Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2002-03-26
2004-08-10
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S061000, C429S071000, C429S082000, C429S093000
Reexamination Certificate
active
06773842
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a battery with a long operating life. In particular, this invention relates to a multiple-cell battery with the constituent metal-air cells being activated in a programmed-timing manner to achieve an extended operating life and better utilization of the capacity of individual cells.
2. Brief Description of the Prior Art
Metal-air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having one surface exposed to the atmosphere and another surface exposed to the aqueous electrolyte of the cell. During cell operation oxygen is reduced within the cathode while anode metal is oxidized, providing a usable electric current flow through an external circuit connected between the anode and the cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuit can be connected. Commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, zinc, lithium, aluminum, magnesium and alloys of these elements are considered especially advantageous owing to their low cost, light weight, and ability to function as anodes in metal-air batteries using a variety of electrolytes.
As an example, a typical aluminum-air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. Aqueous electrolytes for metal-air batteries consist of two basic types, namely a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
In neutral-pH electrolyte, the cell discharge reaction may be written as:
4Al+3O
2
+6H
2
O→4Al(OH)
3
(solid)
In alkaline electrolyte, the cell discharge reaction may be written:
4Al+3O
2
+6H
2
O+4KOH→4Al(OH)
4
−
+4 K
+
(liquid solution),
followed, after the dissolved potassium (or sodium) aluminate exceeds a saturation level, by:
4Al(OH)
4
−
+4K
+
→4Al(OH)
3
(solid)+4KOH
In addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows:
2Al+6H
2
O→2Al(OH)
3
+3H
2
(gas).
Th above equations and similar equations for other types of metal-air cells indicate the importance of regulating the ingress rate of oxygen. Once oxygen is admitted into a metal-air cell, discharge reactions will proceed regardless if the cell is being used or not to power an external device.
There is a need for a metal-air battery which can be used as an emergency power source at locations where electric supply lines do not exist. Such a battery must have a high energy capacity and a high power density and be capable of running for a long period of time under high load. There is also a need for a metal-air battery that can provide much extended “talk time” and “stand-by” time for a mobile phone. A need also exists for a battery that can power a notebook computer for a much longer period of time (e.g., 12 hours being needed to last for a trans-Pacific flight).
State-of-the-art metal-air batteries have exhibited the following drawbacks:
(1) Severe “anode passivation” problem: When the battery is run under high load, large amounts of aluminum hydroxide accumulate on the aluminum anode surface blocking the further access of anode by the electrolyte. In the case of zinc-air cells, zinc oxide layers prevent further access of zinc anode by the electrolyte. Such an anode passivation phenomenon tends to prevent the remaining anode active material (coated or surrounded by a ceramic layer) from contacting the electrolyte. Consequently, the electron-generating function ceases and the remaining active anode material can no longer be used (hence, a low-utilization anode). All metal anodes used in state-of-the-art metal-air batteries suffer from the anode passivation problem to varying degrees.
(2) Severe self-discharge and current leakage problems: “Self-discharge” is due to a chemical reaction within a battery that does not provide a usable electric current. Self-discharge diminishes the capacity of a battery for providing a usable electric current. For the case of a metal-air battery, self-discharge occurs, for example, when a metal-air cell dries out and the metal anode is oxidized by the oxygen that seeps into the battery during periods of non-use. Leakage current can be characterized as the electric current that is supplied to a closed circuit by a metal-air cell even when air is not continuously provided to the cell. These problems also result in a low-utilization anode.
(3) Severe corrosion problem: Four metals have been studied extensively for use in metal-air battery systems: zinc (Zn), aluminum (Al), magnesium (Mg), and lithium (Li). Despite the fact that metals such as Al, Mg, and Li have a much higher energy density than zinc, the three metals (Al, Mg, and Li) suffer from severe corrosion problems during storage. Hence, Mg-air and Al-air cells are generally operated either as “reserve” batteries in which the electrolyte solution is added to the cell only when it is decided to begin the discharge, or as “mechanically rechargeable” batteries which have replacement anode units available. The presence of oxygen tends to aggravate the corrosion problem. Since the serious corrosion problem of Zn can be more readily inhibited, Zn-air batteries have been the only commercially viable metal-air systems. It is a great pity that high energy density metals like Al, Mg and Li have not been extensively used in a primary or secondary cell.
Due to their high energy-to-weight ratio, safety of use, and other advantages, metal-air, and particularly zinc-air, batteries have been proposed as a preferred energy source for use in electrically-powered vehicles. However, just like aluminum-air cells, zinc-air batteries also suffer from the problem of “passivation”, in this case, by the formation of a zinc oxide layer that prevents the remaining anode active material (Zn) from contacting the electrolyte.
A number of techniques have been proposed to prevent degradation of battery performance caused by zinc oxide passivation or to somehow extend the operating life of a metal-air battery. In one technique, a sufficient (usually excessive) amount of electrolyte was added to allow most of the zinc to dissolve (to become Zn ion and thereby giving up the desired electrons). The large amount of electrolyte added significantly increased the total weight of the battery system and, thereby, compromising the energy density.
In a second approach, anodes are made by compacting powdered zinc onto brass current collectors to form a porous mass with a high surface/volume ratio. In this configuration, the oxide does not significantly block further oxidation of the zinc, provided that the zinc particles are sufficiently small. With excessively small zinc particles, however, zinc is rapidly consumed due to self-discharge and leakage (regardless if the battery is in use or not) and even more serious corrosion problems and, hence, the battery will not last long.
In a third approach, part
Huang Wayne
Liu Jean
Nanotek Instruments, Inc.
Parsons Thomas H.
Ryan Patrick
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