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Our Technologies: Innovative Energy Storage and Conversion Technologies (Fuel Cells, Solar Cells, and Batteries)

(B) Smart Metal-Air Batteries

1.                  W. C. Huang, J. Liu and L. W. Wu, ¡§Battery with a Controlled Release Anode,¡¨ U.S. Patent No. 6,864,018 (03/08/2005).

2.                  J. Liu and W. C. Huang, ¡§Metal-Air Battery with an Extended Service Life,¡¨ U.S. Pat. No. 6,773,842 (08/10/2004).

3.                  W. C. Huang, ¡§Metal-Air Battery with Programmed-Timing Activation,¡¨ U.S. Patent Pending (10/431,661) 05/09/2003.

4.                  Kevin L. Jang and W. C. Huang, ¡§Actively Controlled Electrochemical Cell,¡¨ U.S. patent pending (10/702,003) 11/6/2003.

            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.

            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. However, none of them have been very effective.

            We have developed a metal-air battery (control unit shown in Fig.5) that overcomes the aforementioned shortcomings. This battery comprises a control means and a plurality of metal-air cell assemblies that are electronically connected in parallel. Each cell assembly comprises a casing with a controllable air vent thereon and at least a metal-air cell inside the casing. The controllable air vent is closed during a battery storage period and is opened in response to a programmed signal in order to allow outside air or oxygen to enter the assembly through the air vent to activate the operation of the corresponding cell assembly. The control means, preferably including a sampling unit, a power control unit and a logic control unit, is capable of sensing the battery output voltage and sending programmed signals to open or close up the air vents at the same time or at different moments of time in a programmed fashion.