JP7848652B2 - lead acid battery - Google Patents

Description

This invention relates to a lead-acid battery.

Lead-acid batteries are used in a variety of applications, including automotive and industrial uses. A lead-acid battery consists of a positive electrode plate, a negative electrode plate, a separator between them, and an electrolyte. The separator in a lead-acid battery requires various performance characteristics. Generally, porous polyolefin films are used as separators.

Patent Document 1 proposes a separator for a lead-acid battery comprising a polyolefin microporous membrane, wherein the polyolefin microporous membrane comprises polyethylene, preferably ultra-high molecular weight polyethylene, a particulate filler, and a treatment plasticizer, wherein the particulate filler is present in an amount of 40% or more by weight, and the polyethylene comprises a shish-kebab forming polymer containing a plurality of elongated chain crystals (shishi formation) and a plurality of folded chain crystals (kebab formation), and the average repetition or period of the kebab formation is from 1 nm to 150 nm, preferably less than 120 nm.

Patent Document 2 proposes a lead-acid battery characterized by containing a reducing organic substance in the electrolyte at a concentration of 0.5 mg/L to 3 mg/L.

Patent Document 3 proposes a ribbed separator for lead-acid batteries containing 5 to 30% by mass of oil, obtained by heating and melting a raw material composition consisting of a mixture of 20 to 60% by mass of polyolefin resin, 80 to 40% by mass of inorganic powder, and 40 to 240% by mass of mineral oil relative to these components, kneading it, forming it into a sheet with ribs, immersing it in an immersion tank of an organic solvent capable of dissolving the oil to extract and remove a portion of the oil, and then heating and drying it, characterized in that the difference in oil content between the rib portion and the base portion of the separator is 5% by mass or less.

Special table 2019-514173 publication Japanese Patent Publication No. 2005-251394 Japanese Patent Publication No. 2001-338631

The inventors have found that reducing the chemical oxygen demand (COD), which is an indicator of the concentration of organic components in the electrolyte, improves charge acceptance.

However, the inventors have newly discovered that while reducing the amount of organic components in the electrolyte improves charge acceptance, this comes at the cost of a shorter lifespan in high-temperature overcharge tests. For example, in light-load life tests (e.g., the 75°C JIS light-load life test (two-part method)), the charge amount is greater than the discharge amount. Therefore, during the test, the lead-acid battery becomes overcharged, generating hydrogen at the negative electrode and oxygen at the positive electrode due to electrolyte decomposition. Reducing the amount of organic components in light-load life tests improves charge acceptance, which tends to further overcharge the lead-acid battery.

Oxygen generated on the positive electrode side attacks the separator's constituent materials, causing oxidative degradation of the separator. This leads to cracks or tears in the separator, causing the positive and negative electrode materials to come into contact at the damaged area, resulting in a short circuit and ultimately ending the separator's lifespan.

One aspect of this disclosure relates to a lead-acid battery comprising a positive electrode plate, a negative electrode plate, a porous resin film interposed between the positive electrode plate and the negative electrode plate, and an electrolyte, wherein the porous film includes a crystalline region and an amorphous region, and the degree of crystallinity in the X-ray diffraction spectrum of the separator, expressed as 100 × I _c / (I _c + I _a ), is 18% or more, where I _c is the integrated intensity of the diffraction peak with the maximum peak height among the diffraction peaks corresponding to the crystalline region, and I _a is the integrated intensity of the halo corresponding to the amorphous region, and the chemical oxygen demand in the electrolyte is 160 mg/L or less.

According to this disclosure, it is possible to improve the lifespan of a lead-acid battery in high-temperature overcharge conditions while simultaneously improving its charge acceptance.

This is a partially cutaway perspective view showing the external appearance and internal structure of a lead-acid battery according to one embodiment of the present invention.

The following describes embodiments of this disclosure with examples, but this disclosure is not limited to the examples described below. While specific numerical values and materials may be given as examples in the following description, other numerical values and materials may be applied as long as the effects of this disclosure are achieved. In this specification, the phrase "numerical value A to numerical value B" includes numerical values A and B, and can be interpreted as "greater than or equal to numerical value A and less than or equal to numerical value B." In the following description, when lower and upper limits of numerical values relating to specific physical properties or conditions are given as examples, either the given lower limit or either the given upper limit can be arbitrarily combined, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, one may be selected and used alone, or two or more may be used in combination.

Furthermore, this disclosure encompasses combinations of matters described in two or more claims arbitrarily selected from the multiple claims set forth in the attached claims. In other words, as long as no technical inconsistency arises, any combination of matters described in two or more claims arbitrarily selected from the multiple claims set forth in the attached claims is possible.

In this specification, the vertical direction of a lead-acid battery or its components (plates, case, separator, etc.) refers to the vertical direction of the lead-acid battery as it is positioned in its intended use. Each of the positive and negative plates is provided with tabs for connecting to external terminals. For example, in a liquid-type battery, the tabs are provided on the upper part of the plate, protruding upward.

The lead-acid battery relating to this disclosure may be a valve-regulated lead-acid battery (VRLA type battery), but a liquid-type battery (vented type battery) is preferred because it allows for effective utilization of the effects of COD reduction.

A lead-acid battery comprises a positive electrode plate, a negative electrode plate, a separator interposed between the positive and negative electrode plates, and an electrolyte. The electrolyte contains sulfuric acid. Charging and discharging proceed through the movement of sulfate ions between the positive and negative electrode plates and the electrolyte. During discharge, sulfate ions move to the positive and negative electrode plates, decreasing the density of the electrolyte. During charging, sulfate ions move from the positive and negative electrode plates into the electrolyte, increasing the density of the electrolyte.

The separator includes a porous film made of resin. The porous film can be used as a separator on its own. Therefore, the porous film may also be referred to as a separator. The separator may also include a nonwoven fabric in addition to the porous film.

The positive electrode plate, negative electrode plate, and separator constitute an electrode group. The electrode group, together with the electrolyte, constitutes a cell. One electrode group constitutes one cell. A lead-acid battery comprises one or more cells by having one or more electrode groups. There is no particular limit to the number of positive and negative electrode plates included in one electrode group. An electrode group comprising a lead-acid battery according to this disclosure, for example, includes a total of 12 or more positive and negative electrode plates. Multiple electrode groups are usually housed in separate cell chambers and connected in series with one another.

The positive electrode plate includes a positive electrode material. The positive electrode material, as a positive electrode active material that exhibits capacity through oxidation-reduction reactions, contains at least lead dioxide during charging and at least lead sulfate during discharge.

The negative electrode plate includes a negative electrode material. The negative electrode material, as a negative electrode active material that exhibits capacity through oxidation-reduction reactions, contains at least lead during charging and at least lead sulfate during discharge.

(1) A lead-acid battery according to one embodiment of the present disclosure comprises a positive electrode plate, a negative electrode plate, a porous resin film interposed between the positive electrode plate and the negative electrode plate, and an electrolyte, wherein the porous film includes a crystalline region and an amorphous region, and the degree of crystallinity in the X-ray diffraction spectrum of the separator, represented by 100 × I _c / (I _c + I _a ), is 18% or more, where I _c is the integrated intensity of the diffraction peak with the maximum peak height among the diffraction peaks corresponding to the crystalline region, I _a is the integrated intensity of the halo corresponding to the amorphous region, and the chemical oxygen demand (COD) in the electrolyte is 160 mg/L or less.

COD (Chemical Oxygen Demand) is the amount of oxygen required to oxidize oxidizable substances in the electrolyte. The COD in the electrolyte can be considered an indicator of the concentration of organic components contained in the electrolyte. In this specification, COD refers to the COD in the electrolyte sampled from a fully charged lead-acid battery in its initial state of use. An "initial-use battery" refers to a battery that has not been in use for very long and has shown little degradation.

The lead-acid battery described in (1) above exhibits excellent charge acceptance and superior lifespan performance in high-temperature overcharge tests.

(2) In the lead-acid battery described in (1) above, the chemical oxygen demand may be 100 mg/L or less.

The lead-acid battery described in (2) above has superior charge acceptance and its lifespan performance does not degrade easily under high-temperature overcharge conditions.

(3) In the lead-acid battery described in (1) or (2) above, the chemical oxygen demand may be 15 mg/L or more.

The lead-acid batteries described in (3) above exhibit less degradation in lifespan performance during high-temperature overcharge tests.

(4) In the lead-acid battery described in any one of (1) to (3) above, the degree of crystallinity may be 40% or less.

The porous film provided in the lead-acid battery described in (4) above is not only highly flexible but also easy to manufacture.

(5) In the lead-acid battery described in any one of (1) to (4) above, the porous film may have a thickness of 0.1 mm or more and 0.3 mm or less.

In the lead-acid battery described in (5) above, the porous film is sufficiently thin yet possesses high strength, thus enabling higher charge acceptance and superior lifespan performance in high-temperature overcharge tests.

The porous film comprising the lead-acid battery according to this disclosure includes a crystalline region and an amorphous region. In the X-ray diffraction spectrum of the separator, the degree of crystallinity, expressed as 100 × I _c / (I _c + I _a ), is 18% or more. Here, I _c is the integrated intensity of the diffraction peak with the maximum peak height among the diffraction peaks corresponding to the crystalline region. _{I a} is the integrated intensity of the halo corresponding to the amorphous region.

Reducing the COD in the electrolyte decreases side reactions caused by oxidation of organic components, thereby facilitating the charging reaction of the positive and negative electrode active materials, and improving charge acceptance. Reducing the COD in the electrolyte to 160 mg/L or less significantly improves charge acceptance.

On the other hand, while charge acceptance is improved, a trade-off is that the battery life tends to be shorter in high-temperature overcharge tests. The reason for this is presumed to be as follows: Higher charge acceptance makes lead-acid batteries more susceptible to overcharging, leading to increased generation of hydrogen and oxygen due to electrolyte decomposition. The large amount of oxygen generated on the positive electrode side attacks the porous film. As a result, the porous film oxidizes and deteriorates, causing cracks or tears, leading to internal short circuits and ultimately the battery's lifespan.

In contrast, when the crystallinity of the porous film is 18% or higher, even when the COD in the electrolyte is reduced to 160 mg/L or less to improve charge acceptance, the deterioration of lifespan performance in high-temperature overcharge conditions is significantly suppressed. This is because, once the crystallinity reaches a certain level, the oxidation resistance of the porous film improves significantly. As a result, the time until cracks or tears occur in the porous film is extended, improving its lifespan performance.

Furthermore, increasing the crystallinity of the porous film to 23% or higher significantly improves its lifespan performance in high-temperature overcharge tests when the COD in the electrolyte is reduced to 160 mg/L or less. This is because the time until cracks or tears occur in the porous film becomes even longer, making it rare for the porous film to reach the end of its lifespan due to degradation.

The ratio of the mass of the positive electrode material to the mass of the negative electrode material (hereinafter also referred to as the "Mp/Mn ratio") is, for example, 1.2 or higher, and may also be 1.3 or higher. The Mp/Mn ratio may also be 1.4 or lower. A preferred range for the Mp/Mn ratio is, for example, 1.2 to 1.4, and may also be 1.3 to 1.4. The mass of the positive electrode material refers to the mass of the positive electrode material contained in one positive electrode plate. The mass of the negative electrode material refers to the mass of the negative electrode material contained in one negative electrode plate. Increasing the Mp/Mn ratio to 1.2 or higher means reducing the amount of negative electrode material used. In other words, by increasing the Mp/Mn ratio to 1.2 or higher, lead-acid batteries can be made lighter and less expensive. Furthermore, when the Mp/Mn ratio is 1.2 or higher, the load on the positive electrode plate can be reduced, making it easier to suppress softening and detachment of the positive electrode plate.

The lead-acid battery described herein is also suitable for use in vehicles with idle stop-start (ISS) control.

In this specification, the fully charged state of a liquid-type lead-acid battery is defined according to the definition in JIS D 5301:2019. More specifically, the fully charged state is defined as the state in which the lead-acid battery is charged at a current of 2 I²⁰ (unit: A), which is twice the 20-hour rate current _I²⁰ , for three consecutive times in a water bath at 25°C ± 2°C, until the terminal voltage (unit: V) during charging, measured _{every 15} minutes, or the electrolyte density converted to 20°C, shows a constant value with three significant figures. The 20-hour rate current _I²⁰ is 1/20 of the Ah value stated in the rated capacity (A). The value stated as the rated capacity is a value in Ah (ampere-hour). The unit of the current set based on the value stated as the rated capacity is A (ampere). Furthermore, in the case of valve-regulated lead-acid batteries, a fully charged state is defined as a state where charging is performed in an air chamber at 25°C ± ₂ °C with a constant current and constant voltage of 2.67V/cell (16.00V for a lead-acid battery with a rated voltage of 12V) at a current of _5I²⁰ (unit: A), five times the 20-hour rate current I²⁰, and charging is completed when the total charging time reaches 24 hours.

A fully charged lead-acid battery is a lead-acid battery that has been charged to its full capacity after chemical formation. The timing of charging the lead-acid battery to its full capacity can be immediately after chemical formation or after some time has passed. For example, a lead-acid battery in use (preferably in the early stages of use) after chemical formation may be charged.

In this specification, a "battery in its initial use" refers to a battery that has not been in use for very long and has shown little to no degradation.

The following describes a lead-acid battery according to an embodiment of the present invention with reference to the drawings. However, the present invention is not limited to the following embodiments.

The following describes examples of components of a lead-acid battery.

(Positive plate)
The positive electrode plate comprises a positive electrode current collector and a positive electrode material. The positive electrode material is held by the positive electrode current collector. The positive electrode material is the portion of the positive electrode plate excluding the positive electrode current collector. Note that adhesive members such as conductive layers, mats, and pasting paper may be attached to the positive electrode plate. Since the adhesive members are used integrally with the positive electrode plate, they are included as components of the electrode plate. When the positive electrode plate includes adhesive members, the positive electrode material is the portion of the positive electrode plate excluding the positive electrode current collector and the adhesive members.

The positive electrode current collector may be formed by casting lead (Pb) or a lead alloy, or by processing a lead or lead alloy sheet. The processing method may include, for example, expansion or punching. Using a grid-like current collector as the positive electrode current collector makes it easier to support the positive electrode material.

As the lead alloy used for the positive electrode current collector, Pb-Ca alloys and Pb-Ca-Sn alloys, which have excellent corrosion resistance and mechanical strength, are preferred. The positive electrode current collector may have metal layers with different compositions, and the metal layers may be a single layer or multiple layers.

The positive electrode material contains a positive electrode active material that exhibits capacity through a redox reaction. The positive electrode active material includes lead dioxide, lead sulfate, etc. The positive electrode material may also contain additives as needed. These additives may include reinforcing materials, antimony compounds, etc. Examples of reinforcing materials include inorganic fibers and organic fibers.

Unformed positive electrode plates are obtained by maturing and drying a positive electrode current collector and a positive electrode paste filled in the current collector. The positive electrode paste is prepared by kneading a mixture containing lead powder, water, and sulfuric acid. The positive electrode paste may contain additives as needed. These additives may include reinforcing materials, antimony compounds, etc. Such positive electrode plates are also called paste-type positive electrode plates.

A positive electrode plate can be obtained by chemically treating an untreated positive electrode plate. Chemical treatment may be carried out by immersing the electrode plate group, including the untreated positive electrode plate, in an electrolyte containing sulfuric acid within the lead-acid battery case, and then charging the electrode plate group. Chemical treatment may also be performed before the assembly of the lead-acid battery or the electrode plate group.

(Negative electrode plate)
The negative electrode plate comprises a negative electrode current collector and a negative electrode material. The negative electrode material is held by the negative electrode current collector. The negative electrode material is the portion of the negative electrode plate excluding the negative electrode current collector. Note that adhesive members such as conductive layers, mats, and pasting paper may be attached to the negative electrode plate. The adhesive members are included as components of the negative electrode plate. When the negative electrode plate includes adhesive members, the negative electrode material is the portion of the negative electrode plate excluding the negative electrode current collector and the adhesive members.

The negative electrode current collector may be formed by casting lead (Pb) or a lead alloy, or by processing a lead or lead alloy sheet. The processing method may be expansion or punching. Using a grid-like current collector as the negative electrode current collector makes it easier to support the negative electrode material.

The lead alloy used in the negative electrode current collector may be any of the following: Pb-Sb alloy, Pb-Ca alloy, or Pb-Ca-Sn alloy. The lead alloy used in the negative electrode current collector may also contain at least one additive element selected from the group consisting of Ba, Ag, Al, Bi, As, Se, Cu, etc. The negative electrode current collector may have metal layers with different compositions, and the metal layers may be a single layer or multiple layers.

The negative electrode material contains a negative electrode active material that exhibits capacity through a redox reaction. The negative electrode active material includes lead, lead sulfate, etc. The negative electrode material may also contain 100 ppm to 300 ppm of element Bi by mass. The negative electrode material may contain other additives as needed. These additives may include organic shrinkage inhibitors, carbonaceous materials, barium sulfate, etc.

Examples of organic shrinkage inhibitors include lignin, lignin sulfonic acid, and synthetic organic shrinkage inhibitors. Synthetic organic shrinkage inhibitors may include, for example, formaldehyde condensates of phenolic compounds. Organic shrinkage inhibitors may be used individually or in combination of two or more. The content of the organic shrinkage inhibitor in the negative electrode material is, for example, 0.01% by mass or more and 1% by mass or less.

As carbonaceous materials, carbon black, artificial graphite, natural graphite, hard carbon, soft carbon, etc., can be used. One type of carbonaceous material may be used alone, or two or more types may be used in combination. The carbonaceous material content in the negative electrode material is, for example, 0.1% by mass or more and 3% by mass or less.

The barium sulfate content in the negative electrode material is, for example, 0.1% by mass or more and 3% by mass or less.

The unformed negative electrode plate is obtained by aging and drying the negative electrode current collector and the negative electrode paste filled in the negative electrode current collector. Aging is preferably carried out in an atmosphere with a temperature higher than room temperature and high humidity. The negative electrode paste is prepared by kneading a mixture containing lead powder, water, and sulfuric acid. The negative electrode paste may contain additives as needed. These additives may include bismuth compounds (e.g., bismuth sulfate), organic shrinkage inhibitors, carbonaceous materials, barium sulfate, etc.

A negative electrode plate is obtained by chemically treating an untreated negative electrode plate. Chemical treatment may be carried out by immersing the electrode plate group, including the untreated negative electrode plate, in an electrolyte containing sulfuric acid in the lead-acid battery case, and charging the electrode plate group. Chemical treatment may also be carried out before the assembly of the lead-acid battery or the electrode plate group. The charged negative electrode active material contains spongy lead.

(electrolyte)
The electrolyte is an aqueous solution containing sulfuric acid. The electrolyte may be gelled as needed. The electrolyte may further contain at least one metal ion selected from the group consisting of Na ions, Li ions, Mg ions, and Al ions. The density of the electrolyte at 20°C is, for example, 1.10 or higher. The density of the electrolyte at 20°C may be 1.35 or lower. These densities are values for the electrolyte of a fully charged lead-acid battery.

To ensure high power acceptance, the COD in the electrolyte should be 160 mg/L or less, but it may also be 130 mg/L or less, 100 mg/L or less, or 80 mg/L or less. Furthermore, the COD in the electrolyte may be 15 mg/L or more, 25 mg/L or more, 50 mg/L or more, or 80 mg/L or more. A preferred range for the COD in the electrolyte is, for example, 15 mg/L to 160 mg/L, but it may also be 25 mg/L to 130 mg/L, or 50 mg/L to 100 mg/L.

The amount of COD in the electrolyte can be controlled, for example, by the following methods. One of these methods may be used, or a combination of several may be used.
(1) Adjust the concentration of organic additives in the electrolyte.
(2) Adjust the content of organic components in components other than the electrolyte. "Components other than the electrolyte" include separators, current collectors for positive or negative electrode plates, and positive or negative electrode materials. In other words, the method in (2) can be broadly divided into the following three methods.
(2-1) Control the content of relatively low molecular weight organic additives in the porous film. Examples of organic additives include penetrants and oils.
(2) Remove at least a portion of the cutting oil adhering to the current collector of the positive or negative electrode plate, or to the metal plate before it is processed into a current collector, by washing or other means.
(2-3) Control the content of at least one of the organic component and carbonaceous material contained in the positive electrode material or the negative electrode material. The organic component contained in the electrode material also includes an organic shrinkage inhibitor.

Organic solvents may be used to clean the current collector or metal plate. Examples of organic solvents include at least one selected from alcohols, ketones, esters, ethers, amides, and sulfoxides. Examples of alcohols include ethanol. Examples of ketones include acetone and ethyl methyl ketone. Examples of esters include ethyl acetate. Examples of ethers include tetrahydrofuran. Examples of amides include dimethylformamide and N-methyl-2-pyrrolidone. Examples of sulfoxides include dimethyl sulfoxide.

To prevent an excessive increase in COD in the electrolyte, it is desirable to use organic solvents that are easily removed by rinsing with water, or organic solvents that are miscible with water, as the organic solvents used to clean the current collector or metal plate. The cleaning time is preferably 3 seconds or more in an organic solvent that is miscible with water. There is no particular upper limit to the cleaning time; for example, it may be 60 seconds or less.

The electrolyte used in the assembly of lead-acid batteries may contain organic additives. Examples of organic additives include surfactants. However, from the viewpoint of keeping the COD (Chemical Oxide) content in the electrolyte of lead-acid batteries low, it is preferable that the electrolyte used in the assembly of lead-acid batteries does not contain organic additives.

The COD of the electrolyte is measured in accordance with JIS K 0102-1:2021, "17.2 Oxygen consumption by acidic potassium permanganate (CODMn)".
The COD (CODMn) is calculated using the following formula. In this calculation, CODMn is calculated with two significant figures, one decimal place, and a lower limit of <0.5.
CODMn=(titration value-BL)×F×1000/V×0.2
Titration value: The volume (mL) of 5 mmol/L potassium permanganate aqueous solution required to titrate the sample prepared from the electrolyte.
Blank (BL): The volume (mL) of 5 mmol/L potassium permanganate aqueous solution required for titration in a test using distilled water.
F: Factor of a 5 mmol/L potassium permanganate aqueous solution. V: Volume (mL) of the sample prepared from the electrolyte (the sample used for titration).
Oxygen equivalent (mg) in 1 mL of 0.2:5 mmol/L potassium permanganate aqueous solution

The titration sample is prepared using the following procedure. First, the electrolyte is collected from a fully charged lead-acid battery into a 300 mL Erlenmeyer flask. The amount of electrolyte collected should be a maximum of 100 mL, adjusted so that the titration volume is between 3.5 mL and 5.5 mL. If the collected amount is less than 100 mL, measure the amount and add distilled water until the diluted volume reaches 100 mL. In this way, the electrolyte sample is prepared. Additionally, 100 mL of distilled water is prepared in a separate 300 mL Erlenmeyer flask as a sample for BL (Body Line). BL measurement is performed each time the sample prepared from the electrolyte is titrated.

Prepare titration samples from 100 mL of electrolyte solution and distilled water for BL using the following procedure. First, add 10 mL of 5 mmol/L potassium permanganate aqueous solution to the above sample using a volumetric pipette and stir. Next, place each Erlenmeyer flask in a boiling water bath and heat for 30 minutes. During this time, ensure that the water in the water bath is always boiling and that the liquid level in the water bath does not fall below the liquid level in the Erlenmeyer flask. After 30 minutes of heating, remove the Erlenmeyer flask and immediately add 10 mL of 12.5 mmol/L sodium oxalate aqueous solution to the liquid in the Erlenmeyer flask using a volumetric pipette. Prepare the titration sample by cooling the liquid until the temperature is within the range of 50°C to 60°C. If the sample contains chloride ions, add 2 mL of 500 g/L silver nitrate aqueous solution using a volumetric pipette and stir the resulting mixture well until no precipitate remains and the liquid becomes clear. If the cloudiness persists, continue adding silver nitrate solution little by little while stirring until the cloudiness disappears. The amount of silver nitrate solution added should be such that the total amount of silver nitrate is 1g more than the equivalent amount of chloride ions contained in each sample. The silver nitrate solution is then added to the sample described above.

Each prepared sample for titration is titrated with a 5 mmol/L potassium permanganate aqueous solution. During titration, when the liquid in the Erlenmeyer flask turns slightly red, stop the titration and allow it to stand for approximately 30 seconds to check if the red color has disappeared. If the red color disappears, repeat the titration and standing process until the red color is no longer present. For both the sample prepared from the electrolyte and the sample prepared from distilled water, the amount of potassium permanganate aqueous solution (mL) required for titration is used as the titration value and BL (Block Line) in the above formula to determine the COD of the electrolyte. If the electrolyte is diluted with distilled water during sample preparation, the COD of the undiluted electrolyte should be calculated taking the dilution into account.

(Porous film)
A porous film is a resin film having pores. A porous film contains a polymer material. A porous film may optionally contain components such as oil, inorganic particles, penetrating agents, and pore-forming agents. The polymer material constituting the resin film (hereinafter also referred to as the base polymer) includes, for example, polyolefins. A polyolefin is a polymer containing at least olefin units (monomer units derived from olefins).

The base polymer typically includes crystalline polymers. Crystalline polymers include, for example, polyolefins. A polyolefin is a polymer containing at least olefin units (i.e., a polymer containing at least monomer units derived from olefins).

Polyolefins may be used in combination with other base polymers as the base polymer. The ratio of polyolefins to the total base polymer in the crystalline polymer is, for example, 50% by mass or more, may be 80% by mass or more, or 90% by mass or more. The ratio of polyolefins is, for example, 100% by mass or less. The base polymer may also be composed solely of polyolefins. While such a high ratio of polyolefins tends to lower the oxidation resistance of the crystalline polymer, even in such cases, high high-temperature overcharge life performance can be ensured by maintaining the degree of crystallinity within the above range.

Porous films contain crystalline regions where the base polymer molecules are relatively regularly arranged (i.e., highly aligned) and amorphous regions with low alignment. Therefore, in the XRD spectrum of a porous film, diffraction peaks from the crystalline regions are observed, as well as scattered light from the amorphous regions, which is observed as a halo. In the XRD spectrum of a porous film, a crystallinity of 18% or more, expressed as 100 × I _c / (I _c + I _a ), results in excellent high-temperature overcharge life performance even with low COD and high charge acceptance.

Here, _Ic is the integrated intensity of the diffraction peak with the maximum peak height among the diffraction peaks corresponding to the crystalline region (the first diffraction peak), and _Ia is the integrated intensity of the halo corresponding to the amorphous region.

For example, in the XRD spectrum of a separator containing a polyolefin with ethylene units, diffraction peaks corresponding to the (110) plane in the crystalline region are observed in the range where 2θ is between 20° and 22.5°, and diffraction peaks corresponding to the (200) plane in the crystalline region are observed in the range where 2θ is between 23° and 24.5°. Furthermore, a halo in the amorphous region is observed in the range where 2θ is between 17° and 27°. Among the diffraction peaks due to the crystalline region, the diffraction peak corresponding to the (110) plane has the largest peak height and corresponds to the first diffraction peak.

The degree of crystallinity should ideally be 18% or higher, but from the perspective of ensuring higher high-temperature overcharge lifespan performance, it may be 23% or higher, or even 25% or higher. The degree of crystallinity may also be 40% or lower, or 37% or lower, or 35% or lower. When the degree of crystallinity is within this range, it is easier to ensure the flexibility of the porous film, and manufacturing is also easier.

The degree of crystallinity of the porous film may be 18% to 40%, 23% to 37%, 25% to 35%, 23% to 40%, or 23% to 35%.

The integrated intensities of the diffraction peak and halo are obtained by fitting the diffraction peak from the crystalline region and the halo from the amorphous region in the XRD spectrum of the separator. Using the obtained integrated intensity _Ic of the first diffraction peak and the integrated intensity _Ia of the halo, the degree of crystallinity can be determined from the above formula.

Polyolefins include, for example, homopolymers of olefins, copolymers containing different olefin units, and copolymers containing olefin units and copolymerizable monomer units. Copolymers containing olefin units and copolymerizable monomer units may contain one or more types of olefin units. Furthermore, copolymers containing olefin units and copolymerizable monomer units may contain one or more types of copolymerizable monomer units. Copolymerizable monomer units are monomer units derived from polymerizable monomers other than olefins that are copolymerizable with olefins.

Examples of polyolefins include polymers containing at least _C2-3 olefins as monomer units. Examples of _C2-3 olefins include at least one selected from the group consisting of ethylene and propylene. More preferably, polyolefins include polyethylene, polypropylene, and copolymers containing _C2-3 olefins as monomer units (e.g., ethylene-propylene copolymers). Among polyolefins, it is preferable to use polyolefins containing at least ethylene units (e.g., polyethylene, ethylene-propylene copolymers). Polyolefins containing ethylene units (e.g., polyethylene, ethylene-propylene copolymers) may be used in combination with other polyolefins.

The porous film preferably contains oil. When the porous film contains oil, the effect of suppressing oxidative degradation of the porous film can be further enhanced, thereby ensuring higher high-temperature overcharge life performance. Oil refers to a hydrophobic substance that is liquid at room temperature (20°C to 35°C) and separates from water. Oils include naturally derived oils, mineral oils, and synthetic oils. Mineral oils and synthetic oils are preferred. Examples of oils include paraffin oil and silicone oil. The porous film may contain one type of oil, or a combination of two or more types.

The oil content in the porous film is preferably 11% by mass or more and 18% by mass or less. When the oil content is within this range, the effect of suppressing oxidative degradation of the porous film is further enhanced. Furthermore, the resistance of the porous film can be kept relatively low.

The porous film may be in sheet form. Alternatively, a sheet folded in an accordion-like manner may be used as the porous film. The porous film may also be formed into a bag shape. Either the positive or negative electrode plate may be wrapped in a bag-shaped porous film.

A porous film may or may not have ribs. A porous film with ribs, for example, comprises a base portion and ribs erected from the surface of the base portion. The ribs may be provided on only one surface of the porous film or each base portion, or on both surfaces. The base portion of a porous film refers to the sheet-like portion that defines the outer shape of the porous film, excluding protrusions such as ribs.

The thickness of the porous film is, for example, 0.09 mm or more. From the viewpoint of obtaining higher high-temperature overcharge life performance, 0.1 mm or more or 0.15 mm or more is preferable. The thickness of the porous film is, for example, 0.3 mm or less. From the viewpoint of keeping the resistance of the porous film low, the thickness of the porous film may be 0.25 mm or less or 0.2 mm or less. The thickness of the porous film refers to the average thickness of the portion of the porous film facing the electrode material. If the porous film comprises a base portion and ribs erected from at least one surface of the base portion, the thickness of the porous film refers to the average thickness of the base portion. If an adhesive member (mat, pasting paper, etc.) is attached to the porous film, the thickness of the adhesive member is not included in the thickness of the porous film.

The thickness of the porous film may be 0.09 mm or more and 0.3 mm or less (or 0.25 mm or less), 0.09 mm or more and 0.2 mm or less, 0.1 mm or more (or 0.15 mm or more) and 0.3 mm or less, 0.1 mm or more (or 0.15 mm or more) and 0.25 mm or less, or 0.1 mm or more (or 0.15 mm or more) and 0.2 mm or less.

If the porous film has ribs, the rib height may be 0.05 mm or more. Alternatively, the rib height may be 1.2 mm or less. The rib height is the height of the portion protruding from the surface of the base (projection height).

The height of the ribs provided in the region of the porous film facing the positive electrode plate may be 0.4 mm or more. The height of the ribs provided in the region of the porous film facing the positive electrode plate may be 1.2 mm or less.

Porous films are obtained, for example, by extruding a resin composition containing a base polymer, a pore-forming agent, and a penetrating agent (surfactant) into a sheet, stretching it, and then removing at least a portion of the pore-forming agent. Removing at least a portion of the pore-forming agent forms micropores in the matrix of the base polymer. After the removal of the pore-forming agent, the sheet-like porous film is dried as needed. For example, the degree of crystallinity is adjusted by adjusting at least one selected from the group consisting of the cooling rate of the sheet during extrusion, the stretching ratio during stretching, and the temperature during drying. For example, rapidly cooling the sheet during extrusion, increasing the stretching ratio, or lowering the drying temperature tends to increase the degree of crystallinity. Stretching may be performed by biaxial stretching, but is usually performed by uniaxial stretching. The sheet-like porous film may be folded into an accordion shape or processed into a bag shape as needed.

In porous films with ribs, the ribs may be formed on the sheet during the extrusion molding of the resin composition. Alternatively, the ribs may be formed after the resin composition has been formed into a sheet or after the porosizing agent has been removed, by pressing the sheet with a roller having grooves corresponding to each rib.

Examples of pore-forming agents include liquid pore-forming agents and solid pore-forming agents. Preferably, the pore-forming agent contains at least oil. Using oil allows for the production of a porous film containing oil, further enhancing the effect of suppressing oxidative degradation. The pore-forming agent may be used alone or in combination of two or more types. Oil may be used in combination with other pore-forming agents. Liquid pore-forming agents and solid pore-forming agents may also be used in combination. Note that at room temperature (20°C to 35°C), liquid pore-forming agents are classified as liquid pore-forming agents, and solid pore-forming agents are classified as solid pore-forming agents.

As a liquid pore-forming agent, the oils mentioned above are preferred. As a solid pore-forming agent, polymer powders are examples.

The amount of pore-forming agent in a porous film may vary depending on the type. The amount of pore-forming agent in a porous film is, for example, 30 parts by mass or more per 100 parts by mass of the base polymer. Alternatively, the amount of pore-forming agent may be, for example, 60 parts by mass or less per 100 parts by mass of the base polymer.

For example, a porous film containing oil is formed by extracting and removing some of the oil from a sheet formed using oil as a pore-forming agent, using a solvent. The solvent is selected, for example, depending on the type of oil. The oil content in the porous film can be adjusted by controlling, for example, the type and composition of the solvent, and the extraction conditions (extraction time, extraction temperature, solvent supply rate, etc.).

The surfactant used as a penetrating agent may be either an ionic surfactant or a nonionic surfactant. The surfactant may be used alone or in combination of two or more types.

The content of the penetrant in the porous film is, for example, 0.01% by mass or more, and may be 0.1% by mass or more. The content of the penetrant in the porous film may be 10% by mass or less. From the viewpoint of suppressing COD in the electrolyte and improving charge acceptance, the content of the penetrant in the porous film may be 3% by mass or less, or 1% by mass or less, preferably 0.5% by mass or less, and more preferably 0.2% by mass or less.

Porous films (or resin compositions used in the manufacture of porous films) may contain inorganic particles.

As inorganic particles, ceramic particles are preferred, for example. Examples of ceramics constituting the ceramic particles include at least one selected from the group consisting of silica, alumina, and titania.

The inorganic particle content in the porous film may be, for example, 40% by mass or more. Alternatively, the inorganic particle content may be, for example, 80% by mass or less, or 70% by mass or less.

The evaluation and measurement methods are described below.
(1) Analysis or size measurement of porous film (Preparation of porous film)
For the analysis or size measurement of porous films, porous films extracted from unused separators or from fully charged lead-acid batteries in their initial stages of use are used. The porous films extracted from lead-acid batteries are washed and dried prior to analysis or measurement.

The cleaning and drying of the porous film removed from the lead-acid battery is performed using the following procedure: The porous film is immersed in pure water for one hour to remove sulfuric acid from the film. Then, the porous film is removed from the liquid and allowed to stand at 25°C ± 5°C for at least 16 hours to dry.

(XRD spectrum)
The XRD spectrum of a porous film is measured by irradiating the porous film surface with X-rays perpendicular to it. A sample for measurement is prepared by processing the portion of the porous film facing the electrode material into a strip. For porous films with ribs, the sample is prepared by processing the base portion into a strip, excluding the ribs. XRD spectrum measurement and fitting are performed under the following conditions.

(Measurement conditions)
Measurement device: RINT-TTR2, manufactured by Rigaku Corporation. Fitting: FT (step scan) method. Measurement angle range: 15-35°
Step width: 0.02°
Measurement speed: 5°/min
XRD data processing: XRD pattern analysis software (PDXL2, manufactured by Rigaku) was used.

(Thickness of the porous film and height of the ribs)
The thickness of a porous film can be determined by measuring the thickness at five arbitrarily selected locations in a cross-sectional photograph of the porous film and averaging the results.

The rib height is determined by averaging the heights from one surface of the rib's base, measured at 10 arbitrarily selected locations on the rib in a cross-sectional photograph of the porous film.

(Oil content in porous film)
A sample (hereinafter referred to as Sample A) is prepared by processing the portion of the porous film facing the electrode material into a strip shape. In the case of a porous film with ribs, Sample A is prepared by processing the base portion into a strip shape so as not to include the ribs.

Approximately 0.5 g of sample A is taken, accurately weighed, and the initial sample mass (m0) is determined. The weighed sample A is placed in a glass beaker of appropriate size, and 50 mL of n-hexane is added. Next, the beaker and sample are subjected to ultrasound for approximately 30 minutes to dissolve the oil contained in sample A into the n-hexane. Then, the sample is removed from the n-hexane, dried in the air at room temperature (temperature between 20°C and 35°C), and weighed to determine the mass of the sample after oil removal (m1). The oil content is then calculated using the following formula. The oil content is determined for 10 samples of sample A, and the average value is calculated. The resulting average value is taken as the oil content in the separator.
Oil content (mass %) = (m0 - m1) / m0 × 100

(Content of inorganic particles in porous film)
A portion of sample A, prepared in the same manner as described above, is taken, accurately weighed, placed in a platinum crucible, and heated with a Bunsen burner until no more white smoke is emitted. Next, the resulting sample is heated in an electric furnace (in an oxygen stream, 550°C ± 10°C) for approximately one hour to ash, and the ashed material is weighed. The ratio (percentage) of the mass of the ashed material to the mass of sample A is calculated and used as the inorganic particle content (mass %). The inorganic particle content is determined for 10 samples of sample A, and the average value is calculated. The resulting average value is used as the inorganic particle content in the porous film.

(Percentage of penetrant in porous film)
A portion of sample A, prepared in the same manner as described above, is taken, accurately weighed, and dried at room temperature (20°C to 35°C) under reduced pressure below atmospheric pressure for at least 12 hours. The dried material is placed in a platinum cell and set in a thermogravimetric analyzer, and the temperature is raised from room temperature to 800°C ± 1°C at a heating rate of 10 K/min. The weight loss when the temperature is raised from room temperature to 250°C ± 1°C is taken as the mass of the penetrant, and the ratio (percentage) of the mass of the penetrant to the mass of sample A is calculated and taken as the penetrant content (mass %). A Q5000IR manufactured by T. A. Instruments Corporation is used as the thermogravimetric analyzer. The penetrant content is determined for 10 samples of sample A, and the average value is calculated. The obtained average value is taken as the penetrant content in the porous film.

(2) Evaluation of charge acceptance The charge acceptance of the lead-acid battery shall be evaluated in accordance with JIS D5301:2019, by performing discharge and charging under the following conditions, and based on the charging current I _ca2 at 10 minutes after the start of charging.
1) After confirming that the electrolyte temperature of one cell near the center of the lead-acid battery is 25±2℃, discharge it for 2.5 hours at a current 3.42 times the 20-hour rate current _I20 .
2) Immediately after the discharge described in 1) above, move the lead-acid battery to a cooling chamber at 0±1°C and leave it there until the electrolyte temperature of one of the cells near the center reaches 0±1°C.
3) After confirming that the electrolyte temperature of one of the cells near the center is 0±2℃, the lead-acid battery is charged at the same temperature with a constant voltage of 2.4±0.015V/cell and a limiting current of 100A, and the charging current I _ca2 is measured 10 minutes after the start of charging.

(3) Light Load Life Test The light load life is evaluated in accordance with JIS D5301:2019, based on the number of cycles until the life is reached in the following charge-discharge cycle test. However, for convenience, the ambient temperature of the test is changed from 40°C liquid phase to 75°C gas phase, and the time of 25A discharge in the charge-discharge cycle is changed from 4 minutes to 2 minutes.
(a) Discharge the test battery at a discharge current of 25A ± 0.1A for 2 minutes in a 75°C environment.
(b) Then, charge at a charging voltage of 14.80V (limiting current of 25.0A) for 10 minutes.
(c) Repeat (a) and (b) above for 480 cycles, with each cycle counting as one cycle. After every 480 cycles, allow for a 40 to 60-hour rest period, then discharge for 30 seconds at the rated cold cranking current I _cc . The number of cycles at which the voltage drops to 7.2V after 30 seconds is defined as the life cycle. Here, the cold cranking current I _cc is the current value corresponding to the performance rank specified in JIS D 5301:2019.

Since discharge at the rated cold cranking current I _cc is performed every 480 cycles, the number of cycles at which the voltage at 30 seconds reaches 7.2V can be determined by linear interpolation from a graph with the vertical axis representing the voltage at 30 seconds during the I _cc discharge and the horizontal axis representing the number of cycles, and this can be used as the life cycle count.

The matters described herein can be combined in any way.

Figure 1 shows the external appearance of an example of a lead-acid battery according to an embodiment of the present invention.
The lead-acid battery 1 comprises a battery case 12 that houses an electrode plate group 11 and an electrolyte (not shown). The battery case 12 is divided into a number of cell chambers 14 by a partition wall 13. Each cell chamber 14 houses one electrode plate group 11. The opening of the battery case 12 is closed with a lid 15 equipped with a negative electrode terminal 16 and a positive electrode terminal 17. The lid 15 is provided with a vent plug 18 for each cell chamber. When replenishing with water, the vent plug 18 is removed and the water is supplied. The vent plug 18 may also have a function of discharging gas generated in the cell chamber 14 to the outside of the battery.

Each electrode plate group 11 is constructed by stacking multiple negative electrode plates 2 and positive electrode plates 3 with separators 4 in between. In the cell chamber 14 located at one end of the battery case 12, a negative electrode shelf 6, which connects multiple negative electrode plates 2 in parallel, is connected to a through-connector 8, and a positive electrode shelf 5, which connects multiple positive electrode plates 3 in parallel, is connected to a positive electrode column 7. The positive electrode column 7 is connected to a positive electrode terminal 17 on the outside of the lid 15. In the cell chamber 14 located at the other end of the battery case 12, a negative electrode column 9 is connected to the negative electrode shelf 6, and a through-connector 8 is connected to the positive electrode shelf 5. The negative electrode column 9 is connected to a negative electrode terminal 16 on the outside of the lid 15. Each through-connector 8 passes through a through-hole in the partition wall 13, connecting the electrode plate groups 11 of adjacent cell chambers 14 in series.

[Examples]
The present invention will be described in detail below based on examples and reference examples, but the present invention is not limited to the following examples.

《Lead-acid batteries R1 to R10》
Each lead-acid battery was manufactured using the following procedure.
(1) Preparation of separators A resin composition containing polyethylene, silica particles, paraffin-based oil as a pore-forming agent, and a penetrating agent was extruded into a sheet, stretched, and then a portion of the pore-forming agent was removed to produce a porous film having ribs on one side. The cooling rate of the extruded sheet and the stretching ratio were adjusted so that the degree of crystallinity (%) (100 × I _c / (I _c + I _a )) of the porous film obtained by the procedure described above was the value shown in Table 1 (13%).

The oil content of the porous film obtained by the procedure described above was 11-18% by mass, and the silica particle content was 60% by mass.
The rib height obtained using the previously described procedure was 0.55 mm.
The thickness of the porous film (base thickness) obtained using the previously described procedure was standardized to 0.2 mm.

Next, a sheet of porous film was folded in half so that ribs were positioned on the inner surface to form a bag. The overlapping ends were then compressed to obtain a bag-shaped porous film (size when laid flat: 117 mm long x 152 mm wide). The compressed section was 3 mm wide and located 2 mm inward from the side edge of the porous film.

Furthermore, the crystallinity, oil content, silica particle content, base thickness, and rib height of the porous film were determined from the porous film before the lead-acid battery was fabricated. These values were almost identical to those measured using the previously described procedure for porous film removed from the fabricated lead-acid battery.

(2) Preparation of positive electrode plate A positive electrode paste was prepared by mixing lead oxide, reinforcing material (synthetic resin fiber), water, and sulfuric acid. The positive electrode paste was filled into the mesh of an expanded grid made of Pb-Ca-Sn alloy that did not contain antimony, and by aging and drying, an unformed positive electrode plate with a width of 137 mm, a height of 110 mm, and a thickness of 1.6 mm was obtained.

(3) Preparation of the negative electrode plate A negative electrode paste was prepared by mixing lead oxide, carbon black, barium sulfate, lignin, reinforcing material (synthetic resin fiber), water, and sulfuric acid. The negative electrode paste was filled into the mesh of an expanded grid made of Pb-Ca-Sn alloy that did not contain antimony, and aged and dried to obtain an unformed negative electrode plate with a width of 137 mm, a height of 110 mm, and a thickness of 1.3 mm. The amounts of carbon black, barium sulfate, lignin, and synthetic resin fiber used were adjusted so that the content of each component in the negative electrode plate taken from a fully charged lead-acid battery was 0.3 mass%, 2.1 mass%, 0.1 mass%, and 0.1 mass%, respectively.

(4) Fabrication of lead-acid battery Untreated negative electrode plates were placed in a porous film bag-shaped separator. The negative electrode plates and positive electrode plates were stacked with the separator in between. In this way, an electrode plate group was formed with seven untreated negative electrode plates and six untreated positive electrode plates.

The tabs of the positive and negative electrode plates were welded to the positive and negative electrode trays, respectively, using a cast-on-strap method. The electrode plates were inserted into a polypropylene battery case, electrolyte was added, and a chemical conversion process was performed within the case to assemble a liquid-type lead-acid battery with a rated voltage of 12V and a 5-hour rate capacity of 30Ah. The 5-hour rate capacity refers to the capacity when discharged at a current (A) equal to 1/5 of the Ah value indicated in the rated capacity. Six electrode plate groups are connected in series within the battery case.

A sulfuric acid aqueous solution was used as the electrolyte. The density of the electrolyte after chemical conversion at 20°C was 1.285. The COD of the electrolyte extracted from a fully charged lead-acid battery was adjusted to the values shown in Table 1 (based on mass). The COD was controlled by changing the amount of penetrant used when preparing the porous film.

《Lead acid battery R11, E1 to E9》
A lead-acid battery was fabricated in the same manner as in Comparative Examples 1 to 10, except that a porous film was used that was adjusted so that the degree of crystallinity (%) obtained by the procedure described above was the value shown in Table 2 (18%).

《Lead-acid batteries E10 to E16》
A lead-acid battery was fabricated in the same manner as in Comparative Examples 4 to 10, except that a porous film was used that was adjusted so that the degree of crystallinity (%) obtained by the procedure described above was the value shown in Table 2 (23%).

《Lead-acid batteries E17-E23》
A lead-acid battery was fabricated in the same manner as in Comparative Examples 4 to 10, except that a porous film was used that was adjusted so that the degree of crystallinity (%) obtained by the procedure described above was the value shown in Table 2 (25%).

(5) Evaluation The charge acceptance and life performance of the lead-acid battery in the light load life test at 75°C were evaluated using the procedure described above. Charge acceptance was evaluated as a relative value with the evaluation result of lead-acid battery R1 set to 100, and life performance in the light load life test was evaluated as a relative value with the life cycle of lead-acid battery R1 set to 100.

The evaluation results are shown in Tables 1-4. E1-E23 are examples. R1-R11 are comparative examples.

Comparing batteries R1 and R11, when the COD is high at 170 mg/L, increasing the crystallinity from 13% to 18% improves the light-load life performance from 100% to 140%, a 40% improvement. In contrast, comparing batteries R10 and E9, when the COD is low at 15 mg/L, increasing the crystallinity from 13% to 18% improves the light-load life performance from 40% to 104%, a 64% improvement, and a 160% improvement compared to the 13% crystallinity baseline. In other words, the lower the COD, the greater the improvement in light-load life performance from increasing the crystallinity. Furthermore, battery E9 achieves a life performance of 104%, exceeding battery R1's 100%. The same can be said when comparing Table 1 with Tables 3 and 4.

Furthermore, it can be understood that if the COD is 80 mg/L or higher when the crystallinity is 23% or higher, and 50 mg/L or higher when the crystallinity is 25% or higher, even better lifespan performance can be stably obtained. This is presumed to be because the oxidative degradation of the porous film caused by the reduction of COD is suppressed by increasing the crystallinity of the porous film, thereby suppressing internal short circuits.

The lead-acid battery described in this disclosure is suitable for applications such as idle stop systems (e.g., lead-acid batteries for vehicles with idle stop systems) and as a starting power source for various vehicles (trucks, commercial vehicles such as taxis, motorcycles, etc.). Furthermore, the lead-acid battery is also suitable for use as a power source for industrial energy storage devices such as those for electric vehicles (e.g., forklifts). These applications are merely examples. The applications of the lead-acid battery described in this disclosure are not limited to these. Idling reduction (also known as IS) is sometimes referred to as IS.

1: Lead-acid battery 2: Negative electrode plate 3: Positive electrode plate 4: Separator 5: Positive electrode shelf 6: Negative electrode shelf 7: Positive electrode column 8: Through connector 9: Negative electrode column 11: Electrode plate group 12: Battery case 13: Partition wall 14: Cell chamber 15: Cover 16: Negative electrode terminal 17: Positive electrode terminal 18: Electrode cap

Claims (5)

Positive plate and
The negative electrode plate and
A porous resin film interposed between the positive electrode plate and the negative electrode plate,
Electrolyte and
Equipped with,
The porous film includes a crystalline region and an amorphous region.
In the X-ray diffraction spectrum of the porous film, the degree of crystallinity, expressed as 100 × I _c / (I _c + I _a ), is 18% or more.
_Ic is the integrated intensity of the diffraction peak with the maximum peak height among the diffraction peaks corresponding to the crystalline region.
I _a is the integrated intensity of the halo corresponding to the amorphous region,
A lead-acid battery in which the chemical oxygen demand in the electrolyte is 160 mg/L or less. The lead-acid battery according to claim 1, wherein the chemical oxygen demand is 100 mg/L or less. The lead-acid battery according to claim 1, wherein the chemical oxygen demand is 15 mg/L or more. The lead-acid battery according to claim 1, wherein the degree of crystallinity is 40% or less. The lead-acid battery according to claim 1, wherein the porous film has a thickness of 0.1 mm or more and 0.3 mm or less.