Chemistry: electrical current producing apparatus – product – and – Plural concentric or single coiled electrode
Reexamination Certificate
1998-04-28
2001-05-15
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Plural concentric or single coiled electrode
C429S129000, C429S163000, C429S186000
Reexamination Certificate
active
06232012
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electrochemical cells having a lithium anode and more particularly to a primary lithium electrochemical cell adapted for high reliability and high rates of current discharge.
Implantable cardiac defibrillators are used to treat patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the defibrillator device continuously monitors the electrical activity of the heart of the patient, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore a normal heart rhythm. Shocks as large as 30-35 joules may be needed. Shocks are delivered from capacitors capable of providing that energy to the patient in a fraction of a second. In order to provide timely therapy to the patient after the detection of ventricular fibrillation, it is necessary to charge the capacitors with the required amount of energy in only a few seconds. Thus, the power source must have a high rate capability to provide the necessary charge to the capacitors, it must also possess low self-discharge in order to have a useful life of many months, and it must be highly reliable to provide an urgently needed therapy whenever necessary. In addition, since cardiac defibrillators are implanted, the battery must be able to supply energy from a minimum packaged volume.
One battery suitable for defibrillator use is disclosed in U.S. Pat. No. 4,830,940 to Keister et al, which patent is incorporated herein by reference. As disclosed therein, the anode material of the battery is lithium and the reactive cathode material is silver vanadium oxide. The anode is constructed in a serpentine-like fashion with cathode plates inserted between each of the convolutions thereof on both sides thereof. The electrolyte for a lithium battery or cell is a liquid organic type which comprises a lithium salt and an organic solvent. Both the anode and the cathode plates are encapsulated in an electrically insulative separator material. However, a disadvantage of this battery design is that the serpentine anode is not efficiently used since anode material at the bends is not faced by cathode material and is therefore not fully utilized. An improvement which addresses this problem is disclosed in U.S. Pat. No. 5,147,737 to Post et al, in which the active material on the serpentine-type electrode is positioned so that the sections of the serpentine-like structure which do not face cathode plates do not contain anode active material. However, the serpentine bends of the anode are still present to the detriment of volumetric efficiency. Additional problems with these battery designs include the number of piece parts and connections required to make the battery which can affect both the manufacturability and the reliability of the battery; and the difficulty of achieving good current distribution and utilization of reactive material due to the unmatched configurations of the anode and cathode.
Conventional lithium batteries can also employ an electrode body in which anode and cathode elements are combined in spiral wound form. A strip sheet of lithium or lithium alloy comprises the anode, a cathode material supported on a charge collecting metal screen comprises the cathode, and a sheet of non-woven material separates the anode and cathode elements. These elements are combined and wound to form a spiral. Typically, the battery configuration for such a wound electrode would be cylindrical. For example, such configurations can be found in U.S. Pat. Nos. 3,373,060; 3,395,043; 3,734,778; 4,000,351; 4,184,012; 4,332,867; 4,333,994; 4,539,271; 4,550,064; 4,663,247; 4,668,320; 4,709,472; 4,863,815; 5,008,165; 5,017,442; and 5,053,297. Unlike the battery of the '940 patent, there need not be anode material which is not mated to cathode material. Such designs therefore have the potential for an improved match between the cathode and anode components and improved uniformity of anode and cathode utilization during discharge. However, cylindrical cells would not achieve the same space utilization inside the case of an implantable defibrillator as a prismatic cell shape.
It has also been known to adapt wound electrodes to a prismatic case configuration by departing from a true spiral winding. For example, U.S. Pat. No. 2,928,888 discloses in FIGS. 5
a
and 5
b
therein an oblong electrode assembly wound on an elongated mandrel for use in a rectangular case. Also, for example, U.S. Pat. No. 4,051,304 discloses in FIG. 2 therein another oblong wound electrode assembly for use in a rectangular case. However, these patents do not indicate that such structures could be advantageously used for a high current rate capability lithium battery or that they provide a uniform utilization of reactive anode and cathode material during discharge.
It is therefore an object of the present invention to provide a high current rate capability lithium battery having a coiled electrode suitable for use in a prismatic case.
It is also an object of the present invention to provide a high current rate capability lithium battery which provides uniform utilization of cathode and anode materials during discharge.
It is also an object of the present invention to provide a high current rate capability lithium battery for use in implantable cardiac defibrillators which employs a reduced number of piece parts and welds when compared with prior art devices.
SUMMARY OF THE INVENTION
These and other objects are accomplished by the electrochemical cell and electrode assembly of the present invention. We have discovered an electrode assembly for an electrochemical cell which includes an alkali metal anode and a cathode assembly which are wound together in a unidirectional winding having substantially straight sides such that the winding will fit into a prismatic cell. The anode includes an elongated strip of alkali metal having a first portion with a first uniform thickness of alkali metal and a second portion with a second, lesser uniform thickness of alkali metal and at least one connector tab in electrical contact with the alkali metal on an edge of the anode. The cathode assembly includes a cathode current collector having preferably two connector tabs spaced apart on an edge of the current collector and a cathode material bonded to the current collector at a uniform density of reactive material. The cathode assembly is shorter than the anode so that the anode and cathode can be wound by superimposing them with a separator material placed between them and then winding them by making an initial fold in the anode over the end of the cathode assembly and a mandrel to establish the length of the straight sides of the winding. The anode and cathode are then wound together unidirectionally until at the outermost portion of the winding the thin portion of the anode is the outer layer of the electrode assembly. The mandrel is then removed to allow both sides of the initial portion of the anode assembly to contact the cathode assembly. This electrode assembly allows for even utilization of reactive material by placing cathode and anode material in close proximity throughout the electrode assembly in the proportions in which they are utilized. It preferably also contributes to even utilization of reactive material by employing multiple tabs on the cathode assembly to ensure that cathode material is utilized throughout the electrode assembly. This is particularly important in the cathode since the current collector is being relied upon to provide conductivity throughout the cathode; conductivity that may not be inherently good in metals chosen largely for their corrosion-resistance. Preferably, the tabs are placed on the anode and cathode such that in the final winding the anode tabs are on one side of the winding while the cathode tabs are on the opposite side of the winding. This facilitates connection between the electrode and other battery components and avoids short circuits.
In this assembly, it is also preferred to incl
Berkcwitz Fred J.
Crespi Ann M.
Howard William G.
Kelm Roger W.
Skarstad Paul M.
Alejandro Raymond
Chaney Carol
Medtronic Inc.
Patton Harold R.
Wolde-Michael Girma
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