JP7287884B2 - Positive plate for lead-acid battery, lead-acid battery - Google Patents

Description

The present invention relates to lead-acid batteries.

There are liquid lead-acid batteries and valve-regulated lead-acid batteries, and the flooded lead-acid batteries include a container with a cell chamber and a group of electrode plates housed in the cell chamber together with an electrolyte. The electrode plate group has a laminate composed of positive plates and negative plates alternately arranged, and separators arranged between the positive plates and the negative plates.

In recent years, many of the new vehicles sold in Japan are replacing conventional engine-powered vehicles with ISS (idling stop and start) vehicles, and the need for ISS lead-acid batteries capable of controlling these vehicles will continue to increase in the future. expected to rise.
Since lead-acid batteries for ISS vehicles are subjected to a larger load than ordinary lead-acid batteries, they are required to have high life performance, and various studies are being conducted to improve these characteristics.
Since the life performance of a lead-acid battery mainly depends on the performance of the positive electrode, there have been four methods for obtaining high life performance, such as optimizing the density of the positive electrode active material, preparing a paste in a high-temperature environment, or aging it. Methods have been proposed to produce basic lead sulfate.

For example, in US Pat. No. 5,300,000, as additives for the production of positive electrode active compositions for lead-acid batteries, micronized tetrabasic lead sulfate having an average particle size of less than 3 μm and micronized silica are used as a base, with an average Disclosed is one containing red lead with a particle size of less than 1.5 μm. Patent Document 1 describes that by including this additive, stable capacity can be obtained and improvement in life can be expected even when a grid substrate made of an alloy that does not contain antimony is used. .
Further, Patent Document 2 describes that a mixture of lead powder and red lead having a red lead conversion rate of 20 to 80% by weight is used as a raw material for a positive electrode active material.
Furthermore, Patent Document 3 describes that the electrolytic solution impregnated in the positive electrode plate and the negative electrode plate contains silica having an average particle size of 0.01 to 0.1 μm at a rate of 1.0% by mass or less. ing.

Japanese Patent No. 5851491 JP-A-2008-152968 JP-A-8-153535

However, in the methods described in Patent Documents 1 to 3, the positive electrode mixture contains red lead and silica, so it is not possible to obtain a lead-acid battery with high life performance while maintaining good discharge capacity.
An object of the present invention is to obtain a lead-acid battery with high life performance while maintaining good discharge capacity.

In order to solve the above problems, a first aspect of the present invention provides a positive electrode plate for a lead-acid battery, characterized by having the following configurations (a) to (c).
(a) A cell chamber and an electrode plate group housed in the cell chamber together with an electrolyte, the electrode plate group being arranged between the negative electrode plate and the positive electrode plate alternately arranged between the negative electrode plate and the positive electrode plate. A positive electrode plate of a lead-acid battery having a laminate comprising a separator disposed thereon.
(b) A current collector made of a lead alloy and a positive electrode material mixture held on a grid-like substrate of the current collector, satisfying the following configuration (c).
(c) The positive electrode mixture has a median pore diameter of 0.61 μm or more and 0.70 μm or less, and a density of the positive electrode mixture is 3.70 g/cm ³ or more and 3.89 g/cm ^{3 or} less, and constitutes the positive electrode mixture. The ratio (α/β) of the mass of α-PbO ₂ to the mass of β-PbO ₂ contained in the positive electrode active material is 0.12 or less.

A second aspect of the present invention provides a lead-acid battery characterized by having the following configurations (d) and (e).
(d) a cell chamber and an electrode plate group housed in the cell chamber together with an electrolyte, the electrode plate group being between the negative electrode plate and the positive electrode plate alternately arranged and the negative electrode plate and the positive electrode plate; and a separator disposed thereon.
(e) The positive electrode plate is composed of a current collector made of a lead alloy and a positive electrode mixture held on a grid-like substrate of the current collector, and satisfies the above configuration (c).

According to the lead-acid battery provided with the positive electrode plate for lead-acid battery of the present invention, it can be expected that the life performance is improved while maintaining a good discharge capacity.

Embodiments of the present invention will be described below, but the present invention is not limited to the embodiments shown below. In the embodiments shown below, technically preferred limitations are made for implementing the present invention, but the limitations are not essential to the present invention.

[composition]
The lead-acid battery of this embodiment has a monoblock type container, a lid, and a group of six electrode plates. The container is partitioned into six cell chambers by partition walls. Six cell chambers are arranged along the longitudinal direction of the container. One electrode plate group is arranged in each cell chamber. An electrolyte is injected into each cell chamber.
Each electrode plate group has a laminate including a plurality of positive electrode plates and negative electrode plates alternately arranged, and a separator disposed between the positive electrode plates and the negative electrode plates.

The positive electrode plate is formed by holding a positive electrode mixture (a mixture containing a positive electrode active material) on a grid-like substrate of a current collector made of a lead alloy having a grid-like substrate and ears protruding upward from the grid-like substrate. It is. The negative electrode plate is formed by holding a negative electrode mixture (a mixture containing a negative electrode active material) on a grid-like substrate of a current collector made of a lead alloy having a grid-like substrate and ears protruding upward from the grid-like substrate. It is. The plurality of positive electrode plates and negative electrode plates are alternately arranged with separators interposed therebetween. The number of negative electrode plates constituting the laminate may be one more than the number of positive electrode plates, or may be the same.
The positive electrode mixture has a median pore diameter of 0.61 μm or more and 0.70 μm or less, and a density of the positive electrode mixture is 3.70 g/cm ³ or more and 3.89 g/cm ³ or less. The ratio (α/β) of the mass of α-PbO ₂ to the mass of β-PbO ₂ contained in the substance is 0.12 or less.

The negative electrode mixture has the same structure as the conventional product. Specifically, it contains lead, which is a negative electrode active material, and reinforcing fibers.
The negative electrode plate is housed in a bag-shaped separator. By alternately stacking the bag-like separators containing the negative electrode plates and the positive electrode plates, the separators are arranged between the positive electrode plates and the negative electrode plates. In addition, the positive electrode plate may be accommodated in the bag-like separator and alternately stacked with the negative electrode plate.

Each electrode plate group includes a positive electrode strap and a negative electrode strap that connect the positive electrode plate and the negative electrode plate of the laminate at different positions in the width direction, respectively, and a positive electrode intermediate column and a negative electrode intermediate electrode that stand up from the positive electrode strap and the negative electrode strap, respectively. have pillars. A positive strap and a negative strap connect the ears of the positive plate and the negative plate, respectively. The positive electrode strap and the negative electrode strap arranged in the cell chambers at both ends in the cell arrangement direction are formed with a positive electrode pole and a negative electrode pole, respectively, which serve as external terminals via small pieces.

[Manufacturing method]
The lead-acid battery of the embodiment can be manufactured, for example, by the following method. Conventionally known methods can be employed except for the method of manufacturing the positive electrode plate.
First, a kneaded material containing lead powder, tetrabasic lead sulfate having an average particle size of 5 μm or less, sulfuric acid, bismuth oxide, and water is prepared as a kneaded material used when producing a positive electrode plate before chemical conversion. The amount of bismuth oxide to be added is 0.03 parts by mass or more and 0.10 parts by mass or less with respect to 100 parts by mass of lead powder.

Next, after the produced kneaded material is filled into a grid-like substrate of a current collector, it is aged at a temperature of 70° C. or higher and 90° C. or lower and a humidity of 90% or higher and 98% or lower, and then dried. As a result, the content of tetrabasic lead sulfate in the positive electrode mixture before chemical conversion is set to 70% or more and 75% or less. In addition, depending on the aging temperature selected within the range of 70° C. or higher and 90° C. or lower so that the content of tetrabasic lead sulfate in the positive electrode mixture before chemical conversion is 70% or higher and 75% or lower, The amount of sulfuric acid and tetrabasic lead sulfate to be added when preparing the kneaded product is adjusted.
The above is the process for obtaining the positive electrode plate before anodization.

Next, using the obtained positive electrode plate before chemical conversion, the negative electrode plate before chemical conversion produced by a normal method, and the separator, a laminate before chemical conversion is produced.
Next, using a COS (cast-on-strap) type casting apparatus, the laminate before chemical conversion is formed into a positive electrode strap in which the tabs of the positive plates are connected and a negative strap in which the tabs of the negative plates are connected, A positive electrode middle pole, a negative electrode middle pole, a positive electrode pole and a negative electrode pole are formed. After forming them, the laminate is placed in each cell chamber of the battery case.

Next, the adjacent cells are electrically connected in series by resistance welding the positive intermediate poles or the negative intermediate poles of the adjacent cell chambers. Next, the upper surface of the container and the lower surface of the lid are melted by heat, the lid is placed on the container, and the lid is fixed to the container by thermal welding. When the lid is placed on the container, the positive pole and the negative pole are passed through the through hole of the bushing insert-molded in the lid. After that, the positive pole and the negative pole protruding from the through-hole of the bushing are heated with a burner or the like to be integrated with the bushing, thereby forming the positive terminal and the negative terminal.

After that, an electrolytic solution containing aluminum ions at a concentration of 20 mmol/L or more and 200 mmol/L or less (an electrolytic solution in which aluminum sulfate is added to sulfuric acid) is poured into the cell chamber through an injection hole provided as a hole penetrating the lid. After the injection, the assembly of the lead-acid battery is completed by carrying out normal processes such as plugging the injection hole. After that, a lead-acid battery is obtained by forming a container under normal conditions.
By this container formation, the lead powder held in the current collector is changed into a positive electrode active material, the median pore diameter of the positive electrode mixture is 0.61 μm or more and 0.70 μm or less, and the density of the positive electrode mixture is 3.70 g/cm ³ or more and 3.89 g/cm ³ or less, and the ratio (α/β) of the mass of α-PbO ₂ to the mass of β-PbO ₂ contained in the positive electrode active material constituting the positive electrode mixture is 0.12 or less.

[action, effect]
The larger the median pore diameter and the higher the density of the positive electrode mixture, the better the adhesion between the positive electrode active materials or between the positive electrode active material and the lattice substrate. Efficiency decreases. The smaller the median pore diameter and the lower the density of the positive electrode mixture, the more efficiently the active material is used. sexuality declines.
Also, the more electrochemically inactive α-PbO ₂ is, the more advantageous it is in terms of durability (lifetime performance), but it is disadvantageous in terms of utilization efficiency of the active material. A larger amount of electrochemically active β-PbO ₂ is advantageous in terms of utilization efficiency (discharge capacity) of the active material, but disadvantageous in terms of durability (lifetime performance).

In the positive electrode plate constituting the lead-acid battery of the present embodiment, the positive electrode mixture has a median pore diameter of 0.61 μm or more and 0.70 μm or less, and the density of the positive electrode mixture is 3.70 g/cm ³ or more and 3.89 g/cm. ³ or less, and the ratio (α/β) of the mass of α-PbO ₂ to the mass of β-PbO ₂ contained in the positive electrode active material constituting the positive electrode mixture is 0.12 or less. As a result, the lead-acid battery of the present embodiment has a high life performance while maintaining a good discharge capacity.
Further, in the method of the present embodiment, as the step of obtaining the positive electrode mixture before chemical conversion, a kneaded product containing lead powder, tetrabasic lead sulfate having an average particle size of 5 μm or less, sulfuric acid, and water is filled into the lattice substrate. After that, the kneaded material filled in the lattice-like substrate is aged at a temperature of 70° C. or more and 90° C. or less and then dried, so that the content of tetrabasic lead sulfate in the positive electrode mixture before chemical conversion is reduced to 70% or more and 75%. % or less, chemical conversion is performed. Thereby, the positive electrode plate of this embodiment can be obtained.

In the method of the present embodiment, by using fine powder of tetrabasic lead sulfate having an average particle size of 5 μm or less, coarse columnar crystals of tetrabasic lead sulfate can be obtained even when aging is performed in a high temperature environment of 70° C. or higher. does not generate In addition, by controlling the amount of tetrabasic lead sulfate after aging to 70% or more and 75% or less, the positive electrode active material of the positive electrode plate after chemical conversion does not cause a decrease in discharge capacity due to a decrease in reaction specific surface area. Life performance of the lead-acid battery can be improved.
Furthermore, the greater the specific surface area of the positive electrode mixture, the greater the contact area between the active material and the electrolytic solution, so the utilization rate of the active material tends to increase. However, if the specific surface area is too large, the durability will be low. In order to achieve both high utilization of the active material and high durability, the specific surface area of the positive electrode mixture is preferably in the range of 5.5 to 6.0 m ² /g.

[Preparation of test battery]
As lead-acid batteries having the same structure as the lead-acid batteries of the embodiment, sample No. 1 to No. 9 lead-acid batteries were produced by the conventionally known method described in the embodiment. Specifically, a B-size lead-acid battery with a rated capacity of 32 Ah and an operating voltage of 12 V was produced.

[Preparation of positive electrode plate (before chemical conversion)]
<No.1>
First, 2000 g of lead powder for a storage battery (mixed powder of lead and lead oxide with a particle size of several μm to 30 and several μm, the mixing ratio of lead:lead oxide=approximately 25:75 in terms of mass ratio) was added with water. 370 g, 172 g of sulfuric acid having a specific gravity of 1.37, 1 g of bismuth oxide, and 20 g of tetrabasic lead sulfate (4BS) having an average particle size of 5 μm or less were added and kneaded to obtain a positive electrode mixture forming paste (kneaded product). .
Next, this paste was filled in a grid-like substrate of a B-size battery current collector made of a lead alloy made of a Pb—Sn lead alloy, and placed in an environment at a temperature of 70° C. and a humidity of 95% or more for 48 hours. It was aged by leaving it for hours, and then dried at 60° C. for 24 hours. Thus, a positive electrode plate before anodization was obtained.
The content of tetrabasic lead sulfate contained in the positive electrode mixture (hereinafter referred to as "unformed positive electrode material") of the obtained positive electrode plate before chemical formation was examined by powder X-ray diffraction measurement. Specifically, the positive electrode unformed substance was scraped into powder form from the obtained positive electrode plate before chemical formation, and the scraped-off positive electrode unformed substance powder was set in an X-ray diffractometer to obtain an X-ray diffraction chart. Then, the content of tetrabasic lead sulfate in the unformed positive electrode material was calculated from the intensity ratio of the diffraction lines of each material. As a result, it was 70.4%.

<No.2>
A positive electrode plate before formation was obtained in the same manner as in No. 1, except that the amount of water added in obtaining the positive electrode mixture forming paste was 380 g. The content of tetrabasic lead sulfate contained in the positive electrode mixture of the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1 and found to be 70.6%.
<No.3>
A positive electrode plate before chemical conversion was obtained in the same manner as in No. 1, except that 10 g of tetrabasic lead sulfate (4BS) having an average particle size of 5 μm or less was added. The content of tetrabasic lead sulfate contained in the positive electrode mixture of the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1 and found to be 66.8%.

<No.4>
A positive electrode plate before anodization was obtained in the same manner as in No. 1, except that the aging temperature was 80°C. The content of tetrabasic lead sulfate contained in the positive electrode mixture of the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1 and found to be 75.0%.
<No.5>
A positive electrode plate before formation was obtained in the same manner as in No. 1, except that the amount of water added in obtaining the positive electrode mixture forming paste was 380 g and the temperature during aging was 90°C. The content of tetrabasic lead sulfate contained in the positive electrode mixture of the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1 and found to be 72.5%.

<No.6>
A positive electrode plate before formation was obtained in the same manner as in No. 1, except that the amount of water added when obtaining the positive electrode mixture forming paste was 380 g and the temperature during aging was 95°C. The content of tetrabasic lead sulfate contained in the positive electrode mixture contained in the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1 and found to be 72.2%.
<No.7>
A positive electrode plate before anodization was obtained in the same manner as in No. 1, except that the aging temperature was 60°C. The content of tetrabasic lead sulfate contained in the positive electrode mixture of the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1 and found to be 67.0%.

<No.8>
A positive electrode plate before anodization was obtained in the same manner as in No. 1, except that the aging temperature was 65°C. The content of tetrabasic lead sulfate contained in the positive electrode mixture contained in the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1 and found to be 68.6%.
<No.9>
Except that the amount of water added when obtaining the positive electrode mixture-forming paste was 450 g, and tetrabasic lead sulfate (4BS) having an average particle size of 5 μm or less was not added when obtaining the positive electrode mixture-forming paste, No A positive electrode plate before formation was obtained in the same manner as in .1. When the content of tetrabasic lead sulfate contained in the positive electrode mixture of the obtained positive electrode plate before chemical conversion was examined by the same method as in No. 1, it was less than 3%, which was within the range of measurement error. It was confirmed that tetrabasic lead sulfate was substantially absent.

[Preparation of negative electrode plate (before chemical conversion)]
400 g of water, 1.8 g of polyester fiber (reinforcing fiber), 20 g of barium sulfate, 4 g of conductive carbon, and 4 g of lignin were added to 2000 g of the same lead powder for a storage battery that was used in the preparation of the positive electrode mixture forming paste. Add and mix. To the mixture thus obtained, 228 g of an aqueous sulfuric acid solution having a specific gravity D at 20° C. of 1.37 was added and kneaded to obtain a negative electrode mixture forming paste (kneaded product).
This paste was applied to a grid-like substrate of a current collector for a B-size battery made of a Pb--Ca lead alloy, and then subjected to a maturing drying process under normal conditions to obtain a negative electrode plate before anodization.

[Assembly of lead-acid battery]
First, in order to produce electrode plate groups for lead-acid batteries No. 1 to No. 9, each of 36 positive electrode plates before chemical conversion of No. 1 to No. 9 produced by the above method and the above method. 378 (9×42) sheets of the negative electrode plate before chemical conversion and the same number of bag-shaped separators as the negative electrode plate before chemical conversion were prepared.
Next, the negative electrode plate before chemical conversion is housed in a bag-shaped separator, and the seven separators containing the negative electrode plate before chemical conversion and the six positive electrode plates before chemical conversion are alternately laminated to obtain six positive electrode plates before chemical conversion. Samples Nos. 1 to 9 each had 6 laminates each having 7 negative electrode plates before anodization.

Next, six laminates obtained for each sample No. were placed in a COS (cast-on-strap) type casting apparatus, and molten metal (lead alloy) was supplied into the cavity, and the ears were placed downward. First, a positive electrode strap and a negative electrode strap that connect the respective ears were formed by inserting the ears of the laminate in a state of facing toward each other. Subsequently, a small piece and a pole column are formed on the negative electrode straps and the positive electrode straps arranged in the cell chambers at both ends in the arrangement direction, and the positive intermediate electrode column and the negative electrode intermediate column are formed on the other positive electrode straps and negative electrode straps, respectively. formed. Next, they were placed in six cell chambers of a polypropylene monoblock type battery case of "SBA (Battery Manufacturers Association Standard) S 0101" external shape section M.

Next, the positive electrode intermediate electrode column and the negative electrode intermediate electrode column facing each other across the partition wall that partitions the cell chambers of the battery case were connected by resistance welding at the portion of the through hole provided in the partition wall. In this state, the electrode plate group before formation is arranged in each cell of the container.
By heat-sealing the container and the lid in this state by the method described in the embodiment, No. 1 to No. 9 lead-acid batteries before chemical conversion were obtained.
Next, a dilute sulfuric acid electrolyte solution (aluminum ion concentration is 117 mmol/L) to which 20 g/L of aluminum sulfate has been added is poured from the injection hole of the lid of the lead-acid battery before chemical conversion of No. 1 to No. 9 into the container. was injected into each cell chamber. Thereafter, container formation was performed under normal conditions to obtain No. 1 to No. 9 lead-acid batteries.

[Measurement of Median Pore Size, Density, and Ratio (α/β)]
The median pore size, density, and ratio (α/β) of the positive electrode mixture were measured for the positive electrode plates of No. 1 to No. 9 lead-acid batteries by the following methods.
A positive electrode plate was taken out from each lead-acid battery after container formation, washed with water and dried, and then a solid positive electrode mixture was collected from the positive electrode plate. The obtained solid positive electrode mixture was set in a mercury intrusion porosimeter (Shimadzu Corporation, Autopore IV9520), and the median pore diameter and density of the positive electrode mixture were measured by the mercury intrusion method.
In addition, the obtained solid positive electrode mixture was pulverized with a mortar or the like, and the obtained powder was set in an X-ray diffraction apparatus (Rigaku, RINT-Ultima+) to obtain an X-ray diffraction chart and obtain β-PbO ₂ . A ratio (α/β) was calculated from the intensity ratio of the diffraction line of β and α-PbO ₂ .

[Test to check discharge capacity: Discharge test]
The active material utilization rate of the positive plate of the lead-acid batteries No. 1 to No. 9 was examined by the following method.
According to JIS D 5301, each lead-acid battery was placed in a water tank at 25° C. and discharged at a constant current rate of 5 hours until the final voltage reached 10.5V. The discharge capacity when reaching 10.5 V was divided by the theoretical capacity of each lead-acid battery, and the obtained value was defined as the discharge capacity of each lead-acid battery.

[Test to examine life performance: life test]
The capacity retention rate of No. 1 to No. 9 lead-acid batteries was examined by the following method.
In accordance with the heavy load life test described in JIS D 5301 9.5 (b), a fully charged lead-acid battery was discharged to a depth of discharge of 40% in an environment with an ambient temperature of 40 ° C. in 1 hour, and then at a rate of 10 hours. A cycle of charging with current to 125% of the discharge capacity was repeated 24 cycles. The discharge at the 25th cycle was used as a judgment discharge, and the battery was discharged until it reached a final voltage of 10.2 V, and then charged at a rate of 10 hours to 140% of the discharge capacity. After 25 cycles, the lead-acid battery was taken out, and the electrolytic solution was replenished with the amount of water reduced in the 25 cycles. This charge-discharge cycle test was continued until the discharge capacity at the 25th cycle was less than 50% of the 5-hour rate capacity of each lead-acid battery. Then, the number of cycles of lead-acid battery No. 9 (conventional product) was divided by the number of cycles of lead-acid batteries No. 1 to No. 8, and the obtained values were compared as relative values of the number of cycles.

These measurements and test results are shown in Table 1 below, together with the method of manufacturing the positive electrode mixture before chemical conversion, the content of tetrabasic lead sulfate in the positive electrode mixture before chemical conversion, and the composition of the positive electrode mixture. shown. The discharge capacity and the number of cycles are shown as relative values with the No. 9 lead-acid battery (conventional product) set to 100.

As shown in Table 1, the lead-acid batteries No. 1, No. 4, and No. 5, which correspond to the examples of the present invention, have a discharge capacity of 97% or more that of the conventional product, and a cycle number that is greater than that of the conventional product. It was as high as 127% or more. In other words, it can be seen that the No. 1, No. 4, and No. 5 lead-acid batteries, which correspond to the examples of the present invention, can achieve a high number of cycles (lifetime performance) while maintaining the same discharge capacity as the conventional product. .

Claims (2)

A cell chamber, and an electrode plate group accommodated in the cell chamber together with an electrolyte, the electrode plate group including a negative electrode plate and a positive electrode plate alternately arranged, and a space between the negative electrode plate and the positive electrode plate. a separator disposed in the positive electrode plate of a lead-acid battery having a laminate consisting of
Consists of a current collector made of a lead alloy and a positive electrode mixture held on a grid-like substrate of the current collector,
The positive electrode mixture has a median pore diameter of 0.61 μm or more and 0.70 μm or less, and a density of the positive electrode mixture is 3.70 g/cm ³ or more and 3.89 g/cm ³ or less, and constitutes the positive electrode mixture. A positive electrode plate for a lead-acid battery, wherein the ratio (α/β) of the mass of α-PbO ₂ to the mass of β-PbO ₂ contained in the positive electrode active material is 0.12 or less. A cell chamber, and an electrode plate group accommodated in the cell chamber together with an electrolyte, the electrode plate group including a negative electrode plate and a positive electrode plate alternately arranged, and a space between the negative electrode plate and the positive electrode plate. A lead-acid battery having a laminate consisting of a separator arranged in
The positive electrode plate is composed of a current collector made of a lead alloy and a positive electrode mixture held on a grid-like substrate of the current collector,
The positive electrode mixture has a median pore diameter of 0.61 μm or more and 0.70 μm or less, and a density of the positive electrode mixture is 3.70 g/cm ³ or more and 3.89 g/cm ³ or less, and constitutes the positive electrode mixture. A lead-acid battery in which the ratio (α/β) of the mass of α-PbO ₂ to the mass of β-PbO ₂ contained in the positive electrode active material is 0.12 or less.