 discharge reaction at low; , . by promoting buLlc diffusion of hydrogen at low i , and also by catalyzing the reaction of hydrogen and hydroxyl ions at the alloy surface. Because of the low ~ properties of l~ these alloys, it appears that Mn's catalytic properties are . ,' ' when Fe is not present.
In the present invention, Mn can also be substituted for Co. In the resulting Ovonic Base Alloys, one can observe that hydrogen stosge capacity increases while g excellent charge retention. Though not wishing to be bound by theory, it is believed as 20 discussed above, that Mn alters the 1ll;w~Llu.,lulc and acts as a catalyst at the .lr~ 11y active surface oxide.
P~u. ' ~!r preferred ' ' of the present invention contain negative dectrodes of Ovonic Base Alloys modified with 7 to 8 atomic percent Mn; and l to 2 atomic percent Fe. Such Ovonic Base Alloy materials have,; ~ ly of the .

wo 93/22801 2 1 1 ~ 7 ~ 8 PCI/US93/042,53 'i~

separator a long cycle life and improved charge retenion.
The beneficial effects of Mn and Fe have been detailed in U.S. Patent No.
5,096,66'1, U.S. Patent No. No.5,104,617, and U.S. Patent No. 5,238,756.

It is noted in U.S. Patent No.5,104,617 that it was widely believed that the mclusion 5 of Fe in metal hydride hydrogen storage alloy materials would 1~'. t i~ effect ,LIu~ ,al F~ r..",.- i This belief was due to the knowledge that Fe readily oxidizes and corrodes, particularly in the presence of an alkaline electrolyte. Oxidation reduces the 1~ r.,".-~ ~r~ of a metal hydride electrode in many ways, and oxides of Fe were known in th~ ~rior art to adversely affect the nickel hydroxide positive dectrode, 10 I~Li~,ul~ly with respect to charging efficiency and thus capacity and cycle life.
Still other ~mboriin~^ntc of the present invention contain Ovonic lBase Alloy negative electrodes that contain 4.5 to 55 atomic percent Co; 7.5 to 8 atomic percent Mn; and 5.5 to 6 atomic percent Mo.
The effects of the addition of Mn are discussed in detail in U.S. Patent No.
5,096,667. The addition of Mn usually results in improved charging efficiency. Though not wishing to be bound by theory, this effect appears to result from Mn's ability to improve the charging efficiency of alloys it is addeo to by improving the oxidation resistance and oxygen rernmhin~tif~n It has been observed that oxygen gas generated at t~Le nickel hydroxide positive electrode 20 recombined at the surface of the metal hydride electrode. Oxygen r~cr.mhin~ n is an especially aggressive oxidizer of its ~,llViUUlllll~,llL, even compared to the aLcaline electrolyte.
It is possible that the modifier elements, particularly Mn and Fe, and most , . , _ .

~O 93~22801 I, uLik,ul~uly Co, either alone, or in ~ ;..., with Mn and/or Al for example, act to catalyze the oxygen reduction, thereby avoiding or reducing the oxidation of the ~Ull~/_.ll]illg elements im the metal hydride alloy. It is believed that this function of the modified alloys reduces or even eliminates the formation and build up of ~1~nim~-surface oxide, thereby providing a thinner and more stable surface.
Thongh not wishing to be bound by theory, it is believed that several additionalfactors may explain the , ~ behavior of Mn and Fe in the alloys of the present invention:
(I) The rr ~' ' of Mn and Fe may affect the bulk alloy by inhibiting the bulk diffusion rate of hydrogen within the metal through the formation of complex phase structures, either by effecting the grlun boundaries or by affecting the C~ - bond strength of hydrogen witbin the metal. In other words, the ~ r - ~ of the hydrogen bond strength may be increased thereby decreasing the available voltage and capacity available under low h "~ r, discharge.
(2) It is believed that the ~"" 1,:" ;,~" of Mn and Fe may result in a lower electrode surface area for metallurgical reasons by increasing the ductility of the alloy and thereby reducing the amount of crack formation during the activation process.
20 (3) It is believed that the c~ :.. of Mn and excessive Fe m these alloys may inhibit low L~ LIuc discharge through the alteration of the oxide layer itself with respect to ~ ' -vi~y, porosity, thickness, andlor catalytic activity. The oxide layer is an important factor in the discharge reaction that must promote the reaction of hydrogen from the alloy and WO 93122801 2 1 ~ 7 4 8 PCr/US93/04253 hydroxyl ion from the electrolyte. In addition, it is believed that this reaction is promoted by the dhin, conductive, porous portion of this oxide layer which has some degree of catalytic activity.
The ~ .. of excess Fe and Mn does not appear to be a problem under room 5 h , G discharge, but has shown a sulprising tendency to retard the low i reaction. The formation of a complex oxide could result in a subde change in oxide structure such as pore size ~ ;.... or porosity. Since the discharge reaction produces water at the metal hydride surface and within the oxide itself, a small pore size may be causing a slow diffusion of K~ and OH ions from the buL1c of the electrolyte to the oxide.
10 Under room i , discharge where p~ iS almost entirely ohmic to low tl.. ~.. ll c discharge where activation and - r~ r dominate, physical structure of the oxides with Fe and Mn compared to Mn alone could be dhe ,~
Compared to Mn above, it is also possible that Mn and Fe have multivalent 15 oxidation states. It is considered possible that some elements witbin the oxide may in fact change oxidation state during upward electrolyte state of charge variance and rate of discharge. It is equally possible that each of these multiple oxidation states has a different catalytic activity as well as different densities dhat together effect oxide porosity.
A possible problem with a complex oxide containing both Mn and excess Fe could 20 be that the Fe component retards the ability of the Mn to change oxidation state if present in large quantities.
r ~ the preceding discussion with respect to the oxide it should be noted dhat the oxide also corltains other ~ . of the Ovorlic Base Alloy such as V, Ti, Zr, Ni, and Cr as well as any other modifier elements added. The discussion of a wo 93/22801 2 1 1 7 7 ~ 8 PC~US93/04253 .
complex oxide of Mn and Fe is merely for the sake of brevity and one skilled in the art should not infer that the actual mechanism cannot also include a more complex involving other elements.
Alloy Ill~..i;r;.A~;..,. offers ~Ir11.11.1.~11~ cost advantages as well as l. rl.".~.l.
5 advantages. These cost advantages can be up to 30%. The price of V is a ll.r l~ll~;. - ,l rrmrrm. nt in the cost of Ovonic alloys. In U.S. Patent No. 5,002,730, ill~,ulr ' by reference, V in the form of V-Ni offers significant cost advamtages over pure V in cost.
Such cost illl~JlU..,~ l can, of course, be increascd t_rough the use of V-Fe.
The Ovonic Base Alloys of the present invention when used in r~..j ... l;.. . with 10 the separator materials described below, have .l . ,.. ,~l t 1 improved 1 r ~ over prior art alloys for el~ lu~ ical .l.~.lir~ .,lc Using the Ovonic Base Alloys describcd above, it was ri~trrmin~ ~ through cell failure analysis that nylon separators cause high cell ~ . as a result of the loss of electrolyte from the separator which is primarily caused by the absorption of electrolyte 15 by the elecrrodes. In virtually all sealcd ~ cells, the separator and electrodes are disposcd in direct contact with one amother. Thus, the relative capillary action of the electrodes and the separator determme the e~ iihri-lm amoumt of elcctrolyte retained within each ~,U1111.1UII~ . After repeated use, however, the capillary action ~r~hi~iti~5 of Ovonic Base Alloy negative electrodes of the present invention, as well as the nickel 20 hydroxide positive electrodes, increase. It is believed that this occurs because repeated charge and discharge cycling increases the porosity of the electrodes of the present invention by creating new pores and/or making finer pores. This increase in porosity appears related in some degree to the changes in surface area and surface roughness .. discussed in c.etail in U.S. Patent No. ~,728,586 to Venkatesan, et al.

WO 93/22801 2 1 1 7 7 4 8 PCrJU593~04253

18 Similarly, the capillary action of the positive electrode also increases. As a result, the electrodes become capable of absorbing more electrolyte and ~r~ ihri~ml amounts of electrolyte shift toward the electrodes.
Without wishing to be bound by theory, it is believed that the following 5 . I A I Al, . " ~ I ;' ~ of the separators of the present invention contribute to improved cell cycle life as a result of grcater electrolyte capacity and bctter electrolyte retention I) separators of the present invention have a weight to unit area ratio less than that of the standard nylon separator, 2) separators of the present invention have an - ~u ~ d thickness greater than the ~ ull~,u~ cd thickness of the standard nylon separator;
and/or 3) separators of the present invention have pores smaller than the pores of prior art separators. (This approach is in conttast to the teaching in U.S.
Patent No. 5,077,149 which uses zinc oxide to alter the absorption IlAlAr . 1~1; 5 of the positive electrode.) During the cell assembly process, a standard nylon separator is usually ~UIll~Ulcr~
tO Al,~ y 6 mils. Standard nylon separators typically have a weight to unit area ratio of 70 g/m2 and an Ulll..Ullll~ler~:~Cd thickness of 9 mils.
~0 The "high loft" nylon and polyylu~ylc.. ~, separators of the present invention are of uniform thickness and have a weight to unit ratio that is usually less than that of a standard nylon separator. By definition, "high loft" separators are separators capable of absorbing and retaining more electrolyte than standard nylon separators while retaining e~cellent rcsistance to electrical short circuit. High loft separ~tors of the present invention .

_, . ., . ... , . . , .. . . . , . . . .... . . ,, . .. ... , ... . , .,, . , _ . ,, . , . ,,, . , . ,,, _ _ _, . _ _ _ _ _ 93/22801 2 11 7 7 ~ 8 PCr/US93/042~3

19 have a layer weighing less than about 70 g/m2; preferably nol more than 60 g/m2.
The separators of the present invention also have an ~ thickness greater than the , J thickness of standard nylon separators Preferably, the high loft separators of the present invention have an , ' thickness greater than 9 mils;
5 most preferably about 14 mils. The present invention also includes separators that when . .. ~l,lr ~.. 1 are 14% lighter and 15% thicker than the standard nylon separator. As a result of these features, separators of the present invention are capable of absorbing and retaining 15% more electrolyte solution tham a standard nylon separator.
Ovonic Base Alloy cells of the preænt invention using improved separators have 10 an increased cycle life and an increased life span compared to a stanflard ~ f ~ Ni-MH cell. The best prior art Ovonic alloy ~ Ni-MH cells using standard nylon separators are capable of WiLh tL~uldil~ rr ' ' ly 800 cycles at 100% discharge depth.
In contrast, the Ovonic Base Alloy cells of the present invention cam have a life spam of at least 1,fJ00 cycles at 100% depth of discharge. (See, Table 2, below.) A standard nylon separator is typically formed from nonwoven 18 micron thick fibers resulting in material having pores "~ J~ r Iy 15-18 microns in size. In contrast, a nylon separator of the present invention has pores that are less than 5 microns and the separator itself is formed from nonwoven fibers 5-12 microns dlick.
In the present imvention, pore si2e direcdy affects dhe capillaty action associated

20 widh the separator. Pores of reduced size aUow for increased adhesion force between the - molecules of the electrolyte solution and dhe molecules of dhe separator. This adhesion force tends to draw more of the electrolyte solution into dhe separator and away from dhe electrodes and. because of its relative strengdh, tends to more cffectively retain the electrolyte solution in the separator. Thus, the smaller pore size slows the saturation of WO 93/22801 2 1 1 7 7 4 8 Pcr/US93/04253 the electrodes with electrolyte and improves the cycle life of the overaU ceU.
Preferably, the f~bers used to fabricate separators of the present invention are less than or equal to 12 microns thick and have pores less than or equal to 3 microns in siæ.
Such fibers are capable of absorbing and retaining 15% more dectrolyte solution than 5 prior art separators which results in imcreased ceU life spans.
Even finer fibers, about 6 to 8 microns thick, can be utilized. Separators of the present invention made with such fibers have pores of about I micron. Such separators are capable of absorbing and retaining even more electrolyte solution than the 12 micron fiber material of the standard nylon separator.
Another aspect of the present invention is the Ub~ ,iU - that Ovonic aUoy cells have a greater sensitivity to self discharge than NiCd and Ni-~l batteries using AB5 type aUoys. Quite l , "~" the present inventors have found that when the Ovonic Base Alloys of the present invention are combined with the separators of the present invention that this prvblem cam be largely overcome. It is preferred that wettable pOI,y~llulJ~lulle 15 separators of the present invention be used to attain the maximum charge retention because wettable pVIy~!lU~ e, as described herein, is extremely stable in Ovonic Base AUoy ceUs of the present invention.
The greatest problem with untreated pol,yy~v~,~lull~, fibers is thu in contrast to nylon fibers, which are very ll~ u~L;lic, umtreated pvl~ ul~jlu.l~ fibers are very 20 l~ydlu~ vbh,. In order to use pvly~v~yl~, fibers for the separators of the present invention, they must be treated to make them "wettable" so that they wiU effectively absorb and retain electrolyte solution. This is normally ~ h d using radiation graft techniques (using a variety of radiation sources such as ultraviolet radiation, cobalt source radiation, or gamma rays), etching techniques using various chemicals (such as sulfuric 93/22~01 ~ PCr~VS93J~4253 acid), or treatment with a chemical su~factant to produce a wettable material.
Some care must be used to chose a technique that will render the surface of the pol~lu~ , "wettable" within the context of this term as defined above. That is, the treatment must be one that does not produce particles, barbs, residue, etc. as illustrated S in Figure 1. It is theorized that such residues have the capability of "poisoning" the Ovonic Base Alloys of the present invention by depositing on the nickel hydroxide positive dectrode and lowering oxygen stability or forming redox shuttle ' or forming i , products affecting one or both of the above self discharge - ' that lower thc overall charge retention of the cell. Prcferred wettable ~u~ u~Jl~,n~ separators of the present invention are ones m which the individual fibers have a continuous coating as illustrated in Figure 2. Ovonic Base Alloy cells of the prcsent invention using a wettable l~ulylJlu~ separator having such a continuouscoating over the individual fibers have si~ irl,~ ly increased charge retention over the prior art.
Methods of providing wettability, or absorption of electrolyte to the pul~l~lu~y~
are also critical in the self-discharge behavior of the batteTy. Residual sulfates from the sulfonation process, F '' , and residual impurities from the radiation grafting proccsses can all effect self-discharge.
The wettable ~ol~lu~ , separators of the present invention arc quite different from the sulfonated polyl,luyyl.,.l~, separator described in U.S. Patent No. S,û77,149 discussed above. The '149 patent makes no mention of improved charge retention, describes the use of a misch metal alloy rather than an Ovonic Base Alloy of the present invention, and describes treating a pul~lu~yl~, separator , ~ ' with zinc oxide with a ~1" ~ - resin rather than a wetting agent.

WO 93/22801 2 1 17 7 4~ PCr/US93/04253 In particular, the inventors of the present invention have discovered that the mere use of a ~ulylJIu~yhl~ separator, even a sulfonated l~UIy~JlU~Jyl~ separator, is by no means a guatantee of low self-discharge and that self-discharge is effected by the separator material ' ~ process as well as the l~ind of negaive alloy materials used.
It is preferred that radiaion grafted wettable ~I,c.ly~Jlu~ , separators that are continuous and i ~ as described above (~, Figure I and Figure 2~ be used in the Ovonic Base Alloy cells of the present invention to attain cells having a minimum self discharge. Without wishing to be bound by theory, it is bdieved that this is because these 10 kind of grafted ~o~y~lu~yhll~ separators represent the highest purity wettable ~oly~lu~yl~,.le currently available, that is they have minimal particles, barbs, residue, or "poisons" to effect the Ovonic Base Alloys of the present invention; and that the grafted coaing is applied in such a way as to produce the most cr~ in~o~lc, ;,l 1~ ~ h~ , aLIcali resistant, and hydrogen gas resistant wettable coaing for the ~oly~lu~,yl~"~ separator used 15 with the Ovonic Base Alloys of the present invenion. It is believed that these previously , ' ' impurities affect self-oischarge in the Ovûnic Base Alloy when they are ,l.. ,l.. ~J by the a1.1~aline electrolyte and the hydrogen gas that is present during the normal operaion of these cells.
Preferred ~.mhoflim~ntc of the present invention employ the appropriate 20 of a described Ovonic Base Alloy with a described separator to attain the maximum cycle life and/or charge retenion.
Most preferred ~ of the present invention employ a "high loft" wettable poly~,lu~.yl~,ll~, separator having fibers and pores of the preferred sizes described above.

~0 93/22801 21 17 7 ~ 8 PCI`/US93/04253 The Ovonic Base Alloy cells of the present invention can be used in a variety of f.. .f;~,...,l;.. ~ Containers for these cells may be any suitable housing, such as a plastic or metal, which does not deteriorate or react with the cell electrolyte and which allows venting of the cell should it produce an ~ .~C beyond a I ' ' limit during S its operation.
The Ovonic Base Alloy cells of the present invention can be fr~nfi~ 1 for example, as flat cells that include a 5~ ct~n~ y flat plate negative electrode, a cur~ent collector in dectrical contact with the active material of the electrode amd a contact tab in electrical with an electrical lead, a positive clectrode or counter-10 electrode that is ' 11y flat and aligned with the negative elcctrode; or as jelly-roll type cells made by spirally winding a flat cell about an axis.
The dectrodes of the cells of the present invention are immersed in an ~
electrolyte. A preferred electrolyte is a 30 weight percent a~ueous solution of potassium hydroxide. The Ovonic Base Alloys of the present invention are fo~mulated into negaive 15 electrode materials without the use of birders, such as pc~ .,. Binders have becn shown to promote high rates of self dischar~e in the Ovonic Base Alloys of the present invention. See, U.S. Patent No. 4,915,898, discussed further below.
Clearly, various C-.. fih. ,"~ of cells and batteries may be structured in afff~anf.- with the described invention. Thus, the present invention is not intended to 20 be limited to the, ~.o~ ;,'; d in this arFlifatif~n This invention is illustrated further by reference to the following non-limiting examples.

WO 93/22801 2 f ~ 7 7 ~ ~ PCr/US93/04253 EXAMPLES

Preparation of Negative Electrode Materials The Ni-MH materials shown in the Exarnples below were prepared by weighing and 5 mixing powders of the component elements into a graphite crucible. The crucible and its contents were placed in a vacuum furnace which was evacuated and then pressurized with -rr ' 1~ one ~ ' of argon. The crucible contents were mdted by high frequency imduction heating while umder the argon r ~~ 1 The melting was ca~ried out at a of about 1500~C umtil a umform melt was obtained. At that time, the 10 heating was terminated and the melt was aUowed to solidify under a blanket inert The imgot of aUoy material was then reduced in size in a multi-step process. The first step involved a lly~Lidill~/~hy~idil~g process ' llry as described in U.S.
PateM No. 4,983,756 entitled Hydrlde Reacror Apparatus for Hydrogen r~ n of 15 Metal Hydride Hydrogen Storage Alloy Material, the disclosure of which is ~",ir~ uy iUculJ,uldt~ d by reference. In this first step, the alloy was reduced in size to less than 100 m~sh. .S- ~ ly, the material obtained from the ;..~ /d.,l,y~idillg process was further reduced in size by an impact miUing process in which the particles were tangentially and radially ~rr~l~r~ d against an impact block. This process is described in U.S. Patent No. 4,915,898 entitled rnproved Method for the Contmuous Fabricat~on of Cormninuted Hydrogen Storage Alloy Negat~ve Electrode Maserial, the disclosure of which is specificaUy U.Cu.J ' by reference A fractiûn of the auûy material having a particle size of less than 200 mesh and a mass average patlicle size of about 400 mesh (38 microns) was recovered from the impact WO 93/22801 2 1 1 7 7 4 8 PC~'lUS93J04253 milling process and bonded to a nickel SCreen current collector by disposing a layer of alloy material onto the current collector and compacting the powder and collector. This method does not use a binder. ~`f\mr~tin~ was carried out under an inert ~LIIU~ C
with two separate ~.,,.,.~.1;,.., steps, each at a pressure of about 16 tons per square inch.
5 After f ~ ;. . the current collector and the powder adhered to it were sintered irl an , of about 2 atomic percent hydrogen with the balance argon to form negative electrode materials.
These negative electrode materials were activated using the alkaline etch treatment described in U.S. Patent No. 4,716,088.

û Preparation of Cells The prepared negative electrodes, separator, nickel hydroxide pf~sitive electrodes, and 30 % KOH electrolyte were assembled into "C" cells. The specific separator chosen is indicated in the Examples.
Example 1 Cells were prepared as described above using alloy #5 of Table I and a standard nylon separator, a high loft nylon separator, and a fine fiber diameter nylon separator.
The finished cells were subjected to charging and discharging conditions and the cycle life ~if~t~rminf ~1 It is to be ~Inf~f rStflnrl that the concepts of high loft and fine fiber diameter can be combined.
~ ,~

WO 93/22801 PCI`/US93~04253 2~ 17748 Allo S arator Cycle Life y ep (TCO 32DC) nylon 550 high loft 1200 nylon fine fiber 1000 nylon Examplc 2 C cells were prepared using alloy #6 of claim 1 with tho separator materials shown in Table 3. The resulting cells were subjected to a 30 day charge retention testing at 10 room i The results obtained are set forth in Table 3, below.

ALLOY Separator Charge Retention %

30 days 6 radiation 80 grafted pp 6 radiation 76 grafted pp 6 chemically 70 treated pp 6 standard pp 60 6 propylene 10 blend 6 standard 40 nylon 6 fine fiber 50 nylon ~093/2280l 2I1 7748 Pcr/US93~04253 l~xample 3.
Cells were prepared as described above using the alloys listed in Table 4, standard nylon separators, and a variety of poly~lu~ , separators. The "polyprol" separators are standard u~treated poly~lu~ separators. Tbe "polypro2" separators are S poly~,lu~,yl.,..c separators treated with chemical surfactants. The "treated polypro"
separators are radiation grafted pùlyplulJJL,~.~, separators having a continuous and , surface (as shown in Figure 2) r 1 by SCIMAT. The f~nished cells were subjected to charging and ~ conditions and their charge retention t~rmin~
The data obtained from these tests is set forth in Table 4, below.

Table 4 ALLOY separ~tor Charge Retention (22C) type 3 days 14 days 28 days nylon 72 32 8 15 9 nylon 76 37 20 9polyprol 78 42 22 9treated 88 67 47 polyp~
14nylon 83 61 42 14treated 91 82 72 polypro 2û15 nylon 91 71 54 15polyp~2 92 76 61 15treated 95 89 81 polyp~l~
17nylon 92 88 83 17treated 96 94 89 polypro WO 93/22801 PCI'IUS93/04253 7 4 ~ --~ ach of the above examples has been desclibed in detail apart from one another.
However, the various ~n ~ ; n-nre of the present invention can also be practiced in ~r - t ' '' Most easily combined are the "high loh" ~ and the "frne fiber"
is also possible to combine tbese features with a wettable pol~
separator to produce a cell having excellent cycle life as well as excellent charge retention. A~l possible c~ of the features desdbed in the DETAILED
DESCRIP~ON OF THE INVENTION and the EXAMPLES are considered to be within tne scope of the present invenion.
Further, it is obvious to those skilled in the art that the invention may be prepared 10 by additional methods, using additional ~ , ' and different ~ I';c ~rli~.. (such as other cell sizes) without departing from its spirit and scope.
The drawings, discussion"~ and examples of this ~ are merely illustrative of particular r ' ' of the mvention and are not meant as limitadons upon its practice. It is the following claims, including all equivalents, that 15 define the scope of the invention.
What is Claimed is:

Claims

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

1. A rechargeable hydrogen storage cell comprising:
a negative electrode having the following composition:
(Ovonic Base Alloy) aMb where Ovonic Base Alloy represents an Ovonic alloy that contains 0.1 to 60 atomic percent Ti, 0.1 to 25 atomic percent Zr, 0.1 to 60 atomic percent V, 0.1 to 57 atomic percent Ni, and 0.1 to 56 atomic percent Cr:
.alpha. is at least 70 atomic percent;
M represents at least one modifier chosen from the group consisting of Co, Mn, Al. Fe, W, La, Mo, Cu, Mg, Ca, Nb, Si, and Hf;
0 < b < 30;
.alpha. + b = 100 atomic percent;
a positive electrode; and a separator chosen from the group consisting of:
an electrolyte retentive nylon separator having fine fibres and a thickness of approximately 5-12 microns; and a wettable polypropylene separator resistant to reaction with H2 gas and alkaline electrolyte.

2. The rechargeable hydrogen storage cell of claim 1, wherein said negative electrode has the following comprising:
(Ovonic Base Alloy) aCobMncMd where .alpha. is at least 70 atomic percent;
b is 0 to 7 atomic percent;
c is 0.1 to 8 atomic percent;
M represents at least one modifier chosen from the group consisting of 0.1 to 2 atomic percent Al, 0.1 to 6 atomic percent Fe, 0.1 to 4 atomic percent La, and 0.1 to 6 atomic percent Mo; d is 0 to 8 atomic percent;
b + c + d >0; and a + b + c + d = 100 atomic percent.

3. The rechargeable hydrogen storage cell of claim 1, wherein said separator is a high loft separator of uniform thickness having a weight of less than about 70 g/m2.

4. The rechargeable hydrogen storage cell of claim 1, wherein said separator is formed from fibers having a thickness of less than 18 microns.

5. The rechargeable hydrogen storage cell of claim 1, wherein said separator has a thickness of 5 to 12 microns and is formed of fibers oriented to produce pores less than 5 microns.

6. The rechargeable hydrogen storage cell of claim 1, wherein said separator has a weight of less than 60 g/m2.

7. The rechargeable hydrogen storage cell of claim 1, wherein said separator has an uncompressed thickness greater than 9 mils.

8. The rechargeable hydrogen storage cell of claim 1, wherein said separator has an uncompressed thickness greater than 14 mils.

9. The rechargeable hydrogen storage cell of claim 1, wherein said fibers are less than or equal to 12 microns thick.

10. The rechargeable hydrogen storage cell of claim 1, wherein said fibers are less than or equal to 8 microns thick.

11. The rechargeable hydrogen storage cell of claim 5, wherein said separator has pores less than 3 microns in size.

12. The rechargeable hydrogen storage cell of claim 5 wherein said separator has pores less than 1 micron in size.

13. The rechargeable hydrogen storage cell of claim 12, wherein said separator is a stable nylon.