Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-02-25
2002-06-04
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218200, C429S248000, C429S344000, C429S207000, C252S062200, C252S182100
Reexamination Certificate
active
06399247
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nickel-metal hydride secondary battery. More particularly, the present invention is concerned with a nickel-metal hydride secondary battery which is advantageous not only in that it exhibits high charging efficiency in a high temperature environment and high capacity maintaining rate due to the excellent self-discharge characteristics thereof in a high temperature storage, high utilization of the active material for positive electrode, and further it can exhibit a large current discharge even at the initial stage of the discharge, but also in that it has excellent low-temperature charging characteristics.
2. Prior Art
In recent years, various electronic appliances, such as pocketable telephones and hand-held personal computers, have been developed in respect of being cordless, multifunctionality and compactness as well as weight reduction. In accordance with these developments, the demand for high capacity of a second battery which is a power source of the electronic appliance has greatly increased.
Conventionally, as the power sources of these electronic appliances, nickel-cadmium secondary batteries have been mainly used. However, in accordance with the above-mentioned great demand for high capacity, recently, a nickel-metal hydride secondary battery is beginning to be widely used because the nickel-metal hydride secondary battery is interchangeable with the voltage of the nickel-cadmium secondary battery and has a higher capacity than that of the nickel-cadmium secondary battery.
This nickel-metal hydride secondary battery generally comprises an electrode group comprising: a positive electrode comprising a current collector having carried thereon a mixture paste containing, as a main component, a nickel hydroxide powder which is an active material and a binder such as carboxymethyl cellulose; a negative electrode comprising a current collector having carried thereon a paste containing, as a main component, a powder of a hydrogen storage alloy and a binder such as carboxymethyl cellulose or polytetrafluoroethylene; and a separator comprising, for example, a polyamide fiber nonwoven fabric, having electrical insulating properties and a liquid retaining property, which separator is disposed between the positive electrode and the negative electrode, and has a structure such that this electrode group is accommodated in a battery casing which also serves as a negative electrode terminal, together with an alkali electrolyte liquid which is generally comprised mainly of an aqueous potassium hydroxide solution, and then, the battery casing is sealed up so that the electrode group and the alkali electrolyte liquid are sealed in the casing.
Then, an initial charge is conducted with respect to the assembled battery, so that nickel hydroxide as an active material for positive electrode is treated for activation. That is, this is a treatment such that nickel hydroxide which itself has no conductivity is converted into &bgr;-nickel oxyhydroxide being of trivalent and having a conductivity by subjecting to an initial charge for oxidation, making it possible to exhibit a function as an active material.
By the way, this nickel-metal hydride secondary battery is operated utilizing the characteristics of the hydrogen storage alloy such that it absorbs and desorbs hydrogen electrochemically and reversibly.
However, when this nickel-metal hydride secondary battery in a charging state is allowed to stand or stored in a high temperature environment, generally, the equilibrium pressure of the hydrogen storage alloy in the negative electrode increases, and thus, the amount of hydrogen which can be occluded in the negative electrode is reduced. Therefore, the hydrogen which cannot be stored any more in the negative electrode is emitted to the inside of the battery, so that the hydrogen partial pressure in the battery is increased. Then, this hydrogen passes through the separator to the positive electrode, and at the positive electrode, promotes the reduction of the nickel oxyhydroxide present in the positive electrode in a state of &bgr;-nickel oxyhydroxide which is an oxide product from nickel hydroxide by the above-mentioned initial charge. That is, the self-discharge of the nickel oxyhydroxide is promoted, and as a result, lowering of the discharge capacity occurs.
For solving such a problem, there has been proposed a method in which a vinyl monomer having a carboxyl group is subjected to graft polymerization on the surface of the separator comprising a fiber of a polyolefin resin (see Japanese Unexamined Patent Publication No. Hei 10-69898). By this method, the separator is provided with high hydrophilicity. Therefore, on the surface containing the spaces between the fiber constituting the separator, a film of an alkali electrolyte liquid is formed, and thus, this film prevents a diffusion of hydrogen into the positive electrode even when the hydrogen partial pressure within the battery is increased. As a result, the above-mentioned self-discharge of the positive electrode is suppressed.
However, recently, the use of the nickel-metal hydride secondary battery has been expanded in the application fields, such as an electric power tool, an electric vehicle and an electric power-assist bicycle, as a power source which requires a large current discharge. In view of the strong demand for the improvement of the self-discharge characteristics in the above application field, the method proposed in the above-mentioned prior art document has a problem in that a satisfactory improvement of the self-discharge characteristics cannot be achieved.
On the other hand, from the viewpoint of the charging problem of the nickel-metal hydride secondary battery, when the environment at the charging is a normal temperatures environment, the overpotential at the charging reaction of nickel hydroxide is larger than that required for the oxygen generation reaction from the alkali electrolyte liquid. Therefore, the charging reaction of nickel hydroxide first proceeds, and after almost completion of the charging reaction, the reaction is transferred to the oxygen generation reaction. Accordingly, in a normal temperatures environment, it is possible to advance the charging of the positive electrode surely and satisfactorily.
However, when the charging is conducted in a high temperature environment, the overpotential at the oxygen generation reaction is lowered. Therefore, the difference between the overpotential at the oxygen generation reaction and that at the charging reaction of nickel hydroxide becomes small. For this reason, from the relatively early stage of the charging, the competitive relationship between the charging reaction and the oxygen generation reaction occurs, causing a disadvantage that the charging reaction of nickel hydroxide does not satisfactorily proceed. That is, in a high temperature environment, problems arise in that the charging efficiency is lowered, and as a result, the discharge capacity of the battery is lowered.
For solving such problems, there have been proposed a method using, as an active material, nickel hydroxide having a cobalt component coprecipitated therein during the synthesis of the nickel hydroxide for lowering the equilibrium potential of the positive electrode (see Japanese Unexamined Patent Publication No. Sho 50-132441), and a method using nickel hydroxide having cadmium or the like coprecipitated therein for increasing the oxygen overpotential of the positive electrode (see Japanese Unexamined Patent Publication No. Sho 62-108458).
However, the positive electrodes produced by these methods do not exhibit a satisfactory level of charging efficiency in a high temperature environment.
In addition, in Japanese Unexamined Patent Publication No. Hei 5-28992, a positive electrode produced by adding to nickel hydroxide a compound of Y, In, Sb, Ba, Ca, Be or the like for increasing the oxygen overpotential is disclosed. However, in this positive electrode, due to the addition of the above-mention
Bando Naomi
Hayashida Hirotaka
Kitayama Hiroshi
Miyamoto Kunihiko
Suzuki Hideharu
Alejandro Raymond
Kalafut Stephen
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Toshiba Battery Co., Ltd.
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