JP3033430B2 - Hydrogen storage alloy powder and nickel-hydrogen battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen-absorbing alloy powder for a nickel-hydrogen battery having excellent high-efficiency discharge characteristics and a nickel-hydrogen secondary battery using the same as a negative electrode active material.

[0002]

2. Description of the Related Art At present, global environmental problems such as air pollution and global warming caused by fossil fuels have become an issue in the energy situation, and hydrogen has attracted attention as a clean fuel replacing these fossil fuels. This means that hydrogen is made from water, the combustion products are water,
This is because the material is applicable to an energy conversion system for heat and power.

[0003] As a hydrogen storage material, a hydrogen storage alloy exists, and its application to storage, heat pumps, actuators, and the like has been developed. In recent years, nickel-hydrogen secondary batteries (hereinafter, Ni-H
The battery has been put to practical use as a power source for portable electric and electronic products.
In particular, in this field, Ni-Hd has been responded to the problems of pollution of Ni-Cd secondary batteries and resource problems of Cd, which have been the mainstream until now, and the need for higher capacity accompanying the portable equipment.
Demand for batteries is soaring.

[0004] The major alloy systems that have been studied as a hydrogen storage alloy, and AB ₅ type, such as LaNi ₅ and MmNi _5, an AB ₂ type Laves phase-type crystalline represented by ZrV _2. AB ₅ alloys are generally used in secondary batteries that have already been put to practical use, but AB and AB ₂ alloys are also promising. Ni−
In an H battery, a powder of a hydrogen storage alloy is mixed with an appropriate binder to form a paste, and the paste fixed to a current collector is used as a negative electrode.

[0005]

As the practical use of Ni-H batteries progresses, there are several problems, one of which is to improve the discharge characteristics. That is, in general, Ni-H batteries are inferior in high-efficiency discharge characteristics as compared with conventional Ni-Cd batteries, and improvement thereof is desired.

[0006] Here, the high-efficiency discharge characteristics refer to discharge characteristics when a large current is discharged. In general, as the current value (discharge current value) taken out during discharge increases, the discharge capacity becomes smaller than when discharging with a small current. When the decrease in discharge capacity when discharged at a large current is small, it is said that the battery has excellent high-efficiency discharge characteristics.

On the other hand, to increase the discharge capacity, there is a method of increasing the filling rate of the alloy powder to increase the hydrogen storage capacity. In this regard, Japanese Patent Application Laid-Open No. 3-116655 proposes to increase the packing ratio by using spherical powder. However, the present inventors have studied and found that a hydrogen-absorbing alloy powder produced by a gas atomizing method or a rotating disk method, in which a spherical powder can be obtained relatively easily, does not necessarily have a sufficient high-efficiency discharge characteristic when used as it is. Turned out to be impossible.

It is an object of the present invention to provide a hydrogen storage alloy powder capable of easily absorbing and releasing hydrogen, having excellent high-efficiency discharge characteristics, and obtaining a sufficient discharge capacity even in a situation where absorption and release of hydrogen is difficult (eg, low temperature). An object of the present invention is to provide a Ni-H battery using the same. Another object of the present invention is to provide a hydrogen storage alloy powder capable of constituting a Ni-H battery having a high alloy filling amount per unit volume, a large hydrogen storage amount, and an increased volume energy density.

[0009]

Means for Solving the Problems As a result of repeated studies aimed at achieving the above object, the present inventors have found that when an acid treatment is performed on a hydrogen storage alloy powder with an aqueous acid solution , a Ni-H battery is formed. It has been noticed that the high-efficiency discharge characteristics of P.I. After exploring the reason, the chemical surface treatment improved the efficiency of high-efficiency discharge characteristics because some elements were more easily eluted than other elements, forming a layer enriched with specific elements on the surface. And reached the present invention.

[0010] Here, the present invention is, AB-type, a hydrogen-absorbing alloy powder of the type ₂ or AB ₅ type AB, aqueous acid
Acid treatment and the B site element concentration
1.015 or more B-site element-enriched layer
Above, characterized in that it has the following thickness 500 Å, a high
The gist of the present invention is a hydrogen storage alloy powder for a Ni-hydrogen secondary battery with improved efficiency discharge characteristics .

In a preferred embodiment, the hydrogen storage alloy powder is a substantially spherical powder obtained by a production method such as a gas atomization method. Here, the term “substantially spherical” refers to a spherical shape or a substantially spherical shape, and corresponds to, for example, a powder produced by a gas atomizing method, a rotating electrode method, or the like. Generally, the ratio between the maximum and minimum particle diameter (aspect ratio)
Is about 1.5 or less.

According to the present invention, there is also provided a nickel-hydrogen secondary battery using such a hydrogen storage alloy powder as a negative electrode active material.

An example of the AB ₂ type alloy is ZrNi _y (y is 1.9 to 2.
25) as a basic structure, part or all of Zr is Ti, and part of Ni is one or more elements such as V, Mn, Cr, Co, Fe, Al, Mo, Cu, Be, etc. It has been replaced. As specific examples, Zr _1.0 V _0.4 Ni _1.6 , Zr _1.0 Mn _0.6 Cr _0.2 Ni _1.2 , Zr
_1.0 Ni _1.2 Mn _0.6 V _0.2 Co _0.1 , Zr _1.0 Ni _1.2 Mn _0.6 V _0.2 Fe _0.1 ,
There are Zr _1.0 V _0.4 Ni _1.6 and Zr _0.5 Ti _0.5 Mn _0.6 V _0.2 Ni _1.2 .

Examples of AB ₅ type alloys are LaNi _x or MmNi _x
(x is 4.7 to 5.2) (Mm is a rare earth metal alloy <
E.g., alloy of La, Ce, Pr, Nd>
Part of Ni is Co, Mn, Al, Fe, Cr, Cu, V, Be, Zr, Ti,
It is substituted with one or more elements such as Mo. Since LaNi _x is expensive and has a short life, MmNi _x is practically preferable.

An example of an AB type alloy has a basic structure of TiNi _X (x is 0.9 to 1.3), and a part of Ni is Co, Mn, V, Cr, Al, Fe, M
It is substituted with one or more elements such as o, Cu, Zr, and Be.

The method for producing the hydrogen storage alloy powder is not particularly limited, and any conventionally known method can be employed. For example, an ordinary melting method, that is, a hydrogen storage alloy is melted in an arc melting furnace, a high-frequency induction furnace, or the like, and an ingot obtained by casting the alloy is pulverized by a suitable pulverizer such as a ball mill to form a hydrogen storage alloy. A powder can be obtained. The alloy powder obtained by this method is an irregularly shaped pulverized powder.

As another method, there is a method of directly producing a hydrogen storage alloy powder without a pulverizing step as described in JP-A-2-253558 and JP-A-3-116655. Specifically, alloy powder can be directly produced by a rotating disk method, a rotating electrode method, a gas atomizing method, or the like. In these methods, generally spherical hydrogen storage alloy powder can be obtained. In addition, the cooling rate is remarkably faster than that of the melting method, and rapid solidification results in less segregation of components and problems associated with segregation (partial deterioration of corrosion resistance and embrittlement, resulting in capacity reduction due to repeated charging and discharging. ) Can be obtained. However, since the alloy powder thus produced has quenching strain, it is preferable to subject the powder to a heat treatment for removing strain. This heat treatment is performed, for example, at 550 to 950 ° C. in a vacuum or inert atmosphere.
For 2 to 5 hours.

The shape of the hydrogen storage alloy powder is not particularly limited, but a substantially spherical powder is desirable because a high filling rate can be obtained. In this sense, a particularly preferable method for producing a hydrogen storage alloy powder is a gas atomization method capable of inexpensively mass-producing spherical alloy powder having a shape close to a perfect circle. In the gas atomization method, a molten metal adjusted to a predetermined composition is caused to flow down from a small-diameter nozzle, and a high-pressure inert gas is sprayed from a gas nozzle toward the molten metal stream to powderize the molten metal, and the powder is cooled and solidified to form a powder. It is a method of obtaining and is used industrially in the production of various metal and alloy powders. The powder obtained by the gas atomization method is spherical or substantially spherical.

The particle size of the hydrogen storage alloy powder of the present invention is not particularly limited. However, if the particle size is too large, the filling rate is reduced, and if the particle size is too small, the life is shortened. Therefore, it is generally desirable that the average particle size is in the range of 10 to 150 μm, particularly 30 to 70 μm.

[0020]

According to the present invention, an AB type, AB ₂ type or AB ₅ type hydrogen storage alloy powder is subjected to an acid treatment with an aqueous acid solution.
A B-site element-enriched layer having a thickness of 20 to 500 mm from the surface. Where B
The site element enriched layer means that the B site element enrichment rate is 1.015
The above means the surface layer portion.

The concentration of the B-site element is the ratio of the concentration of the B-site element at that point to the average concentration of the B-site element in the entire powder, and the concentration of the B-site element is the total content of the B-site element. (Atomic%). The average concentration (atomic%) of the B site element in the entire powder is calculated from the chemical analysis value of the powder. On the other hand, elemental analysis of the powder surface can be performed by ESCA (X-ray photoelectron spectroscopy).

In order to examine the element distribution in the depth direction of the powder, the surface is analyzed by ESCA while removing the powder surface by an appropriate surface grinding method (eg, sputtering method). ES measured at many points at different depths from the surface
The CA analysis value is corrected to match the chemical analysis value, and the concentration (atomic%) of the B site element at that depth is determined. From the ratio between the concentration of the B-site element (ESCA analysis value) and the average concentration of the B-site element calculated from the chemical analysis value, the concentration of the B-site element at each depth is calculated. The surface layer portion up to the depth where the concentration ratio of the B-site element thus determined is 1.015 or more is defined as the B-site concentrated layer, and the thickness of the B-site concentrated layer is determined. In the hydrogen storage alloy powder of the present invention, the thickness of the B-site-enriched layer is not less than 20 mm and not more than 500 mm. That is, the concentration of the B-site element becomes 1.015 or less within a depth of 20 to 500 mm from the surface.

As described above, the B-site element refers to A
For B ₂ type alloy, Ni and V, Mn, Cr, Co, Fe, Al, Mo, Cu,
Elements such as Be, the AB type and AB ₅ type alloys Ni and Co, M
Elements such as n, Al, Fe, Cr, Cu, V, Be, Zr, Ti, and Mo. As can be seen, most of the B-site elements are transition metal elements having a high catalytic action.

Charge / discharge of a Ni-H battery is a chemical reaction for absorbing and releasing hydrogen between the hydrogen storage alloy and the electrolyte. In the hydrogen storage alloy, the B site element exerts a catalytic action to assist the hydrogen absorption / desorption reaction. In the hydrogen storage alloy powder of the present invention, since the B site element having a catalytic action is concentrated on the powder surface, the reaction between the hydrogen storage alloy and the electrolytic solution is promoted, and the reaction efficiency is increased. as a result,
The charge / discharge rate is increased to improve the high-efficiency discharge characteristics, and even in a situation where charging / discharging is difficult (eg, low temperature), excellent charging / discharging characteristics as in a steady state can be obtained.

The hydrogen-absorbing alloy powder of the present invention having a B site element concentrated layer on the surface, AB type produced by any known method as described above, the AB ₂ type or AB ₅ type hydrogen-absorbing alloy powder, A It can be obtained by performing an acid treatment with an aqueous acid solution that can elute site elements preferentially .

When the hydrogen-absorbing alloy powder is treated with an acid, the element is easily eluted (A-site elements such as Zr and Ti in AB ₂ type alloys, rare-earth elements and B-sites as A-site elements in AB type and AB ₅ type alloys). Al) is preferentially eluted, and the transition metal element at the B site, which is difficult to elute, is enriched on the surface. The concentration of the transition metal element on the surface enhances the catalytic action on the electrochemical reaction of the Ni-H battery, and contributes to the promotion of the reaction and the improvement of the electrical conductivity. It shows excellent characteristics in charging / discharging under difficult conditions.

The thickness of the B-site element concentrated layer on this surface is 20
More than Å and less than 500 よ い is good. When the thickness of the B-site element-concentrated layer exceeds 500 mm, the amount of hydrogen occlusion decreases remarkably due to elution of components, and when it is less than 20 mm, almost no effect is obtained. In order to sufficiently obtain the above effects, it is preferable that the thickness of the B-site element concentrated layer on the surface is within a range of 40 to 100 °.

The acid used for the acid treatment is not particularly limited.
Non-oxidizing acids such as hydrochloric acid and / or hydrofluoric acid are preferred. With other acids (eg, nitric acid, sulfuric acid, etc.), an oxidizing function of the acid may easily cause an oxide film to be formed on the alloy surface during the immersion treatment with the acidic aqueous solution, which may reduce the reaction effect. In view of the dissolution property of the A-site element, AB
The ₂ type alloys is suitably hydrofluoric acid, the AB ₅ type alloys using hydrochloric acid is preferred.

The aqueous acid solution used for the immersion is a stock solution of the reagent grade or the first grade or a similar concentration thereof (hydrochloric acid is 35 to 35%).
(36%, hydrofluoric acid at 44-46% concentration) can be prepared by diluting with water. The condition of the acid treatment may be adjusted so that a B-site element-concentrated layer having a thickness of 20 to 500 mm is generated. For example, the acid concentration of the acid solution is preferably 1 to 20% with hydrochloric acid and 0.1 to 4% with hydrofluoric acid, more preferably 2 to 15% with hydrochloric acid, as the content (% by weight) of the stock solution. , And 0.3 to 2% of hydrofluoric acid. The temperature of the acid treatment is preferably from 0 to 80C.

The hydrogen-absorbing alloy powder after the acid treatment is then washed with water and dried. Drying is preferably performed in a vacuum or an inert gas atmosphere .

As described above, the shape of the hydrogen storage alloy powder is preferably a substantially spherical powder from which a high filling rate can be obtained. However, even in the case of irregularly shaped powders, such as pulverized powders produced by the melting method, the capacity cannot be expected to be improved as much as spherical powders due to the low filling factor. In addition, the discharge characteristics at low temperatures are significantly improved, which is significant.

The hydrogen storage alloy powder of the present invention can be used as a negative electrode of a Ni-H battery by forming an electrode in the same manner as in the prior art. For example, use a hydrogen storage alloy powder with a suitable binder.
(Resin such as polyvinyl alcohol) and water (or other liquid) to form a paste, fill in a nickel porous material serving as a current collector, dry, and then press-mold into the desired electrode shape. , Electrodes can be manufactured. Alternatively, the electrode can also be manufactured by forming a molded body by pressure molding and hot pressing after mixing with a binder. In this case, the current collector is attached during or after the molding.

[0033]

EXAMPLES The effects of the hydrogen storage alloy powder of the present invention are demonstrated by the following examples. In Examples,% is% by weight unless otherwise specified. The concentration of the acid aqueous solution used for the acid treatment is the content (% by weight) of the stock solution described above.

The hydrogen storage alloy powder used in the examples is shown in Table 1.
It was AB ₅ type and AB ₂ type alloys having compositions shown in. In Table 1, Mm is La: 27%, Ce: 48%, Pr: 7%, Nd:
Lm is a rare earth metal alloy (Misch metal) containing 17%
It is a La-enriched rare earth metal alloy (La-enriched misch metal) containing 57% La: 15% Ce: 8% Pr: 19% Nd.

[0035]

[Table 1]

Example 1 Using the compositions of alloys A and C shown in Table 1, a hydrogen storage alloy powder was produced by an argon gas atomizing method. The obtained alloy powder was heat-treated at 900 ° C. for 4 hours, and then acid-treated with various acid aqueous solutions shown in Table 2 to obtain test materials. The acid treatment conditions were a temperature of 25 ° C. and an immersion time of 40 minutes. The alloy powder obtained after each acid treatment is sieved to a particle size range of 20 to 45 μm (average particle size 30
μm) of powder.

For each of these alloy powders, the presence / absence of a B-site element-concentrated layer on the surface and the thickness thereof were investigated by surface analysis using ESCA. Analysis was performed by Shimadzu ESCA-7
Using 50, MgKα (1.25 keV) was used as an X-ray source. In order to examine the element distribution in the depth direction, the powder surface was analyzed while being sputtered and ground with 2 keV Ar ⁺ ions. After correcting the analysis result every 20 ° from the surface obtained in this way with an average composition value by chemical analysis, the B / A atomic ratio (A site element / B site element atomic ratio) and the B site element enrichment ratio were calculated. did. The results are shown in Table 2.

On the other hand, battery performance when each alloy powder was used as a negative electrode active material of a Ni-H battery was evaluated by the following method. To 5 g of the alloy powder, a 10% Teflon binder (tetrafluoroethylene-hexafluoropropylene copolymer) was added, and pressed into a plate by a cold press. Then, both surfaces were sandwiched between nickel meshes, and then 5 tons at 300 ° C. /
Hot pressing under a pressure of cm ² was performed for 1 minute to produce a test electrode. Next, this electrode was used as the negative electrode, a commercially available sintered nickel electrode (nominal capacity of 4000 mAh) was used as the positive electrode, and a polyamide non-woven fabric was placed in the container with a separator interposed between them. An aqueous solution to which 20 g / L of lithium oxide was added was injected as an electrolytic solution to form a negative electrode capacity-regulated type Ni-H battery.

The test battery was charged at 500 ° C at 25 ° C.
The charge of mA × 4 hours and the discharge to 0.85 V at 500 mA were repeated, the discharge capacity at that time was measured, and the initial capacity (maximum discharge capacity in charge / discharge cycle mAh / g) was obtained.

At 25 ° C., 1500 mA, 3000 mA,
At 4500 mA and 4500 mA at -20 ° C, respectively
The capacity (mAh / g) when discharging up to 0.85 V (filling was 500 mA × 4 hours) was measured, and high-efficiency discharge characteristics were evaluated.
Table 3 summarizes these battery characteristics.

[0041]

[Table 2]

[0042]

[Table 3]

As can be seen from Tables 2 and 3, the hydrogen storage alloy powder of the present invention in which a B-site element-enriched layer having a thickness of about 20 to 500 mm was formed from the surface by the acid treatment had an initial capacity of The large discharge capacity was maintained during high-efficiency discharge, and the discharge capacity was very large even during high-efficiency discharge at a low temperature such as -20 ° C. where discharge was difficult. That is, it exhibited extremely excellent high efficiency discharge characteristics at both room temperature and low temperature. These effects were more remarkable when a B element-enriched layer having a thickness of 40 mm or more and 100 mm or less was formed from the surface.

On the other hand, the conventional alloy powder (untreated powder) not subjected to the acid treatment has a low initial capacity itself,
The high-efficiency discharge characteristics were inferior, and the capacity was particularly remarkably reduced during high-efficiency discharge at −20 ° C.

In the charge / discharge cycle test, the discharge capacity maintenance ratio (500 cycles) showed no significant difference between the present invention example, the conventional example, and the comparative example.

(Example 2) Using the compositions of alloys A and B shown in Table 1, a normal melting method (the alloy was vacuum-melted by high-frequency induction heating and cast into a water-cooled iron mold, and the obtained ingot was ball milled The pulverized powder of the hydrogen storage alloy produced by the pulverization was treated with various acid aqueous solutions shown in Table 4 to obtain test materials. The acid treatment conditions were a temperature of 25 ° C. and an immersion time of 40 minutes.

The acid-treated hydrogen storage alloy powder was sieved to collect powder having a particle size range of 20 to 75 μm (average particle size 50 μm). The surface analysis and battery performance of each of these powders were investigated in the same manner as in Example 1 (high-efficiency discharge characteristics were 25 ° C.
Measurement only). The results of the surface analysis are shown in Table 4, and the results of the battery performance are shown in Table 5.

[0048]

[Table 4]

[0049]

[Table 5]

It can be seen that the effect of the B-site element-enriched layer on the powder surface according to the present invention is remarkable also in the case of a powder of a hydrogen storage alloy produced by a usual melting method. Also in this example, there was no difference in the retention rate (500 cycles) of the discharge capacity in the charge / discharge cycle test between the present invention example, the conventional example, and the comparative example.

Although the present invention has been confirmed to be effective with AB ₂ type and AB ₅ type, TiNi _X (x = 0.9 to 1.3) has a basic structure, and a part of Ni is Co, Mn, Al, Fe, Cr, Cu, V, B
A substituted with one or more elements such as e, Zr, Mo
The B type has the same effect.

[0052]

The hydrogen-absorbing alloy powder for a Ni-H battery of the present invention has a thin B-site element-enriched layer having a thickness of not less than 20 mm and not more than 500 mm on its surface. The efficiency of the hydrogen absorption / desorption reaction is increased, the initial capacity is high, and this high discharge capacity is maintained even during highly efficient discharge. In addition, the excellent high-efficiency discharge characteristics can be exhibited even in a situation where discharge is difficult at low temperatures or the like. However, since the thickness of the B-site element-concentrated layer is very small, there is no adverse effect on the battery life when the capacity is reduced or charge / discharge is repeated. Accordingly, the present invention can significantly improve the initial capacity and the high-efficiency discharge characteristics at room temperature and low temperature without reducing the life of the Ni-H battery.
It contributes to quality improvement of H batteries and expansion of applications.

The effect of the present invention can be achieved irrespective of the particle size and shape of the hydrogen storage alloy powder. In particular, by applying the present invention to a substantially spherical hydrogen storage alloy powder,
Increased filling rate due to close packing of alloy powder, high volume energy density and extremely high efficiency discharge characteristics
A Ni-H battery can be configured, which is industrially very advantageous. Of course, even when applied to pulverized powder, the high-efficiency discharge characteristics are greatly improved, and therefore, their industrial significance is great.

Continuation of the front page (72) Inventor Koichi Jindai 4-5-33 Kitahama, Chuo-ku, Osaka City Inside Sumitomo Metal Industries, Ltd. (56) References JP-A-3-269953 (JP, A) JP-A-4-348849 (JP, A) JP-A-6-88150 (JP, A) WO 95/23435 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/24-4/26 H01M 4/38 H01M 10/24-10/30

Claims (2)

(57) [Claims] An AB type, AB ₂ type or AB ₅ type hydrogen storage alloy powder, which is subjected to an acid treatment with an aqueous acid solution.
Things and B site element and B-site element concentrated layer thickening rate is 1.015 or more, from the surface 20Å or more, and having thickness of less than 500 Å, modified high efficiency discharge characteristics
Goodness the Ni- hydrogen secondary battery hydrogen storage alloy powder. 2. A Ni-hydrogen secondary battery, wherein the negative electrode active material comprises the hydrogen storage alloy powder according to claim 1.