Separator for nonaqueous electrochemical cells

A combination separator comprising a single layer of a non-woven, polyolefinic cloth superposed with a single layer of a polyolefinic, microporous film for use in an electrochemical cell, is described. A preferred polyolefinic material for both the non-woven cloth and the microporous film is polypropylene. The redundancy of using two layers of separator is an enhanced safety characteristic of the cell; however, the use of the polypropylene web/film combination adds another dimension to the cell's safety characteristics by imparting the benefits of each type of material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to the conversion of chemical 
energy to electrical energy and, more particularly, to a combination 
separator provided to electrically insulate the anode from the cathode in 
an electrochemical cell. The combination separator comprises at least one 
layer of a flexible, non-woven polyolefinic cloth superposed with at least 
one layer of a flexible, microporous polyolefinic film. A preferred 
polymeric material for both the non-woven cloth and the film is 
polypropylene. A preferred superposed combination separator according to 
the present invention has a surfactant such as a non-ionic surfactant 
coated thereon, and more preferably coated on the film. An electrochemical 
cell includes an alkali metal primary or secondary cell having the present 
combination separator intermediate the electrodes to provide physical 
separation therebetween. 
2. Prior Art 
Conventionally, separators have fallen into two general categories--those 
made of microporous films and those in the form of a cloth made from 
various materials including polymeric fibers. Whether a separator is 
comprised of a microporous film or a cloth material, a fundamental 
requirement is that the material of construction be resistant to 
degradation in the cell environment, have sufficient thickness to maintain 
interelectrode separation without interfering with cell discharge 
performance, and exhibit sufficient surface energy such that electrolyte 
wettability and absorption are augmented. The separator material must also 
have a relatively high electrical resistivity in order to prohibit the 
establishment of short circuit currents flowing directly between the 
electrodes through the separator. 
An example of a film separator is shown in U.S. Pat. No. 4,629,666 to 
Schlaikjer, which discloses partially halogenated microporous polymeric 
films for use as separators in electrochemical cells containing alkali 
metals, such as lithium, and inorganic electrolytes. Similarly, a 
microporous film separator comprising polytetrafluoroethylene (PTFE) is 
disclosed in U.S. Pat. No. 3,661,645 to Strier et al. The benefit is that 
microporous films can be made very thin which contributes to volumetric 
efficiency in that the separator does not detract appreciably from the 
volume of cathode and anode active materials and therefore the energy 
density. The problem is that reduction in separator thickness is 
accompanied by a reduction in material strength as microporous films, 
including those made of polytetrafluoroethylene, can be weak and 
susceptible to tearing. As previously discussed, tensile properties are 
important in selecting a separator, and it is not uncommon for film 
separators to rupture during the manufacturing process, which can lead to 
contact between the electrode materials and result in an internal short 
circuit condition. 
The separator also must have sufficient porosity such that electrode 
separation is maintained while allowing ionic transfer within the 
electrolyte to occur unimpeded during intended cell discharge. Cloth 
separators are relatively porous structures. An exemplary cloth separator 
is shown in U.S. Pat. No. 5,002,843 to Cieslak et al., which discloses a 
lithium/thionyl chloride electrochemical cell system having a separator 
made of aramid fibers provided in a non-woven mat form. Although non-woven 
mats are highly porous and, therefore, not a detriment to ionic transfer 
within the depolarizer/catholyte, their inherent porosity may allow small 
particles of electrode material to migrate through the separator. As is 
the case with microporous films, the use of a non-woven cloth as a 
separator in an electrochemical cell can result in direct physical contact 
between the electrodes, which would give rise to an internal short circuit 
condition. 
The above requirements are balanced by the need that the separator have 
sufficiently strong tensile properties to facilitate cell fabrication and 
to further withstand internal cell stresses due to changes in electrode 
volume during discharge, and during re-charging cycles in secondary 
electrochemical cells. Polytetrafluoroethylene has high tensile strength 
and is, therefore, desirable for use as a separator material in some cell 
chemistries, especially when provided in a cloth form. To complicate 
matters, however, it is known that alkali metals such as lithium are 
reactive in contact with fluorinated carbon (CF.sub.x) electrode active 
materials and polytetrafluoroethylene separators. Contact between lithium 
metal electrodes and polytetrafluoroethylene separators can generate 
sparks and possibly sufficient heat to cause fire. Furthermore, while 
fluorinated carbons (CF.sub.x) are useful as cathode active materials, and 
especially in cells intended to be discharged under a light load for 
extended periods of time such as for routine monitoring of cardiac 
functions by an implantable cardiac defibrillator and the like, it is 
imperative that physical separation is maintained between lithium and the 
cathode material without the provision of polytetrafluoroethylene as a 
separator material due to the potential for excessive heat generation when 
fluorinated carbon active materials are contacted with alkali metals. 
The separator combination of the present invention has excellent tensile 
properties while allowing ionic transfer within electrolyte to occur, but 
it does not include polytetrafluoroethylene. In comparison to separators 
made of that material, the use of one non-woven polyolefinic cloth 
superposed with a microporous polyolefinic film according to the present 
invention provides a separator combination for alkali metal cells having 
all of the desirable above-described attributes of a separator structure 
including the prevention of internal short circuit conditions without 
interfering with discharge performance while also allowing for the 
fabrication of thinner and/or smaller cell constructions. 
It should be pointed out that an alkali metal electrochemical cell that 
does not use polytetrafluoroethylene as a separator material is described 
in U.S. Pat. No. 4,552,821 to Gibbard et al. However, this patent 
describes a separator comprising at least two layers of polypropylene film 
used in conjunction with a non-woven fabric wicking layer. The double 
layers of polypropylene film are intended to minimize dendritic short 
circuit conditions during recharging of the disclosed nickel-zinc 
secondary cell. In practice, the double layers of polypropylene separator 
film provide a degree of redundancy that allows large pores or holes, due 
to imperfections produced during manufacture or subsequently, in each film 
to be non-aligned to minimize short circuit conditions caused by dendrite 
growth during cell recharging. 
Hoechst Celanese Corporation, Charlotte, N.C., sells a combination 
separator under the designation CELGARD.RTM. 5550 comprising a layer of 
non-woven polypropylene laminated with a layer of polypropylene film 
having a blend of non-ionic and cationic surfactants coated thereon. 
However, it is believed that the lamination process reduces this 
combination separator's ability to support ion transfer therethrough. 
As previously discussed, the present alkali metal electrochemical cell can 
comprise either a primary cell or a secondary, rechargeable cell. A most 
preferred form of the present electrochemical cell includes a Li/CF.sub.x 
couple. In that case, it is imperative that physical separation is 
maintained between the respective electrode active materials to not only 
prevent the formation of short circuit conditions but also to preclude 
internal heat generation. Such an eventuality could be catastrophic. 
In the present invention, redundant film layers are not desired. The 
present superposed non-woven cloth and microporous film provide improved 
strength characteristics in comparison to one or the other layer used 
alone and the present separator structure also prevents migration of 
fluorinated carbon cathode materials into short circuit contact with the 
anode material while still providing for cells of reduced thickness and 
smaller size. 
SUMMARY OF THE INVENTION 
The present invention is directed to a combination separator comprising a 
single layer of a polyolefinic, non-woven cloth superposed with a single 
layer of a microporous, polyolefinic film. Preferably a non-ionic 
surfactant is coated on the microporous film. This separator combination 
is thinner than conventional separators using two layers of non-woven 
material, and that allows for the fabrication of thinner and/or smaller 
cells than were previously known prior to the present invention. The 
superposed separator combination is especially useful in Li/CF.sub.x cells 
to preclude the possibility of short circuit conditions between the active 
electrode components. Not only does a short circuit destroy the cell's 
electrical discharge functionality, but it is known that alkali metals 
contacted to certain fluorinated carbonaceous materials including 
electrode active fluorinated carbons can generate heat inside the cell. 
For that reason, prevention of contact between lithium metal anodes and 
fluorinated carbonaceous electrode active materials without the provision 
of a polytetrafluoroethylene separator is paramount for proper cell 
discharge. 
These and other aspects of the present invention will become increasingly 
more apparent to those skilled in the art by reference to the following 
description and to the drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, FIG. 1 shows an exemplary electrochemical 
cell 10 according to the present invention constructed having a separator 
12 comprising one layer of a non-woven polyolefinic cloth 14 superposed 
with one layer of a polyolefinic film 16 to provide separation between an 
anode electrode 18 and a cathode electrode 20. The term "polyolefinic" is 
intended to describe a polymer prepared by polymerizing olefin monomers 
through their unsaturation. 
It is known that when used alone, non-woven, polymeric cloths are very 
porous and being unconsolidated, may allow particles of the electrode 
active material to migrate therethrough. Microporous, polymeric films are 
themselves relatively weak; however, they are generally thinner than 
non-woven cloths which benefits increased cell energy density. According 
to the present invention, the problems of high porosity inherently 
characteristic in non-woven polymeric cloths and the relative weakness of 
microporous films are overcome by the provision of the two materials 
superposed in contact with each other. While such a structure is not as 
thin as two layers of microporous polymeric film, the somewhat increased 
separator thickness substantially benefits tear resistance, which is 
critical to proper cell operation. 
As shown in FIG. 1, the combination separator 12 of the present invention 
is thus placed between the anode 18 and cathode 20 of the electrochemical 
cell 10 in a manner preventing physical contact between the electrodes. 
For reasons that will be explained in more detail hereinafter, the cloth 
layer 14 preferably faces the cathode 20 and the film 16 preferably faces 
the anode 18. 
Anode active materials acceptable for use with the present combination 
separator 12 comprise metals selected from Group IA of the Periodic Table 
of the Elements, including lithium, sodium, potassium, calcium, magnesium 
or their alloys, or any alkali metal or alkali-earth metal capable of 
functioning as an anode. Alloys and intermetallic compounds include, for 
example, Li--Si, Li--Al, Li--Mg, Li--Al--Mg, Li--B and Li--Si--B alloys 
and intermetallic compounds. In that case, the anode active material 
preferably comprises lithium. The form of the anode may vary, but 
typically, the anode comprises a thin sheet or foil of the anode metal, 
and a current collector 22 contacted to the anode material. The current 
collector 22 includes an extended tab or lead (not shown) for connection 
to the negative terminal. 
The cathode electrode 20 of the exemplary electrochemical cell 10 has a 
current collector 24 including a lead (not shown) for connection to the 
positive cell terminal. The conductor portion of the cathode current 
collector 24 is in the form of a thin sheet of metal screen, for example, 
a titanium, stainless steel, aluminum or nickel screen having the lead 
extending therefrom. 
The cathode electrode 20 can comprise solid active materials such as are 
typically used in alkali metal/solid cathode electrochemical cells. 
Suitable electrode active materials include a metal, a metal oxide, a 
metal sulfide and carbonaceous materials, and mixtures thereof. Examples 
of electrode active materials that may be formed into a cathode component 
include, but are not limited to, manganese dioxide, copper silver vanadium 
oxide, silver vanadium oxide, copper vanadium oxide, titanium disulfide, 
copper oxide, copper sulfide, iron sulfide, iron disulfide, carbon and 
fluorinated carbon, and mixtures thereof. Preferably, the cathode 
comprises about 80 to about 99 weight percent of the electrode active 
material. 
The combination separator 12 of the present invention is particularly 
useful with carbonaceous active materials prepared from fluorine and 
carbon including graphitic and nongraphitic forms of carbon, such as coke, 
charcoal or activated carbon. The fluorinated carbon is represented by the 
formula (CF.sub.x).sub.n wherein x varies between about 0.1 to 1.9 and 
preferably between about 0.5 and 1.2, and (C.sub.2 F).sub.n wherein the n 
refers to the number of monomer units which can vary widely. The preferred 
cathode active mixture comprises CF.sub.x combined with a discharge 
promoter component such as acetylene black, carbon black and/or graphite. 
Metallic powders such as nickel, aluminum, titanium and stainless steel in 
powder form are also useful as conductive diluents when mixed with the 
cathode active mixture of the present invention. Up to about 10 weight 
percent of the conductive diluent is added to the mixture to improve 
conductivity. 
Solid cathode active components for incorporation into a cell according to 
the present invention may be prepared by rolling, spreading or pressing a 
mixture of one or more of the above listed electrode active materials, a 
discharge promoter component and/or one or more of the enumerated 
conductive diluents onto the current collector 24 with the aid of a binder 
material. Preferred binder materials include a powdered fluoro-resin such 
as powdered polytetrafluoroethylene (PTFE) or powdered polyvinylidene 
fluoride present at about 1 to about 5 weight percent of the electrode 
active material. Cathodes prepared as described above may be in the form 
of one or more plates operatively associated with at least one or more 
plates of anode material, or in the form of a strip wound with a 
corresponding strip of anode material in a structure similar to a 
"jellyroll". 
The exemplary cell 10 further includes the combination separator 12 of the 
present invention disposed intermediate the Group IA anode 18 and the 
cathode 20 to provide physical separation therebetween. The combination 
separator has a thickness of about 3 to 4 mils and comprises at least one 
layer 14 of a non-woven, synthetic polyolefinic cloth superposed with at 
least one layer 16 of a synthetic polyolefinic film. To prevent movement 
of the non-woven cloth with respect to the film during cell fabrication, 
the present separator is provided as two superposed structures, one placed 
on either side of one of the electrodes such as the cathode. The opposed 
superposed separator structures are then heat sealed to each other at 
their peripheries to envelope the electrode. 
Both the polyolefinic non-woven cloth 14 and the polyolefinic film 16 are 
electrically insulative, chemically unreactive with the anode and cathode 
active materials and are chemically unreactive with and insoluble in the 
electrolyte. In addition, both separator layers 14, 16 have a degree of 
porosity sufficient to allow flow therethrough of the electrolyte during 
the electrochemical reaction of the electrochemical cell 10. 
In particular, the non-woven polyolefinic cloth 14 has an open mesh or open 
weave characteristic that does not act as a barrier to ion flow 
therethrough. By way of example, the non-woven cloth can be a polyethylene 
or a polypropylene, open mesh cloth material. A preferred form of the 
non-woven separator material is a non-woven polypropylene web cloth 
commercially available from Schuller Web Dynamics, Stroudsburg, Pa., under 
the designation DYNAWEB.RTM. 902x. This material is a melt-blown 
polypropylene non-woven web with a nominal thickness of 2.5 mils by TMI 
549M. Fiber diameter is approximately 3 .mu.m, material porosity is &gt;50%, 
with a maximum pore size of &lt;15 .mu.m. Air permeability is 6 CFM (cubic 
feet per minute) using the Frazier method. An alternate non-woven 
polypropylene cloth useful in the present invention is commercially 
available from Tonen Tapyrus under the designation TAPYRUS.RTM., part 
number PO225SW-OCS. The web has a basis weight of 22.+-.2 g/m.sup.2 and a 
nominal thickness of 3 mil. Its resistance to air flow is &lt;&lt;1 sec/30 cc 
air (1 square inch sample). 
The film layer 16 is of a material having mechanical characteristics 
including tensile strength and percent elongation that compensate for 
cathode swelling during discharge without the need for increased amounts 
of film material. The film material has a microporous structure that 
provides for flow of alkali metal ions therethrough. By way of example, 
the microporous film material can be a polyethylene or a polypropylene 
film of the type that may be produced by an extruding, blow molding or 
casting process. In particular, a preferred form of the synthetic 
polyolefinic film 16 is a polypropylene film commercially available from 
Hoechst Celanese Corporation under the designation CELGARD.RTM. 3500. The 
film has a nominal thickness of about 1 mil with a wettability in water of 
&lt;30 seconds and an electrical resistance of .ltoreq.5 m.OMEGA. in.sup.2. 
Its resistance to air flow is .ltoreq.15 seconds/in.sup.2. The preferred 
film is a microporous continuous sheet membrane coated with a nonionic 
surfactant. Preferred surfactants include silicon glycol copolymers, such 
as polyoxyethylene polymethyl siloxane, either alone or in combination 
with an imidazoline tertiary amine, phosphate esters such as ethoxylated 
2-ethyl-hexyl phosphate and hydrophilic organic hydrocarbon monomers such 
as acrylic acid, methacrylic acid, vinyl acetate and mixtures thereof, as 
described in U.S. Pat. Nos. 4,438,185 to Taskier and 4,359,510 to Taskier, 
the disclosures of which are hereby incorporated by reference. The 
nonionic surfactant is provided to improve the wettability of the film to 
the electrolyte. 
An alternative polypropylene film useful in the present invention is also 
commercially available from Hoechst Celanese under the designation 
CELGARD.RTM. 3501. This film is coated with a cationic and nonionic 
surfactant. 
In some cases it may be desired to use polyethylene microporous film and 
non-woven cloth in the present electrochemical cell. Polyethylene has a 
lower melting point than polypropylene which is beneficial for shutting 
down cell discharge when excessive internal heat is generated. 
The electrochemical cell 10 of the present invention further includes a 
nonaqueous, ionically conductive electrolyte which serves as a medium for 
migration of ions between the anode and the cathode electrodes during the 
electrochemical reactions of the cell. The electrochemical reaction at the 
electrodes involves conversion of ions in atomic or molecular forms which 
migrate from the anode to the cathode. Thus, nonaqueous electrolytes 
suitable for the present invention are substantially inert to the anode 
and cathode materials, and they exhibit those physical properties 
necessary for ionic transport, namely, low viscosity, low surface tension 
and wettability. 
A suitable electrolyte has an inorganic or organic, ionically conductive 
salt dissolved in a nonaqueous solvent, and more preferably, the 
electrolyte includes an ionizable alkali metal salt dissolved in a mixture 
of aprotic organic solvents comprising a low viscosity solvent and a high 
permittivity solvent or, a single solvent. The ionically conductive salt 
serves as the vehicle for migration of the anode ions to intercalate or 
react with the cathode active material. Preferably the ion-forming alkali 
metal salt is similar to the alkali metal comprising the anode. 
In a solid cathode/electrolyte system, the ionically conductive salt 
preferably has the general formula MM'F.sub.6 or MM'F.sub.4 wherein M is 
an alkali metal similar to the alkali metal comprising the anode and M' is 
an element selected from the group consisting of phosphorous, arsenic, 
antimony and boron. Examples of salts yielding M'F.sub.6 are: 
hexafluorophosphate (PF.sub.6), hexafluoroarsenate (AsF.sub.6) and 
hexafluoroantimonate (SbF.sub.6), while tetrafluoroborate (BF.sub.4) is 
exemplary of salts yielding M'F.sub.4. Alternatively, the corresponding 
sodium or potassium salts may be used. Other salts useful with the present 
invention include LiClO.sub.4, LiC(SO.sub.2 CF.sub.3).sub.3, LiN(SO.sub.2 
CF.sub.3).sub.2 and LiCF.sub.3 SO.sub.3, and mixtures thereof. 
Low viscosity solvents include tetrahydrofuran (THF), methyl acetate (MA), 
diglyme, trigylme, tetragylme, dimethyl carbonate (DMC), 
1,2-dimethoxyethane (DME), diethyl carbonate and mixtures thereof, and 
high permittivity solvents include cyclic carbonates, cyclic esters and 
cyclic amides such as propylene carbonate (PC), ethylene carbonate (EC), 
acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, 
.gamma.-butyrolactone (GBL) and N-methyl-pyrrolidinone (NMP) and mixtures 
thereof. In the preferred electrochemical cell comprising the Li/CF.sub.x 
couple, the preferred electrolyte is 1.0M to 1.4M LiBF.sub.4 in 
.gamma.-butyrolactone (GBL). 
The combination separator 12 of the present invention is also useful in an 
alkali metal ion cell. Such rechargeable cells are typically constructed 
in a discharged state. The alkali metal ion, such as lithium, comprising a 
lithiated cathode is then intercalated into a carbonaceous anode by 
applying an externally generated electrical potential to recharge the 
cell. The applied recharging electrical potential serves to draw the 
alkali metal ions from the cathode, through the combination separator via 
the electrolyte and into the carbonaceous material of the anode to 
saturate the same. The cell is then provided with an electrical potential 
and is discharged in a usual manner. Particularly useful lithiated 
materials that are stable in air and readily handled include lithiated 
nickel oxide, lithiated manganese oxide, lithiated cobalt oxide and 
lithiated mixed oxides of cobalt with nickel or tin. The more preferred 
oxides include LiNiO.sub.2, LiMn.sub.2 O.sub.4, LiCoO.sub.2, LiCo.sub.0.92 
Sn.sub.0.08 O.sub.2 and LiCo.sub.1-x Ni.sub.x O.sub.2. 
It is known that oxyhalide electrolytes including such catholytes as 
thionyl chloride and sulfuryl chloride rapidly degrade polyolefins. In 
that case, in a cell having an alkali metal anode coupled with a 
catholyte/depolarizer such as sulfur dioxide or oxyhalides including 
phosphorized chloride, thionyl chloride and sulfuryl chloride, used 
individually or in combination with each other or in combination with 
halogens and interhalogens, such as bromine trifluoride, or other 
electrochemical promoters or stabilizers, the combination separator 12 of 
the present invention can include ethylene tetrafluoroethylene copolymer 
(ETFE) as both the non-woven cloth 14 and the film 16. A 
polyethylenetetrafluoroethylene separator material is commercially 
available under the name Tefzel, a trademark of the DuPont Company. In 
combination with an alkali metal-anode suitable catholytes include thionyl 
chloride, sulfuryl chloride, chlorinated sulfuryl chloride (CSC), thionyl 
chloride-bromine chloride (BCX), and mixtures thereof having an alkali 
metal salt dissolved therein. In such systems, the cathode is comprised of 
a high surface area carbonaceous admixture formed from a mixture of carbon 
black/PTFE materials. The carbonaceous admixture including the binder and 
conductive diluent is formed into a free-standing sheet in a manner 
similar to that described in U.S. Pat. No. 5,543,249 to Takeuchi et al., 
which is assigned to the assignee of the present invention and 
incorporated herein by reference. 
While not shown in the drawings, those skilled in the art will readily 
understand that the casing is provided with a cell header comprising a 
metallic disc-shaped body with a first hole to accommodate a 
glass-to-metal seal/terminal pin feedthrough and a hole for 
electrolyte/catholyte filling. The glass used is a corrosion resistant 
type, for example, having from about 0% to about 50% by weight silicon 
such as CABAL 12, TA 23, CORNING 9013, FUSITE 425 or FUSITE 435. The 
positive terminal pin feedthrough preferably comprises titanium although 
molybdenum and aluminum, such as an aluminum 52 alloy pin, can also be 
used. The cell header comprises elements having compatibility with the 
other components of the electrochemical cell and is resistant to 
corrosion. The cathode lead is welded to the positive terminal pin in the 
glass-to-metal seal and the header is welded to the case containing the 
electrode stack. 
The cell is thereafter filled with the electrolyte/catholyte described 
hereinabove and hermetically sealed such as by close-welding a stainless 
steel ball over the fill hole, but not limited thereto. This above 
assembly describes a case-negative cell which is the preferred 
construction of the cell of the present invention having the combination 
separator 12 intermediate the anode electrode 18 and the cathode electrode 
20 to provide physical separation therebetween, as described above. 
The preferred Li/CF.sub.x is constructed having the non-woven cloth 14 
facing the cathode 20 and the microporous film 16 facing the anode 18. 
This configuration is believed to provide an electrolyte rich environment 
proximate the cathode which benefits improved depth of discharge in the 
cathode and provides the cell with enhanced pulse discharge 
characteristics. The nonionic surfactant coated on the microporous film 16 
facing the anode 18 is believed to benefit wetting of the 
anode/electrolyte interphase. The cell of the present invention is 
particularly adapted for discharge under a light load for extended periods 
of time such as for routine monitoring of cardiac functions by an 
implantable cardiac defibrillator and the like. As is well known to those 
skilled in the art, the exemplary electrochemical system of the present 
invention can also be constructed in a case-positive configuration. 
The following examples describe the manner and process of manufacturing an 
electrochemical cell according to the present invention, and they set 
forth the best mode contemplated by the inventors of carrying out the 
invention, but they are not to be construed as limiting. 
EXAMPLE I 
Thirty-two prismatic Li/CF.sub.x cells of a central cathode design were 
constructed, each having 3.15.+-.0.01 gram cathodes consisting essentially 
of, by weight, 98% carbon monofluoride, 1% PTFE binder, and 1% acetylene 
black, pressed to chemically-etched titanium screens. Separators were then 
fashioned into bags and heat-sealed around the individual cathodes. The 
separator structures investigated are listed in tables 1 and 2 and 
consisted essentially of: 
1) two layers of superposed TAPYRUS.RTM. non-woven polypropylene cloth, 
designated as separator No. 1; 2) two layers of superposed DYNAWEB.RTM. 
902x non-woven polypropylene cloth, designated as separator No. 2; 3) one 
layer of CELGARD.RTM. 3500 microporous film, designated as separator No. 
3; 4) one layer of CELGARD.RTM. 3501 microporous film with a surfactant 
modified surface, designated as separator No. 4; 5) one layer of 
TAPYRUS.RTM., non-woven polypropylene cloth facing the anode and 
superposed with one layer of CELGARD.RTM. 3500 microporous polypropylene 
film facing the cathode, designated as separator No. 5; 6) one layer of 
CELGARD.RTM. 3500 microporous film facing the anode and superposed with 
one layer of TAPYRUS.RTM. non-woven cloth facing the cathode, designated 
as separator No. 6; 7) one layer of DYNAWEB.RTM. 902x non-woven 
polypropylene cloth facing the anode and superposed with one layer of 
CELGARD.RTM. 3500 microporous film facing the cathode, designated as 
separator No. 7; and 8) one layer of CELGARD.RTM. 3500 microporous 
polypropylene film coated with a non-ionic surfactant facing the anode and 
superposed with one layer of DYNAWEB.RTM. 902X non-woven polypropylene 
cloth facing the cathode, designated as separator No. 8. 
Four cells were built having each of the eight separator structures. 
Separator No. 7 is the preferred superposed combination in a non-preferred 
orientation with the non-woven polypropylene cloth facing the anode and 
the microporous film coated with a non-ionic surfactant facing the 
cathode. Separator No. 8 is the preferred superposed combination in the 
preferred orientation according to the present invention. 
Anodes were fabricated from lithium metal (nominal weight of 0.74 grams) 
pressed to chemically-etched nickel screens. Anode halves were placed on 
opposing sides of the central cathode. 
Each cell was filled with 1M LiBF.sub.4 in GBL electrolyte at a nominal 
weight of 3.80 grams. The hermetically-sealed cells were housed in 
stainless steel cases. 
Cell burn-in was carried out by discharging the cells at 37.degree. C. 
under 2 kohm loads for 72 hours. The cells were then placed on open 
circuit for a minimum of one week prior to the initiation of discharge 
testing at 37.degree. C. under 1 kohm loads. 
TABLE 1 
__________________________________________________________________________ 
Separator: 1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
Average capacity to 2 V, mAh 
1821.93 
1976.72 
1853.03 
1801.98 
1727.59 
1772.97 
1881.00 
1899.43 
.+-.1 std. dev. 
50.69 
12.23 
55.57 
78.71 
35.93 
12.52 
11.06 
39.69 
Average capacity to 1.7 V, mAh 
1844.20 
2048.50 
1948.54 
1909.12 
1753.26 
1804.36 
1957.69 
1988.55 
.+-.1 std. dev. 
51.15 
11.24 
54.49 
73.74 
36.74 
12.27 
15.77 
28.83 
Average capacity to 1.5 V, mAh 
1857.50 
2106.12 
2035.18 
1993.18 
1770.21 
1823.61 
2026.53 
2069.21 
.+-.1 std. dev. 
51.20 
7.29 
70.56 
78.39 
37.66 
11.32 
17.68 
27.65 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Separator: 1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
Average impedance at 2 V, .OMEGA. 
127.39 
56.08 
40.73 
38.13 
144.25 
144.49 
65.41 
51.33 
.+-.1 std. dev. 
17.42 
1.57 
6.11 
3.65 
10.98 
17.86 
5.13 
9.64 
Average impedance at 1.7 V, .OMEGA. 
304.43 
127.58 
57.95 
51.50 
340.83 
322.74 
136.85 
100.14 
.+-.1 std. dev. 
38.29 
3.69 
10.23 
12.38 
16.13 
37.98 
8.50 
15.78 
Average impedance at 1.5 V, .OMEGA. 
444.21 
180.41 
58.34 
53.97 
515.12 
483.54 
177.17 
123.52 
.+-.1 std. dev. 
68.13 
5.74 
6.48 
11.84 
15.02 
49.35 
12.14 
23.10 
__________________________________________________________________________ 
Average capacity measurements for each of the cell groups were calculated 
at 2 Volt, 1.7 Volt and 1.5 Volt cut-offs. The results are presented in 
Table 1. Average impedance was also calculated for each cell group at the 
respective cut-offs, and the results are presented in Table 2. 
Furthermore, FIG. 2 is a graph constructed from the discharge of the 
Li/CF.sub.x cells provided with separator No. 2 (two layers of superposed 
DYNAWEB.RTM. 902x non-woven polypropylene cloth), wherein curve 30 was 
constructed from the average discharge of those cells and curve 32 was 
constructed from the average impedances. FIG. 3 is a graph constructed 
from the discharge of the Li/CF.sub.x cells provided with separator No. 3 
(one layer of CELGARD.RTM. 3500 microporous film), wherein curve 40 was 
constructed from the average discharge of those cells and curve 42 was 
constructed from the average impedances. FIG. 4 is a graph constructed 
from the discharge of the Li/CF.sub.x cells provided with separator No. 7 
(one layer of TAPYRUS.RTM., non-woven polypropylene cloth facing the anode 
and superposed with one layer of CELGARD.RTM. 3500 microporous 
polypropylene film facing the cathode), wherein curve 50 was constructed 
from the average discharge of those cells and curve 52 was constructed 
from the average impedance. FIG. 5 is a graph constructed from the 
discharge of the Li/CF.sub.x cells provided with separator No. 8 according 
to the present invention (one layer of CELGARD.RTM. 3500 microporous 
polypropylene film facing the anode and superposed with one layer of 
DYNAWEB.RTM. 902X non-woven polypropylene cloth facing the cathode) 
wherein curve 60 was constructed from the average discharge of those cells 
and curve 62 was constructed from the average cell impedance. 
The preferred separator configuration of the present invention provides a 
Li/CF.sub.x cell with up to a 60% to 70% reduction in internal impedance 
and a 4% to 11% increase in delivered capacity over the use of a 
conventional non-woven cloth separator (FIG. 2). As is apparent from the 
tables and FIGS. 2 to 5, there is some degradation in delivered capacity 
(2% at a 1.5V cutoff which is statistically significant) when the 
preferred microporous film (CELGARD.RTM. 3500) is positioned facing the 
cathode and the superposed preferred non-woven cloth (DYNAWEB.RTM. 902x) 
faces the anode, which is a non-preferred configuration (separator No. 7, 
FIG. 4) in comparison to placing the microporous film facing the anode and 
the superposed non-woven web facing the cathode (separator No. 8, FIG. 5), 
which is the preferred separator configuration. The average impedance in 
the cells used to construct FIGS. 2 and 4 is also greater at each voltage 
cutoff when just the cloth is used or the anode-cloth/film-cathode 
configuration is used. 
EXAMPLE II 
To determine the resistance to ionic transfer of the superposed separator 
of the present invention, five (5) samples of the combination separator 
were subjected to air flow resistance testing. The combination separator 
had a thickness of about 2.5 to 3.0 mils. Air flow resistance was carried 
out on the samples with the air passing first through the non-woven cloth 
then the microporous film (Test No. 1) and with the air passing first 
through the microporous film then the non-woven cloth (Test No. 2). The 
results are indicated in seconds in Table 3. 
TABLE 3 
______________________________________ 
Test No. 1 
Test No. 2 
Separator (seconds) 
(seconds) 
______________________________________ 
1 9.624 10.035 
2 9.739 9.923 
3 9.797 9.945 
4 9.729 10.181 
5 9.771 10.027 
Avg. 9.732 10.022 
______________________________________ 
Similar air flow resistance tests were performed on samples of combination 
separator material obtained from Hoechst Celanese Corporation under the 
designation CELGARD.RTM. 5550 comprising one layer of non-woven propylene 
cloth laminated with one layer of polypropylene film having a blend of 
non-ionic and cationic surfactants coated thereon. The prior art laminated 
separator had a thickness of about 3.0 to 4.0 mils. The mean resistance to 
air flow for the air passing first through the microporous film then the 
laminated non-woven cloth was 19.3.+-.4.0 seconds. With the air passing 
first through the microporous film then the laminated non-woven cloth, the 
mean air flow resistance was 19.2.+-.3.3 seconds. 
It should be pointed out that the 5550 separator included the film 
component having a blend of non-ionic and cationic surfactants while the 
superposed combination separator of the present invention only has a 
non-ionic surfactant coated on the polypropylene film. However, Hoechst 
Celanese also manufactures a laminated separator designated K460 
consisting of one layer of polypropylene film laminated to one layer of 
non-woven polypropylene cloth. The K460 separator has no surfactant 
component. The air flow resistance for the K460 separator is listed in the 
Hoechst Celanese literature as 9.3 seconds, which is similar to that of 
the present separator, as set forth in Table 3. In other words, the 
present combination separator with a polypropylene film having a non-ionic 
surfactant has a similar air flow resistance to that of the K460 laminated 
separator devoid of any surfactants. This is likely the result of the 
present separator being a superposed combination in comparison to the K460 
separator being a laminate. Then, when a surfactant is added to a prior 
art laminated separator, that consequently increases the air flow 
resistance. The Hoechst Celanese 3501 separator, which is a 1 mil 
polypropylene film provided with a blend of non-ionic and cationic 
surfactants and having an air flow resistance of &gt;200, gives an indication 
of the effect that surfactants have on the ionic transfer characteristics 
of a separator structure. 
Thus, the superposed combination separator of the present invention is 
resistant to degradation in the cell environment, possesses sufficient 
thickness to maintain interelectrode separation, has sufficient surface 
energy to promote electrolyte wettability and absorption, has relatively 
high electrical resistivity to prohibit the establishment of short circuit 
currents flowing directly between the electrodes through the separator, 
and additionally has sufficient porosity to allow ionic transfer within 
the electrolyte to occur unimpeded during intended cell discharge. 
It is intended that the foregoing description of the preferred embodiments 
of the present invention be only representative, and that the present 
invention be only limited by the hereinafter appended claims.