JP2015037050A - Method of manufacturing separator for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a separator for a lithium ion secondary battery capable of producing a long separator having excellent battery characteristics, in which coating amount distribution of coating liquid is uniform, stably even when the substrate is thin.SOLUTION: A method of manufacturing a separator for a lithium ion secondary battery by giving a coating liquid, containing inorganic particles or organic particles, to a porous substrate includes a step for squeezing the coating liquid by means of at least two vent blades, after the coating liquid is carried by the substrate.

Description

  The present invention relates to a method for producing a separator for a lithium ion secondary battery (hereinafter sometimes abbreviated as “separator”).

  With the recent spread of portable electronic devices and higher performance, secondary batteries having high energy density are desired. As this type of battery, a lithium ion secondary battery using an organic electrolyte (non-aqueous electrolyte) has attracted attention. Since non-aqueous electrolytes are flammable, there is a risk of ignition and the like, and careful attention is paid to safety in their use.

  The porous film made of polyolefin such as polyethylene, which is often used as a separator, melts and closes the micropore when the temperature inside the battery is around 130 ° C. There is a thermal fuse function (shutdown function) that shuts off the power. However, it has been suggested that if the temperature further increases due to some situation, the polyolefin itself may melt and short-circuit, causing thermal runaway. In order to prevent such a short circuit due to thermal contraction of the separator, a separator in which inorganic particles or organic particles are applied to a base material has been proposed. As the substrate, a porous film made of polyolefin may be used, or a non-woven fabric with good heat resistance may be used.

  As a method for producing such a separator, a liquid containing inorganic particles or organic particles (hereinafter also referred to as “coating liquid”) is applied to a base material, and then a predetermined peripheral speed in a direction opposite to the transport direction of the base material. By passing the base material between two rolls arranged so that the straight line connecting the rotation axes of each other rotated at right angles to the traveling direction of the base material, the excess coating liquid on the base material is removed, A proposal has been made to make the surface of the coating film uniform (see, for example, Patent Document 1).

  Further, a roll that rotates in a direction opposite to the traveling direction of the base material is brought into contact with only one side of the base material, or a plurality of rolls that rotate in a direction opposite to the traveling direction of the base material, By sequentially bringing both surfaces of the base material into contact with a plurality of rolls arranged so that the straight lines connecting the rotation axes of the rolls facing the base material are not orthogonal to the traveling direction of the porous base material, There has also been a proposal to make the thickness of the coating film uniform (for example, see Patent Document 2).

  However, according to the techniques of Patent Documents 1 and 2, the coating amount distribution of the coating liquid tends to be non-uniform in the width direction, pinholes and high resistance portions exist in the separator, and the ion permeability becomes non-uniform. It was difficult to obtain a separator with good battery characteristics. Furthermore, when the base material is thin, wrinkles enter the base material, resulting in coating spots, and it is difficult to continuously produce a long separator with high thickness uniformity.

JP 2002-166218 A JP 2008-179903 A

  An object of the present invention is to provide a separator for a lithium ion secondary battery that can produce a long and stable separator even when the substrate is thin, with a uniform coating amount distribution of the coating liquid and good battery characteristics. It is in providing the manufacturing method of.

As a result of intensive studies to solve the above problems, the present inventors have
(1) A method for producing a separator for a lithium ion secondary battery obtained by applying a coating liquid containing inorganic particles or organic particles to a porous substrate, and at least after the coating liquid is supported on the substrate, A method for producing a separator for a lithium ion secondary battery, comprising a step of squeezing the coating liquid with two vent blades;
(2) The method for producing a separator for a lithium ion secondary battery according to (1), wherein the coating liquid is squeezed simultaneously on both sides with a vent blade centering on the base material,
(3) The manufacturing method of the separator for lithium ion secondary batteries as described in (1) or (2) whose base material is a nonwoven fabric,
I found.

  With the method for manufacturing a separator for a lithium ion secondary battery according to the present invention, a separator having a uniform coating amount distribution and good battery characteristics can be produced stably in a long length even when the substrate is thin. It is possible to manufacture a lithium ion secondary battery separator that does not have pinholes or high resistance.

It is the schematic which showed an example of the one part process concerning the manufacturing method of the separator for lithium ion secondary batteries in this invention. It is explanatory drawing regarding the double-sided vent blade of the process of restrict | squeezing the coating liquid of the apparatus concerning the manufacturing method of the separator for lithium ion secondary batteries in this invention. It is sectional drawing of the double-sided vent blade part of the process of restrict | squeezing the coating liquid of the apparatus concerning the manufacturing method of the separator for lithium ion secondary batteries in this invention.

  Hereinafter, the manufacturing method of the separator for lithium ion secondary batteries of this invention is demonstrated, using drawing.

  FIG. 1 is a schematic view showing an example of a part of steps relating to a method for producing a separator for a lithium ion secondary battery in the present invention. First, the base material 2 is supported on the base material 2 while the base material 2 is pulled out and conveyed from the base material roll 3 around which the base material 2 is wound. The method for supporting the coating liquid on the base material 2 is not particularly limited. For example, as shown in FIG. 1, a method of passing the base material 2 through the impregnation pad 5 containing the coating liquid is used. Can do. In FIG. 1, reference numeral 4 denotes an impregnation roll.

  Next, the supporting amount of the coating liquid on the base material 2 is uniformly adjusted by leaving at least two vent blades 1 so as to sandwich the base material 2 while leaving a necessary amount. FIG. 2 is an explanatory diagram relating to a double-sided vent blade in the step of squeezing the coating liquid of the apparatus according to the method for producing a separator for a lithium ion secondary battery in the present invention. FIG. 3 is a cross-sectional view of the double-sided vent blade part in the step of squeezing the coating liquid of the apparatus according to the method for producing a separator for a lithium ion secondary battery in the present invention. In the width direction coating amount profile adjustment, the gap between the base material 2 and the vent blade 1 is adjusted with the blade profiler bar 8 in the width direction. By having a step of squeezing the coating liquid with at least two vent blades 1, it is possible to apply a uniform tension in the width direction of the substrate 2, so that the generation of wrinkles is suppressed and the ion permeability is deteriorated. In addition, it has become possible to obtain a lithium ion secondary battery separator with a long length in which deterioration of battery characteristics due to coating peeling is suppressed. Next, the base material 2 whose surface is smoothed is dried by a dryer 7, and taken up by a reeler through a transport roll 6. In FIG. 1 to FIG. 3, dotted arrows indicate the conveyance direction of the substrate 2. In FIGS. 1 to 3, the coating liquid is squeezed by two vent blades, but the number of vent blades may be three or more.

  In the present invention, the material of the vent blade is not particularly limited, and various resin plates such as polyester, polypropylene, soft vinyl chloride, low foam vinyl chloride, fluororesin, acrylic resin, ABS (acrylonitrile butadiene styrene copolymer) resin, polycarbonate, aluminum, It is preferable to appropriately select materials such as various metal plates such as iron, copper, steel, and SUS, and other carbon fiber plates according to the type of coating liquid to be used. Further, in the case of an aqueous coating solution, the contact surface of the vent blade with the coating solution is preferably hydrophobic because of the effect of suppressing the occurrence of lateral steps due to friction. In the case of hydrophilicity, it is preferable to perform a hydrophobizing treatment as necessary. You may coat | cover the material (For example, a ceramic, a metal oxide, a metal carbide etc.) which is hard to wear as needed.

  In the present invention, it is preferable that the thickness of the vent blade is appropriately selected from 0.1 mm to 2 mm depending on the apparatus width. Since the vent blade is used with the blade being largely distorted, it is preferable to change the thickness appropriately according to the material of the vent blade. Since the distortion of the blade is large, the pressure distribution in the width direction becomes more uniform as the force in the width direction increases. Moreover, the vent blade can adjust the coating amount on both sides of the substrate and in the width direction with a blade profiler bar, if necessary. The blade profiler bar is preferably installed at a position of 1/4 to 1/2 from the cutting edge of the vent blade. Also. The position of the blade profiler bar can be adjusted as appropriate depending on the material, thickness, and hardness of the vent blade.

  In this invention, although the width | variety of a vent blade is suitably adjusted according to a base-material width, it is preferable to set 10-20 mm wider than a base-material width. The length of the vent blade is preferably approximately twice the horizontal distance from the fixing source of the vent blade to the base material, preferably 4 to 7 cm, and more preferably 5 to 6 cm.

  In the present invention, at least two vent blades are installed so that the blade edge and the width match so as to be sandwiched between the base material and the vent blade, and the coating liquid is squeezed simultaneously on both sides, so that the coating amount is uniform. Thus, a separator with a small difference between the front and the back can be produced.

  In the present invention, examples of the substrate include porous films, nonwoven fabrics, woven fabrics, and woven fabrics. As a porous film, porous films, such as polyethylene, a polypropylene, an olefin, an aramid, a polyimide, a fluorine, can be used. The porous film may be a single layer or a multilayer of two or more layers. In the case of multiple layers, porous films of the same kind of material may be stacked, or films of different kinds of materials may be used in combination. Further, the porous film may contain fibers, inorganic fillers, organic fillers, and the like.

  In the present invention, the base material is preferably a nonwoven fabric in order to improve heat resistance or reduce the internal resistance of the lithium ion secondary battery. Non-woven fabric is not particularly limited, but polyolefin resin, polyester resin, polyamide resin, acrylic resin, polyvinyl ether resin, polyvinyl ketone resin, polyether resin, polyurethane resin, unsaturated resin Nonwoven fabrics containing fibers such as polyester resins and fine cellulose can be used. These fibers can be incorporated not only in one type but also in a mixture.

  The thickness of the substrate is preferably 6 μm or more, more preferably 8 μm or more, and further preferably 10 μm or more in order to ensure the strength as a separator. Moreover, 30 micrometers or less are preferable from the surface of a battery characteristic, 20 micrometers or less are more preferable, and 15 micrometers or less are further more preferable. In this invention, even if it is a thin base material of 6-30 micrometers, the separator for lithium ion secondary batteries can be produced stably long.

The basis weight of the substrate is preferably 6 g / m ² or more, more preferably 7 g / m ² or more, and even more preferably 8 g / m ² or more in order to ensure the strength as a separator. . Moreover, from the surface of a battery characteristic, 20 g / m < ² > or less is preferable, 15 g / m < ² > or less is more preferable, and 12 g / m < ² > or less is more preferable.

The density of the base material, in order to sufficiently ensure the ion permeability, preferably from 0.85 g / cm ³ or less, preferably further 0.75 g / cm ³ or less, 0.70 g / cm ³ or less is most preferred. Moreover, it is difficult to obtain smoothness of the substrate, 0.30 g / cm ³ or more, more preferably more 0.40 g / cm ³ or more, 0.45 g / cm ³ or more is most preferable. In the present invention, the density is a value obtained by dividing the basis weight of the substrate by the thickness of the substrate.

Inorganic particles used in the present invention include, but are not limited to, _{boehmite, SiO 2, Al 2 O 3} , alumina - may be used silica composite oxide.

  The organic particles used in the present invention are not particularly limited, and examples thereof include SBR (styrene-butadiene rubber) resins, acrylic resins, cellulose derivatives, and fluorine resins.

  The medium for the coating liquid in the present invention is not particularly limited, but water or an organic medium can be used. Examples of the organic medium include a solvent having a hydroxyl group such as methanol, ethanol and isopropyl alcohol, and a medium having a carbonyl group such as acetone and methyl ethyl ketone.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example.

Example 1
A microporous polyethylene (PE) film (basis weight: 10 g / m ² , thickness: 18 μm, width: 250 mm) was prepared as the base material 2, and a lithium ion secondary battery separator was manufactured. The double-sided vent blade 1 was installed on both sides of the base material 2 so that a high-density polyethylene film (thickness 0.2 mm, width 300 mm, length 100 mm) was sandwiched around the base material 2. The blade profiler bar 8 was adjusted and attached so as to be 28 mm from the cutting edge of the vent blade 1.

  The substrate 2 is transported and immersed in an impregnation pad 5 containing a coating solution containing boehmite and acrylic resin so that the coating solution is supported on the substrate 2 and then touches both surfaces of the substrate 2. The coating amount was adjusted by the blade profiler bar 8 so that the coating solution was uniformly and appropriate amount with the vent blade 1 installed in the above. The separator for lithium ion secondary batteries was produced by drying with a dryer 7.

Example 2
A separator was produced in the same manner as in Example 1 except that the substrate 2 was a microporous polypropylene (PP) film (basis weight 10 g / m ² , thickness 18 μm, width 250 mm).

Example 3
A separator was produced in the same manner as in Example 1 except that the base material 2 was a non-woven fabric (basis weight 8 g / m ² , thickness 15 μm, width 250 mm) made of long polyethylene terephthalate (PET).

Example 4
A separator was manufactured in the same manner as in Example 3 except that the material of the vent blade 1 was a polyester film (thickness 0.2 mm, width 300 mm, length 100 mm).

Example 5
A separator was produced in the same manner as in Example 3 except that the material of the vent blade 1 was an acrylic film (thickness 0.2 mm, width 300 mm, length 100 mm).

Example 6
A separator was produced in the same manner as in Example 3 except that the vent blade 1 was made of metal steel (thickness 0.1 mm, width 300 mm, length 100 mm).

Example 7
A separator was produced in the same manner as in Example 3 except that the vent blade 1 was a high-density polyethylene film (thickness 0.5 mm, width 300 mm, length 100 mm).

Example 8
A separator was produced in the same manner as in Example 3 except that the vent blade 1 was made of a high-density polyethylene film (thickness 1.0 mm, width 300 mm, length 100 mm).

Example 9
A separator was produced in the same manner as in Example 3, except that the vent blade 1 was a high-density polyethylene film (thickness 2.0 mm, width 300 mm, length 100 mm).

Comparative Example 1
A separator was produced in the same manner as in Example 3 except that the vent blade 1 was omitted.

Comparative Example 2
A separator was produced in the same manner as in Example 3 except that the coating liquid was squeezed using a SUS roll (φ30 mm, width 300 mm, clearance 0.1 mm) instead of the vent blade 1.

<Evaluation>
The following evaluation was performed about the separator for lithium ion secondary batteries obtained by the Example and the comparative example, and the result was shown in Table 2.

[Separator film thickness]
The produced separator was measured at 20 points using a contact-type film thickness meter (made by Mitutoyo, the contact terminal has a cylindrical shape with a bottom having a diameter of 0.5 cm), and the difference between the maximum value and the minimum value was less than 2 μm. “◯” represents 2 μm or more and less than 3 μm, and “△” represents 3 μm or more.

[Separator basis weight]
About the produced separator, it cut into a 20 cm x 25 cm angle | corner, performed mass measurement using the electronic balance (the product made from METLER, AJ150), and made it 20 times, and made it basic weight (g / m < ² >).

[Presence of wrinkles]
About the produced separator, the presence or absence of wrinkles was confirmed visually. Those with no wrinkles in the width of 250 mm are indicated with “◯”, those with 1 or more and 3 or less confirmed “Δ”, and those with 4 or more confirmed with “x”.

[Separator impedance measurement]
In order to evaluate the uniformity of ion permeability, impedance measurement was performed. Using 1M LiPF ₆ / EC: DEC (30:70 vol%) as the electrolyte, incorporating the separators of each example and comparative example into a commercially available assembled battery evaluation cell, and measuring the impedance at 20 kHz for 20 samples. did. The difference between the maximum value and the minimum value was expressed as “◯”, the difference between 0.5Ω and less than 1.0Ω as “Δ”, and the difference between 1.0Ω and 1.0 as “×”. A portion where the film thickness is thick or a portion where the particles are densely packed has a large value because the ion movement distance between the electrodes becomes long. On the other hand, the portion where the film thickness is thin or the portion where the particle is coarsely filled has a small value because the ion movement distance becomes short. For this reason, a large difference between the maximum value and the minimum value means that the coating amount is non-uniform and the ion permeability is non-uniform.

[Separator pinhole detection]
In order to evaluate the presence or absence of pinholes in the separator, light was applied from the back of the separator, and the light transmission location was visually confirmed. “◯” indicates that there is no transmission portion within a 20 cm × 20 cm square, “Δ” indicates that the transmission location is 1 to less than 10, and “×” indicates that the transmission location is 10 or more.

[Production of evaluation battery]
(1) A positive electrode agent paste composed of 89.5 parts by mass of lithium manganate powder, 4.5 parts by mass of acetylene black, 6 parts by mass of polyvinylidene fluoride (PVdF), and N-methylpyrrolidone as a solvent was 2.0 mAh / cm. A positive electrode plate having a thickness of 97 μm was obtained by applying, drying and pressing on an aluminum foil having a thickness of 20 μm so that a capacity of ² was obtained.
(2) A negative electrode agent paste composed of 87 parts by mass of mesophase carbon microbead powder, 3 parts by mass of acetylene black, 10 parts by mass of PVdF, and N-methylpyrrolidone as a solvent so that a capacity of 2.1 mAh / cm ² can be obtained. The coating was dried and pressed on a copper foil having a thickness of 18 μm to produce a negative electrode plate having a thickness of 90 μm.
(3) 1M LiPF ₆ / ethylene carbonate (EC): diethyl carbonate (DEC) (30:70 vol%) (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolytic solution.
(4) As a separator, the separator produced by the Example and the comparative example was used.
(5) The positive electrode and the negative electrode (3 cm × 5 cm) were opposed to each other through a separator (4 cm × 6 cm). This was impregnated with an electrolytic solution and vacuum-sealed in an exterior made of an aluminum laminate film to produce a lithium ion secondary battery with a design capacity of 30 mAh. Five batteries were prepared for each separator.

[Battery characteristics: Separator C-rate discharge test (discharge capacity)]
The battery for evaluation was subjected to three-cycle aging at 1C, followed by constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V, and then a constant current discharge test (2.8V cut) at 5C. The average value of the discharge capacity was calculated. The unit is mAh.

  Table 1 summarizes the base materials used, the drawing means, and their materials.

  Table 2 will be described in detail. In Examples 1 to 9, after the coating liquid is supported on the base material, the coating surface is excessively supported on the base material by using an at least two vent blades and squeezed into an appropriate amount to squeeze the coating surface. Was able to produce a separator that was clean, hardly wrinkled, had little unevenness in ion permeability, and had few pinholes.

  From comparison between Examples 1 to 9 and Comparative Example 1, it can be seen that adjustment to an appropriate thickness is possible due to the effect of the vent blade, there is little unevenness in ion permeability, and generation of wrinkles can be suppressed. In Comparative Example 1, it was difficult to adjust the coating amount, and it was difficult to suppress thickness and wrinkles. Moreover, it turns out from the comparison with Examples 1-9 and the comparative example 2 by the effect of a vent blade that there are few non-uniformities in ion permeability, there are few pinholes, and generation | occurrence | production of a wrinkle can be suppressed. In Comparative Example 2, it is difficult to suppress pinholes. Over time, the coating liquid gradually accumulates from the end of the metal roll, the coating amount and the thickness vary widely, and wrinkles are gradually generated. It was.

  In Examples 1 to 3, separators that are less likely to wrinkle and have few pinholes are obtained. In particular, in Example 3 in which a non-woven fabric was used as the base material, it was possible to produce a separator with less unevenness in ion permeability.

  From the comparison of Examples 3 to 6, there was almost no difference in performance due to the difference in the material of the vent blade, but in the case of using the metal steel of Example 6, slight thickness unevenness occurred.

  From Examples 3 and 7 to 9, almost no difference in performance due to the difference in thickness of the vent blade was observed, but as the thickness increased, the tendency for the generation of thickness and wrinkles to increase slightly was observed.

  The present invention can be suitably used as a method for producing a separator for a lithium ion secondary battery.

DESCRIPTION OF SYMBOLS 1 Vent blade 2 Base material 3 Base material roll 4 Impregnation roll 5 Impregnation pad 6 Transport roll 7 Dryer 8 Blade profiler bar

Claims (3)

  A method for producing a separator for a lithium ion secondary battery, wherein a coating liquid containing inorganic particles or organic particles is applied to a porous substrate, and after the coating liquid is supported on the substrate, at least two sheets The manufacturing method of the separator for lithium ion secondary batteries characterized by having the process of restrict | squeezing a coating liquid with a vent blade.   The method for producing a separator for a lithium ion secondary battery according to claim 1, wherein the coating liquid is squeezed simultaneously on both sides by a vent blade centering on the substrate.   The method for producing a separator for a lithium ion secondary battery according to claim 1 or 2, wherein the substrate is a nonwoven fabric.