Nickel zinc secondary battery

Abstract

To provide a nickel zinc secondary battery which can maintain a capacity as well after cycles.SOLUTION: A nickel zinc secondary battery includes a positive electrode, a negative electrode, and an alkaline electrolyte. The positive electrode includes composite particles including base particles containing a nickel composite hydroxide and including a coating layer containing a cobalt compound covering the surface of the base particles. The composite particles satisfy the relation of Δy / Δx=0 under any circumstances, in the range in which the energy x of incident x-rays is at least 7730 and is 7736 at a maximum, x denoting the energy of the incident X-rays and y denoting the absorption amount of X-rays in an X-ray absorption fine structure (XAFS) spectrum of cobalt measured by a conversion electron yield technique.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a nickel-zinc secondary battery.

Background Art

[0002] A nickel-zinc secondary battery is known as a battery in which the hydrogen storage alloy negative electrode of a nickel-metal hydride secondary battery is replaced with a negative electrode containing zinc or a zinc compound. Zinc is a material that is rich in resources, inexpensive, and has a low environmental load. In addition, the nickel-zinc secondary battery has advantages such as a high open-circuit voltage of 1.8 V, a high theoretical energy density, and a high output.

[0003] As the positive electrode active material of the nickel-zinc secondary battery, nickel composite hydroxide such as nickel hydroxide is used. For example, Patent Document 1 discloses a nickel-zinc secondary battery using particles containing nickel hydroxide as the positive electrode active material.

[0004] Nickel hydroxide is also used as the positive electrode in nickel-metal hydride secondary batteries. In nickel-metal hydride secondary batteries, for the purpose of improving the utilization rate and cycle characteristics of nickel hydroxide, various elements such as Zn, Mg, and Co are dissolved in nickel hydroxide, or a base particle containing a nickel composite hydroxide is coated with a coating layer containing a cobalt compound.

[0005] For example, Patent Document 2 discloses a nickel-metal hydride battery containing nickel hydroxide powder in which 1.5 to 2.8% by mass of zinc is dissolved as the positive electrode active material. In this document, it is said that by setting the solid solution amount of zinc to 1.5 to 2.8% by mass, swelling of the positive electrode active material can be suppressed while maintaining the packing volume density of nickel hydroxide.

[0006] In addition, Patent Document 3 discloses a positive electrode active material including a coating layer containing cobalt oxide, and in X-ray absorption fine structure (XAFS), Co L 3A nickel-hydrogen battery containing composite particles having a peak top in the region where the -edge is 780.5 eV or more and 783 eV or less is disclosed. Patent Document 3 states that since these composite particles have high electron conductivity, they can improve the high-rate characteristics of the nickel-hydrogen battery. Patent Document 3 describes that zinc was dissolved so that the molar ratio of nickel to zinc was Ni:Zn = 0.96:0.04 to obtain this positive electrode active material.

[0007] Thus, in nickel-hydrogen secondary batteries, solid solution of zinc in nickel hydroxide and the like is generally performed. However, if the amount of solid solution of zinc becomes excessive, a decrease in capacity or the like occurs, so it is common to make it less than 5% by mass with respect to nickel hydroxide.

[0008] Note that Non-Patent Document 1 describes the difference between a higher-order cobalt compound in which cobalt is in a higher valence state of 3 or more and a general cobalt oxyhydroxide in which the valence is 3 or less. Non-Patent Document 1 describes that the above higher-order cobalt compound shows disappearance of a peak near the energy of the incident X-ray of 7736 eV in the XAFS spectrum. Also, Non-Patent Document 1 describes that the above higher-order cobalt compound shows a shift of the diffraction peak of the (110) plane of CoOOH to the high-angle side in the X-ray diffraction (XRD) profile.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0010]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] However, according to the studies of the present inventors, in nickel-zinc secondary batteries, when using the nickel composite hydroxide as described above, the capacity reduction due to repeated charge and discharge is remarkable, and it has been clarified that the capacity retention rate after cycling is low.

[0012] An object of the present invention is to provide a nickel-zinc secondary battery having a good capacity retention rate after cycling.

Means for Solving the Problems

[0013] [1] A nickel-zinc secondary battery having a positive electrode, a negative electrode, and an alkaline electrolyte, wherein the positive electrode includes composite particles having base particles containing a nickel composite hydroxide and a coating layer containing a cobalt compound covering the surface of the base particles, in the X-ray absorption fine structure (XAFS) spectrum of cobalt obtained by measurement using the conversion electron yield method for the composite particles, when the energy of the incident X-ray is x and the X-ray absorption amount is y, in the range where the energy x of the incident X-ray is 7730 eV or more and 7736 eV or less, Δy / Δx is always 0 or less, Nickel-zinc secondary battery. [2] A nickel-zinc secondary battery having a positive electrode, a negative electrode, and an alkaline electrolyte, wherein the positive electrode includes composite particles having base particles containing a nickel composite hydroxide and a coating layer containing a cobalt compound covering the surface of the base particles, In the X-ray diffraction (XRD) profile obtained with the energy E of X-rays being 20 keV, the diffraction peak of the (110) plane of CoOOH appears at 2θ = 25.52° or more in the composite particles. Nickel-zinc secondary battery. [3] The coating layer contains a cobalt compound doped with lithium. The nickel-zinc secondary battery according to [1] or [2]. [4] The base particles include nickel hydroxide in which at least zinc is solid-dissolved as the nickel composite hydroxide. The amount of zinc solid-dissolved in the composite particles is 5 to 10% by mass based on the composite particles. The nickel-zinc secondary battery according to any one of [1] to [3]. [5] The negative electrode contains particles of zinc or zinc oxide. The nickel-zinc secondary battery according to any one of [1] to [4]. [6] The alkaline electrolyte contains zinc ions. The concentration of the zinc ions in the alkaline electrolyte is 4% by mass or more in terms of zinc oxide. The nickel-zinc secondary battery according to any one of [1] to [5].

Advantages of the Invention

[0014] According to the present invention, a nickel-zinc secondary battery with a good capacity retention rate after cycling can be provided.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Best Mode for Carrying Out the Invention

[0016] The inventors of the present invention mainly focused on the positive electrode active material to examine the cause of the low capacity retention rate after cycling of nickel-zinc secondary batteries. In nickel-zinc secondary batteries, for example, since zinc oxide and zinc hydroxide contained in the negative electrode elute, a large amount of zinc ions are dissolved in the alkaline electrolyte. The inventors of the present invention focused on the fact that the capacity of the nickel composite hydroxide, which is the positive electrode active material, decreases when the charge-discharge cycle is repeated in such an alkaline electrolyte. Then, the inventors of the present invention found that the capacity decrease is caused by the intercalation (invasion) of zinc ions in the electrolyte into the interlayer of nickel hydroxide, which is a layered compound (also referred to as "zinc poisoning"). It is presumed that the zinc ions that have invaded the interlayer of nickel hydroxide not only cause swelling of the positive electrode active material but also inhibit the diffusion of protons, which is a normal charge-discharge reaction, thereby reducing the capacity retention rate after cycling.

[0017] On the other hand, in the present invention, by increasing the valence of the cobalt compound in the coating layer that coats the nickel composite hydroxide to trivalent or higher, it is found that zinc ions are less likely to invade the base particles containing the nickel composite hydroxide, and thereby, the decrease in the capacity retention rate after cycling can be suppressed. The reason why zinc is less likely to invade the base particles due to the increase in the valence of the cobalt compound contained in the coating layer is not clear, but it is presumed, for example, that due to the change in the valence of cobalt, electrostatic repulsion is likely to occur with zinc ions.

[0018] Composite particles having such a coating layer, after activation, in the X-ray absorption fine structure (XAFS) spectrum of Co obtained by measurement using the conversion electron yield method, the absorption peak attributed to the cobalt oxyhydroxide (CoOOH) component observed in the range where the energy of the incident X-ray is 7730 eV or more and 7736 eV or less disappears. In other words, in the above XAFS spectrum, the slope of the spectrum in the region where it is 7730 eV or more and 7736 eV or less does not become a positive value. Further in other words, in the above region of the XAFS spectrum, when the energy of the incident X-ray is x and the X-ray absorption amount is y, the first derivative coefficient Δy / Δx is always 0 or less. Also, composite particles having such a coating layer, after activation, in the X-ray diffraction (XRD) profile obtained with the X-ray energy E being 20 keV, the position of the diffraction peak of the (110) plane of CoOOH shifts to the high angle side. Specifically, the above diffraction peak appears at 2θ = 25.52° or more. And according to the findings of the present inventors, if the cobalt compound in the coating layer is thus highly ordered, the intrusion of zinc ions into the base particles can be sufficiently prevented, and a decrease in the capacity retention rate due to the intrusion of zinc ions can be suppressed.

[0019] Hereinafter, the nickel-zinc secondary battery according to the present embodiment will be described. In this specification, the description of "~" means a numerical range including the lower limit and the upper limit which are endpoints unless otherwise specified.

[0020] 1. Nickel-zinc secondary battery FIG. 1 is a perspective view showing a partially broken nickel-zinc secondary battery 10 according to an embodiment of the present invention. In the figure, a part of the wound body 16 is not shown.

[0021] As shown in FIG. 1, the nickel-zinc secondary battery 10 according to the present embodiment is, for example, a cylindrical battery of FA size, and has an outer can 12, a sealing body 14, a wound body 16 (electrode group), an electrolytic solution (not shown), an upper insulating member 18, and a lower insulating member 20.

[0022] The outer can 12 is a container for storing the wound body 16. In the present embodiment, it is a bottomed cylindrical container with an open upper end. The outer can 12 has conductivity, and its bottom wall 12A functions as a negative electrode terminal.

[0023] The material constituting the outer can 12 may be any material that has conductivity and corrosion resistance against the electrolyte and the electrochemical reaction inside the battery, and usually includes metal materials such as iron and steel.

[0024] The sealing body 14 is fixed to the opening of the outer can 12 via an insulating packing 22, seals the outer can 12, and provides a positive electrode terminal. The sealing body 14 includes a cover plate 24, a valve body 26, and a positive electrode terminal 28.

[0025] The cover plate 24 is a disc-shaped member having conductivity and has a through hole 24A at the center. The insulating packing 22 has a ring shape surrounding the cover plate 24 and is interposed between the outer can 12 and the sealing body 14. The insulating packing 22 is fixed to the opening edge 12C of the outer can 12 by caulking the opening edge 12C of the outer can 12. Thereby, the cover plate 24 and the insulating packing 22 cooperate with each other to airtightly close the opening of the outer can 12.

[0026] The valve body 26 is a member made of rubber and is disposed on the outer surface of the cover plate 24 so as to close the through hole 24A.

[0027] The positive electrode terminal 28 is a member having a cylindrical shape with a flange made of metal, and is electrically connected to the outer surface of the cover plate 24 so as to cover the valve body 26. The positive electrode terminal 28 presses the valve body 26 toward the cover plate 24. The positive electrode terminal 28 has a gas vent hole (not shown) opened.

[0028] During normal operation, the through-hole 24A is hermetically closed by the valve body 26. On the other hand, when gas is generated inside the outer can 12 and its internal pressure increases, the valve body 26 is compressed by the internal pressure and opens the through-hole 24A. As a result, gas is released to the outside from inside the outer can 12 through the through-hole 24A and the gas vent hole (not shown) of the positive electrode terminal 28. That is, the through-hole 24A, the valve body 26, and the positive electrode terminal 28 form a safety valve for the battery.

[0029] The wound body 16 includes a positive electrode 30, a negative electrode 32, and a separator 34. That is, the wound body 16 is an electrode group wound in a state where the positive electrode 30 and the negative electrode 32 are laminated with the separator 34 interposed therebetween. Specifically, the wound body 16 is wound with the laminated separator 34, positive electrode 30, separator 34, and negative electrode 32 such that the negative electrode 32 is on the outside.

[0030] The negative electrode 32 is disposed on the outermost peripheral surface of the wound body 16, and the negative electrode 32 is in contact with the inner wall surface of the outer can 12. That is, the negative electrode 32 and the outer can 12, which is the negative electrode terminal, are electrically connected to each other.

[0031] On the other hand, a positive electrode lead 36 is connected to the positive electrode 30 of the wound body 16. One end of the positive electrode lead 36 is connected to the positive electrode 30, and the other end is connected to the cover plate 24. Thereby, the positive electrode 30 and the positive electrode terminal 28 are electrically connected to each other via the positive electrode lead 36 and the cover plate 24.

[0032] The upper insulating member 18 is disposed between the wound body 16 and the cover plate 24. Thereby, the negative electrode 32 of the wound body 16 is prevented from contacting the sealing body 14. Further, the upper insulating member 18 has a slit 18A, and the positive electrode lead 36 passes through the slit 18A.

[0033] The lower insulating member 20 is disposed between the wound body 16 and the bottom of the outer can 12. Thereby, the positive electrode 30 of the wound body 16 is prevented from contacting the inner wall surface of the outer can 12.

[0034] An alkaline electrolyte (not shown) is enclosed within the exterior can 12. The alkaline electrolyte is an aqueous solution containing an alkali metal hydroxide. Examples of the alkali metal hydroxide include potassium hydroxide, sodium hydroxide, lithium hydroxide, etc. Among these, potassium hydroxide and lithium hydroxide are preferable. The alkali metal hydroxide may be of one type or a combination of two or more types.

[0035] The concentration of the alkali metal hydroxide in the alkaline electrolyte is not particularly limited, but it is preferably 25 to 45 mass%. The higher the concentration of the alkali metal hydroxide in the alkaline electrolyte, the higher the ionic conductivity, and it is suitable for high output density applications. However, elution of zinc oxide from the negative electrode is likely to occur, and as a result, a decrease in capacity due to zinc poisoning of the above particles is likely to occur. On the other hand, the lower the concentration of the alkali metal hydroxide in the alkaline electrolyte, the less elution of zinc oxide from the negative electrode, and it is suitable for high capacity density applications. However, the ionic conductivity is low, and the output density is likely to be small. Also, the more the amount of zinc dissolved in the particles containing nickel hydroxide in which zinc is dissolved, the more zinc poisoning can be suppressed. However, since the proportion of nickel as the active material decreases, the capacity density decreases. Therefore, it is preferable to adjust the concentration of the alkali metal hydroxide in the alkaline electrolyte and the amount of zinc dissolved in the composite particles according to the use of the battery.

[0036] For example, for high capacity density applications, it is preferable to combine an alkaline electrolyte containing 25 to 35 mass% of an alkali metal hydroxide and composite particles containing nickel hydroxide in which 5 to 7 mass% of zinc is dissolved. On the other hand, for high output density applications, it is preferable to combine an alkaline electrolyte containing 35 to 45 mass% of an alkali metal hydroxide and composite particles containing nickel hydroxide in which 7 to 10 mass% of zinc is dissolved.

[0037] Further, zinc compounds such as zinc oxide and zinc hydroxide may be further dissolved in the alkaline electrolyte. This is to further suppress the elution of zinc and zinc oxide from the negative electrode into the electrolyte. For example, it is preferable that the alkaline electrolyte is one in which zinc oxide is dissolved to the saturation concentration. That is, the concentration of zinc ions in the alkaline electrolyte is preferably 4% by mass or more in terms of zinc oxide. The concentration of zinc ions in the alkaline electrolyte can be measured by high-frequency inductively coupled plasma optical emission spectrometry (ICP).

[0038] In addition, in the nickel-zinc battery, water is generated and consumed during the charge-discharge reaction. Therefore, the change in the concentration of the alkali metal hydroxide in the electrolyte causes the solubility of zinc oxide to increase or decrease, leading to the precipitation of zinc oxide on the positive electrode and promoting the intrusion of zinc ions into the interlayer. Thus, the total water content of the alkaline electrolyte is preferably 1.5 to 2.5 g per 1 Ah of the battery capacity (nominal capacity). When the total water content of the alkaline electrolyte is 1.5 g or more, the change in the concentration of the alkali metal hydroxide in the electrolyte is less (the amount of water generated during charging of 1 Ah is 0.34 g or less), so that the precipitation of supersaturated zinc oxide on the positive electrode can be more effectively suppressed. On the other hand, when the total water content of the alkaline electrolyte is 2.5 g or less, the proportion of the electrolyte in the battery is not too high, so that the decrease in the capacity density of the battery can be reduced more. The total water content in the alkaline electrolyte can be calculated from the mass change or the like when the battery is disassembled and heated or vacuum dried.

[0039] Next, each member constituting the wound body 16 will be described.

[0040] (Positive electrode) The positive electrode 30 includes a positive electrode current collector and a positive electrode mixture.

[0041] The positive electrode current collector can be, for example, a metal foil, a metal porous body in the form of a net, sponge, fiber or felt, or a punched metal or expanded metal. The material of the positive electrode current collector may be any metal material that is stable at the reaction potential of the positive electrode. For example, it can be nickel or stainless steel, and preferably nickel. That is, the positive electrode current collector can be a foamed nickel or a metal body in the form of a net, sponge or fiber made of nickel or nickel-plated.

[0042] The positive electrode mixture is held by the positive electrode current collector and contains a positive electrode active material.

[0043] (Positive electrode active material) The positive electrode active material contains composite particles having base particles containing a nickel composite hydroxide and a coating layer covering the surface of the base particles and containing a cobalt compound.

[0044] The nickel composite hydroxide may be nickel hydroxide, or nickel hydroxide in which one or more foreign metal elements other than nickel such as zinc, cobalt, magnesium 、 manganese, cadmium, iron, aluminum, chromium, yttrium are solid-solved may also be acceptable.

[0045] The base particles may contain nickel hydroxide in which zinc (Zn) is solid-solved. The solid-solved zinc is the zinc added during the preparation of the composite particles. Zinc is solid-solved, for example, in a form that replaces a part of the nickel atomic sites in the base particles. By solid-solubilizing zinc in a larger amount than before during the preparation of the composite particles, it is possible to more easily make the cobalt compound of a higher order than before.

[0046] In addition to or instead of zinc, the base particles may contain nickel hydroxide in which cobalt (Co) is solid-solved. The solid-solved cobalt is the cobalt added during the preparation of the composite particles. Cobalt is solid-solved, for example, in a form that replaces a part of the nickel atomic sites in the base particles.

[0047] The coating layer is arranged to cover at least a part of the surface of the base particles. The coating layer may contain a cobalt compound doped with lithium (Li). The lithium is the lithium added during the preparation of the coating layer. The cobalt compound includes a higher-valent cobalt compound of trivalent or higher such as cobalt oxyhydroxide (CoOOH). Thus, the higher-valent cobalt compound with an alkali metal incorporated into the crystal can extremely effectively suppress the intrusion of zinc. Further, the higher-valent cobalt compound has extremely high conductivity. And by adding lithium during the preparation of the composite particles, it is possible to more easily make the cobalt compound have a higher valence than before.

[0048] Further, the cobalt compound in the coating layer may be doped with sodium (Na). The sodium is the lithium added during the preparation of the coating layer. The sodium is incorporated into the crystal of the cobalt compound to change the crystal structure and enhance the conductivity of the coating layer.

[0049] The thickness of the coating layer is not particularly limited, but can be, for example, 0.05 to 0.5 μm. When the thickness of the coating layer is 0.05 μm or more, it is possible to make it less likely for zinc ions to penetrate between the layers of nickel hydroxide in which zinc is solid-solved. Also, when the thickness of the coating layer is 0.05 μm or more, the conductivity of the positive electrode active material can be further enhanced. Therefore, the utilization rate of the positive electrode active material and the capacity retention rate after cycling can be further increased. When the thickness of the coating layer is 0.5 μm or less, the decrease in the packing volume density can be made less.

[0050] The presence or absence of the coating layer and the thickness of the coating layer can be confirmed by preparing a cross-section sample of the composite particles as the positive electrode active material using a cross-section polisher and observing the cross-section with a scanning electron microscope (SEM).

[0051] The amount of cobalt (Co) in the composite particles can be 2 to 5% by mass with respect to the composite particles.

[0052] The amount of lithium (Li) in the composite particles can be 0.05 to 0.5% by mass based on the composite particles. The lithium is usually doped in the coating layer.

[0053] The amount of sodium (Na) in the composite particles can be 0.1 to 1.0% by mass based on the composite particles. The sodium is usually doped in the coating layer.

[0054] When zinc is dissolved in nickel hydroxide, the amount of zinc dissolved in the composite particles is preferably 3 to 10% by mass, more preferably 5 to 10% by mass, still more preferably 5.1 to 10% by mass, and particularly preferably 6.7 to 10% by mass based on the composite particles. When the amount of zinc dissolved is 3% by mass or more, swelling of the positive electrode accompanying the progress of charge-discharge cycles can be suppressed. At this time, by setting the amount of zinc dissolved to 5% by mass or more, which is more than that in conventional nickel-metal hydride batteries, presumably, the interface structure between the base particles and the coating layer can be changed, making it easier to make the cobalt compound higher-order than before. When the amount of zinc dissolved is 10% by mass or less, the filling capacity density of the nickel composite hydroxide does not decrease too much, so a decrease in discharge capacity density can be suppressed.

[0055] The amounts of cobalt, lithium, sodium, and zinc in the composite particles can be measured by the following procedure using high-frequency inductively coupled plasma optical emission spectrometry (ICP). 1) Prepare a plurality of standard solutions with known concentrations of the measured elements, measure the emission intensity using a high-frequency inductively coupled plasma optical emission spectrometer (ICP apparatus), and create a calibration curve. 2) On the other hand, a sample solution is prepared by dissolving particle powder containing nickel composite hydroxide in nitric acid. The emission intensity of this sample solution is measured using the above apparatus. 3) By comparing the measurement results of the sample solution with the calibration curve, the amounts of the respective elements in the composite particles can be specified. The amount of zinc measured corresponds to the amount of zinc dissolved.

[0056] The above measurement can be performed on composite particles as raw materials for nickel-zinc batteries, or on composite particles recovered by washing the positive electrode obtained by disassembling the battery with ion-exchanged water or the like, drying it to remove the alkaline electrolyte, and then peeling it off from the positive electrode by ultrasonic treatment. Note that it is preferable to remove nickel foam fragments from the recovered composite particles by sieving. In addition, the amount of metal elements other than zinc can also be measured by the same method as described above.

[0057] The average particle size of the particles containing the above nickel composite hydroxide is not particularly limited, but is preferably, for example, 10 to 20 μm. That is, when the average particle size of the above particles is within the above range, the surface area of the particles increases more, so that the utilization rate of the positive electrode active material can be further increased. Note that the average particle size means the average particle size (median diameter) at which the integration based on the mass standard is 50%. The average particle size of the above particles can be measured by the laser diffraction / scattering method using a particle size distribution measuring device.

[0058] (Method for manufacturing positive electrode active material) The above composite particles can be prepared by any method, and can be prepared, for example, by the following method by the reaction crystallization method. 1) First, prepare an aqueous solution containing nickel sulfate and cobalt sulfate. While controlling the pH, ammonium ion concentration, reaction temperature, etc. in this aqueous solution, gradually add an alkaline aqueous solution such as an aqueous sodium hydroxide solution and react to precipitate base particles containing nickel hydroxide. When zinc and / or cobalt are further solid-dissolved in the composite particles, an aqueous solution containing nickel sulfate, zinc sulfate and / or cobalt sulfate is used as the above aqueous solution. Due to the solid solution of zinc, a coating layer containing a cobalt compound of a higher order than before is formed in the next step. 2) Put the obtained base particles into an aqueous ammonia solution, and add an aqueous cobalt sulfate solution to this aqueous solution. Thereby, using the base particles as nuclei, cobalt hydroxide is precipitated on the surface of the nuclei to obtain intermediate particles provided with a layer of cobalt hydroxide. The obtained intermediate particles are convected in air under a high-temperature environment and heat-treated (chemically oxidized) at a predetermined heating temperature for a predetermined heating time. The heat treatment is preferably carried out, for example, at 80 to 100 °C for 30 minutes to 2 hours. By this treatment, cobalt hydroxide on the surface of the intermediate particles is converted into a higher-order cobalt compound (such as cobalt oxyhydroxide). When a cobalt compound doped with lithium (Li) is included in the coating layer, the above heat treatment is carried out while spraying an aqueous lithium hydroxide solution. Note that, during the heat treatment, an aqueous sodium hydroxide solution may be sprayed to enhance the conductivity of the cobalt compound.

[0059] At this time, for example, by the following method, a coating layer containing a higher-order cobalt compound than before is formed. Note that the method for making the cobalt compound higher-order is not limited to these. Also, any one of these methods may be carried out alone, or both may be carried out. The amount of zinc solid-solubilized in the step 1) is 5 to 10% by mass, preferably 5.1 to 10% by mass, more preferably 6.7 to 10% by mass with respect to the composite particles. Thereby, presumably, the interface structure between the base particles and the coating layer is changed, and a coating layer containing a higher-order cobalt compound than before can be formed. In the step 2), by performing the above heat treatment while spraying an aqueous lithium hydroxide solution, lithium can be incorporated into the crystal, and a coating layer containing a higher-order cobalt compound than before can be formed.

[0060] (Other materials) The positive electrode active material may further contain a positive electrode additive and a binder as necessary.

[0061] Examples of the positive electrode additive include yttrium oxide; cobalt compounds such as cobalt oxide, metallic cobalt, and cobalt hydroxide; zinc compounds such as metallic zinc, zinc oxide, and zinc hydroxide; rare earth compounds such as erbium oxide; and niobium oxide. For example, zinc compounds such as zinc oxide and zinc hydroxide may be added for the purpose of suppressing positive electrode swelling. The content of the zinc compound can be 0.1 to 5% by mass with respect to the total mass of the positive electrode active material.

[0062] The binder functions to bind the positive electrode active material and the positive electrode additive to each other and to bind the positive electrode active material and the positive electrode additive to the positive electrode current collector. Examples of the binder include hydrophilic or hydrophobic polymers, such as hydroxypropyl cellulose, carboxymethyl cellulose (CMC), sodium polyacrylate, and fluorine-based polymers (such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), etc.).

[0063] (Negative electrode) The negative electrode 32 includes a negative electrode current collector and a negative electrode mixture.

[0064] The negative electrode current collector may be a non-porous current collector or a porous current collector. The non-porous current collector can be a non-porous foil. The porous current collector can be a net-like, sponge-like, fibrous or felt-like metal porous body, punched metal, expanded metal, etc. The material of the negative electrode current collector may be a metal material having conductivity and being stable even at the reaction potential of the negative electrode, and includes copper and copper alloys (such as brass, etc.), iron, and preferably copper. That is, the negative electrode current collector is preferably a copper foil.

[0065] The surface of the negative electrode current collector may be plated. The metal constituting the plating film is a metal having a higher hydrogen overvoltage than the metal constituting the main body of the negative electrode current collector, preferably a metal having a higher hydrogen overvoltage, more preferably a metal having a higher hydrogen overvoltage than copper, such as tin.

[0066] The negative electrode mixture is held on the negative electrode current collector and contains a negative electrode active material. The negative electrode active material includes at least one of zinc, zinc alloy, and zinc-containing compound. Examples of the metal constituting the zinc alloy include bismuth, aluminum, indium, etc. in addition to zinc. Examples of the zinc-containing compound include zinc oxides such as zinc oxide (1 type / 2 types / 3 types), zinc hydroxide, zinc sulfide, tetrahydroxy zinc ion salt, zinc halide, zinc carboxylate compounds such as zinc acetate, zinc tartrate, and zinc oxalate, magnesium zincate, calcium zincate, barium zincate, zinc borate, zinc silicate, zinc aluminate, zinc fluoride, zinc carbonate, zinc hydrogen carbonate, zinc nitrate, zinc sulfate, etc. Among them, it is preferable that the negative electrode active material contains zinc oxide as a main component. Containing as a main component means that it can be, for example, in an amount of 50% by mass or more based on the total mass of the negative electrode active material. Also, it is preferable that the negative electrode active material further contains zinc (metallic zinc). Zinc can serve as both a discharge reserve and a conductive material.

[0067] The form of the negative electrode active material is, for example, powdery. The particle size of the particles of the negative electrode active material is not particularly limited, but when using zinc or zinc alloy, the average particle size is preferably 10 μm or more and 1000 μm or less, and when using a zinc-containing compound, it is preferably 0.1 μm or more and 100 μm or less. The average particle size can be measured by the same method as described above.

[0068] The negative electrode mixture may further contain a negative electrode additive and a binder.

[0069] The negative electrode additive may be, for example, a component that reduces the elution of the negative electrode active material into the electrolyte. Examples of such negative electrode additives include bismuth oxide, bismuth hydroxide, indium oxide, indium hydroxide, potassium oxalate, and their hydrates. For example, when potassium oxalate and their hydrates elute into the electrolyte, they dissociate into oxalate ions. Thereby, the zinc ions eluted into the electrolyte form a sparingly soluble salt with the oxalate ions and cover the surface of the negative electrode active material, so that the contact between the metallic zinc of the negative electrode active material and the electrolyte can be reduced.

[0070] The binder functions to bind the negative electrode active material and the negative electrode additive to each other, and at the same time, to bind the negative electrode active material, the negative electrode additive, etc. to the negative electrode current collector. Examples of the binder include hydroxypropyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylic acid, sodium polyacrylate, polyimide, polyamideimide, polyamide, styrene butadiene rubber, polyethylene oxide, polytetrafluoroethylene, polyvinylidene fluoride, perfluoroalkoxy fluororesin, ethylene tetrafluoride - hexafluoropropylene copolymer, and the like. Among them, styrene butadiene rubber is preferable from the viewpoints of high binding effect and alkali resistance.

[0071] The content of the binder only needs to be such that the negative electrode mixture can be sufficiently bound to the negative electrode current collector. For example, it can be 1% by mass or more and 5% by mass or less, preferably 1% by mass or more and 3% by mass or less, based on the total amount of the negative electrode mixture.

[0072] (Separator) As described above, the separator 34 is disposed between the positive electrode 30 and the negative electrode 32 (see FIG. 1). The separator 34 may be a non - woven fabric or a microporous film.

[0073] The materials of the non - woven fabric and the microporous film are not particularly limited, and can be polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyphenylene sulfide, polyamide, etc. Among them, polyolefin is preferable, and polypropylene is more preferable, from the viewpoints of mechanical strength and shutdown characteristics.

[0074] The non - woven fabric and the microporous film may each be provided with hydrophilic functional groups by a hydrophilization treatment. For example, the non - woven fabric or the microporous film may be sulfonated by immersion in an acid containing a sulfonic acid group such as sulfuric acid or fuming sulfuric acid, etc., so that a sulfone group is imparted. Thereby, the non - woven fabric and the microporous film can be made more wettable with the alkaline electrolyte.

[0075] The separator 34 may be used alone or in combination of two or more types. For example, as the separator 34, a laminate of a nonwoven fabric and a microporous film subjected to a hydrophilic treatment may be used.

[0076] (Physical properties after activation) The above battery is activated by charge and discharge such as activation treatment. After activation means a state in which a battery assembled by injecting and sealing an electrolyte has undergone an activation treatment process of performing charge and discharge a plurality of times. For example, it means a state in which charge and discharge about 3 to 5 cycles are performed also for quality inspection of the battery. And, for the composite particles obtained from the activated battery, it can be confirmed from the XAFS spectrum or the XRD profile that the cobalt compound in the coating layer is in a higher order than before.

[0077] (XAFS spectrum) The X-ray absorption fine structure (XAFS) spectrum of the cobalt K absorption edge measured by the conversion electron yield method for the composite particles after activation is represented by a graph with the horizontal axis being the energy (eV) of the incident X-ray and the vertical axis being the normalized absorbance (see FIGS. 2 and 3 described later). And, if the peak (absorption peak due to the cobalt oxyhydroxide (CoOOH) component) appearing in the region where x (energy of the incident X-ray) is 7730 eV or more and 7736 eV or less of the above spectrum disappears, it can be said that the cobalt compound in the coating layer is in a higher order than before. Specifically, in the above XAFS spectrum, the slope of the spectrum in the region where it is 7730 eV or more and 7736 eV or less does not become a positive value. In other words, in the above region of the XAFS spectrum, when the energy of the incident X-ray is x and the X-ray absorption amount is y, the first derivative coefficient Δy / Δx is always 0 or less.

[0078] The X-ray absorption spectrum after activation can be measured by the conversion electron yield method using the synchrotron radiation X-ray of the large synchrotron radiation facility SPring-8. Specifically, the positive electrode is taken out from the battery in the discharged state after activation, and the nickel composite hydroxide powder is recovered. The recovered nickel composite hydroxide powder is used as a sample. Then, after applying an appropriate amount of the sample onto a carbon tape, it is placed on the table of a conversion electron yield detector. Next, helium (He) gas is introduced into the detector, and a high voltage of 1 kV is applied. In this state, the sample is irradiated with X-rays, and He atoms ionized by the collision with the battery emitted from the sample are collected by the electrode to measure the XAFS spectrum. At this time, the intensity I before the X-ray irradiation 0 and the ratio of the intensity I after transmission are used to measure the normalized absorbance I / I 0 . I / I 0 The spectrum of is normalized by the jump amount by fitting the pre-edge line and the post-edge line.

[0079] Δy / Δx in the above region is always 0 or less. In other words, the maximum value is 0 or less, but it is preferably -0.002 or less.

[0080] (XRD profile) When the energy E of the X-ray is 20 keV in the XRD profile, the diffraction peak of the (110) plane of CoOOH appears at 2θ = 25.52° or more for the composite particles after activation. Usually, the diffraction peak of the (110) plane appears at 2θ = 25 to 25.50°. From the shift of the position of this peak, it can be seen that the cobalt compound in the coating layer is more highly ordered than before.

[0081] The XRD profile of the composite particles after activation can be measured by X-ray diffraction using synchrotron radiation from Spring8. Specifically, the positive electrode is taken out from the battery in the discharged state after activation, and the nickel composite hydroxide powder is recovered. The recovered nickel composite hydroxide powder is used as a sample. Then, with the energy E of the X-ray being 20 keV, an X-ray diffraction (XRD) profile is obtained.

[0082] The diffraction peak of the above-mentioned mirror index (110) plane preferably appears at 2θ = 25.52 to 26.00°, more preferably at 2θ = 25.55 to 26.00°.

[0083] (Amount of zinc in the composite particles) It can be, for example, 10 to 15% by mass with respect to the total amount of nickel and zinc contained in the total amount of zinc in the composite particles after activation. The total amount of zinc means the total amount of zinc including dissolved zinc and free zinc.

[0084] The total amount of zinc contained in the particles containing the nickel composite hydroxide after activation can be measured by composition analysis by SEM / EDX and composition analysis by ICP. Specifically, the composite particles are recovered from the battery after activation, and 25 or more SEM / EDX measurements are performed by point analysis and averaged to obtain the ratio of the amount of zinc to the total amount of nickel and zinc (Ni / (Ni + Zn)).

[0085] 2. Method for manufacturing a nickel-zinc secondary battery The above nickel-zinc secondary battery can be manufactured by any method. For example, the above nickel-zinc secondary battery can be manufactured through 1) a step of preparing a positive electrode, a negative electrode, and a separator member, and 2) a step of obtaining a nickel-zinc secondary battery using the prepared positive electrode, negative electrode, and separator member.

[0086] Regarding the step of 1) First, a positive electrode, a negative electrode, and a separator member are prepared.

[0087] The positive electrode 30 can be manufactured, for example, by the following procedure. First, a positive electrode active material containing the above composite particles, a conductive assistant, a positive electrode additive, a binder, and water or a solvent are mixed and kneaded to obtain a positive electrode mixture slurry. Then, the obtained positive electrode mixture slurry is applied and dried on a positive electrode current collector, and then rolled and cut to a predetermined size to obtain a positive electrode. The negative electrode 32 can be obtained in the same manner.

[0088] Regarding the step of 2) Next, a nickel-zinc secondary battery is fabricated using the prepared positive electrode, negative electrode, and separator member.

[0089] Specifically, the prepared positive electrode 30 and negative electrode 32 are wound in a stacked state through a separator 34 (for example, a laminate of a nonwoven fabric and a microporous film) to form a wound body. A positive electrode lead 36 is welded to one end of the positive electrode 30 in the length direction. Then, for example, after laminating the separator 34, the positive electrode 30, the separator 34, and the negative electrode 32 in this order, they are wound along the length direction so that the negative electrode 32 is on the outside to obtain a wound body 16.

[0090] The obtained wound body 16 is housed in an exterior can 12, an electrolytic solution is injected, and then the opening of the exterior can 12 is sealed with a sealing body 14.

[0091] After leaving it for a certain period of time, an activation treatment is performed by charging under predetermined conditions. The activation conditions can be adjusted according to the properties of the electrode active materials (positive electrode active material and negative electrode active material). In this embodiment, for example, after charging to the nominal capacity at a constant current - constant voltage of 1.9 V, a cycle of discharging to 1.3 V can be performed 5 times. Thereby, the nickel-zinc secondary battery 10 can be obtained.

[0092] 3. Modification In the above embodiment, an example of a cylindrical nickel-zinc secondary battery is shown, but it is not limited thereto, and a square or laminated nickel-zinc secondary battery may also be used.

Example

[0093] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

[0094] 1. Preparation of Positive Electrode Active Material Particles (Composite Particles) <Preparation of Positive Electrode Active Material Particles A> The positive electrode active material particles A were prepared by a reaction crystallization method. Specifically, nickel sulfate, zinc sulfate, and cobalt sulfate were mixed to prepare an aqueous solution such that the ratios of nickel, zinc, and cobalt were the ratios described in Table 1. To this, the pH, reaction temperature, and ammonium ion concentration were adjusted to predetermined conditions, and an aqueous sodium hydroxide solution was added and continuously reacted to gradually grow crystals, thereby obtaining base particles of nickel hydroxide in which zinc and cobalt were solid-soluted.

[0095] The obtained base particles were put into an aqueous ammonia solution, and an aqueous cobalt sulfate solution was added while adjusting the pH to precipitate cobalt hydroxide on the surface of the base particles. An aqueous 12N (normality) sodium hydroxide solution was sprayed under an atmosphere containing oxygen and heat-treated for chemical oxidation to convert the precipitated cobalt hydroxide into cobalt oxyhydroxide (a conductive cobalt compound). Thereby, positive electrode active material particles A having a coating layer containing cobalt oxyhydroxide into which sodium was introduced on the surface of the base particles were obtained.

[0096] <Preparation of Positive Electrode Active Material Particles B> In the preparation of the base particles, the blending amounts of nickel sulfate, zinc sulfate, and cobalt sulfate were changed to the ratios of nickel, zinc, and cobalt in Table 1, and positive electrode active material particles B were prepared in the same manner as positive electrode active material particles A except that a 4N (normality) lithium hydroxide aqueous solution was sprayed together with the aqueous sodium hydroxide solution during the heat treatment.

[0097] <Preparation of Positive Electrode Active Material Particles C> In the preparation of the base particles, positive electrode active material particles C were prepared in the same manner as positive electrode active material particles A except that the blending amounts of nickel sulfate, zinc sulfate, and cobalt sulfate were changed to the ratios of nickel, zinc, and cobalt in Table 1.

[0098] <Measurement of Physical Properties>

[0099] (Bulk Density) The tap bulk density was measured as the apparent density when the positive electrode active material particles were filled in a container and tapped to fill the gaps between the particles.

[0100] (Average particle diameter) Using the laser diffraction / scattering method, the average particle diameter (median diameter) corresponding to 50% of the mass-based integration was measured.

[0101] The compositions and physical properties of the prepared positive electrode active material particles A to C are shown in Table 1.

Table 1

[0102] (Cross-sectional observation of positive electrode active material particles) For the obtained positive electrode active material particles, cross-sectional samples were prepared using a cross-section polisher (CP), and the cross-sections of the particles were observed with a scanning electron microscope (SEM). As a result, it was confirmed that all of the obtained positive electrode active material particles were composite particles in which the surface of the base particles was covered with a coating layer. Also, the thickness of the coating layer was about 0.1 μm for all of them.

[0103] 2. Fabrication of battery [Example 1] (Fabrication of positive electrode) 100 parts by mass of the positive electrode active material powder composed of the prepared positive electrode active material particles A, 1 part by mass of zinc oxide powder, cobalt hydroxide powder, yttrium oxide powder, niobium oxide powder, thickener, PTFE, and water were mixed in predetermined amounts to prepare a positive electrode mixture slurry.

[0104] The obtained positive electrode mixture slurry was filled into foamed nickel, dried, rolled, and cut to a predetermined size. Thereby, a positive electrode having a capacity of 13.4 mAh / cm 2 per unit area was fabricated.

[0105] (Fabrication of negative electrode) 100 parts by mass of zinc oxide powder, 25 parts by mass of metallic zinc powder, bismuth oxide powder, potassium oxalate monohydrate, thickener, water, and styrene-butadiene rubber were mixed in predetermined amounts to prepare a negative electrode mixture slurry.

[0106] The obtained negative electrode mixture slurry was applied to a copper non-porous foil with tin plating on its surface. After drying this electrode plate, it was rolled with a rolling roll and cut to a predetermined size. Thereby, a negative electrode having a capacity of 25 mAh / cm per unit area was produced. 2 was produced.

[0107] (Preparation of electrolyte solution) An electrolyte solution was prepared by dissolving 4 mass% of zinc oxide in an aqueous solution containing 6 mol / L of potassium hydroxide and 0.5 mol / L of lithium hydroxide.

[0108] (Preparation of battery) The above-prepared positive electrode, negative electrode, and a separator group in which a non-woven fabric separator and a polypropylene microporous membrane subjected to a hydrophilic treatment were integrated were wound together to obtain an electrode group. Specifically, those laminated in the order of separator group / positive electrode / separator group / negative electrode were wound to obtain an electrode group with the negative electrode disposed on the outermost peripheral surface. This was inserted into an exterior can with tin plating. Then, a predetermined amount of the above-prepared electrolyte solution was injected to produce a cylindrical nickel-zinc secondary battery with a nominal capacity of 2000 mAh. Two nickel-zinc secondary batteries were produced for analysis and cycle characteristic evaluation.

[0109] (Activation treatment) The obtained battery was subjected to an activation treatment by performing 5 cycles of charging up to the nominal capacity and discharging to 1.3 V. After charging up to the nominal capacity at a constant current-constant voltage of 1.9 V, it was discharged to 1.3 V.

[0110] [Example 2] A positive electrode was produced in the same manner as in Example 1 except that the positive electrode active material particles A were changed to positive electrode active material particles B, and a battery was produced.

[0111] [Comparative Example 1] A positive electrode was produced in the same manner as in Example 1 except that the positive electrode active material particles A were changed to positive electrode active material particles C, and a battery was produced.

[0112] [Comparative Example 2] The positive electrode was fabricated in the same manner as in Example 1, except that the positive electrode active material particles A were changed to positive electrode active material particles B, and the amount of zinc oxide powder added during the fabrication of the positive electrode was increased to 4.3 parts by mass so that the total amount of zinc in the positive electrode was the same as in Example 1, and then the battery was fabricated.

[0113] 3. Evaluation (1) Evaluation of the positive electrode active material powder (Sampling of the positive electrode active material powder) The positive electrode active material particles were sampled from the activated battery by the following procedure. That is, the activated battery was disassembled in the discharged state to recover the positive electrode, which was then washed with ion-exchanged water and dried at room temperature under vacuum to remove the electrolyte. Thereafter, the active material was detached from the positive electrode using an ultrasonic generator, and coarse particles were removed using a sieve with a mesh size of 75 μm. Then, for the positive electrode active material particles recovered from the activated battery, X-ray absorption spectrum (ZAFS) measurement and X-ray diffraction (XRD) profile measurement were performed.

[0114] (Measurement of the X-ray absorption spectrum) The X-ray absorption fine structure (XAFS) spectrum of cobalt in the composite particles was measured by the conversion electron yield method using the synchrotron radiation X-ray of the large-scale synchrotron radiation facility SPring-8. After an appropriate amount of the sample was applied onto a carbon tape, it was placed on the table of the conversion electron yield detector. Next, helium (He) gas was introduced into the detector, and a high voltage of 1 kV was applied. In this state, the sample was irradiated with X-rays, and the He atoms ionized by the collision with the battery emitted from the sample were collected by the electrode to measure the XAFS spectrum. At this time, the normalized absorbance I / I was measured from the ratio of the intensity I before the X-ray irradiation and the intensity I after the transmission. 0 and the intensity I after the transmission, and the normalized absorbance I / I 0 was measured. I / I 0The spectrum is normalized by the jump amount by fitting the pre-edge line and the post-edge line. In Fig. 2, the horizontal axis represents the X-ray energy (eV), and the vertical axis represents the normalized absorbance (a.u.). Thus, a spectrum as shown in Fig. 2 was obtained. Fig. 3 is an enlarged view of the spectrum shown in Fig. 2 in the range where the X-ray energy is 7730 eV or more and 7736 eV or less. The horizontal axis (x-axis) represents the X-ray energy (eV), and the vertical axis (y-axis) represents the absorbance normalized by the main axis (a.u.), and the second axis represents the value of Δy / Δx, respectively.

[0115] (Measurement of XRD profile) The XRD spectrum of the composite particles was measured using synchrotron radiation X-rays from Spring8 with the X-ray energy E set to 20 keV. Thus, an XRD profile as shown in Fig. 3 was obtained.

[0116] (2) Cycle test The above battery was subjected to a cycle test in an environment at 25°C. First, constant current constant voltage (CCCV) charging was performed at a rate of 0.5C and an upper limit voltage of 1.9V over 24 hours. Then, after providing a 15-minute rest, constant current (CC) discharge at 0.5C was performed until the voltage reached 1.3V to obtain the initial discharge capacity.

[0117] Next, after a 15-minute rest, constant current (CC) charging was performed at a rate of 0.5C up to 1.95V, and after a 3-minute rest, constant current constant voltage (CCCV) charging was performed at a rate of 1.0C with an upper limit voltage of 1.9V and an end current of 0.2C. Then, after providing a 15-minute rest, constant current (CC) discharge at 1.0C was performed until the voltage reached 1.3V, and this was defined as one cycle. Charge and discharge were performed 49 cycles under these conditions. Finally, constant current constant voltage (CCCV) charging was performed at a rate of 0.5C and an upper limit voltage of 1.9V over 24 hours. Then, after providing a 15-minute rest, constant current (CC) discharge at 0.5C was performed until the voltage reached 1.3V. Taking this 50-cycle charge and discharge as one set, 10 sets (a total of 500 times) of charge and discharge were performed.

[0118] Based on the following formula, the initial capacity ratio was calculated from the discharge capacity at the 500th cycle and the initial discharge capacity. Initial capacity ratio (%) = [(Discharge capacity at the 500th cycle) / (Initial discharge capacity)] × 100

[0119] The evaluation results of Example 1 and 2, and Comparative Example 1 and 2 are shown in Table 2. Table 2 shows the amount of zinc oxide added during the preparation of the positive electrode mixture slurry (the addition amount of zinc oxide (mass %) when the amount of positive electrode active material particles is 100 parts by mass), the value of Δy / Δx calculated from the XAFS spectrum, the diffraction peak position of the 110 plane of CoOOH calculated from the XRD spectrum, the initial capacity ratio, and the number of evaluated batteries. The initial capacity ratio is the average value of the evaluated values for a plurality of batteries. In addition, the XAFS spectra of each battery are shown in FIGS. 2 and 3, and the XRD profiles of each battery are shown in FIG. 4, respectively.

[0120]

Table 2

[0121] (Regarding the cycle test) As shown in Table 2, it can be seen that the batteries of Example 1 and 2 using composite particles in which the cobalt compound of the coating layer is higher-ordered than before have a high initial capacity ratio of 70% or more.

[0122] In comparison, it can be seen that the battery of Comparative Example 1 using composite particles in which the cobalt compound of the coating layer is of the same level as before has a low initial capacity ratio of 62%. Also, it can be seen that the battery of Comparative Example 2 in which the addition amount of zinc oxide to the positive electrode was increased to make the total zinc amount of the positive electrode the same as that of Example 1 also has a low initial capacity ratio of 60%.

[0123] From these facts, it can be understood that it is important to make the cobalt compound of the coating layer higher-ordered than before to improve the capacity retention rate of the battery.

[0124] (Regarding XAFS spectra and XRD profiles) As shown in FIGS. 2 and 3, in Examples 1 and 2, when the energy of the incident X-ray in the XAFS spectrum is x and the X-ray absorption amount is y, in the range where the energy x of the incident X-ray is 7730 eV or more and 7736 eV or less, the absorption peak due to the cobalt oxyhydroxide (CoOOH) component disappears, and Δy / Δx is always 0 or less. On the other hand, in Comparative Examples 1 and 2, in the above range, the absorption peak due to the cobalt oxyhydroxide (CoOOH) component exists, and Δy / Δx is partially positive. Also, as shown in FIG. 4, in Examples 1 and 2, the diffraction peak position of the 110 plane of CoOOH in the XRD profile appears at 2θ = 25.52° or more, while in Comparative Examples 1 and 2, the diffraction peak position of the 110 plane is less than 2θ = 25.52°. From these facts, it can be seen that by increasing the amount of zinc dissolved in the positive electrode active material or doping lithium into the coating layer, the cobalt compound in the coating layer has been made higher-order than in the past (for example, Comparative Examples 1 and 2).

[0125] And in Examples 1 and 2, the battery capacity after 500 cycles is less likely to decrease than in Comparative Examples 1 and 2. From this, it can be seen that by making the cobalt compound in the coating layer higher-order, the capacity retention rate of the nickel-zinc battery after cycling can be increased.

[0126] By the way, when comparing Comparative Examples 1 and 2, in Comparative Example 2 where the amount of zinc oxide added to the positive electrode active material is large, the capacity retention rate after cycling is lower than that in Comparative Example 1 where the amount of added zinc oxide is small. From this, it can be seen that zinc in the vicinity of the electrode active material reduces the capacity of the nickel-zinc battery after cycling. Regarding this point, the present inventors further studied, and OH in the electrolytic solution - and water molecules coordinated with tetracoordinated zinc ions (for example, Zn(OH) 4 2- or Zn(H 2 O) 4 2+)It has been found that similar free zinc penetrates into the Ni - O layer of nickel composite hydroxide contained in base particles, and due to this zinc poisoning, the positive electrode active material (composite particles) deteriorates and the capacity decreases after cycling. In nickel - zinc batteries, unlike nickel - hydrogen batteries, zinc or zinc oxide is used for the negative electrode, so zinc atoms eluted from the negative electrode exist in the electrolyte as a large amount of free zinc. Therefore, in nickel - zinc batteries, capacity decrease due to the above - mentioned free zinc is likely to occur. By increasing the order of the cobalt compound in the coating layer, it is considered that the penetration of free zinc into the base particles is suppressed, and the capacity decrease after cycling due to this is suppressed.

Industrial Applicability

[0127] According to the present invention, a nickel - zinc secondary battery with a good capacity retention rate after cycling can be provided.

Explanation of Signs

[0128] 10 Nickel - zinc secondary battery 12 Exterior can 12A Bottom wall 12C Opening edge 14 Sealing body 16 Wound body (electrode group) 18 Upper insulating member 20 Lower insulating member 22 Insulating packing 24 Cover plate 26 Valve body 28 Positive electrode terminal 30 Positive electrode 32 Negative electrode 34 Separator

Claims

1. A nickel-zinc secondary battery having a positive electrode, a negative electrode, and an alkaline electrolyte, wherein the positive electrode includes composite particles having base particles containing a nickel composite hydroxide and a coating layer containing a cobalt compound that covers the surface of the base particles, and in the X-ray absorption fine structure (XAFS) spectrum of cobalt obtained by measurement using the conversion electron yield method for the composite particles, when the energy of the incident X-ray is x and the X-ray absorption amount is y, Δy / Δx is always 0 or less in the range where the energy x of the incident X-ray is 7730 eV or more and 7736 eV or less. A nickel-zinc secondary battery.

2. A nickel-zinc secondary battery having a positive electrode, a negative electrode, and an alkaline electrolyte, wherein the positive electrode includes composite particles having base particles containing a nickel composite hydroxide and a coating layer containing a cobalt compound that covers the surface of the base particles, and in the X-ray diffraction (XRD) profile obtained with the energy E of the X-ray being 20 keV for the composite particles, the diffraction peak of the (110) plane of CoOOH appears at 2θ = 25.52° or more. A nickel-zinc secondary battery.

3. The coating layer contains a cobalt compound doped with lithium. The nickel-zinc secondary battery according to Claim 1 or 2.

4. The base particles include nickel hydroxide in which at least zinc is solid-dissolved as the nickel composite hydroxide, and the solid-dissolved amount of zinc in the composite particles is 5 to 10% by mass with respect to the composite particles. The nickel-zinc secondary battery according to Claim 1 or 2.

5. The negative electrode includes particles of zinc or zinc oxide. The nickel-zinc secondary battery according to Claim 1 or 2.

6. The alkaline electrolyte contains zinc ions, and the concentration of the zinc ions in the alkaline electrolyte is 4% by mass or more in terms of zinc oxide conversion. The nickel-zinc secondary battery according to Claim 1 or 2.

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