Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-03-12
2001-07-31
Dunn, Tom (Department: 1725)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218200, C429S206000, C429S223000, C029S623100
Reexamination Certificate
active
06268082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a production method of an active material for a positive electrode of an alkaline secondary battery, a positive electrode using the produced active material, and a production method of an alkaline secondary battery using the positive electrode, more specifically to a production method of an active material for a positive electrode having high utilization as an active material, unsusceptible to deterioration even in a long term storage, a positive electrode using the active material, and a production method of an alkaline secondary battery having an excellent high ratio discharge characteristic and capable of restraining the decline of the discharge capacity even at the time of recharging after leaving for a long time in an over discharge state.
2. Prior Art
Typical examples of alkaline secondary batteries include a nickel-hydrogen secondary battery and a nickel-cadmium secondary battery. In these batteries, a nickel electrode mainly comprising nickel hydroxide as the positive electrode active material is assembled as the positive electrode.
As the nickel electrode, two kinds, that is, a sintered type and a non-sintered type are known.
Among them, a non-sintered type nickel electrode is produced as follows.
A viscous paste mixture for positive electrode is prepared by kneading a nickel hydroxide powder which functions as an active material for a positive electrode and a binder such as carboxy methyl cellulose, methyl cellulose, sodium polyacrylate, and polytetrafluoroethylene with water. Then, the paste is filled or applied to a collector such as a three-dimensional substrate of a foamed nickel substrate, a net-like sintered substrate made of metal fibers or a non-woven fabric with the surface applied with nickel plating, and a two-dimensional substrate of a nickel punching sheet and an expand nickel, followed by a drying treatment and a press forming so that the above-mentioned paste mixture is filled and supported in the collector in the dry state.
Since a non-sintered type nickel electrode produced by above mentioned method has a higher filling density of a nickel hydroxide (active material) compared with a sintered type, it is advantageous in that a battery with a high discharge capacity can be provided.
With the above-mentioned nickel electrode as the positive electrode, an alkaline secondary battery is assembled as follows.
A generating element is produced by placing a separator having the electric insulating property and the liquid maintaining property between the above-mentioned nickel electrode and a predetermined negative electrode.
When the purposed alkaline secondary battery to be produced is a nickel-cadmium secondary battery, a negative electrode supporting a mixture for negative electrode having a cadmium compound such as a metal cadmium and a cadmium hydroxide as the negative electrode active material is used. When the purposed alkaline secondary battery to be produced is a nickel-hydrogen secondary battery, a negative electrode supporting a mixture for negative electrode mainly comprising a hydrogen absorbing alloy is used. As a separator, a non-woven fabric of a polyamide fiber or a non-woven fabric of a polyolefin fiber such as a polyethylene fiber and a polypropylene fiber, applied with a hydrophilic treatment can be used commonly.
The above-mentioned generating element is placed in a battery can with the bottom also serving as a negative terminal comprising a nickel plating steel plate, for example, and a predetermined amount of an alkaline electrolyte is filled therein. Examples of the alkaline electrolyte include, in general, an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous solution of lithium hydroxide, and an optional mixture thereof.
Then, after placing a positive electrode terminal at the opening of the battery can, the entirety is sealed so as to provide a battery.
The initial charging is conducted to the assembled battery so as to apply the activating treatment for the nickel hydroxide, which is the activate material, prior to the shipment. In general, the initial charging is conducted under the condition where quantity of electricity more than 100% of the theoretical capacity of the assembled nickel electrode can be charged.
In the above-mentioned nickel electrode, it is important to improve the conductivity within the active material (nickel hydroxide), and between the active material and the collector for improving the utilization of the active material.
In order to achieve the task, the following treatment has been adopted conventionally.
In preparing a paste mixture for a positive electrode, a predetermined amount of particles of a metal cobalt, a cobalt compound such as cobalt hydroxide, cobalt trioxide, cobalt tetroxide, and cobalt monoxide, or a mixture thereof are added as a conducting material so as to produce a powdery material mixed with the nickel hydroxide particles by a predetermined ratio to be used as the active material.
If a nickel electrode supporting the active material produced by above mentioned method is assembled in an alkaline secondary battery as the positive electrode, the metal cobalt or the cobalt compound contained in the above-mentioned powdery material is dissolved temporarily in the alkaline electrolysis solution as complex ions, and is distributed on the surface of the nickel hydroxide particles, which are the active material. At the time of the initial charging of the battery, the complex ions are oxidized earlier than the nickel hydroxide so as to be converted to an oxide of higher order such as oxycobalt hydroxide. It is precipitated among the nickel hydroxide particles, which are the active material, and between the active material layer and the collector so as to form a conductive matrix. Therefore, the collecting efficiency at the nickel electrode can be improved, and consequently, the utilization of the active material can be improved. In that case, with a larger number of contacting points between the above-mentioned conductive matrix and the nickel hydroxide particles, the utilization of the nickel hydroxide particles (active material) can further be improved.
Moreover, a method of treating the powdery material produced as mentioned above with a heat alkaline aqueous solution is also known.
Specifically, the above-mentioned powdery material is soaked in an alkaline aqueous solution so that the alkaline aqueous solution is applied or impregnated to the powdery material. Then, the entirety is filtrated so that the substance obtained by the filtration is heated at a predetermined temperature. According to the method, a part of the metal cobalt or the cobalt compound contained in the powdery material is dissolved in the heat alkaline aqueous solution as complex ions so that they homogeneously cover the surface of the particles mainly comprising the nickel hydroxide. Then, it is changed into an active material having factors of forming the above-mentioned conductive matrix.
However, in this method of treating the above-mentioned powdery material with the heat alkaline aqueous solution, the cobalt complex ions temporarily dissolved in the heat alkaline aqueous solution may be precipitated again in the cooling process of the heat alkaline aqueous solution so as to form particle aggregates by bonding adjacent particles with each other. Therefore, since the obtained treated substance is an assembly of the above-mentioned particle aggregates, if paste mixture for a positive electrode is prepared with the treated substance, the distribution state of the nickel hydroxide particles, which are the positive electrode active material in the paste, becomes uneven, and thus it is problematic in that the effective use as the active material cannot be realized.
As another method, for example, a method of using a powdery material prepared by introducing particle mainly comprising nickel hydroxide into an alkaline aqueous solution controlled to be in the range of pH 11 to 13 and gradually adding
Bando Naomi
Hiruma Masayoshi
Miyamoto Kunihiko
Wakabayashi Makoto
Dunn Tom
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pittman Zidia T.
Toshiba Battery Co., Ltd.
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