JP5473442B2 - Battery separator and secondary battery - Google Patents

Description

  The present invention relates to a battery separator and a secondary battery suitable for a secondary battery, for example, a battery separator and an alkaline secondary battery optimal for an alkaline secondary battery.

  Alkaline secondary batteries such as nickel metal hydride secondary batteries have excellent charge / discharge characteristics and overcharge / discharge characteristics, and can be used repeatedly with a long life. In addition, since it has low internal resistance and excellent large current characteristics, it is expected to be used for batteries such as electric vehicles and electric tools.

  The separator used in these batteries is required to have a lyophilic property and an ability to retain an electrolyte, and a mechanical strength sufficient to prevent a short circuit between the positive electrode and the negative electrode due to dendrites or the like. The mechanical strength is also an important characteristic from the viewpoint of improving the handling properties in the process.

  Conventionally, as a separator for this type of secondary battery, a highly lyophilic polyamide nonwoven fabric has been used. However, the polyamide-based nonwoven fabric has a separator effect in which the separator is gradually decomposed in the alkaline electrolyte, and the ammonia generated during the decomposition is oxidized to nitrate ions on the positive electrode and reduced on the negative electrode to return to ammonia. It has been found that there are problems such as increased self-discharge (Non-Patent Document 1).

  Therefore, instead of polyamide nonwoven fabrics, polyolefin nonwoven fabrics with excellent chemical stability have come to be used as separators. However, polyolefin resins are inferior in lyophilicity, so various lyophilic treatments such as the following: Has been proposed.

(1) Surfactant treatment (2) Corona discharge treatment, plasma treatment, UV ozone treatment (3) Fluorine gas treatment (4) Acrylic acid graft polymerization treatment (5) Sulfonation treatment

The surfactant treatment (1) is a relatively easy method of applying a surfactant to the separator. However, as the battery is repeatedly charged and discharged, the surfactant dissolves in the electrolyte and the separator itself is the parent. Loss of liquidity and the electrolyte holding capacity rapidly decreases.

  The (2) corona discharge treatment, plasma treatment, and UV ozone treatment are inexpensive treatment methods in which polar groups such as carboxyl groups are introduced into the resin surface by radicals generated by the respective methods, but only on the nonwoven fabric surface. It cannot be lyophilic and its lyophilic persistence is poor.

  In the fluorine gas treatment (3), polar groups are introduced into the resin using fluorine gas. In this treatment, since the lyophilic layer is formed up to the inside of the resin, the lyophilic durability is good, but the self-discharge suppressing effect described below is inferior to the sulfonation treatment.

  In the acrylic acid graft polymerization treatment (4), acrylic acid is graft polymerized with fibers to impart lyophilic properties. The lyophilic property is higher than that of the sulfonation treatment, which is inferior to the fluorine gas treatment, but the lyophilic property is high.

In the sulfonation treatment of (5), a sulfonic acid group is introduced into the fiber by concentrated sulfuric acid, chlorosulfuric acid, fuming sulfuric acid, SO ₃ gas, etc., to impart lyophilicity. However, in general, lyophilicity is not so high.

  In particular, it has been found that (4) acrylic acid graft polymerization treatment and (5) sulfonation treatment not only provide lyophilic properties but also exert an effect of suppressing self-discharge of the battery. This is considered to be because the separator adsorbs ammonia which is a causative substance of the shuttle effect. For this reason, among these treatments, the sulfonation treatment that is excellent in the balance between the lyophilic durability and the self-discharge suppression effect is mainly used.

Yoshiharu Matsuda, Zenichiro Takehara, Battery Manual (Heisei 13), p237-238

Japanese Patent No. 29987920 Japanese Patent No. 3561302 JP 11-297293 A Japanese Patent No. 2762443

However, the sulfonation treatment has a side effect that the carbonization of the separator proceeds with the introduction of the sulfonic acid group, and there is a problem that the mechanical strength decreases when the degree of treatment is increased to improve lyophilicity.
As a means for solving the mechanical strength reduction accompanying the sulfonation treatment, several separators that combine a material that is not easily sulfonated and a material that is easily sulfonated have been proposed.

  By keeping the sulfonation treatment conditions mild and mixing materials that are easily sulfonated and highly lyophilic with materials that are not easily sulfonated and retain strength, both lyophilicity and strength can be achieved. want to be.

  Here, a polyolefin resin typified by polypropylene and polyethylene is a resin that is crystalline and is not easily sulfonated. In general, when polypropylene and polyethylene are compared, polyethylene is more easily sulfonated. In addition, examples of materials that are easily sulfonated include polystyrene, polyvinyl naphthalene, and polyether sulfone.

  For example, Patent Document 1 discloses a separator in which a polyolefin resin and a polystyrene resin are mixed at a resin level and then made into a fiber and sulfonated with concentrated sulfuric acid at a low temperature.

  However, since the fiber is not a single component of polyolefin but a mixed component, incompatible resin is deposited inside the fiber and the fiber becomes non-uniform, so that the strength of the fiber itself before sulfonation is insufficient. There was a trend.

  Moreover, since only the polystyrene resin exposed on the surface is sulfonated, the exposure of the polystyrene resin to the fiber surface is not sufficient, so the efficiency of sulfonation tends to be low, and sufficient lyophilicity is obtained. Was not done.

  In addition, the separator described in Patent Document 2 was sulfonated into a nonwoven fabric using a polyolefin resin and polystyrene split fibers. In the separator described in Patent Document 3, a nonwoven fabric obtained by mixing continuous fibers of polystyrene, polyvinyl naphthalene or polyethersulfone and polyolefin fibers is sulfonated.

  However, these separators contain a mixture of materials that are easily sulfonated and materials that are not easily sulfonated on the fiber surface. Therefore, when the degree of sulfonation is small, the material that is not easily sulfonated impairs lyophilicity. When the degree of sulfonation is large, a material that is easily sulfonated promotes deterioration in strength, and it has not been possible to achieve both strength and lyophilicity.

  Patent Document 4 proposes a separator sulfonated into a non-woven fabric produced by applying a polyethylene dispersion to a polypropylene fiber to impart thermal binding properties. In addition, there is a separator sulfonated into a wet nonwoven fabric using core and sheath fibers of polypropylene and polyethylene that are generally distributed. These non-woven fabrics are highly lyophilic because most of the surface is made of polyethylene, but because the majority of the fiber entanglement points are formed by thermal fusion of polyethylene, it is difficult to achieve high strength in terms of material. It was a thing.

  This invention is made | formed in view of said subject, Comprising: It aims at providing the battery separator using the battery separator which makes high lyophilic property and high mechanical strength compatible, and this battery separator.

  As a means for solving the above-described problem, an embodiment of the present invention according to the present invention has the following configuration. That is, a battery separator that is used by being sandwiched between a positive electrode and a negative electrode of a battery, wherein a polypropylene resin is a main constituent material, and a polyethylene-based surface is formed by binding the polypropylene resins together. The battery separator is characterized in that a resin surface is formed, and then a treatment for introducing a carboxyl group is applied to the fiber surface of the nonwoven fabric, followed by a sulfonation treatment.

  Alternatively, any one of the battery separators described above is used between the positive electrode and the negative electrode. For example, the battery is a nickel metal hydride storage battery.

  According to the present invention, it is possible to provide a battery separator that achieves both high hydrophilicity and high strength and a battery using the battery separator.

It is a figure for demonstrating the manufacturing process of the embodiment battery separator of one invention concerning this invention.

  Hereinafter, an embodiment of an invention according to the present invention will be described in detail. The present embodiment is a battery separator that is used by being sandwiched between a positive electrode and a negative electrode, and has a polypropylene resin as a main constituent material, and a non-woven fabric formed by binding the polypropylene resins together. The battery separator is characterized in that a polyethylene-based resin surface is formed, and then a non-woven fiber surface is subjected to a treatment for introducing a carboxyl group and further subjected to a sulfonation treatment.

  With reference to FIG. 1, the outline of the manufacturing process of the separator of the present embodiment will be described below. FIG. 1 is a process diagram for explaining an outline of a method for manufacturing a battery separator according to an embodiment of the present invention.

  First, in step S1, a sheet is formed by an arbitrary method to form a base fabric. As for the base fabric, a polypropylene resin is used as the main constituent material, and most of the entanglement points only need to be formed by binding of the polypropylene resins. Specifically, a spunbonded nonwoven fabric composed of stretched continuous fibers is suitable for obtaining high mechanical properties.

  As the polypropylene resin, as long as the basic skeleton is made of polypropylene, any shape and size may be used as long as they do not hinder the basic performance of the battery.

  However, a structure having a nitrogen-containing functional group or bond such as amine is not preferable because it causes a shuttle effect and increases self-discharge like the above-mentioned polyamide-based nonwoven fabric.

  Next, in step S2, a polyethylene resin layer is formed on the surface of the base fabric by applying a polyethylene resin emulsion.

  The polyethylene resin is not particularly limited as long as the polyethylene resin layer can be formed on the surface of the base fabric made of polypropylene resin. Can be used. That is, the molecular structure of the polyethylene resin may have a functional group such as an ester group or a phenyl group, or a double bond. However, it is not preferable to have a functional group or bond containing nitrogen, such as amine, because the shuttle effect described above is caused.

  As a dispersion medium for the polyethylene resin emulsion, it is preferable to use water in view of safety in handling, availability, and stability during storage. However, as described above, when nitrogen is contained, the shuttle effect is promoted, so that the surfactant or the dispersion medium containing nitrogen is not preferable.

  As a means for applying the polyethylene resin emulsion, a known means such as a method of immersing the base fabric in the polyethylene resin emulsion or dip impregnation may be used. For example, spraying a polyethylene resin emulsion on the base fabric is also effective as a means.

  As the adhesion amount of the polyethylene resin to the base fabric made of polypropylene resin, the polyethylene resin is preferably 0.1 to 10% by weight with respect to the basis weight of the base fabric.

  A more preferable range is 1 to 5% by weight, and an optimum adhesion amount is 3% by weight. If the amount is less than 0.1% by weight, the desired effect cannot be obtained. If the amount is more than 10% by weight, the gap between the fibers constituting the base fabric is filled, and the airtightness of the separator is increased, so that the battery There is concern that the internal resistance becomes too high when incorporated.

  Thereafter, surface treatment is performed in step S3, and carboxyl groups are introduced into the surface of the polyethylene resin layer. As the surface treatment, for example, any one of corona discharge, plasma discharge treatment, UV ozone treatment, or a combination of these treatments may be performed. By performing the surface treatment by any method, it is possible to improve the degree of sulfonation in the subsequent step S4.

  The purpose of introducing carboxyl groups into a base fabric made of polypropylene resin is to make the hydrophobic resin surface hydrophilic by introducing carboxyl groups into the hydrophobic resin surface because the resin surface of the polypropylene resin is hydrophobic. This is to improve the efficiency of the sulfonation treatment in the subsequent step. The treatment method for introducing a carboxyl group may be any known means such as corona discharge treatment, plasma treatment, UV ozone treatment, and is not particularly limited.

  In this way, by further increasing the sulfonation treatment efficiency of the polyethylene resin on the surface layer, it becomes possible to obtain sufficient lyophilicity under extremely mild processing conditions, and the deterioration of mechanical strength can be minimized. .

Thereafter, in step S4, the base fabric is subjected to sulfonation treatment. As a treatment method for the sulfonation treatment, any treatment method can be suitably used as long as it is a known treatment method such as a method using hot concentrated sulfuric acid, fuming sulfuric acid, or SO ₃ gas.
In this embodiment, since the polyethylene-based resin coating layer is uniformly formed on the entire surface of the base fabric in step S2, the polyethylene-based resin coating layer can effectively prevent deterioration of the base fabric due to sulfonation. it can.

  In this embodiment, sulfonation is performed under milder conditions in order to further suppress deterioration of the base fabric. This is because the surface treatment is performed in step S3. This is because it is easy to sulphonate even under milder conditions.

  In this way, a non-woven fabric in which polypropylene resins having high mechanical strength are bonded together is used as a base fabric, and a sulfonation treatment is performed under extremely mild conditions by forming a polyethylene resin coating layer. A separator having high mechanical strength can be realized by preventing a decrease in strength due to crystallization and leaving the strength of the base fabric as it is.

In addition, by forming a polyethylene-based resin layer that is easily sulfonated on the entire surface of the nonwoven fabric, it becomes possible to impart high lyophilicity to the entire surface of the nonwoven fabric by sulfonation treatment, realizing a highly lyophilic separator. can do.
That is, it is possible to obtain a battery separator that achieves both high mechanical strength and high lyophilicity.

Moreover, the following effects can be expected by using this separator in a battery.
(A) Improvement of cycle life Since the lyophilicity to the electrolytic solution is high, improvement of the cycle life can be expected.
(B) Reduction of defective rate in battery manufacturing process Since the mechanical strength is high, line suitability and burr resistance are high, and the defective rate in the battery manufacturing process can be reduced.
Next, an embodiment according to the present invention will be described together with a comparative example.

PE emulsion liquid (Chemical M200, manufactured by Mitsui Chemicals ) with a coating weight of 0.1% by weight on a polypropylene spunbond nonwoven fabric (hereinafter referred to as “PP spunbond nonwoven fabric”) having a basis weight of 53 g and a thickness of 125 μm . Was coated by a dip method, dried and fixed at 125 ° C., and coated with polyethylene (hereinafter referred to as “PE”) .
Next, a corona discharge treatment was performed at a treatment density of 220 kW / mm / min. Thereafter, a sulfonation treatment is performed by reacting with nitrogen gas containing 10 vol% SO ₃ gas at 25 ° C. for 2 minutes, immersing in about 10 wt% NaOH aqueous solution for 5 minutes, washing with water, drying at 70 ° C. and sulfone. A separator was prepared.

  A sulfonated separator was obtained in the same manner as in Example 1, except that the coating rate of the PE emulsion liquid was changed to 1% by weight.

  A sulfonated separator was obtained in the same manner as in Example 1 except that the coating rate of the PE emulsion liquid was changed to 3% by weight.

  A sulfonated separator was obtained in the same manner as in Example 1 except that the coating rate of the PE emulsion liquid was changed to 5% by weight.

  A sulfonated separator was obtained in the same manner as in Example 1, except that the coating rate of the PE emulsion liquid was changed to 10% by weight.

  A sulfonated separator was obtained in the same manner as in Example 1 except that the PE emulsion liquid was “Maycatex HP-70” (manufactured by Meisei Chemical Co., Ltd.) and the coating rate was changed to 3% by weight.

The coating rate of the PE emulsion was 3 wt%, the ultraviolet intensity of about 15mV / cm ² instead of the corona discharge treatment, except that UV ozone treatment for 3 minutes was performed under conditions of an ozone concentration of about 300 pp m is Example 1 A sulfonated separator was obtained in the same manner as described above.

Comparative Example 1

Basis weight of 53 g, the PP made spunbonded nonwoven fabric having a thickness of 125 [mu] m, was subjected to corona discharge treatment in the process density 220k W · m / min. Then, sulfonation is performed by reacting with nitrogen gas containing 10 vol% SO ₃ gas at 25 ° C. for 2 minutes, immersing in about 10 wt% NaOH aqueous solution for 5 minutes, washing with water, drying at 70 ° C. and sulfonation. A separator was produced.

Comparative Example 2

Using a polypropylene / polyethylene core-sheath fiber having a fiber diameter of 11 μm and a fiber length of 5 mm, a nonwoven fabric having a basis weight of 53 g and a thickness of 125 μm was obtained by a wet method. It was subjected to corona discharge treatment in this process the density 220k W · m / min. After that, sulfonation is performed by reacting with nitrogen gas containing 10 vol% SO ₃ gas at 5 ° C. for 2 minutes, soaking in about 10 wt% NaOH aqueous solution for 5 minutes, washing with water, drying at 70 ° C. and sulfonation. A separator was produced.

Comparative Example 3

  A PE emulsion liquid was applied to a spunbond nonwoven fabric made of PP having a basis weight of 53 g and a thickness of 125 μm by a dip method so that the coating rate was 20% by weight, and dried and fixed at 125 ° C. to coat PE.

  In order to compare the strength of the separator and the degree of strength deterioration due to the sulfonation treatment, the tensile strength of the separator was measured before and after the sulfonation and the strength maintenance ratio was calculated. The tensile test was performed using a sample having a width of 15 mm at a grip interval of 180 mm and a tensile speed of 200 mm / min.

Furthermore, from the obtained data, the elongation of the separator and the strength retention before and after sulfonation were calculated using the following formula.
Difficult to stretch (kgf /%) = {Tensile strength (kgf) / Elongation (%)}
Strength retention ratio (%) = {strength after sulfonation (kgf) / strength before sulfonation (kgf)} × 100

  Further, in order to compare the degree of hydrophilization by the sulfonation treatment, a 30 mm square separator was floated on a 30% KOH aqueous solution at 70 ° C., and the immersion time until the separator was completely wetted with the electrolyte was measured. The higher the affinity with the electrolytic solution, the shorter the time taken for immersion.

  In order to evaluate the gas permeability of the separator, the gas density was measured. The air density is JIS P 8117 (paper and paper board air permeability test method) with a 6 mmφ adapter attached to the lower test piece of the B-type measuring instrument, and the separator paper is pressed down to 100 ml. It was measured by the time (minute / 100 ml) required for the air to pass through.

Table 1 shows a list of the results. Table 1 is a table for explaining the strength after sulfonation, the maintenance ratio and the immersion time.

  From Table 1, the PE coated separators of Examples 1 to 5 are less affected by the sulfonation treatment than the uncoated PP spunbonded nonwoven fabric of Comparative Example 1 by PE coating. It was. Furthermore, since the base fabric made of PP was used, the strength was higher than that of the separator made of wet nonwoven fabric of Comparative Example 2.

  On the other hand, also in terms of hydrophilicity, the separator coated with PE was more compatible with the electrolyte than the PP spunbonded nonwoven fabric of Comparative Example 1. In particular, the sample of Example 5 in which the coating rate of the PE emulsion was 10% showed almost the same hydrophilicity as the wet nonwoven fabric separator of Comparative Example 2.

  However, when the coating rate of the PE emulsion is increased, the PE emulsion fills the gaps between the fibers, and the air density tends to increase. In particular, in the sample in which the coating rate of the PE emulsion of Comparative Example 3 was 20%, the air density rapidly increased, and the sample could not be used as a separator.

  In addition, the immersion time also tended to increase when the coating rate exceeded 10% because the capillary phenomenon did not work well. From the balance between the immersion time and the air density, it was found that the sample having a coating rate of 3% in Example 3 was suitable as a battery separator.

Compared with the base fabric of Comparative Example 1, the sample of Example 6 coated with the ester-modified PE emulsion “Mecatex HP-70” instead of “Chemipearl M200” also had a strength retention ratio and hydrophilicity after the sulfonation treatment. Improved.
Similarly, instead of the corona treatment, the sample of Example 7 subjected to UV ozone treatment also had improved strength retention and hydrophilicity after the sulfonation treatment as compared with the base fabric of Comparative Example 1.

Next, a sealed nickel-metal hydride battery was produced using the obtained separator. As battery members, a sintered nickel electrode was used for the positive electrode, a sintered hydrogen storage alloy was used for the negative electrode, and a 30 wt% aqueous potassium hydroxide solution was used for the electrolyte. The sample of Comparative Example 3 was excluded because it has a high air density and is clearly unsuitable as a battery separator.
The produced sealed nickel-metal hydride battery was initially charged and discharged for 10 cycles with a charging 0.1 C rate of 12 hours, a rest period of 0.5 hour, a discharge of 0.1 C rate and a final voltage of 1.0 V.

[Defect rate]
100 batteries using each separator were prepared by the above method, and the defect rate was examined.

[Self-discharge test]
The sealed nickel-metal hydride battery that was initially activated was charged for 12 hours at a charging rate of 0.1 C, suspended for 0.5 hours, discharged at a discharging rate of 0.1 C, and a final voltage of 1.0 V. The ratio of the remaining capacity (0.1 C rate discharge, final voltage 1.0 V) when left at 45 ° C. for 14 days after charging under the same conditions (0.1 C rate) is the capacity storage rate after self-discharge. did. In addition, all charging / discharging was performed at 25 degreeC.

[Cycle life test]
The sealed nickel-metal hydride battery that was initially activated was charged at 1.0 C rate for 1.1 hours at 25 ° C., rested for 1 hour, and then discharged at a 1.0 C rate with a final voltage of 1.0 V. The number of cycles when the utilization ratio with respect to the capacity became 80% or less was determined.

電池試験の結果を表２に示す。不良率は、比較例２のみ２％で、他は０％であった。不良の原因は、極材料のバリによってセパレータが破れたことによるショートと、電池作製時にセパレータに張力を掛けすぎたためセパレータの幅が収縮し、正負極同士が接触してしまったことによるショートであった。容量保持率は、各セパレータともあまり差異はなかった。一方、ＰＥコーティングを施したセパレータを使用した電池は、比較例１のＰＥコーティング前の基布を用いた電池と比べてサイクル寿命が高くなっていた。

The results of the battery test are shown in Table 2. The defective rate was 2% only for Comparative Example 2 and 0% for the others. The cause of the defect is a short circuit due to the separator being broken due to the burr of the electrode material, and a short circuit due to the separator being contracted due to excessive tension applied to the separator during battery production, and the positive and negative electrodes were in contact with each other. It was. The capacity retention was not so different from each separator. On the other hand, the battery using the separator coated with PE had a higher cycle life than the battery using the base fabric before PE coating in Comparative Example 1.

  This seems to be because the PE coating improved the affinity with the electrolytic solution, thereby suppressing the dry-up of the electrolytic solution.

  As described above, according to this example, a PP spunbond nonwoven fabric having a PE layer formed on the surface was subjected to a treatment for introducing a carboxyl group into the nonwoven fabric surface, such as corona discharge treatment, plasma treatment, and UV ozone treatment. Subsequent sulfonation treatment can provide a battery separator having both high mechanical strength and high affinity with the electrolyte.

Claims (7)

A battery separator used by being sandwiched between a positive electrode and a negative electrode of a battery,
A polypropylene resin is used as a main constituent material, and 0.1 to 10.0% by weight of a polyethylene resin surface is formed on the nonwoven fabric surface formed by binding the polypropylene resins together. Then, the battery separator is characterized in that the fiber surface of the nonwoven fabric is subjected to a treatment for introducing a carboxyl group and further subjected to a sulfonation treatment.   The battery separator according to claim 1, wherein the polypropylene resin constituting the nonwoven fabric is made of dry fiber or continuous fiber.   The battery separator according to any one of claims 1 to 2, wherein a polyethylene-based resin surface is formed by coating a polyethylene-based emulsion on the surface of the nonwoven fabric. The battery separator according to any one of claims 1 to 3 , wherein the introduction of the carboxyl group is performed by performing at least one of corona discharge treatment, plasma treatment, and UV ozone treatment. The non-woven fabric, a battery separator according to any one of claims 1 to 4, characterized in that it is produced by a spunbond method. A secondary battery using the battery separator according to any one of claims 1 to 5 . The secondary battery according to claim 6, which is a nickel metal hydride storage battery.

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