JP2022134586A - Nickel metal hydride storage battery - Google Patents

Abstract

To provide a nickel metal hydride storage battery capable of reducing self-discharge.SOLUTION: A nickel metal hydride storage battery comprises: a positive electrode plate including a positive electrode mixture; a negative electrode plate including a negative electrode mixture; and a separator disposed between the positive electrode plate and the negative electrode plate. Therein: the positive electrode mixture contains cobalt; the negative electrode mixture contains a hydrogen storage alloy; the hydrogen storage alloy includes a matrix phase containing misch metal, cobalt, nickel, and manganese, and a segregation phase containing nickel and manganese; in the segregation phase, a mass ratio of the manganese to the nickel is 0.17 or more and 0.4 or less; and a mass ratio of the cobalt contained in the positive electrode mixture and in the hydrogen storage alloy to the manganese contained in the hydrogen storage alloy is 1.2 or more and 1.6 or less.SELECTED DRAWING: Figure 3

Description

The present invention relates to nickel-metal hydride storage batteries.

Nickel-metal hydride storage batteries are known as an example of secondary batteries (see Patent Document 1, for example). A nickel-hydrogen storage battery includes a positive electrode plate using nickel hydroxide as a positive electrode active material, a negative electrode plate using a hydrogen storage alloy as a negative electrode active material, and a separator interposed between the positive electrode plate and the negative electrode plate. The hydrogen storage alloy for the nickel-metal hydride storage battery described in Patent Document 1 contains misch metal, which is a mixture of rare earth elements, and nickel, and part of the nickel is replaced with aluminum, manganese, cobalt, or the like.

JP 2015-203119 A

In nickel-metal hydride storage batteries, self-discharge occurs in which the discharge capacity of the battery decreases even when the battery is not in use due to the deposition of conductive deposits on the separator. Nickel-metal hydride storage batteries have a large amount of self-discharge compared to other secondary batteries, and it is desired to reduce the amount of self-discharge.

A nickel-metal hydride storage battery for solving the above problems is a nickel-metal hydride storage battery comprising a positive electrode plate comprising a positive electrode mixture, a negative electrode plate comprising a negative electrode mixture, and a separator disposed between the positive electrode plate and the negative electrode plate. , the positive electrode mixture contains cobalt, the negative electrode mixture contains a hydrogen-absorbing alloy, and the hydrogen-absorbing alloy contains a matrix containing misch metal, cobalt, nickel, and manganese, and nickel and manganese. a segregation phase, wherein the segregation phase has a mass ratio of manganese to nickel of 0.17 or more and 0.4 or less; The mass ratio of cobalt contained is 1.2 or more and 1.6 or less.

According to the above configuration, the mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy is 1.2 or more, so that the hydrogen-absorbing alloy contains an excessive amount of manganese. Since an increase in the segregation phase is suppressed, excessive pulverization of the hydrogen-absorbing alloy is suppressed. As a result, the increase in the surface area of the hydrogen-absorbing alloy due to excessive pulverization is optimized, and the elution of cobalt contained in the hydrogen-absorbing alloy is suppressed, thereby reducing the amount of self-discharge. When the mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy is 1.6 or less, the amount of cobalt deposited on the separator can be reduced, and manganese deposited on the separator can be reduced. By suppressing the high conductivity of cobalt, the self-discharge amount of the nickel-metal hydride storage battery can be reduced. Also, in the segregation phase, the mass ratio of manganese to nickel is 0.17 or more and 0.4 or less, so that excessive pulverization in the hydrogen storage alloy due to reduction of manganese in the matrix phase is suppressed. As a result, the amount of self-discharge can be reduced.

In the nickel-metal hydride storage battery, the segregation phase has a ratio of the area of the segregation phase to the total area of the mother phase and the segregation phase in the cross section of the hydrogen-absorbing alloy in the range of 1.5% to 3.0%. The following are preferable. According to the above configuration, the segregation phase accounts for 1.5% or more of the cross section of the hydrogen-absorbing alloy, so that the amount of manganese deposited on the separator from the segregation phase can be increased. As a result, manganese deposited on the separator suppresses the increase in conductivity of cobalt, thereby reducing the amount of self-discharge. When the ratio of the segregation phase in the cross section of the hydrogen storage alloy is 3.0% or less, pulverization is suppressed and corrosion resistance can be improved. In addition, by suppressing pulverization, the increase in the surface area of the hydrogen storage alloy is optimized, so the elution of cobalt contained in the hydrogen storage alloy is suppressed. As a result, the amount of self-discharge in the nickel-metal hydride storage battery can be reduced.

In the nickel-hydrogen storage battery, the segregation phase preferably has an average area of 30 square μm or more and 65 square μm or less per segregation phase in the cross section of the hydrogen storage alloy. According to the above configuration, the average area per segregation phase in the cross section of the hydrogen storage alloy is 30 square μm or more, so that the segregation phase in the hydrogen storage alloy is aggregated. It is possible to reduce the number of segregation phases that cause cracks. Therefore, the corrosion resistance of the hydrogen storage alloy can be improved. As a result, excessive pulverization of the hydrogen storage alloy is suppressed and the increase in the surface area of the hydrogen storage alloy is optimized, so the elution of cobalt contained in the hydrogen storage alloy is suppressed, thereby reducing the amount of self-discharge. can. In addition, when the average area per segregation phase in the cross section of the hydrogen storage alloy is 65 square μm or less, the elution of manganese contained in the segregation phase is impaired due to excessive enlargement of the segregation phase. can be suppressed.

In the nickel-metal hydride storage battery, the hydrogen storage alloy is preferably AB ₅ type. According to the above configuration, since the AB ₅ type hydrogen storage alloy easily elutes the B element, when the nickel-metal hydride storage battery includes the AB ₅ type hydrogen storage alloy, cobalt and manganese, which are the B element, are added to the separator. By controlling the amount of precipitation, the amount of self-discharge can be suitably reduced.

In the above nickel-metal hydride storage battery, the hydrogen storage alloy further includes aluminum as an element constituting the parent phase and the segregation phase, and the composition ratio of the hydrogen storage alloy in terms of atomic ratio is MmNi _a Co _b Mn _c Al _d. , the values of a, b, c, and d in the composition ratio are 4.5 ≤ a + b + c + d ≤ 5.5, 3.5 ≤ a ≤ 5.0, 0 < b ≤ 0.7, And it is preferable to satisfy the range of 0.1≦c≦0.7. According to the above configuration, by configuring the B element contained in the AB ₅ type hydrogen storage alloy with the above composition, it is possible to suitably control the amount of cobalt and manganese, which are B elements, deposited on the separator. Become. As a result, the self-discharge amount of the nickel-metal hydride storage battery can be suitably reduced.

ADVANTAGE OF THE INVENTION According to this invention, the self-discharge amount in a nickel metal hydride storage battery can be reduced.

The perspective view of a nickel-hydrogen storage battery. FIG. 2 is a cross-sectional view of an electrode plate group provided in the nickel-metal hydride storage battery; 4 is a graph showing the correspondence relationship between the ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy and the amount of self-discharge in the nickel-hydrogen storage battery. 4 is a graph showing the correspondence relationship between the average area per segregation phase in the hydrogen storage alloy and the corrosion resistance of the hydrogen storage alloy. 4 is a graph showing the correspondence relationship between the ratio of manganese to nickel in the segregation phase in the hydrogen-absorbing alloy and the corrosion resistance of the hydrogen-absorbing alloy.

An embodiment of the present invention will be described below with reference to FIGS. 1 to 5. FIG.
As shown in FIG. 1 , nickel-metal hydride storage battery 11 includes a plurality of battery cells 12 , container 13 , and lid portion 14 . The battery case 13 houses a plurality of battery cells 12 electrically connected in series. The lid portion 14 seals the opening of the container 13 . Electric power of the battery cells 12 housed in the battery case 13 is taken out from the positive terminal 13 a and the negative terminal 13 b provided in the battery case 13 . Note that in FIG. 1 , six battery cells 12 are accommodated in the container 13 .

As shown in FIG. 2 , the battery cell 12 has an electrode plate group 20 . The electrode plate group 20 is configured by alternately stacking a plurality of positive electrode plates 15 and a plurality of negative electrode plates 16 with separators 17 interposed therebetween. An end portion 15 a of the positive electrode plate 15 is joined to the positive current collector 21 . An end portion 16 a of the negative electrode plate 16 is joined to the negative current collector 22 .

<Positive plate>
The positive electrode plate 15 will be explained. The positive electrode plate 15 includes a base material and a positive electrode mixture provided on the base material. The base material is preferably made of foamed metal such as foamed nickel. The base material has a function of holding the positive electrode mixture and a function of a current collector.

The positive electrode mixture contains a positive electrode active material, a conductive material, a thickener, a binder, and the like. The positive electrode active material is mainly composed of nickel hydroxide (Ni(OH) ₂ ). The positive electrode active material is provided with a coating layer in which the surface of the particles is coated with cobalt hydroxide (Co(OH) ₂ ). The conductive material is mainly composed of a cobalt compound such as cobalt hydroxide or cobalt oxide (CoO).

When the nickel-metal hydride storage battery is charged for the first time, the cobalt compound contained in the positive electrode mixture is electrochemically oxidized and deposited on the surface of the positive electrode active material as highly conductive cobalt oxyhydroxide (CoOOH). A high-density layer is formed by the coating layer formed before charging and the cobalt oxyhydroxide deposited after charging.

<Separator>
Next, the separator 17 will be explained. A separator 17 is arranged between the positive plate 15 and the negative plate 16 . Separator 17 holds an electrolytic solution. Although the material of the separator 17 is not particularly limited, it may be, for example, a nonwoven fabric, a resin film provided with a large number of fine holes, a sheet capable of retaining other liquids, or a combination thereof. The electrolytic solution is an alkaline aqueous solution containing potassium hydroxide (KOH) as the main component of the solute.

<Negative plate>
The negative plate 16 will be explained. The negative electrode plate 16 includes a base material and a negative electrode mixture provided on the base material. The negative electrode mixture contains a hydrogen storage alloy, which is an alloy that reversibly absorbs and releases hydrogen. The negative electrode plate 16 is made by adding a thickening agent such as carbon black and a binder such as styrene-butadiene copolymer to a hydrogen storage alloy and processing the negative electrode mixture into a paste to adhere it to a base material made of a metal material. manufactured by drying, rolling and cutting.

Hydrogen storage alloys are AB type, AB ₅ type, AB ₂ type, and A ₂ B ₇ type, where "A" is an element with high affinity with hydrogen and "B" is an element with low affinity with hydrogen. Either one or a combination thereof can be used. TiCo, ZrCo, or the like can be used as the AB type hydrogen storage alloy. MmNi ₅ , which is an example of a MmNi alloy, can be used as the AB ₅ type hydrogen storage alloy. "Mm" refers to misch metal, which is an alloy containing a plurality of rare earth elements. The misch metal contains at least one of lanthanum (La), cerium (Ce), neodymium (Nd), samarium (Sm), and the like. Also, instead of or in addition to the alloys described above, vanadium (V)-based alloys and magnesium (Mg)-based alloys may be used.

In particular, as the AB5 type hydrogen storage alloy, an _MmNixMyy alloy obtained _by substituting a part of nickel ₍ Ni) in MmNi5 with another _element can be suitably used. At least one element selected from cobalt (Co), manganese (Mn), aluminum (Al), iron (Fe), copper (Cu), and titanium (Ti) can be used as the element that replaces nickel. can. Note that x and y represent the atomic ratios of nickel and elements substituting nickel when the atomic ratio of misch metal is 1.

The hydrogen storage alloy of the present embodiment is an _AB5 type _{MmNixMy alloy} , and contains at least cobalt, manganese, and aluminum as elements substituting nickel. Specifically, the composition ratio of each element in the hydrogen storage alloy of the present embodiment can be _{expressed as MmNiaCobMncAld .} Note that a, b, c, and d represent the atomic ratio of each element when the atomic ratio of the misch metal is 1. The atomic ratio of nickel, cobalt, manganese, and aluminum in the hydrogen storage alloy of this embodiment is preferably 4.5≦a+b+c+d≦5.5.

Further, the hydrogen storage alloy of this embodiment has a matrix phase containing misch metal, nickel, cobalt, manganese and aluminum, and a segregation phase containing nickel, manganese and aluminum. The segregation phase is a region in which aluminum and manganese are present in relatively higher concentrations than in the matrix phase. Specifically, the segregation phase is a region in which manganese and aluminum are present at a concentration that is at least twice the average concentration in the matrix. Incidentally, in the segregation phase, nickel is generally present at the same concentration as in the matrix phase.

The concentration of each element contained in the matrix phase and the segregation phase is measured using known measurement means such as electron probe microprobe analyzer (EPMA) and SEM-EDX. Specifically, in the present embodiment, the concentration of each element contained in the parent phase and the segregation phase is measured using the element distribution measured under predetermined conditions by EPMA with respect to the cross section of the hydrogen-absorbing alloy. An electron beam microprobe analyzer "JXA-8100" (manufactured by JEOL Ltd.) is used for the measurement of the elemental distribution. In this embodiment, the acceleration voltage=15 kV, the irradiation current=0.1 μA, and the observation magnification=400 are adopted as the predetermined conditions. Under these predetermined conditions, an X-ray intensity signal peculiar to aluminum is detected from the cross section of the hydrogen storage alloy, and if the value is 60 CPS or more, aluminum segregates in that region and the concentration is relatively higher than that of the matrix. is determined to exist in Under these predetermined conditions, an X-ray intensity signal specific to manganese is detected from the cross section of the hydrogen storage alloy. It is determined that it exists at high concentration. That is, the segregation phase is a region in the cross section of the hydrogen storage alloy in which the X-ray intensity signal specific to aluminum is 60 CPS or more and the X-ray intensity signal specific to manganese is 20 CPS or more. . The values of 60 CPS and 20 CPS are values set so as to detect a region where the X-ray intensity signal specific to aluminum and manganese is twice or more the average level.

<Self-discharge in nickel-metal hydride storage battery>
Here, the self-discharge of the nickel-metal hydride storage battery will be described. In general, nickel-metal hydride storage batteries undergo self-discharge, in which the discharge capacity of the battery decreases even when the battery is not in use. One of the causes of self-discharge is a minute short circuit between the positive electrode plate and the negative electrode plate due to the metal contained in the positive electrode plate and the negative electrode plate being eluted and deposited as a conductive deposit on the separator.

In the AB ₅ type hydrogen storage alloy, the ratio of the B element to the A element is large, so the B element is easily eluted into the electrolytic solution. In particular, in a nickel-metal hydride storage battery equipped with an _AB5 type hydrogen storage alloy, cobalt, which is the B element in the hydrogen storage alloy, is eluted and deposited as a conductive deposit on the separator, thereby causing a connection between the positive electrode plate and the negative electrode plate. Self-discharge is likely to occur due to minute short circuits.

_In the AB5 type hydrogen storage alloy, manganese, which is a B element, also precipitates as a conductive deposit on the separator, similarly to cobalt, thereby causing self-discharge. However, since the electrical conductivity of manganese is lower than that of cobalt, the amount of self-discharge is smaller when manganese is deposited on the separator than when cobalt is deposited on the separator. In addition, manganese deposited on the separator suppresses high conductivity of cobalt deposited on the separator. Therefore, in a nickel-metal hydride storage battery comprising an _AB5 type hydrogen storage alloy, by increasing the amount of manganese deposited on the separator, the self-discharge amount in the nickel-metal hydride storage battery is reduced more than when only cobalt is deposited as a segregation phase. be.

The amount of cobalt and manganese deposited on the separator depends on the amount of manganese contained in the hydrogen storage alloy and the amount of cobalt contained in the positive electrode mixture and the hydrogen storage alloy. Therefore, by controlling these amounts, the amounts of manganese and cobalt deposited on the separator can be controlled, and as a result, the amount of self-discharge in the nickel-metal hydride storage battery can be reduced.

<Relationship between Cobalt and Manganese in Nickel Metal Hydride Storage Battery and Self-Discharge>
A graph 100 shown in FIG. 3 shows the relationship between the mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy in the nickel-hydrogen storage battery and the amount of self-discharge in the nickel-hydrogen storage battery.

The horizontal axis of the graph 100 indicates the mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy. The amount of manganese and cobalt contained in the hydrogen-absorbing alloy here means the amount of manganese and cobalt contained in the entire hydrogen-absorbing alloy without distinguishing between the matrix phase and the segregation phase of the hydrogen-absorbing alloy.

The vertical axis of the graph 100 indicates the relative amount of self-discharge, and the higher the plot points 101 are, the larger the amount of self-discharge. The amount of self-discharge here refers to the amount of decrease in discharge capacity when the nickel-metal hydride storage battery is left at 45° C. for one week, based on the discharge capacity when the nickel-metal hydride storage battery is charged to 60% SOC (State Of Charge). is.

Each plotted point 101 in the graph 100 indicates the self-discharge amount of the nickel-hydrogen storage battery with respect to the mass ratio of manganese contained in the hydrogen-absorbing alloy and cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy. A correlation curve 102 in the graph 100 is a curve approximating the correspondence relationship between the mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy and the self-discharge amount of the nickel-hydrogen storage battery. .

The self-discharge amount has a maximum value when the value on the horizontal axis in the graph 100 is 0.9, decreases as the value on the horizontal axis increases, and reaches a minimum value near 1.4. In particular, the self-discharge amount sharply decreases as the value on the horizontal axis increases in the range of 0.9 to 1.2 on the horizontal axis. Further, when the value on the horizontal axis in graph 100 exceeds 1.4, the self-discharge amount increases again as the value on the horizontal axis increases.

When the value of the horizontal axis in the graph 100 is less than 1.2, the hydrogen storage alloy contains excessive manganese, so the segregation phase in the hydrogen storage alloy increases, and cracks originating from the segregation phase occur. Pulverization of the hydrogen storage alloy is promoted by the occurrence of As a result, the increase in the surface area of the hydrogen-absorbing alloy promotes the elution of cobalt contained in the hydrogen-absorbing alloy into the electrolytic solution, which is thought to increase the self-discharge amount.

When the value of the horizontal axis in the graph 100 exceeds 1.4, particularly when the value of the horizontal axis exceeds 1.6, the amount of cobalt contained in the positive electrode mixture and the hydrogen storage alloy is large. elution is accelerated, and the amount of precipitation on the separator increases. For the above reasons, when the value on the horizontal axis in graph 100 exceeds 1.6, the amount of self-discharge is considered to have increased.

As described above, in the present embodiment, the mass ratio of cobalt contained in the positive electrode mixture and the negative electrode mixture to manganese contained in the hydrogen storage alloy is 1.2 or more and 1.63 or less, more preferably 1.2 or more and 1.6. The following range is the preferred range. By setting the mass ratio to such a range, the amount of cobalt deposited on the separator can be reduced, and the manganese deposited on the separator suppresses the high conductivity of cobalt, thereby increasing the self-discharge amount of the nickel-metal hydride storage battery. can be reduced.

The mass ratio of the cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to the manganese contained in the hydrogen-absorbing alloy is the amount of manganese and cobalt contained in the hydrogen-absorbing alloy. It can be controlled by adjusting the amount of the cobalt compound that is added.

<Relationship between the ratio of the segregation phase in the hydrogen storage alloy and the amount of self-discharge>
The segregation phase of the hydrogen storage alloy contains manganese at a higher concentration than the matrix phase. Therefore, manganese contained in the segregation phase is more likely to be eluted into the electrolytic solution than manganese contained in the matrix phase. Therefore, by increasing the segregation phase of the hydrogen storage alloy, the amount of manganese deposited on the separator can be preferably increased. On the other hand, the segregation phase of the hydrogen-absorbing alloy serves as a starting point for cracks in the hydrogen-absorbing alloy, thus promoting pulverization of the hydrogen-absorbing alloy. When the pulverization is excessively promoted, the surface area of the hydrogen-absorbing alloy increases, which promotes the elution of cobalt contained in the hydrogen-absorbing alloy into the electrolytic solution. Therefore, when the segregation phase is excessively present in the hydrogen storage alloy, the self-discharge amount in the nickel-metal hydride storage battery increases. That is, it is preferable that the segregation phase of the hydrogen-absorbing alloy is present to the extent that the amount of manganese precipitated on the separator is appropriately increased, and that the hydrogen-absorbing alloy is not excessively pulverized.

Specifically, in the cross section of the hydrogen-absorbing alloy, the ratio of the area of the segregation phase to the area of the entire hydrogen-absorbing alloy is preferably 1.5% or more and 3.0% or less. Here, the area of the entire hydrogen-absorbing alloy is the sum of the area of the parent phase and the area of the segregation phase, and is the area that does not include the area of the alloy-free region in the cross section of the hydrogen-absorbing alloy. .

When the ratio of the segregation phase is 1.5% or more, the amount of manganese deposited on the separator can be increased. As a result, the manganese deposited on the separator suppresses the increase in conductivity of cobalt, thereby reducing the amount of self-discharge in the nickel-metal hydride storage battery.

When the ratio of the segregation phase is 3.0% or less, the occurrence of cracks originating from the segregation phase is suppressed, and excessive pulverization is suppressed. Thereby, the corrosion resistance of the hydrogen storage alloy can be improved. Therefore, when the ratio of the segregation phase is 3.0% or less, the elution of cobalt contained in the hydrogen storage alloy is suppressed, so the self-discharge amount in the nickel-metal hydride storage battery can be reduced.

<Relationship between the average area per segregation phase and the amount of self-discharge>
If the average area per segregation phase is excessively small, the hydrogen-absorbing alloy will be in a state in which many small segregation phases are scattered. In this case, since the number of segregation phases, which are the starting points of cracks in the hydrogen-absorbing alloy, has increased, pulverization is excessively promoted and the surface area of the hydrogen-absorbing alloy increases, resulting in a decrease in the corrosion resistance of the hydrogen-absorbing alloy. do. In addition, the increase in the surface area of the hydrogen storage alloy promotes the elution of cobalt into the electrolytic solution, thereby increasing the self-discharge amount of the nickel-metal hydride storage battery. On the other hand, when the average area per segregation phase is excessively large, the segregation phases in the hydrogen storage alloy are aggregated. In this case, the number of segregated phases is reduced, so pulverization is suppressed, but the ratio of the contact area between the segregated phase and the electrolytic solution to the cross-sectional area of the segregated phase is reduced. In other words, in each segregation phase, the ratio of the surface area to the volume is reduced, so that the elution amount of manganese contained in the segregation phase is reduced when considering the hydrogen storage alloy as a whole. From the above, the average area per segregation phase is the size of the segregation phase aggregated to the extent that the number of segregation phases does not increase excessively, and the elution of manganese contained in the segregation phase is not impaired. It is preferred that the sizes are evenly distributed.

A graph 200 shown in FIG. 4 shows the relationship between the average area per segregation phase and the corrosion resistance of the hydrogen storage alloy. The corrosion resistance in this embodiment means the difficulty of pulverization in the hydrogen storage alloy. The difficulty of pulverization can be evaluated by the amount of change in the magnetic susceptibility of the hydrogen storage alloy when charging and discharging are repeated a predetermined number of times. Magnetic susceptibility can be determined by measuring the magnetic moment of a sample using a VSM (Vibrating Sample Magnetometer). Corrosion resistance in the present embodiment is evaluated by the amount of change (slope) of magnetic susceptibility with respect to the amount of discharged electricity in charge/discharge cycles under predetermined conditions. It means that the smaller the amount of change in magnetic susceptibility in the hydrogen-absorbing alloy, the smaller the progress of pulverization. In other words, the smaller the amount of change in magnetic susceptibility, the higher the corrosion resistance.

The horizontal axis of graph 200 is the average area per segregation phase, which is the value obtained by dividing the total area of segregation phases contained in an arbitrary cross section in the hydrogen storage alloy by the number of segregation phases contained in the cross section. indicates The vertical axis of graph 200 indicates the relative level of corrosion resistance in the hydrogen storage alloy. The corrosion resistance here is evaluated based on the amount of change in the magnetic susceptibility of the hydrogen storage alloy subjected to charge-discharge cycles under predetermined conditions, and the higher the plot point 201 is, the higher the corrosion resistance is. .

Each plotted point 201 in the graph 200 represents the corrosion resistance of the nickel-metal hydride storage battery with respect to the average area per segregation phase. A correlation curve 202 in the graph 200 is a curve approximating the correspondence relationship between the average area per segregation phase and the corrosion resistance in the nickel-metal hydride storage battery.

When the average area per segregation phase is less than 30 square μm, the hydrogen-absorbing alloy is in a state in which many segregation phases are scattered. That is, the corrosion resistance of the hydrogen-absorbing alloy decreases due to an increase in the number of segregation phases that act as starting points for cracks in the hydrogen-absorbing alloy.

Therefore, the average area per segregation phase in the cross section of the hydrogen storage alloy is preferably 30 square μm or more. When the average area per segregation phase is 30 square μm or more, the segregation phases in the hydrogen storage alloy are aggregated, so the number of segregation phases that cause cracks in the hydrogen storage alloy can be reduced. . As a result, the corrosion resistance of the hydrogen storage alloy can be improved, and excessive pulverization can be suppressed. Therefore, the promotion of cobalt elution due to excessive pulverization is prevented, thereby reducing the amount of self-discharge of the nickel-metal hydride storage battery.

Moreover, the average area per segregation phase in the cross section of the hydrogen storage alloy is preferably 65 square μm or less. When the average area per segregation phase is 65 square μm or less, it is possible to suppress deterioration in the dissolution of manganese contained in the segregation phase due to excessive enlargement of the segregation phase.

<Relationship between the mass ratio of manganese in the segregation phase and the amount of self-discharge>
A graph 300 shown in FIG. 5 shows the correspondence between the ratio of manganese to nickel in the segregation phase in the hydrogen storage alloy and the corrosion resistance of the hydrogen storage alloy. The horizontal axis of graph 300 represents the ratio of manganese to nickel in the segregation phase, which is the mass of manganese in the segregation phase divided by the mass of nickel in the segregation phase. The vertical axis of graph 300 indicates the relative level of corrosion resistance in the nickel-metal hydride storage battery. Corrosion resistance here is evaluated based on the amount of change in magnetic susceptibility in the hydrogen storage alloy subjected to charge-discharge cycles under predetermined conditions, and the higher the plot point 301 is, the higher the corrosion resistance is. .

Each plotted point 301 in the graph 300 shows the relationship between the ratio of manganese to nickel in the segregation phase in the hydrogen storage alloy and the corrosion resistance in the nickel-metal hydride storage battery. A correlation curve 302 in the graph 300 is a curve approximating the corresponding relationship between the ratio of manganese to nickel in the segregation phase in the hydrogen storage alloy and the corrosion resistance of the hydrogen storage alloy.

If the mass ratio of manganese to nickel in the segregation phase is more than 0.4, manganese is excessively segregated, and the content of manganese in the matrix phase decreases, so pulverization is likely to occur. Therefore, when the mass ratio of manganese to nickel in the segregation phase is 0.4 or less, promotion of excessive pulverization of the hydrogen-absorbing alloy due to the reduction of manganese in the matrix phase is suppressed. As a result, an increase in the surface area of the hydrogen-absorbing alloy due to pulverization can be suppressed, and the amount of cobalt eluted from the hydrogen-absorbing alloy is reduced, thereby reducing the amount of self-discharge. The mass ratio of manganese to nickel in the segregation phase should be at least 0.17, which is at least twice the average concentration of manganese in the matrix phase. When the mass ratio of manganese to nickel in the segregation phase is at least twice the average concentration of manganese in the matrix phase, the elution of manganese contained in the segregation phase is promoted.

<Manufacturing method of nickel-metal hydride storage battery>
An example of a method for manufacturing the nickel-metal hydride storage battery 11 will be described below.
An example of a method for manufacturing the negative electrode plate 16 will be described. First, misch metal, nickel, cobalt, manganese, and aluminum are blended so as to have a predetermined composition as raw materials for the hydrogen storage alloy.

Specifically, the hydrogen storage alloy is blended so as to satisfy the range of 4.5≦a+b+ _c + _d ≦5.5 when the composition ratio in terms of atomic ratio in the hydrogen storage alloy is _expressed as _{MmNiaCobMncAld} . be. The atomic ratio of nickel in the hydrogen storage alloy is preferably 3.5≦a≦5.0. The atomic ratio of cobalt in the hydrogen storage alloy is preferably 0<b≦0.7. The atomic ratio of manganese in the hydrogen storage alloy is preferably 0.1≦c≦0.7. The atomic ratio "d" of aluminum in the hydrogen storage alloy is determined based on the numerical values of a to c so as to satisfy the condition of 4.5≤a+b+c+d≤5.5. By configuring the B element contained in the AB ₅ type hydrogen storage alloy with the above composition, the composition of the segregation phase, the ratio of the area of the segregation phase in the cross section of the hydrogen storage alloy, and the segregation phase in the cross section It becomes easy to control the average area of to a suitable value.

Furthermore, a hydrogen storage alloy is produced by melting the raw material of the hydrogen storage alloy and performing a so-called melt quenching method in which the cooling rate from the molten state to solidification is set to 1000° C./sec or more. In this case, the melted raw material of the hydrogen-absorbing alloy is rapidly cooled to produce a hydrogen-absorbing alloy with a small variation in the distribution of raw material components. Then, by pulverizing the produced hydrogen-absorbing alloy with a ball mill, hydrogen-absorbing alloy powder is produced.

By adjusting the mass ratio of aluminum and manganese to the entire hydrogen storage alloy, the ratio of the area of the segregation phase in the cross section of the hydrogen storage alloy is controlled. Also, by adjusting the mass ratio of nickel and manganese in the hydrogen storage alloy, the mass ratio of manganese to nickel in the segregation phase can be controlled.

Next, the hydrogen-absorbing alloy powder is immersed in an alkaline aqueous solution, stirred, washed with water, and dried for alkali treatment. By adjusting the treatment temperature and treatment time in the alkali treatment, the ratio of the area of the segregation phase in the cross section of the hydrogen storage alloy and the average area per segregation phase in the cross section can be controlled.

Further, a thickener such as carboxymethyl cellulose and a binder such as a styrene-butadiene copolymer are added to the dried hydrogen storage alloy powder and kneaded to prepare a negative electrode mixture paste. Then, the negative electrode plate 16 is manufactured by applying the negative electrode mixture paste to a punching metal, drying, rolling and cutting. The mass ratio of manganese and cobalt in the hydrogen absorbing alloy powder in the negative electrode mixture paste and the amount of negative electrode mixture paste put into the punching metal determine the mass of manganese and cobalt contained in the hydrogen absorbing alloy in the negative electrode plate. be done.

An example of a method for manufacturing the positive electrode plate 15 will be described. First, a positive electrode mixture paste is prepared by mixing a positive electrode active material, a conductive material, a thickener, a binder, and the like as raw materials for the positive electrode mixture. The positive electrode active material is nickel hydroxide coated with cobalt hydroxide. The conductive material is a cobalt compound such as cobalt hydroxide or cobalt oxide. The positive electrode plate 15 is produced by filling a foamed nickel substrate with a positive electrode mixture paste, drying, rolling and cutting.

The mass of cobalt contained in the positive electrode mixture in the positive electrode plate 15 is determined by the concentration of cobalt in the positive electrode mixture paste and the amount of the positive electrode mixture paste put into the foamed nickel substrate. Therefore, by controlling the concentration of cobalt in the positive electrode mixture paste and the input amount of the positive electrode mixture paste, and the mass ratio of manganese and cobalt in the negative electrode mixture paste and the input amount of the negative electrode mixture paste, The mass ratio of cobalt contained in the positive electrode mixture and hydrogen storage alloy to manganese contained can be controlled.

Then, a plurality of the positive electrode plates 15 and the negative electrode plates 16 are stacked with separators 17 made of non-woven fabric made of an alkali-resistant resin interposed therebetween, and placed in a battery container together with an alkaline electrolyte containing potassium hydroxide as a main component. Then, the nickel-metal hydride storage battery 11 is produced. The output of the nickel-hydrogen storage battery 11 is improved by activating the battery by repeating charging and discharging. In the activation process, by controlling the charged electricity amount, SOC, environmental temperature, and the number of times of charging, etc., the ratio of the area of the segregation phase in the cross section of the hydrogen storage alloy and the average per segregation phase in the cross section You can also control the area.

[Effects of Embodiment]
According to the above embodiment, the following effects can be obtained.
(1) The mass ratio of the cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to the manganese contained in the hydrogen-absorbing alloy is 1.2 or more, so that the hydrogen-absorbing alloy contains an excessive amount of manganese, thereby forming a segregation phase. Since the increase is suppressed, excessive pulverization of the hydrogen storage alloy is suppressed. This optimizes the increase in the surface area of the hydrogen-absorbing alloy due to excessive pulverization, thereby suppressing the elution of cobalt contained in the hydrogen-absorbing alloy. As a result, the amount of cobalt deposited on the separator 17 is reduced, so that the amount of self-discharge in the nickel-metal hydride storage battery 11 can be reduced.

(2) Since the mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy is 1.6 or less, the amount of cobalt deposited on the separator 17 can be reduced. The amount of self-discharge in the storage battery 11 can be reduced. In addition, the manganese deposited on the separator 17 suppresses the increase in conductivity of cobalt, thereby reducing the self-discharge amount in the nickel-metal hydride storage battery 11 . In addition, by setting the mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy to 1.6 or less, the content of cobalt, which is relatively expensive compared to other metal elements, is reduced. can do.

(3) When the mass ratio of manganese to nickel in the segregation phase is 0.17 or more and 0.4 or less, excessive pulverization of the hydrogen storage alloy due to a decrease in manganese content in the matrix phase is suppressed. Then, the increase in the surface area of the hydrogen-absorbing alloy accompanying the pulverization is optimized, so that the elution of cobalt contained in the hydrogen-absorbing alloy is suppressed as much as possible. As a result, the amount of cobalt deposited on the separator 17 is reduced, so that the amount of self-discharge in the nickel-metal hydride storage battery 11 can be reduced.

(4) The segregation phase accounts for 1.5% or more of the cross section of the hydrogen-absorbing alloy, so that the amount of manganese deposited on the separator 17 from the segregation phase can be increased. As a result, the manganese deposited on the separator 17 suppresses the increase in conductivity of cobalt, thereby reducing the amount of self-discharge in the nickel-metal hydride storage battery 11 .

(5) When the ratio of the segregation phase in the cross section of the hydrogen storage alloy is 3.0% or less, excessive generation of cracks originating from the segregation phase is suppressed, thereby improving the corrosion resistance. As a result, the amount of self-discharge in the nickel-hydrogen storage battery 11 can be reduced by suppressing the elution of cobalt contained in the hydrogen storage alloy as much as possible.

(6) When the average area per segregation phase in the cross section of the hydrogen storage alloy is 30 square μm or more, the segregation phase in the hydrogen storage alloy is aggregated, so that cracks start in the hydrogen storage alloy. can reduce the number of segregation phases. Therefore, the corrosion resistance of the hydrogen storage alloy can be improved. As a result, excessive pulverization of the hydrogen storage alloy is suppressed and the increase in the surface area of the hydrogen storage alloy is optimized, so the elution of cobalt contained in the hydrogen storage alloy is suppressed, thereby reducing the amount of self-discharge. can. In addition, when the average area per segregation phase in the cross section of the hydrogen storage alloy is 65 square μm or less, the elution of manganese contained in the segregation phase is impaired due to excessive enlargement of the segregation phase. can be suppressed.

(7) In the present embodiment, the composition of the hydrogen storage alloy and the segregation phase, the ratio of the area of the segregation phase in the cross section of the hydrogen storage alloy, etc. are controlled, and the amount of cobalt and manganese, which are B elements, deposited on the separator 17 is controlled. By controlling, the amount of self-discharge in the nickel-hydrogen storage battery 11 is reduced. Since the AB ₅ type hydrogen storage alloy easily elutes the B element, the above configuration can more preferably reduce the self-discharge amount when the nickel-metal hydride storage battery 11 includes the AB ₅ type hydrogen storage alloy.

(8) When the composition ratio of the hydrogen storage alloy is expressed as _{MmNiaCobMncAld} , 4.5 ≤ a + _b + _c + _d ≤ 5.5, 3.5 ≤ a ≤ 5.0, 0 < b ≤ 0 .7 and the range 0.1≤c≤0.7. This makes it easier to control the composition of the segregation phase, the area ratio of the segregation phase in the cross section of the hydrogen storage alloy, and the average area per segregation phase in the cross section to suitable values. Therefore, it is possible to suitably control the amount of B elements such as cobalt and manganese deposited on the separator. As a result, the self-discharge amount of the nickel-metal hydride storage battery can be suitably reduced.

It should be noted that the above embodiment can be implemented with the following modifications.
In the above embodiments, when the composition ratio of the hydrogen storage alloy is expressed as MmNiaCobMncAld, _4.5≤a + _b + _c +d≤5.5, 3.5≤a≤5.0, 0< _b Configurations satisfying the ranges of ≦0.7 and 0.1≦c≦0.7 are illustrated. However, it is not limited to this, and for example, the value of a+b+c+d may be outside the above range. That is, the hydrogen storage alloy included in the nickel-metal hydride storage battery 11 is not limited to the _AB5 type. Then, for example, the mass ratio of manganese to nickel in the segregation phase can be adjusted within the range of 0.17 or more and 0.4 or less, and by adjusting the manufacturing conditions in the alkali treatment and activation process, the segregation phase Each value of a, b, and c may be out of the above range as long as the ratio of the area of and the average area per segregation phase can be controlled to a suitable value. In addition, the hydrogen storage alloy may not contain aluminum. In this case, instead of aluminum, B elements other than nickel, cobalt, and manganese may be used, and nickel, cobalt, and , the manganese content may be increased.

- In the above-described embodiment, the configuration in which the nickel-metal hydride storage battery 11 includes an AB ₅ type hydrogen storage alloy was exemplified. However, the hydrogen storage alloy included in the nickel-metal hydride storage battery 11 is not limited to the AB ₅ type, and any one of the AB type, the AB ₂ type, and the A ₂ B ₇ type or a combination thereof can be used. Even if the nickel-metal hydride storage battery 11 includes a hydrogen storage alloy other than type AB ₅ , the amount of self-discharge in the nickel-metal hydride storage battery 11 can be reduced by controlling the amount of cobalt and manganese, which are B elements, deposited on the separator 17. reduction is possible.

- In the above-described embodiment, the segregation phase is defined as a region where nickel is present at a concentration similar to that of the parent phase, and where manganese and aluminum are present at a concentration equal to or higher than twice the average concentration in the parent phase. However, the hydrogen storage alloy may not contain aluminum. In this case, a region in which manganese is present at a relatively high concentration than in the mother phase, and a region in which manganese is present at a concentration of at least twice the average concentration in the mother phase becomes the segregation phase. . Even in this case, by controlling the ratio of the segregation phase in the cross section of the hydrogen storage alloy, the average area per segregation phase, and the mass ratio of manganese to nickel in the segregation phase, Self-discharge can be reduced.

- In the above embodiment, in the cross section of the hydrogen storage alloy, the ratio of the area of the segregation phase to the total area of the parent phase and the segregation phase is 1.5% or more and 3.0% or less. Alternatively, if the self-discharge amount can be reduced by adjusting the mass ratio of manganese to nickel in the segregation phase within the range of 0.17 or more and 0.4 or less, the ratio of the area of the segregation phase to 1 It may be out of the range of 0.5% or more and 3.0% or less.

- In the above embodiment, the average area per segregation phase is 30 square μm or more and 65 square μm or less in the cross section of the hydrogen storage alloy. Alternatively, when the self-discharge amount can be reduced by adjusting the mass ratio of manganese to nickel in the segregation phase within the range of 0.17 or more and 0.4 or less, the average area per segregation phase is It may be out of the range of 30 square μm or more and 65 square μm or less. As a result, at least the effect of reducing the self-discharge amount of the nickel-hydrogen storage battery 11 can be obtained.

- In the above-described embodiment, the nickel-metal hydride storage battery 11 is a laminated battery in which a plurality of positive electrode plates 15 and a plurality of negative electrode plates 16 are alternately laminated with separators 17 interposed therebetween. Alternatively, a wound type battery in which one long positive electrode sheet and one long negative electrode sheet are laminated and wound with a separator interposed therebetween may be used, or a battery having another structure may be used. .

- Although six battery cells 12 are housed in the battery case 13 in the above-described embodiment, the number of battery cells 12 may be one, or a plurality other than six.
- The nickel-metal hydride storage battery 11 may constitute an assembled battery consisting of a plurality of nickel-metal hydride storage batteries 11 . In addition, the nickel-metal hydride storage battery 11 may be installed in a computer, other electronic equipment, an automatic transport machine, a special vehicle for cargo handling, an electric vehicle, a hybrid vehicle, or the like, or may constitute a system other than these. may be For example, it may be provided in a moving body such as a ship or an aircraft, and it is a power supply system that supplies power from a power plant to a building or home in which a secondary battery is installed via a substation. good too.

DESCRIPTION OF SYMBOLS 11... Nickel metal hydride storage battery 12... Battery cell 13... Battery case 13a... Positive electrode terminal 13b... Negative electrode terminal 14... Cover part 15... Positive electrode plate 15a... End 16... Negative electrode plate 16a... End 17... Separator 20... Electrode plate group 21 ... Positive electrode current collector 22 ... Negative electrode current collector

Claims (5)

A nickel-metal hydride storage battery comprising a positive electrode plate including a positive electrode mixture, a negative electrode plate including a negative electrode mixture, and a separator disposed between the positive electrode plate and the negative electrode plate,
The positive electrode mixture contains cobalt,
The negative electrode mixture contains a hydrogen storage alloy,
The hydrogen storage alloy has a matrix phase containing misch metal, cobalt, nickel, and manganese, and a segregation phase containing nickel and manganese,
The segregation phase has a mass ratio of manganese to nickel of 0.17 or more and 0.4 or less,
A nickel-hydrogen storage battery, wherein a mass ratio of cobalt contained in the positive electrode mixture and the hydrogen-absorbing alloy to manganese contained in the hydrogen-absorbing alloy is 1.2 or more and 1.6 or less. 2. The ratio of the area of the segregation phase to the total area of the matrix phase and the segregation phase in the cross section of the hydrogen storage alloy is 1.5% or more and 3.0% or less. The nickel-metal hydride storage battery according to . 3. The nickel-hydrogen storage battery according to claim 1, wherein the segregation phase has an average area of 30 square μm or more and 65 square μm or less per segregation phase in a cross section of the hydrogen storage alloy. The nickel-hydrogen storage battery according to any one of claims 1 to ₃ , wherein the hydrogen storage alloy is type AB5. The hydrogen storage alloy further contains aluminum as an element constituting the parent phase and the segregation phase,
When the composition ratio in atomic ratio of the hydrogen storage alloy is expressed as MmNiaCobMncAld, the values of a, b, _c , and _d in the composition ratio are 4.5≤a+ _b + _c +d≤5. 5, 3.5≦a≦5.0, 0<b≦0.7, and 0.1≦c≦0.7.

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