Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-11-19
2004-04-27
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S245000, C429S128000, C429S219000, C429S220000, C429S224000, C429S231100, C429S231500, C429S231700, C429S231950, C429S329000, C429S330000, C429S332000, C429S322000
Reexamination Certificate
active
06727022
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the current invention relates to a new sandwich electrode design and a process for manufacturing the same. Sandwich electrodes are useful as the cathode in primary lithium cells and as the positive electrode in secondary lithium ion cells. These designs make such cells particularly useful for powering implantable medical devices.
2. Prior Art
Early medical devices in many cases used at least two lithium electrochemical cells in series as their power source. However, the electronic circuits in these devices now consume less energy than before. This makes it currently possible to use a single lithium cell as a reliable power source. With a unitary cell design, the requirement for high power density in many applications is even greater as the result of lowered pulsing voltage. Thus, a large electrode surface area is needed to accomplish this requirement. However, as the electrode surface area increases, more inert materials (current collector, separator, etc.) are introduced into the system. As a result, the cell's volumetric capacity is decreased. Another concern is medical device longevity, which is dependent on the cell's capacity and power efficiency.
An attempt to use high capacity materials, such as CF
x
, by mixing it with a high rate cathode material, such as SVO, is reported in U.S. Pat. No. 5,180,642 to Weiss et. al. However, electrochemical cells made with these cathode composites have relatively lower rate capability. The benefit of increasing the cell theoretical capacity by using CF
x
as part of the cathode mix is balanced, in part, by lowering its power capability in a high rate discharge application, such as is encountered in an implantable cardiac defibrillator.
A significant solution to this problem is described in U.S. Pat. NO. 6,551,747 to Gan entitled Sandwich Cathode Design For Alkali Metal Electrochemical Cell With High Rate Capability by Gan et al., which is assigned to the assignee of the current invention and is incorporated herein by reference. This application describes a new sandwich electrode design using silver vanadium oxide (SVO) and a fluorinated carbon (CF
x
). An exemplary sandwich electrode has the following configuration:
SVO/current collector screen/CF
x
/current collector screen/SVO.
However, if one or both of the active materials is in a powdered form and the openings in the current collector screen are too large, there can be communication of one of them to the other side of the current collector during the manufacturing process. This “contamination” is undesirable as it detracts from discharge performance. Specifically, SVO is of a higher rate capability, but a lower energy density than CF
x
. Therefore, contamination of the interface between the current collector and one of the active materials by the other is undesirable as it defeats the purpose of having the respective active materials segregated on opposite sides of the current collector in the first place.
SUMMARY OF THE INVENTION
To maintain the improved discharge capability of a cell containing a sandwich electrode, it is necessary to maintain direct contact of both the first and second electrode materials with the opposed sides of the current collector. A good contact or adhesion translates into good interfacial conductivity during discharge. Although it is clear in theory, in practice this interfacial conductivity is highly influenced by the manufacturing methods or processes. When the current collector is a screen, it is possible for some of one of the powdered electrode materials to pass through the current collector openings and become trapped between the other electrode material and the current collector. This leads to decreased interfacial conductivity between the current collector and the “contaminated” first electrode material.
Thus, the present process consists of having one of the electrode active materials in a cohesive form incapable of moving through the current collector to the other side thereof. The other or second active material is in a powdered form capable of communication through the current collector. Then, the assembly of first active material/current collector/second active material is pressed from the direction of the first, cohesive electrode active material, which causes it to seal off the current collector as the pressing force moves the current collector against the second, powdered electrode active material.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The electrochemical cell of the present invention is of either a primary chemistry or a secondary, rechargeable chemistry. For both the primary and secondary types, the cell comprises an anode active metal selected from Groups IA, IIA and IIIB of the Periodic Table of the Elements, including lithium, sodium, potassium, etc., and their alloys and intermetallic compounds including, for example, Li—Si, Li—Al, Li—B, Li—Mg and Li—Si—B alloys and intermetallic compounds. The preferred metal comprises lithium. An alternate negative electrode comprises a lithium alloy, such as lithium-aluminum alloy. The greater the amount of aluminum present by weight in the alloy, however, the lower the energy density of the cell.
For a primary cell, the anode is a thin metal sheet or foil of the lithium material, pressed or rolled on a metallic anode current collector, i.e., preferably comprising nickel, to form the negative electrode. In the exemplary cell of the present invention, the negative electrode has an extended tab or lead of the same material as the current collector, i.e., preferably nickel, integrally formed therewith such as by welding and contacted by a weld to a cell case of conductive material in a case-negative electrical configuration. Alternatively, the negative electrode may be formed in some other geometry, such as a bobbin shape, cylinder or pellet to allow an alternate low surface cell design.
In secondary electrochemical systems, the anode or negative electrode comprises an anode material capable of intercalating and de-intercalating the anode active material, such as the preferred alkali metal lithium. A carbonaceous negative electrode comprising any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glassy carbon, etc.) which are capable of reversibly retaining the lithium species, is preferred for the anode material. A “hairy carbon” material is particularly preferred due to its relatively high lithium-retention capacity. “Hairy carbon” is a material described in U.S. Pat. No. 5,443,928 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated herein by reference. Graphite is another preferred material. Regardless of the form of the carbon, fibers of the carbonaceous material are particularly advantageous because they have excellent mechanical properties which permit them to be fabricated into rigid electrodes that are capable of withstanding degradation during repeated charge/discharge cycling. Moreover, the high surface area of carbon fibers allows for rapid charge/discharge rates.
A typical negative electrode for a secondary cell is fabricated by mixing about 90 to 97 weight percent “hairy carbon” or graphite with about 3 to 10 weight percent of a binder material, which is preferably a fluoro-resin powder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylenetetrafluoroethylene (ETFE), polyamides, polyimides, and mixtures thereof. This negative electrode admixture is provided on a current collector such as of a nickel, stainless steel, or copper foil or screen by casting, pressing, rolling or otherwise contacting the admixture thereto.
In either the primary cell or the secondary cell, the reaction at the positive electrode involves conversion of ions which migrate from the ne
Gan Hong
Takeuchi Esther S.
Scalise Michael F.
Weiner Laura
Wilson Greatbatch Ltd.
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