Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-08-17
2001-01-23
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S223000, C429S220000, C429S231300, C429S231800
Reexamination Certificate
active
06177213
ABSTRACT:
FIELD OF THE INVENTION
The instant invention relates generally to positive electrode materials for rechargeable batteries such as nickel hydroxide materials. More specifically, the instant invention relates to composite nickel hydroxide particulate having increased conductivity over the prior art material.
BACKGROUND OF THE INVENTION
In rechargeable alkaline cells, weight and portability are important considerations. It is also advantageous for rechargeable alkaline cells to have long operating lives without the necessity of periodic maintenance. Rechargeable alkaline cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable alkaline cells can also be configured as larger cells that can be used, for example, in industrial, aerospace, and electric vehicle applications.
There are many known types of Ni based cells such as nickel cadmium (“NiCd”), nickel metal hydride (“Ni-MH”), nickel hydrogen, nickel zinc, and nickel iron cells. NiCd rechargeable alkaline cells are the most widely used although it appears that they will be replaced by Ni-MH cells. Compared to NiCd cells, Ni-MH cells made of synthetically engineered materials have superior performance parameters and contain no toxic elements.
Ni-MH cells utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte. Upon application of an electrical potential across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, as shown in equation (1):
M
+
H
2
O
+
e
-
⟷
discharge
charge
M—H
+
OH
-
(
1
)
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron. The reactions that take place at the nickel hydroxide positive electrode of a Ni-MH cell are shown in equation (2):
Ni
(
OH
)
2
+
OH
-
⟷
discharge
charge
NiOOH
+
H
2
O
+
e
-
(
2
)
Ni-MH materials are discussed in detail in U.S. Pat. No. 5,277,999 to Ovshinsky, et al., the contents of which are incorporated by reference.
In alkaline rechargeable cells, the discharge capacity of a nickel based positive electrode is limited by the amount of active material, and the charging efficiencies. The charge capacities of a Cd negative electrode and a MH negative electrode are both provided in excess, to maintain the optimum capacity and provide overcharge protection. Thus, a goal in making the nickel positive electrode is to obtain as high an energy density as possible. The volume of a nickel hydroxide positive electrode is sometimes more important than weight. The volumetric capacity density is usually measured in mAh/cc and specific capacity is written as mAh/g.
At present, sintered or pasted nickel hydroxide positive electrodes are used in NiCd and Ni-MH cells. The process of making sintered electrodes is well known in the art. Conventional sintered electrodes normally have an energy density of around 480-500 mAh/cc. In order to achieve significantly higher capacity, the current trend has been away from sintered positive electrodes and toward foamed and pasted electrodes.
Sintered nickel electrodes have been the dominant nickel electrode technology for several decades for most applications. These consist of a porous nickel plaque of sintered high surface area nickel particles impregnated with nickel hydroxide active material either by chemical or electrochemical methods. While expensive, sintered electrodes provide high power, high reliability, and high cycle life, but not the highest energy density. They are likely to remain important for high reliability military and aerospace applications for some time.
Pasted nickel electrodes consist of nickel hydroxide particles in contact with a conductive network or substrate, preferably having a high surface area. There have been several variants of these electrodes including the so-called plastic-bonded nickel electrodes which utilize graphite as a microconductor and also including the so-called foam-metal electrodes which utilize high porosity nickel foam as a substrate loaded with spherical nickel hydroxide particles and cobalt conductivity enhancing additives. Pasted electrodes of the foam-metal type now dominate the consumer market due to their low cost, simple manufacturing, and higher energy density relative to sintered nickel electrodes.
Conventionally, the nickel battery electrode reaction has been considered to be a one electron process involving oxidation of divalent nickel hydroxide to trivalent nickel oxyhydroxide on charge and subsequent discharge of trivalent nickel oxyhydroxide to divalent nickel hydroxide, as shown in equation 2 hereinbelow.
Some recent evidence suggests that quadrivalent nickel is involved in the nickel hydroxide redox reaction. This is not a new concept. In fact, the existence of quadrivalent nickel was first proposed by Thomas Edison in some of his early battery patents. However, full utilization of quadrivalent nickel has never been investigated.
In practice, electrode capacity beyond the one-electron transfer theoretical capacity is not usually observed. One reason for this is incomplete utilization of the active material due to isolation of oxidized material. Because reduced nickel hydroxide material has a high resistance, the reduction of nickel hydroxide adjacent the current collector forms a less conductive surface that interferes with the subsequent reduction of oxidized active material that is farther away.
As discussed in U.S. Pat. No. 5,348,822, nickel hydroxide positive electrode material in its most basic form has a maximum theoretical specific capacity of 289 mAh/g, when one charge/discharge cycles from a &bgr;II phase to a &bgr;III phase and results in one electron transferred per nickel atom. It was recognized in the prior art that greater than one electron transfer could be realized by deviating from the &bgr;II and &bgr;III limitations and cycling between a highly oxidized &ggr;-phase nickel hydroxide phase and the &bgr;II phase. However, it was also widely recognized that such gamma phase nickel hydroxide formation destroyed reversible structural stability and therefore cycle life was unacceptably degraded. A large number of patents and technical literature disclosed modifications to nickel hydroxide material designed to inhibit and/or prevent the destructive formation of the transition to the &ggr;-phase, even though the higher attainable capacity through the use of &ggr;-phase is lost.
Attempts to improve nickel hydroxide positive electrode materials began with the addition of modifiers to compensate for what was perceived as the inherent problems of the material. The use of compositions such as NiCoCd, NiCoZn, NiCoMg, and their analogues are described, for example, in the following patents:
U.S. Pat. No. Re. 34,752, to Oshitani, et al., reissued Oct. 4, 1994, describes a nickel hydroxide active material that contains nickel hydroxide containing 1-10 wt % zinc or 1-3 wt % magnesium to suppress the production of gamma-NiOOH. The invention is directed toward increasing utilization and discharge capacity of the positive electrode. Percent utilization and percent discharge capacity are discussed in the presence of various additives.
Oshitani, et al. describe the lengths that routineers in the art thought it was necessary to go to in order to inhibit &ggr;-NiOOH. The patent states:
Further, since the current density increased in accordance with the reduction of the specific surface area, a large amount of higher oxide &ggr;-Ni
Fetcenko Michael A.
Fierro Christian
Mays William
Ovshinsky Stanford R.
Reichman Benjamin
Energy Conversion Devices Inc.
Schlazer Philip H.
Weiner Laura
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