JPH10125350A - Alkaline secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain corrosion of an electrode base material at the time of overcharge, and extend a battery service life by applying the constitution that salt composed of a strong base and a weak acid as well as soluble in an electrolyte is contained in the coat layer of a battery positive electrode, or in an alkaline electrolyte. SOLUTION: A coated layer containing an electrode active material, such as nickel hydroxide, is formed on the surface of an electrode base material such as nickel, and a positive electrode is thereby prepared. In an alkaline secondary battery containing the positive electrode so prepared and an alkaline electrolyte, at least one of the coated layer and the alkaline electrolyte is made to contain salt, composed of strong base and weak acid as well as soluble in the alkaline electrolyte. In this case, at least one type of sodium acetate, potassium phosphate and sodium formate is preferable as the salt. According to this construction, H<+> -ions which increased on the reaction surface of the positive electrode are arrested by the salt in an overcharged state and made into a stable weak acid, thereby preventing the corrosion of the electrode base material due to the H<+> -ions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alkaline secondary battery such as a nickel-cadmium storage battery and a Ni-MH storage battery, and more particularly to an alkaline secondary battery having a long life.

[0002]

2. Description of the Related Art Alkaline secondary batteries are widely used as power sources for various portable devices and industrial devices because of their high reliability, small size, light weight and high capacity. The anode of this alkaline secondary battery has
A nickel hydroxide electrode having an active material mainly composed of nickel hydroxide is used.

[0003] The nickel hydroxide electrode is manufactured by, for example, immersing a porous metal base material serving as a current collector in an aqueous solution of an acidic nickel salt such as nickel nitrate and then immersing it in an alkaline aqueous solution. This method is generally called an immersion method. According to this manufacturing method, nickel hydroxide impregnated in the pores of the porous metal substrate comes into contact with the alkali to generate nickel hydroxide, and the active material is filled in the pores of the porous metal substrate. It becomes possible.

However, in the above-mentioned immersion method, it is difficult to sufficiently fill the active material with a single process, so that the above process must be repeated several times to obtain a desired capacity, and the number of steps becomes large. There was a problem. Therefore, as another method of filling the active material, for example,
Japanese Patent Application Publication No. JP-A-2005-17795 discloses that an active material mainly composed of nickel hydroxide is made into a paste together with a binder such as carboxymethyl cellulose (CMC) and a solvent, and the paste is made of a porous metal electrode using a spatula or an applicator. Describes a method of physically filling the material. This method is called a paste filling method, in which a larger amount of active material can be easily filled in the pores than in the above-described immersion method, and a high-capacity electrode can be easily manufactured.

[0005] As a material of the electrode substrate, nickel which is stable in an alkaline solution as an electrolytic solution and has high corrosion resistance is generally used. For example, JP-A-59-1
Japanese Patent No. 28766 discloses an anode plate for an alkaline battery in which a sponge-like porous nickel body is filled with an active material mainly composed of nickel hydroxide powder and cobalt powder.

[0006]

However, in the conventional alkaline secondary battery, there is a case where corrosion occurs on the electrode substrate made of nickel during use, and there is a problem that the life is shortened due to a decrease in conductivity. Was. The reason why corrosion occurs in the electrode substrate made of nickel as described above is explained as follows.

[0007] For example, the voltage at the electrode in a Ni-MH battery is
The gas-chemical reaction is represented by the following equation. (1) During charging Anode: Ni (OH)_Two+ OH^- → NiOOH +
 H_TwoO + e^- Cathode: M + H⁺ + E^- → MH (2) During discharge Anode: NiOOH + H_TwoO + e^- → Ni
(OH)_Two+ OH^- Cathode: MH → M + H⁺ + E^- In other words, when charging, as shown in the above formula, OH^-Disappears
Spent at the reaction interface ⁺The concentration increases. Therefore
In the charged state, H⁺Concentration increases significantly,
As shown schematically in FIG.⁺1 is nickel
Attacks the Ni electrode substrate 2 to remove electrons and_TwoBecome
As a result, the Ni electrode substrate 2 is corroded. Also,
The electrochemical reaction of corrosion involves impurities in the alkaline electrolyte.
NO that exists_Three ^-Promoted by strong acid ions such as
I know that.

Therefore, when the battery is used for a long time in an overcharged state, corrosion of the electrode substrate proceeds, and the life of the battery is shortened due to a decrease in conductivity. The present invention has been made in view of such circumstances, and an object of the present invention is to suppress the corrosion of an electrode substrate during overcharge and prolong the life of a battery.

[0009]

According to a first aspect of the present invention, there is provided an alkaline secondary battery comprising an electrode substrate and an anode comprising a coating layer formed on the surface of the electrode substrate and containing an electrode active material. And at least one of the coat layer and the alkaline electrolyte contains a salt composed of a strong base and a weak acid and soluble in the alkaline electrolyte.

[0010] A feature of the alkaline secondary battery according to claim 2 is that the alkaline secondary battery according to claim 1 is characterized in that:
The salt is to be at least one of sodium acetate, potassium phosphate and sodium formate.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION As an electrode substrate, a sintered metal body in the form of fibers or powder, or a porous metal substrate made of foamed metal or the like can be used. The material of the electrode substrate is not particularly limited as long as it is a conductive material that functions as a current collector. In the case of an electrode for an alkaline battery, nickel hydroxide is generally used as an active material. It is preferable to use nickel which has an excellent affinity for the compound.

Nickel hydroxide (Ni (OH) ₂ ) is generally used as the anode active material powder. It is also preferable to include a cobalt compound such as metal cobalt, cobalt oxide (CoO), or cobalt fluoride (CoF) together with the active material. Due to the presence of metallic cobalt or a cobalt compound, cobalt hydroxide and CoOOH are generated when charge and discharge are repeated, and the electrical contact between the porous metal electrode base material and the active material is improved, so that a so-called conductive network is formed. Thereby, the utilization efficiency of the active material is improved.

The anode active material is generally made into a paste and coated on an electrode substrate with an applicator or a spatula to form a coat layer. As the alkaline electrolyte, an aqueous solution of potassium hydroxide (KOH) is generally used. Cadmium, zinc, iron, manganese or the like is used as the cathode. The greatest feature of the present invention is that at least one of the coat layer and the alkaline electrolyte contains a salt composed of a strong base and a weak acid and soluble in the alkaline electrolyte.
H ⁺ generated during overcharging can be captured by this salt, and corrosion of the electrode substrate due to H ⁺ attack can be prevented.

That is, since this salt is a salt of a strong base and a weak acid, the dissociation constant of the salt is larger than the dissociation constant of the weak acid, and the weak acid ions are combined with H ^{+ in the} alkaline electrolyte to become a weak acid and become stable. Try to make it. Thereby, H ⁺ in the electrolytic solution can be captured. For example, when sodium acetate is used as such a salt, a reaction represented by the following formula (A) occurs, and the weak acid ion (CH ₃ COO ⁻ ) absorbs H ⁺ to become a weak acid (CH ₃ COOH). Thus, corrosion of the electrode substrate is prevented.

CH ₃ COONa + H ⁺ → CH ₃ COOH + Na ⁺ (A) Since the weak acid formed by absorbing H ⁺ is a weak acid, it does not significantly affect the alkaline electrolyte and is almost harmless. And stably exist in the alkaline electrolyte. Such a salt is not particularly limited as long as it is a salt of a strong base and a weak acid and can be dissolved in an alkaline electrolyte, but has a high solubility in an electrolyte such as sodium acetate, potassium phosphate, and sodium formate. Are preferred, with sodium acetate being the most effective. This salt may be used alone or in combination of two or more.

The addition amount of this salt can provide a certain effect if it is added as little as possible, but when it is contained in the coat layer, it is preferably in the range of 1 to 5 parts by weight with respect to 100 parts by weight of the active material. An addition amount of about 3 parts by weight is particularly desirable.
If the amount of the salt added is less than 1 part by weight, the amount of H ⁺ trapped is small and it may be difficult to prevent corrosion of the electrode substrate.
If the amount is more than 5 parts by weight, the effect is saturated, and if the amount is more than the saturation concentration, the amount of H ⁺ trapped decreases, and the battery capacity may decrease due to a relative decrease in the amount of active material.

In order to include this salt in the coat layer, a method of impregnating a salt solution with an aqueous solution of the salt after forming the coat layer, mixing the salt with the paste of the active material and applying it to the electrode base material, And the like. In the latter method, the paste may be formed with a solvent in which the salt such as alcohol does not dissolve, and the salt may be contained in a solid state, or the salt may be contained in a dissolved state using water.

The salt may be contained in a state of being dissolved in an alkaline electrolyte. In this case, the amount of addition
The concentration in the alkaline electrolyte is preferably in the range of 0.1 to 5 mol / L, and particularly preferably in the range of 2 to 3 mol / L.

[0019]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) 93 parts by weight of nickel hydroxide powder, 4.8 parts by weight of cobalt oxide powder, 1.9 parts by weight of metal cobalt, 1.5 parts by weight of carboxymethyl cellulose (CMC), and ethanol dispersion of carbon black 11 parts by weight of a product (carbon black concentration 10% by weight) and 3 parts by weight of sodium acetate were mixed well to prepare an anode active material paste.

Next, an electrode substrate made of foamed nickel (thickness 1.6 mm, width 50 mm, length 50 mm, porosity 96
% By volume), and after filling the above paste with a spatula, excess paste on the surface was removed. And 60 ° C
And dried by heating for 5 hours, and then press-molded to a thickness of 0.6 mm using a roller press to form an anode. The anode is filled with 74% by weight of the anode active material. Sodium acetate is contained in the anode active material in an amount of 3% by weight.

The obtained anode was combined with a general-purpose paste-type cadmium cathode, and an alkaline battery was prepared using a 6N aqueous KOH solution as an alkaline electrolyte. Then, in an atmosphere of 25 ° C., the battery is charged with a current of 1 C for 1.5 hours,
A charge / discharge pattern for discharging to 1.0 V by a current of 1 C was defined as one cycle, and this was repeated a plurality of cycles.
At this time, the 5-hour rate capacity of the battery was measured every 10 cycles, and the point in time when the 5-hour rate capacity was less than 80% was determined as the battery life. The change in capacity with respect to the number of charge / discharge cycles at this time is shown in FIGS.

(Example 2) An anode was produced in the same manner as in Example 1 except that the content of sodium acetate in the anode active material was 1% by weight, and an alkaline battery was produced in the same manner.
Then, similarly, the change in capacity with respect to the number of charge / discharge cycles was measured, and the results are shown in FIG. Example 3 An anode was produced in the same manner as in Example 1 except that the content of sodium acetate in the anode active material was changed to 5% by weight, and an alkaline battery was produced in the same manner. Then, similarly, the change in capacity with respect to the number of charge / discharge cycles was measured, and the results are shown in FIG.

Comparative Example An anode was produced in the same manner as in Example 1 except that sodium acetate was not contained in the anode active material, and an alkaline battery was produced in the same manner. Similarly, the change in capacity with respect to the number of charge / discharge cycles was measured, and the results are shown in FIGS. 1 and 2. (Evaluation) From FIG. 1, it is apparent that each of the examples including sodium acetate in the anode active material has a larger number of cycles until the capacity becomes 80% and a longer life than the comparative example. .

However, the degree differs depending on the content of sodium acetate, and the life of Example 2 containing 1% by weight is improved by 5% as compared with the comparative example, and Example 1 containing 3% by weight is compared with the comparative example. The life was improved by 20%, and the life of Example 3 containing 5% by weight was improved by 8% as compared with the comparative example. In other words, it can be understood that the effect of extending the life is maximized when the amount of sodium acetate added is about 3% by weight, and that the effect is reduced when the amount is less or more.

Example 4 An anode was prepared in the same manner as in Example 1 except that 3% by weight of potassium phosphate was contained in place of sodium acetate in the anode active material, and an alkaline battery was similarly prepared. . Then, similarly, a change in capacity with respect to the number of charge / discharge cycles was measured, and the results are shown in FIG.

Example 5 An anode was produced in the same manner as in Example 1 except that sodium formate was contained in the anode active material in an amount of 3% by weight instead of sodium acetate, and an alkaline battery was produced in the same manner. Then, similarly, a change in capacity with respect to the number of charge / discharge cycles was measured, and the results are shown in FIG.

(Evaluation) As shown in FIG. 2, all of sodium acetate, potassium phosphate and sodium formate are effective in extending the life. However, sodium acetate has the greatest effect of extending the life of 20%, followed by sodium formate of 14%. It can be seen that the effect decreases in this order, with potassium phosphate being 11%. Therefore, it is understood that it is particularly preferable to use sodium acetate.

[0028]

According to the alkaline secondary battery of the present invention, the electrode substrate is prevented from being corroded at the time of overcharging, so that a long life is achieved. In addition, if a salt consisting of a strong base and a weak acid is included in the coating layer of the anode, elution of the salt into the alkaline electrolyte causes the coating layer to become porous and increase the surface area, thereby improving the utilization of the active material. Output is improved.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the number of charge / discharge cycles and the battery capacity.

FIG. 2 is a graph showing the relationship between the number of charge / discharge cycles and the battery capacity.

FIG. 3 is a schematic explanatory view showing a corrosion mechanism of a conventional electrode substrate.

[Explanation of symbols]

 1: H + 2: Electrode substrate

Claims (2)

[Claims] 1. An alkaline secondary battery comprising: an anode composed of an electrode substrate and a coating layer formed on the surface of the electrode substrate and containing an electrode active material; and an alkaline electrolyte, wherein the coating layer and the alkaline electrolyte Wherein at least one of the above comprises a salt comprising a strong base and a weak acid and soluble in the alkaline electrolyte. 2. The alkaline secondary battery according to claim 1, wherein the salt is at least one of sodium acetate, potassium phosphate and sodium formate.