Patent Description:
After the Great East Japan Earthquake, there have been increasing cases of storing alkaline batteries over a long period of time for the purpose of disaster prevention usage and similar usage in our country. Thus, a demand for improving leakage resistance of the alkaline battery is increasing. Meanwhile, in association with improved performances and downsizing of electronic devices, such as digital cameras, video cameras, mobile phones, and smart phones recently, there is increasing demand for the alkaline battery to improve heavy load discharge performance.

As known art relating to the leakage resistance improvement and the heavy load discharge performance improvement of the alkaline battery, for example, Patent Literature <NUM> discloses a steel plate for forming a battery can in order to ensure the leakage resistance performance and the heavy load discharge performance. The steel plate for forming the battery can has a structure having an Fe-Ni alloy plated layer or an Fe-Ni diffusion alloy layer, a recrystallized Ni layer, and an Fe-Ni diffusion alloy layer. The Fe-Ni alloy plated layer or the Fe-Ni diffusion alloy layer has an outermost layer Fe concentration on a surface that serves as an inner surface of the can in a range of equal to or more than <NUM> atomic% and equal to or less than <NUM> atomic%. The recrystallized Ni layer is formed below the Fe-Ni alloy plated layer or the Fe-Ni diffusion alloy layer and has a thickness of equal to or more than <NUM>. The Fe-Ni diffusion alloy layer is formed below the recrystallizedNi layer. Patent Literature <NUM> discloses that, as a surface treated steel plate for an alkaline primary battery casing, one that has a surface roughness Ra in a predetermined range is used in order to reduce a contact resistance between an inner surface of the battery casing and a positive electrode mixture filled in the battery casing.

As a material of a battery can, a steel plate (such as Nickel Plated Steel (NPS)) that generally includes a corrosion resistant plated layer, such as nickel plate, on a surface is used in order to ensure good leakage resistance. Here, the steel plate is composed principally of iron. However, the iron reacts with, for example, nickel, manganese dioxide, and oxygen, and dissolves. Therefore, a method, for example, of thickening the plated layer (such as a nickel plated layer) provided on the inner surface of the battery can and decreasing an average diameter of crystal grains of the steel plate, is effective to improve the leakage resistance of the alkaline battery.

On the other hand, in order to improve the heavy load discharge performance of the alkaline battery, it is effective to make the inner surface of the battery can rough (increase the average diameter of the crystal grains of the steel plate) to expose the iron and reduce the contact resistance with a power generating element (such as an electrode active material) housed in the battery can. However, increasing an exposure amount of the iron lowers the leakage resistance.

Thus, it has not always been easy to achieve both the leakage resistance improvement and the heavy load discharge performance improvement of the alkaline battery.

The present disclosure describes an embodiment of an alkaline battery having superior leakage resistance and heavy load discharge performance.

This specification describes a method for producing an improved steel plate for forming a battery can, as set forth in claim <NUM>. The specification also describes an alkaline battery as set forth in claim <NUM>, and a manufacturing method of an alkaline battery as set forth in claim <NUM>.

Problems disclosed by this application and a method to solve the problems will be apparent from Description of Embodiments and the drawings.

According to the present invention, an alkaline battery excellent in a leakage resistance and a heavy load discharge performance can be achieved.

This patent application claims priority to <CIT> in the Japan Patent Office.

<FIG> illustrates a configuration (hereinafter referred to as an alkaline battery <NUM>) of a common cylindrical alkaline battery (LR6 type (AA size) alkaline battery) that is an application target of the present disclosure. It should be noted that in <FIG>, the alkaline battery <NUM> is illustrated as a vertical cross-sectional view (a cross-sectional view when an extension direction of a cylinder axis of the alkaline battery <NUM> is an up-down (vertical) direction).

As illustrated in <FIG>, the alkaline battery <NUM> includes a battery can, a separator <NUM>, a negative electrode mixture <NUM>, a negative electrode terminal plate <NUM>, and a negative electrode current collector <NUM>. The battery can (hereinafter referred to as a positive electrode can <NUM>) is made of metal and is in the shape of a cylinder with a closed bottom. The separator <NUM> is in the shape of a circular cylinder with a closed bottom and is disposed in an inner peripheral side of a positive electrode mixture <NUM> (constituted of three pellets 21a to 21c in hollow cylindrical shapes) that is inserted into the positive electrode can <NUM>. The negative electrode mixture <NUM> fills in an inner peripheral side of the separator <NUM>. The negative electrode terminal plate <NUM> is fitted to an opening of the positive electrode can <NUM> via a sealing gasket <NUM> made of resin. The negative electrode current collector <NUM> is in the shape of a rod and made of a material, such as brass. The negative electrode current collector <NUM> is fixedly secured to the inside of the negative electrode terminal plate <NUM> by, for example, spot welding. The positive electrode mixture <NUM>, the separator <NUM>, and the negative electrode mixture <NUM> constitute a power generating element <NUM> of the alkaline battery <NUM>.

The positive electrode can <NUM> has a conductive property. The positive electrode can <NUM> is formed, for example, by pressing a metal material, such as a nickel plated steel plate. The positive electrode can <NUM> doubles as the positive electrode current collector and the positive electrode terminal. The positive electrode can <NUM> has a bottom portion at which a convex shaped positive electrode terminal portion <NUM> is integrally formed.

The three pellets 21a to 21c, which constitute the positive electrode mixture <NUM>, have identical shapes and sizes. Components of these are common. The components include, for example: electrolytic manganese dioxide (EMD) as a positive-electrode active material; graphite as a conductive material; polyacrylic acid as a binder; an electrolyte containing mainly of potassium hydroxide (KOH); and a surfactant (for example, an anionic surfactant). It should be noted that in this embodiment, as the three pellets 21a to 21c, a product produced by the following method is used. The electrolytic manganese dioxide (EMD), the graphite, and the polyacrylic acid are mixed (dry blending) to obtain the mixture. The electrolyte (KOH solution of <NUM> mass%) containing mainly of the potassium hydroxide (KOH) and the surfactant (liquid) are mixed (wet blending) to the obtained mixture. Furthermore, after the mixture is processed through processes, such as rolling, disintegration, granulation, and classification, the mixture is compressed and shaped into a ring-shape.

As illustrated in <FIG>, in the positive electrode can <NUM>, the three pellets 21a, 21b, and 21c are laminated in a vertical direction and press-fitted such that cylinder axes of the three pellets 21a, 21b, and 21c are coaxial with a cylinder axis of the positive electrode can <NUM>. At least a part of outer peripheral surfaces of the three pellets 21a, 21b, and 21c are in contact with the positive electrode can <NUM>.

The negative electrode mixture <NUM> is zinc alloy powders as a negative electrode active material that has been gelatinized. The zinc alloy powders are made by a gas atomization method or a centrifugal spray method. The zinc alloy powders include zinc, alloy components (such as bismuth, aluminum, and indium) added for the purpose of suppressing a generation of gas (leakage prevention) or similar purpose, and potassium hydroxide (KOH) as an electrolyte. The negative electrode current collector <NUM> penetrates a center of the negative electrode mixture <NUM>.

According to this embodiment, the alkaline battery <NUM>, which has excellent leakage resistance and heavy load discharge, includes a steel plate for forming a battery can (the positive electrode can <NUM>) having the following configuration.

<FIG> illustrates a layer structure of a steel plate (hereinafter referred to as a battery can forming steel plate <NUM>) used for the alkaline battery <NUM> (a partial cross-sectional view of the battery can forming steel plate <NUM>). In <FIG>, a lower side of the paper corresponds to an outer peripheral surface side of the positive electrode can <NUM> and an upper side on the paper corresponds to an inner peripheral surface side of the battery can <NUM>.

As illustrated in <FIG>, the battery can forming steel plate <NUM> has a structure that laminates an outer peripheral side plated layer <NUM>, an outer peripheral side diffusion layer 101a, a steel plate (hereinafter referred to as a base material steel plate <NUM>) that serves as a base material of the battery can forming steel plate <NUM>, an inner peripheral side diffusion layer 103a, and an inner peripheral side plated layer <NUM> in this order from the lower side toward the upper side on the paper.

The base material steel plate <NUM> is, for example, a cold-rolled steel plate (such as, low carbon aluminum killed steel, ultra-low carbon steel, and non-aging ultra-low carbon steel) excellent for presswork (such as deep drawn presswork). A layer thickness of the base material steel plate <NUM> is, for example, approximately <NUM> to <NUM>. The base material steel plate <NUM> is manufactured by, for example, the following method. After a hot-rolled plate is pickled and an oxide film (scale) is removed, cold-rolling is performed and the rolling oil is electrolytically cleaned. Afterwards, annealing (continuous annealing and box annealing) and temper rolling are performed.

As illustrated in <FIG>, the outer peripheral side diffusion layer 101a and the inner peripheral side diffusion layer 103a in the battery can forming steel plate <NUM> are, as illustrated in <FIG>, both formed by performing the heat diffusion treatment after the plated layers (the outer peripheral side plated layer <NUM> and the inner peripheral side plated layer <NUM>) are formed on surfaces of the base material steel plate <NUM> (an iron and nickel diffusion layer or an iron and nickel-cobalt alloy diffusion layer is formed). It should be noted that the above-described plated layer (the outer peripheral side plated layer <NUM> and the inner peripheral side plated layer <NUM>) are formed by, for example, using a plating bath (a nickel alloy plating bath or a nickel-cobalt alloy plating bath). The above-described heat diffusion treatment is performed, for example, by the continuous annealing method and the box annealing method under a predetermined condition (heat treatment temperature, heat treatment processing time).

It should be noted that the diffusion layer (heat diffusion layer) is only needed to be formed at least on the inner peripheral surface side of the positive electrode can <NUM> where the battery element <NUM> (the positive electrode mixture <NUM>) contacts the positive electrode can <NUM>. The outer peripheral surface side of the positive electrode can <NUM> does not necessarily have a configuration similar to the inner peripheral surface side. For example, the outer peripheral surface side of the positive electrode can <NUM> may include a nickel plated layer instead of the diffusion layer.

For the battery can forming steel plate <NUM> having the above-mentioned configuration, in order to find one that ensures both the leakage resistance improvement and the heavy load discharge performance improvement of the alkaline battery <NUM> at the same time, the inventors manufactured a plurality of kinds of the battery can forming steel plates <NUM> that have varied average numbers of the crystal grains of the base material steel plates <NUM> (crystal grain diameters of the base material steel plates <NUM>) and varied thicknesses of the inner peripheral side plated layers <NUM> before the heat diffusion treatment. The inventors then manufactured a plurality of samples of the alkaline battery <NUM> using the positive electrode cans <NUM> manufactured using the respective battery can forming steel plates <NUM>, and examined the leakage resistance and the heavy load discharge performance for the respective samples.

It should be noted that every sample described above included the base material steel plate <NUM> of the positive electrode can <NUM> with a thickness of <NUM>. Every sample described above included the outer peripheral side plated layer <NUM> before the heat diffusion treatment with a thickness of <NUM>. Every sample described above included the outer peripheral side plated layer <NUM> whose composition was common to a composition (nickel or nickel-cobalt alloy) of the inner peripheral side plated layer <NUM> on the inner peripheral surface.

First, each of the manufactured samples was stored for <NUM> days under drying at <NUM>, and leakage resistance characteristics were examined for the respective samples. The test results of when the inner peripheral side diffusion layers 103a were the iron and nickel diffusion layers (when the inner peripheral side plated layers <NUM> before the heat diffusion treatment (<FIG>) were the nickel plated layers) are shown in Table <NUM>. The test results of when the inner peripheral side diffusion layers 103a were the iron and nickel-cobalt alloy diffusion layers (when the inner peripheral side plated layers <NUM> before the heat diffusion treatment (<FIG>) were the nickel-cobalt alloy plated layers) are shown in Table <NUM>.

In Table <NUM> and Table <NUM>, "Average Number of Crystal Grains" is an average number of the crystal grains per unit area (area of <NUM><NUM> (square shape)) on a section plane of the base material steel plate <NUM>. In Table <NUM> and Table <NUM>, "Good" is given when leakage was not visually confirmed and "Poor" is given when the leakage was visually confirmed.

From Table <NUM> and Table <NUM>, in both cases where the inner peripheral side diffusion layers 103a of the battery can forming steel plates <NUM> were the iron and nickel diffusion layer and the iron and nickel-cobalt alloy diffusion layer, the leakage was not confirmed when the average number of the crystal grains of the base material steel plate <NUM> was in a range of <NUM> to <NUM> and the thickness of the inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) was in a range of <NUM> to <NUM>.

On the other hand, leakage was confirmed when the average number of the crystal grains of the base material steel plate <NUM> was in a range of <NUM> to <NUM>. It is conceivable that this leakage occurs because an exposure amount of the iron on the surface of the battery can forming steel plate <NUM> increases when the average number of the crystal grains of the base material steel plate <NUM> decreases (the average grain diameter of the crystal grains increases).

Next, discharge characteristics were examined for the respective samples. The discharge characteristics were examined by performing a cycle discharge test assuming the heavy load discharge, for example, during the use of a digital camera (a cycle of discharge for two seconds at <NUM> mW and discharge for <NUM> seconds at <NUM> mW was performed ten times per hour (an idle period per hour was <NUM> minutes)) under a room temperature (<NUM>). Then, the number of cycles until reaching a cutoff voltage (<NUM> V) was counted.

The results of when the inner peripheral side diffusion layers 103a of the battery can forming steel plates <NUM> were the iron and nickel diffusion layers (when the inner peripheral side plated layers <NUM> before the heat diffusion treatment (<FIG>) were the nickel plated layers) are shown in Table <NUM>. The results of when the inner peripheral side diffusion layers 103a were the iron and nickel-cobalt alloy diffusion layers (when the inner peripheral side plated layers <NUM> before the heat diffusion treatment (<FIG>) were the nickel-cobalt alloy plated layers) are shown in Table <NUM>. It should be noted that in Table <NUM> and Table <NUM>, "Average Number of Crystal Grains" is an average number of crystal grains per unit area (area of <NUM><NUM> (square shape)) on a section plane of the base material steel plate <NUM>. In Table <NUM> and Table <NUM>, a criterion (<NUM>%) is set to the discharge performance (the discharge performance of a conventional and general alkaline battery) of when the average number of the crystal grains of the base material steel plate <NUM> was <NUM> and the thickness of the inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) was <NUM> (thick frame portions in Table <NUM> and Table <NUM>). "Good" is given when the discharge performance is <NUM> ± <NUM>%, "Excellent" is given when the discharge performance exceeds the upper limit of the above-described range, and "Poor" is given when the discharge performance is less than the lower limit of the above-described range.

From Table <NUM> and Table <NUM>, in both cases where the inner peripheral side diffusion layers 103a of the battery can forming steel plates <NUM> were the iron and nickel diffusion layer and the iron and nickel-cobalt alloy diffusion layer, the discharge performance of equal to or more than <NUM>% was confirmed when the average number of the crystal grains of the base material steel plate <NUM> was in a range of <NUM> to <NUM> and the thickness of the inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) was in a range of <NUM> to <NUM>. When the inner peripheral side diffusion layer 103a was the iron and nickel-cobalt alloy diffusion layer (Table <NUM>), the average number of the crystal grains of the base material steel plate <NUM> was in a range of <NUM> to <NUM>, and the thickness of the inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) was in a range of <NUM> to <NUM>, particularly satisfactory results were obtained.

It should be noted that when the thickness of the inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) was <NUM>, a sufficient discharge performance could not be obtained except when the average number of the crystal grains of the base material steel plate <NUM> was <NUM>. It is conceivable that this phenomenon is due to an increase of an electrical resistance (contact resistance) caused by the thickened inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) decreasing an exposure amount of the iron on the surface of the battery can forming steel plate <NUM>.

Subsequently, the following Table <NUM> and Table <NUM> are made by combining contents of Table <NUM> to Table <NUM> in order to perform comprehensive evaluations for the leakage resistance characteristics and the discharge characteristics for the respective cases where the inner peripheral side diffusion layer 103a of the battery can forming steel plate <NUM> was the iron and nickel diffusion layer and the inner peripheral side diffusion layer 103a was the iron and nickel-cobalt alloy diffusion layer. It should be noted that Table <NUM> summarizes the results of the leakage resistance characteristic test (Table <NUM>) and the results of the discharge characteristic test (Table <NUM>) for the case where the inner peripheral side diffusion layer 103a was the iron and nickel diffusion layer. Table <NUM> summarizes the results of the leakage resistance characteristic test (Table <NUM>) and the results of the discharge characteristic test (Table <NUM>) for the case where the inner peripheral side diffusion layer 103a was the iron and nickel-cobalt alloy diffusion layer. In Table <NUM> and Table <NUM>, "Good" is given when "Good" or "Excellent" is given for both the leakage resistance characteristics and the discharge characteristics (furthermore, "Excellent" is given when "Excellent" is given for the discharge characteristics) and "Poor" is given when "Poor" is given for at least any one of the leakage resistance characteristics and the discharge characteristics.

As illustrated in Table <NUM> and Table <NUM>, in both cases where the inner peripheral side diffusion layer 103a of the battery can forming steel plate <NUM> was the iron and nickel diffusion layer and was the iron and nickel-cobalt alloy diffusion layer, satisfactory results were obtained in both the leakage resistance and the discharge characteristics when the average number of the crystal grains of the base material steel plate <NUM> was in a range of <NUM> to <NUM> and the thickness of the inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) was in a range of <NUM> to <NUM>.

In the case where the inner peripheral side diffusion layer 103a of the battery can forming steel plate <NUM> was the iron and nickel-cobalt alloy diffusion layer, the discharge performance is significantly improved without losing the leakage resistance when the average number of the crystal grains of the base material steel plate <NUM> was in a range of <NUM> to <NUM> and the thickness of the inner peripheral side plated layer <NUM> before the heat diffusion treatment (<FIG>) was in a range of <NUM> to <NUM>.

It should be noted that when the average number of the crystal grains of the base material steel plate <NUM> exceeds <NUM>, an extension of the base material steel plate <NUM> becomes worse to make the presswork (deep drawn presswork) difficult. Therefore, from the aspect of productivity, it is preferred to have the average number of the crystal grains of the base material steel plate <NUM> in a range of <NUM> to <NUM>.

It has been confirmed that it is possible to achieve the alkaline battery in which both the leakage resistance improvement and the heavy load discharge performance improvement are provided by the following method. As the base material steel plate <NUM> of the battery can forming steel plate <NUM>, one that has the average number of the crystal grains per <NUM><NUM> unit area of equal to or more than <NUM> is used, the inner peripheral side plated layer <NUM> (nickel plated layer or nickel-cobalt alloy plated layer) is formed with a thickness of <NUM> to <NUM> on a surface on the side that is the inner surface of the positive electrode can <NUM> of the base material steel plate <NUM>, and the inner peripheral side diffusion layer 103a (iron and nickel diffusion layer or iron and nickel-cobalt alloy diffusion layer) is formed by performing the heat diffusion treatment.

Claim 1:
A method for producing a steel plate (<NUM>) suitable for forming a battery can by presswork, the method comprising the steps of:
providing a steel plate (<NUM>) as a base material, wherein an average number of crystal grains per <NUM><NUM> unit area of the steel plate (<NUM>) as the base material, as set in the description, is equal to or more than <NUM> and the average number of the crystal grains per <NUM><NUM> unit area of the steel plate (<NUM>) as the base material is equal to or less than <NUM>; and
forming an iron and nickel diffusion layer or an iron and nickel-cobalt alloy diffusion layer (103a) on the steel plate (<NUM>) by
forming a nickel plated layer (<NUM>) or a nickel-cobalt alloy plated layer (<NUM>) with a thickness of <NUM> to <NUM> on an inner surface of a battery can of the steel plate (<NUM>) as a base material, and subsequently
performing a heat diffusion treatment under a predetermined temperature and a predetermined time on the nickel plated layer (<NUM>) or the nickel-cobalt alloy plated layer (<NUM>).