JP2925604B2 - Processing method of hydrogen storage alloy for alkaline secondary battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for treating a hydrogen storage alloy used for a negative electrode of an alkaline secondary battery.

2. Description of the Related Art In recent years, as a new alkaline secondary battery replacing a nickel-cadmium secondary battery using cadmium for the negative electrode, research and development of a nickel-metal hydride battery using a hydrogen storage alloy for the negative electrode have been actively performed. The nickel-hydrogen battery can have a long life and a high energy density by selecting the composition type of the hydrogen storage alloy for the negative electrode.

However, the hydrogen storage alloy is susceptible to surface oxidation during the alloy pulverization step and the electrode preparation step, and a dense oxide film is formed on the surface particularly during the pulverization. And
When the hydrogen storage alloy powder on which such a dense oxide film is formed is used as an electrode, the initial activation of the alloy is impaired or the electrical conductivity of the electrode is reduced, and the charge and discharge during rapid charge and discharge are performed. Problems such as a decrease in efficiency will occur.

Therefore, the following methods have been proposed to suppress oxidation in the alloy pulverizing step and the electrode manufacturing step.

A method in which both of the above steps are performed under an inert atmosphere.

As disclosed in JP-A-61-285658, a method in which a pulverized hydrogen-absorbing alloy is treated with an aqueous alkali solution to remove in advance metals that are easily dissolved on the alloy surface.

Problems to be Solved by the Invention However, in the above-described method, the steps are complicated and productivity is reduced, so that the manufacturing cost of the battery is increased.

In addition, the above-mentioned method has a problem that a dense oxide film cannot be sufficiently removed due to the chemical properties of the oxide, and the initial activation of the alloy and the electrical conductivity of the electrode cannot be sufficiently improved. Had.

The present invention has been made in view of such circumstances, and a hydrogen storage alloy for an alkaline secondary battery capable of sufficiently improving the initial activation of the alloy and the electrical conductivity of the electrode without lowering the productivity. It is an object to provide a processing method.

Means for Solving the Problems In order to achieve the above object, the present invention provides a first step of preparing a hydrogen storage alloy and then pulverizing the hydrogen storage alloy, and converting the pulverized hydrogen storage alloy into hydrochloric acid, fluorine, and phosphoric acid. And a third step of treating the hydrogen storage alloy after completion of the acid treatment with an alkaline aqueous solution, wherein the second step is a treatment with at least one acidic aqueous solution selected from boric acid.

Action As shown in the above second step, if the pulverized hydrogen storage alloy is treated with an acidic aqueous solution, the dense oxide film formed on the alloy surface during the pulverization of the alloy in the first step can be sufficiently reduced due to its chemical properties. Will be removed.

Further, as shown in the third step, if the hydrogen storage alloy after the acid treatment is treated with an alkaline aqueous solution, the alloy surface is covered with a porous film mainly composed of hydroxide. Thereby, even if the alloy is subsequently exposed to air, a dense oxide film is not formed on the alloy surface. Since the porous film is very similar to a film formed in the battery, the electrochemical activity of the battery is not impaired even when used as a negative electrode.

[Example I]

First, using commercially available misch metal Mm (a mixture of rare earth elements such as La, Ce, Nd, and Pr), Ni, Co, and Mn, Mm: Ni:
It was weighed and mixed so that Co: Mn was 1: 3: 1.25: 0.75.
Next, this mixture was melted in an arc furnace with an inert atmosphere of argon to prepare an alloy represented by MmNi ₃ Co _1.25 Mn _0.75 . Next, the alloy is mechanically pulverized to a particle size of 30 μm or less, and the alloy powder is treated in a hydrochloric acid aqueous solution having a pH of 3 for about 8 hours while stirring. Thereafter, after removing the supernatant, the mixture is treated with an excess amount of an aqueous KOH solution at pH = 15. Thereafter, the treated alloy powder is washed with pure water and dried.

Thereafter, polytetrafluoroethylene (PTFE) as a binder was added to the alloy powder that had been subjected to the above treatment, and these were kneaded to prepare a paste. Next, this paste is pressed against both surfaces of the current collector to form a hydrogen storage alloy negative electrode (hereinafter abbreviated as a hydrogen electrode). The hydrogen electrode and a known sintered nickel positive electrode (capacity: 0.6 Ahr) Was wound through a separator made of a non-woven fabric to produce an electrode body.
Next, after inserting the electrode body into the battery can, an electrolyte (30 wt% KOH solution) is injected into the battery can. Finally, the battery can was sealed to produce a sealed nickel-hydrogen battery.

The battery fabricated in this manner is hereinafter referred to as (A ₁ ) battery.

(Examples II and III)

As shown in Table 1 below, a battery was prepared in the same manner as in Example I except that phosphoric acid and hydrofluoric acid were used as the acidic solution.

The batteries fabricated in this manner are referred to below as (A ₂ )
Battery, referred to as (A ₃ ) battery.

[Comparative Example I]

As shown in Table 1 below, a battery was produced in the same manner as in Example I above, except that no acid or alkali treatment was applied after the pulverization of the hydrogen storage alloy.

The battery fabricated in this manner is hereinafter referred to as (X ₁ ) battery.

(Comparative Examples II and III)

As shown in Table 1 below, a battery was fabricated in the same manner as in Example I, except that the alkali treatment was not performed after the pulverization of the hydrogen storage alloy. In Comparative Example III, the pH of the acid was set to 3 as in Example I, but in Comparative Example II, the pH of the acid was set to 1.

The batteries fabricated in this way are referred to below as (X ₂ )
Cell, referred to as (X ₃₎ batteries.

(Comparative Examples IV to VI)

As shown in Table 1 below, a battery was produced in the same manner as in Example II except that the alkali treatment was not performed after the pulverization of the hydrogen storage alloy. In Comparative Example V, the pH of the acid was set to 3 as in Example II. However, Comparative Example IV and Comparative Example VI
In, the pH of the acid is set to 1 and 6, respectively.

The batteries fabricated in this manner are referred to below as (X ₄ )
Battery, (X ₅₎ cell, referred to as (X ₆₎ cells.

(Comparative Examples VII to IX)

As shown in Table 1 below, a battery was manufactured in the same manner as in Example III except that the alkali treatment was not performed after the pulverization of the hydrogen storage alloy. In Comparative Example VIII, the pH of the acid was set to 3 as in Example III, but in Comparative Examples VII and IX, the pH of the acid was set to 1 and 6, respectively.

Thus the battery thus fabricated is hereinafter respectively (X ₇₎
Battery, (X ₈₎ cell, referred to as (X ₉₎ cells.

[Experiment I] The number of cycles and the batteries in the (A ₁ ) battery using the hydrogen storage alloy treated according to the present invention and the (X ₁ ) battery using the hydrogen storage alloy not treated with acid and alkali. The relationship between the capacity and the decrease in battery weight was examined, and the results are shown in FIG. The experimental conditions were such that the hydrogen storage alloy was charged at a current per unit weight of 250 mA to a charge amount of 0.72 Ah, and then discharged until the battery voltage reached 1.0 V.

As apparent from FIG. 1, the battery capacity of the (X ₁ ) battery in the first cycle is small, whereas the battery capacity of the (A ₁ ) battery is 1
From the cycle, it is recognized that the battery capacity has increased. This indicates that the activation of the alloy in the (A ₁ ) battery has progressed rapidly from the beginning.

In addition, when the charge and discharge cycle is repeated, (X ₁ )
In the case of the battery, the battery capacity is significantly reduced and the battery weight is also significantly reduced, whereas in the case of the (A ₁ ) battery, the battery capacity is not significantly reduced and the battery weight is only slightly reduced. Is recognized. This indicates that the charge / discharge efficiency of the negative electrode during charge / discharge is improved in the (A ₁ ) battery.

(Experiment II)

(A ₁ ) to (A ₃ ) batteries using the hydrogen storage alloy treated according to the present invention, (X ₁ ) batteries using the hydrogen storage alloy not subjected to acid and alkali treatment, only alkali treatment (X ₂ ) to (X ₉ ) using hydrogen-absorbing alloys that were not subjected to hydrogen storage, the battery weight loss was investigated.
The results are shown in Table 1 above. Note that the value of the battery weight reduction amount is the value at the 100th cycle.

As is clear from Table 1, the battery weight reduction of (A ₁ ) to (A ₃ ) is 0.01 to 0.03 g,
(X ₁ ) battery: 0.54 g, acid-treated (X ₂ ) battery ~
(X ₉ ) It is also recognized that the battery weighs 0.06 to 0.14 g.
As described above, in the (A ₁ ) battery to the (A ₃ ) battery, since the battery weight reduction amount is significantly reduced, it can be seen that the charge / discharge efficiency is increased.

In addition, the battery using the hydrogen storage alloy subjected to the acid treatment with nitric acid or sulfuric acid is the same as the acid (hydrochloric acid,
(Phosphoric acid, hydrofluoric acid), the amount of weight loss of the battery is increased. Although most of the anions are removed by the alkali treatment, the anions may be slightly mixed into the battery system as impurities. In this case, for example, nitric acid, with sulfuric acid, NO ₃ ^-, self-discharge occurs in the cell by SO ₄ ^2-ions. As a result, in the battery obtained by using nitric acid and sulfuric acid, the amount of reduction in battery weight increases. Therefore, acid composed of ions that do not adversely affect the battery system as the acid,
That is, it is necessary to use hydrochloric acid, phosphoric acid, hydrofluoric acid or boric acid which is not shown in the above-described embodiments but has been confirmed to have the same effect.

Further, from the results of the above example, the pH of the acidic aqueous solution is preferably in the range of 1 to 6.

Furthermore, as an alkaline aqueous solution, it can be easily handled.
Those having a pH of about 8 to 16 are preferred. As the alkaline aqueous solution, an aqueous solution mainly composed of potassium hydroxide, which is generally used as an electrolyte for a battery, is desirable.

In addition, the hydrogen storage alloy is not limited to the one shown in the above-described embodiment, and it goes without saying that the present invention can be applied to alloys of all compositions.

Effects of the Invention As described above, according to the present invention, hydrogen-absorbing alloy powder is subjected to acid treatment with at least one acidic aqueous solution selected from hydrochloric acid, hydrofluoric acid, phosphoric acid, and boric acid,
During the pulverization, a dense oxide film formed on the alloy surface is removed. Therefore, if the electrode is composed of such powder,
Since the initial activation can be sufficiently achieved, the contact resistance between the particles is reduced, and the electric conductivity of the electrode is improved, so that the charging / discharging efficiency at the time of rapid charging can be improved.

In addition, after the acid treatment is completed, an alkali treatment is performed to remove anions, but a porous hydroxide film is formed on the surface of the alloy. Therefore, even if the alloy is subsequently exposed to air, a dense oxide film is not formed on the alloy surface, and the productivity can be improved.

In addition, both processes do not need to be performed in an inert atmosphere, so that the productivity can be improved from this point as well.

[Brief description of the drawings]

FIG. 1 shows the number of cycles and the number of batteries in the battery (A ₁ ) using the hydrogen storage alloy treated according to the present invention and the battery (X ₁ ) using the hydrogen storage alloy not treated with acid or alkali. 5 is a graph showing a relationship between capacity and battery weight reduction.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Mikiro Tadokoro 2-18-18 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (56) References JP-A-64-54669 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) H01M 4/38 H01M 4/24-4/26

Claims (1)

(57) [Claims] A first step of pulverizing the hydrogen storage alloy after preparing the hydrogen storage alloy; and removing the pulverized hydrogen storage alloy from at least one acid selected from hydrochloric acid, hydrofluoric acid, phosphoric acid and boric acid. A method for treating a hydrogen storage alloy for an alkaline secondary battery, comprising: a second step of treating with an aqueous solution; and a third step of treating the hydrogen storage alloy after completion of the acid treatment with an alkaline aqueous solution.