Positive electrode active material for alkaline storage batteries, and alkaline storage battery

Abstract

[Problem] To improve an alkaline storage battery. [Solution] Provided is a positive electrode active material for alkaline storage batteries which comprises particles that each include a core layer and a coating layer disposed on the surface of the core layer, wherein the core layer comprises a hydroxide 1 of at least one metal including nickel (Ni), the coating layer comprises a hydroxide 2 of at least one metal including nickel (Ni), and the elemental composition of the hydroxide 1 and the elemental composition of the hydroxide 2 differ. Also provided is an alkaline storage battery using said positive electrode active material.

Description

Positive electrode active material for alkaline storage batteries and alkaline storage batteries

[0001] This invention relates to a positive electrode active material for alkaline storage batteries and an alkaline storage battery. This invention relates to a novel positive electrode active material and an alkaline storage battery using the same that can significantly reduce the amount of cobalt used in alkaline storage batteries such as nickel-metal hydride batteries, or enable the complete cobalt-free and high-performance use of the above-mentioned storage batteries.

[0002] Batteries are essential industrial products for modern life and industry. Batteries can be broadly classified into primary batteries and secondary batteries. Rechargeable secondary batteries (storage batteries) are widely used in industrial products such as automobiles and electrical equipment. Alkaline storage batteries, which use an alkaline aqueous solution as the electrolyte, account for a large proportion of secondary battery products.

[0003] The history of alkaline batteries can be said to have begun with nickel-cadmium batteries (Ni-Cd batteries) and nickel-iron batteries. Ni-Cd batteries were used for a long time as the representative of alkaline rechargeable batteries, but the commercialization of nickel-metal hydride batteries in 1990 brought about changes in the market for alkaline batteries and rechargeable batteries as a whole.

[0004] Nickel-metal hydride (NMH) batteries use nickel hydroxide as the positive electrode material and a hydrogen-absorbing alloy (a metal hydride in which hydrogen enters and exits the metal crystal lattice) as the negative electrode, and their cadmium-free design is considered environmentally advantageous, leading to their widespread adoption as a replacement for conventional nickel-cadmium (Ni-Cd) batteries. Nickel-metal hydride batteries have a high power density, allowing them to power electrical devices with high power consumption. Furthermore, their battery voltage is approximately 1.2V, allowing for compatibility with dry cell batteries depending on the device design. These advantages have led to increased demand for nickel-metal hydride batteries. Currently, nickel-metal hydride batteries are widely used as power sources for small portable devices, power tools, electric vehicles, and hybrid vehicles.

[0005] Today, products such as small portable devices, power tools, electric vehicles, and hybrid vehicles face fierce competition (a so-called battleground) to win the preferences of today's consumers and users. The performance of rechargeable batteries such as nickel-metal hydride batteries is directly linked to the quality of these various products. For this reason, there is a strong demand for improved performance in various types of rechargeable batteries.

[0006] Alkaline batteries have already been improved in performance through various approaches. One such approach is increasing capacity by adding cobalt compounds as conductive materials to the positive electrode active material. However, the use of expensive cobalt makes it difficult to reduce the manufacturing cost of battery products. Therefore, measures to avoid the use of expensive cobalt have been proposed.

[0007] Patent document 1 describes how coating nickel hydroxide particles with β-cobalt hydroxide can improve battery capacity with a smaller amount of cobalt added. However, this technology does not achieve complete cobalt-free operation.

[0008] Patent Document 2 describes a method for producing low-resistance nickel hydroxide for cathode materials by reducing the surface of nickel hydroxide to a highly conductive nickel metal in the presence of a catalyst containing a metal selected from palladium (Pd), platinum (Pt), and ruthenium (Ru), thereby imparting conductivity between nickel hydroxide particles. However, this method requires a catalyst containing an expensive platinum element, which presents further problems in terms of cost reduction.

[0009] Patent Document 3 also describes a method for producing low-resistance nickel hydroxide particles for cathode materials by immersing nickel hydroxide particles in an alkaline solution to dissolve the surface of the nickel hydroxide particles, then adding a reduction catalyst to deposit and fix a highly conductive nickel-based metal onto the surface of the nickel hydroxide particles. However, this method also has problems in that it uses a catalyst containing expensive platinum group elements.

[0010] Furthermore, Patent Document 4 describes a cobalt-free alkaline secondary battery that uses a negative electrode active material that does not contain cobalt and a positive electrode active material consisting of nickel hydroxide coated with carbon. When manufacturing the above positive electrode active material, a carbon black dispersion is sprayed onto nickel hydroxide in a fluidized bed coating apparatus. Such a spraying process requires operations and equipment not used in the production of conventional positive electrode active materials. Controlling the formation of a uniform carbon coating layer is not easy. Therefore, this method still has practical problems.

[0011] Japanese Patent Publication No. 11-067198, Japanese Patent Publication No. 2002-298841, Japanese Patent Publication No. 2001-325953, Japanese Patent Publication No. 2012-204177

[0012] Thus, an ideal method for achieving cobalt-free alkaline batteries, such as nickel-metal hydride batteries, has yet to be found.

[0013] The inventors sought a new cathode material for alkaline batteries that could reduce the amount of cobalt compounds used and improve the performance of alkaline batteries. As a result, they succeeded in obtaining a new cathode active material that can significantly reduce the amount of cobalt used or achieve complete cobalt-free operation by coating the surface of nickel hydroxide particles with a specific nickel-containing metal hydroxide. In other words, the present invention is as follows.

[0014] (Invention 1) A positive electrode active material for an alkaline storage battery comprising particles including a core layer and a coating layer disposed on the surface of the core layer, wherein the core layer is made of a hydroxide 1 of one or more metals including nickel (Ni), and the coating layer is made of a hydroxide 2 of one or more metals including nickel (Ni), and the elemental composition of hydroxide 1 and the elemental composition of hydroxide 2 are different. (Invention 2) The positive electrode active material for an alkaline storage battery according to Invention 1, wherein hydroxide 1 and / or hydroxide 2 are composite hydroxides of nickel (Ni) and one or more metals selected from zinc (Zn), magnesium (Mg), manganese (Mn), aluminum (Al), and cobalt (Co). (Invention 3) The positive electrode active material for alkaline storage batteries according to Invention 1, wherein the hydroxide 1 is a composite metal hydroxide consisting of nickel (Ni), zinc (Zn), and / or magnesium (Mg), and the ratio of Ni in the hydroxide 1 to the total metal elements contained in the hydroxide 1 is 85 mol% or more and 95 mol% or less. (Invention 3) The positive electrode active material for alkaline storage batteries according to Invention 1, wherein the hydroxide 2 is nickel hydroxide, or a composite metal hydroxide consisting of nickel (Ni), zinc (Zn), and / or manganese (Mn), and the ratio of Ni in the hydroxide 2 to the total metal elements contained in the hydroxide 2 is 70 mol% or more and 100 mol% or less. (Invention 4) The positive electrode active material for alkaline storage batteries according to Invention 1, wherein the ratio of the coating layer to the particles is 2% by mass or more and 50% by mass or less. (Invention 5) When an alkaline storage battery 1 using a positive electrode active material consisting of hydroxide 1 and an alkaline storage battery 2 using a positive electrode active material consisting of hydroxide 2 are charged and discharged under the same conditions, provided that there are no differences between the alkaline storage battery 1 and the alkaline storage battery 2 other than hydroxide 1 and hydroxide 2, the relationship: Battery voltage 2 < Battery voltage 1 holds true between the battery voltage 1 of the alkaline storage battery 1 when the discharge amount of the alkaline storage battery 1 reaches 50% of its discharge capacity and the battery voltage 2 of the alkaline storage battery 2 when the discharge amount of the alkaline storage battery 2 reaches 50% of its discharge capacity. This is the positive electrode active material for nickel-metal hydride batteries according to Invention 1. (Invention 6) An alkaline storage battery using the positive electrode active material of Invention 1.

[0015] The positive electrode active material of the present invention can replace conductive agents made of cobalt compounds and improve the utilization rate of nickel hydroxide contained in the positive electrode active material. The positive electrode active material of the present invention is effective as a positive electrode material for alkaline storage batteries such as nickel-metal hydride batteries.

[0016] A schematic diagram showing particles of the positive electrode active material of the present invention.

[0017] [Positive Electrode Active Material for Alkaline Storage Batteries] The positive electrode active material for alkaline storage batteries of the present invention consists of particles comprising a core layer and a coating layer disposed on the surface of the core layer. The elemental composition of the core layer and the elemental composition of the coating layer are different. Here, "elemental composition" refers to a composition that takes into account not only the number and combination of constituent elements, but also the content ratio of each constituent element. The present invention is characterized in that the compositional formula of the material constituting the core layer and the compositional formula of the material constituting the coating layer are not the same.

[0018] The proportion of the coating layer to the particles is generally 2% by mass or more and 50% by mass or less, preferably 10% by mass or more and 20% by mass or less.

[0019] Figure 1 shows a cross-section of the positive electrode active material particles of the present invention. Figure 1 is a schematic diagram for understanding the present invention. Figure 1 does not depict the actual shape and dimensions of the positive electrode active material particles (10), core layer (1), and coating layer (2), and may contain exaggerations or omissions.

[0020] [Hydroxide 1] The core layer described above consists of a hydroxide 1 of one or more metals including nickel (Ni). Hydroxide 1 may be, for example, nickel hydroxide, or it may be a composite hydroxide of nickel (Ni) and one or more metals selected from zinc (Zn), magnesium (Mg), manganese (Mn), aluminum (Al), and cobalt (Co).

[0021] Hydroxide 1 is preferably a composite metal hydroxide consisting of nickel (Ni), zinc (Zn), and / or magnesium (Mg). In this case, the proportion of Ni in hydroxide 1 to the total metal elements contained in hydroxide 1 is generally 85 mol% or more and 95 mol% or less.

[0022] [Hydroxide 2] The coating layer is composed of hydroxide 2 of one or more metals containing nickel (Ni). Hydroxide 2 may be, for example, nickel hydroxide, or may be a composite hydroxide of nickel (Ni) and one or more metals selected from zinc (Zn), magnesium (Mg), manganese (Mn), aluminum (Al), and cobalt (Co).

[0023] Hydroxide 2 is preferably nickel hydroxide or a composite hydroxide of a metal composed of nickel (Ni) and zinc (Zn) and / or manganese (Mn). In this case, the ratio of Ni contained in hydroxide 2 to all metal elements contained in hydroxide 2 is generally 70 mol% or more and 100 mol% or less.

[0024] [Conductive layer] In the present invention, the case where the coating layer functions as a kind of conductive layer with respect to the core layer can be considered as follows.

[0025] During the charge and discharge of the alkaline storage battery, nickel hydroxide, which is the main component of the positive electrode active material, reacts as follows.

[0026] Charge reaction: Ni(OH) ２ , ２ , － ,

[0028] ,

[0027] , , + OH － → NiOOH + H ２ O + e － Discharge reaction: NiOOH + H ２ O + e － → Ni(OH) ２ + OH －

[0027] The positive electrode active material of the present invention is particles in which the surface of the core layer composed of hydroxide 1 is formed of a coating layer composed of hydroxide 2. If, during discharge, the above discharge reaction preferentially proceeds in the core layer and the discharge reaction of the core layer is completed and the reduction of Ni(OH) of nickel hydroxide contained in the core layer to ２ is completed, and then the discharge reaction starts in the coating layer, the nickel hydroxide in the core layer can be completely utilized. In this case, a conventional cobalt-based conductive agent is not required.

[0028] Assume an alkaline storage battery 1 using a positive electrode active material composed of hydroxide 1 and an alkaline storage battery 2 using a positive electrode active material composed of hydroxide 2, and assume that the battery voltage 1 of the alkaline storage battery 1 is higher than the battery voltage 2 of the battery 2 (battery voltage 2 < battery voltage 1). However, in this assumption, there is no difference between the alkaline storage battery 1 and the battery 2 other than the positive electrode active material, that is, the difference between hydroxide 1 and hydroxide 2.

[0029] When discharging an alkaline storage battery using such particles as the positive electrode active material, it is considered that the discharge of the core layer proceeds preferentially (first) in accordance with the above relationship: battery voltage 2 < battery voltage 1. On the other hand, it is considered that nickel hydroxide remains in the state of NiOOH in the coating layer while the discharge reaction of the core layer is proceeding in accordance with the above relationship: battery voltage 2 < battery voltage 1. When NiOOH in the core layer is reduced to Ni(OH) ２ and the discharge reaction is completed, it is considered that the discharge reaction of the coating layer starts and proceeds. Through such a discharge reaction, the nickel hydroxide in the core layer is completely utilized.

[0030] Therefore, when the compositions of hydroxide 1 and hydroxide 2 are different and the relationship: battery voltage 2 < battery voltage 1 holds, it can be said that the coating layer composed of hydroxide 2 functions as a conductive layer with respect to the core layer. In this case, the coating layer can replace the cobalt compound that has been conventionally used as a conductive agent. In the present invention, in order to improve the utilization rate of nickel hydroxide, it is preferable to use hydroxide 1 and hydroxide 2 for which the relationship: battery voltage 2 < battery voltage 1 holds.

[0031] In order to form a coating layer having excellent function as a conductive layer, it is more preferable that the relationship: 10 mA ≤ (battery voltage 1 - battery voltage 2) holds. Whether the relationship: 10 mA ≤ (battery voltage 1 - battery voltage 2) holds can be determined, for example, by performing the following steps 1 to 6 for hydroxide 1 and hydroxide 2. When hydroxide 1 and hydroxide 2 for which the relationship: 10 mA ≤ (battery voltage 1 - battery voltage 2) holds are combined, the function of the above conductive layer can be surely imparted to the coating layer.

[0032] (Step 1) Prepare particles 1 consisting of hydroxide 1 that pass through a sieve with a mesh size of 150 μm (100 mesh according to ASTM-E11) according to JIS Z8801-1. Similarly, prepare particles 2 consisting of hydroxide 2.

[0033] (Step 2) A positive electrode plate 1 conforming to a JIS or IEC standard HR6 type (US type AA) nickel-metal hydride battery is prepared from a positive electrode active material slurry 1, which is made by mixing 0.2 parts by mass of hydroxypropyl cellulose, 30 parts by mass of water, and 0.3 parts by mass of polytetrafluoroethylene per 100 parts by mass of particles 1, and a foamed nickel positive electrode core. Similarly, a positive electrode plate 2 for a nickel-metal hydride battery containing particles 2 is prepared.

[0034] (Step 3) Composition: La ０．３０ Sm ０．７０ Mg ０．１０ Ni ３．３３ Al ０．１７ A negative electrode paste is prepared by mixing 0.4 parts by mass of sodium polyacrylate, 0.1 parts by mass of carboxymethylcellulose, 1.0 part by mass of styrene-butadiene rubber latex, 1.0 part by mass of carbon black, and 30 parts by mass of water per 100 parts by mass of hydrogen storage alloy having a volume-average particle size of 60 μm, and a perforated steel plate to produce a negative electrode plate suitable for an HR6 type (US type AA) nickel-metal hydride battery according to JIS standards.

[0035] (Step 4) As a separator, a thickness of 0.1 mm and a basis weight of 53 g / m² is used. ２ Prepare a nonwoven fabric made of sulfonated polypropylene fibers. Prepare an aqueous solution containing KOH, NaOH, and LiOH as an alkaline electrolyte in a mass ratio (KOH:NaOH:LiOH) of 0.8:7.0:0.2.

[0036] (Step 5) Assemble a nickel-metal hydride battery 1 of type HR6 (American type AA) conforming to JIS or IEC standards, with a capacity of 1000 mAh, using the positive electrode plate 1, negative electrode plate, separator, and alkaline electrolyte. However, the above capacity refers to the capacity when the battery is charged at an ambient temperature of 25°C with a current of 0.2 It amperes (A) for 16 hours and then discharged to 1.0V with a current of 0.4 It A. Similarly, assemble a nickel-metal hydride battery 2 using the positive electrode plate 2, negative electrode plate, separator, and alkaline electrolyte.

[0037] (Step 6) The nickel-metal hydride battery 1, which has been initially activated, is charged at an ambient temperature of 25°C with a current of 0.1 ItA for 16 hours, and then discharged to 1.0V with a current of 0.2 ItA. The battery voltage of the nickel-metal hydride battery 1 when the discharge amount reaches 50% of the discharge capacity is defined as battery voltage 1 (unit: mA). Similarly, the battery voltage 2 (unit: mA) of the nickel-metal hydride battery 2 is measured.

[0038] The embodiments described later support the existence of a conductive layer in the positive electrode active material of the present invention when the above relationship: battery voltage 2 < battery voltage 1, preferably the relationship: 10 mA ≤ (battery voltage 1 - battery voltage 2) holds true. In the alkaline storage battery of the present invention where these relationships hold true, a battery capacity comparable to conventional alkaline storage batteries containing cobalt compounds is achieved despite not containing cobalt compounds.

[0039] In the present invention, in order to satisfy the relationship 10mA ≤ (battery voltage 1 - battery voltage 2) and improve the nickel utilization rate in the positive electrode material, the following combination of hydroxide 1 and hydroxide 2 is preferred.

[0040] Hydroxide 1 is a composite metal hydroxide consisting of nickel (Ni), zinc (Zn), and / or magnesium (Mg), wherein the proportion of Ni in hydroxide 1 to the total metal elements contained in hydroxide 1 is 85 mol% or more and 95 mol% or less.

[0041] Hydroxide 2 is either nickel hydroxide or a composite metal hydroxide consisting of nickel (Ni), zinc (Zn), and / or manganese (Mn), wherein the proportion of Ni in hydroxide 2 to the total metal elements contained in hydroxide 2 is 70 mol% or more and 100 mol% or less.

[0042] In particular, positive electrode active material particles are preferred in which hydroxide 1 is a composite hydroxide of Ni and Zn with a molar ratio of Ni to Zn (Ni:Zn) in the range of 90:10 to 95:5, hydroxide 2 is a composite hydroxide of Ni and Mn with a molar ratio of Ni to Mn (Ni:Mn) in the range of 87.5:12.5 to 92.5:17.5, and the proportion of the coating layer to the positive electrode active material particles (total mass of core layer and coating layer) is 10% by mass or more and 20% by mass or less.

[0043] [Method for producing positive electrode active material] Generally, the positive electrode active material of the present invention is produced by a coprecipitation method. Known raw materials and equipment can be used without limitation in the coprecipitation reaction.

[0044] First, the particles that will form the core layer are manufactured. An aqueous solution is prepared containing one or more metal salts, including nickel (Ni), at a concentration that achieves a hydroxide 1 having a predetermined metal atomic composition. Generally, aqueous solutions of nitrate or sulfate salts of each metal are used. While controlling the pH and stirring intensity of this aqueous solution, the hydroxide 1 of the metal is precipitated as particulate coprecipitates. Then, the particles consisting of hydroxide 1 are recovered from the slurry solution by filtration. The recovered hydroxide 1 particles are washed and dried as necessary.

[0045] Next, the hydroxide particle is mixed with an aqueous solution containing a salt of one or more metals including nickel (Ni) at a concentration that realizes hydroxide 2 having a predetermined metal atom composition to prepare a slurry solution consisting of the hydroxide particle and the aqueous solution. The hydroxide particle and the salt concentration of the aqueous solution are set according to a predetermined mass ratio of the core layer and the coating layer. While controlling the pH and stirring intensity of this slurry solution, hydroxide 2 is precipitated as a coprecipitate on the surface of the hydroxide particle. In this way, metal composite hydroxide particles are produced in which the surface of the hydroxide particle is coated with a layer of hydroxide 2. The metal composite hydroxide particles are recovered from the slurry solution by filtration. The recovered metal composite hydroxide particles are washed and dried. The dried metal composite hydroxide particles are crushed and classified as needed. In this way, the positive electrode active material of the present invention in powder form is obtained.

[0046] [Alkaline Storage Batteries] The positive electrode active material of the present invention described above is effective as a positive electrode material for alkaline storage batteries such as nickel-metal hydride batteries and nickel-cadmium storage batteries. Using the positive electrode active material of the present invention described above, the alkaline storage battery of the present invention can be manufactured according to a conventional method including the coprecipitation method described above.

[0047] For example, a positive electrode active material mixture containing the positive electrode active material powder of the present invention is applied to and impregnated onto a positive electrode plate substrate, dried, and a positive electrode plate material with the positive electrode active material arranged is manufactured. The obtained positive electrode plate material is molded into a predetermined shape to obtain a positive electrode plate for the alkaline storage battery of the present invention.

[0048] For example, a negative electrode active material mixture containing particles of the negative electrode active material is applied to and impregnated onto a predetermined positive electrode plate substrate, dried, and a negative electrode plate material with the negative electrode active material arranged on it is manufactured. The obtained negative electrode plate material is molded into a predetermined shape to obtain a negative electrode plate for the alkaline battery of the present invention. If the alkaline battery of the present invention is a nickel-metal hydride battery, hydrogen storage alloy particles are used as the negative electrode active material. If the alkaline battery of the present invention is a nickel-cadmium battery, a cadmium compound is used as the negative electrode active material. Known raw materials can be used without limitation as additives for the negative electrode active material and the non-localized active material mixture.

[0049] The positive electrode plate, negative electrode plate, and separator described above are stacked and housed in a battery casing. Alkaline electrolyte is then filled in, and the battery casing is sealed. Generally, a polypropylene nonwoven fabric is used as the separator. This nonwoven fabric may be surface-treated as appropriate. Any known product can be used as such a separator without limitation.

[0050] Inside and outside the battery casing, according to the type of battery intended, components such as a positive electrode, negative electrode, separator, electrolyte, sealing plate, insulating plate, gasket, safety valve, positive electrode terminal, and negative electrode terminal are arranged.

[0051] [Examples of Cathode Active Material Production] In the following examples and comparative examples, metal raw materials selected from the following metal salts were used. The metal salts include those available as hydrates. The following metal raw materials were dissolved in deionized water.

[0052] Nickel (Ni) salts: Nickel sulfate (NiSO4) ４ ) ・Cobalt (Co) salt: Cobalt sulfate (CoSO) ４ ) Zinc (Zn) salt: Zinc sulfate (ZnSO4) ４ ) ・Magnesium (Mg) salt: Magnesium sulfate (MgSO) ４ ) Manganese (Mn) salt: Manganese sulfate (MnSO4) ４ )

[0053] In the following examples and comparative examples, aqueous sulfuric acid solution, aqueous sodium hydroxide solution, and aqueous ammonia solution were used as pH adjusters for the coprecipitation reaction.

[0054] [Example 1] (Production of core particles) A ​​metal salt aqueous solution 1, in which nickel salt and zinc salt were dissolved as metal salts, was filled into an overflow reactor. The amount of the metal salts to be charged was determined so that nickel and zinc were present in aqueous solution 1 in a molar ratio (Ni:Zn) of 93.3:6.7. The pH of the solution in the reactor was adjusted to 11.0 and the temperature of the solution in the reactor was adjusted to 50°C, and the stirrer in the reactor was driven to start the coprecipitation reaction of nickel hydroxide and zinc hydroxide. The reaction solution was stirred for 30 hours while maintaining the pH at 11.0 and the temperature at 50°C, and it was confirmed that the coprecipitation hydroxide concentration in the slurry in the reactor had reached a steady state. Subsequently, the reaction solution was stirred for 20 hours while maintaining the pH at 11.0 and the temperature at 50°C, and the slurry in the reactor was removed by overflow. The solid components recovered from the slurry were dehydrated and washed with water to obtain the hydrated core particles consisting of hydroxide 1 (composite hydroxide of nickel and zinc). The water content of the above core particles is 15% by mass, and 85% of the mass of the water-containing material is the mass of hydroxide 1.

[0055] (Coating of core particles) The aqueous hydroxide 1 was suspended in distilled water, a pH adjusting agent was added to prepare a hydroxide 1 slurry with a pH of 9.0, and this slurry was packed into the reactor. To avoid oxidation of hydroxide 1, nitrogen gas was supplied to the reactor to replace the dissolved oxygen.

[0056] Separately, aqueous solution 2 was prepared by dissolving nickel salt and manganese salt as metal salts. The amount of the above metal salts added was determined so that nickel and manganese were present in aqueous solution 2 in a molar ratio (Ni:Mn) of 90:10.

[0057] The stirrer in the reactor was activated, and the supply of aqueous solution 2 to the reactor was started. Aqueous solution 2 was supplied continuously, maintaining the pH of the solution in the reactor at 9.0 and the temperature of the solution in the reactor at 75°C. The inflow and stirring of aqueous solution 2 were also maintained. As a result, nickel hydroxide and manganese hydroxide originating from aqueous solution 2 coprecipitated on the surface of the hydroxide 1 particles. After the coprecipitation reaction of aqueous solution 2 was completed, the solid components were recovered from the reaction solution. The recovered material was dehydrated, washed with water, and dried to obtain positive electrode active material particles in which a coating layer of hydroxide 2 (a composite hydroxide of nickel and manganese) was formed on a core layer of hydroxide 1.

[0058] (Percentage of coating layer) The percentage (mass%) of the coating layer in the obtained positive electrode active material particles was calculated using the following formula.

[0059] ((Mass of positive electrode active material particles) - (Mass of hydroxide 1)) ÷ (Mass of positive electrode active material particles) × 100

[0060] Table 1 shows the metallic element composition (ratio of each metal to the total metal, in mole%) of the core layer (hydroxide 1) and coating layer (hydroxide 2) of the positive electrode active material, and the proportion of the coating layer.

[0061] [Examples 2-10] The amount of metal salt used as raw material was changed, and the coprecipitation reaction was carried out under the same conditions as in Example 1 to form core particles and a coating layer. The proportion of the coating layer was calculated in the same manner as in Example 1. Table 1 shows the metal element composition (ratio of each metal to the total metal, unit: mol%) of the core layer (hydroxide 1) and coating layer (hydroxide 2) of the positive electrode active material in Examples 2-10, and the proportion of the coating layer.

[0062] [Comparative Example 1] In Comparative Example 1, a positive electrode active material equivalent to a conventional product was manufactured.

[0063] (Core Particle Production) A metal salt aqueous solution containing nickel salt, cobalt salt, and zinc salt was filled into an overflow reactor. The amount of metal salt to be charged was determined so that nickel, cobalt, and zinc were present in the above metal salt aqueous solution in a molar ratio (Ni:Co:Zn) of 91.7:1.7:6.6. The pH of the solution in the reactor was adjusted to 11.0 and the temperature of the solution in the reactor was adjusted to 50°C, and the stirrer in the reactor was driven to start the coprecipitation reaction of nickel hydroxide and zinc hydroxide. The reaction solution was stirred for 30 hours while maintaining the pH at 11.0 and the temperature at 50°C, and it was confirmed that the coprecipitation hydroxide concentration in the slurry in the reactor had reached a steady state. Subsequently, the reaction solution was stirred for 20 hours while maintaining the pH at 11.0 and the temperature at 50°C, and the slurry in the reactor was removed by overflow. The solid components recovered from the slurry were dehydrated and washed to obtain a hydrated core particle composed of hydroxide 1 (a composite hydroxide of nickel, cobalt, and zinc). The water content of the core particle was 15% by mass, and 85% of the mass of the hydrated material was the mass of hydroxide 1.

[0064] (Coating of Core Particles) A ​​slurry liquid obtained by suspending aqueous hydroxide 1 in distilled water was filled into a reactor. The solution in the reactor was maintained at pH: 9.0 to 10.0 and temperature: 75°C, and a cobalt salt aqueous solution was supplied to the slurry liquid under stirring. As a result, cobalt hydroxide precipitated on the surface of the core particles. The solid components were recovered from the slurry liquid, dehydrated, and washed with water to obtain positive electrode active material precursor particles consisting of core particles and a coating layer made of cobalt hydroxide. These precursor particles were heated in an airflow at 80°C while in contact with a spray of 12 N sodium hydroxide aqueous solution, thereby converting the cobalt hydroxide on the surface of the precursor particles to sodium (Na)-containing cobalt oxyhydroxide. The obtained particles were separated, washed with water, and dried. In this way, positive electrode active material particles were obtained in which a coating layer containing cobalt oxyhydroxide was formed on a core layer made of hydroxide 1.

[0065] (Percentage of coating layer) The percentage of the coating layer was calculated in the same manner as in Example 1. Table 1 shows the metal element composition (ratio of each metal to the total metal, unit: mol%) of the core layer (hydroxide 1) and coating layer (cobalt oxyhydroxide) of the positive electrode active material in Comparative Example 1, and the percentage of the coating layer.

[0066] However, Table 1 lists the hydroxides that make up the core layer and the coating layer. Table 1 does not list the sodium (Na) element contained in the coating layer of Comparative Example 1.

[0067] [Comparative Examples 2-4] The amount of metal salt used as raw material was changed, and the coprecipitation reaction was carried out under the same conditions as in Example 1 to form core particles. The core particles were used as the positive electrode active material particles. Table 1 shows the metal element composition (ratio of each metal to the total metal, unit: mole%) of the positive electrode active material (hydroxide 1) in Comparative Examples 2-4.

[0068] [Confirmation of conductive layer] Steps 1 to 6 below were performed on hydroxide 1 and hydroxide 2 used in Examples 1 to 10, and the battery voltage 1 and battery voltage 2 mentioned above were measured.

[0069] (Step 1) Particles 1 consisting of hydroxide 1 that pass through a sieve with a mesh size of 150 μm (100 mesh according to ASTM-E11) according to JIS Z8801-1 were selected. Similarly, particles 2 consisting of hydroxide 2 were prepared.

[0070] (Step 2) A positive electrode active material slurry 1 was prepared by mixing 0.2 parts by mass of hydroxypropyl cellulose powder, 30 parts by mass of water, and 0.3 parts by mass of polytetrafluoroethylene per 100 parts by mass of particles 1. The slurry 1 was filled into a tape-shaped positive electrode core made of foamed nickel and dried. The dried positive electrode core tape was roll-rolled and cut to produce a positive electrode plate 1 conforming to JIS or IEC standards for an HR6 type (US type AA) nickel-metal hydride battery. Similarly, a positive electrode plate 2 for a nickel-metal hydride battery containing particles 2 was produced.

[0071] (Step 3) Composition: La ０．３０ Sm ０．７０ Mg ０．１０ Ni ３．３３ Al ０．１７ To 100 parts by mass of hydrogen storage alloy having a volume-average particle size of 60 μm, 0.4 parts by mass of sodium polyacrylate, 0.1 parts by mass of carboxymethylcellulose, 1.0 part by mass of styrene-butadiene rubber latex, 1.0 part by mass of carbon black, and 30 parts by mass of water were mixed and kneaded. Thus, a negative electrode mixture paste was obtained. This negative electrode mixture paste was applied to both sides of a nickel-plated perforated steel sheet with a thickness of 60 μm, and the negative electrode mixture paste layer was dried. The obtained perforated steel sheet was roll-rolled and cut to produce a negative electrode plate suitable for an HR6 type (US type AA) nickel-metal hydride battery according to JIS standards.

[0072] (Step 4) As a separator, a thickness of 0.1 mm and a basis weight of 53 g / m² is used. ２ A nonwoven fabric made of sulfonated polypropylene fibers was prepared. As an alkaline electrolyte, an aqueous solution containing KOH, NaOH, and LiOH in a mass ratio (KOH:NaOH:LiOH) of 0.8:7.0:0.2 was prepared.

[0073] (Step 5) Using the positive electrode plate 1, negative electrode plate, separator, and alkaline electrolyte, an HR6 type (US type AA) nickel-metal hydride battery 1 with a capacity of 1000 mAh, conforming to JIS or IEC standards, was assembled. However, the above capacity refers to the capacity when the battery is charged at an ambient temperature of 25°C with a current of 0.2 It amperes (A) for 16 hours and then discharged to 1.0 V with a current of 0.4 It A. Similarly, a nickel-metal hydride battery 2 was assembled using the positive electrode plate 2, negative electrode plate, separator, and alkaline electrolyte.

[0074] (Step 6) The nickel-metal hydride battery 1, which had been initially activated, was charged at an ambient temperature of 25°C with a current of 0.1 ItA for 16 hours, and then discharged to 1.0V with a current of 0.2 ItA. The battery voltage of the nickel-metal hydride battery 1 when the discharge amount reached 50% of the discharge capacity was defined as battery voltage 1 (unit: mA). Similarly, the battery voltage 2 (unit: mA) of the nickel-metal hydride battery 2 was measured.

[0075] Table 1 shows the results of calculating the difference in battery voltage (mA): (battery voltage 1 - battery voltage 2) for hydroxide 1 and hydroxide 2 combined in Examples 1 to 10.

[0076] In all of Examples 1 to 10, the relationship: Battery voltage 2 < Battery voltage 1 was true, and the relationship: 10mA ≤ (Battery voltage 1 - Battery voltage 2) was true.

[0077] [Manufacturing of Nickel-Metal Hydride Batteries] The alkaline storage battery of the present invention was manufactured. Using the positive electrode active materials obtained in Examples 1 to 10 and Comparative Examples 1 to 4, nickel-metal hydride batteries were manufactured according to the following procedure.

[0078] (1) From the positive electrode active material particles, particles that passed through a sieve with a mesh size of 150 μm (100 mesh according to ASTM-E11) in accordance with JIS Z8801-1 were selected.

[0079] (2) For every 100 parts by mass of positive electrode active material particles obtained in step 1, 0.2 parts by mass of hydroxypropyl cellulose powder, 30 parts by mass of water, and 0.3 parts by mass of polytetrafluoroethylene were mixed to prepare a positive electrode active material slurry. This positive electrode active material slurry 1 was filled into a tape-shaped positive electrode core made of foamed nickel and dried. The dried positive electrode core tape was roll-rolled and cut to produce a positive electrode plate conforming to HR6 type (US type AA) nickel-metal hydride battery according to JIS or IEC standards.

[0080] (3) Composition: La ０．３０ Sm ０．７０ Mg ０．１０ Ni ３．３３ Al ０．１７ To 100 parts by mass of hydrogen storage alloy having a volume-average particle size of 60 μm, 0.4 parts by mass of sodium polyacrylate, 0.1 parts by mass of carboxymethylcellulose, 1.0 part by mass of styrene-butadiene rubber latex, 1.0 part by mass of carbon black, and 30 parts by mass of water were mixed and kneaded. Thus, a negative electrode mixture paste was obtained. This negative electrode mixture paste was applied to both sides of a nickel-plated perforated steel sheet with a thickness of 60 μm, and the negative electrode mixture paste layer was dried. The obtained perforated steel sheet was roll-rolled and cut to produce a negative electrode plate suitable for an HR6 type (US type AA) nickel-metal hydride battery according to JIS standards.

[0081] (4) As a separator, thickness 0.1 mm, basis weight 53 g / m ２ A nonwoven fabric made of sulfonated polypropylene fibers was prepared. As an alkaline electrolyte, an aqueous solution containing KOH, NaOH, and LiOH in a mass ratio (KOH:NaOH:LiOH) of 0.8:7.0:0.2 was prepared.

[0082] (5) Using the positive electrode plate 1, negative electrode plate, separator, and alkaline electrolyte described above, an HR6 type (American type AA) nickel-metal hydride battery conforming to JIS or IEC standards was assembled.

[0083] [Unit Capacity] The unit capacity of the obtained nickel-metal hydride batteries was measured. The results are shown in Table 1.

[0084] [Battery Voltage] The nickel-metal hydride battery described above was initially activated. Next, the nickel-metal hydride battery was charged at an ambient temperature of 25°C with a current of 0.1 ItA for 16 hours, and then discharged to 1.0V with a current of 0.2 ItA. Table 1 shows the battery voltage of the nickel-metal hydride battery when the discharge amount reached 50% of the discharge capacity.

[0085]

[0086] [Evaluation] The positive electrode active materials produced in Examples 1 to 10 do not contain cobalt compounds. The unit capacity of the nickel-metal hydride batteries produced in Examples 1 to 10 is equivalent to that of a conventional nickel-metal hydride battery (Comparative Example 1) that uses a cobalt compound as a conductive agent. The positive electrode active material of the present invention and the alkaline storage battery using it have succeeded in both cobalt-free design and increased battery capacity.

[0087] In Examples 1 to 10, the relationship: Battery voltage 2 < Battery voltage 1 holds true. This suggests that the coating layer in the positive electrode active material particles of the present invention functions as a conductive layer, improving the utilization rate of nickel hydroxide in the positive electrode active material.

[0088] The nickel-metal hydride batteries of the present invention (Examples 1-10) exhibit a significantly higher unit capacity compared to nickel-metal hydride batteries using positive electrode active material without a coating layer (Comparative Examples 1-4). This demonstrates that the coating layer present in the positive electrode active material particles of the present invention greatly contributes to increasing the battery capacity.

[0089] The results from Examples 1 to 10 demonstrate that even when the proportion of the coating layer in the positive electrode active material particles varies from 4% to 30% by mass, equivalent high capacity is achieved. It is noteworthy that an effect comparable to that of conventional conductive layers mainly composed of cobalt oxyhydroxide can be obtained with a relatively small amount of coating layer.

[0090] The conductive function of the coating layer of the positive electrode active material in this invention can also be expected in other alkaline batteries that use similar positive electrode active materials, such as nickel-cadmium batteries.

[0091] This invention is effective as a means of reducing the amount of cobalt compounds conventionally used as raw materials for alkaline batteries, or as a means of improving the utilization rate of nickel hydroxide. This invention can also achieve completely cobalt-free alkaline batteries. This invention is attracting attention as a means of supplying low-cost and highly practical alkaline battery products.

[0092] 1 Core layer made of hydroxide 1 2 Coating layer made of hydroxide 2 10 Particles of positive electrode active material

Claims

1. A positive electrode active material for alkaline storage batteries, comprising particles including a core layer and a coating layer disposed on the surface of the core layer, wherein the core layer is made of a hydroxide 1 of one or more metals including nickel (Ni), and the coating layer is made of a hydroxide 2 of one or more metals including nickel (Ni), and the elemental composition of hydroxide 1 and the elemental composition of hydroxide 2 are different.

2. The positive electrode active material for alkaline storage batteries according to claim 1, wherein the hydroxide 1 and / or hydroxide 2 is a composite hydroxide of nickel (Ni) and one or more metals selected from zinc (Zn), magnesium (Mg), manganese (Mn), aluminum (Al), and cobalt (Co).

3. The positive electrode active material for alkaline storage batteries according to claim 1, wherein the hydroxide 1 is a composite metal hydroxide consisting of nickel (Ni), zinc (Zn), and / or magnesium (Mg), and the proportion of Ni in the hydroxide 1 to the total metal elements contained in the hydroxide 1 is 85 mol% or more and 95 mol% or less.

4. The positive electrode active material for alkaline storage batteries according to claim 1, wherein the hydroxide 2 is nickel hydroxide, or a composite metal hydroxide consisting of nickel (Ni), zinc (Zn), and / or manganese (Mn), and the ratio of Ni in the hydroxide 2 to the total metal elements contained in the hydroxide 2 is 70 mol% or more and 100 mol% or less.

5. The positive electrode active material for alkaline storage batteries according to claim 1, wherein the proportion of the coating layer to the particles is 2% by mass or more and 50% by mass or less.

6. When an alkaline battery 1 using a positive electrode active material consisting of hydroxide 1 and an alkaline battery 2 using a positive electrode active material consisting of hydroxide 2 are charged and discharged under the same conditions, provided that there are no differences between the alkaline battery 1 and the alkaline battery 2 other than hydroxide 1 and hydroxide 2, the following relationship holds: battery voltage 2 < battery voltage 1 between the battery voltage 1 of the alkaline battery 1 when the discharge amount of the alkaline battery 1 reaches 50% of its discharge capacity and the battery voltage 2 of the alkaline battery 2 when the discharge amount of the alkaline battery 2 reaches 50% of its discharge capacity, the positive electrode active material for a nickel-metal hydride battery according to claim 1.

7. An alkaline storage battery using the positive electrode active material described in claim 1.

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