HK1022992A - Cathode active material for an alkaline storage battery and cathode using the same - Google Patents

Description

Cathode active material for alkaline storage battery and cathode using the same

The present invention relates to a cathode active material for a high-capacity alkaline storage battery, which is mainly composed of a metal oxide containing nickel as a main metal element, and a cathode using such an active material.

With recent advances in semiconductor technology, development of miniaturized, lightweight, and multifunctional electronic devices (e.g., mobile phones, notebook computers, etc.) is also rapidly underway. There is therefore a great need for compact, lightweight alkaline storage batteries for use as a power source in such portable equipment.

Nickel oxide (NiOOH) is commonly used as the active material for the cathode of alkaline storage batteries. Highly porous (95%) three-dimensional foamed nickel porous substrates have replaced the commonly used sintered substrates. Electrodes (foamed metal electrodes) obtained by densely packing nickel oxide powder into a foamed nickel porous substrate have been used in industrial applications (U.S. patent 4,251,603). This significantly increases the energy density of the nickel cathode.

An important technology for providing nickel cathodes with high energy density improves the process for producing the active material, i.e., nickel oxide powder. A common method for producing nickel oxide powder is to react an aqueous alkaline solution, such as a sodium hydroxide solution, with an aqueous nickel salt solution to precipitate nickel hydroxide. After the precipitate grows crystal by aging, the nickel hydroxide crystal is mechanically crushed. This method requires a complicated process, and the resulting nickel hydroxide powder has an unfixed shape. Thus, it is difficult to provide an electrode having a high packing density.

As described in japanese examined patent publication No. hei 4-80513, an improved production method is proposed which repeats a process of reacting ammonia with an aqueous nickel salt solution to form a nickel-ammonium complex and reacting a base with the complex to produce nickel hydroxide, thereby growing nickel hydroxide. I.e. this method causes nickel hydroxide to be deposited on the nickel hydroxide present. The method can continuously generate nickel oxide and reduce production cost. The resulting quasi-spherical oxide can achieve high density packing.

The nickel oxide active material thus obtained is a high-density particle that has been grown to a particle diameter of up to several tens of micrometers. This reduces the electron conductivity of the active material itself and thus reduces the electrode charging and discharging efficiency. Some measures have been proposed, such as adding metallic cobalt, cobalt oxide, or metallic nickel to compensate for electronic conductivity, or adding other metals than cobalt or nickel to the active material to form a solid solution, to improve charge and discharge efficiency.

Examples of metal elements known to be added to crystalline nickel oxide to improve charge and discharge efficiency are Cd and Co, as described in Power Sources 12, p203 (1988). However, there is a demand for cadmium-free batteries from an environmental point of view. It has been proposed to add Zn and to add three elements, Co, Zn and Ba, in place of cadmium (us patent 5,366,831). A technique of adding different metal elements to nickel oxide and forming a solid solution to achieve high charge and discharge efficiency has been known, for example, as disclosed in japanese unexamined patent publication No. 51-122737.

Improvements in the structure of the substrate, the shape of the particles of the active material, the composition of the active material, and the additives significantly increase the energy density of the cathode. The energy density of the cathode in practical applications has reached about 600 mAh/cc. However, as described above, there is an increasing demand for increasing the energy density of energy sources applied to miniaturized portable devices. Another direction to increase the energy density of a battery is to improve the structure of the anode and cathode, the electrolyte, the separator, and the battery.

The practical use of high energy density metal hydrides instead of the commonly used cadmium anodes (Power Sources 12, p393(1988)) has increased the volumetric energy density of the anode to at least twice the cathode energy density. Such technological advances, such as the formation of thinner film separators or high density packing of electrode materials, have dramatically increased energy density, but have essentially reached a limit.

The most effective way to further increase the energy density is to increase the energy density of the cathode, since the cathode occupies nearly half the cell volume.

There are some methods of increasing the packing density of the electrode material to increase the energy density of the cathode; for example, increasing the tap density of the active material particles, reducing the amount of additives, and reducing the amount of metal included in the foamed nickel substrate. However, these methods have substantially reached their limits. Therefore, there is a need to improve the reactive metal itself from the viewpoint of increasing the reactivity and the number of reaction stages.

Nickel oxide, commonly used as a cathode active material, is beta-Ni (OH) when filled into an electrode substrate₂(divalent oxide) structure. Said beta-Ni (OH)₂By means of charge-discharge reactions, it is possible to convert reversibly to β -NiOOH (trivalent oxide) by exchanging one electron. The beta-NiOOH in the charged state carries an excess charge and is oxidized to a highly oxidized gamma-NiOOH structure (valence: 3.5-3.8). γ -NiOOH is an irreversible stoichiometric substance with a disordered crystalline structure (J. Power Sources 8, p229 (1982)).

This γ -NiOOH is electrochemically inert, resulting in a voltage drop and capacity reduction. The wider interlayer distance of γ -NiOOH expands the volume of the electrode, causing many problems such as poor contact of the active material with the conductive agent or the substrate, release of the active material from the substrate, and drying of the electrolyte by the uptake of water molecules. Thus requiring interference with the production of gamma-NiOOH.

In order to obtain a high energy density active material comprising nickel oxide as a main material, it is particularly important to utilize the higher order oxide γ -NiOOH. One proposed material has a structure similar to that of alpha-type hydroxide (U.S. Pat. nos. 5,348,822 and 5,569,562), which is obtained by substituting a part of Ni with another metal element such as mn (iii), al (iii), or fe (iii), with anions and water molecules between layers. It is considered that this oxide can be reversibly converted into a higher-order oxide having a structure similar to γ -NiOOH by charge and discharge. However, such an oxide has a wide interlayer distance and low density (true density), making high-density filling difficult, and thus such an oxide is not practical.

The present inventors have noted an active material of γ -NiOOH which has a β -type crystal structure during filling of an electrode and is reversibly converted into a higher-order oxide by charge and discharge energy. The inventors propose that in order to achieve charge and discharge reactions with more than one electron exchange, another metal element may be added to modify the nickel oxide. It has also been proposed to use a composition comprising Mn as a main component for the metal element added to nickel oxide (Japanese unexamined patent publication Hei 9-115543). As disclosed in this document, the addition of Mn to nickel oxide enhances proton mobility and electron conductivity, thereby improving the utilization rate thereof.

Solid solution nickel oxide to which Mn is added has been proposed in Japanese unexamined patent publication Nos. Sho 51-122737, Hei 4-179056 and Hei 5-41212. The present inventors also noticed solid solution nickel oxide with Mn added. The present inventors have found that such solid solution nickel oxide is easily charged and oxidized to the γ phase by adjusting the valence of Mn added, and discharged to a high order reaction having a valence of not less than 1.2. The present inventors also propose a method of synthesizing such solid solution nickel oxide to obtain high density.

As described above, a method is proposed in which solid solution nickel oxide or eutectic mixture nickel oxide to which Mn is added is used as a cathode active material to improve charge and discharge efficiency and the number of reaction stages. However, among the proposed materials, γ -NiOOH is generated during normal charging, and it can be reversibly charged and discharged. The conductive network of cobalt compounds is destroyed by the expansion and contraction of the active material in the electrode. This interferes with the formation of the gamma phase, resulting in lower cycle stability, compared to conventional nickel oxide in which charging and discharging is performed by approximately one electron exchange.

Accordingly, it is an object of the present invention to obtain a remarkably high energy density by effectively using a gamma phase for charge and discharge reactions, and to provide a cathode active material for an alkaline storage battery having excellent cycle-life characteristics.

The present invention provides a cathode active material for alkaline storage batteries comprising solid solution nickel oxide or low melting mixture nickel oxide particles having beta-Ni (OH)₂And at least Mn, where the Mn has an average valence of not less than 3.3, and a coating layer of a solid solution or a low-melting mixture of cobalt oxide containing at least one element of Ni and Mn is formed on the surface of the particles.

It is required that the content of the at least one element Ni and Mn in the coating layer is not less than 0.5% by mole, not more than 20% by mole, preferably in the range of not less than 0.5% by mole to not more than 10% by mole of all the metal elements included in the coating layer.

The content of the above coating layer is required to be not less than 1 mol% but not more than 20 mol% based on the hydroxide.

It is also preferred that the cathode material is a spherical or quasi-spherical powder having a tap density of not less than 1.7 g/cc.

The present invention also provides a cathode for an alkaline storage battery comprising the above active material.

Preferred cathodes also include yttria particles.

The novel features believed characteristic of the invention are set forth in the appended claims and will be better understood from the following detailed description when considered in connection with the accompanying drawings.

Fig. 1 is a graph showing the relationship between the number of charge-discharge cycles and the utilization rate of an active material in a nickel-metal hydride storage battery using a cathode active material in an example of the invention.

Fig. 2 is a graph of charge-discharge cycle times versus active material utilization in a nickel-metal hydride battery using active materials having coatings of different nickel content.

The present inventors have found that in nickel oxide containing at least Mn in a solid solution state or a low melting mixture state and having an average valence of Mn of not less than 3.3, at least two electron exchanges occur when a charge-discharge reaction proceeds. Although the detailed mechanism has not been explained so far, this oxide is oxidized by charge to the gamma phase of a higher oxidation state, which is easily discharged. This is attributed to the fact that the addition of Mn, which has a valence different from that of Ni, to nickel oxide improves the mobility of protons and oxygenElectronic conductivity in nickel. The oxide has beta-Ni (OH)₂A structure having a high density in a discharge state and thus having an excellent filling property, and a cathode having a high energy density can be obtained.

In the active material of the present invention, the cobalt oxide coating layer includes at least one element of Ni and Mn. Ni and/or Mn enhances the physical and chemical stability of the cobalt oxide. It is believed that this effectively prevents the coating from dissolving due to repeated charge and discharge cycles, or physical destruction of the particles due to expansion and contraction. This arrangement allows the cobalt oxide coating to maintain a conductive network over long charge and discharge cycles, thereby maintaining the effect of increasing the utilization of the chemical material.

The above effect is particularly remarkable when the cobalt oxide coating layer contains at least one element of Ni and Mn in an amount of not less than 0.5% by mole of all the metal elements in the coating layer. The content of more than 20% (mol) slightly lowers the conductivity, possibly resulting in lowering the number of reaction stages. Therefore, it is preferable that the content is not more than 20 mol%.

If the content of at least one element of Ni and Mn exceeds 10 mol%, crystallization is disturbed and the formed particles become bulky. Therefore, the content is more preferably in the range of not less than 0.5% (by mol) and not more than 10% (by mol).

The amount of cobalt oxide to be coated on the nickel oxide is preferably in the range of not less than 1% by weight and not more than 20% by weight based on the hydroxide.

An amount less than 1% by weight may prevent the cobalt oxide from sufficiently functioning as a conductive network, which may reduce the discharge efficiency. On the other hand, an amount of more than 20% by weight increases the production cost and interferes with crystallization, making the formed particles bulky. Therefore, when the amount of cobalt oxide exceeds the above range of not less than 1% by weight and not more than 20% by weight, there is a significant disadvantage.

The addition of yttria to the cathode using the above active material can significantly improve the charging efficiency at high temperatures. In order to obtain a high energy density of the cathode, it is preferable to improve the number of reaction stages, and to make the packing density of the electrode at least equal to that of a conventional electrode. In order to make the packing density of the electrode at least equal to that of a conventional electrode, the particles of the active material should be spherical or quasi-spherical powder having a tap density of not less than 1.7 g/cc.

In the following paragraphs, the present invention will be described in more detail with reference to examples.

Example 1

First, a method of synthesizing an active material is described. The method provides a NiSO content of 2.16 mol/l₄And 0.24 mol/l MnSO₄An aqueous solution containing 5.52 mol/l NaOH, and an aqueous solution containing 4.8 mol/l NH₃And continuously feeding these aqueous solutions into a reactor maintained at 40 ℃. The feed rate of each aqueous solution was adjusted so that the pH of the mixed solution in the reactor was in the range of 11.5 to 12.5. Meanwhile, gaseous Ar was continuously introduced into the solution in the reactor at a flow rate of 800 ml/min so that the oxygen concentration dissolved in the solution was kept at not more than 0.05 mg/l. Stirring blades provided in the reactor were rotated at a fixed speed to uniformly mix the solution and gas. The total input rate of Ni ions and Mn ions to the reactor was calculated to be 1.2X 10 from the concentration of the aqueous solution and the feed rate^-3。

When the pH of the solution in the reactor is kept substantially constant and the metal salt concentration and the resulting oxide particle concentration are substantially at steady state, the process collects the suspension which overflows the reactor and separates the precipitate by decantation. The precipitate was washed with water and the water-wetted precipitate, i.e. the metal oxide powder, was dried in air at 80 c for 72 hours.

The metal oxide particles "a" were obtained in the form of spheres and had an average particle diameter of 10 μm. ICP emission spectroscopy analysis showed that the ratio of the metal elements Ni to Mn in the metal particles was 9: 1 (atomic ratio). The total valency of all metals was determined by iodometric titration and the average valency of Mn was calculated from the total valency to be 3.5. XRD pattern showed nickel oxide as a single phase beta- (OH)₂. Due to the correlation between the average valence or content of Mn and the lattice constant (Vegard's law), it is certain that Mn replaces part of Ni.

While stirring the metal oxide particles "a" in water, a solution containing 0.09 mol/L of CoSO was added dropwise₄And 0.01 mol/l NiSO₄An aqueous solution containing 0.23 mol/l NaOH and an aqueous solution containing 0.4 mol/l NH₃An aqueous solution of (a). This causes the cobalt oxide containing Ni to be gradually deposited on the surface of the metal oxide particles "a". At this point, gaseous argon is introduced into the reactor to prevent oxidation of the cobalt by dissolved oxygen.

The process then filters the suspension containing these particles, washes the particles with water, and dries to obtain active material a. The amount of the coating layer covering the particle "a" in the active material a and the composition thereof were calculated from ICP emission spectrum analysis and the composition of the particle "a". The ratio of the metal elements Co to Ni in the coating was 9: 1 (atomic ratio) and the amount of the coating was 5% by weight based on the hydroxide.

Example 2

An active material B was obtained in the same manner as in example 1 for preparing a coating of active material A, wherein a coating of Mn-containing cobalt oxide was formed on the surface of the metal oxide particles "a", except that the coating was formed of a material containing 0.09 mol/L of CoSO₄And 0.1 mol/l MnSO₄Mixed aqueous solution of (3) in place of CoSO₄And NiSO₄The mixed aqueous solution of (1). The ratio Co: Mn of the metal elements in the coating was calculated as 9: 1 (atomic ratio) in the above manner, and the amount of the coating was 7% by weight based on the hydroxide.

Comparative example

An active material X in which metal oxide particles "a" were coated with cobalt oxide was obtained in the same manner as in example 1 except that the coating layer of active material A was prepared by containing 0.1 mol/L of CoSO₄In place of CoSO₄And NiSO₄The mixed aqueous solution of (1).

From the obtained active material A, B and X, a battery was assembled. The method is that, first, 2 g of yttria powder and 30 g of water are added to 100 g of each active material powder, and the mixture is kneaded into a paste. The paste was filled into a foamed nickel substrate having a porosity of 95%, dried and press-molded. Obtaining the nickel cathode plate. The cathode plate is cut into a predetermined size, and an electrode lead is spot-welded to the predetermined size of the cathode plate. A nickel cathode with a theoretical capacity of 1300mAh was obtained. The theoretical capacity of the nickel electrode is calculated assuming that Ni in the active material undergoes an electronic reaction.

A known anode for an alkaline storage battery is used as the anode. Here, the hydrogen-absorbing alloy MmNi prepared by the following method was used_3.55Co_0.75Mn_0.4Al_0.3As an anode. The method is to melt a mixture of Mm, Ni, Co, Mn and Al which are mixed according to the required proportion in an electric arc furnace to obtain the alloy which absorbs hydrogen and has the required composition. The alloy ingot was mechanically pulverized into a powder having a particle size of 30 μm in an inert atmosphere. Water and carboxymethyl cellulose were added to the powder as a binder, and the mixture was kneaded into a paste. The paste was filled into an electrode substrate under pressure to obtain an anode plate made of a hydrogen absorbing alloy. The anode plate is cut to a predetermined size. An anode with a capacity of 2000mAh was obtained.

The anode and cathode were combined with each other to form a set of spiral electrodes through a separator composed of a 0.15 mm thick sulfonated polypropylene nonwoven fabric. The set of electrodes is inserted into the battery case. 2.2 ml of an electrolyte (9 mol/L KOH) was injected, and the opening of the battery can was sealed with a sealing plate having a safety valve having an operating pressure of about 20kgf/cm². An AA size cylindrical sealed nickel-metal hydride battery was obtained.

The performance of each sealed battery including as a cathode the active material a of example 1, the active material B of example 2, and the active material X of comparative example, respectively, was evaluated in accordance with the following method. The evaluation method was that each cell was charged at a current of 130mA for 18 hours and discharged at 260mA to a cell voltage of 1.0V at 20 ℃. The charge and discharge cycles are repeated. The discharge capacity was measured to determine the relationship between the number of charge and discharge cycles and the utilization rate of the active material. The utilization of the active material is calculated as the ratio of the actual discharge capacity to the theoretical capacity assuming that Ni undergoes an electron reaction.

Fig. 1 is a graph showing the relationship between the number of charge-discharge cycles and the utilization rate of an active material. As is clear from the graph, the utilization rate of the battery including the active material X of the comparative example was reduced after 300 cycles, and the utilization rate of the active material of the batteries including the active materials a and B of examples 1 and 2 was not significantly reduced after 400 cycles.

Example 3

This example varied the mixing ratio of the Co salt to the Ni salt in the method of preparing the coating of the active material a of example 1, and produced active materials having various ratios of Ni to all the metal elements in the coating. The ratio of Ni to all metal elements included in the coating layer varies in 0, 0.3, 0.5, 10, 20 and 25% (mol).

Cylindrical sealed cells like example 1 were prepared with these active materials. The relationship between the number of charge and discharge cycles and the utilization rate of the active material was determined under the same conditions.

Fig. 2 is a graph showing the relationship between the number of charge-discharge cycles and the utilization rate of various active materials. As is clear from the graph, when the Ni content is not less than 0.5 mol%, the utilization of the active material is not significantly decreased after 400 cycles. Ni content more than 20 mol% results in lower utilization than the initial state. This is due to the fact that the substantial reduction of the Co content in the coating reduces the conductivity of the coating. That is, the Ni content in the coating layer should be suitably not less than 0.5% (mol), and not more than 20% (mol).

The same results were obtained for active material B of example 2. This indicates that the suitable content of Mn in the coating should be not less than 0.5% (mol) and not more than 20% (mol).

Although the coating layer in the above embodiment includes only one of Ni or Mn, the same effect is obtained by the improved structure of the coating layer including both Ni and Mn.

In the above embodiment, the coating layer includes cobalt hydroxide as a main component. The cobalt oxide layer obtained by oxidizing cobalt hydroxide has the same effect. In this case, too, the content of Ni or Mn should be suitably not less than 0.5 mol% and not more than 20 mol%.

Foamed nickel may be used as the cathode substrate in the above embodiments. Other porous metal substrates, such as three-dimensional porous metal substrates like nickel felt, and two-dimensional porous metal substrates like perforated metal sheets, all have the same effect.

As described above, the present invention provides a cathode active material for an alkaline storage battery, which can maintain a high utilization rate for a long time. Thus, an alkaline storage battery having improved energy density and cycle performance is obtained.

While the invention has been described in connection with the preferred embodiments set forth, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Therefore, it is intended that the appended claims cover all such variations and modifications as fall within the true spirit and scope of this present invention.

Claims (7)

1. A cathode active material for an alkaline storage battery, comprising:

particles of solid solution or eutectic mixture of nickel oxide having beta-Ni (OH)₂And contains at least Mn, wherein the average valence of Mn is not less than 3.3; and

a coating layer of a solid solution or a low melting mixture of cobalt oxide containing at least one element of Ni and Mn is formed on the surface of the particles.

2. The cathode active material for alkaline storage batteries according to claim 1, characterized in that the molar content of at least one of the elements Ni and Mn in said coating layer is not less than 0.5% and not more than 20% of all the metal elements in said coating layer.

3. The cathode active material for an alkaline storage battery as claimed in claim 2, wherein the molar content of at least one of the elements Ni and Mn in the coating layer is not more than 10% of all the metal elements in the coating layer.

4. The cathode active material for alkaline storage batteries according to claim 1, wherein the amount of said cobalt oxide used for coating said nickel oxide is in the range of not less than 1% by weight and not more than 20% by weight of nickel oxide based on hydroxide.

5. The cathode active material for alkaline storage batteries according to claim 1, wherein the cathode active material is a spherical or quasi-spherical powder having a tap density of not less than 1.7 g/cc.

6. A cathode for an alkaline storage battery comprising the active material according to claim 1.

7. The cathode for an alkaline storage battery according to claim 6, wherein said cathode further comprises yttria particles.