JPH0973898A - Processing method for hydrogen storage alloy for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a processing method for a hydrogen storage alloy which has high initial activity when used in an Ni-hydrogen battery and from which a battery that self-discharges little and is therefore excellent in long-range shelf life can be fabricated. SOLUTION: This hydrogen storage alloy processing method is characterized by dipping an Ni-containing hydrogen storage alloy in a nonoxidizing acid solution (e.g. hydrochloric acid, hydrofluoric acid, or a mixture of these acids) containing a 40wt.% or more organic solvent (e.g. alcohol, ketone, or amine). The hydrogen storage alloy to be processed by the method is an AB5 or AB2 type alloy having a composition shown in Figure 1, containing Ni in particular as a component element, for use in an Ni-hydrogen secondary battery. Therefore, the processing method is applicable to both AB5 and AB2 types without depending on the alloy composition only if the alloy contains Ni.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a hydrogen storage alloy for a battery containing Ni, and more specifically, to obtain a hydrogen storage alloy for a Ni-hydrogen battery having high initial activity and less self-discharge. Regarding the processing method of.

[0002]

2. Description of the Related Art Currently, secondary batteries used for power supplies of portable AV equipment and computer memory backup are
Ni-Cd batteries are the mainstream. However, from the viewpoints of pollution problems of Cd, resource limitation problem that Cd is a by-product of zinc refining, and development of higher capacity secondary batteries, a hydrogen storage alloy is used instead of Cd as a negative electrode material (strictly speaking, negative electrode A secondary battery called Ni-hydrogen battery was developed.

The Ni-hydrogen battery has the features that it has a higher capacity than Ni-Cd batteries and Ni-Zn batteries, and that the electrodes do not contain harmful elements. For this reason, Ni-hydrogen batteries have begun to be used in portable AV devices and popular ones, and also as secondary batteries for electric vehicles whose use is expanding as pollution-free vehicles and energy-saving vehicles due to global environmental problems. It is under consideration and is already in mass production.

The major alloy systems that have been studied as hydrogen storage alloys for Ni-hydrogen batteries are LaNi ₅ system and MmNi ₅ system (Mm is Misch metal which is a rare earth metal mixture).
An intermetallic compound having a B ₅ type crystal structure and ZnV _0.4 Ni _1.6
Is an intermetallic compound having an AB ₂ type Laves phase structure. That is, both contain Ni as a main component. AB ₅ type hydrogen storage alloys are more promising for practical use, but AB ₂ type hydrogen storage alloys are also promising because they show high capacity.

However, when mass production of these hydrogen storage alloys for Ni-hydrogen batteries began, some new problems came up. One is that the initial activation process (the process of increasing the discharge capacity of the battery to a predetermined steady-state value) after constructing the Ni-hydrogen battery takes a very long time, and productivity is significantly impaired. .

The initial activation process currently performed is a process of discharging at low current for a long time and then discharging (charging for 15 to 20 hours and discharging for several hours) until a predetermined discharge capacity is obtained. It consists of repeating several times. Therefore, it is necessary to repeat charging and discharging in the factory for several days as an initial activation process from the time the battery is assembled to the time it is shipped.

As a means for solving this problem, it has been attempted to improve the initial activity by controlling the grain boundaries of the hydrogen storage alloy. For example, JP-A-3-219036 proposes that boron is added to a hydrogen storage alloy to generate a boron-rich phase that is easily pulverized, and the initial activation efficiency is improved by increasing the specific surface area by pulverization. There is. However, this is accompanied by pulverization of the alloy, so that the battery life of the Ni-hydrogen battery (charge / discharge repeated cycle life) is significantly reduced. Therefore, it is difficult to achieve both the initial activity and the battery life by such means.

As another activation means, there is known a method of immersing a hydrogen storage alloy in an aqueous solution containing an acid, an alkali or a salt before producing an electrode. For example, JP-A-4
-328252 discloses a dipping treatment using an aqueous solution of hypophosphite, and JP-A-3-49154 discloses a phosphate, silicate, arsenate, chromate, hypophosphite, Immersion treatments using tetrahydroborate and the like have been proposed. However, in such a dipping treatment using an aqueous salt solution, the chemical components used for the treatment remain on the powder surface, the electrochemical characteristics on the positive electrode side (the positive electrode characteristics of the battery) deteriorate, and the battery capacity decreases early. There is a problem of doing.

On the other hand, Japanese Patent Laid-Open No. 4-179055 and 6
-223827 discloses an immersion treatment using an acid aqueous solution for the purpose of removing oxides on the surface of the alloy, and JP-A-5-195007 describes coating the surface of the alloy with a hydroxide layer having a large specific surface area. A dipping treatment using an alkaline aqueous solution is proposed in Japanese Patent Laid-Open Nos. 3-152868 and 4-98760. First, an dipping treatment using an acid aqueous solution and then an alkaline aqueous solution is proposed.

[0010]

When the hydrogen storage alloy powder treated with the acid aqueous solution and / or the alkaline aqueous solution as described above is used for the negative electrode of the Ni-hydrogen battery, the initial activity is certainly improved, but the self-discharge is self-discharged. It has been found that there is a problem that the characteristics are adversely affected, and the capacity decreases greatly after long-term storage after charging. This problem significantly reduces the practicality of the Ni-hydrogen battery because it is not uncommon to leave the Ni-hydrogen battery unused and unused after charging.

An object of the present invention is to provide a method for treating a hydrogen storage alloy, which is capable of producing a battery having a high initial activity when used in a Ni-hydrogen battery and having little self-discharge and excellent long-term storage stability. It is to be.

[0012]

The present invention is characterized in that a hydrogen storage alloy containing Ni is immersed in a non-oxidizing acid solution containing an organic solvent in an amount of 40% by weight or more,
It is a method of treating a hydrogen storage alloy for a battery.

The present inventors have developed a hydrogen storage alloy containing Ni.
When (hereinafter simply referred to as hydrogen storage alloy) is used for the negative electrode of a Ni-hydrogen battery, the initial activity of the battery depends on the amount of oxide on the surface of the alloy powder, and the amount of self-discharge depends on the amount of hydroxide on the surface of the alloy powder. Confirmed to do. That is, the smaller the oxide content, the higher the initial activity, and the smaller the hydroxide content, the smaller the self-discharge amount.

Of these, the oxide is formed by the reaction of the metal on the surface of the alloy with oxygen in the atmosphere or oxygen in the treatment atmosphere in each of the steps of melting, heat treatment and crushing performed in the production of the hydrogen storage alloy powder. It is a thing. Oxides have poor conductivity and therefore reduce the initial activity of the battery. However, as known in the prior art, oxides can be removed with an aqueous acid solution, so treatment with an aqueous acid solution significantly improves the initial activity.

On the other hand, hydroxide is formed on the surface of the alloy when the hydrogen storage alloy is treated with an alkaline aqueous solution, and especially Ni (OH)
₂ is generated a lot. Although it has been considered that hydroxide is not formed by the treatment with the acid aqueous solution, the present inventors have found that Ni (OH) ₂ is also present on the surface of the acid-treated hydrogen storage alloy.

Ni (OH) ₂ is a substance used in the positive electrode of a Ni-hydrogen battery, and when it adheres to a hydrogen storage alloy that is the negative electrode active material, it undergoes a self-discharge reaction (electricity stored in the negative electrode acts as the positive electrode). (A reaction that is consumed by a chemical reaction other than the discharge reaction of) occurs. Therefore, when the amount of hydroxide is large, self-discharge occurs during long-term storage after charging, the amount of electricity charged decreases, and the discharge capacity of the battery decreases, which deteriorates the long-term storability of the battery.

From the above, in order to achieve the object of the present invention,
The hydrogen storage alloy is treated with acid to remove oxides on the alloy surface,
At that time, it was found necessary to suppress the formation of hydroxide. This is because when the oxide is removed by acid treatment, the initial activity of the battery is improved, and if the amount of hydroxide produced is small at that time, the long-term storage stability of the battery due to self-discharge can be prevented. Alkali treatment is disadvantageous for long-term storability of Ni-hydrogen battery because it produces a large amount of hydroxide on the alloy surface.

Based on this finding, the present inventors have investigated the reason why hydroxide, especially Ni (OH) _2, is produced on the surface of the alloy by the treatment with the acid aqueous solution, and as a result, the following points have been clarified. That is, when a hydrogen storage alloy is immersed in an aqueous acid solution, the oxides on the alloy surface and the component metals of the alloy are dissolved as a result of the anode reaction, and as a result, metal ions are generated in the liquid, but the metal ions eluted in the liquid are The closer to, the higher the concentration. A hydrogen generation reaction occurs as a cathode reaction that is paired with this anodic reaction, which consumes hydrogen ions in the liquid and generates hydroxide ions. Higher ion concentration. Of the metal ions eluted in the liquid, Ni ions in particular tend to combine with hydroxide ions to precipitate as hydroxide [Ni (OH) ₂ ] when the pH rises, so even under acid treatment conditions, the nearest neighbors to the alloy. Then Ni
The conditions under which precipitation of (OH) ₂ is likely to occur are complete, so that Ni (OH) ₂ will be present on the alloy surface.

Therefore, in order to improve the long-term storage stability of the Ni-hydrogen battery, in order to prevent the formation of Ni (OH) ₂ , Ni ions eluted in the liquid during the acid treatment are combined with hydroxide ions. Therefore, it was found that it is effective to use an organic solvent as a part of the solvent, and the present invention described above was completed.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The hydrogen storage alloy to be treated by the method of the present invention is AB ₅ type or A type used for Ni-hydrogen secondary batteries.
It is an alloy of B ₂ type or the like, and particularly contains Ni as a constituent element. The effect of the present invention is not affected by the alloy composition, and the treatment method of the present invention can be applied to both AB ₅ type and AB ₂ type as long as Ni is contained.

Examples of AB ₅ type alloys are LaNi _x or MmNi _x
(x is 4.7 to 5.2) as the basic composition, and in some cases, a part of Ni
Co, Mn, Al, Fe, Cr, Cu, V, Be, Zr, Ti, Mo, etc. 1
It has been replaced with one or more elements. LaNi
_x is expensive and its lifespan decreases quickly, so Mm is practically
Ni _x is preferred.

An example of an AB ₂ type alloy is ZrNi _y (y is 1.9 to 2.
25) as a basic composition, and if necessary, a part of Ni is V, M
One or two of n, Cr, Co, Fe, Al, Mo, Cu, Be, etc.
It is the one that is replaced by one or more elements. As a specific example, Zr
_1.0 V _0.4 Ni _1.6 , Zr _1.0 Mn _0.4 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 , Zr _1.0 V _0.4 N
i _1.6 etc. It should be noted that these are merely examples, and other compositions can be used.

As a method for producing a hydrogen storage alloy, in addition to a usual ingot method (a crushed ingot obtained by casting a molten alloy), a rapid solidification method such as a rotating electrode method, a roll quenching method or an atomizing method is used. Various methods used are known. The method of the present invention can be applied to the hydrogen storage alloy produced by any of these methods. Since the hydrogen storage alloy is used in the form of powder in the production of the electrode, it is generally necessary to grind the produced alloy for the hydrogen storage alloy produced by a method other than the atomization method in which the powder is directly obtained. Oxides are generated on the surface of the alloy powder in this crushing process,
Since the initial activity is lowered, it is desirable to apply the treatment method of the present invention to a powdery hydrogen storage alloy pulverized to a final particle size. Further, the hydrogen storage alloy produced by the rapid solidification method is preferably heat-treated in a non-oxidizing atmosphere before the treatment of the present invention in order to remove the strain generated during the rapid cooling.

For the treatment of the hydrogen storage alloy according to the present invention, a non-oxidizing acid solution is used. The oxides formed on the surface of the hydrogen storage alloy during the production and pulverization are mainly Ni oxide and rare earth oxides (for AB ₅ type) or Zr oxides (A
In the case of B ₂ type). Non-oxidizing acids are effective in removing these oxides from the alloy surface, which significantly improves the initial activity of the cell.

Examples of non-oxidizing acids suitable for use in the present invention are hydrochloric acid and hydrofluoric acid, and mixed acids of hydrochloric acid and hydrofluoric acid can also be used. If non-oxidizing, other acids can be used alone or in a mixture with hydrochloric acid and / or hydrofluoric acid. When an oxidizing acid such as nitric acid or sulfuric acid is used, an oxide film is likely to be newly formed due to its oxidizing power, and the initial activity of the alloy cannot be sufficiently improved.

The hydrogen storage alloy is immersed in such a non-oxidizing acid solution. In the present invention, this solution contains a large amount of a water-miscible organic solvent. Thereby,
Both Ni ions, which are eluted from the alloy in the liquid by reacting with an acid, and hydroxide ions, which are generated by the counter reaction, dissolve in the organic solvent, and unlike water, the organic solvent does not dissociate into hydroxide ions and hydrogen ions. As a result, it is considered that the concentrations of these ions are diluted, and the reaction in which these ions combine and precipitate as Ni (OH) ₂ on the alloy surface is suppressed. Therefore, self-discharge when used as a negative electrode of a Ni-hydrogen battery is reduced, and long-term storage stability is improved.

The type of organic solvent is not particularly limited, but when the acid solution contains water (for example, concentrated hydrochloric acid or concentrated hydrofluoric acid is used as the stock solution of the acid as described below),
It is desirable to use a water-miscible organic solvent so that the solvent does not phase separate. Examples of water-miscible organic solvents suitable for use in the present invention include monohydric or polyhydric alcohols having a total carbon number of 10 or less (eg, methanol, ethanol,
Propanol, isopropanol, n-butanol, i
-Butanol, t-butanol, hexanol, decanol, ethylene glycol, cyclopropanol, cyclohexanol, etc.), ketones having a total carbon number of 10 or less (eg,
Acetone, cyclohexanone, etc.), amines having a total carbon number of 10 or less (eg, methylamine, ethylamine, butylamine, amylamine, diethylamine, pyridine, etc.) and the like. The organic solvent can use 1 type (s) or 2 or more types.

The content of the organic solvent in the acid solution used for the treatment is 40% or more by weight. The balance of the solution is acid and water. When the amount of organic solvent in the acid solution is less than 40%, the water content of the solution increases, so Ni (OH) ₂
Cannot be sufficiently suppressed. The higher the content of the organic solvent and the lower the water content, the higher the effect of suppressing self-discharge due to the formation of Ni (OH) ₂ . Therefore, it may be a solution consisting only of an acid and an organic solvent, but industrially available hydrochloric acid and hydrofluoric acid contain 50% or more of water, so when industrially carried out It is difficult to reduce the water content to zero.

The acid solution used in the treatment of the present invention is a stock solution of commercially available reagent grade or first grade or a nonoxidizing acid having a similar concentration (concentration is generally 35 to 36% with hydrochloric acid, with hydrofluoric acid. 44-46%) can be prepared by diluting with water and a water-miscible organic solvent or only a water-miscible organic solvent. The acid concentration in the acid solution is the content of this stock solution.
(% By weight), hydrochloric acid 1-15%, hydrofluoric acid
0.2-10%, mixed acid of hydrochloric acid and hydrofluoric acid 0.2-15%
The range of is desirable. When the acid concentration is lower than the lower limit, the reactivity between the oxide and the acid solution is low, and it is difficult to obtain a sufficient improvement in the initial activity even after the immersion treatment. On the other hand, when the acid concentration exceeds the upper limit, the dissolution reaction rapidly occurs, not only the oxide film on the surface of the alloy is removed, but also the dissolution of the alloy itself proceeds, and the loss amount increases.

Other treatment conditions with the acid solution are not particularly limited, but the temperature is preferably in the range of 10 to 50 ° C. At 10 ° C. or lower, the oxide removal reaction does not proceed sufficiently and the treatment takes too long. On the other hand, when the temperature is 50 ° C. or higher, the vaporization of the organic solvent becomes vigorous, and the concentration of the organic solvent decreases remarkably during the treatment, so that stable treatment may not be performed.

The acid treatment time (immersion time in the acid solution) varies depending on the temperature and the type and concentration of the acid solution, but is generally several minutes to several hours, and in most cases, one hour or less is sufficient. Is. Hydrofluoric acid requires a shorter treatment time than hydrochloric acid. It is also desirable to stir the acid solution during processing.

The hydrogen storage alloy soaked in a non-oxidizing acid solution containing an organic solvent may be washed sufficiently with water to remove the attached acid, and then dried. Drying is preferably performed in vacuum or in an inert gas.

An electrode is prepared from the hydrogen storage alloy thus treated by a method well known to those skilled in the art and used as a negative electrode of a Ni-hydrogen battery. For the electrode, hydrogen storage alloy powder is used as a suitable binder.
It can be prepared by mixing (resin such as polyvinyl alcohol) and water (or other liquid) to form a paste, filling a nickel porous body, drying and then pressure-molding into a desired electrode shape.

[0034]

EXAMPLE The following example illustrates the constitution and effect of the present invention. In the examples,% means% by weight unless otherwise specified.
It is.

The hydrogen storage alloy powders used in the examples are shown in Table 1.
It was an AB ₅ type or AB ₂ type alloy having the composition shown in. Raw materials used for casting of these alloys are flake Ni having a purity of 99.9%, electrolytic Co having a purity of 99.8%, shot Al having a purity of 99.9%, plate Mn having a purity of 99.8%, Ni-56.9% V mother alloy,
Spur-like Zr with a purity of 99.5% or higher, rare earth metal purity 99.8
% Or more of misch metal (Mm) (La = 28%, Ce = 48%, Nd
= 18%, Pr = 6%).

[0036]

[Table 1]

75 kg / ch of Ar gas atomizing method (cooling rate from molten liquid = 1 × 10 ³ to 1 × 10 ⁵⁾ was used by using an alloy melt melted from these raw materials by high frequency induction heating in vacuum.
A hydrogen storage alloy powder having a predetermined composition was produced by the ingot method of ⁴ ° C./sec) or 100 kg / ch (cooling rate from molten liquid = 1.0 ° C./sec). The hydrogen storage alloy obtained by the ingot method was then mechanically crushed by a stainless steel ball mill in an Ar atmosphere into a powder having an average particle size of 40 μm.
Hydrogen storage alloy powder obtained by atomization method (average particle size 40μ
m) and the hydrogen storage alloy powder obtained by the ingot method and crushed are both 900 ° C x 10hr in an Ar atmosphere with a purity of 99.99%.
Was heat-treated.

500 g of these hydrogen storage alloy powders were mixed with hydrochloric acid (HCl) and / or hydrofluoric acid containing an organic solvent.
The solution was immersed in 3 kg of the solution of (HF) with stirring at 30 rpm. The treatment conditions were a temperature of 25 ° C. and an immersion time of 10 minutes. The acid solution used for the dipping treatment is an undiluted solution of acid (HCl = concentrated hydrochloric acid for reagent grade, concentration 36%; HF = concentrated hydrofluoric acid for reagent grade, concentration 46%) and organic solvent (grade 1 reagent) and Prepared by diluting with deionized water as needed. Table 2 shows the content of the organic solvent and the acid concentration (content of each stock solution) in the used acid solution. The amount of water in the acid solution is 100
It is the amount obtained by adding the amount of water contained in the acid concentration (content of the stock solution) to the remainder obtained by subtracting the content of the organic solvent shown in Table 2 from%.

The hydrogen-absorbing alloy powder soaked as described above was washed with 10 times the weight ratio of water and then vacuum dried. This alloy powder was classified into 74 μm or less and 32 μm or more, and a binder (5% polyvinyl alcohol aqueous solution) was added and kneaded. The obtained alloy powder paste was filled in a foam metal porous body made of nickel (Sumitomo Electric Celmet), dried and then pressurized at a pressure of 1.5 ton / cm ² ,
The alloy powder was supported in a porous Ni body to prepare a negative electrode for a battery. At this time, the supported amount of the hydrogen storage alloy powder was 12 g.

A commercially available Ni electrode with a nominal value of 2000 mA was used as the positive electrode, and a nylon nonwoven fabric impregnated with an alkaline electrolyte of 6N-KOH was sandwiched between the positive electrode and the negative electrode as a separator.
A 2000mA Ni-hydrogen battery was fabricated. This battery was sealed in a C2 type case to obtain a battery to be tested. This battery is a positive electrode regulated battery having a large negative electrode capacity.

Table 2 shows the results of the following battery performance investigations using this battery. Initial activity The prepared positive electrode capacity-regulated Ni-hydrogen battery was charged at 80 mA / g (current for 1 g of hydrogen storage alloy supported on the negative electrode) at 25 ° C for 3 hours, then at 160 mA / g for terminal voltage.
Discharge to 0.9 V, repeat charging and discharging 10 times,
Measure the discharge capacity of the 1st time and the discharge capacity of the 10th time and
The initial activity was determined by (first discharge capacity / 10th discharge capacity × 100%). If the initial activity is 95% or more, it can be evaluated as passing.

Self-Discharge Characteristics (Capacity after Storage) Using the manufactured Ni-hydrogen battery, the battery was repeatedly charged and discharged 10 times under the same conditions as above, then charged at 80 mA / g for 3 hours, and then at 50 ° C. After 10 days of storage at 160 mA / g, the capacity when discharged to a terminal voltage of 0.9 V was measured and the ratio to the discharge capacity after 10 times of the above repeated charging / discharging (discharge capacity after storage / The discharge capacity after storage was determined by the 10th discharge capacity × 100%). If this value is 85% or more, it can be evaluated as a pass.

[0043]

[Table 2]

As can be seen from Table 2, in the comparative examples (Test Nos. 38 to 49) in which the acid solution used contained no organic solvent or contained less than 40% of organic solvent, the initial activity and storage None of the later discharge capacities reached the acceptable level. On the other hand, in the present invention examples (Test No. 1 to 37) treated with an acid solution containing 40% or more of an organic solvent, the activity was 95% or more and the discharge capacity after storage was 85% or more in any case. And the effect of adding an organic solvent is clear. When an organic solvent is added, not only the capacity after storage but also the initial activity is improved because the Ni hydroxide formed on the surface of the alloy powder,
It is thought that this is because it inhibits the battery reaction, though not as much as the oxide. Also, the better the content of the organic solvent, the better the results obtained, and if a larger amount of the organic solvent than the amount tested this time is included (e.g., water is not included at all, all the solvents are organic solvents). It is considered that further effects can be obtained.

[0045]

EFFECTS OF THE INVENTION According to the method of the present invention, before producing a battery, the hydrogen storage alloy is immersed in an acid solution containing an organic solvent so that the initial activation characteristics of the hydrogen storage alloy in the case of constructing the battery are improved. It is possible to achieve improvement and long-term storage stability by suppressing self-discharge.

Front page continuation (72) Inventor Kouki Takeshita 4-53-3 Kitahama, Chuo-ku, Osaka Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] 1. A method for treating a hydrogen storage alloy for a battery, comprising immersing a hydrogen storage alloy containing Ni in a non-oxidizing acid solution containing an organic solvent in a weight ratio of 40% or more.