Nickel-metal hydride secondary battery

Abstract

To provide a nickel-metal hydride battery that maintains the corrosion resistance of a hydrogen storage alloy while suppressing deterioration of low-temperature discharge characteristics.SOLUTION: A nickel-metal hydride battery 1 includes a non-sintered positive electrode plate 3 containing a positive electrode active material including nickel hydroxide, and a non-sintered negative electrode plate 4 containing a hydrogen storage alloy and facing the positive electrode plate via a separator 5, and is housed in an exterior can 2 together with an alkaline electrolyte. The positive electrode plate contains an additive containing either an yttrium compound or an ytterbium compound. The negative electrode plate contains the same type of additive. The additive added to the negative electrode plate is less soluble in the alkaline electrolyte than the additive added to the positive electrode plate.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a nickel-hydrogen secondary battery, and more particularly to a non-sintered nickel-hydrogen secondary battery. [Background technology]

[0002] Nickel-metal hydride secondary batteries are used in a variety of applications, and in particular in industrial applications, such as backup power sources and vehicle-mounted power sources, there is a demand for use in a wide range of environmental temperatures, from high to low.

[0003] In general, in a secondary battery containing an aqueous electrolyte, oxygen gas is generated at the positive electrode in a high-temperature environment, and side reactions become dominant, making it difficult to sufficiently charge the positive electrode active material, which is called a decrease in charge acceptance. Therefore, attempts are being made to improve the charge acceptance by adding yttrium, ytterbium, erbium compounds, etc. to the positive electrode.

[0004] Furthermore, yttrium, ytterbium, erbium compounds, and the like have the effect of suppressing corrosion of hydrogen storage alloys and contribute to extending the life of batteries, so they are sometimes added not only to the positive electrode but also to the negative electrode. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-28992 [Patent Document 2] Japanese Patent Application Publication No. 6-215765 [Patent Document 3] Japanese Patent Application Publication No. 9-92279 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when yttrium, ytterbium, erbium compounds, etc. are added to the negative electrode, the reactivity of the hydrogen storage alloy after initial activation is reduced, and the low-temperature discharge performance of the battery is reduced. This is because the addition of yttrium, ytterbium, erbium compounds, etc. suppresses corrosion, reducing the catalytic layer on the surface of the hydrogen storage alloy, and resulting in a shortage of electrons at the electrode plate interface during discharge, especially in a low-temperature discharge environment.

[0007] Furthermore, when yttrium, ytterbium, or erbium compounds were added only to the positive electrode, the effect of suppressing corrosion of the hydrogen storage alloy in the negative electrode was observed, but the contribution to improving the battery life was small.

[0008] An object of the present invention is to provide a nickel-metal hydride secondary battery which is capable of maintaining high-temperature charge-discharge efficiency while improving low-temperature discharge characteristics. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the nickel-hydrogen secondary battery of the present invention is a nickel-hydrogen secondary battery comprising a non-sintered positive electrode plate containing a positive electrode active material including nickel hydroxide, and a non-sintered negative electrode plate containing a hydrogen storage alloy and facing the positive electrode plate via a separator, the nickel-hydrogen secondary battery being housed in an outer can together with an alkaline electrolyte, wherein the positive electrode plate contains an additive containing either an yttrium compound or an ytterbium compound, the negative electrode plate contains the additive, and the additive added to the negative electrode plate has lower solubility in the alkaline electrolyte than the additive added to the positive electrode plate. Effect of the Invention

[0010] According to the nickel-metal hydride secondary battery of the present invention, the corrosion resistance of the hydrogen storage alloy can be improved to maintain a long battery life, while suppressing deterioration of low-temperature discharge characteristics. [Brief description of the drawings]

[0011] [Figure 1]1 is a partially cutaway perspective view of a nickel-metal hydride battery according to an embodiment of the present invention; [Diagram 2] 1 is a table showing the types, particle sizes, particle size ratios, low-temperature discharge characteristics, and life spans of additives contained in the electrode plates of the batteries of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The nickel-metal hydride battery according to this embodiment is applied to a cylindrical battery.

[0013] 1. Nickel-metal hydride battery structure 1, a nickel-metal hydride battery (hereinafter referred to as "battery") 1 has a spiral electrode group 6 housed together with an alkaline electrolyte in a cylindrical exterior can 2 of 4 / 3FA size. The exterior can 2 has a cylindrical shape with a bottom and an open upper end, and the upper end is sealed with a sealing body 7.

[0014] The bottom wall 8 of the exterior can 2 is conductive and functions as a negative electrode terminal. The sealing body 7 includes a cover plate 9 and a positive electrode terminal 10. The cover plate 9 is conductive and has a gas vent hole 11 in the center, and a rubber valve body 12 that closes the gas vent hole 11 is disposed on the outer surface of the cover plate 9. The cover plate 9 is disposed on the opening end of the exterior can 2 via a ring-shaped gasket 13, and the opening edge of the exterior can 2 is crimped to close the opening. A positive electrode terminal 10 is attached to the cover plate 9.

[0015] The electrode group 6 is composed of a strip-shaped positive electrode plate 3, a negative electrode plate 4, and a separator 5, which are wound in a spiral shape with the separator 5 sandwiched between the positive electrode plate 3 and the negative electrode plate 4, forming a substantially cylindrical shape. That is, the positive electrode plate 3 and the negative electrode plate 4 face each other via the separator 5, and are overlapped in the radial direction of the outer casing 2.

[0016] In the exterior can 2, a positive electrode lead 14 is disposed between one end of the electrode group 6 and the cover plate 9, and each end of the positive electrode lead 14 is electrically connected to the positive electrode plate 3 and the cover plate 9, respectively.

[0017] The positive electrode plate 3 is composed of a conductive positive electrode core having a porous structure, and a positive electrode mixture that mainly contains nickel hydroxide and is applied to the surface and inside the pores of the positive electrode core. For example, a nickel-plated mesh-like, sponge-like, or fibrous metal body or foamed nickel is used as the positive electrode core.

[0018] The positive electrode mixture includes positive electrode active material particles, a conductive material, a positive electrode additive, and a binder. The positive electrode active material particles are nickel hydroxide (Ni(OH)2) particles or high-order nickel hydroxide particles. Cobalt hydroxide (Co(OH)2) is used as the conductive material. Either yttrium oxide (Y2O3) or ytterbium oxide (Yb2O3) is used as the positive electrode additive.

[0019] The negative electrode plate 4 is composed of a conductive negative electrode core in the form of a strip, and a negative electrode mixture applied to the negative electrode core. The negative electrode core is made of a sheet-like metal material with through holes distributed therein, for example, a punched metal sheet made of iron with a nickel-plated surface. When the negative electrode mixture is applied to the negative electrode core, it forms a negative electrode mixture layer. The negative electrode mixture contains particles of a hydrogen storage alloy, a negative electrode additive, a conductive material, and a binder. The negative electrode additive uses the same type of additive as the positive electrode additive. The additive added to the negative electrode plate 4 is added to the negative electrode plate 4 so that its solubility in an alkaline electrolyte is lower than the solubility of the additive added to the positive electrode plate 3 in an alkaline electrolyte.

[0020] The hydrogen storage alloy is an alloy capable of absorbing and releasing hydrogen, which is a negative electrode active material. As the hydrogen storage alloy, a general hydrogen storage alloy can be used. Here, in the present disclosure, it is preferable to use a rare earth-Mg-Ni based hydrogen storage alloy containing rare earth elements, Mg, and Ni. As the rare earth elements, for example, La, Ce, Pr, Nd, Sm, Gd, and Y are used.

[0021] Separator 5 is made of, for example, a nonwoven fabric made of polypropylene fibers that have been subjected to a fluorine treatment or a sulfonation treatment.

[0022] An alkaline electrolyte is poured into the exterior can 2. The alkaline electrolyte is impregnated into the positive electrode plate 3, the negative electrode plate 4, and the separator 5, and contributes to an electrochemical reaction, that is, a charge / discharge reaction, between the positive electrode plate 3 and the negative electrode plate 4. The alkaline electrolyte is an aqueous solution containing KOH, NaOH, and LiOH.

[0023] Solubility is a qualitative characteristic. In the present disclosure, the "solubility" refers to the degree of solubility of the additive particles contained in the mixture layer of each electrode plate in an alkaline electrolyte. For example, when a predetermined amount of a positive electrode mixture containing an additive and an equal amount of a negative electrode mixture containing an additive are separately placed in an alkaline electrolyte, the mixture with a higher concentration (wt%) of the additive dissolved in the alkaline electrolyte has a higher solubility than the other mixture.

[0024] For example, if the surface area per unit volume of the additive particles in contact with the alkaline electrolyte is large, the solubility in the alkaline electrolyte is greater than that of those with a small surface area. A method for increasing the surface area of ​​the additive particles per unit volume is to reduce the particle diameter and increase the density of the particles per unit volume. Alternatively, from the moment when the additive contained in the mixture layer of each electrode plate is immersed in the alkaline electrolyte, the additive with a larger concentration increase rate in the alkaline electrolyte has a higher solubility than the additive with a smaller concentration increase rate.

[0025] 2. Battery manufacturing Next, a method for producing the battery 1 will be described below. (1) Preparation of the positive electrode plate Nickel sulfate, aluminum sulfate, and cobalt sulfate are weighed out to a predetermined composition, and the weighed nickel sulfate, aluminum sulfate, and cobalt sulfate are added to an aqueous sulfuric acid solution to prepare a mixed aqueous solution. This mixed aqueous solution is gradually added to an aqueous sodium hydroxide solution while stirring, and the pH of the aqueous solution during the reaction is stabilized at 13 to 14, producing nickel hydroxide particles mainly made of nickel hydroxide with Al and Co dissolved therein. As a result, nickel hydroxide powder, which is a mixture of nickel hydroxide particles with Al and Co dissolved therein, is obtained as a positive electrode active material powder.

[0026] A coating layer made of cobalt hydroxide is formed on this nickel hydroxide powder. The active material powder with the coating layer made of cobalt hydroxide is subjected to a thermal alkali treatment in air to make the surface cobalt hydroxide 3.2-valent, which is higher than trivalent. In addition, yttrium oxide (Y2O3) or ytterbium oxide (Yb2O3) is used as an additive.

[0027] When yttrium oxide was used as the additive, the positive electrode active material and yttrium oxide were mixed in a weight ratio of 100:0.5, and a binder made of a hydrophilic resin was further added in an amount of 0.2% by weight based on the total weight of the positive electrode active material and additives, and the mixture was mixed with 30 parts by weight of water added to 100 parts by weight of this mixture and kneaded to prepare a positive electrode active material slurry.

[0028] On the other hand, when ytterbium oxide was used as the additive, the above-mentioned positive electrode active material and ytterbium oxide were mixed in a weight ratio of 100:1.0, and a binder made of a hydrophilic resin was further added and mixed in an amount of 0.2% based on the total weight of the positive electrode active material and additives. 30 parts by weight of water was added to 100 parts by weight of this mixture and kneaded to prepare a positive electrode active material slurry.

[0029] The positive electrode active material slurry was filled into a positive electrode core made of foamed nickel and dried. After that, the dried positive electrode core was rolled and cut to prepare a positive electrode plate 3. This positive electrode plate 3 is a so-called non-sintered electrode plate.

[0030] (2) Preparation of negative electrode plate A mixture of rare earth elements including lanthanum (La) and samarium (Sm) is mixed with Mg, Ni, and Al. This mixture is melted in an induction melting furnace, and the molten metal is poured into a mold and cooled to room temperature to become an ingot of hydrogen storage alloy.

[0031] The hydrogen storage alloy ingot is subjected to a heat treatment at 1000° C. for 10 hours in an argon gas atmosphere. After the heat treatment, the ingot is cooled to room temperature and mechanically crushed in an argon gas atmosphere to obtain hydrogen storage alloy powder consisting of hydrogen storage alloy particles.

[0032] Furthermore, when yttrium oxide is used as the positive electrode additive, 1 part by weight of yttrium oxide having a particle size larger than that of the yttrium oxide particles added to the positive electrode is added per part by weight of carbon. 0.3 parts by weight of a binder made of a hydrophilic resin and 40 parts by weight of water are mixed to prepare a paste. 100 parts by weight of the hydrogen storage alloy powder is added to this paste and kneaded to prepare a negative electrode active material slurry. The reason for using yttrium oxide having a particle size larger than that of the yttrium oxide added to the positive electrode as the negative electrode additive is to make the solubility of the yttrium oxide contained in the negative electrode lower than that of the yttrium oxide contained in the positive electrode.

[0033] On the other hand, when ytterbium oxide is used as the positive electrode additive, 1 part by weight of ytterbium oxide having a larger particle size than the ytterbium oxide added to the positive electrode is added.

[0034] This was further mixed with 0.3 parts by weight of a binder made of a hydrophilic resin and 40 parts by weight of water to prepare a paste, and 100 parts by weight of the hydrogen storage alloy powder was added to this paste and kneaded to prepare a negative electrode active material slurry.

[0035] The negative electrode active material slurry is applied to both sides of the punched metal sheet and into the through holes. After that, the applied negative electrode active material slurry is dried, and the punched metal sheet is rolled and cut to a predetermined size to produce the negative electrode plate 4. This negative electrode plate is a non-sintered type electrode plate.

[0036] (3) Battery assembly The positive electrode plate 3 and the negative electrode plate 4 are stacked together so that their longitudinal directions are aligned, and then wound in a spiral shape with a separator 5 sandwiched between the two electrode plates to form an electrode group 6.

[0037] The electrode group 6 is housed in the outer can 2 such that the negative electrode plate 4 is in contact with the bottom wall 8 of the outer can 2. After the electrode group 6 is housed in the outer can 2, an alkaline electrolyte is poured in. In this embodiment, KOH, NaOH, and LiOH are mixed to prepare the alkaline electrolyte.

[0038] Thereafter, the opening of the exterior can 2 was sealed with a sealing body 7 to assemble a 4 / 3FA size (diameter 18 mm, length 68 mm) battery 1 (theoretical capacity 3200 mAh). After that, the battery 1 was subjected to an initial activation process to become usable.

[0039] 3. Batteries of Examples and Comparative Examples In order to evaluate the battery characteristics according to the type and particle size of the additive added to each electrode plate, four examples and six comparative examples shown in the table in FIG. 2 were produced. The examples are batteries in which the same type of additive is used for the positive and negative electrodes, and the ratio (hereinafter referred to as "particle size ratio R") of the particle size of the additive added to the negative electrode (hereinafter referred to as "negative electrode particle size") to the particle size of the additive added to the positive electrode (hereinafter referred to as "positive electrode particle size") is 1.5 or more. In this disclosure, the numerical value of the particle size is expressed as D50. D50 is the so-called median diameter, and means the diameter at which the larger side and the smaller side are equal when a powder is divided into two from one particle diameter. In this disclosure, no upper limit is particularly set for the particle size ratio R, but if the negative electrode particle ratio is set to a value exceeding 4.6 shown in the table with respect to the positive electrode particle diameter, the discharge reaction may be reduced. (Regarding the upper limit of the particle size ratio R, in your instruction dated August 24th, you commented that "We believe that there is a risk of reducing productivity by adding an additive with a particle size larger than the particle size of the hydrogen alloy being used." However, since the particle size of the hydrogen storage alloy has not been mentioned in the description of this specification up to this point, there is a concern that the description of the upper limit of the additive particle size is insufficient to bring up the particle size of the hydrogen storage alloy. If it is customary to use additives with the same particle size when adding the same additive to each electrode plate, we believe that there is no problem even if there is no upper limit, and have therefore deleted this description. We ask for your confirmation.)

[0040] On the other hand, the comparative example is a battery in which the same type of additive is used in the positive and negative electrodes, and the ratio of the negative electrode particle size to the positive electrode particle size is less than 1.5.

[0041] Example 1 Yttrium oxide was used as the positive and negative electrode additives, the particle diameter of the yttrium oxide added to the positive electrode was 2.3 μm, and the particle diameter of the yttrium oxide added to the negative electrode was 3.5 μm. The particle diameter ratio R was 1.5.

[0042] Example 2 Yttrium oxide was used as the positive and negative electrode additives, the particle diameter of the yttrium oxide added to the positive electrode was 2.3 μm, and the particle diameter of the yttrium oxide added to the negative electrode was 9.5 μm. The particle diameter ratio R was 4.1.

[0043] Example 3 Yttrium oxide was used as the positive and negative electrode additives, the particle diameter of the yttrium oxide added to the positive electrode was 2.3 μm, and the particle diameter of the yttrium oxide added to the negative electrode was 10.5 μm. The particle diameter ratio R was 4.6.

[0044] Example 4 Ytterbium oxide was used as the positive and negative electrode additives, the particle size of the ytterbium oxide added to the positive electrode was 1.0 μm, and the particle size of the ytterbium oxide added to the negative electrode was 3.5 μm. The particle size ratio R was 3.5.

[0045] Comparative Example 1 Yttrium oxide was used as the positive and negative electrode additive, and yttrium oxide with a particle size of 2.3 μm was added to both the positive and negative electrodes. The particle size ratio R was 1.0.

[0046] Comparative Example 2 Yttrium oxide was used as the positive and negative electrode additives, the particle diameter of the yttrium oxide added to the positive electrode was 3.5 μm, and the particle diameter of the yttrium oxide added to the negative electrode was 2.3 μm. The particle diameter ratio R was 0.7.

[0047] Comparative Example 3 Yttrium oxide was used as the positive and negative electrode additive, and yttrium oxide with a particle size of 3.5 μm was added to both the positive and negative electrodes. The particle size ratio R was 1.0.

[0048] Comparative Example 4 Yttrium oxide was used as the positive and negative electrode additives, the particle diameter of the yttrium oxide added to the positive electrode was 9.5 μm, and the particle diameter of the yttrium oxide added to the negative electrode was 2.3 μm. The particle diameter ratio R was 0.2.

[0049] Comparative Example 5 Ytterbium oxide was used as the positive and negative electrode additive, and ytterbium oxide particles with a particle size of 1.0 μm were added to both the positive and negative electrodes. The particle size ratio R was 1.0.

[0050] Comparative Example 6 Ytterbium oxide was used as the positive and negative electrode additive, and ytterbium oxide particles with a particle size of 3.5 μm were added to both the positive and negative electrodes. The particle size ratio R was 1.0.

[0051] 4. Evaluation of battery characteristics As the battery characteristics, the low-temperature discharge characteristics and cycle characteristics were evaluated. 4-1.Low temperature discharge characteristics At an ambient temperature of 0°C, the battery was repeatedly pulse-discharged, with one cycle consisting of an on-period during which a discharge current of 36A flows for 2 seconds, followed by an off-period during which the discharge current is stopped for 8 seconds. When the battery pulse-discharge begins, the battery voltage starts to drop from the discharge start voltage value over time, but when the battery temperature rises due to heat generation by the internal resistance caused by the discharge, it starts to increase, and then stabilizes at an almost constant value, with the battery voltage dropping rapidly towards the end of the discharge. The battery voltage at the point at which the voltage, which dropped at the beginning of the discharge, starts to increase, was taken as the "bottom voltage" and was measured for each battery. It is believed that batteries with a higher bottom voltage have better low-temperature discharge characteristics.

[0052] 4-2. Cycle characteristics The battery is charged at a constant current of 1C (delta peak value 10mV) at an ambient temperature of 25°C, and then discharged at a discharge current of 15A until the final voltage reaches 0.8V. This cycle of charging and discharging is repeated, and the battery is charged and discharged. The AC internal resistance of the battery is measured at 1kHz for each cycle, and when the measured value increases to 20mΩ, the battery is considered to have reached the end of its life. The number of cycles until the measured internal resistance reaches 20mΩ is considered the battery's life.

[0053] The low-temperature pulse characteristics and cycle characteristics of the batteries of Examples 1 to 4 and Comparative Examples 1 to 6 were measured and are shown in the table of FIG.

[0054] Evaluation Regarding low-temperature discharge characteristics, the batteries of Example 1-4, in which the ratio of the negative electrode particle diameter to the positive electrode particle diameter is 1.5 or more while the same additive is added to the positive electrode plate and the negative electrode plate, all have bottom voltages of 0.75V or more, which is higher than the bottom voltage of Comparative Example 1-6. In addition, it was confirmed that the batteries of Example 1-4 all have a long life with a cycle number of 480 cycles or more. In other words, it was found that by setting the particle diameter ratio R to 1.5 or more, it is possible to suppress the deterioration of low-temperature discharge characteristics while improving the cycle life.

[0055] Furthermore, when yttrium oxide was used as the additive, if the particle size ratio R was 1.5 or more, the bottom voltage was high at 0.75 or more and the life was also long, regardless of the absolute value of the particle size.

[0056] Even when ytterbium oxide is used as an additive, Example 4 with a particle size ratio R of 3.5 has a high bottom voltage of 0.750 V and a long life of 680 cycles. This is because, in the batteries of Comparative Examples 5 and 6 with a particle size ratio R of 1, regardless of the particle size, the bottom voltage is small at less than 0.750 V even though the life is long at 720 cycles and 690 cycles, and sufficient discharge voltage is not obtained compared to Example 4. Therefore, it can be seen that, regardless of whether yttrium oxide or ytterbium oxide is used as an additive, the low-temperature discharge characteristics of the battery can be improved and the life can be extended by setting the particle size ratio to 1.5 or more.

[0057] 5. Discussion The additive made of yttrium oxide or ytterbium oxide contained in the mixture layer of the negative electrode plate dissolves in the alkaline electrolyte as the battery is charged and discharged, and acts on the surface of the hydrogen storage alloy in the process of becoming a hydroxide, suppressing corrosion of the alloy surface. The particle diameter of the yttrium oxide added to the negative electrode plate is 1.5 times larger than the particle diameter of the yttrium oxide added to the positive electrode plate, so it is difficult to dissolve in the alkaline electrolyte. Therefore, from the time after the assembly of the battery until the end of the initial activation treatment, the additive has little effect on the surface of the hydrogen storage alloy, so the hydrogen storage alloy is easily corroded. As a result, a catalyst layer made of nickel is formed on the surface of the hydrogen storage alloy. This catalyst layer can suppress the drop in bottom voltage during pulse discharge in a low-temperature environment. In addition, as the time elapsed from the start of discharge increases, the additive dissolves from the negative electrode plate into the alkaline electrolyte and the concentration increases, so it acts on the surface of the hydrogen storage alloy, contributing to a long life while maintaining a high high-temperature charging rate.

[0058] A similar phenomenon occurs when ytterbium oxide is added as an additive; a catalytic layer made of nickel is formed on the surface of the hydrogen storage alloy between the assembly of the battery and the end of the initial activation treatment, and the additive then acts on the hydrogen storage alloy, thereby suppressing a decrease in the bottom voltage during pulse discharge in a low-temperature environment.

[0059] As described above, by making the additive added to the negative plate less soluble in the alkaline electrolyte than the additive added to the positive plate, it is possible to improve the low-temperature charging characteristics in addition to extending the battery's life and stabilizing the high-temperature charging efficiency.

[0060] In addition, by making the particle diameter of the additive added to the negative electrode plate at least 1.5 times larger than the particle diameter of the additive added to the positive electrode plate, it is possible to achieve both low-temperature discharge characteristics and a long life.

[0061] Furthermore, by using yttrium oxide as the yttrium compound and ytterbium oxide as the ytterbium compound, both the positive electrode active material slurry and the negative electrode active material slurry can be easily prepared, and non-sintered positive electrode plates and negative electrode plates can be easily prepared.

[0062] Furthermore, by coating the nickel hydroxide particles that make up the positive electrode with a cobalt compound with a valence of more than three, the battery capacity can be increased. [Explanation of symbols]

[0063] 1 Nickel-metal hydride secondary battery 2 Outer can 3 Positive plate 4 Negative plate 5. Separator

Claims

1. A non-sintered positive electrode plate containing a positive electrode active material including nickel hydroxide; a non-sintered negative electrode plate containing a hydrogen storage alloy and facing the positive electrode plate via a separator; A nickel-metal hydride secondary battery, which is housed in an exterior can together with an alkaline electrolyte, The positive electrode plate includes an additive including either an yttrium compound or an ytterbium compound, The negative electrode plate contains the additive, A nickel-metal hydride secondary battery, wherein the additive added to the negative electrode plate has a lower solubility in the alkaline electrolyte than the additive added to the positive electrode plate.

2. 2. The nickel-metal hydride secondary battery in accordance with claim 1, wherein the particle diameter of the additive added to the negative electrode plate is at least 1.5 times larger than the particle diameter of the additive added to the positive electrode plate.

3. The yttrium compound is yttrium oxide, 3. The nickel-metal hydride secondary battery in accordance with claim 1, wherein the ytterbium compound is ytterbium oxide.

4. 3. The nickel-hydrogen secondary battery in accordance with claim 1, wherein the nickel hydroxide particles are coated with a cobalt compound having a valence of more than three.

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