Patent Application
20080220316
The invention relates to an end cap assembly for sealing electrochemical cells, particularly lithium primary cells having wound electrodes, more particularly lithium wound cells having an anode comprising lithium and a cathode comprising iron disulfide. The invention relates to rupturable devices within the end cap assembly which allow gas to escape from the interior of the cell to the environment.
Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in commercial use. The cell casing, commonly of steel, may typically be cylindrical having an open end and opposing closed end. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in a nonaqueous solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. They are typically in the form having spirally wound electrodes, that is, a sheet of anode material, a sheet of cathode material, and electrolyte permeable separator therebetween spirally wound before insertion into the cell casing.
Alternative primary lithium cells with lithium metal anodes but having different cathodes are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite. The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically an AA size cell or ⅔A size cell, with a sheet of anode material, separator, and sheet of cathode material spirally wound before insertion into the cell casing. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.5 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is also much higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.7 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons reducing the valence state of Fe+2 in FeS2 to Fe and the remaining 2 electrons reducing the valence of sulfur from −1 in FeS2 to −2 in Li2S.
Overall the Li/FeS2 cell is more powerful than the same size Zn/MnO2 alkaline cell. That is for a given continuous current drain, particularly for higher current drain over 200 milliAmp, in the voltage vs. time profile the voltage drops off much less quickly for the Li/FeS2 cell than the Zn/MnO2 alkaline cell. This results in a higher energy obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is also clearly shown more directly in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. In such tests the power drain is maintained at a constant continuous power output selected between 0.01 Watt and 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is considerably above that for the same size Zn/MnO2 alkaline cell. This is despite that the starting voltage of both cells (fresh) is about the same, namely, between about 1.2 and 1.5 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cells, for example, AAA (44×10 mm), AA (50×14 mm), C (49×25.5 mm) or D (60×33 mm) size or any other size cell in that the Li/FeS2 cell may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is primary (nonrechargeable) cell can be used as a replacement for the same size rechargeable nickel metal hydride cells, which have about the same voltage (fresh) as the Li/FeS2 cell.
After the spirally wound electrodes for the Li/FeS2 cell are inserted into the typically cylindrical casing, electrolyte is added, and the open end of the casing must then be closed with an end cap assembly. The end cap assembly is multifunctional. There is a terminal end cap or end plate within the end cap assembly which provides a contact terminal. For the Li/FeS2 cell the end cap is in electrical contact with cell's cathode and provides the cell's positive terminal. The end cap assembly must include a reliable seal to prevent leakage of electrolyte and withstand levels of cell internal pressure due to gassing during cell storage or discharge. The cell should include a venting system which is activated when gas pressure within the cell builds up to predetermined level. The venting system is desirably included within the end cap assembly.
The electrochemical cell art discloses vents that may be formed within the cell casing wall itself, that is, by weakening the casing wall so that it will rupture when the cell internal pressure reaches a given level. The art teaches that this may be achieved by scoring or etching the cell metal casing wall to provide a thinned rupturable portion within the casing wall itself. Such scored regions are shown in the cell casing side wall or casing bottom (closed end), so that the scored region faces the external environment. Examples of electrochemical cells which disclose such scored or weakened regions on the cell casing wall are U.S. Pat. Nos. 2,478,798; U.S. Pat. No. 2,525,436; U.S. Pat. No. 4,484,691; U.S. Pat. No. 4,256,812; U.S. Pat. No. 4,789,608; U.S. Pat. No. 4,175,166. and U.S. Pat. No. 6,159,631.
In U.S. application 2006/0228620 A1 is shown a wound Li/FeS2 cell which includes a separate thin metal foil or polymeric membrane within the end cap assembly. The separate membrane is designed to rupture when gas within the cell builds up to a predetermined level.
Electrochemical cells may be provided with a rupturable venting mechanism which typically includes a rupturable membrane integrally formed within a plastic insulating sealing disk, e.g. of nylon, polypropylene or polyethylene, within an end cap assembly. The rupturable membrane may be formed from grooved or thinned portions within the plastic insulating disk as described, for example, in U.S. Pat. No. 3,617,386. Such membranes are designed to rupture when gas pressure within the cell exceeds a predetermined level. The end cap assembly may be provided with vent holes for the gas to escape to the environment when the membrane is ruptured.
The electrochemical cell art discloses rupturable vent membranes which are integrally formed as thinned areas within a plastic insulating sealing disk included within the end cap assembly. Such vent membranes are normally oriented such that they lay in a plane perpendicular to the cell's longitudinal axis, for example, as shown in U.S. Pat. No. 5,589,293. In U.S. Pat. No. 4,227,701 the rupturable membrane is formed of an annular “slit or groove” located in an arm of the insulating disk which is slanted in relation to the cell's longitudinal axis. The plastic insulating disk is slid ably mounted on an elongated current collector running therethrough. As gas pressure within the cells builds up the center portion of the insulating disk slides upwards towards the cell end cap, thereby stretching the thinned membrane “groove” until it ruptures. U.S. Pat. Nos. 6,127,062 and 6,887,614 B2 disclose an insulating sealing disk and an integrally formed rupturable membrane therein which is inclined. The rupturable membrane portion in the sealing disk abuts an aperture in the overlying metal support disk. When the gas pressure within the cell rises the membrane ruptures through the aperture in the metal support disk thereby releasing the gas pressure which passes to the external environment.
U.S. Pat. Nos. 6,127,062 and 6,887,614 B2 disclose a plastic insulating sealing disk and an integrally formed rupturable membrane wherein the rupturable membrane abuts an aperture in the overlying metal support disk. In U.S. Pat. No. 6,887,614 the rupturable membrane is integrally formed within the plastic insulating sealing disk. The rupturable membrane abuts an opening in an overlying metal support disk. In U.S. Pat. No. 6,887,614 there is an undercut groove on the underside of the membrane. The groove circumvents the cell's longitudinal axis. The groove creates a thinned membrane portion at its base which ruptures through the opening in the overlying metal support disk when the cell's internal gas pressure reaches a predetermined level.
The rupturable membrane can be in the form of one or more “islands” of thin material integrally formed within the plastic insulating disk as shown in U.S. Pat. No. 4,537,841; U.S. U.S. Pat. No. 5,589,293; and U.S. Pat. No. 6,042,967. Alternatively, the rupturable membrane as integrally formed within the plastic insulating disk can be in the form of a thin portion circumventing the cell's longitudinal axis as shown in U.S. Pat. No. 5,080,985 and U.S. Pat. No. 6,991,872. The circumventing thinned portion forming the rupturable membrane can be in the form of slits or grooves within the plastic insulating disk as shown in U.S. Pat. No. 4,237,203 and U.S. Pat. No. 6,991,872. The rupturable membrane may also be a separate piece of polymeric film which is sandwiched between the metal support disk and the plastic insulating disk and facing apertures therein as shown in Patent Application Publication US 2002/0127470 A1. A pointed or other protruding member can be oriented above the rupturable membrane to assist in rupture of the membrane as shown in U.S. Pat. No. 3,314,824. When gas pressure within the cell becomes excessive, the membrane expands and ruptures upon contact with the pointed member, thereby allowing gas from within the cell to escape to the environment through apertures in the overlying terminal end cap.
The above described end cap assemblies which include venting mechanisms such as rupturable membranes which are an integral part of a plastic insulating sealing disk are generally not suitable for use in the end cap assembly for wound primary lithium cells because of assembly and connection requirements which are specific to such wound cells.
Accordingly, it is desirable to have an end cap assembly of components that can be readily manufactured and assembled and which provides a tight seal for a wound primary lithium cell during normal operation and extremes in both hot and cold climate.
It is desired to have a reliable rupturable venting mechanism within the end cap assembly which activates and functions reliably in a wound lithium cell when gas pressure within the cell rises to a predetermined level.
It is desirable that the end cap assembly include a current interrupter such as a PTC (positive temperature coefficient) device to provide additional protection against short circuit or abnormally high current drain.
It is desirable that the end cap be tamper proof, that is, cannot be readily pried from the end cap assembly.
The invention is directed to an end cap assembly for closing and sealing cells having a wound electrode therein. The end cap assembly is inserted into the open end of the cell casing (housing) to seal and close the casing and also provides a venting device therein which activates should gas pressure within the cell rise to a predetermined level. The venting device preferably includes a rupturable metal surface which is designed to rupture if the gas pressure within the cell builds to a predetermined level. The end cap assembly may also include a current interrupter such as a PTC (positive temperature coefficient) device. The PTC device activates to abruptly increase resistance therethrough to quickly reduce current drain, if the cell is subjected to short circuit, abnormally high current drain or abnormally high temperatures. The end cap assembly of the invention is principally intended for lithium primary (non rechargeable) cells, that is, wherein the anode comprises lithium. The cell may typically have an anode comprising a sheet of lithium or lithium alloy and a cathode comprising manganese dioxide (MnO2) or iron disulfide (FeS2). In particular the end cap assembly of the invention has a principal application for primary (nonrechargeable) wound electrode cells wherein the anode comprises a sheet of lithium or lithium alloy and the cathode comprises a layer, normally a coating comprising iron disulfide (FeS2). The cell casing is typically cylindrical.
In a principal aspect the end cap assembly comprises a metal end cap which forms the positive terminal, and an underlying metal cathode contact cup with an optional PTC (positive temperature coefficient) device therebetween. The cathode contact cup is electrically connected to both the underlying cathode and overlying end cap so that the cathode contact cup becomes a part of the electrical pathway between cathode and end cap. The cathode contact cup has an open end, opposing closed end or base with integral side walls therebetween. The end cap assembly also includes an insulating sealing disk, preferably of plastic, into which the cathode contact cup is inserted so that it is insulated from electrical contact with the cell casing. The insulating sealing disk has an aperture running longitudinally therethrough resulting in a pair of opposing open ends. The aperture is bounded by the side walls or peripheral edge of said insulating sealing disk.
In a principal aspect the cathode contact cup, which is of metal, is provided with an integral rupturable thinned portion which is designed to rupture and thereby release gas therethrough should the cell's internal pressure rise to a predetermined level. The rupturable thinned portion is an integral part of one of the walls of the cathode contact cup, desirably located within the cup's closed end or base facing the cell interior. The thinned portions are preferably formed by impacting a die having a sharp edge onto the closed end of the cathode contact cup. (The die edge may be preheated before impact.) Other methods of forming the thinned portions may be possible and are not excluded. Preferably the die is impacted against the closed end of the cathode contact cup thereby forming grooves therein. The grooves may be segmented or continuous and may be straight or curvilinear or a combination of both. The remaining metal underlying the grooves at the base of said grooves forms the thinned metal portions in the cathode cup closed end. The grooves are preferably made on the side of cathode contact cup closed end facing away from the cell interior. Alternatively, the grooves can be made on the opposite side of the cathode contact closed end, namely on the side facing the cell interior. The remaining metal underlying the grooves in the cathode cup base are designed to be thin enough so that they will rupture when gas pressure within the cell builds up to a predetermined level. A preferred metal for the cathode cup and thus also for the rupturable metal underlying said grooves has been determined to be an alloy of aluminum. The preferred metal of construction for the cathode contact cup is preferably an aluminum alloy that has been subjected to annealing so that it is sufficiently malleable that said rupturable metal portions underlying the grooves can be reliably manufactured at the small thicknesses required. The aluminum alloy also provides excellent electrical conductivity between the cathode material, cathode contact cup, and end cap.
The cathode contact cup desirably has a support disk or washer, preferably of metal, inserted therein to enhance the strength of said cup. The support disk or washer is typically of flat shape with a central aperture. Alternatively, the support disk may be built into the cathode contact cup, that is, formed as an integral part of the cathode contact cup. This in turn increases the annular thickness of the cathode contact cup and eliminates the need for a separate support disk to be inserted therein.
In assembly the wound electrodes are inserted into the cell casing and an insulating cover or insulating washer may be inserted to cover the top of the wound electrodes. An anode tab extending from the anode is welded to the closed end of the casing. The end cap assembly of the invention is formed outside of the casing. In forming the end cap assembly the metal cathode contact cup with optional support disk therein is inserted into the insulating sealing disk. The metal end cap with optional underlying PTC device is then also inserted into the insulating sealing disk over the cathode contact cup, so that the side walls or peripheral edge of said insulating sealing disk extends over the edge of the metal end cap. This completes formation of the end cap assembly. A cathode tab is joined with the base of the cathode contact cup through an opening at the base of the insulating sealing disk. Electrolyte is added to the wound electrodes within the casing. The end cap assembly is then inserted into the cell casing open end to close the casing. The edge of the casing is crimped over the insulating sealing disk peripheral edge which in turn crimps over the end cap assembly permanently locking the end cap assembly in place and tightly sealing the casing.
The invention will be better understood with reference to the drawings in which:
The end cap assembly 14 of the invention has application to wound electrode cells. The principal application for end cap assembly 14 is for use in closing, sealing, providing a venting system, and an electrical safety cut off, for a cylindrical casing (housing) 70. End assembly 14 also provides an end terminal for the cell. The casing 70 may be of a standard cylindrical size AAA (44×10 mm), AA (50×14 mm), C (49×25.5 mm) or D (60×33 mm) or other cell sizes.
The end cap assembly 14 herein described is principally intended for lithium primary (non rechargeable) cells, that is, wherein the anode comprises lithium. The cell may typically have an anode 240 comprising a sheet of lithium and a cathode comprising a coating or layer 260 comprising manganese dioxide (MnO2) or iron disulfide (FeS2). Anode 240 can be an alloy of lithium and an alloy metal, for example, an alloy of lithium and aluminum. In such case the alloy metal, is present in very small quantity, preferably less than 1 percent by weight of the lithium alloy. Thus, the term “lithium” or “lithium metal” as used herein and in the claims is intended to include in its meaning such lithium alloy. The lithium sheet forming anode 240 does not require a substrate. The lithium anode 240 can be advantageously formed from an extruded sheet of lithium metal having a thickness desirably between about 0.05 and 0.30 mm.
In particular the end cap assembly 14 of the invention has a principal application for wound electrode cells in particular wound electrode primary (nonrechargeable) cells as in cell 10 wherein the anode 240 comprises a sheet of lithium or lithium alloy and the cathode comprises a coating or layer 260 comprising iron disulfide (FeS2). The cathode coating 260 comprising FeS2 powder is desirably applied onto a grid or mesh or foil 265 thus forming a cathode composite 262 sheet (
In a principal embodiment end cap assembly 14 (
The cathode cup 40 desirably has a support disk or washer 140, preferably of metal, inserted therein as shown in representative
End cap assembly 14 (
The metal cathode contact cup 40 may be disk shaped having an open end 41 and opposing closed end or base 49 and integral side walls forming peripheral edge 48 therebetween. The base 49 may be stepped or recessed down from peripheral edge 48 as shown best in
The cathode contact cup 40 is characterized by having one or more thinned portions 43, preferably die cut into base 49. The thinned portions 43 are preferably formed by impacting a die having a sharp edge onto the top surface of the metal cathode contact cup base 49 thereby forming one or more grooves 44 which dig into the surface of said base 49. Grooves 44 have an open end and opposing closed base 42 and side walls 47a and 47b therebetween as shown in
The grooves 44 which are formed into the cathode contact cup base 49 may be of varying shape and pattern. The grooves 44 may be continuous or segmented. They may be linear (straight) or curvilinear or a combination of both. There may be one or a plurality of such grooves 44 cut into the cathode cup base 49. The grooves 44 side walls 47a and 47b may be vertical or slanted thus forming a V shape as shown in
An example of grooves 44 having a straight line pattern is shown in
An example of grooves 44 having a curvilinear pattern is circumferential groove 44 which circumvents the cathode contact cup base 49 as shown best in
By way of a specific non limiting example, if cell 10 has a lithium or lithium alloy anode 240 and cathode coating 260 comprising iron disulfide (FeS2), then a suitable rupture pressure for the thin portions 43 underlying grooves 44 for an AA size cell may be between about 50 and 1000 psi (345 and 6894 kilo pascal, desirably between about 300 and 800 psi (2068 and 5515 kilo pascal), preferably between about 350 and 500 psi (2413 and 3447 kilo pascal). In order to achieve such burst pressure in the context of the present invention, a cathode contact cup 40 formed of aluminum alloy (2.5% Mg; 0.25% Cr) can be advantageously employed. Such aluminum alloy, for example, is available under the ASTM designations 5052-H34 or 5052-H38, wherein H is the strain hardening designation. (Other aluminum alloys of different alloy composition and degree of heat treatment could also be sufficiently suitable material for cathode cup 40.) The cathode contact cup 40 wall thickness may typically be between about 0.2 and 1.5 mm. The base 49 portions adjacent grooves 44 (
When gas pressure within the cell 10 builds up to a predetermined pressure the remaining metal portions 43 underlying grooves 44 in the cathode contact cup base 49 will burst allowing gas from within the cell to escape to the environment through vent apertures 65 in end cap 60.
When the cathode contact cup 40 is formed of the above designated preferred aluminum alloy materials, e.g., ASTM designated 5052-H34 or 5052-H38 aluminum alloy, it has been determined that the remaining metal portion 43 underlying grooves 44 should have a reduced thickness in order to achieve burst pressure in the range between about 50 and 1000 psi (345 and 6894 kilo pascal) or more preferably burst pressures in the range between about 300 and 800 psi (2068 and 5515 kilo pascal) employing the above designated aluminum alloys. In order to achieve burst pressures in the range between about 50 psi and 1000 psi (345 and 6894 kilo pascal), preferably between about 350 and 500 psi (2413 and 3447 kilo pascal) employing the above designated aluminum alloys, the remaining metal portion 43 underlying grooves 44 should have a thickness between about 0.02 and 0.12 mm, typically between about 0.02 and 0.06 mm. More specifically, to achieve a burst pressure between about 350 and 500 psi (2413 and 3447 kilo pascal) when using aluminum alloy 5052-H38 ASTM designation for cathode contact cup 40, a preferred thickness of the remaining metal portion 43 underlying grooves 44 is between about 0.02 and 0.04 mm. When aluminum alloy 5052-H34 ASTM designation is employed for cathode contact cup 40, a preferred thickness of the remaining metal portion 43 underlying grooves 44 is between about 0.04 and 0.06 mm to achieve the same burst pressure between about 350 and 500 psi (2413 and 3447 kilo pascal). The groove width is defined herein as the width of groove 44 at its base 42, that is, at its closed end abutting underlying remaining metal 43 (
The end cap assembly 14 may be provided with a PTC (positive thermal coefficient) device 160 located under the end cap 60 and electrically connected in series between the cathode 260 and end cap 60 (
The cell 10, which may be a primary Li/FeS2 cell, may be assembled in the following manner:
An electrode assembly 213 is formed by spirally winding an anode sheet 240 and cathode composite 262 with separator sheet 250 therebetween. The initial layered configuration before winding is shown in
In assembly the anode tab 244 is passed against the flat or truncated edge portion 172 of bottom insulating disk 170 (
The cap assembly 14 is then formed in the following manner: A subassembly 14a may be formed first comprising cathode contact cup 40 with support washer 140, preferably of metal, inserted therein (
Various configurations of the subassembly 14a comprising cathode contact cup 40 with metal support washer 140 (or equivalent) therein are possible. Three embodiments of subassembly 14a are provided herein by way of example. In the first embodiment a metal support washer 140 (
In a second embodiment (single piece fabrication) shown in
In a third embodiment (
Once the subassembly 14a comprising the cathode contact cup 40 and metal support disk 140 (or equivalent) is completed it may be inserted directly into the body of insulating sealing disk 20 so that at least a portion of base 49 of the cathode contact cup 40 is exposed. The cathode tab 264 may then be welded to base 49 by laser welding or equivalent. The PTC disk 160 is inserted within the insulating sealing disk 20 so that it rests on the contact cup edge 48 as shown in
The following are suitable materials of construction for the above indicated components of cell 10 and end cap assembly 14, although it is not intended that the invention be necessarily limited to any particular materials:
The casing 70 may suitably be of nickel plated cold rolled steel of wall thickness typically between about 0.1 and 0.5 mm, preferably between 0.2 and 0.3 mm, for example about 0.25 mm. Alternatively, the casing 70 may be composed of aluminum, aluminum alloy, nickel, or stainless steel, or may include a plastic shell. The cathode contact cup 40 is preferably constructed of an aluminum alloy, in particular an aluminum alloy which has been heat treated (annealed) to make it more malleable. Suitable aluminum alloys for cathode contact cup 40 may be selected from the ASTM designated 1000 to 7000 series which have been subjected to heat treatment (annealing). A preferred aluminum alloy for cathode contact cup 40 is aluminum alloyed with magnesium and chromium, subjected to heat treatment (annealing), available under ASTM designation 5052-H38 or 5052-H34 as above described. The support washer 140 may desirably be of nickel plated cold rolled steel. Alternatively, support washer 140 may be of the same preferred composition as cathode contact cup 40, namely the above indicated aluminum alloys. The support washer 140 may have a wall thickness typically between about 0.1 and 1.5 mm, desirably between about 0.2 and 1.5 mm. Contact cup 40 may have wall thicknesses ranging typically between about 0.2 and 1.2 mm. End cap 60 may desirably be of nickel plated cold rolled steel having a wall thickness of between about 0.1 and 0.5 mm. The insulating sealing disk 20 for the lithium cell 10 is preferably of polypropylene but may be of other durable plastics including polyethylene, copolymers of polyethylene and copolymers of polypropylene, silicone rubber, and polybutyleneterephthalate, or other materials. Similarly the insulating disks 150 and 170 (
For a representative Li/FeS2 primary (nonrechargeable) wound electrode cell 10 employing the end cap assembly 14 of the invention, the cathode coating 260 having the following dry content is initially mixed with a hydrocarbon solvents such as ShellSol A100 hydrocarbon solvent (Shell Chemical Co.) and Shell Sol OMS hydrocarbon solvent (Shell Chemical Co.). The mixture is applied to conductive substrate (carrier) 265 (
FeS2 powder (89.0 wt. %); Binder Kraton G1651 elastomer from Kraton Polymers, Houston, Tex.) (3.0 wt. %); conductive carbon particles, high crystalline graphite Timrex KS6 from Timcal Ltd (7 wt. %) and carbon black, e.g., acetylene black (1 wt %). The dried cathode coating 260 adheres to conductive substrate 265 such as a foil or grid, preferably a sheet of aluminum, or stainless steel expanded metal foil to form the cathode composite 262 (
Anode 240 may be a sheet of lithium metal (99.8% pure). Alternatively, the anode sheet 240 can be an alloy of lithium and an alloy metal, for example, an alloy of lithium and aluminum. In such case the alloy metal, is present in very small quantity, preferably less than 1 percent by weight of the lithium alloy. Thus the lithium alloy functions electrochemically nearly as pure lithium. The separator sheet 250 for the Li/FeS2 cell may be a microporous polypropylene.
The wound electrode assembly 213 comprising anode sheet 240, cathode composite 262 (cathode coating 260 on conductive substrate 265) with separator sheet 250 therebetween is formed and inserted into cell casing 70. A suitable electrolyte is then added to the electrode assembly 213 after it has been inserted into the cell casing 70. A desirable electrolyte is an electrolyte solution comprising 0.8 molar (0.8 mol/liter) Li(CF3SO2)2N (LiTFSI) salt dissolved in an organic solvent mixture comprising about 75 vol. % methyl acetate (MA), 20 vol. % propylene carbonate (PC), and 5 vol. % ethylene carbonate (EC) as recited in commonly assigned U.S. patent application Ser. No. 11/516,534.
Although the present invention has been described with respect to specific embodiments, it should be appreciated that variations are possible within the concept of the invention. Accordingly, the invention is not intended to be limited to the specific embodiments but is within the claims and equivalents thereof.
