JPH09259870A - Hydrogen absorbing alloy electrode, and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize hydrogen absorbing alloy electrode of high capacity, and of excellent cycle characteristics and high-rate discharge characteristics. SOLUTION: High crystalline carbon of a crystal face interval (d) of 3.45Å or less such as graphite is attached having a diffusion layer by mechanical stress of a planetary ball mill or the like to the surface of a hydrogen absorbing alloy. Peak of carbon exists in powder X-ray diffraction measurement of the hydrogen absorbing alloy to which the high crystalline carbon is attached, and a ration of carbon peak strength/hydrogen absorbing alloy main peak strength is set to be 1/2 or less to a strength ratio at the time of simple mixture.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a hydrogen storage alloy electrode used for a negative electrode of a nickel-hydrogen storage battery and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, with the development of portable equipment and cordless equipment, a battery as a power source thereof has been required to have higher energy density. In order to achieve this requirement, a nickel-hydrogen storage battery using a hydrogen storage alloy electrode as a negative electrode has been drawing attention and has been put into practical use today.

Among the hydrogen storage alloys capable of reversibly absorbing and releasing hydrogen, the ones currently in practical use are multi-elements of the Mm (mixed rare earth) -Ni system which are excellent in high rate discharge characteristics and storage characteristics. Of AB ₅ type alloys (eg MmMn _0.4 A)
1 _0.3 Co _0.7 Ni _3.6 ) is the center.

As a hydrogen storage alloy for a negative electrode of a nickel-hydrogen storage battery, one having a high capacity, a high electrode activity and an excellent cycle life is required, but rare earth metals and Z are required for increasing the capacity.
It is effective to increase the ratio of A site elements such as r, but addition of B site elements such as Ni and copper is effective in terms of electrode activity and cycle life. In order to achieve both these contradictory properties, the composition of hydrogen storage alloys and various surface treatments have been studied.

In the case of the mixed rare earth AB ₅ type alloy, these characteristics are well balanced, but in recent years, there has been a great demand from the equipment side for higher capacity, and a hydrogen storage alloy material with a large discharge capacity is desired. It is rare.

On the other hand, the theoretical hydrogen storage amount is M
CaNi ₅ alloy, AB ₂ type Laves phase alloy, solid solution type alloy of body-centered cubic structure (bcc) such as TiVNi type and Mg ₂ Ni etc. as hydrogen storage alloys that can be expected to have a higher capacity than m-Ni type AB ₅ type alloy The MgNi-based alloy is being studied (for example, Japanese Patent Laid-Open No. 6-228699).

[0007]

However, these high-capacity hydrogen storage alloys are not suitable for conventional Mm-Ni type AB ₅ alloys.
Although it is superior in terms of discharge capacity compared to those of type alloys, it has poor cycle characteristics, the problem that the discharge capacity drops to less than half in several tens of cycles, the electrode activity is low, and the high rate discharge characteristics are poor. is there. The main causes of these problems are that the hydrogen storage alloy is corroded by KOH of the electrolytic solution and that the amount of Ni in the alloy is small and electrochemical storage and release of hydrogen is difficult. In the case of the Mm-Ni AB ₅ type alloy, the composition is stable to alkali and has a large amount of Ni that is active in the electrode, and addition of Co suppresses pulverization and causes La oxide to corrode the electrolytic solution. Is also preventing. Therefore, it has excellent cycle characteristics and high rate discharge characteristics.

As measures against these problems, a method of plating Ni or Cu, which is stable even in a strong alkaline electrolyte, on the alloy surface (JP-A-2-79369), mechanical alloying of Ni fine powder, etc. (JP-A-64- No. 6366, Japanese Patent Laid-Open No. 3-
A method of coating an alloy surface according to Japanese Patent No. 155049) and a coating of carbonaceous material (Japanese Patent Laid-Open No. 61-185863) has been devised. However, the coating of the surface of the hydrogen storage alloy with Ni or Cu was effective in improving the cycle characteristics and the high rate discharge characteristics, but on the other hand, its specific gravity is large (8.9 for Ni), so if the added amount is too large, the capacity of Caused a decline.

On the other hand, the carbonaceous material produced by the method of immersing the hydrogen-absorbing alloy powder in sucrose and then firing or the sputtering method has low crystallinity (there is no carbon peak in powder X-ray diffraction measurement), and the conductivity is low. The hydrogen storage alloy coated with such a carbonaceous material has improved cycle characteristics due to its low temperature, but has little effect on improving discharge capacity and high rate discharge characteristics due to its poor current collecting property. It is also widely known to add highly crystalline carbon such as graphite to the negative electrode as a conductive material, but in this case, the graphite or the like does not cover the surface of the hydrogen storage alloy, but simply the hydrogen storage alloy particles. Since it only effectively works to improve the current collection property during the period, it is effective in improving the discharge capacity, but not in improving the high rate discharge characteristics and the cycle characteristics.

In view of the above problems, the present invention aims to improve a nickel-hydrogen storage battery having a high capacity and excellent cycle characteristics and high rate discharge characteristics, in which highly crystalline carbon such as graphite is strongly adhered to the surface of a hydrogen storage alloy (carbon (1) is diffused in the alloy), the cycle characteristics are improved, and since the conductivity is good, the current collecting ability is improved, and the discharge capacity and the high rate discharge characteristics are improved.

[0011]

The present invention is a hydrogen storage alloy electrode characterized by having a carbon diffusion layer on the surface of a hydrogen storage alloy. The present invention also provides an X-ray diffraction measurement of hydrogen storage alloy powder having carbon diffused,
A hydrogen storage alloy electrode comprising a hydrogen storage alloy having a carbon peak and having a carbon peak strength / hydrogen storage alloy main peak strength ratio of 1/2 or less of the strength ratio during simple mixing. Further, it is preferably characterized by being manufactured by using a highly crystalline carbon having a crystal plane spacing d value of 3.45 Å or less.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The hydrogen storage alloy electrode of the present invention comprises C
aNi ₅ type AB ₅ type alloy, AB ₂ Laves phase alloy, solid solution type alloy having a body-centered cubic structure or hydrogen storage alloy such as MgNi type alloy and highly crystalline carbon such as glite are mixed in a planetary ball mill or the like. Mechanical stress is applied to form a carbon diffusion layer. The carbon diffusion layer can also be obtained by heating in a non-oxidizing atmosphere, and the diffusion layer can be easily obtained by heating in a pressurized state.

In this case, as the highly crystalline carbon mixed with the hydrogen storage alloy (which indicates carbon having a crystal plane spacing d value of 3.7 Å or less), a crystal plane spacing d value of 3.45 Å or less is more preferable. It is valid. The particle size of the highly crystalline carbon attached to the surface of the hydrogen storage alloy is preferably 5 μm or less.

At this time, there is a carbon peak in the X-ray diffraction measurement of the hydrogen storage alloy powder in which carbon is diffused, but the carbon peak intensity / hydrogen storage alloy main peak intensity ratio is simply high crystalline carbon and hydrogen. It is 1/2 or less of the strength ratio just by mixing the storage alloy.

The surface of the hydrogen storage alloy is partially covered.
Particles of highly crystalline carbon in which bon has diffused adhere in a finely dispersed state. The highly crystalline carbon and the carbon diffusion layer thus adhered improve the current collecting property. It is considered that this improves the electrode activity and promotes the improvement of the discharge capacity and the high rate discharge characteristics.

Hydrogen storage alloy and 5 wt% or more and 30 wt% or more
Highly crystalline carbon such as graphite below is applied to the hydrogen storage alloy surface by applying mechanical stress using a ball mill, planetary ball mill or mechanofusion method in an inert gas to attach the highly crystalline carbon to the carbon. Part of the hydrogen-diffusing alloy diffuses into the alloy, and a hydrogen storage alloy electrode having high capacity and excellent cycle characteristics and high rate discharge characteristics can be obtained.

That is, the coating of the surface of the hydrogen storage alloy with a carbonaceous material having low crystallinity has been studied so far, but amorphous or low crystalline carbon is effective for improving the cycle characteristics by improving the corrosion resistance by coating. However, since the conductivity is as low as 1/1000 of that of the highly crystalline carbon and the current collecting property is not improved, the discharge capacity and the high rate discharge characteristics cannot be improved. Further, in the method of mixing graphite having high conductivity into the negative electrode paste as a conductive material, the current collecting property is improved and the discharge capacity is increased, but the bond between the hydrogen storage alloy and graphite is weak (there is no diffusion layer). Therefore, it did not become an active point, the electrode activity was not improved, and the high rate discharge characteristics could not be improved.
Further, the effect of covering the hydrogen storage alloy was small and the effect of improving the corrosion resistance was small, and the cycle characteristics were not improved.

Therefore, by making the surface of the hydrogen-absorbing alloy finer and more firmly attached to the surface of the hydrogen-absorbing alloy with highly crystalline carbon such as graphite having excellent conductivity, the corrosion resistance of the hydrogen-absorbing alloy can be greatly improved and the current collecting property can be improved. It becomes possible to improve the high-rate discharge characteristics by making the carbon be deposited on the surface of the hydrogen storage alloy to become an electrode active point.

Examples of the present invention will be described in detail below. (Example 1) Hydrogen storage alloys are commercially available Ti, V, Cr, L
a, Ni metal as raw material, Ti by arc melting method
A bcc alloy having a composition ratio of _0.3 V _0.43 Cr _0.13 La _0.05 Ni _0.09 was prepared. Next, this alloy was allowed to sufficiently occlude hydrogen, hydro-pulverized, and mechanically pulverized and classified to obtain hydrogen-occluding alloy particles of 45 μm or less.

90 g of the hydrogen-absorbing alloy particles thus prepared and 10 g of graphite having a particle size of about 10 μm (manufactured by Nippon Graphite Co., Ltd., artificial graphite SP-10, d value = 3.35 Å) were filled with nitrogen gas in a ball mill 10 After mixing for a while, graphite was allowed to adhere to the surface of the hydrogen storage alloy particles almost uniformly. As a result of electron microscopic (EPMA) observation, the particle size is 5μ.
It was found from the composition analysis of the cross section that a small amount of graphite fine particles of approximately m adhered to the surface of the hydrogen storage alloy almost uniformly, and that some of the adhered carbon had diffused into the alloy (about 0.1 μm). .

Further, FIG. 1 shows the result of powder X-ray diffraction measurement, and a peak due to carbon was recognized at around 27 °. However, the peak intensity of carbon at this time was higher than that of the simple peak mixture (Comparative Example 2) in which the same amount of graphite was mixed for 10 minutes in the mortar, based on the main peak of bcc alloy (around 40 °). 1 compared to peak intensity
It was about / 3. The cause of the decrease in peak intensity is considered to be that graphite was pulverized and the crystal plane was destroyed, and that carbon was diffused into the hydrogen storage alloy.

3% by weight of polyethylene powder as a binder was added to the hydrogen-absorbing alloy particles thus produced, and ethanol was added to form a paste, which had a porosity of 95%.
Fill a foamed nickel plate with a thickness of 0.6 mm, length 2 cm, width 2 cm, dry, pressurize at 800 kgf / cm ² , and heat in vacuum for 1 hour at 130 ° C to melt the binder and hydrogen. A storage alloy negative electrode was produced.

The procedure for producing an open system liquid-rich negative electrode regulated battery will be described below. A nickel lead wire was welded to the above negative electrode to prepare a negative electrode (alloy weight: about 1.0 g), and a positive electrode mixture containing a conventional positive electrode mixture containing nickel hydroxide as a main component was used as the positive electrode, and the porosity was 95%, like the negative electrode. Thickness 1mm, length 2cm, width 2
cm foamed nickel plate was filled and a nickel lead wire was welded to produce a positive electrode.

The two positive electrodes were each wrapped with a polypropylene (PP) separator having a thickness of 0.15 mm and made hydrophilic, and the negative electrode was similarly wrapped with the separator, and one negative electrode was sandwiched between two positive electrodes. The ones fastened with acrylic plates from both sides are put in a battery case, and a potassium hydroxide aqueous solution (density 1.30
An electrolytic solution (about 200 cc) containing g / cm ³ ) as a main component was poured into the battery case to prepare an open type liquid-rich battery in which the negative electrode capacity was regulated.

FIG. 2 shows the change in the discharge capacity per 1 g of the negative electrode alloy when the charge / discharge cycle was carried out at 20 ° C. for 5 hours at a charge of 0.1 A for discharge of 0.1 A and 0.8 V cut. Further, FIG. 3 shows high rate discharge characteristics, which is 0.1 A
When the discharge capacity is 1, the capacity ratio at each discharge current is shown.

Here, in Comparative Example 1, the negative electrode hydrogen storage alloy was not surface-treated, and in Comparative Example 2, the same amount of the graphite used in this Example was mixed in a mortar for 10 minutes. In Comparative Example 1, the maximum discharge capacity was as small as 320 mAh / g, the cycle characteristics were poor, and the high rate discharge characteristics were 0.5.
In A, almost no discharge was possible. In Comparative Example 2, the discharge capacity was about 340 mAh / g, and the cycle characteristics were slightly improved as compared with those of the untreated type, but they were not so remarkable, and the high rate discharge characteristics were hardly improved.

On the other hand, the maximum discharge capacity of this embodiment is 4
It was significantly improved to 50 mAh / g, the cycle characteristics were improved, and the high rate discharge characteristics were also improved. This is because graphite, which has excellent conductivity and corrosion resistance, coats the surface of the hydrogen storage alloy to improve the current collection performance and increase the discharge capacity, and the corrosion resistance improves the cycle characteristics, as well as the chemical bond with the alloy surface. It is considered that the electrode activity increased and the high rate discharge characteristics were also improved.

As in Comparative Example 2, since mechanical stress is weak with simple mixing, the crushing of graphite is not sufficient.
Further, no diffusion of graphite into the alloy was observed. Therefore, the conductivity was slightly improved and the discharge capacity was slightly improved, but the high rate discharge characteristics were hardly improved. It is considered that this is because graphite attached to the alloy surface with a diffusion layer serves as an active point to facilitate electrochemical hydrogen absorption / desorption.

Also, when the mixing time in the ball mill was set to 1 hour, a large improvement in capacity was not observed, as in Comparative Example 2. This is because the crystallinity of graphite itself is good, but the decrease rate of the carbon peak intensity based on the main peak intensity of the hydrogen storage alloy in powder X-ray diffraction after mixing is 1 /
It stayed at about 2. This is considered to be due to insufficient electrode activity due to insufficient pulverization and no diffusion of carbon into the alloy. When the mixing time was 6 hours, the peak intensity ratio of carbon was slightly lower than 1/2 of that in the simple mixing.

Since the high rate discharge characteristics of this alloy were considerably improved, the peak intensity ratio of carbon as seen on the basis of the main peak of the alloy in the powder X-ray diffraction measurement of the hydrogen storage alloy to which graphite after mixing was adhered. It was found that the electrode activity was not improved unless the ratio became less than 1/2 of that in simple mixing. Further, FIG. 4 shows the relationship between the graphite content rate and the maximum discharge capacity, and it was confirmed that the discharge capacity was improved in the range of 5 wt% or more and 25 wt% or less.

(Example 2) Mg powder having a particle size of 5 μm and carbonyl Ni having a particle size of several μm were ball-milled in an argon gas atmosphere for 24 hours to perform mechanical alloying to obtain a particle size of 1
First, a MgNi alloy powder of about 0 μm was prepared.

Next, 80 g of this alloy was mixed with carbon micro beads (PC-1028 manufactured by Nippon Carbon Co., d value = 3.
20 g of 40 Å) was added, and hybridization was carried out for 10 minutes using a planetary ball mill in an argon gas atmosphere to deposit highly crystalline carbon on the surface of the MgNi alloy. From EPMA observation, carbon with a particle size of about 2 μm was distributed almost uniformly on the surface of the alloy, and some carbon was diffused (about 0.1 μm) in the alloy.

An open system liquid-rich negative electrode regulated battery having the same structure as in Example 1 was produced. (Fig. 5) Charged at 20 ℃ 0.1
The change of the discharge capacity per 1 g of the negative electrode alloy when the charge / discharge cycle was performed under the conditions of discharge of 0.1 A and 0.8 V cut for 5 hours at A is shown. In Comparative Example 3, no treatment was performed, and in Comparative Example 4, Ketjen Black having a particle size of about 0.1 μm (low crystalline carbon, d value = 3.8Å) was added in the same amount by the same method.

In Comparative Example 3, the maximum discharge capacity was 380 mAh.
/ G, the cycle characteristics were poor, and in Comparative Example 4, the cycle characteristics were slightly improved, but the discharge capacity was 400 mA.
About h / g, it was almost the same as the untreated one.
On the other hand, the maximum discharge capacity of this example is 500 mAh /
g, and the cycle characteristics and high rate discharge characteristics were also improved.

FIG. 6 shows the relationship between the carbon microbead content and the maximum discharge capacity.
It was confirmed that the discharge capacity was improved in the range of 0 wt% or less. When the low crystalline carbon as in Comparative Example 4 is attached, the conductivity is lower than 1/1000 or less as compared with the crystalline carbon, so that the current collecting property is not improved and the discharge capacity is not improved so much. It was

(Embodiment 3) Predetermined amounts of Zr, Mn, V and C
After putting r and Ni into the crucible of the high frequency melting furnace and melting them,
Molten alloy is poured into a water-cooled mold, and the main alloy phase is C15 type Laves phase, AB ₂ type ZrMn.
_0.5 V _0.1 Cr _0.2 Ni _1.2 was produced. The alloy thus produced was heat-treated in vacuum at 1100 ° C. for 6 hours, and then mechanically pulverized and classified to obtain alloy particles having a particle diameter of 38 μm or less.

To 70 g of the hydrogen-absorbing alloy particles, 30 g of carbon microbeads (PC-1020 manufactured by Nippon Carbon Co., d value = 3.45 Å) was added, and the amount was 2 in an argon atmosphere.
Mechanofusion (made by Hosokawa Micron) was performed for 0 minutes to deposit carbon on the alloy surface. EPM
From the observation of A, it was found that carbon having a particle size of about 0.3 μm was distributed almost uniformly on the surface of the hydrogen storage alloy.

An open system liquid-rich negative electrode regulated battery having the same structure as in Example 1 was produced. (Fig. 7) Charged at 20 ℃ 0.1
The change of the discharge capacity per 1 g of the negative electrode alloy when the charge / discharge cycle was performed under the conditions of discharge of 0.1 A and 0.8 V cut for 5 hours at A is shown. In Comparative Example 5, no treatment was applied, and in Comparative Example 6, Ketjen Black was added in the same amount by the same method.

In Comparative Example 5, the maximum discharge capacity is 360 mAh.
/ G, the cycle characteristics were poor, and in Comparative Example 6, the cycle characteristics were slightly improved, but the discharge capacity was 380 mA.
About h / g, it was almost the same as the untreated one.
On the other hand, the maximum discharge capacity of this example is 410 mAh /
g, and the initial activity and cycle characteristics were also improved. Also, the high rate discharge characteristics were improved. FIG. 8 shows the relationship between the content rate of carbon microbeads and the maximum discharge capacity. The improvement of the discharge capacity was observed in the range of 10 wt% or more and 30 wt% or less.

Example 4 Predetermined amount of Ca (particle size 25 μm
m) and Ni (particle size: several μm) were mixed in argon using a ball mill for 20 hours in the same manner as in Example 2 to produce CaNi ₅ alloy particles having a particle size of about 35 μm.

Next, 20 g of graphite (SP-10 manufactured by Nippon Graphite Co., Ltd.) was added to 80 g of the hydrogen storage alloy particles,
The mixture was mixed using a planetary ball mill for 8 minutes in an argon atmosphere. From EPMA observation, it was found that carbon having a particle size of about 1 μm was distributed almost uniformly on the alloy surface, and carbon was diffused into the alloy by about 0.2 μm by qualitative analysis of the cross section.

An open system liquid-rich negative electrode regulated battery was produced from the alloy powder thus produced in the same structure as in Example 1. As shown in FIG.
4 shows changes in discharge capacity per 1 g of the negative electrode alloy when a charge / discharge cycle was performed under the conditions of 1 A and 0.8 V cut. Comparative Example 7 is one in which graphite was not added.

In Comparative Example 7, the discharge capacity in the first cycle was 3
Although it was considerably large at about 80 mAh / g, the cycle characteristics were extremely poor. It is considered that this is because Ca in the alloy reacted with the electrolytic solution. On the other hand, in the case of this example, the maximum discharge capacity was slightly improved to 400 mAh / g, and the cycle characteristics were also greatly improved.

As described above, by adhering graphite having a d value of 3.45 Å or less to the surface of the hydrogen storage alloy of 5 wt% or more and 30 wt% or less, an electrode having a high capacity and excellent cycle characteristics and high rate discharge characteristics was obtained. . At this time, if the carbon peak intensity / hydrogen storage alloy main peak intensity ratio in the powder X-ray diffraction measurement of the hydrogen storage alloy to which the highly crystalline carbon is adhered is not less than 1/2 of the intensity ratio during simple mixing, the effect is obtained. It was found that when high crystalline carbon having a d value of 3.5 Å or more was used, which did not appear remarkably, the conductivity was slightly low, and therefore the characteristic improving effect (particularly, the discharge capacity) was slightly lowered. Further, when the particle size of carbon after the mixing treatment was larger than 5 μm, the mixing and adhesion were not sufficient, and the high rate discharge characteristics were hardly improved.

[0045]

As is clear from the above examples, the hydrogen storage alloy electrode of the present invention has a higher current collecting ability than conventional hydrogen storage alloy electrodes, improved corrosion resistance and electrode activity, high capacity, and cycle characteristics. And an electrode excellent in high rate discharge characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the results of powder X-ray diffraction measurements of Example 1 and Comparative Example 2.

FIG. 2 is a diagram showing a change in discharge capacity per 1 g of a negative electrode alloy during charge / discharge cycles of Example 1 and Comparative Examples 1 and 2.

FIG. 3 is a diagram showing high rate discharge characteristics of Example 1 and Comparative Examples 1 and 2.

FIG. 4 is a graph showing the relationship between the graphite content and the maximum discharge capacity in Example 1.

FIG. 5 is a graph showing changes in discharge capacity per 1 g of negative electrode alloy in charge / discharge cycles of Example 2 and Comparative Examples 3 and 4.

FIG. 6 is a graph showing the relationship between the carbon microbead content and the maximum discharge capacity in Example 2.

FIG. 7 is a graph showing changes in discharge capacity per 1 g of negative electrode alloy during charge / discharge cycles of Example 3 and Comparative Examples 5 and 6.

8 is a diagram showing the relationship between the carbon microbead content and the maximum discharge capacity in Example 3. FIG.

FIG. 9 is a graph showing changes in discharge capacity per 1 g of negative electrode alloy during charge / discharge cycles of Example 4 and Comparative Example 7.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toyoguchi ▲ Yoshi ▼ No. 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. Issue (72) Inventor Chiaki Iwakura 3-3-4-105 Shin-Hinodai, Sakai City, Osaka Prefecture

Claims (5)

[Claims] 1. A hydrogen storage alloy electrode, comprising a carbon diffusion layer on the surface of the hydrogen storage alloy. 2. X of a hydrogen storage alloy powder in which carbon is diffused
A hydrogen storage alloy comprising a hydrogen storage alloy having a carbon peak in the line diffraction measurement and having a carbon peak intensity / hydrogen storage alloy main peak intensity ratio of 1/2 or less of the intensity ratio during simple mixing. electrode. 3. Highly crystalline carbon is added to the hydrogen storage alloy particles.
A method for producing a hydrogen storage alloy electrode, which comprises a hydrogen storage alloy in which a carbon is diffused on the surface of the hydrogen storage alloy by adding mechanical stress to the hydrogen storage alloy in an amount of 30 wt% or more. 4. The crystal plane spacing d value of highly crystalline carbon is 3.
The method for producing a hydrogen storage alloy electrode according to claim 3, wherein the method is 45 Å or less. 5. The method for producing a hydrogen storage alloy electrode according to claim 3, wherein mechanical stress is applied by using a ball mill, a planetary ball mill, or a mechanofusion method in an inert gas atmosphere.