WO2001022511A1

WO2001022511A1 - Electrochemical cell

Info

Publication number: WO2001022511A1
Application number: PCT/US2000/025687
Authority: WO
Inventors: William T. Mchugh; Yelena Kouznetsova; Fred J. Berkowitz
Original assignee: The Gillette Company
Priority date: 1999-09-22
Filing date: 2000-09-20
Publication date: 2001-03-29
Also published as: AU7592500A

Abstract

A non-circular electrochemical cell (10) with a housing (20) having an elongated surface, an anode (40) containing lithium, a cathode (62) containing manganese dioxide, and a nonaqueous electrolyte wherein the anode and cathode are spirally wound. In one embodiment the cell has overall dimensions of length, width and thickness about the same as the overall dimensions of two AA size alkaline cells placed in side by side arrangement with edges aligned. In such configuration the cell of the invention can be used in place of two AA alkaline cells in high power electronic devices, such as digital cameras, which were designed to house two AA alkaline cells in side by side arrangement, and electrically connected in series. The cell of the invention preferably has a cross section, taken perpendicular to its central longitudinal axis, which is of elliptic or approximately elliptic shape.

Description

ELECTROCHEMICAL CELL The invention relates to spirally wound primary cells having an anode comprising lithium and a cathode comprising manganese dioxide.

Primary (non-rechargeable) electrochemical cells having an anode comprising lithium and a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF₃SO₃) dissolved in a non-aqueous solvent, are known and are in widespread commercial use. These cells are commonly in the form of button cells or cylindrical cells having about 2/3 the height of a conventional AA size alkaline cell. (Alkaline cells as referenced herein shall be understood to be conventional commercial alkaline cells having an anode comprising zinc, a cathode comprising manganese dioxide, and an electrolyte comprising potassium hydroxide). 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. Therefore, Li/MnO₂ cells have been used in compact electronic equipment, especially photographic cameras, which require operation at higher voltage and at higher power demand than individual alkaline cells.

Recently electronic devices, such as digital cameras, as well as some high power toys and electronic games have appeared in the commercial market. These devices, which require operational load voltage at the level of about 2 to 3 Volt were intended to operate with two conventional Zn/MnO₂ alkaline cells electrically connected in series. The devices are provided with a compartment that is designed to receive and hold two alkaline cells in side by side (parallel) arrangement, but with the cells electrically connected in series. The cylindrical bodies of the two alkaline cells are typically in side by side relationship with one end of the first alkaline cell aligned to the same level or height (same plane) as one end of the second cell. The compartment typically does not have any raised surface or other material to separate the cell bodies. (The cylindrical body of each alkaline cell is well insulated electrically by virtue of the cell label and therefore no further insulation is required between the two cells placed in side by side physical contact with each other). The compartment is provided with a pair of positive and negative connection terminals located at the compartment ends as well as fixed internal connectors which electrically connect the cells in series. A pair of alkaline cells is simply inserted, in side by side (parallel) alignment, into the compartment, so that the cells are electrically connected in series between the positive and negative terminals within the compartment. In certain devices requiring higher voltage or additional capacity, the device is provided with a second compartment which can house a second pair of alkaline cells similarly arranged in side by side orientation, but connected in series with each other. The first pair of cells can be connected in parallel or in series with the second pair of cells.

It is a requirement of the IEC (International Electrotechnical Commission) that primary cylindrical cells of given voltage have a characteristic cell length and diameter. The standard overall dimensions which IEC follows for cylindrical AA size cells is given by the American National Standards Institute (ANSI) battery specification ANSI C18.1M, Part 1-1999. Since the Li/MnO₂ primary cell has a higher voltage than conventional alkaline cells, its length must be different from an alkaline cell and conventionally has been restricted to a length which is 2/3 the length of an AA alkaline cell. Such standards are set to prevent a user from inadvertently using a Li/MnO₂ cell in place of an alkaline cell, since the higher voltage and higher power of the Li/MnO₂ cell could damage or destroy an electronic device which is intended to operate only under the alkaline cell voltage. In the case of a 2/3 A Li/MnO₂ cell, the cell's casing is a cylindrical stainless steel casing having a closed end and open end. Li/MnO₂ primary cells are conventionally formed of spirally wound material comprising an anode formed of a sheet of lithium, a cathode formed of a coating of cathode active material comprising manganese dioxide on a conductive metal substrate (cathode substrate) and a sheet of separator material therebetween, as shown, for example, in U.S.

Patent 4,707,421. The cathode substrate is typically a stainless steel expanded metal foil. The separator sheet is typically placed on opposite sides of the lithium anode sheet and the cathode sheet is placed against one of the separator sheets, thereby separating the anode from the cathode. The layered material, thus formed, is spirally wound and inserted into the open end of the casing. The spiral of material can be oriented so that outermost layer of active material comprises anode active material which comes into electrical contact with the inside surface of the casing and the closed end of the can forms the cell's negative terminal. An end cap is inserted into the open end of the casing. A conductive tab is provided to electrically connect the cathode substrate with the end cap. The end cap thus functions as the cell's positive terminal. The casing is crimped over the peripheral edge of the end cap to seal the casing's open end. The Li/MnO₂ cell is typically provided with PTC (positive thermal coefficient) device located under the end cap and connected in series between the cathode and end cap. Such device protects the cell from discharge at a current drain higher than a predetermined level. Thus, if the cell is drained at an abnormally high current, e.g., higher than about 2 Amp, the PTC device expands and heats causing its resistance to increase dramatically thus preventing the abnormally high drain.

The Li/MnO₂ cell is a non-aqueous cell. The manganese dioxide powder used to form the cathode active material can be conventionally heat treated at temperatures of between about 200-350°C in vacuum as taught in U.S. Patent 4,133,856 (Ikeda). It is preferable to heat the MnO₂ in two steps once to temperatures above 250°C to drive off non-crystalline water during which step gamma MnO₂ is gradually converted to gamma-beta structure and then again at temperatures between 250 and 350°C as described in U.S. Patent 4,133,856 prior to insertion of the MnO₂ into the cell. The treatment results in better cell performance and higher capacity. The second heating helps to prevent electrolyte leakage. The treated MnO₂ is mixed with suitable binders, for example, tetrafluoroethylene (Teflon) binders, and conductive agents, for example, carbon black and graphite. The cathode mixture can be coated onto a metallic substrate such as a stainless steel expanded metal foil. The anode can be formed by coating a layer of lithium on a metallic substrate such as copper. However, it is preferable that the anode is formed of a sheet of lithium without any substrate.

The electrolyte used in a Li/MnO₂ cell is formed of a lithium salt dissolved in an organic solvent. Typically, the salt is lithium perchlorate (LiClO₄) or lithium trifluoromethanesulfonate (LiCF₃SO₃). Other salts which are conventionally used include LiPF₆, LiAsF₆ and LiCF₃CO₂ and LiBF₄. Organic solvents can typically include ethylene carbonate/propylene carbonate (EC/PC) dimethoxyethane (DME), dioxolane, gamma-butyrolactone, and diglyme.

Accordingly, it is desirable to use one Li MnO₂ primary cell in place of two same sized cylindrical alkaline cells in devices intended to operate with two such alkaline cells electrically connected in series. It is desirable to use one Li/MnO₂ primary cell in place of two cylindrical alkaline cells electrically connected in series in devices that have a battery compartment intended to hold the two alkaline cells in a side by side arrangement.

An aspect of the invention is directed to a primary electrochemical cell having an anode comprising lithium and a cathode comprising manganese dioxide wherein the cell has an elongated non-circular casing (housing). The cell casing (housing) is preferably formed of a continuous non-circular cylindrical surface having an open end and a closed end and preferably has an elliptic or approximately elliptic shape when viewed in cross section taken perpendicular to the casing's longitudinal axis.

The cell of the invention comprises a spirally wound electrode composite. The electrode composite comprises an anode sheet comprising lithium, a cathode sheet formed of a mixture comprising manganese dioxide coated onto a conductive substrate, and a separator sheet between anode and cathode sheets. A second separator sheet is applied to cover the exposed anode surface. The electrode composite is spirally wound and inserted into the open end of the casing. An end cap assembly comprising a metallic end cap and underlying insulating disk is inserted into the casing and the end cap is sealed within the casing by crimping. The anode is preferably electrically connected to the body of the casing and the cathode is electrically connected to the end cap. The cell, when fresh (before first used), has an open circuit voltage (OCV) of about 3 volt.

The cell casing has a non-circular cylindrical continuous surface characterized by a pair of opposing wide sides and a pair of opposing short sides. The pair of wide sides preferably comprise two flat, opposing parallel surfaces and the short sides comprise a pair of opposing curved surfaces. The opposing curved surfaces are convex as viewed from outside the cell. Although the preferred shape of the pair of opposing wide sides are flat, the shape of the cell in cross section has an elliptic appearance in that its overall shape is approximately elliptical. (The term elliptic as used herein shall have its ordinary dictionary definition as "as having the shape of an ellipse or resembling or having the approximate shape of an ellipse"). The shape of the cell in cross section (perpendicular to the longitudinal axis) can be a true elliptical or oval shape. In such case the pair of opposing wide surfaces will be curved convex as viewed from outside the cell.

The overall cell configuration having an elliptic cross section can be adjusted so that it can fit into a battery compartment of electronic devices intended to house two conventional cylindrical AA alkaline cells electrically connected in series but arranged in side by side orientation within the compartment. This is possible in measure because the running load voltage of one cell of the invention is about equal to two AA cells in series. The cell of the invention, however, has the advantage that its capacity (mAmp-hrs) and service life are greater than that of a the two cylindrical AA cells which it replaces, particularly, in high power applications, for example, electronic digital cameras. Similarly, the cell of the invention having an elliptic cross section configuration can be sized to replace any two same size alkaline cells, for example, two alkaline cells of AAAA, AAA, C or D size in those applications where the battery compartment was intended to house two of such same size cells electrically connected in series but arranged in side by side orientation. The invention will be better understood with reference to the drawings in which:

Fig. 1 is an isometric view of a preferred embodiment of the invention.

Fig. 2A is a partial cross sectional elevation view of the cell taken through sight lines 2-2 of Fig. 1 to show the top and interior portion of the cell.

Fig. 2B is a partial cross sectional elevation view of the cell taken through sight lines 2-2 of Fig. 1 to show the bottom and interior portion of the cell.

Fig. 3 is a cross sectional view (taken perpendicular to the cell's longitudinal axis) of the electrode spiral in the cell. Fig. 4 is a schematic showing the placement of the layers comprising the electrode spiral.

Fig. 5 is a plan view of the electrode spiral of Fig. 4 with each of the layers thereof partially peeled away to show the underlying layer.

A preferred embodiment of a primary (non-rechargeable) cell of the invention having an anode comprising lithium and a cathode comprising manganese dioxide is shown in Figures 1-5. As shown in the figures the salient structural feature of the cell is that it has a non-circular cylindrical casing (housing) 20 which preferably has an elliptic or approximately elliptic shape in cross section taken perpendicular to the central longitudinal axis 67. (The term elliptic as used herein shall have its ordinary dictionary definition "as having the shape of an ellipse or resembling or having the approximate shape of an ellipse"). In this regard the overall appearance of the cell is approximately as a non-circular cylindroid. The preferred shape of the cell casing (housing) 20 is shown in Fig. 1. Casing 20 is preferably formed of nickel plated steel. The cell casing 20 (Fig. 1) has a continuous non-circular cylindrical surface characterized by a pair of opposing wide sides (90a and 90b) and a pair of opposing short sides (95 a and 95b). The pair of wide sides preferably comprise two flat, opposing parallel wide surfaces (90a and 90b) and the short sides comprise a pair of opposing curved surfaces (95 a and 95b). The opposing curved surfaces 95 a and 95b are convex as viewed from outside the cell. Although the preferred shape of the opposing wide sides 90a and 90b are flat and parallel, the shape of cell 10 and casing 20 in cross section (Figure 3) has an elliptic appearance in that its overall shape is elliptic or approximately elliptic. It will be appreciated that the cross section of cell 10 and casing 20 can also be a true elliptical or oval shape. In such case opposing wide surfaces 90a and 90b will have a convex curved shape as viewed from outside the cell.

In an alternative embodiment, one of wide sides 90a can be flat and the other opposing wide side 90b can have a convex curved shape when viewed from outside the casing and the opposing short sides 95a and 95b preferably both have a convex curved shape as shown. In this latter embodiment the overall appearance of the cell in cross section taken along a plane perpendicular to the cell's longitudinal axis 67 will still have an overall elliptic appearance, since the shape of the cross section will appear approximately as the shape of an ellipse, despite the fact that one of the wide sides 90a is flat and the other 90b is curved. The short sides 95a and 95b preferably have a convex curvature as shown in Figure 3 so that the cell 10 can fit into a standard battery compartment of a digital camera or other electronic device intended to accommodate a pair of same size cylindrical alkaline cells in side by side arrangement but electrically connected in series. The phrase "side by side relationship of the pair of same size cylindrical alkaline cells" as used herein is meant that the cell cylindrical bodies are intended to be parallel to each other with said bodies contacting or very nearly contacting each other and with one end of the first alkaline cell aligned to the same level or height (same plane) as one end of the second alkaline cell and the other end of the first alkaline cell aligned to the same level or height (same plane) as the other end of the second alkaline cell. Such orientation is obtained, for example, by simply placing two same sized cylindrical alkaline cells upright and side by side on a flat table surface. In such orientation the negative terminals of the two cells can be aligned so that they are one plane and the positive terminals of the cells are in another plane. Alternatively, of the two cells can be reversed so that the negative terminal of the first cell and the positive terminal of the second cell are in one plane and the positive terminal of the first cell and the negative terminal of the second cell are in another plane. The above referenced battery compartment, for example, of a digital camera, typically has one pair of side edges curved to conform to the curvature of the pair of same size cylindrical alkaline cells to be inserted therein. The bottom of such compartment is typically flat or very nearly flat. The cell 10 of the invention being a Li/MnO₂ cell having a voltage of approximately 3.0 volts (twice that of a conventional 1.5 volt alkaline cell) is thus designed to be inserted into such compartments instead of the pair of alkaline cells. The Li/MnO₂ cell 10 of the invention has a higher energy density than an alkaline cell and is more suitable for high power application than an alkaline cell. It has been determined that the cell 10 of the invention has a total capacity, which is very much greater than that of a pair of alkaline cells electrically connected in series and occupying about the same space, especially at high pulsed power demands above 2 watt.

In the preferred embodiment shown in the figures, the cell 10 has a width approximately that of AA cylindrical cell diameter and the curvature of opposing short sides 95a and 95b is about the same as a conventional AA cell. In such embodiment the overall size of cell 10 as measured by length (distance between positive and negative terminals), width (largest distance between short sides 95a and 95b) and thickness (largest distance between opposing wide sides 90a and 90b) is about the same as the overall size of two standard AA cells whose cylindrical bodies are placed side by side and with one end of the first alkaline cell aligned to the same level or height as one end of the second alkaline cell. The standard AA cylindrical cell overall dimensions as referenced herein are given by the American National Standards Institute (ANSI) battery specification ANSI C18.1M, Part 1-1999 as follows: The standard AA size cell is cylindrical. The overall length from positive and negative terminal tips of the standard AA cell are between 49.2 mm and 50.5 mm and overall outside cell diameter is between 13.5 mm and 14.5 mm. Thus, in the embodiment shown in Fig. 1 the single cell 10 of the invention desirably has an overall width of between about 27.0 and 29.0 mm (twice the diameter of a single AA cell) and a length (between terminal tips) of between about 49.2 mm and 50.5 mm (same a single AA cell) and thickness of between about 13.5 mm and 14.5 mm (same as the outside diameter of an AA cell). It should be appreciated that the standard size of the AA cell could be changed by the American National Standards Institute and the present invention is intended to apply as well to any such changed standard of the AA size cell. The term AA size as used herein shall also apply to cell sizes at least within 10 percent of the standard AA size. In the embodiment shown in Fig. 1, the single cell 10 of the invention can be inserted into the above referenced battery compartment in place of a pair of AA alkaline cells intended to be oriented in side by side arrangement but electrically connected in series. It will be appreciated that the overall size of cell 10 can be adjusted so that it will replace a pair of like cylindrical alkaline cells for the referenced battery compartment regardless of the cell size. For example, the width of cell 10 and curvature of opposing sides 95a and 95b can be adjusted to conform to the diameter and curvature, respectively, of AAAA, AAA, C or D size cylindrical alkaline cells.

The spiral wound electrode composite 70 (Fig. 3) comprising anode 40 and cathode 62 with separator 50 therebetween can be prepared in the following manner: A cathode mixture comprising manganese dioxide can be prepared and coated onto a substrate sheet 65 of stainless steel expanded metal foil to form a cathode composite sheet 62 (Fig. 4). The expanded stainless steel sheet 65 is available under the trade designation EXMET metal from Delkeo Co. It appears as a porous mesh or screen. Alternatively, the cathode substrate 65 can be a stainless steel foil. The cathode coating 60 on the substrate 65 desirably has a thickness of between about 0.38 and 0.42 mm, preferably about 0.4 mm. The stainless steel substrate 65 secures the cathode coating 60 and functions as a cathode current collector during cell discharge.

The anode 40 is prepared from a solid sheet of lithium metal. The lithium sheet does not require a substrate. The lithium anode 40 can be advantageously formed from an extruded sheet of lithium metal having a thickness of desirably between about 0.15 and 0.20 mm desirably between about 0.16 and 0.17 mm, preferably about 0.165 mm. A separate sheet of separator material 50, preferably of polypropylene having a thickness of about 0.025 mm is inserted on each side of the lithium anode sheet 40 (Figs. 4 and 5). One separator sheet 50 can be arbitrarily designated the outer separator sheet and the other can be designated the inner separator sheet. The cathode composite sheet 62 comprising cathode coating 60 on conductive substrate 65 is then placed against the inner separator sheet 50a to form the flat composite shown in Fig. 4. The flat composite (Fig. 4) is spirally wound to form electrode spiral 70 (Fig. 3). As may be seen from Fig. 3 the electrode spiral 70 has separator material 50 between anode sheet 40 and cathode composite 62. The spirally wound electrode 70 is wound into an elliptic configuration (Fig. 3) conforming to the shape of the casing body. The spirally wound electrode composite 70 with empty core 98 is inserted into the open end 30 of casing 20. As wound, the outer layer of the electrode spiral 70 comprises separator material 50 as best shown in Figs. 2A, 2B, and 3. An additional insulating layer 72, for example, a plastic film such as polyester tape, can desirably be placed over the outer separator layer 50, before the electrode composite is wound. In such case the spirally wound electrode 70 will have insulating layer 72 in contact with the inside surface of casing 20 (Fig. 2A and 2B) when the wound electrode composite is inserted into the casing. Alternatively, the inside surface of the casing 20 can be coated with electrically insulating material 72 before the wound electrode spiral 70 is inserted into the casing. Also it is desirable to insert an electrical insulating material 74 (Fig. 2B) to line the bottom (closed end 35) of the casing before the spirally wound electrode composite 70 is inserted into the casing. Insulating material 74 prevents cathode material from inadvertently contacting the casing bottom 35 and thus provides additional assurance that the cell will not short.

An end cap 18 forming the cell's positive terminal has a metal tab 25 (cathode tab) which is welded on one of its sides to inside surface of end cap 18. Metal tab 25 is preferably of stainless steel. A portion of the cathode substrate 65 is flared along its top edge forming an extended portion 64 extending from the top of the wound spiral as shown in Figure 2 A and 5. The flared cathode substrate portion 64 can be welded to the exposed side of metal tab 25 before the casing peripheral edge 22 is crimped around the end cap 18 to close the cell's open end 30. Alternatively, the end cap 18 can be welded along its peripheral edge to the inside surface of the casing peripheral edge 22. In such case the terminal 17 is desirably an integral part of end cap 18. Alternatively, terminal 17 can be formed as the top of an end cap assembly of the type described in U.S. Patent 5,879,832 which assembly can be inserted into an opening in the surface of end cap 18 and then welded thereto.

A metal tab 44 (anode tab), preferably of nickel can be pressed into a portion of the lithium metal anode 40. Anode tab 44 can be pressed into the lithium metal at any point within the spiral but is conveniently pressed in to the lithium metal at the outermost layer of the spiral as shown in Fig. 2B. Anode tab 44 can be embossed on one side forming a plurality of raised portions on the side of the tab to be pressed into the lithium. The opposite side of tab 44 can be welded to the inside surface of the casing as illustrated in Fig. 2B.

A plastic insulating disk 80 can be inserted into the open end 30 of casing 20. The peripheral edge 85 of insulating disk 80 fits snugly into the open end 30 and presses against the inside surface of the peripheral edge 22 of casing 20. The positive end cap 18 (cover) is then inserted into the open end 30 so that edge 85 of the insulating disk 80 electrically insulates end cap 18 from casing 20.

Alternatively, an end cap assembly 15 can be formed of end cap 18 and insulating disk 80 and the end cap assembly 15 can be inserted as a separate unit into the casing open end 30. End cap 18 desirably has a vent 19 which can contain a rupturable membrane designed to rupture and allow gas to escape if the gas pressure within the cell exceeds a predetermined level. The end cap 18 has metal tab 25 (cathode tab) welded to a portion of its under surface (Fig. 2A). The tab penetrates through an opening in insulating disk 80 so that it can be welded to the extended portion 64 of the cathode substrate 65 as above described. After metal tab 25 is welded to the extended portion 64 of cathode substrate 65, the peripheral edge of the casing can then be crimped around the peripheral edge 85 of the insulating disk 80 to permanently close open end 30 and to hold end cap 18 firmly sealed within the cell as shown in Fig. 1.

In the preferred embodiment shown in the figures, the cathode coating 60 can be prepared have the following desirable formulation: manganese dioxide (electrolytic manganese dioxide, EMD), 90.9 wt.%, tetrafluoroethylene (Teflon polymer), 3.0 wt.%, Shawinigan carbon black, 4.1 wt.%, and particulate graphite, 2.0 wt.%. The manganese dioxide was heat treated in conventional manner to remove non-crystalline water therefrom before the cathode coating 60 was prepared. The cathode mixture can be mixed in a conventional electric blender at room temperature until a homogeneous mixture is obtained. The cathode mixture 60 can be coated on one side of a cathode substrate 65 to form cathode composite sheet 62. The cathode substrate 65 is preferably a stainless steel expanded metal foil (EXMET stainless steel foil) having a basis weight of about 0.024 g/cm². If the cell 10 of the invention is to be used in place of 2 AA alkaline cells in side by side arrangement, the cathode composite sheet 62 can desirably be about 441.0 mm (length), 40.7 mm (width) and 0.4 mm (thick). In such application the cathode coating 60 can desirably have a weight of about 19 grams and cathode substrate 65 can desirably have a weight of about 4 grams.

The anode 40 is desirably formed of a continuous sheet of lithium metal (99.8% pure). If the cell 10 of the invention is to be used in place of 2 AA alkaline cells in side by side arrangement, the anode dimensions are 483.0 mm (length), 38.0 mm (width), and 0.165 mm (thick) with the anode weight about 1.6 grams.

Electrolyte is added to the wound electrode spiral 70 before the insulating disk 80 and end cap is inserted into the cell. The electrolyte desirably has the following composition: Lithium salt lithium trifluoromethanesulfonate (LiCF₃SO₃), 9.3 wt.%), LiNO₃, 500 ppm, solvents ethylene carbonate/propylene carbonate (EC/PC) 34.5 wt.%, dimethoxyethane (DME), 56.2 wt.%. The total electrolyte can desirably be about 5.2 grams.

The separator 50 can desirably be a sheet of microporous polypropylene membrane of basis weight between about 13.5 to 16.5 g/m² and about 0.025 mm thick.

After the cell is assembled it is conditioned by predischarging at 1 Amp for 3.6 minutes followed by storage for 24 hours at 55°C. Performance tests of the cell of the invention were made. The cell of the invention has a discharge profile at different current drain, similar to that of a 2/3 AA size Li MnO₂ cell having like chemical composition and like anode and cathode thickness. However, since the cell of the invention can be much larger, that is, if it is designed to occupy the space taken by two AA alkaline cells in side by side arrangement, the cell of the invention has a capacity (mAmp-hrs) which is about 3 times that of a conventional 2/3 A size Li/MnO₂ cell.

When compared to the performance of two AA cells electrically connected in series, the Li/MnO₂ cell 10 of the invention has a greater capacity and exhibits better performance, especially at high power applications. For example, at a constant high current drain at about 1.5 Amp, the capacity (mAmp-hrs) of the cell of the invention, designed to occupy the space of two conventional AA alkaline cells in side by side arrangement, is about 4 times that of such two AA alkaline cells electrically connected in series. PERFORMANCE TEST

A performance test has been made using a NEC Picona PC-DC200 electronic digital camera. This camera, is representative of digital cameras which are generally high power drain devices. The power drain of the NEC Picona PC-DC200 digital camera is about 2.38 Watt (pulse) for non-flash photos. The camera's battery compartment is designed to hold two AA cells in side by side arrangement, with the cells electrically connected in series. There is no raised partition within the compartment separating the AA cell bodies. The camera's performance was first examined using a pair of AA alkaline cells. The camera's performance was then examined using the cell of the invention (Li/MnO₂ cell with elliptic cross section as above described and as shown in the above figures). No modification was made to the battery compartment. The test was conducted by the following protocol: 1) Turn the camera on, 2) take a flash photo every 1 minute, 3) clear memory disk every 40 photos. Thus, the batteries were subjected to high power pulsed discharge. The camera was operated in this manner until the batteries discharged to the point that the camera no longer performed. The actual number of photo exposures obtained with the camera loaded with a pair of AA alkaline (Duracell Ultra 1.5 Volt AA cells) electrically connected in series was 37. The actual number of flash photo exposures obtained with the camera loaded with the above described cell of the invention was 271. Thus, the camera loaded with the cell of the invention delivered, at least more than 5 times (actually more than 7 times) the number of flash photo exposures. Although the present invention was described with respect to one or more specific embodiments, it should be appreciated that variations are possible within the concept of the invention. Accordingly, the scope of the invention is not intended to be limited by the specific embodiments, but rather is better reflected the claims and equivalents thereof.

Claims

C L A I M S 1. An electrochemical cell comprising a housing having a surface, a positive and a negative terminal, an anode, a cathode comprising manganese dioxide, and a non-aqueous electrolyte, wherein the cell has overall dimensions of length, width and thickness about the same as the overall dimensions of a first and second AA size cylindrical alkaline cell in side by side arrangement with one end of said first AA cell aligned in the same plane as one end of said second AA cell and the other end of said first AA cell aligned in the same plane as the other end of said second AA cell.

2. The cell of claim 1, where said anode comprises lithium.

3. The cell of claim 1, wherein said housing has an elongated surface and said housing has a non-circular shape when viewed in cross section taken through a plane perpendicular to the cell's longitudinal axis.

4. The cell of claim 1, wherein said housing has an elongated surface and said housing has an elliptic shape when viewed in cross section taken through a plane perpendicular to the cell's longitudinal axis.

5. An electrochemical cell comprising a housing having a positive and a negative terminal, an anode comprising lithium, a cathode comprising manganese dioxide, and a non-aqueous electrolyte, wherein the housing comprises an elongated surface forming the body of said housing, a central longitudinal axis defined along the length of said housing, said housing having an elliptic shape when viewed in cross section taken through a plane perpendicular to said central longitudinal axis.

6. The cell of claim 5, wherein the anode and cathode are in spirally wound configuration.

7. The cell of claim 5, wherein the housing is formed of a continuous elongated surface having an open end and a closed end.

8. The cell of claim 7, wherein the said elongated surface has a pair of opposing short sides and a pair of opposing wide sides between said pair of opposing short sides.

9. The cell of claim 8, wherein said pair of opposing short sides have a convex curved shape when viewed from outside the housing.

10. The cell of claim 8, wherein the pair of opposing wide sides are flat.

11. The cell of claim 10, wherein the pair of opposing wide sides are parallel.

12. The cell of claim 8, wherein the pair of opposing wide sides have a convex curved shape when viewed from outside the housing.

13. The cell of claim 8, wherein one of the pair of opposing wide sides is flat and the other has a convex curved shape when viewed from outside the housing.

14. The cell of claim 5, wherein the cell has an overall size as measured by length, width and thickness which is about the same as the overall size of a first and second cylindrical alkaline AA cell placed in side by side arrangement with one end of said first AA cell aligned in the same plane as one end of said second AA cell and the other end of said first AA cell is aligned in the same plane as the other end of said second AA cell.

15. The cell of claim 14, wherein said cell can be used in place of two AA alkaline cells electrically connected in series and arranged in side by side relationship.

16. The cell of claim 5, wherein the anode comprises a sheet of lithium and the cathode comprises a coating comprising manganese dioxide on an electrically conductive substrate (cathode substrate).

17. The cell of claim 16, wherein the conductive substrate is a stainless steel expanded metal foil.

18. The cell of claim 16, wherein the anode and cathode are spirally wound with a separator sheet therebetween.

19. The cell of claim 7, comprising an end cap inserted into the open end of said housing, said end cap forming the positive terminal of said cell and the closed end of said housing forming the negative terminal.

20. The cell of claim 19, wherein said electrically conductive cathode substrate is electrically connected to said positive terminal.

21. The cell of claim 19, wherein said anode is electrically connected to said negative terminal.

22. The cell of claim 5, wherein said cell, when fresh, has an open circuit voltage (OCV) of about 3 volt.

23. The cell of claim 14, wherein when said cell is applied to power a NEC Picona PC-DC200 digital camera, said cell yields over 250 flash photo exposures (test performed with flash photo taken every 1 minute, and memory disk cleared every 40 photos).

24. The cell of claim 14, wherein when said cell is applied to power a NEC Piconal PC-DC200 digital camera, said cell yields over 5 times the number of flash photo exposures than two Duracell Ultra AA (1.5 Volt) alkaline batteries connected in series (test performed with flash photo taken every 1 minute, and memory disk cleared every 40 photos).

25. The cell of claim 14, wherein when said cell is applied to power a NEC Piconal PC-DC200 digital camera, said cell yields over 7 times the number of flash photo exposures than two Duracell Ultra AA (1.5 Volt) alkaline batteries connected in series (test performed with flash photo taken every 1 minute, and memory disk cleared every 40 photos).