CA2103733A1 - Apparatus for pinhole detection and process for improving yield in the manufacture of batteries - Google Patents
Apparatus for pinhole detection and process for improving yield in the manufacture of batteriesInfo
- Publication number
- CA2103733A1 CA2103733A1 CA002103733A CA2103733A CA2103733A1 CA 2103733 A1 CA2103733 A1 CA 2103733A1 CA 002103733 A CA002103733 A CA 002103733A CA 2103733 A CA2103733 A CA 2103733A CA 2103733 A1 CA2103733 A1 CA 2103733A1
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- Prior art keywords
- sample
- gas
- film
- grid plate
- periphery
- Prior art date
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- Abandoned
Links
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- 229910052744 lithium Inorganic materials 0.000 claims abstract description 19
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 claims abstract description 15
- 229920000098 polyolefin Polymers 0.000 claims abstract description 7
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- 238000011144 upstream manufacturing Methods 0.000 claims description 21
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- 210000004027 cell Anatomy 0.000 description 26
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
- H01M10/4285—Testing apparatus
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/411—Organic material
- H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
- H01M50/417—Polyolefins
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M3/00—Investigating fluid-tightness of structures
- G01M3/02—Investigating fluid-tightness of structures by using fluid or vacuum
- G01M3/04—Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point
- G01M3/12—Investigating fluid-tightness of structures by using fluid or vacuum by detecting the presence of fluid at the leakage point by observing elastic covers or coatings, e.g. soapy water
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M3/00—Investigating fluid-tightness of structures
- G01M3/02—Investigating fluid-tightness of structures by using fluid or vacuum
- G01M3/26—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
Landscapes
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Manufacturing & Machinery (AREA)
- Cell Separators (AREA)
Abstract
APPARATUS FOR PINHOLE DETECTION AND PROCESS FOR
IMPROVING YIELD IN THE MANUFACTURE OF BATTERIES
ABSTRACT
This invention pertains to microporous plastic films and to methods of inspecting such films. A signifi-cant number of rechargeable lithium batteries can be defective as fabricated and must be rejected. Flaws in the microporous polyolefin film separator, consisting of pinholes ?10 µm in size, can be a cause for many defective cells. A novel apparatus and process is disclosed to detect the rare occurrence of pinholes over large areas in microporous polyolefin film separators used in rechargeable lithium batteries to improve battery yield.
IMPROVING YIELD IN THE MANUFACTURE OF BATTERIES
ABSTRACT
This invention pertains to microporous plastic films and to methods of inspecting such films. A signifi-cant number of rechargeable lithium batteries can be defective as fabricated and must be rejected. Flaws in the microporous polyolefin film separator, consisting of pinholes ?10 µm in size, can be a cause for many defective cells. A novel apparatus and process is disclosed to detect the rare occurrence of pinholes over large areas in microporous polyolefin film separators used in rechargeable lithium batteries to improve battery yield.
Description
~103 ~33 APPARATU~ FOR PINHOLE DETECTION AND PROCE~8 FOa INPROVING YIELD IN THE MANUFACTURE OF BATTERIE~
FIELD OF THE INVENTION
This invention pertains to microporous plastic films and specifically to methods of inspecting such films.
More particularly, the invention pertains to detecting the occurrences of pinholes in microporous polyolefin film separators and the use of such film in rechargeable bat-teries and manufacturing methods thereof.
BACKGROUND OF THE INVENTION
Microporous film, in which microscopic holes or pores typically less than 1 micrometer in size are created in a plastic film, has been commercially available for many years. The applications for such films are numerous and have found use in the biological or medical industries and clothing industry amongst others. A potential defect in these films is the occasional occurrence of an oversize pore, often referred to as a pinhole. In some applica-tions, the occasional occurrence of a pinhole is not a serious concern and can be accepted, for example, water vapour breathable clothing. In other applications, the presence of pinholes is a serious concern, for example, when complete separation of bacteria from a ~edium is desired. Typically the number of microscopic pores per unit area in such microporous films is of the order of 101/cm2. When the application requires the use of signifi-cant areas of film the number of pores can exceed 10l2.
A significant number of rechargeable lithium batteries can be defective as fabricated and must be rejected. Flaws in the microporous polyolefin film separ-ator, consisting of pinholes <10 ~m in size, can be a cause for many defective cells.
FIELD OF THE INVENTION
This invention pertains to microporous plastic films and specifically to methods of inspecting such films.
More particularly, the invention pertains to detecting the occurrences of pinholes in microporous polyolefin film separators and the use of such film in rechargeable bat-teries and manufacturing methods thereof.
BACKGROUND OF THE INVENTION
Microporous film, in which microscopic holes or pores typically less than 1 micrometer in size are created in a plastic film, has been commercially available for many years. The applications for such films are numerous and have found use in the biological or medical industries and clothing industry amongst others. A potential defect in these films is the occasional occurrence of an oversize pore, often referred to as a pinhole. In some applica-tions, the occasional occurrence of a pinhole is not a serious concern and can be accepted, for example, water vapour breathable clothing. In other applications, the presence of pinholes is a serious concern, for example, when complete separation of bacteria from a ~edium is desired. Typically the number of microscopic pores per unit area in such microporous films is of the order of 101/cm2. When the application requires the use of signifi-cant areas of film the number of pores can exceed 10l2.
A significant number of rechargeable lithium batteries can be defective as fabricated and must be rejected. Flaws in the microporous polyolefin film separ-ator, consisting of pinholes <10 ~m in size, can be a cause for many defective cells.
- 2 - '~103733 SUMMARY OF THE INVENTION
An apparatus has been developed that adapts conventional bubble point determination equipment for gas flow analysis of micr/oporous polyolefin film to detect undesirable pinholes. Since the pinholes are very small and are rare in occurrence, the flow of gas through such pinholes is also small. Very small gas flows must be detected so it is critical to have a reliable leak free seal between the sample to be tested and the sample holder.
The flow of gas around a faulty seal into the downstream side of the sample holder could wrongfully be interpreted as the presence of pores by flow rate measurement alone.
The simple application of a grease commonly employed in vacuum applications effectively seals the irregular porous surface of such film to the holder. Additionally, use of upstream and downstream flow measurement devices allows confirmation that said seal is generally sound and that some of the applied gas does not leak out of the apparatus.
Another essential requirement for accuracy is that of im-proved support for the film under application of signifi-cant gas pressure. Relatively fragile microporous film is easily stretched if inadequately supported. Such stretching causes the pores to enlarge thus defeating the purpose of the test. Careful attention to the flatness of the support is sufficient to achieve this result.
Generally such apparatus, if adapted to readily test large areas of microporous sample, can be gainfully used to determine the probable presence of pinhole defects per unit area of film. For applications where such defects are of critical importance, quantification of this prob-ability is useful in choice of product design (based on typical probability values) or process decisions (based on specific probability values). In particular, batches of film with a relatively high proportion of pinholes may be rejected for use in a critical application for economic - 3 _ ~a3733 reasons and instead be used in applications where such relatively high proportion is not so critical.
One such critical application is in the manufac-ture of rechargeable lithium batteries which utilizemicroporous film as a separator material. We have dis-covered that a substantial number of such batteries can be defective after manufacture as a result of internal elec-trical current shorts. In turn, we have determined that these internal shorts are associated with pinhole defects in the microporous film used as the separator material in such batteries. With this forehand knowledge, we have invented a method that uses this probability as a rejection criterion on incoming microporous film. The result is a substantial yield improvement with respect to internal shorts, with a corresponding reduction in waste and overall savings in time and money.
The invention is directed to an apparatus for the non-destructive detection of pinhole defects in a sample of microporous film by flowing gas through the sample compris-ing: (a) gas supply means; (b) gas regulator means con-nected with the gas supply means; (c) sample holder means which seals the sample to the holder means around the periphery of the sample; (d) gas pressure means for measur-ing the gas pressure in the apparatus;
The sample holder can have a grid plate with a plurality of holes therein, the sample being sealed around its periphery to the upstream side of the grid plate. A
steel screen can be positioned between the downstream side of the sample and the upstream side of the grid plate.
Said sample holder can be constructed of a sight frame and a base plate which are releasably secured together by a plurality of toggle clamps.
An apparatus has been developed that adapts conventional bubble point determination equipment for gas flow analysis of micr/oporous polyolefin film to detect undesirable pinholes. Since the pinholes are very small and are rare in occurrence, the flow of gas through such pinholes is also small. Very small gas flows must be detected so it is critical to have a reliable leak free seal between the sample to be tested and the sample holder.
The flow of gas around a faulty seal into the downstream side of the sample holder could wrongfully be interpreted as the presence of pores by flow rate measurement alone.
The simple application of a grease commonly employed in vacuum applications effectively seals the irregular porous surface of such film to the holder. Additionally, use of upstream and downstream flow measurement devices allows confirmation that said seal is generally sound and that some of the applied gas does not leak out of the apparatus.
Another essential requirement for accuracy is that of im-proved support for the film under application of signifi-cant gas pressure. Relatively fragile microporous film is easily stretched if inadequately supported. Such stretching causes the pores to enlarge thus defeating the purpose of the test. Careful attention to the flatness of the support is sufficient to achieve this result.
Generally such apparatus, if adapted to readily test large areas of microporous sample, can be gainfully used to determine the probable presence of pinhole defects per unit area of film. For applications where such defects are of critical importance, quantification of this prob-ability is useful in choice of product design (based on typical probability values) or process decisions (based on specific probability values). In particular, batches of film with a relatively high proportion of pinholes may be rejected for use in a critical application for economic - 3 _ ~a3733 reasons and instead be used in applications where such relatively high proportion is not so critical.
One such critical application is in the manufac-ture of rechargeable lithium batteries which utilizemicroporous film as a separator material. We have dis-covered that a substantial number of such batteries can be defective after manufacture as a result of internal elec-trical current shorts. In turn, we have determined that these internal shorts are associated with pinhole defects in the microporous film used as the separator material in such batteries. With this forehand knowledge, we have invented a method that uses this probability as a rejection criterion on incoming microporous film. The result is a substantial yield improvement with respect to internal shorts, with a corresponding reduction in waste and overall savings in time and money.
The invention is directed to an apparatus for the non-destructive detection of pinhole defects in a sample of microporous film by flowing gas through the sample compris-ing: (a) gas supply means; (b) gas regulator means con-nected with the gas supply means; (c) sample holder means which seals the sample to the holder means around the periphery of the sample; (d) gas pressure means for measur-ing the gas pressure in the apparatus;
The sample holder can have a grid plate with a plurality of holes therein, the sample being sealed around its periphery to the upstream side of the grid plate. A
steel screen can be positioned between the downstream side of the sample and the upstream side of the grid plate.
Said sample holder can be constructed of a sight frame and a base plate which are releasably secured together by a plurality of toggle clamps.
- 4 _ 2~03 i 3 ~
The apparatus can include sealing means for sealing the periphery of the sample to the periphery of the grid plate and the apparatus. The sealing means can be grease. The upstream side of the grid plate can be flat.
The upstream side of the grid plate can be flat to a toler-ance of plus or minus 25 micrometers.
An oil trap means can be connected downstream of the sample holder means. The sample holder can have a sample observation window.
First gas flow measuring means can be connected downstream of the gas supply means and upstream of the sample holder means. Second gas flow measuring means can be connected downstream of the gas supply means and the sample holder means.
The invention is also directed to a method of detecting the existence of pinhole defects in a sample of microporous film comprising: sealing the periphery of the sample with a sealing means to prevent seepage of gas around the periphery of the sample, maintaining the sample in a flat configuration, applying a wetting agent to a downstream side of the sample, applying a gas pressure on an upstream side of the sample, and determining the rate of flow of gas through the sample. The gas pressure can be between 5 and 25 psig.
The method can comprise observing the downstream side of the sample for the occurrence of gas bubbles in the wetting fluid. Equivalent diameter of pinhole defects can be in the range from less than about 10 micrometers to greater than about 0.1 micrometers.
The invention is also directed to a method wherein the yield of internal electrical short free rechargeable lithium batteries during manufacture is ':
~ 2~03733 improved by quantifying the probability of the presence of pinhole defects per unit area of a microporous polyolefin film batch and discarding an unacceptable batch, which comprises employing the principle of bubble point determi-nation to analyse a substantially large area of a filmsample, or a group of samples, from said batch and applying predetermined criteria for rejecting a film batch based on said probability.
Gas pressure can be applied to random samples from the film batch and the rate of gas flow is measured upstream of and downstream from said sample. The equival-ent diameter of said pinhole defects can be in the range from less than about 10 micrometers to greater than about lS 0.1 micrometers.
A wetting fluid can be applied to a downstream side of a sample and observing the presence of bubbles on the downstream side of the sample. A periphery of the sample can be sealed and the sample can be maintained in a flat configuration.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate specific embodiments of the invention, but which should be not regarded as restricting or limiting the spirit or scope of the inven-tion in any way:
Figure 1 depicts a schematic of prior art appar-atus used for bubble point and flow pore testing of mem-brane filters (reprinted from ASTM F316-86).
Figure 2 depicts a sample holder apparatus from the prior art used for bubble point and flow pore testing of membrane filters (reprinted from ASTM F316-86).
2io3733 Figure 3a depicts a cross-sectional end view of the invention sample holder apparatus.
Figure 3b depicts an enlargement of Figure 3a showing the sample near the seal area.
Figure 3c depicts a top view of the invention sample holder apparatus.
Figure 3d depicts a side view of the invention sample holder apparatus.
Figure 4a depicts a histogram of frequency at varied flow rates for samples tested at 10 psig in inven-tion Example 3.
Figure 4b depicts a histogram of frequency atvaried flow rates for samples tested at 25 psig in inven-tion Example 3.
DETAILED DESCRIPTION OF SPECIFIC
EMBODIMENTS OF THE INVENTION
While it has often been possible to achieve a zero number of pinholes in an individual item made with a microporous film, it has been commercially impractical to ensure that microporous film used in mass produced items meets such a requirement. Where zero pinholes in such a mass produced item is an absolute requirement, alternative solutions must be found. One method is to use an aligned similar redundant film in the product design. Thus, while each individual film may have an unacceptable presence of pinholes, the probability of such pinholes aligning in two films placed in series is so small that it appears zero.
The negligible probability of there being an effective hole through both is acceptable in such situations. Another method is to inspect the film or manufactured item for 2~ l33 defects and only use those that are defect free. The redundant film method is commonly accepted practice.
However, twice as much film is required to provide redun-dancy. As microporous film is reasonably expensive, the redundant film solution can be undesirable. The latter inspection method is effective if the defects can be readily detected but this method too can be prohibitive if the cost of the inspection and/or the rejected items is too high.
Use of microporous film in commercial battery construction has been increasing in recent years since such film can serve as a thin, low ionic impedance, inert, consistent separator. For rechargeable lithium metal anode batteries, microporous film is a critical component.
During the charging process in such batteries, lithium metal is extracted from the cathode and must be plated back onto the lithium metal anode. The plating is not 100%
efficient and dendritic lithium deposits can form during this process. Typically, these batteries are made with excess lithium metal such that even though lithium is lost due to inefficient plating, a reasonable number of re-charges is possible. A minimum stack pressure is also required to obtain a reasonable number of recharges as described in U.S. patent No. 5,114,804.
Most importantly, a microporous film is used as a separator in order to prevent the formation of dendritic lithium deposits that bridge the gap between the anode and the cathode in such batteries. Batteries that use separ-ators with an average effective pore size exceeding 1 micron in size typically cannot recharge an adequate integrated amount of lithium per unit electrode area to be acceptable as a practical rechargeable battery product.
Failure to recharge in such batteries occurs at early cycle numbers as a result of a plated dendritic lithium deposit bridging the cathode and anode. This bridge electrically ~10373~
connects the two electrodes forming an internal short circuit. Non-woven separators and solid polymer electro-lytes used as separators are examples of separators with effective pore sizes exceeding 1 micron.
On the other hand, the use of microporous film suppresses the onset of dendrite bridging significantly such that practical rechargeable batteries can be made.
Microporous film does not completely prevent dendrite bridging but merely delays it. The common failure mode for such batteries is the formation of an internal short due to dendrite bridging. The Molicel~ B06 rechargeable lithium battery of the applicant is an example of such a product where failure occurs typically after a few hundred recharges.
In the manufacture of any rechargeable battery, it is important that the finished battery does not have any intsrnal shorts. A poor, unpredictable yield due to a significant number of such internal short defects is a serious economic problem. Such a problem has been experi-enced in the manufacture of the applicant's Molicel~ B06 battery (nominally a 800 mAh battery). Up to 17% of batteries made per production batch were rejected due to the existence of internal shorts after manufacture. In this battery product, the microporous film Celgard 2500~ is used as the separator. Part of the assembly process in~
volves an electrical conditioning step wherein the cathode material undergoes a phase transition by incorporating lithium into the cathode. This transition is simply achieved by discharging the battery. Then, a single re-charge is performed in order to put the battery in a fully charged condition ready for sale. It was after this elec-trical conditioning step that the majority of internal shorting defects could be detected. Thus, an inspection for internal shorting was performed on each battery so made by determining the voltage drop of each battery after a 14 day a3~33 g storage or qualification period. Typica]ly, batteries with no internal short showed a drop of only about 10 mV after a fourteen day period which corresponds to a 0.5 mAh drop in capacity. The speculated sources of these defects were numerous ranging from pressure related mechanical reasons, lithium surface condition, separator defects, and the like.
However, it was not possible to directly determine the cause of shorting by analysis before or after battery disassembly.
After an exhaustive preliminary investigati~e effort, during which several possible causes were rejected, pinhole defects in the microporous film separator remained the prime suspected cause of the internal shorting. It was unclear what pinhole size range and frequency would likely create this problem. Pinholes greater than 10 microns in size can typically be detected visually if suitable con-trasting techniques are used. No significant number were detected. Detecting pinholes in the 0.1 to 10 micron size range over large sample areas became the challenge. A
preferred conventional method is to use the one described in ASTM test method F316-86 for determining pore size characteristics of membrane filters by bubble point and mean flow pore tests. Commercially available apparatus obtained from Porous Materials Inc. has been useful for many applications but was found unsuitable for this analy~
sis due to the small sample size holders available, insuf-ficient membrane or film support, and inadequate seal at the sample perimeter. A method and apparatus specific to this problem was developed to overcome these difficulties.
The inventors have been able to confirm using the apparatus of the invention that most of the batteries that were defective contained separator film material from batches that had rare occurrences of pinholes in the size range 0.1 to 10 ~m. On the other hand, the film separators in batteries that were non-defective had close to none.
2~03733 With this data in mind, it was possible to determine the probability of presence of such pinholes per unit area for each supplied separator batch. This in turn could be correlated to yield results with respect to internal shorting. Yield could then be markedly improved at sub-stantial savings by inspecting and rejecting, as is occa-sionally required, separator batches prior to battery assembly.
The preferred apparatus of the invention for flow pore measurements on large microporous film samples is an inventive improvement over the apparatus described in ASTM
F316-86. Figure 1 illustrates a schematic of this prior art apparatus 15 (reprinted from said reference). The sample 2 (not shown in Figure 1, but see Figure 2) to be tested is clamped in the sample holder 3. Gas pressure 4 is applied on the upstream side of holder 3. This gas pressure is kept at a constant level by means of regulator 5. An accurate pressure gauge 6 is used to determine the upstream gas pressure. On the downstream side, an oil trap 7 is connected by line 16 to holder 3 and is used to trap wetting fluid liquid 8 (not shown in this Figure) and/or vapour exiting holder 3. A single rotameter 9 ( two possible alternative locations are shown, one upstream of the holder 3, the other downstream of the trap) or another suitable vapour flow measuring device is used to quantify gas flow rates.
Figure 2 shows a small sample holder 3 reprinted from ASTM F316-86 (maximum diameter mentioned is 4.7 cm).
The holder 3 consists of a base 10, a locking ring 11, an O-ring 12 for sealing, a support disc 13 and an upstream gas inlet 14. The support disc 13 is of two-ply construc-tion consisting of a 100 x 100 mesh or finer screen (not shown) and a perforated plate (not shown) for rigidity.
The film sample 2 to be tested is clamped between the 0-ring 12 and the support disc 13. For use in the apparatus - 11 2~37~3 15 of Figure 1, the exhaust of the holder 3 is connected to line 16.
The apparatus of the invention consists of a considerably altered larger, rectangular version of the sample holder 3 of Figure 2. This novel apparatus is used in the complete apparatus assembly 15 of Figure 1. Figure 3a illustrates a cross-sectional view of the invention holder apparatus 20. A large sample 21 to be tested is clamped in a large holder 20 (approximately 30 cm x 5 cm).
The film sample 21 is located adjacent to the base plate 22 and contacts O-ring 23. A grid plate 24 rests on the top surface of the edges of the sample 21. A window 28 is provided above the grid plate 24 and contacts second O-ring 29. Clamping force to the stack comprising the baseplate 22, grid plate 24, and window 28 is provided by the sample frame 30 and the sight frame 31. Toggle clamps 32 are provided for rapid sample loading and application of -clamping force. Inlet gas is admitted through inlet port 33 and exhaust gas is released through exhaust port 34.
Figure 3b is an enlargement of Figure 3a and shows the sample 21 in the vicinity of the O-ring seal 23 area. The recessed interior surface of the grid plate 24 contains 249 through holes 25, approximately 0.48 cm in diameter, arranged in a hexagonal close packed array. This - array serves to provide rigidity as is required for the perforated support disc described in the prior art.
Underneath the interior surface 26 of the grid plate 24 and attached to it, is a stainless steel wire mesh 27 ,105 spaces by 105 spaces per inch of .003" diameter wire. The mesh 27 serves the purpose of providing a finer screen as is taught in the prior art. Special attention must be paid to the tolerances that determine the smoothness of the surface of the screen 27 contacting the sample 21. Typical-ly, the "out-of-flat" tolerance is set at a maximum of plus or minus 25 micrometers.
~.
2~03733 Figures 3c and 3d show complete external views of the invention apparatus from the top and side respectively.
As seen in Figure 3c, six toggle clamps 32 are used to provide sound sealing around the entire periphery. Figure 3d illustrates the sight frame 31 and sample frame 30 as well as four of the toggle clamps 32.
This large holder 20, which is much more sensi-tive and reliable than the sample holder 3 shown in Figures1 and 2, is used in place of the small sample holder 3 in the complete apparatus 15 shown in Figure 1. In addition, two accurate flowmeters 9 are used in the positions indi-cated in Figure 1.
O~eration of the Invention Typically the sample 21 is approximately 30 cm by 4.6 cm in size (these values being the length used in the applicant's Molicel B06 battery product and the width of the film as supplied respectively).
To provide a sound seal, silicone-based vacuum grease is applied to the top and bottom edges of the sample 21. Additionally, vacuum grease is applied to both O-rings 23 and 29. Grease is applied to the sample 21 edges such that an adequate leak tight seal can be obtained to the irregular porous separator surface. Without grease, it is difficult to achieve a leak tight seal without the!need for enormous clamping force. The stack consisting of sample frame 30, base plate 22, O-rings 23 and 29, grid plate 24 and attached wire mesh 27 is assembled prior to testing.
Then, 30 cc of a low surface tension wetting fluid, FC-40 Freon (product of 3M Company) is poured into the grid plate 24 to wet the sample and seal the pores.
21037~3 The window 28 is put in place and the assembly is clamped by clamps 32 and sealed together by the force applied by the sample frame 30 and sight frame 31. The actual sample 21 area tested as described is about 95 cm2 since some edge area is used for sealing. Nitrogen gas pressure is applied via the inlet port 33.
As per ASTM F316-86, the pressure p, in psig, required to blow gas continuously through a pore of diam-eter d, in micrometers, can be determined from d = -where ~ = 16 dynes/cm (surface tension) for FC-40 Freon and C = 0.415.
Typically, increasing discrete pressure levels such as 5, 10, and 25 psig are applied, corresponding to pore diameters of 1.3, 0.67, and 0.27 microns respectively in this case. Several minutes are allowed for equilibra-tion following each pressure increase after which a leak check is performed. This check includes visually checking for bubbles in the vicinity of the sample sealing area as well as comparing readings of the dual flowmeters 9.
Bubbling near the sample sealing area may indicate leaks around the seal. Different readings from the dual flow-meters may indicate leaks out of the apparatus. Assuming there is no evidence of a leak, the flow rate readings are recorded along with any visual indication of bubbling which can be seen through window 28. The presence of bubbling, or flow readings well above the background level, indicate the presence of a pore or pores greater than or equal to the corresponding diameter derived from the preceding equation.
2~03733 Once the test criteria have been established, it is a straightforward process to determine statistics on the likelihood of having a pinhole greater than or equal to said discrete diameters in size per unit area of micropor-ous film by testing a suitable number of sample pieces.
For purposes of improving battery yield on manu-facture, such statistical tests are performed on incoming batches of microporous film intended for use as separators in such batteries. There are typically definite differ-ences in the average pinhole occurrence associated with a batch of supplied film. This can arise, for example, as a result of particles capable of penetrating the film being present on rollers used in the processing of or subsequent handling of the film at the manufacturer's plant or in transport. Batches with excessive occurrence of pinhole flaws per unit area can be rejected at inspection. This procedure can provide a significant increase in production efficiency with a corresponding reduction in the waste associated with ultimately rejecting entire battery assem-blies. The actual rejection criteria used is determined by specific economic factors, battery design used, and charac-teristics of the specific microporous film used. This determination will be apparent to those skilled in the rechargeable battery art.
Comparative Example 1 To confirm the deficiencies of the prioriart, commercially available apparatus from Porous Materials Inc.
was used to test small circular samples (approximately 1~"
diameter) of Celgard 2500~ microporous film. Using the recommended test method, bubbling due to leaks at the O-ring to sample sealing area was common at application pressures of about 25 psig. This test example demonstrates that the sealing method employed in this apparatus is '~103733 inadequate for low flow rate measurements on thin micropor-ous film.
Comparative ExamPle 2 Samples of Celgard 25000 microporous film were tested in the same apparatus as used in Comparative Example 1 up to 90 psig pressure (corresponding to a critical pore diameter of 0.08 ~m). Initial flow rates measured were of the order of 2 cc/min. However, at constant application of this pressure, the flow rate increased significantly with time. Typically, the flow rate might double after a few minutes.
Upon removal of the sample after testing, stret-ching and damage of the separator could be clearly seen to have occurred. A corresponding enlargement of the pores would be expected as the plastic film stretched under load, leading to an increased measured flow rate.
This test example demonstrates that the support method employed in this apparatus is inadequate for mea-surements of pores of the order of 0.1 ~m in size in thin microporous plastic film samples. Excessive stretching of the sample film, causing an enlargement of the pores, is allowed by the support used in this holder assembly.
Invention Example 1 Several large (approximately 30 cm x 4.6 cm) Celgard 2500~ samples were tested using the method and the apparatus as described in the preceding part of the dis-closure. Typically, leakage at the O-ring to sample sealing area did not occur based on visual observation through the window and on comparison of upstream and downstream flowmeter readings. Also, flow-rate readings were usually relatively stable over a period of several "' '-'"':~
2~3733 minutes under continued application of gas pressures up to 90 psig. No damage to the samples was visually apparent upon subsequent removal.
Thus, the apparatus of the invention was suitable for use in making low flow rate measurements on pores of thin microporous film down to a 0.1 ~ size range.
Invention Example 2 ~.
Eleven Molicel~ B06 cells manufactured by the applicant were obtained for pinhole analysis after the qualification/inspection step used to detect internal shorting had been performed. Four of these cells had no detectable internal shorts. The other seven cells had significant internal shorts. The cells were disassembled and the separators were recovered for analysis. Special methods were used for the disassembly since lithiated debris often sticks to the separator after such cells are electrically conditioned or cycled. Heat generated by reactions of this debris with air can cause damage to the separator. Briefly, the method used relied on rapidly performing the disassembly in very dry air followed by immersion of the separators in a solution that dissolved 25 Li. Typically, a warm bath containing tetrahydrofuran and -naphthalene was used for this purpose. With such treat-ment, Li was extracted at controlled temperature. The plastic separator film swelled slightly in such a bath aiding the removal of debris. The separator film was!then rinsed a few times and dried at ambient temperature where-upon the separator recovered from the slight swelling.
Each of the dual separator pieces from each cell were visually tested for presence of pinholes initially, and then at 5, 10, and 25 psig gas pressures progressively using the apparatus and method describèd in the preceding disclosure. The data from these measurements is summarized 2~03733 in Table 1. For all separator pieces from cells that had no internal shorts, no pinholes could be detected at any of the three pressures used, nor could bubbling be detected visually through the window. In five of the seven cells that did have internal shorts, at least one pinhole in at least one of the two separators from each cell was detected by observation of bubbling. The corresponding flow read-ings at 25 psig ranged from 0.67 cc/min to greater than 10 cc/min (off scale) for these samples.
Bubbling could not be seen in tests on the separators from two of the seven cells with internal shorts. It must be remembered though that not all the active area of the separator in the battery is tested since the active edges are used for sealing purposes. These two cells, #7 and #8 in Table 1, had somewhat higher flow rates at 25 psig pressure than that of the separators from cells with no internal shorts (the "background"). However, "back-ground" readings varied from 0.27 cc/min to 0.52 cc/min.
The reason for this significant variation in "background"
level is not completely understood. (Since gas diffuses through the head of wetting liquid to some extent, it was speculated that head height variations as a result of uncontrolled minor tilting of the sample holder may account for the background variation.) Thus, it was not possible to conclude positively that cell #7 had a pinhole based on the observed flow rate of 0.59 cc/min. Cell #8 gave a flow reading of 0.85 cc/min which was noticeably higher than the "background" range.
Although this example does not conclusively prove that internal shorting mainly occurs as a result of the presence of pinholes in the separator, it does strongly suggest this.
2iO3733 Invention Example 3 Several batches of new Molicel B06 cells were manufactured by the applicant. Both separator segments used in each cell came from the same supplied batch of separator. From these, a batch of cells with low incidence of internal shorting and a batch of cells with high inci-dence of internal shorting were selected. The former batch, hereinafter referred to as the "good" batch, com-prised 490 cells of which only one had an internal short.Thus the reject rate due to internal shorts was only 0.2%.
The latter batch, hereinafter referred to as the "bad"
batch, comprised 299 cells of which 40 had internal shorts.
Thus the reject rate here was 13.4%.
Numerous samples of unused separators used to prepare "good" and "bad" batches were tested at 5, 10, and 25 psig gas pressure using the invention apparatus and method as above described in the disclosure. Histograms of frequency at various flow rates observed for tests done at 10 and at 25 psig on these samples are shown in Figures 4a and 4b respectively. No high flow rate readings were detected on separator from the batch used in the "good"
cells. A large number of high flow rate readings were detected on separator used to make the "bad" cells. Enough incidence of high flow rate readings was measured to account for the observed number of internally shorted cells under the assumption that a flow rate higher than that observed for any of the "good" separator samples tested could correspond to a pinhole which in turn corresponds to an internal shorting site.
This example demonstrates that the presence of pinholes, as evidenced by high flow rates or visual detec-tion of bubbles at pressures lower than 25 psi, is associ-ated with internal shorting in these batteries. By perform-ing such analysis on incoming separator material, batches such as this "bad" one can be rejected before attempting cell manufacture. Such rejected separator is still accept-able in principle for other applications and thus need not be wasted.
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.
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` ` 2~03733 Table 1 __ . .
Cell Voltage Intemal Separator Flow . Flow Flow Obsen~ations# at short? segment (cc/min) (cc/min) (cc/min) inspection @ 5 @ 10 @ 25 l psig pSig . psig 1 2.038 No 1st 0.10 0.19 Q46 _ _ 2nd 0.07 0.14 0.36 2 2.040 No 1st 0.06 0.10 0.27 2nd 0.06 Q18 0.28 I . . . .
3 2.035 No 1st 0.08 Q 18 0.52 I_ 2nd 0.05 0.10 OAO
4 2.037 No 1st 0.07 0.10 0.40 Not avaiL
1.865 Yes 1st 0.05 Q08 0.27 l 2nd 0.20 0.33 0.67 Bubbling ¦
L coO
~nuously@ I
6 1.912 Yes 1st 0.10 0.20 0.47 2cd _ > 10 ~ 10 > 10 2 big bubbles ¦
7 1.906 Yes 1st 0.09 0.22 0.59 ll 2nd O.Q6 0.09 0.19 ¦¦
The apparatus can include sealing means for sealing the periphery of the sample to the periphery of the grid plate and the apparatus. The sealing means can be grease. The upstream side of the grid plate can be flat.
The upstream side of the grid plate can be flat to a toler-ance of plus or minus 25 micrometers.
An oil trap means can be connected downstream of the sample holder means. The sample holder can have a sample observation window.
First gas flow measuring means can be connected downstream of the gas supply means and upstream of the sample holder means. Second gas flow measuring means can be connected downstream of the gas supply means and the sample holder means.
The invention is also directed to a method of detecting the existence of pinhole defects in a sample of microporous film comprising: sealing the periphery of the sample with a sealing means to prevent seepage of gas around the periphery of the sample, maintaining the sample in a flat configuration, applying a wetting agent to a downstream side of the sample, applying a gas pressure on an upstream side of the sample, and determining the rate of flow of gas through the sample. The gas pressure can be between 5 and 25 psig.
The method can comprise observing the downstream side of the sample for the occurrence of gas bubbles in the wetting fluid. Equivalent diameter of pinhole defects can be in the range from less than about 10 micrometers to greater than about 0.1 micrometers.
The invention is also directed to a method wherein the yield of internal electrical short free rechargeable lithium batteries during manufacture is ':
~ 2~03733 improved by quantifying the probability of the presence of pinhole defects per unit area of a microporous polyolefin film batch and discarding an unacceptable batch, which comprises employing the principle of bubble point determi-nation to analyse a substantially large area of a filmsample, or a group of samples, from said batch and applying predetermined criteria for rejecting a film batch based on said probability.
Gas pressure can be applied to random samples from the film batch and the rate of gas flow is measured upstream of and downstream from said sample. The equival-ent diameter of said pinhole defects can be in the range from less than about 10 micrometers to greater than about lS 0.1 micrometers.
A wetting fluid can be applied to a downstream side of a sample and observing the presence of bubbles on the downstream side of the sample. A periphery of the sample can be sealed and the sample can be maintained in a flat configuration.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate specific embodiments of the invention, but which should be not regarded as restricting or limiting the spirit or scope of the inven-tion in any way:
Figure 1 depicts a schematic of prior art appar-atus used for bubble point and flow pore testing of mem-brane filters (reprinted from ASTM F316-86).
Figure 2 depicts a sample holder apparatus from the prior art used for bubble point and flow pore testing of membrane filters (reprinted from ASTM F316-86).
2io3733 Figure 3a depicts a cross-sectional end view of the invention sample holder apparatus.
Figure 3b depicts an enlargement of Figure 3a showing the sample near the seal area.
Figure 3c depicts a top view of the invention sample holder apparatus.
Figure 3d depicts a side view of the invention sample holder apparatus.
Figure 4a depicts a histogram of frequency at varied flow rates for samples tested at 10 psig in inven-tion Example 3.
Figure 4b depicts a histogram of frequency atvaried flow rates for samples tested at 25 psig in inven-tion Example 3.
DETAILED DESCRIPTION OF SPECIFIC
EMBODIMENTS OF THE INVENTION
While it has often been possible to achieve a zero number of pinholes in an individual item made with a microporous film, it has been commercially impractical to ensure that microporous film used in mass produced items meets such a requirement. Where zero pinholes in such a mass produced item is an absolute requirement, alternative solutions must be found. One method is to use an aligned similar redundant film in the product design. Thus, while each individual film may have an unacceptable presence of pinholes, the probability of such pinholes aligning in two films placed in series is so small that it appears zero.
The negligible probability of there being an effective hole through both is acceptable in such situations. Another method is to inspect the film or manufactured item for 2~ l33 defects and only use those that are defect free. The redundant film method is commonly accepted practice.
However, twice as much film is required to provide redun-dancy. As microporous film is reasonably expensive, the redundant film solution can be undesirable. The latter inspection method is effective if the defects can be readily detected but this method too can be prohibitive if the cost of the inspection and/or the rejected items is too high.
Use of microporous film in commercial battery construction has been increasing in recent years since such film can serve as a thin, low ionic impedance, inert, consistent separator. For rechargeable lithium metal anode batteries, microporous film is a critical component.
During the charging process in such batteries, lithium metal is extracted from the cathode and must be plated back onto the lithium metal anode. The plating is not 100%
efficient and dendritic lithium deposits can form during this process. Typically, these batteries are made with excess lithium metal such that even though lithium is lost due to inefficient plating, a reasonable number of re-charges is possible. A minimum stack pressure is also required to obtain a reasonable number of recharges as described in U.S. patent No. 5,114,804.
Most importantly, a microporous film is used as a separator in order to prevent the formation of dendritic lithium deposits that bridge the gap between the anode and the cathode in such batteries. Batteries that use separ-ators with an average effective pore size exceeding 1 micron in size typically cannot recharge an adequate integrated amount of lithium per unit electrode area to be acceptable as a practical rechargeable battery product.
Failure to recharge in such batteries occurs at early cycle numbers as a result of a plated dendritic lithium deposit bridging the cathode and anode. This bridge electrically ~10373~
connects the two electrodes forming an internal short circuit. Non-woven separators and solid polymer electro-lytes used as separators are examples of separators with effective pore sizes exceeding 1 micron.
On the other hand, the use of microporous film suppresses the onset of dendrite bridging significantly such that practical rechargeable batteries can be made.
Microporous film does not completely prevent dendrite bridging but merely delays it. The common failure mode for such batteries is the formation of an internal short due to dendrite bridging. The Molicel~ B06 rechargeable lithium battery of the applicant is an example of such a product where failure occurs typically after a few hundred recharges.
In the manufacture of any rechargeable battery, it is important that the finished battery does not have any intsrnal shorts. A poor, unpredictable yield due to a significant number of such internal short defects is a serious economic problem. Such a problem has been experi-enced in the manufacture of the applicant's Molicel~ B06 battery (nominally a 800 mAh battery). Up to 17% of batteries made per production batch were rejected due to the existence of internal shorts after manufacture. In this battery product, the microporous film Celgard 2500~ is used as the separator. Part of the assembly process in~
volves an electrical conditioning step wherein the cathode material undergoes a phase transition by incorporating lithium into the cathode. This transition is simply achieved by discharging the battery. Then, a single re-charge is performed in order to put the battery in a fully charged condition ready for sale. It was after this elec-trical conditioning step that the majority of internal shorting defects could be detected. Thus, an inspection for internal shorting was performed on each battery so made by determining the voltage drop of each battery after a 14 day a3~33 g storage or qualification period. Typica]ly, batteries with no internal short showed a drop of only about 10 mV after a fourteen day period which corresponds to a 0.5 mAh drop in capacity. The speculated sources of these defects were numerous ranging from pressure related mechanical reasons, lithium surface condition, separator defects, and the like.
However, it was not possible to directly determine the cause of shorting by analysis before or after battery disassembly.
After an exhaustive preliminary investigati~e effort, during which several possible causes were rejected, pinhole defects in the microporous film separator remained the prime suspected cause of the internal shorting. It was unclear what pinhole size range and frequency would likely create this problem. Pinholes greater than 10 microns in size can typically be detected visually if suitable con-trasting techniques are used. No significant number were detected. Detecting pinholes in the 0.1 to 10 micron size range over large sample areas became the challenge. A
preferred conventional method is to use the one described in ASTM test method F316-86 for determining pore size characteristics of membrane filters by bubble point and mean flow pore tests. Commercially available apparatus obtained from Porous Materials Inc. has been useful for many applications but was found unsuitable for this analy~
sis due to the small sample size holders available, insuf-ficient membrane or film support, and inadequate seal at the sample perimeter. A method and apparatus specific to this problem was developed to overcome these difficulties.
The inventors have been able to confirm using the apparatus of the invention that most of the batteries that were defective contained separator film material from batches that had rare occurrences of pinholes in the size range 0.1 to 10 ~m. On the other hand, the film separators in batteries that were non-defective had close to none.
2~03733 With this data in mind, it was possible to determine the probability of presence of such pinholes per unit area for each supplied separator batch. This in turn could be correlated to yield results with respect to internal shorting. Yield could then be markedly improved at sub-stantial savings by inspecting and rejecting, as is occa-sionally required, separator batches prior to battery assembly.
The preferred apparatus of the invention for flow pore measurements on large microporous film samples is an inventive improvement over the apparatus described in ASTM
F316-86. Figure 1 illustrates a schematic of this prior art apparatus 15 (reprinted from said reference). The sample 2 (not shown in Figure 1, but see Figure 2) to be tested is clamped in the sample holder 3. Gas pressure 4 is applied on the upstream side of holder 3. This gas pressure is kept at a constant level by means of regulator 5. An accurate pressure gauge 6 is used to determine the upstream gas pressure. On the downstream side, an oil trap 7 is connected by line 16 to holder 3 and is used to trap wetting fluid liquid 8 (not shown in this Figure) and/or vapour exiting holder 3. A single rotameter 9 ( two possible alternative locations are shown, one upstream of the holder 3, the other downstream of the trap) or another suitable vapour flow measuring device is used to quantify gas flow rates.
Figure 2 shows a small sample holder 3 reprinted from ASTM F316-86 (maximum diameter mentioned is 4.7 cm).
The holder 3 consists of a base 10, a locking ring 11, an O-ring 12 for sealing, a support disc 13 and an upstream gas inlet 14. The support disc 13 is of two-ply construc-tion consisting of a 100 x 100 mesh or finer screen (not shown) and a perforated plate (not shown) for rigidity.
The film sample 2 to be tested is clamped between the 0-ring 12 and the support disc 13. For use in the apparatus - 11 2~37~3 15 of Figure 1, the exhaust of the holder 3 is connected to line 16.
The apparatus of the invention consists of a considerably altered larger, rectangular version of the sample holder 3 of Figure 2. This novel apparatus is used in the complete apparatus assembly 15 of Figure 1. Figure 3a illustrates a cross-sectional view of the invention holder apparatus 20. A large sample 21 to be tested is clamped in a large holder 20 (approximately 30 cm x 5 cm).
The film sample 21 is located adjacent to the base plate 22 and contacts O-ring 23. A grid plate 24 rests on the top surface of the edges of the sample 21. A window 28 is provided above the grid plate 24 and contacts second O-ring 29. Clamping force to the stack comprising the baseplate 22, grid plate 24, and window 28 is provided by the sample frame 30 and the sight frame 31. Toggle clamps 32 are provided for rapid sample loading and application of -clamping force. Inlet gas is admitted through inlet port 33 and exhaust gas is released through exhaust port 34.
Figure 3b is an enlargement of Figure 3a and shows the sample 21 in the vicinity of the O-ring seal 23 area. The recessed interior surface of the grid plate 24 contains 249 through holes 25, approximately 0.48 cm in diameter, arranged in a hexagonal close packed array. This - array serves to provide rigidity as is required for the perforated support disc described in the prior art.
Underneath the interior surface 26 of the grid plate 24 and attached to it, is a stainless steel wire mesh 27 ,105 spaces by 105 spaces per inch of .003" diameter wire. The mesh 27 serves the purpose of providing a finer screen as is taught in the prior art. Special attention must be paid to the tolerances that determine the smoothness of the surface of the screen 27 contacting the sample 21. Typical-ly, the "out-of-flat" tolerance is set at a maximum of plus or minus 25 micrometers.
~.
2~03733 Figures 3c and 3d show complete external views of the invention apparatus from the top and side respectively.
As seen in Figure 3c, six toggle clamps 32 are used to provide sound sealing around the entire periphery. Figure 3d illustrates the sight frame 31 and sample frame 30 as well as four of the toggle clamps 32.
This large holder 20, which is much more sensi-tive and reliable than the sample holder 3 shown in Figures1 and 2, is used in place of the small sample holder 3 in the complete apparatus 15 shown in Figure 1. In addition, two accurate flowmeters 9 are used in the positions indi-cated in Figure 1.
O~eration of the Invention Typically the sample 21 is approximately 30 cm by 4.6 cm in size (these values being the length used in the applicant's Molicel B06 battery product and the width of the film as supplied respectively).
To provide a sound seal, silicone-based vacuum grease is applied to the top and bottom edges of the sample 21. Additionally, vacuum grease is applied to both O-rings 23 and 29. Grease is applied to the sample 21 edges such that an adequate leak tight seal can be obtained to the irregular porous separator surface. Without grease, it is difficult to achieve a leak tight seal without the!need for enormous clamping force. The stack consisting of sample frame 30, base plate 22, O-rings 23 and 29, grid plate 24 and attached wire mesh 27 is assembled prior to testing.
Then, 30 cc of a low surface tension wetting fluid, FC-40 Freon (product of 3M Company) is poured into the grid plate 24 to wet the sample and seal the pores.
21037~3 The window 28 is put in place and the assembly is clamped by clamps 32 and sealed together by the force applied by the sample frame 30 and sight frame 31. The actual sample 21 area tested as described is about 95 cm2 since some edge area is used for sealing. Nitrogen gas pressure is applied via the inlet port 33.
As per ASTM F316-86, the pressure p, in psig, required to blow gas continuously through a pore of diam-eter d, in micrometers, can be determined from d = -where ~ = 16 dynes/cm (surface tension) for FC-40 Freon and C = 0.415.
Typically, increasing discrete pressure levels such as 5, 10, and 25 psig are applied, corresponding to pore diameters of 1.3, 0.67, and 0.27 microns respectively in this case. Several minutes are allowed for equilibra-tion following each pressure increase after which a leak check is performed. This check includes visually checking for bubbles in the vicinity of the sample sealing area as well as comparing readings of the dual flowmeters 9.
Bubbling near the sample sealing area may indicate leaks around the seal. Different readings from the dual flow-meters may indicate leaks out of the apparatus. Assuming there is no evidence of a leak, the flow rate readings are recorded along with any visual indication of bubbling which can be seen through window 28. The presence of bubbling, or flow readings well above the background level, indicate the presence of a pore or pores greater than or equal to the corresponding diameter derived from the preceding equation.
2~03733 Once the test criteria have been established, it is a straightforward process to determine statistics on the likelihood of having a pinhole greater than or equal to said discrete diameters in size per unit area of micropor-ous film by testing a suitable number of sample pieces.
For purposes of improving battery yield on manu-facture, such statistical tests are performed on incoming batches of microporous film intended for use as separators in such batteries. There are typically definite differ-ences in the average pinhole occurrence associated with a batch of supplied film. This can arise, for example, as a result of particles capable of penetrating the film being present on rollers used in the processing of or subsequent handling of the film at the manufacturer's plant or in transport. Batches with excessive occurrence of pinhole flaws per unit area can be rejected at inspection. This procedure can provide a significant increase in production efficiency with a corresponding reduction in the waste associated with ultimately rejecting entire battery assem-blies. The actual rejection criteria used is determined by specific economic factors, battery design used, and charac-teristics of the specific microporous film used. This determination will be apparent to those skilled in the rechargeable battery art.
Comparative Example 1 To confirm the deficiencies of the prioriart, commercially available apparatus from Porous Materials Inc.
was used to test small circular samples (approximately 1~"
diameter) of Celgard 2500~ microporous film. Using the recommended test method, bubbling due to leaks at the O-ring to sample sealing area was common at application pressures of about 25 psig. This test example demonstrates that the sealing method employed in this apparatus is '~103733 inadequate for low flow rate measurements on thin micropor-ous film.
Comparative ExamPle 2 Samples of Celgard 25000 microporous film were tested in the same apparatus as used in Comparative Example 1 up to 90 psig pressure (corresponding to a critical pore diameter of 0.08 ~m). Initial flow rates measured were of the order of 2 cc/min. However, at constant application of this pressure, the flow rate increased significantly with time. Typically, the flow rate might double after a few minutes.
Upon removal of the sample after testing, stret-ching and damage of the separator could be clearly seen to have occurred. A corresponding enlargement of the pores would be expected as the plastic film stretched under load, leading to an increased measured flow rate.
This test example demonstrates that the support method employed in this apparatus is inadequate for mea-surements of pores of the order of 0.1 ~m in size in thin microporous plastic film samples. Excessive stretching of the sample film, causing an enlargement of the pores, is allowed by the support used in this holder assembly.
Invention Example 1 Several large (approximately 30 cm x 4.6 cm) Celgard 2500~ samples were tested using the method and the apparatus as described in the preceding part of the dis-closure. Typically, leakage at the O-ring to sample sealing area did not occur based on visual observation through the window and on comparison of upstream and downstream flowmeter readings. Also, flow-rate readings were usually relatively stable over a period of several "' '-'"':~
2~3733 minutes under continued application of gas pressures up to 90 psig. No damage to the samples was visually apparent upon subsequent removal.
Thus, the apparatus of the invention was suitable for use in making low flow rate measurements on pores of thin microporous film down to a 0.1 ~ size range.
Invention Example 2 ~.
Eleven Molicel~ B06 cells manufactured by the applicant were obtained for pinhole analysis after the qualification/inspection step used to detect internal shorting had been performed. Four of these cells had no detectable internal shorts. The other seven cells had significant internal shorts. The cells were disassembled and the separators were recovered for analysis. Special methods were used for the disassembly since lithiated debris often sticks to the separator after such cells are electrically conditioned or cycled. Heat generated by reactions of this debris with air can cause damage to the separator. Briefly, the method used relied on rapidly performing the disassembly in very dry air followed by immersion of the separators in a solution that dissolved 25 Li. Typically, a warm bath containing tetrahydrofuran and -naphthalene was used for this purpose. With such treat-ment, Li was extracted at controlled temperature. The plastic separator film swelled slightly in such a bath aiding the removal of debris. The separator film was!then rinsed a few times and dried at ambient temperature where-upon the separator recovered from the slight swelling.
Each of the dual separator pieces from each cell were visually tested for presence of pinholes initially, and then at 5, 10, and 25 psig gas pressures progressively using the apparatus and method describèd in the preceding disclosure. The data from these measurements is summarized 2~03733 in Table 1. For all separator pieces from cells that had no internal shorts, no pinholes could be detected at any of the three pressures used, nor could bubbling be detected visually through the window. In five of the seven cells that did have internal shorts, at least one pinhole in at least one of the two separators from each cell was detected by observation of bubbling. The corresponding flow read-ings at 25 psig ranged from 0.67 cc/min to greater than 10 cc/min (off scale) for these samples.
Bubbling could not be seen in tests on the separators from two of the seven cells with internal shorts. It must be remembered though that not all the active area of the separator in the battery is tested since the active edges are used for sealing purposes. These two cells, #7 and #8 in Table 1, had somewhat higher flow rates at 25 psig pressure than that of the separators from cells with no internal shorts (the "background"). However, "back-ground" readings varied from 0.27 cc/min to 0.52 cc/min.
The reason for this significant variation in "background"
level is not completely understood. (Since gas diffuses through the head of wetting liquid to some extent, it was speculated that head height variations as a result of uncontrolled minor tilting of the sample holder may account for the background variation.) Thus, it was not possible to conclude positively that cell #7 had a pinhole based on the observed flow rate of 0.59 cc/min. Cell #8 gave a flow reading of 0.85 cc/min which was noticeably higher than the "background" range.
Although this example does not conclusively prove that internal shorting mainly occurs as a result of the presence of pinholes in the separator, it does strongly suggest this.
2iO3733 Invention Example 3 Several batches of new Molicel B06 cells were manufactured by the applicant. Both separator segments used in each cell came from the same supplied batch of separator. From these, a batch of cells with low incidence of internal shorting and a batch of cells with high inci-dence of internal shorting were selected. The former batch, hereinafter referred to as the "good" batch, com-prised 490 cells of which only one had an internal short.Thus the reject rate due to internal shorts was only 0.2%.
The latter batch, hereinafter referred to as the "bad"
batch, comprised 299 cells of which 40 had internal shorts.
Thus the reject rate here was 13.4%.
Numerous samples of unused separators used to prepare "good" and "bad" batches were tested at 5, 10, and 25 psig gas pressure using the invention apparatus and method as above described in the disclosure. Histograms of frequency at various flow rates observed for tests done at 10 and at 25 psig on these samples are shown in Figures 4a and 4b respectively. No high flow rate readings were detected on separator from the batch used in the "good"
cells. A large number of high flow rate readings were detected on separator used to make the "bad" cells. Enough incidence of high flow rate readings was measured to account for the observed number of internally shorted cells under the assumption that a flow rate higher than that observed for any of the "good" separator samples tested could correspond to a pinhole which in turn corresponds to an internal shorting site.
This example demonstrates that the presence of pinholes, as evidenced by high flow rates or visual detec-tion of bubbles at pressures lower than 25 psi, is associ-ated with internal shorting in these batteries. By perform-ing such analysis on incoming separator material, batches such as this "bad" one can be rejected before attempting cell manufacture. Such rejected separator is still accept-able in principle for other applications and thus need not be wasted.
::. ' :' ' ' ,: ': '.: ' ';::;
.'~
'-~'' ', ~ ~
.
~ `
` ` 2~03733 Table 1 __ . .
Cell Voltage Intemal Separator Flow . Flow Flow Obsen~ations# at short? segment (cc/min) (cc/min) (cc/min) inspection @ 5 @ 10 @ 25 l psig pSig . psig 1 2.038 No 1st 0.10 0.19 Q46 _ _ 2nd 0.07 0.14 0.36 2 2.040 No 1st 0.06 0.10 0.27 2nd 0.06 Q18 0.28 I . . . .
3 2.035 No 1st 0.08 Q 18 0.52 I_ 2nd 0.05 0.10 OAO
4 2.037 No 1st 0.07 0.10 0.40 Not avaiL
1.865 Yes 1st 0.05 Q08 0.27 l 2nd 0.20 0.33 0.67 Bubbling ¦
L coO
~nuously@ I
6 1.912 Yes 1st 0.10 0.20 0.47 2cd _ > 10 ~ 10 > 10 2 big bubbles ¦
7 1.906 Yes 1st 0.09 0.22 0.59 ll 2nd O.Q6 0.09 0.19 ¦¦
8 1936 Yes 1st 0.18 0.35 0.85 ¦_ 2nd 0.09 0.15 0.49 9 1.826 Yes 1st 0.09 0.17 0.49 2nd 0.40 0.70 1.56 2 cor~nuous ¦
. ' bubb!les 1.811 Yes Not avaiL _ _ _ _ 2nd > 10 > 10 > 10 1 large bubble l l 11 1.833 Yes 1st 3.70 > 10 > 10 1 large bubble ~ 2nd 0.17 0.35 0.86 .: .: ~ ~ :, .,: . . ., . .: . . . . .
~03733 As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. While the examples in the foregoing disclosure applied only to pinhole detection down to a size of 0.1 microns in an application involving rechargeable lithium batteries, the apparatus and method will apply to other applications in which the rare occurrence of pinholes of any detectable size is of critical importance. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
'-' ~ ', ~' .
:: :
~.'~ , '
. ' bubb!les 1.811 Yes Not avaiL _ _ _ _ 2nd > 10 > 10 > 10 1 large bubble l l 11 1.833 Yes 1st 3.70 > 10 > 10 1 large bubble ~ 2nd 0.17 0.35 0.86 .: .: ~ ~ :, .,: . . ., . .: . . . . .
~03733 As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. While the examples in the foregoing disclosure applied only to pinhole detection down to a size of 0.1 microns in an application involving rechargeable lithium batteries, the apparatus and method will apply to other applications in which the rare occurrence of pinholes of any detectable size is of critical importance. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
'-' ~ ', ~' .
:: :
~.'~ , '
Claims (22)
1. An apparatus for the non-destructive detection of pinhole defects in a sample of microporous film by flowing gas through the sample comprising:
(a) gas supply means;
(b) gas regulator means connected with the gas supply means;
(c) sample holder means which seals the sample to the holder means around the periphery of the sample and maintains the sample in a flat configuration when the sample is subjected to gas pressure; and (d) gas pressure means for measuring the gas pressure upstream of the sample holder means.
(a) gas supply means;
(b) gas regulator means connected with the gas supply means;
(c) sample holder means which seals the sample to the holder means around the periphery of the sample and maintains the sample in a flat configuration when the sample is subjected to gas pressure; and (d) gas pressure means for measuring the gas pressure upstream of the sample holder means.
2. An apparatus as claimed in claim 1 wherein the sample holder has a grid plate with a plurality of holes therein, the sample being sealed around its periphery to the upstream side of the grid plate.
3. An apparatus as claimed in claim 2 wherein a steel screen is positioned between the downstream side of the sample and the upstream side of the grid plate.
4. An apparatus as claimed in claim 2 wherein said sample holder is constructed of a sight frame and a base plate which are releasably secured together by a plurality of toggle clamps.
5. An apparatus as claimed in claim 2 including sealing means for sealing the periphery of the sample to the periphery of the grid plate and the apparatus.
6. An apparatus as claimed in claim 5 wherein the sealing means is grease.
7. An apparatus as claimed in claim 2 wherein the upstream side of the grid plate is flat.
8. An apparatus as claimed in claim 7 wherein the upstream side of the grid plate is flat to a tolerance of plus or minus 25 micrometers.
9. An apparatus as claimed in claim 1 wherein an oil trap means is connected downstream of the sample holder means.
10. An apparatus as claimed in claim 1 wherein the sample holder has a sample observation window.
11. An apparatus as claimed in claim 1 wherein first gas flow measuring means are connected downstream of the gas supply means and upstream of the sample holder means.
12. An apparatus as claimed in claim 11 wherein second gas flow measuring means are connected downstream of the gas supply means and the sample holder means.
13. A method of detecting the existence of pinhole defects in a sample of microporous film comprising:
sealing the periphery of the sample with a sealing means to prevent seepage of gas around the periphery of the sample, maintaining the sample in a flat configuration, applying a wetting agent to a downstream side of the sample, applying a gas pressure on an upstream side of the sample, and determining the rate of flow of gas through the sample.
sealing the periphery of the sample with a sealing means to prevent seepage of gas around the periphery of the sample, maintaining the sample in a flat configuration, applying a wetting agent to a downstream side of the sample, applying a gas pressure on an upstream side of the sample, and determining the rate of flow of gas through the sample.
14. A method as claimed in claim 13 wherein the gas pressure is between 5 and 25 psig.
15. A method as claimed in claim 14 comprising observing the downstream side of the sample for the occur-rence of gas bubbles in the wetting fluid.
16. A method as claimed in claim 13 wherein equiva-lent diameter of pinhole defects is in the range from less than about 10 micrometers to greater than about 0.1 micro-meters.
17. A method as claimed in claim 13 wherein the sample is maintained flat to a tolerance of plus or minus 25 micrometers.
18. A method wherein the yield of internal electrical short free rechargeable lithium batteries during manufac-ture is improved by quantifying the probability of the presence of pinhole defects per unit area of a microporous polyolefin film batch and discarding an unacceptable batch, which comprises employing the principle of bubble point determination to analyse a substantially large area of a film sample, or a group of samples, from said batch and applying predetermined criteria for rejecting a film batch based on said probability.
19. A method as claimed in claim 18 wherein gas pressure is applied to random samples from the film batch and the rate of gas flow is measured upstream of and downstream from said sample.
20. A method as claimed in claim 18 wherein the equivalent diameter of said pinhole defects is in the range from less than about 10 micrometers to greater than about 0.1 micrometers.
21. A method as claimed in claim 18 wherein a wetting fluid is applied to a downstream side of a sample and observing the presence of bubbles on the downstream side of the sample.
22. A method as claimed in claim 18 wherein a periph-ery of the sample is sealed and the sample is maintained in a flat configuration.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002103733A CA2103733A1 (en) | 1993-08-10 | 1993-08-10 | Apparatus for pinhole detection and process for improving yield in the manufacture of batteries |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002103733A CA2103733A1 (en) | 1993-08-10 | 1993-08-10 | Apparatus for pinhole detection and process for improving yield in the manufacture of batteries |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2103733A1 true CA2103733A1 (en) | 1995-02-11 |
Family
ID=4152131
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002103733A Abandoned CA2103733A1 (en) | 1993-08-10 | 1993-08-10 | Apparatus for pinhole detection and process for improving yield in the manufacture of batteries |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2103733A1 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109406723A (en) * | 2018-10-20 | 2019-03-01 | 武汉惠强新能源材料科技有限公司 | The on-Line Monitor Device of lithium battery diaphragm flaw |
| CN115485536A (en) * | 2020-04-15 | 2022-12-16 | 气体运输技术公司 | Device for monitoring the tightness of a sealing component |
| CN117249961A (en) * | 2023-11-20 | 2023-12-19 | 四川省产品质量监督检验检测院 | Air-tight detection tool for air inlet of hydrogen fuel cell |
-
1993
- 1993-08-10 CA CA002103733A patent/CA2103733A1/en not_active Abandoned
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109406723A (en) * | 2018-10-20 | 2019-03-01 | 武汉惠强新能源材料科技有限公司 | The on-Line Monitor Device of lithium battery diaphragm flaw |
| CN109406723B (en) * | 2018-10-20 | 2023-05-16 | 武汉惠强新能源材料科技有限公司 | On-line monitoring device for flaws of lithium battery diaphragm |
| CN115485536A (en) * | 2020-04-15 | 2022-12-16 | 气体运输技术公司 | Device for monitoring the tightness of a sealing component |
| CN115485536B (en) * | 2020-04-15 | 2025-10-24 | 气体运输技术公司 | Device for monitoring the tightness of sealing components |
| CN117249961A (en) * | 2023-11-20 | 2023-12-19 | 四川省产品质量监督检验检测院 | Air-tight detection tool for air inlet of hydrogen fuel cell |
| CN117249961B (en) * | 2023-11-20 | 2024-02-02 | 四川省产品质量监督检验检测院 | Air-tight detection tool for air inlet of hydrogen fuel cell |
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