JPH0685325B2 - Active material for nickel electrode, nickel electrode and alkaline battery using the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to an active material for a nickel electrode, a nickel electrode, and an alkaline battery using the same.

Conventional technology and its problems In general, an alkaline battery using a nickel electrode as a positive electrode is called a sintered battery, and is a microporous substrate made of sintered nickel powder on a perforated steel plate filled with nickel hydroxide. . This type of electrode is very complicated because the filling process is repeated many times, and the cost is high. Moreover, since the porosity of the substrate used is limited, the packing density of the active material is low, and only an electrode having an electrode energy density of 400 mAh / cc can be manufactured.

Non-sintered electrodes have been widely developed as an attempt to improve this. For example, cobalt hydroxide-coated nickel hydroxide powder is mixed with 20% by weight of graphite powder as a conductive additive to form a sheet, which is then pressure-bonded to a nickel plate serving as a current collector to form an electrode. Since the conductive additive itself does not contribute to the capacity of the electrode, the capacity density decreases,
In addition, a large amount of carbonate radicals are generated due to the decomposition of graphite. For this reason, it cannot be used in a battery with a small amount of electrolyte, such as a sealed nickel-cadmium battery. In order to overcome the above drawbacks, a paste type nickel electrode using a metal fiber substrate having a high porosity of 95% is being put to practical use. The electrode was prepared by adding a nickel hydroxide powder active material prepared from a nickel sulfate aqueous solution and a sodium hydroxide aqueous solution to a CoO powder that forms a conductive network between active materials, and dissolving carboxymethyl cellulose in water. It is prepared by adding a liquid preparation and filling a fiber substrate in a paste state. This type of electrode is cheaper than a sintered electrode and has a high energy density of 500 mAh / cc.

However, as the weight of portable electronic devices has been reduced in recent years, a high energy density of about 600 mAh / cc is required as a market need. To address this,
Due to the limited porosity of the substrate, it is necessary to densify the nickel hydroxide powder itself. The high-density nickel hydroxide powder is used as a part of the raw material for the parkarizing treatment of iron plates. The production method is to dissolve nickel nitrate or nickel sulfate in a weakly basic aqueous ammonia solution to stabilize it as a nickel ammine complex ion, and gradually add sodium hydroxide aqueous solution so that pores do not develop inside the particles. It is deposited as nickel hydroxide.

In this system, unlike the conventional neutralization method, since the disordered precipitation is not carried out, a high-density nickel hydroxide having few grain boundaries and high crystallinity (small pore volume) is obtained.

However, due to this peculiar physical property, there are some problems in using this powder as an active material for a battery.

For example, the charge / discharge reaction of the nickel hydroxide electrode occurs due to free movement of protons in the nickel hydroxide crystal. However, as the density of nickel hydroxide increases, the crystal becomes denser, which limits the freedom of proton transfer within the crystal. Moreover, the current density increases due to the decrease in the specific surface area, and a large amount of higher-order oxide γ-NiOOH is generated, which causes discharges such as two-stage discharge and electrode swelling, and deterioration of life characteristics or a decrease in utilization rate. Cause. Γ-NiOOH of nickel electrode which is a fatal factor of electrode
The swelling mechanism associated with generation is from high density β-NiOOH to low density γ
-This is due to the change in density to NiOOH. γ-NiOOH
The present inventors have already found that a small amount of cadmium is added as a solid solution to nickel hydroxide as an effective means for preventing the formation of methane, but effective additives other than cadmium are desired from the viewpoint of pollution.

OBJECT OF THE INVENTION The present invention densifies nickel hydroxide powder to a higher density, prevents the formation of γ-NiOOH with further densification by a less toxic additive, and prolongs the service life, while increasing the utilization rate of the active material. It is an object of the present invention to provide an improved active material for a nickel electrode, a nickel electrode, and an alkaline battery using the same.

Composition of the Invention The present invention is to add 1 to 3 wt% of magnesium to a nickel hydroxide powder active material, the magnesium is in a solid solution state in the crystal of nickel hydroxide, and the pore radius is 30 Å or more particle internal transition Prevents the development of pores and further increases the total pore volume to 0.05
It is an active material for a nickel electrode, which is controlled to be less than ml / g.

Further, the alkali-resistant metal porous body is used as a current collector, magnesium is added to the nickel hydroxide powder active material in an amount of 1 to 3 wt%, and the magnesium active material for nickel electrode is in a solid solution state in the nickel hydroxide crystal. The nickel electrode is characterized by being filled with a paste as a main component.

In the case of high density nickel hydroxide powder with a minimum internal pore volume, a large amount of higher order oxide γ-NiOOH is produced. However, when dissimilar metal ions, especially magnesium ions, are placed in the nickel hydroxide crystal, distortion occurs in the crystal, which increases the freedom of movement of protons, has the effect of improving the utilization rate and decreasing the production of γ-NiOOH. I found it.

It was generally said that the addition of magnesium adversely affects the nickel electrode, but it has been clarified that a very high performance electrode can be obtained by adding a trace amount of 1 to 3 wt%.

On the other hand, in the crystal outside of the nickel hydroxide to dissolve the cobalt compound additive, the current collector and between nickel hydroxide particles HCoO ₂ ^- charged after → β-Co (OH) is connected by _two reactions. Then, by β-Co (OH) ₂ → CoOOH reaction due to electrochemical oxidation called charging, it changes into cobalt oxyhydroxide with high conductivity, and the electron flow between the current collector nickel fiber and nickel hydroxide particles is changed. It has the effect of smoothing and increasing the utilization rate. This reaction mechanism is modeled and shown in FIG. As shown in the model diagram, an important point of this electrode is that the additive is dissolved and the current collector nickel fiber is connected to the active material.

Examples Hereinafter, details of the present invention will be described with reference to examples.

Ammonium sulfate is added to an aqueous solution of nickel sulfate to which a small amount of magnesium sulfate is added to form an ammine complex ion of nickel and magnesium.

This solution is dripped into an aqueous solution of sodium hydroxide and vigorously stirred to gradually decompose complex ions to precipitate and grow nickel hydroxide particles in the form of solid solution of magnesium. At this time, the sodium hydroxide aqueous solution is made to have a low alkaline concentration of about PH 11 to 13, and the temperature is gradually precipitated in the range of 40 to 50 ° C. Depending on the pH of the precipitation solution, nickel hydroxide particles having various physical properties can be obtained.

Fig. 2 shows the relationship between the internal pore volume of the powder composed of only nickel hydroxide and the PH dependence of the γ-NiOOH production rate.

The lower the internal pore volume, the lower the PH, and the more dense the powder. On the other hand, γ-NiOOH tends to be generated more easily as the pH becomes lower. The region that satisfies the two factors is the region between PH11 and 13 shown by the hatching sandwiched between the inflection points.

Fig. 3 shows the relationship between the pore volume and the specific surface area. The pore volume of nickel hydroxide changed by changing the pH of the precipitation solution, but at the same time, the specific surface area also changed. A to E are nickel hydroxide only, F is 3 wt% of magnesium added in a solid solution state, and G is only nickel hydroxide prepared by the conventional method.

The conventional method is a method in which nickel hydroxide particles are precipitated in a high-concentration alkali having a pH of 14 or higher.

In each case, the pore volume inside the particles tends to increase as the specific surface area increases. That is, there is a correlation between the specific surface area and the pore volume, and a high-density active material with a small pore volume has a small specific surface area regardless of whether magnesium is added or not.

FIG. 4 shows a comparison of the pore size distributions of the conventional nickel hydroxide and the magnesium-added high-density nickel hydroxide active material of the present invention calculated from the desorption side of the nitrogen adsorption isotherm.

Nickel hydroxide G prepared by the conventional method is prepared by dropping a nickel sulfate aqueous solution into a high-concentration alkaline solution having a PH of 14.5 at 50 ° C.

The G particles are present in a large amount in a wide range of a specific surface area of about 66 m ² / g and a pore radius of 15 to 100Å. Its pore volume is 0.
Reaching 136-40 ml / g and 30-40% of particle volume (0.41 ml / g),
It is a particle with fairly large voids. On the other hand, the magnesium-added high density nickel hydroxide F of the present invention has a volume of 0.02
It is as small as 8 ml / g, which is only about 1/4 of G particles. this is,
The F particles are 20 to 30% more dense than the G particles. That is, it is shown that in order for the active material particles to have a high density, the specific surface area and the pore volume should be as small as possible. A small amount of cobalt compound, CoO, a-Co (OH) ₂ , β, which dissolves in an alkaline electrolyte to form a Co (II) complex ion, is added to these nickel hydroxide powders.
Powders such as —Co (OH) ₂ or cobalt acetate were mixed.
Thereafter, an aqueous solution containing 1% of carboxymethyl cellulose dissolved therein was added to prepare a fluid paste solution.
A predetermined amount of this paste solution was filled in an alkali resistant fiber substrate having a porosity of 95%, such as a nickel fiber substrate, and dried to obtain a nickel electrode.

In order to know the utilization rate of the active material and the production rate of γ-NiOOH by charge and discharge, this nickel electrode was used as a positive electrode, a cadmium electrode as a counter electrode was assembled via a polypropylene nonwoven fabric separator, and a potassium hydroxide aqueous solution with a specific gravity of 1.27 was used as an electrolytic solution. Injected. After injecting the electrolytic solution, the battery was left under various conditions in order to dissolve the cobalt compound as an additive at the corrosion potential and to connect the nickel hydroxide powders. FIG. 5 shows the relationship between the standing condition and the active material utilization rate for a battery prepared using CoO as an additive and nickel hydroxide powder having a specific surface area of 66 m ² / g. The storage condition, which is an important process of forming the conductive network, indicates that the higher the concentration of the electrolytic solution and the higher the temperature, the higher the utilization rate can be obtained in a short period of time.
It also shows that the amount of dissolved CoO is effectively acting. This is due to the uniform dispersibility (more complete network formation) due to the dissolution and precipitation of the additive.

FIG. 6 shows the relationship between the various nickel hydroxides and the utilization rate of the active material under the proper standing condition. For A to G in which the composition of the active material is only nickel hydroxide, there is a proportional relationship between the specific surface area and the utilization rate of the active material. This fact indicates that a high specific surface area is required to obtain a high active material utilization rate. Therefore, from the relationship between the specific surface area and the pore volume,
In order to obtain a higher utilization rate of the active material, it means that a low-density active material having a larger pore volume, that is, a lower density active material is better. Ultimately, it is impossible to increase the energy density of the electrode. Become. Since the active material utilization rate is close to the theoretical value, the pore volume of the high-density active material powder satisfying the required energy density of 600 mAh / cc must be 0.05 ml / g or less, at which time the pore volume is The specific surface area correlated with is 15 to 30 m ² / g. However, F in which a small amount of magnesium is added to the nickel hydroxide crystal shows a high utilization rate which is the same as that of the conventional powder G, although the specific surface area is small. Compared with conventional powder, high-density powder can be packed more in the same volume substrate, so the energy density per unit volume of electrode plate is 504 mAh / cc for conventional powder G, 620 mAh / cc for high-density powder F and conventional powder for high-density powder F. 20% more than G
It shows a high value.

Due to the decrease in the specific surface area due to the high density of the active material, the inlet and outlet of the reactive species protons from the electrolytic solution are reduced.
It is considered that the addition of magnesium causes the nickel hydroxide crystal to have a strain, so that the proton transfer in the solid phase becomes smooth. That is, it can be said that the utilization rate means the amount of proton transfer. This is governed by two factors: the specific surface area of the particles and the diffusion rate inside the crystal (solid phase). When the crystals are the same, the specific surface area is dominant, and when the crystals are different, the internal strain is dominant. Is considered to be done. In order for the active material to react, it is necessary to smoothly move electrons from the current collector to the surface of the active material particle, and as described above, the conductive state in the free state (existing on the particle surface without solid solution in nickel hydroxide) A network of CoOOH particles with properties is essential. FIG. 7 shows the relationship between the amount of CoO added, the active material utilization rate, and the energy density per electrode plate volume. Regarding the CoO additive that forms this network, the active material utilization rate increases as the additive amount increases. However, since the additive itself contributes to conductivity but does not actually discharge, the electrode plate energy density is 15
%, It tends to decrease from around.

Charged at a high current density of 1C, γ − in the electrode plate at the end of charging
The correlation between the amount of NiOOH produced and the type of active material powder was examined by X-ray analysis. The X-ray diffraction peak is shown in FIG.

As shown in FIG. 9, when magnesium is added in a solid solution state to nickel hydroxide crystals, γ-
It can be seen that the amount of NiOOH produced decreases.

The effect of suppressing the production of γ-NiOOH by magnesium is also influenced by the method for producing nickel hydroxide, that is, the precipitated PH, and is different from the case of the conventional method as shown in FIG. In particular, in the case of the present invention, among the less reversible γ-NiOOH present at the end of filling compared to the conventional method, 30
Characteristic is that ~ 50% can be discharged. As a result, accumulation of γ-NiOOH due to repeated charging and discharging can be further prevented, and the life of the electrode can be extended. Thus, the effect of the solid solution additive changes depending on the active material precipitation conditions. However, at least in the magnesium addition of the present invention, it is understood that precipitation in a thin alkaline aqueous solution is superior to precipitation in a conventional high-concentration alkaline aqueous solution.

Figure 11 shows the relationship between the γ-NiOOH production rate and the change in electrode thickness in the overcharged state. The higher the γ-NiOOH production rate, the greater the plate thickness. In other words, in order to obtain a long-life electrode, it is necessary to suppress the production of γ-NiOOH,
Also in this respect, it can be seen that addition of magnesium is very effective.

In addition, in the case of the conventional method, when magnesium was added in an amount of 1 wt% or more, a layer of nickel hydroxide and liberated magnesium hydroxide appeared, but in the present invention, it was revealed by various instrumental analyzes that it does not liberate to about 3 wt%. became. Further, as shown in FIG. 12, the addition of a large amount of magnesium shifts the oxidation potential of nickel hydroxide to a noble level and reduces the potential difference from the oxygen generation potential, so that the oxidation reaction of nickel hydroxide during charging does not occur. The competitive reaction of the oxygen generation reaction easily occurs from the early stage of charging, so that the so-called charge acceptance performance deteriorates. Further, the released magnesium becomes Mg (OH) ₂ and also inhibits the charge / discharge reaction. Therefore, as shown in FIGS. 8, 9 and 11, the large effect of preventing the production of γ-NiOOH by adding 6 to 8 wt% seems to be that the active material is not charged. 6 wt% in the figure
With the above addition, the utilization rate of the active material is drastically reduced.

In the case of magnesium-free high-density powder A that produces a large amount of γ-NiOOH, a two-stage discharge is generated as shown in Fig. 14, but the magnesium-added high-density powder prevents γ-NiOOH production. There is no such thing. It is also found that the addition potential of magnesium shifts the reduction potential of NiOOH, which is an active material, to a noble value, as in the case of charging. In the past, there have been few reports of redox potential shifts in the noble direction of nickel electrode additives, and this can be said to be a major feature of magnesium addition. Therefore, when a battery using magnesium-added high-density powder for the positive electrode is produced,
A battery with a high discharge voltage can be obtained.

Although such a redox potential shift has not been studied in detail, as described above, the addition of magnesium causes strain in the nickel hydroxide crystal, resulting in smooth diffusion of protons in the solid phase. Is considered.

The effect of magnesium has the same effect even if other different elements such as cobalt coexist in a solid solution state. 15th
The figure shows the relationship between the active material, the charge / discharge temperature, and the active material utilization rate. In H containing both magnesium and cobalt as a solid solution, an improvement in charging performance was observed at a higher temperature (about 45 ° C.) than in F containing magnesium alone.
Further, as shown in FIG. 11, γ-is similar to the addition of magnesium to cadmium which has been added to the conventional powder.
It was effective in preventing NiOOH generation.

FIG. 16 shows the relationship between the active material utilization rates of the additives that form the CoOOH network.

The rank of the active material utilization rate is CoO> a-Co (OH) ₂ > β-Co (OH) ₂
It is considered that the reason for becoming is due to the solubility in the electrolytic solution. That is, in the case of β-Co (OH) ₂ , it is oxidized by dissolved oxygen after injection of the electrolytic solution, and brown brown Co (OH) _{3 with} low solubility [or CoH
Easily be] is formed represented by O _2, whereas, a-Co (OH)
_In the case of ₂ , C is necessary because it goes through a-Co (OH) ₂ → β-Co (OH) _2.
O (OH) ₃ is more difficult to form. In the case of CoO, it can be said that it is the most excellent additive because Co (OH) ₃ is not formed at all. More specifically, from the viewpoint of the dissolution rate, it is desirable that β-Co (OH) ₂ is used as a starting material and heated and produced in a high temperature inert atmosphere at 200 to 800 ° C. to have low crystallinity.

HCoO ₂ nickel hydroxide ^- was immersed in ion, electrode powder precipitated cobalt hydroxide and paste filling on the surface, even inferior utilization than the electrode obtained by mixing CoO powders, beta-C
It was about the electrode mixed with o (OH) ₂ powder. Further, a powder in which a conductive CoOOH layer was formed on the surface of nickel oxyhydroxide powder (specifically, after charging and discharging an electrode mixed with CoO powder, nickel fiber as a current collector was removed from the electrode). The electrode that is filled with paste) has a poor utilization rate.

That is, the conductive network (Co
It is essential that OOH) be formed in the fabricated electrode. That is, even if it is formed on the surface of the active material particles in advance, the connection between the particles is incomplete. Therefore,
After assembling the electrode as a battery, a process of dissolving and re-precipitating CoO powder is necessary.

The electrode produced by the present invention using the CoO additive reaches a high utilization rate close to the theoretical utilization rate by the dissolution-reprecipitation step without using a conductive additive, and thus, for example,
Like an electrode using graphite powder as a conductive additive,
It can be used for the positive electrode of a density-type nickel-cadmium battery without generation of harmful carbonic acid radicals due to oxidative decomposition.

In addition, although the metal fiber sintered body is shown as the substrate in the above embodiment, the substrate is not limited to these. Further, the effect of adding magnesium is similarly recognized for nickel hydroxide particles having high crystallinity, in addition to the production method of the present invention.

EFFECTS OF THE INVENTION As described above, the present invention densifies nickel hydroxide powder more, preventing the formation of γ-NiOOH accompanying further densification with a less toxic additive, and prolonging the life,
Since it is possible to provide a nickel electrode active material having a high discharge potential and a nickel electrode and an alkaline battery using the same, the industrial value thereof is extremely large.

[Brief description of drawings]

FIG. 1 is a model diagram of a dissolution-precipitation mechanism of a cobalt compound. Fig. 2 shows the precipitation solution PH, the particle internal pore volume and γ-NiOO.
It is a figure showing the correlation with the generation rate of H. FIG. 3 is a diagram showing the relationship between the specific surface area of nickel hydroxide particles and the pore volume. FIG. 4 is a diagram showing curves of pore size distributions of the conventional nickel hydroxide powder and the high-density nickel hydroxide powder of the present invention. FIG. 5 is a diagram showing the relationship between the standing condition and the active material utilization rate. FIG. 6 is a diagram showing the relationship between the type of nickel hydroxide and the active material utilization rate. FIG. 7 is a diagram showing the relationship between the amount of CoO added, the active material utilization rate, and the energy density per electrode plate volume. FIG. 8 shows X-ray diffraction peaks at the end of charging of various magnesium-added high-density powder active materials. FIG. 9 shows the relationship between the amount of magnesium added and the amount of γ-NiOOH produced. Figure 10 shows γ at the end of charge and discharge of various nickel hydroxides.
It is a figure showing the generation ratio of -NiOOH. FIG. 11 is a diagram showing a γ-NiOOH production rate and an electrode thickness increase rate when an electrode using an active material containing various additives is overcharged. FIG. 12 shows charge potential characteristics of various magnesium-added electrodes. FIG. 13 is a graph showing the relationship between the magnesium addition amount and the active material utilization rate. FIG. 14 shows discharge potential characteristics of various magnesium-added electrodes. FIG. 15 is a diagram showing the relationship between the active material, the charge / discharge temperature and the active material utilization rate. FIG. 16 is a diagram showing the relationship between various cobalt compound additives and the utilization rate of the active material.

Claims (7)

[Claims] 1. A nickel hydroxide powder active material containing 1 to 3 wt% of magnesium in a crystal in a solid solution state, having an internal pore radius of 30 Å or less and a total pore volume of 0.05 ml / g or less. Active material for nickel electrode mainly composed of 2. A nickel-hydroxide and a small amount of a magnesium sulfate aqueous solution are used as starting materials, and caustic soda or potassium caustic and ammonium sulfate are precipitated in an aqueous solution controlled to PH 11 to 13 to give 1 to 3 wt% of magnesium. A method for producing an active material for a nickel electrode mainly containing a nickel hydroxide powder active material, which is contained in a crystal in a solid solution state, and has an inner pore radius of 30Å or less and a total pore volume of 0.05 ml / g or less. . 3. A nickel electrode in which an alkali-resistant metal porous body is filled with a paste containing a nickel hydroxide powder active material as a main component and containing 1 to 3 wt% of magnesium in a solid solution state in a crystal. 4. A nickel compound which is dissolved in an alkaline electrolyte to form a cobalt complex ion is added to the nickel hydroxide powder active material in the range of 5 to 15 wt%, and the cobalt compound is liberated from the active material. The nickel electrode according to claim 3, which is in a state. 5. The nickel electrode according to claim 3, wherein, in addition to magnesium, a small amount of cobalt coexists as a solid solution in the nickel hydroxide powder active material. 6. The nickel electrode according to claim 3, wherein the electroconductivity between the alkali-resistant metal porous body and the active material is maintained only by the cobalt compound additive without containing the electroconductive additive. 7. A nickel electrode in which an alkali-resistant metal porous body is filled with a paste containing a magnesium hydroxide in a solid solution state in a crystal as a solid solution and a cobalt compound added in a free state as a main component of a nickel hydroxide powder active material. A method for producing an alkaline battery, which is prepared, assembled into a battery using the nickel electrode without being formed, and allowed to stand for a certain period of time after injecting an electrolytic solution to completely dissolve and re-precipitate the cobalt compound and then charge the battery for the first time.