Patent Application
20080026288
The invention will be better understood and other features and advantages will become apparent by reading the detailed description of the invention, taken together with the drawings, wherein:
The electrochemical cells of the present invention are preferably primary cells, that each include a positive electrode that makes electrical contact with a container of a cell thereby providing the container with a positive polarity and a negative electrode that makes electrical contact with a portion of a cover thereby providing the cover with a negative polarity, wherein the container is free of electrical contact with the cover. In one embodiment, the negative electrode comprises lithium as the negative electrode active material, preferably with the positive electrode comprising iron disulfide (FeS2). The positive electrode and negative electrode may be provided in the form of strips, which are joined together with a separator in an electrode assembly, preferably in a jellyroll or spiral-wound configuration, and placed in the container with the positive electrode making electrical contact with the container.
The electrochemical cells of the invention are normally cylindrical in shape and preferably have a maximum height greater than the maximum diameter, with the cylindrical container having a greater interior volumetric capacity than the cover or end cap. Preferably, the dimensions of the cells will match IEC standardized sizes, including but not limited to “AA”, “AAA” and “AAAA” sizes. However, the invention can also be adapted to other cell sizes and shapes and to cells with alternative electrode assembly, housing, seal and pressure relief vent designs, etc.
A preferred embodiment of the invention will be better understood with reference to
Cell closure 114 is affixed over the open end of the container 112 according to any number of known mechanisms. In a preferred embodiment, cell closure 114 comprises pressure relief vent 113, negative terminal cover 115, gasket 116 and PTC 142. Negative terminal cover 115 may be held in place by the inwardly crimped top edge of container 112 and gasket 116. In a preferred embodiment, container 112 may have a bead or reduced diameter step near the top end which axially and/or radially compresses the container 112 and the cell closure 114, thereby forming an essentially leak-proof seal. Notably, cell closure 114 (and in a more specific and preferred embodiment, gasket 116) must provide electrical insulation between the container 112 and the terminal cover 115 in order to avoid unwanted shorting of the cell 110. Cell closure 114 and container 110 work in conjunction with one another to provide a leak-proof seal for the cell internals, including electrodes 118, 120 and the non-aqueous electrolyte (not shown in
Cell container 112 is preferably a metal can with an integral closed bottom, although in some embodiments a metal tube that is initially open at both ends can be used instead of a can. The container 112 can be any suitable material with non-limiting examples including stainless steels, nickel plated stainless steels, nickel clad or nickel plated steels, aluminum and alloys thereof. For example, a diffusion annealed, low carbon, aluminum killed, SAE 2006 or equivalent steel with a grain size of ASTM 9 to 11 and equiaxed to slightly elongated grain shape is preferred in one embodiment of the invention. Choice of container material depends upon factors including, but not limited to, conductivity, corrosion resistance, compatibility with internal and active materials within the cell and cost. As the container 112 of the cell 110 must have a positive polarity, the bottom of the cell must have a shape, such as shown in
The use of aluminum or aluminum alloys as the primary material for the container allows a significant reduction in the overall weight of cell 110. For example, the use of aluminum as the cell container can reduce the container weight by 67% and the overall cell weight by 20%. Notably, use of aluminum to construct a cell having a negative polarity container is not possible since aluminum at the anodic potential can form lithium aluminum alloys which have low mechanical strength. Through the use of aluminum and/or lightweight metals or alloys, significant improvements can be made in the energy density of the overall cell construction, particularly with respect Wh/kg, which is a primary concern for many consumers and users of such electrochemical cells.
Cell closure 114, and including terminal cover 115, must also be made from a conductive material, such as a metal, metal alloy or an appropriate conductive plastic. Suitable examples include, but are not limited to, those used in the construction of the container (discussed above) or other known materials possessing the other qualities discussed herein. In addition to the considerations identified in the preceding paragraph, the complexity of the cover shape, ease of forming/machining/casting/extruding and compatibility with cell internals are all factors for consideration. The cell cover 114 and/or negative terminal cover 115 may have a simple shape, such as a thick, flat disc, or may have a more complex shape, such as the cover shown in
Gasket 116 is a non-conductive portion of the cell cover and is compressed between can 112 and cover 114 to seal the peripheral edges of these components, to prevent corrosion and to inhibit leakage of electrolyte through, around or between these components. Gasket 116 can be made of a polymeric composition, for example, a thermoplastic or thermoset polymer, the composition of which is based in part on the chemical compatibility the electrodes 118, 120 and the electrolyte used in cell 110. Examples of materials that can be used in a gasket 116 include but are not limited to, polypropylene, polyphenylene sulfide, tetrafluoride-perfluoroalkyl vinyl ether co-polymer, polybutylene terephthalate (PBT), ethylene tetrafluoroethylene, polyphthalamide, and blends thereof. A suitable prolypropylene that can be used is PRO-FAX® 6524 from Basell Polyolephins, of Wilmington, Del., USA. A suitable polyphenylene sulfide is available as TECHTRON® PPS from Boedeker Plastics, Inc. of Shiner, Tex., USA. A suitable polyphthalamide is available as Amodel® ET 1001 L from Solvay Advanced Polymers of Alpharetta, Ga. The polymers can also contain reinforcing inorganic fillers and organic compounds in addition to the base resin, such as glass fibers and the like. Significantly, a material with a low vapor transmission rate for the electrolyte is preferred.
The gasket 116 may be coated with a sealant to provide an even better seal. Ethylene propylene diene terpolymer (EPDM) is a suitable sealant material, but other suitable materials can be used.
A positive temperature coefficient (PTC) device 142 may also be disposed between the peripheral flange of terminal cover 115 and cell cover 114. PTC 142 substantially limits the flow of current under abusive electrical conditions. During normal operation of the cell 110, current flows through the PTC device 142. If the temperature of the cell 110 reaches an abnormally high level, the electrical resistance of the PTC device 142 increases to reduces the current flow, thereby allowing PTC device 142 to slow or prevent cell continued internal heating and pressure buildup resulting from electrical abuses such as external short circuiting, abnormal charging and forced deep discharging. Nevertheless, if internal pressure continues to build to the predetermined release pressure, the pressure relief vent 113 may be activated to relieve the internal pressure.
Cell closure 114 includes a pressure relief vent 113 as a safety mechanism to avoid internal pressure build up and to prevent disassembly of the cell under abusive conditions. In one embodiment, cell cover 114 includes a ball vent comprising an aperture with an inward projecting central vent well 128 with a vent hole 130 in the bottom of the well 128. The aperture is sealed by a vent ball 132 and a thin-walled thermoplastic bushing 134, which is compressed between the vertical wall of the vent well 128 and the periphery of the vent ball 132. When the cell internal pressure exceeds a predetermined level, the vent ball 132, or both the ball 132 and bushing 134, is/are forced out of the aperture to release pressurized gasses from cell 110.
The vent busing 134 is made from a thermoplastic material that is resistant to cold flow at high temperatures (e.g., 75° C.). The thermoplastic material comprises a base resin such as ethylene-tetrafluoroethylene, polybutylene terephthlate, polyphenylene sulfide, polyphthal-amide, ethylenechloro-trifluoroethylene, chlorotrifluoroethylene, perfluoroalkoxyalkane, fluorinated perfluoroethylene polypropylene and polyetherether ketone. Ethylene-tetrafluoroethylene copolymer (ETFE), polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) and polyphthalamide are preferred. The resin can be modified by adding a thermal-stabilizing filler to provide a vent bushing with the desired sealing and venting characteristics at high temperatures. The bushing can be injection molded from the thermoplastic material. TEFZEL® HT2004 (ETFE resin with 25 weight percent chopped glass filler) is a preferred thermoplastic material.
The vent ball 132 can be made from any suitable material that is stable in contact with the cell contents and provides the desired cell sealing and venting characteristic. Glasses or metals, such as stainless steel, can be used.
In an alternative embodiment, vent 113 may comprise a single layer or laminar foil vent. Such foil vents prevent vapor transmission and must be chemically compatible with the electrodes 118, 120 and the electrolyte. Optionally, such foil vents may also include an adhesive component activated by pressure, ultrasonic energy and/or heat in order to further perfect the seal. In a preferred embodiment, a four layered vent consisting of oriented polypropylene, polyethylene, aluminum and low density polyethylene may be used, although other materials are possible, as well as varying the number of layers in the laminate. The vent may be crimped, heat sealed and/or otherwise mechanically held in place over an aperture in the cell closure 114. Notably, use of such a vent increases the internal volume of the cell 110 available for electrochemically active materials. In particular and understanding that appropriate materials are utilized and electrical connections are maintained, a foil vent similar to that disclosed in U.S. Patent Application Publication No. 2005/0244706, which is incorporated by reference herein, may be used.
The cell 110 includes positive electrode 118 and negative electrode 120 that are spirally-wound together in a jellyroll configuration, with a separator disposed between positive electrode 118 and negative electrode 120. Negative electrode 120 comprises a foil or sheet of pure lithium or an alloy of lithium selected to enhance the conductivity, ductility, processing capabilities or mechanical strength of the electrode 120. In a preferred embodiment, the lithium may be alloyed with 0.1% to 2.0% aluminum by weight, with most preferred alloy having 0.5% aluminum by weight. This most preferred material is available from Chemetall Foote Corp., Kings Mountain, N.C., USA. Negative electrode 118 may be provided in an axial excess at the top terminal edge so as to make an electrical connection to the inner surface of cover 114 through contact spring 124. In a preferred embodiment, an electrically conductive member 122 may be affixed to the negative electrode 120 itself. Most advantageously, the member 122 is affixed along the inner-most surface of negative electrode 120 so as to avoid unwanted contact with positive electrode 118, although so long as the member 122 is in electrical contact with negative electrode 120 and is also electrically separated from positive electrode 118, preferably through the use of a separator (not show in
As indicated above, electrically conductive member 122 serves as an electrical lead or tab to electrically connect the negative electrode 120 to a portion of cell closure 114, which in turn imparts a negative polarity to closure 114 and more specifically terminal cover 115. The electrically conductive member 122 is made from a material, preferably a metal or metal alloy selected for its ductility, mechanical strength, conductivity and compatibility with the electrochemically active materials inside cell 110, including the electrolyte. The electrically conductive member is preferably formed from a strip of metal sized to fit the particular dimensions of cell closure 114, preferably at thickness between 0.025-0.125 mm and a width between 4.5-6.5 mm with the length being sufficient to bridge the space between the electrode 118 and the cell closure 114 while accommodating the particular shape utilized (see below). One of the preferred materials is nickel plated cold rolled steel, although steel, nickel, copper and other similar materials may be possible.
The electrically conductive member 122 is fixedly connected to the negative electrode 118 along at least one portion of the electrode 118. Owing to the properties of lithium, this connection can be accomplished by way of a simple pressure contact which embeds one end of the electrically conductive member 122 within a portion of the negative electrode or by pressing an end of the member onto a surface of the lithium foil. In a preferred embodiment, the electrically conductive member 122 is connected to the negative electrode near the center or core of the spiral winding, although the member may be connected at other and/or multiple locations on electrode 118.
A second portion of the electrically conductive member, preferably its opposing end, is connected to a portion of the cell cover by a fixed connection or by a non-fixed connection. Examples of fixed connections include riveting, crimping, or welding the electrically conductive member to the cell cover, whereas non-fixed connections can be accomplished by pressure contact, interference fits or other engineered solutions that do not require either an adhesive media (e.g., weld melt) or bending/other metal working of both the conductive member and the cell cover (e.g., crimping).
A fixed connection is made, for example as shown in
As seen in
Another example of a non-fixed pressure contact is shown in
Notably, in the preferred embodiments shown in
Positive electrode 118 may comprise an electrochemically active material affixed on one or both sides of an electrically conductive foil, such as aluminum or other suitable materials allowing for appropriate rheological properties to adhere the electrochemically active material. The electrochemically active material is preferably iron disulfide. Notably, positive electrode 118 makes an electrical connection to the container 112 along its axial sidewall and/or through contact with the bottom of the can. As discussed in greater depth below, the electrochemically active material affixed to the foil in a manner that enhances the electrical connection between the positive electrode 118 and the container 112. Insulating material (not shown in
With positive electrode 118 forming the outer-most wind of the jellyroll configuration of electrodes 118, 120, the container 112 will serve as the positive terminal of the electrochemical cell 110, either along the axial sidewalls and/or the bottom of the container as described above. Electrodes 118, 120 have an axial length extending substantially parallel to a longitudinal length of container 112, generally along a central axis thereof. The upper ends of positive electrode 118 and negative electrode 120 are preferably coextensive and positive electrode current collector has an upper axial end substantially equal to the upper axial end height of the separator utilized and does not extend thereabove. Alternatively, one of the electrodes may be deliberately sized larger than the other to advantageously allow for enhanced electrical connection with the cell closure 114 or the bottom of container 112.
The positive electrode 118 for cell 110 may contain one or more active materials, usually in particulate form. Any suitable active cathode material may be used, and can include for example FeS2, CuO, MnO2, CFx and (CF)n, although iron disulfide (FeS2) is preferred as the dominant if not exclusive electrochemically active material. Other cathode materials may be possible, although the choice of cathode material will have direct impact on the optimal electrolyte, both in terms of chemical compatibility and overall cell performance, such that the header assembly must be specifically engineered to the materials selected.
The positive electrode 118 is preferably in the form of foil carrier, such as aluminum coated with chemically active materials, usually in particulate form. Iron disulfide is a preferred active material. In a Li/FeS2 cell the active material comprises greater than 50 weight percent FeS2. The positive electrode 18 can also contain one or more additional active materials, depending on the desired cell electrical and discharge characteristics. The additional active positive electrode material may be any suitable active positive electrode material. Examples include Bi2O3, C2F, CFx, (CF)n, CoS2, CuO, CuS, FeS, FeCuS2, MnO2, Pb2Bi2O5 and S.
More preferably, the active material for a Li/FeS2 cell positive electrode generally comprises at least 95 weight percent FeS2, desirably at least 99 weight percent FeS2, and preferably FeS2 is the sole active positive electrode material. Battery grade FeS2 having a purity level of at least 95 weight percent is available from American Minerals, Inc., Camden, N.J., USA; Chemetall GmbH, Vienna, Austria; Washington Mills, North Grafton, Mass.; and Kyanite Mining Corp., Dillwyn, Va., USA.
In addition to the active material, the positive electrode mixture contains other materials. A binder is generally used to hold the particulate materials together and adhere the mixture to the current collector. One or more conductive materials such as metal, graphite and carbon black powders may be added to provide improved electrical conductivity to the mixture. The amount of conductive material used can be dependent upon factors such as the electrical conductivity of the active material and binder, the thickness of the mixture on the current collector and the current collector design. Small amounts of various additives may also be used to enhance positive electrode manufacturing and cell performance. The following are examples of active material mixture materials for Li/FeS2 cell positive electrodes. Graphite: KS-6 and TIMREX® MX15 grades synthetic graphite from Timcal America, Westlake, Ohio, USA. Carbon black: Grade C55 acetylene black from Chevron Phillips Company LP, Houston, Tex., USA. Binder: ethylene/propylene copolymer (PEPP) made by Polymont Plastics Corp. (formerly Polysar, Inc.) and available from Harwick Standard Distribution Corp., Akron, Ohio, USA; non-ionic water soluble polyethylene oxide (PEO): POLYOX® from Dow Chemical Company, Midland, Mich., USA; and G1651 grade styrene-ethylene/butylenes-styrene (SEBS) block copolymer from Kraton Polymers, Houston, Tex. Additives: FLUO HT® micronized polytetrafluoroethylene (PTFE) manufactured by Micro Powders Inc., Tarrytown, N.Y., USA (commercially available from Dar-Tech Inc., Cleveland, Ohio, USA) and AEROSIL® 200 grade fumed silica from Degussa Corporation Pigment Group, Ridgefield, N.J.
A preferred method of making FeS2 positive electrodes is to roll coat a slurry of active material mixture materials in a highly volatile organic solvent (e.g., trichloroethylene) onto both sides of a sheet of aluminum foil, dry the coating to remove the solvent, calender the coated foil to compact the coating, slit the coated foil to the desired width and cut strips of the slit positive electrode material to the desired length. It is desirable to use positive electrode materials with small particle sizes to minimize the risk of puncturing the separator. For example, FeS2 is preferably sieved through a 230 mesh (63 μm) screen before use. Coating thicknesses of 100 μm and less are common.
In a further embodiment, a positive electrode comprises FeS2 particles having a predetermined average particle size produced by a wet milling method such as a media mill, or a dry milling method using a non-mechanical milling device such as a jet mill. Electrochemical cells prepared with the reduced average particle size FeS2 particles exhibit increased cell voltage at any given depth of discharge, irrespective of cell size. The smaller FeS2 particles also make possible thinner coatings of positive electrode material on the current collector; for example, coatings as thin as about 10 μm can be used. Preferred FeS2 materials and methods for preparing the same are disclosed in U.S. patent application Ser. Nos. 11/020,339 and 11/155,352, both fully incorporated herein by reference.
The foil carrier may serve as a current collector for positive electrode, or a current collector may otherwise be disposed within or imbedded into the positive electrode surface. To the extent a foil carrier is used, the positive electrode mixture may be coated onto one or both sides of a thin metal strip or foil and aluminum is the preferred material. Bare portions of only foil may extend beyond the portion where the positive electrode mixture is coated, so as to allow for better electrical contact with the various portions of the container 12 as described herein (e.g., the axial sidewall of the container, the bottom of the container, etc.).
Electrolytes for lithium cells, and particularly for lithium iron disulfide cells, are non-aqueous electrolytes and contain water only in very small quantities, for example, less than about 500 parts per million by weight, as a contaminant. Suitable non-aqueous electrolytes contain one or more electrolyte salts dissolved in an organic solvent. Any suitable salt may be used depending on the anode and cathode active materials and the desired cell performance. Examples include lithium bromide, lithium perchlorate, lithium hexafluorophosphate, potassium hexafluorophosphate, lithium hexafluoroarsonate, lithium trifluoromethanesulfonate and lithium iodide. Suitable organic solvents include one or more of the following: dimethyl carbonate; diethyl carbonate; dipropyl carbonate; methylethyl carbonate; ethylene carbonate; propylene carbonate; 1,2-butylene carbonate; 2,3-butylene carbonate; methaformate; gamma-butyrolactone; sulfolane; acetonitrile; 3,5-dimethylisoxazole; n,n-dimethylformamide; and ethers. The salt and solvent combination should provide sufficient electrolytic and electrical conductivity to meet the cell discharge requirements over the desired temperature range. When ethers are used in the solvent they provide generally low viscosity, good wetting capability, good low temperature discharge performance and high rate discharge performance. Suitable ethers include, but are not limited to, acyclic ethers such as 1,2-dimethoxyethane (DME); 1,2-diethoxyethane; di(methoxyethyl)ether; triglyme, tetraglyme and diethylether; cyclic ethers such as 1,3-dioxolane (DIOX), tetrahydrofuran, 2-methyl tetrahydrofuran and 3-methyl-2-oxazolidinone; and mixtures thereof.
A nonaqueous electrolyte, containing water only in very small quantities as a contaminant (e.g., no more than about 500 parts per million by weight, depending on the electrolyte salt being used), is used in the battery cell of the invention. Any nonaqueous electrolyte suitable for use with lithium and active positive electrode material may be used. The electrolyte contains one or more electrolyte salts dissolved in an organic solvent. For an Li/FeS2 cell examples of suitable salts include lithium bromide, lithium perchlorate, lithium hexafluorophosphate, potassium hexafluorophosphate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate and lithium iodide; and suitable organic solvents include one or more of the following: dimethyl carbonate, diethyl carbonate, methylethyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, methyl formate, γ-butyrolactone, sulfolane, acetonitrile, 3,5-dimethylisoxazole, n,n-dimethyl formamide and ethers. The salt/solvent combination will provide sufficient electrolytic and electrical conductivity to meet the cell discharge requirements over the desired temperature range. Ethers are often desirable because of their generally low viscosity, good wetting capability, good low temperature discharge performance and good high rate discharge performance. This is particularly true in Li/FeS2 cells because the ethers are more stable than with MnO2 positive electrodes, so higher ether levels can be used. Suitable ethers include, but are not limited to acyclic ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, di(methoxyethyl) ether, triglyme, tetraglyme and diethyl ether; and cyclic ethers such as 1,3-dioxolane, tetrahydrofuran, 2-methyl tetrahydrofuran and 3-methyl-2-oxazolidinone.
Accordingly, various combinations of electrolyte salts and organic solvents can be utilized to form the electrolyte for electrochemical cells. The molar concentration of the electrolyte salt can be varied to modify the conductive properties of the electrolyte. Examples of suitable nonaqueous electrolytes containing one or more electrolyte salts dissolved in an organic solvent include, but are not limited to, a 1 mole per liter solvent concentration of lithium trifluoromethanesulfonate (14.60% by weight) in a solvent blend of 1,3-dioxolane, 1,2-diethoxyethane, and 3,5-dimethyl isoxazole (24.80:60.40:0.20% by weight) which has a conductivity of 2.5 mS/cm; a 1.5 moles per liter solvent concentration of lithium trifluoromethanesulfonate (20.40% by weight) in a solvent blend of 1,3-dioxolane, 1,2-diethoxyethane, and 3,5-dimethylisoxazole (23.10:56.30:0.20% by weight) which has a conductivity of 3.46 mS/cm; and a 0.75 mole per liter solvent concentration of lithium iodide (9.10% by weight) in a solvent blend of 1,3-dioxolane, 1,2-diethoxyethane, and 3,5-dimethylisoxazole (63.10:27.60:0.20% by weight) which has a conductivity of 7.02 mS/cm. Electrolytes utilized in the electrochemical cells of the present invention have conductivity generally greater than about 2.0 mS/cm, desirably greater than about 2.5 or about 3.0 mS/cm, and preferably greater than about 4, about 6, or about 7 mS/cm.
Suitable separator materials are ion-permeable and electrically non-conductive. Examples of suitable separators include microporous membranes made from materials such as polypropylene, polyethylene and ultra high molecular weight polyethylene. A suitable separator material for Li/FeS2 cells is available as CELGARD® 2400 microporous polypropylene membrane from Celgard Inc., of Charlotte, N.C., USA, and Setella F20DHI microporous polyethylene membrane available from Exxon Mobil Chemical Company of Macedonia, N.Y., USA. A layer of a solid electrolyte or a polymer electrolyte can also be used as a separator.
The separator is a thin microporous membrane that is ion-permeable and electrically nonconductive. It is capable of holding at least some electrolyte within the pores of the separator. The separator is disposed between adjacent surfaces of the anode and cathode to electrically insulate the electrodes from each other. Portions of the separator may also insulate other components in electrical contact with the cell terminals to prevent internal short circuits. Edges of the separator often extend beyond the edges of at least one electrode to insure that the anode and cathode do not make electrical contact even if they are not perfectly aligned with each other. However, it is desirable to minimize the amount of separator extending beyond the electrodes.
To provide good high power discharge performance it is desirable that the separator have the characteristics (pores with a smallest dimension of at least 0.005 μm and a largest dimension of no more than 5 μm across, a porosity in the range of 30 to 70 percent, an area specific resistance of from 2 to 15 ohm-cm.2 and a tortuosity less than 2.5) disclosed in U.S. Pat. No. 5,290,414, hereby incorporated by reference. Suitable separator materials should also be strong enough to withstand cell manufacturing processes as well as pressure that may be exerted on the separator during cell discharge without tears, splits, holes or other gaps developing that could result in an internal short circuit. Additional suitable separator materials are described in U.S. patent application Ser. Nos. 11/020,339 and 11/155,352, which claim priority to U.S. patent application Ser. No. 10/719,425, herein fully incorporated herein by reference.
To minimize the total separator volume in the cell, the separator should be as thin as possible, but at least about 1 μm or more so a physical barrier is present between the cathode and anode to prevent internal short circuits. That said, the separator thickness ranges from about 1 to about 50 μm, desirably from about 5 to about 25 μm, and preferably from about 10 to about 16 or about 20 μm. The required thickness will depend in part on the strength of the separator material and the magnitude and location of forces that may be exerted on the separator where it provides electrical insulation.
Separator membranes for use in lithium batteries are often made of polypropylene, polyethylene or ultrahigh molecular weight polyethylene, with polyethylene being preferred. The separator can be a single layer of biaxially oriented microporous membrane, or two or more layers can be laminated together to provide the desired tensile strengths in orthogonal directions. A single layer is preferred to minimize the cost. Suitable single layer biaxially oriented polyethylene microporous separator is available from Tonen Chemical Corp., available from EXXON Mobile Chemical Co., Macedonia, N.Y., USA. Setela F20DHI grade separator has a 20 μm nominal thickness, and Setela 16MMS grade has a 16 μm nominal thickness.
The cell can be closed and sealed using any suitable process. Such processes may include, but are not limited to, crimping, redrawing, colleting and combinations thereof. For example, for the cell in
By providing an electrochemical cell with an electrode assembly as specified above, the quantity of lithium or separator, and preferably both, can be reduced as compared to a container negative cell of the same size and cell capacity can be increased. One reason that less lithium is required is because the lithium on the outer wrap of the spirally wound electrode of the container negative cell is only consumed or discharged from one side. In fact, the amount of lithium required in a AA size positive container cell may be reduced by approximately 2.5% in comparison to a similarly designed negative container cell, thereby resulting in substantial materials savings.
Three sets of cells were constructed from the preferred materials identified above. The first set were made using a “standard” negative-polarity can, hereafter referred to as the control group. The second set utilized the positive-polarity can in conjunction with an electrical connection between the positive electrode and the can only along the bottom of the can. The third set had a positive-polarity can in conjunction with an axial sidewall electrical connection between the positive electrode and the can.
These cells were then service tested, under continuous drain conditions, as shown in the Table 1 below. Note that results in Table 1 are reported as overall service, with the parenthetical number representing the percentage improvement in comparison to the control group.
Clearly, cells made with a positive-polarity container exhibited increased performance of anywhere from 5-10% over the control group. Other benefits, including increased performance at low temperatures, improved storage life, etc., may also be realized.
It will be understood by those who practice the invention and those skilled in the art that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concepts. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.
