CA1273396A

CA1273396A - Battery separator assembly

Info

Publication number: CA1273396A
Application number: CA000514353A
Authority: CA
Inventors: Mary Roberta Suchanski; Marilyn Ann Faust; Hans Walter Osterhoudt
Original assignee: Eastman Kodak Co
Current assignee: Ultralife Corp
Priority date: 1986-05-19
Filing date: 1986-07-22
Publication date: 1990-08-28
Anticipated expiration: 2007-08-28
Also published as: MX163326A; KR900007730B1; EP0246866B1; EP0246866A1; CN87103680A; JPS62283553A; JPH0433108B2; AU7310887A; DE3767916D1; US4741979A; KR870011714A

Abstract

BATTERY SEPARATOR ASSEMBLY
Abstract of the Disclosure A separator assembly for use in batteries comprising a film bearing a thermal fuse in the form of a layer of wax coated fibers is disclosed.

Description

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BATTERY SEPARATOR ASSEMBLY
Field of the Invention This invention relates to a battery separator as~embly and to batteries containing the assembly.
BACKGROUND OF THE INVENTION
A battery generates electrical energy by chemical action oi` electrodes of opposite polarity in an electrolyte. Sometimes a battery is ~hort circuited, causing the battery to overheat. Over-heatin8 can result in the emission of an electro-lyte, vapor, or molten electrode material. In some situations, explo3ions may occur. Thus, battery overheating can be dangerous to the user and others in the environment in which the battery is used.
U.S. Patent 4,075,400 discloses a battery, comprising a lithium anode, a SOC12 cathode and a woven electrically insulating separator. Overheat-ing i-~ prevented in thi~ battery by the use of a thermal fuse. The fuse consists of a plurality of encap~ulated particles containing a poisoning agent. The encapsulated particle~ are embedded in the ~ibers of the separator &5 particulate mate-rials. When the battery of thi3 patent reaches a predetermined temperature, the encapsulating mate-rials relea~e the poisoning agent. The released poisoning agent then deactivates the battery by ~hutting down current flow or combining chemically with one or more elements of the battery. The poisoning agent may be a wax.
However, tests have shown that the presence of unencapsulated particulate waxes on the separator or adjacent to a battery electrode can degrade the electrical performance of batteries. The average service life, energy delivered and peak power will be below that of batteries which include no parti-culated wax particles on the separator.

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It is desirable to improve the performance of batteries having a thermal fuse.
SUMMARY OF THE INVENTION
The present invention provides a separator a~sembly for use in batterie~ comprising a fllm bearing a thermal fuse in the form of a layer of wax coated fibers.
In batteries using this separator assembly, the service life and energy delivered are not adversely ~ffected compared to batteries of U.S.
Patent 4,075,400. Moraover, the thermal fuse used in the separator assembly of this invention does not require the step of encapsulatlng the material used as the fuse.
In a preferred embodiment, this invention makes pos~ible a battery comprising an anode, a cathode, an electrolyte and a separator assembly according to the present invention between the anode and the cathode.
In a most preferred embodiment, batteries made possible by this invention comprise a lithium anode and a manganese dioxide (MnO2) cathode.
Such batteries exhibit improved lo~d voltage, service life and energy extracted when pulse discharged compared to ~imilar batteries in whlch ~eparators without a thermal fuse are employed. These improvements are unexpected since one skilled in the art would expect that a thermal fuse would increase internal battery resistance, thereby decreasing the electrical performance of batteries.
DETAILS OF THE INVENTION
The anode, the cathode, the separator, the thermal fuse and the technique by which they are brought together to form the battery oE this inven-tion will be described. In this description of the 73~3~

invention, lithium anodes and MnO2 ca~hodes are used. It will be recognized that the separator assembly having the thermal fuse herein described will work in most batteries.
The separator assembly must be sufficiently porous to sllow a continuous flow of ions between the anode and the cathode. This means that both, the thermal fuse (layer of wax coated fibers) and the film bearing the thermal fuse are porous. The materials from which the separator a~sembly are made must also be a) insoluble in the electrolyte b) electrically insulating and c~ capable of physically separating the anode and the cathode to prevent internal shorting.
A wide variety o$ fabrics can be used as the film layer of the separator assembly.
Especially useful fabrics are non-woven polymerics such as Kiara~ 9120 or 9123 (60% polyester, 40%
polyethylene, fabric density of 7 gm/m and 14 gm/m , respectively, Chicopee Industrial Division, New Brunswick, NJ), and Pellon po~yester with a fabric density of 20.9 gm/m . This includes woven and non-woven fabrics.
The layer of waxed fibers does not form a coatin~ over the 5urface of the film that closes the pores of the film. Otherwise the flow of ions which is essentlal to current flow will be inhibited.
Most substances which can be applied to the fibers ~hich will melt and flow at the desired 3~ temperature can be used as the wax to form the thermal fuse.
Waxes having a melting point in the range of 30 to 200C preferably 50 to 150C will be u~eful. Examples of particularly use~ul waxes are Tissue Prep (combination of paraffin and a thermo-plastic polymer, m.p. 56-7C), beeswax (Kodak, ., ."" ,.. . . .

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White, m.p. 63C), microcrystalline wax (Strahl &
Pitsch #96, m.p. 63-65C), candellila (Strahl &
Pitsch, m.p. 67-70C), Polywax~ 500 (Petrolite Corp., m.p. 79.9~C by DSC), rice bran wax (Frank B.
Ros~ Corp., m.p. 81C by DSC), Epolenes C-18 (95-97C) and E-14 (m.p. 100C) (Eastman Kodak Company), Petrolite Bareco hard microcrystalline C700 (m.p. 91C) (Petrolite Corp.) and Ross Wax 160 (Frank B. Ross Co., Inc., m.p. 143-157~C).
Standard costing technology can be used to apply the wax (brushing, spraying, dipping, knife coating, roller coating, electrostatic spraying, airless spray, fluidized bed, etc.) Separator assemblies and batteries, includ-ing such sssemblie~, were generally constructed as follows.
A separ~tor assembly was prepared with a 1.25 inch wide strip of non-woven polyethylens coated polyester fabric (Kiara~ 9123). The fibers of the fabric were coated with rice bran wax. Any of the previously described waxes could also be used. The wax was melted in a 200 cc, three-neck round-~ottom flask by means of a hesting mantle.
The ~lask temperature wa~ controlled in the range of 80 to 90C by a variable autotransformer. The wax was sprayed onto the fibers of the non-woven fabric with an air brush (14 lbs. sir pressure, 15 cm distance from nozzle tip to fabrlc). A 6.35 mm (1/4 inch) o.d. ~tainless steel tube which was immersed in the molten wax served to deliver the wax to the nozzle of sn air brush which was wrapped in heating tape to prevent clogging of the spray nozzle with solid wax. The f&bric traveled past the spray nozzle at a rate ot I.524 m/min (5 ft/min) by means of motor controlled delivery and wind-up rolls.

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After spraylng the wax onto the fabric, the wax was in particulate form on the fibers of the fabric. However the particulate wax psrticles, if left in this condition, would degrade battery performance. To svoid this, an infrsred lamp line heater with an ellipticsl mirror was placed between the air brush and the wind-up roll to fuse the wax on the fabric fibers. The voltage to power the heater was controlled and the distance between the front surface of the lamp housing and the fabric was ad~usted to achieve the desired fusing. When the fusing was completed, the fabric was porous although the individual fibers were wax coated. This formed the thermal fuse.
The above 3.18 cm wide strip of waxed i`iber~ was pressure laminated to a 3.18 cm wide microporous strip of 0.025 mm thick polypropylene film (Celgardl~ 2500) by simultsneously feeding the wsxed flber layer and microporous polypropylene film through a pair oi polypropylene rollers to form a separator sssembly comprising a thermal fuse.
In battery construction the waxed layer can face either the cathode or the anode, and in battery a~sembly the waxed separator may be applied either to the csthode or to the anode. In the example described herein~ the waxed layer faced the snode.
The anode was essentially a two piece laminate comprised of lithium coated on a stainless steel foil current collector. A portion of the stainless steel foil is left uncoated and trimmed to form the anode terminal.
The separator assembly was pressed onto the surface of a lithium anode with the waxed fiber layer fscing the lithium. The anode consisted of a ~trip of 0.203 mm (0.008 inch) thick lithium foil laminated to a 0.025 mm (0.001 inch) thick piece of 304 stainless steel foil with the same dimen~ions.

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~ ~7;3~rCi The MnO2 cathode comprised 2 RtainleS8 steel grid current collector coated on one or both sideq wlth a mixture of MnO2, c~rbon and Teflonm. A
smsll portion of the stainless steel current collector was left uncoated and shaped st one end to function a5 a cathode terminal.
A complete electrode assembly was made by positioning the cathode on top of the separator attached to the anode so that the cathode terminal and the anode terminal are side by side but are not in electrical contact. The cathode, in this embodi-ment of the invention, is about one-half the length of the anode. The entire anode is then folded over the entire cathode to form a laminate structure in which the cathode is sandwiched between the folds of the anode.
Next, the complete electrode as~embly was then accordion folded or rolled into a configuration that would essentially fill the cell space.
After checking for internal electrical shorts with an ohmmeter, three such electrode assemblies are made into a battery by first insert-ing the assembly into a battery case having 8 separate compartment for each assembly. The three electrode assemblies were then electrically connected in ~eries. An electrolyte comprising, for example, a 70:30 volume percent solvent mixture of 4-butyrolactone and dimethoxyethane containing lM
LiBF4 was added to the battery to complete a three cell 9V battery. Other electrolytes are LiCF3S03 in a solvent mixture of propylene carbonate and di-methoxyethane or in a solvent mixture of 4-butyrol-actone and dimethoxyethane.
Other useful anode materials include alkali metals (Na and K), Li-Al alloys, Li-Sl alloys, Li-B
alloys and the metals o~ Groups Ia and IIa of the ~73~3~

periodic table of elements. Metal foils which can be used as the current collector and support include metals such as nickel, stainless steel, aluminum and titanium.
The wide variety of cathode materials which would be useful in the electrode assemblies o~ this invention includes, ln addition to MnO2, FeS2, FeS, CuO, Bi203 and various ~orms of polyfluorocarbons, i.e. (CFX)n wherein x is < 1.2, and n is some indet-erminate large number.
Electrical Behavior of the Batteries of thiS
Invention 1. Shorting Batteries prepared according to the above described procedure with and without ~eparator assemblies containing a thermal fuse were shorted through a 0.04 ohm lead. The battery skin-temperature and current were monitored as a function of time during shorting. The comparative shorting data are summarized in Table I. Batteries wlth and without the thermal fuse had similar initial limit-ing currents (IQ) (here "limiting current" and "~hort circuit current" have the same meaning).
This was indicative of batteries with similar power. However, the battery which contained the thermal fuse (20 gm/m rice bran) had a maximum skin temperature (T) of 74C, whereas the two control batteries without the thermal use had maximum skin temperatures of 112C and 108C, re~pectively. The battery that contained the thermal fuse did not vent or swell. However, both control batteries swelled and vented a significant amount of electrolyte.

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TABLE I
Thermal ProPerties of Batteries with and without a Thermal Fuse ExamPle Control SeparatorKiara~ 9123 fabric Celgard~ 4510*
AssemblYwith fiber~ coated with rice bran WAX
pressure-laminated to Celgard~ 2500 lO wax loading (g/m2) 20 0 0 Shorting IQ (A) 2.6 2.5 2.7 Tskin maximum ~C) 74 112 108 time (minutes3 to Tskin 7 13 11 maximum Case integrity very venting venting slight leak *Celgard~ 4510 is Celgard~ 2500 plus a non-woven f~bric layer without wax.

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2. Continuous Dischar~e Throu~h 90 ohms (Q) The continuous discharge performance of the battery containing the thermal fuse described in T&ble I wa9 compared to that of four batteries made in a similar manner without the thermal fusz.
Discharge was through a 90Q load to a 6 volt cutoff. Average values of load voltage, charge extracted and energy extracted for the pair employ-ing the thermal fuse were Wi thin 3 percent of the v~lues observed for the batteries without the thermal fuse. It can be concluded that continuous discharge performance at a current drain of approxi-mately 80 mA is not adversely affected by the presence of the thermal fuse.

3. Galvanostatic Pul~ed Dischar~e at 0.45 AmPs ~ alvanostatic pulsed discharge at 0.45 amps, lO percent duty cycle (3 seconds on, 27 seconds off)~ to a 3.5 volt cutoff, was employed to evaluate batteries with and without the thermal fuse. Two batteries of each type were tested.
These test conditions were selected to simulate a typical f1ash/charging cycle in a camera. Average values of the test data for each pair of batterie~
in fact fflvored the bAtteries employing the thermal fuse for three critical parameters. LoRd voltage was 5 percent higher; charge extracted was 8 percent higher; and energy extracted was 14 percent higher for batteries employing the thermal fuse compared to the batteries without the thermal fuse. It can be concluded that ~alvano~tatic discharge performance at 0.45 amps, lO percent duty/cycle is not adversely affected by the presence of the thermal fuse.
The foregoing data demonstrAte the separator assembly contsining the thermal fuse providing thermal control of the battery when ~ ~ ~7;~{~i -shorted, and is not detrimental to bat~ery electri-cal performance at moderate current continuous drain or high current pulsed drain.
Performance of Batteries havin~ Particulate Thermal Fuse This example demonstrates the performance of batteries comprising a separator assembly having particulate wax particles as the thermal fuse similar to the thermal fuse of U.S. Paten~ 4,075,400.
A difterent wax wa~ u~ed in this compara-tive example (m.p. 64C) than that ussd in the exemp]ified batteries of this invention. The wax used wa~ lower melting and was expected to shut the battery down sooner than a higher melting wax.
However, we hfive no reason to believe that it would adversely affect the electrical properties of the battery.
Also note that this comparative example employed a commercially prepared prelaminate of a non-woven polypropylene fabric and a microporous polypropylene onto which the wax was sprayed. Such a separator a~sembly is quite similar to that pre-pared in the foregoing exsmple by pres~ure-laminating the waxed non-woven fabric to microporous Celgard~ 2500. In the present example, however, the wax particles were not fused.
A. Fabrication ~ 3.18 cm (1.25 inch) wide strip of separator fabric (Celgard~ 4510) was coated with a 50:50 (wt) mixture of Candellila wax and Tissue Prep~. The wax was applied, using the spray method described previously herein, to the non-woven side of the Celgard separator. No fusing step was u~ed to melt the wax. By this technique it was observed that the wax particulates were not confined to the non-~oven fabrlc fibers.

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The separator containing the particulate wax~ (about 20 gm/m ), was pressure laminated onto the surface of a lithium flnode with the waxed l~yer facing the lithium. A series of such anodes was combined with the previously described MnO2 cathodes. The combinations were made into batteries as previously described.
B. Electrical Behavior 1. Shorting ~o Sample batteries with the separator con-taining particulate wax as described above were shorted through a 0.04 ohm lead and both ~kin temperature and current were monitored. The sXin temperature rose only to 68C with a strong trunca-tion of the current noted due to presence of the wax particles. The batteries did not swell or vent.
2. Continuou~ Discharge Through 90 ohm ~Q) Batteries prepared ~u~t as those for the shorting test above were discharged through A 90 ohm load to a 6 volt cutoff. The average value~ for service life and energy delivered were 10-15 percent below the values obtained for batteries having the separator ~ssembly used in the present invention.
The foregoing data show that batteries comprising the separator assembly of the invention unexpectedly improve the load voltage, service life and the energy extracted from a battery when pulse discharged compared to batteries in which a separator is used without a thermal fuse. One skilled in the art would have expected greater internal battery resistance and a degradation of battery pertormance due to the presence of the thermal fuse.
Moreover, the batteries comprising the separstor or assembly of this invention do not sufter the battery performance losses which occur in batteries using the particulate wax poisoning agents of U.S. Patent 4,075,400.

~'73 In contrast, the example of the batterles of this inventlon required application of the wax to the fibers of a non-woven fabric followed by fusing. While not wishing to be bound to a proposed explsnation of the unexpected advantages from the present invention, fusing is belieY,ed to cause the wax to adhere to the fibers. Thus, any method which adheres the wax to the fibers will be useful in making the 3eparator assembly of thls invention.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations ~nd modifications can be effected within the spirit and scope of the invention.

Claims

WE CLAIM:

1. A porous separator assembly for use in batteries comprising a film bearing a thermal fuse in the form of a layer of wax coated fibers.

2. The separator assembly of claim 1 wherein the film is microporous.

3. The separator assembly of claim 1 or 2 wherein the wax has a melting point of 50 to 150°C.

4. The separator assembly of claim 1 or 2 wherein the wax is selected from the group consist-ing of a) a mixture of paraffin and a thermoplastic polymer, b) beeswax, c) microcrystalline wax, d) candellila wax, e) rice bran wax, f) polyethylene wax, and g) polypropylene wax.

5. A battery comprising an anode, a cathode, an electrolyte and a porous separator assembly between the anode and the cathode; wherein the separator assembly comprises a film bearing a thermal fuse in the form of a layer of wax coated fibers.

6. The battery of claim 5 comprising a lithium anode.

7. The battery of claim 5 comprising a MnO2 cathode.

8. The battery of claim 5, 6 or 7 wherein the wax is selected from the group consisting of a) a mixture of paraffin and a thermoplastic polymer, b) beeswax, c) microcrystalline wax, d) candellila wax, e) rice bran wax, f) polyethylene wax, and g) polypropylene wax.

9. The battery of claim 5, 6 or 7 wherein the wax has a melting point in the range of 50 to 150°C.

10. A battery comprising a lithium anode, an MnO2 cathode, an electrolyte comprising 1M
LiBF4 in a solvent mixture of butyrolactone and dimethoxyethane and a porous separator assembly between the anode and cathode; wherein the separator assembly comprises a porous film bearing a thermal fuse in the form of a porous layer of wax coated fibers.

11. A battery comprising a lithium anode, an MnO2 cathode, an electrolyte comprising 1M
LiCF3SO3 in a solvent mixture of butyrolactone and dimethoxyethane and a porous separator assembly between the anode and the cathode; wherein the separator assembly comprises a porous film bearing a thermal fuse in the form of a porous layer of wax coated fibers.