JP5750033B2 - Lithium ion battery separator - Google Patents

Description

The present invention relates to a lithium ion battery separator.

  A lithium ion secondary battery, which is one of the electrochemical elements, has a feature of high energy density, and is widely used as a power source for portable electric devices such as mobile phones, portable music players, and notebook personal computers. . In addition, the movement to use lithium ion secondary batteries is spreading in large equipment such as electric bicycles, hybrid cars, and electric cars. For this reason, lithium ion secondary batteries are required to have high capacity and charge / discharge characteristics at a large current. However, since lithium ion secondary batteries are non-aqueous batteries, they emit smoke compared to water-based batteries. It is known that there is a high risk of ignition, rupture, etc., and improvement in safety is also required.

  In a lithium ion secondary battery, the risk of smoking increases due to temperature rise due to external heat, overcharge, internal short circuit, external short circuit, and the like. These can be prevented to some extent by an external protection circuit. In addition, the polyolefin resin porous film used as a lithium ion battery separator melts at around 120 ° C., and the pores are blocked to block the flow of current and ions, thereby suppressing battery temperature rise. . This is called a shutdown function. However, when the temperature rises due to external heat or when a chemical reaction occurs inside the battery due to the temperature rise, the battery temperature further rises even if the shutdown function works, and when the battery temperature reaches 150 ° C or higher, The porous film contracted, causing an internal short circuit, which could cause ignition.

  As described above, it is difficult to suppress the ignition of the battery by the shutdown function of the separator. In addition, with the increase in capacity of batteries, the increase in current during charging and discharging is also progressing, and in order to suppress the Joule heat generated at that time, it is also necessary to lower the resistance value of the separator impregnated with the electrolyte. It has become. For this reason, metal oxide fine particles were used for the purpose of suppressing the ignition of the battery by lowering the resistance value while making it difficult to cause an internal short circuit by raising the heat shrinkage temperature than the porous film of polyolefin resin. Separator has been developed. In this separator, the pore diameter is controlled by the metal oxide fine particles, and it is possible to suppress internal short circuit, improve heat resistance, and lower the resistance value.

  For example, a separator having a microporous pseudoboehmite layer obtained as a porous film by mixing pseudoboehmite and a binder, drying and peeling after coating on a separately prepared film has been proposed (for example, patents) Reference 1). However, although this separator has an improved heat shrink temperature, it is prone to powder falling and cracking, so that it is difficult to take out as a wound separator alone, and handling during battery production is poor. It was.

  Also proposed is a separator having a porous inorganic coating on and in the nonwoven fabric, the inorganic coating having aluminum (Al), silicon (Si) and / or zirconium (Zr) oxide particles. (For example, refer to Patent Document 2). Although this separator uses non-woven fabric as a base material, handling is improved, but other metal oxides are detected by silica fine particles generated by the sol-gel method. Drops and cracks are likely to occur, which leads to the generation of pinholes and causes internal short circuits, making it difficult to be a useful separator.

  Furthermore, a coating liquid containing insulating fine particles having heat resistance, a thickener, and a dispersion medium, and a separator coated with the coating liquid on a substrate have been proposed (for example, Patent Document 3). reference). However, the coating solution has a problem that the stability of the coating solution is not sufficient, and a battery using the coating solution has a problem that the capacity decreases while charging and discharging are repeated.

JP-T-2001-527274 JP 2005-536658 Gazette International Publication No. 2009/096451 Pamphlet

  The present invention provides a highly stable coating solution for producing a lithium ion battery separator having high heat resistance, and also provides a lithium ion battery separator with little reduction in capacity even after repeated charge and discharge. is there.

As a result of diligent research to solve the above problems, the inventors of the present invention contain an alumina hydrate, an aqueous dispersion of a binder polymer, an aqueous dispersant, and a thickener. The dispersant is a carboxymethylcellulose salt (hereinafter referred to as “CMC salt”) having a viscosity of 100 mPa · sec to 500 mPa · sec at 25 ° C. of the 1 mass% aqueous solution, and the thickener is 1 mass by mass. A lithium ion battery separator obtained by coating a nonwoven fabric with a coating solution for a lithium ion battery separator, which is a CMC salt having a viscosity at 25 ° C. of a 25% aqueous solution of 3000 mPa · sec or more and 10,000 mPa · sec or less, was found.

  A coating liquid comprising alumina hydrate, an aqueous dispersion of a binder polymer, an aqueous dispersion, and a thickener, and the aqueous dispersion has a viscosity at 25 ° C. of a 1 mass% aqueous solution thereof. It is a CMC salt having a viscosity of 100 mPa · sec to 500 mPa · sec, and the thickener is a CMC salt having a 1% by mass aqueous solution having a viscosity at 25 ° C. of 3000 mPa · sec to 10,000 mPa · sec. The stability of the working fluid can be increased.

  A lithium ion battery separator obtained by applying the coating liquid to a nonwoven fabric and drying the lithium ion secondary battery using the separator provides an effect that the capacity reduction is reduced even when charging and discharging are repeated.

  The coating liquid of the present invention comprises an alumina hydrate, an aqueous dispersion of a binder polymer, an aqueous dispersion, and a thickener, and the aqueous dispersion is a 1% by weight aqueous solution. A CMC salt having a viscosity at 25 ° C. of 100 mPa · sec to 500 mPa · sec, and the thickener is a CMC salt having a 1% by mass aqueous solution having a viscosity at 25 ° C. of 3000 mPa · sec to 10,000 mPa · sec. is there.

  In the prior art, a water-soluble polymer such as sodium polyacrylate is used as an aqueous dispersant, and a water-soluble polymer having a structure different from that of the dispersant such as CMC salt is used as a thickener. In the present invention, it has been found that the reason why the stability of the coating solution is low is due to the use of water-soluble polymers having different structures in the aqueous dispersant and the thickener. According to the estimation in the present invention, when the aqueous dispersant and the thickener have different structures, the dispersion is adsorbed on the surface of the alumina hydrate to improve the dispersion stability of the alumina hydrate. Some of the agent molecules are replaced by thickener molecules over time, and the dispersion becomes unstable.

  In order to avoid this problem, the basic structure is the same, only the molecular weight is different, and a substance that can be used for both the aqueous dispersant and the thickener is searched, and the viscosity at 25 ° C. of the 1 mass% aqueous solution is 100 mPa A CMC salt of sec to 500 mPa · sec can be used as an aqueous dispersion of alumina hydrate, and the viscosity at 25 ° C. of the 1% by mass aqueous solution is 3000 mPa · sec to 10000 mPa · sec. It has been found that a CMC salt can be used as a good thickener for a dispersion using the CMC salt as an aqueous dispersant.

  In the coating liquid of the present invention, since the water-soluble polymer having the same structure as CMC salt is used as the aqueous solution and thickener of alumina hydrate, the dispersion does not become unstable. Furthermore, a lithium ion secondary battery using a separator obtained by applying the coating liquid of the present invention to a nonwoven fabric and drying it is applied using a water-soluble polymer having a different structure for the aqueous dispersant and the thickener. There is less decrease in capacity due to repeated charge / discharge than when the working solution is used. According to the estimation in the present invention, this is because the types of side reactions that deteriorate the battery are reduced because the types of compounds present in the battery are reduced.

  In the present invention, the CMC salt used as the aqueous dispersant is limited to a 1% by mass aqueous solution having a viscosity at 25 ° C. of 100 mPa · sec to 500 mPa · sec. If the viscosity is lower than this, the action as a dispersant is insufficient, and a stable coating solution is not provided. If the viscosity is higher than this, an action as a flocculant is exhibited, and a stable coating solution is not provided. In the present invention, the amount of the CMC salt used as the aqueous dispersant is preferably 0.05% by mass or more and 3.00% by mass or less, and 0.10% by mass or more and 2.00% by mass with respect to the alumina hydrate. The following is more preferable. When the amount of the CMC salt used as the aqueous dispersant is too small, the alumina hydrate may aggregate when the CMC salt used as the thickener is added. When the amount of the CMC salt used as the aqueous dispersant is too large, the high rate charge / discharge characteristics may be deteriorated.

  In the present invention, the CMC salt used as a thickener is limited to a 1% by mass aqueous solution having a viscosity at 25 ° C. of 3000 mPa · sec to 10,000 mPa · sec. If the viscosity is lower than this, the viscosity required for coating will not be given unless added in a large amount, and if it is added in a large amount, the charge / discharge characteristics of the battery will be deteriorated. If the viscosity is higher than this, an action as a flocculant is exhibited, and a stable coating solution is not provided. In the present invention, the amount of the CMC salt used as a thickener is preferably 0.05% by mass or more and 3.00% by mass or less, and 0.10% by mass or more and 2.00% by mass or less with respect to the alumina hydrate. Is more preferable. When the addition amount of the CMC salt used as the thickener is too small, the coating liquid may penetrate the opposite surface of the nonwoven fabric or the stability of the coating liquid may be reduced. If the amount of the CMC salt used as a thickener is too large, the high-rate charge / discharge characteristics may be deteriorated, or the viscosity of the coating solution may become extremely high, making coating difficult.

  In the present invention, the viscosity is 25 ° C., using a B-type viscometer manufactured by Tokyo Keiki Co., Ltd. No. 2 for CMC salt aqueous solution used as a thickener at 30 rpm using a 2 rotor It is measured at 30 rpm using 4 rotors.

  When preparing the coating liquid of the present invention, the CMC salt used as the aqueous dispersant must be mixed with the alumina hydrate prior to the CMC salt used as the thickener. Preferably, after mixing the CMC salt used as the aqueous dispersant with the alumina hydrate, the CMC salt used as the thickener is mixed after a time of 10 minutes or more has elapsed. When the CMC salt used as the thickener is mixed with the alumina hydrate prior to or simultaneously with the CMC salt used as the aqueous dispersant, the alumina hydrate aggregates and a stable coating solution cannot be obtained. .

  In addition, as CMC salt used for this invention, water-soluble salts, such as lithium salt, sodium salt, potassium salt, ammonium salt of carboxymethylcellulose, are used. Among these, sodium salts and lithium salts are preferably used in that they are highly water-soluble and do not adversely affect the charge / discharge characteristics of the lithium ion secondary battery.

  As the alumina hydrate used in the coating liquid of the present invention, boehmite is preferably used from the viewpoint of easily obtaining thermal stability and liquid properties suitable for coating. Examples of such boehmite commercially available products include boehmite manufactured by Daimei Chemical Industry Co., Ltd., and APYRAL (registered trademark) AOH manufactured by Navaltec (Schwandorf, Germany). It is not limited.

  Various latexes such as styrene-butadiene latex can be used as the aqueous dispersion of the binder polymer used in the coating liquid of the present invention. As an example of the aqueous dispersion of the binder polymer, (Registered trademark) 2001 can be exemplified.

  The addition amount of the binder polymer used in the coating liquid of the present invention is preferably 2% by mass or more and 15% by mass or less with respect to the alumina hydrate. From the point of balance of powder fall of the coating layer and voids in the coating layer, 3 mass% or more and 10 mass% or less is more preferable.

  The coating liquid of the present invention is preferably applied to a nonwoven fabric. The nonwoven fabric manufactured by a conventionally known method can be used. For example, what was manufactured by methods, such as the spun bond method, the melt blow method, the dry method, the wet method, and the electrospinning method, can be used.

  In the present invention, the nonwoven fabric may be smoothed by calendaring or thermal calendaring for the purpose of controlling the surface and thickness of the nonwoven fabric.

  As the constituent material of the nonwoven fabric, polyethylene terephthalate, polybutylene terephthalate and derivatives thereof, polyester such as aromatic polyester, wholly aromatic polyester, polyolefin, acrylic, polyacetal, polycarbonate, aliphatic polyketone, aromatic polyketone, aliphatic polyamide, Aromatic polyamide, wholly aromatic polyamide, polyimide, polyamideimide, polyphenylene sulfide, polybenzimidazole, polyetheretherketone, polyethersulfone, poly (para-phenylenebenzobisthiazole), poly (para-phenylene-2,6- Benzobisoxazole), polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyurethane and polyvinyl chloride Like Wei and cellulose fibers. The nonwoven fabric may contain two or more of these constituent materials.

As a nonwoven fabric used for the lithium ion battery separator of the present invention, the basis weight is preferably 5.0 to 30.0 g / m ² . If the basis weight exceeds 30.0 g / m ² , the nonwoven fabric occupies most of the separator, and the effect of the coating layer may be difficult to obtain. If it is less than 5.0 g / m ² , it may be difficult to obtain uniformity as a nonwoven fabric. As a fabric weight of a nonwoven fabric, 7.0-20.0 g / m < ² > is more preferable. The basis weight means the basis weight based on the method defined in JIS P 8124 (paper and paperboard—basis weight measurement method).

  There is no particular limitation on the method of applying the coating liquid of the present invention to the nonwoven fabric, for example, conventionally known air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll coater, Examples include a die coater, a reverse roll coater, a transfer roll coater, and a spray coater.

In the present invention, the adhesion amount of the coating layer is preferably 1.0~20.0g / ^{m 2,} further 4.0~15.0g / ^{m 2} is more preferable. If the amount of the coating layer deposited is less than 1.0 g / m ² , the surface of the nonwoven fabric cannot be sufficiently covered, the pore diameter becomes large, and good battery characteristics such as short circuit may not be exhibited. is there. On the other hand, when the adhesion amount of the coating layer exceeds 20.0 g / m ² , it is difficult to reduce the thickness of the separator, and the large current charge / discharge characteristics of a battery using the separator may be deteriorated. .

  In order to produce a uniform coating layer by the above coating method, an antifoaming agent, a wetting agent, and the like can be appropriately added to the coating solution as necessary.

  In the present invention, the method of drying after coating is not particularly limited, and a known drying method can be used. In particular, a method of drying by heating, such as a method of blowing hot air or a method of irradiating infrared rays, is productivity. Is preferably used.

In the lithium ion battery separator of the present invention, the basis weight of the separator is preferably ^{10.0~50.0g / m 2, 15.0~40.0g / m} 2 is more preferable. Moreover, 10.0-50.0 micrometers is preferable and, as for the thickness of a separator, 15.0-40.0 micrometers is more preferable. Preferably 0.4~1.2g / cm ³ as the density of the separator, 0.6~1.0g / cm ³ is more preferable.

  In the present invention, after coating and drying, the lithium ion battery separator may be smoothed by calendaring for the purpose of controlling the flatness and thickness of the coating layer surface.

  Examples of the present invention will be described below.

<Production of non-woven fabric>
45 parts by mass of oriented crystallized polyethylene terephthalate (PET) short fibers having a fineness of 0.06 dtex (average fiber diameter of 2.4 μm) and a fiber length of 3 mm, an orientation of fineness of 0.1 dtex (average fiber diameter of 3.0 μm) and a fiber length of 3 mm 15 parts by mass of crystallized PET-based short fibers, a fine component of 0.2 dtex (average fiber diameter 4.3 μm), and a fiber length of 3 mm PET-based short fibers for a single-component binder (softening point 120 ° C., melting point 230 ° C.) 40 parts by mass Were mixed together, disintegrated in water with a pulper, and a uniform papermaking slurry having a concentration of 1% by mass was prepared under stirring by an agitator. Using cylinder paper machine, raising the paper making the sheet-forming slurry by a wet method, the cylinder dryer of 120 ° C., and to adhere the PET-based short fibers binders were expressed nonwoven strength, the basis weight 15.2 g / m ² A non-woven fabric was used. Furthermore, this nonwoven fabric was subjected to heat treatment at a roll temperature of 185 ° C., a linear pressure of 740 N / cm, and a conveying speed of 20 m / min using a 1-nip thermal calendar composed of a metal roll and a metal roll. Produced.

Example 1
180 parts by mass of water, alumina hydrate (boehmite, single particles, average particle size 0.9 μm, specific surface area 5.5 m ² / g, aspect ratio 2.5, manufactured by Naval Tech, trade name: APYRAL (registered trademark) Carboxymethylcellulose sodium salt (hereinafter, referred to as “CMC-Na”) having a viscosity of 200 mPa · s at 25 ° C. and a degree of etherification of 0.65 in an aqueous 1 mass% solution as 100 parts by mass of AOH60) as an aqueous dispersant. ) 20 parts by mass of a 2% by weight aqueous solution (0.4 parts by mass as a solid content) was mixed and stirred using a disperser equipped with a saw blade, and then, as a thickener, 80 parts by mass (0.8 parts by mass as a solid content) of a 1% by mass aqueous solution of CMC-Na having a viscosity of 7000 mPa · s and a degree of etherification of 0.7 at 25 ° C. is stirred and then stirred. 18.75 parts by mass (9 parts as a solid content) of a styrene-butadiene latex having a form content of 48% by mass is mixed and stirred, and further, ion-exchanged water is added to adjust the concentration, so that the solid content concentration is 25% by mass. A coating solution was prepared. The coating solution is uniformly applied to one side of the nonwoven fabric and dried so as to have a dry solid content of 10 g / m ² with a gravure coater on the nonwoven fabric after the thermal calendar treatment, and a lithium ion battery separator having a thickness of 30 μm. Got.

Examples 2-11
Lithium ion battery in the same manner as in Example 1 except that the type and amount of CMC-Na as an aqueous dispersant and the type and amount of CMC-Na as a thickener were changed as shown in Table 1. A separator was obtained. The dry solid content allowed a difference of 10 g / m ² to ± 1 g / m ² , and the thickness allowed a difference of 30 μm to ± 2 μm. However, this difference does not significantly affect the charge / discharge characteristics. That has been confirmed.

(Comparative Examples 1-8)
A lithium ion battery separator was obtained in the same manner as in Example 1 except that the types and addition amounts of the aqueous dispersant and thickener were changed as shown in Table 1. The dry solid content allowed a deviation from 10 g / m ² to ± 1 g / m ² , and the thickness allowed a deviation from 30 μm to ± 2 μm. However, this experience has a great influence on the deterioration of charge / discharge characteristics. There is nothing.

<Evaluation>
The following evaluation was performed about the coating liquid and lithium ion battery separator which were obtained by the Example and the comparative example, and the result was shown in Table 1 and Table 2.

[Problems when preparing coating liquid]
Table 1 shows problems that occurred during the preparation of the coating solution.

[Stability of coating solution]
About the produced coating liquid, it was left to stand for 28 days, respectively, the sedimentation state of the alumina hydrate at that time was confirmed visually, and it evaluated by the following degree.
A: No sedimentation is observed at all.
○: Slight sedimentation of alumina hydrate is observed at the bottom, but returns to the original state by stirring.
Δ: Some sedimentation of alumina hydrate was observed at the bottom, and some aggregates of alumina hydrate were observed even when stirred.
X: Precipitation of a large amount of alumina hydrate was observed, and a large amount of aggregate of alumina hydrate was observed even when stirred.

[Coating properties]
The coating liquid of each Example and each Comparative Example was applied to the nonwoven fabric laid on the black drawing paper with a manual rod coater that gave a coating amount of 10 g / m ² after drying. When the nonwoven fabric was removed from the drawing paper, the trace of the coating liquid adhering to the drawing paper was observed and evaluated according to the following degree.
○: No trace of coating liquid on drawing paper.
Δ: There is a trace of the coating liquid that breaks through at a frequency of 1 piece / cm ² or less.
X: There is a trace of the coating liquid that has betrayed at a frequency exceeding 1 piece / cm ² .

[Repetitive charge / discharge characteristics of battery]
Using the produced separator, the positive electrode active material is lithium manganate, the negative electrode active material is artificial graphite, the positive and negative electrode area is 15 cm ² , and the electrolyte is 7/3 of ethylene carbonate and diethyl carbonate (capacity ratio) of lithium hexafluorophosphate. A laminated lithium ion battery having a design capacity of 30 mAh, which is a mixed solvent solution (1 mol / L), is manufactured, and when 30 mA constant current charging → 4.2 V constant voltage charging → charging current 3 mA is reached, constant current discharge at 30 mA → 2. When 8V is reached, charge and discharge is performed for 100 cycles in the sequence of the next cycle, and the capacity reduction rate is obtained as [1- (discharge capacity at the 100th cycle / discharge capacity at the fourth cycle)] × 100 (%). It was.

  The alumina hydrate prepared in each Example, an aqueous dispersion of a binder polymer, an aqueous dispersant, and a thickener were contained, and the aqueous dispersant was 25 ° C. of the 1 mass% aqueous solution. A CMC salt having a viscosity at 100 mPa · sec to 500 mPa · sec, and a thickener is a CMC salt having a 1% by mass aqueous solution having a viscosity at 25 ° C. of 3000 mPa · sec to 10000 mPa · sec. The liquid does not agglomerate during the preparation of the coating liquid, the stability of the coating liquid is high, the coating property is good, and the repeated charge / discharge characteristics of the lithium ion battery produced using this are good. there were.

  On the other hand, in Comparative Example 1 in which the CMC salt as a thickener was not used, the stability of the prepared coating liquid was poor and the coatability was also poor. In Comparative Example 2 in which the CMC salt as the aqueous dispersant was not used, the liquid aggregated when the CMC salt as the thickener was added, and a coating solution could not be prepared. In Comparative Example 3 in which a compound other than the CMC salt was used as the aqueous dispersion agent, the repeated charge / discharge characteristics of the produced lithium ion battery were poor. Even in Comparative Example 4 in which a compound other than the CMC salt was used as the thickening agent, the repeated charge / discharge characteristics of the produced lithium ion battery were poor. In Comparative Example 5 where the aqueous solution viscosity of the CMC salt as the aqueous dispersant was too low, the liquid aggregated when the CMC salt as the thickener was added, and a coating solution could not be prepared. In Comparative Example 6 in which the aqueous solution viscosity of the CMC salt as the aqueous dispersant was too high, the liquid aggregated when the CMC salt as the aqueous dispersant was added, and a coating solution could not be prepared. . In Comparative Example 7 in which the aqueous solution viscosity of the CMC salt as a thickener was too low, the stability of the prepared coating solution was poor and the coating property was also poor. In Comparative Example 8 in which the aqueous solution viscosity of the CMC salt as the thickener was too high, the liquid agglomerated when the CMC salt as the thickener was added, and a coating solution could not be prepared.

Lithium ion battery separator of the present invention, in addition to the lithium ion secondary battery applications, lithium polymer batteries, it can also be used for lithium-ion capacitor or the like.

Claims (1)

An alumina hydrate, an aqueous dispersion of a binder polymer, an aqueous dispersion, and a thickener are contained, and the aqueous dispersion has a viscosity of 100 mPa · sec at 25 ° C. of a 1% by mass aqueous solution thereof. The amount of carboxymethylcellulose salt used as the aqueous dispersant is 0.05% by mass or more and 3.00% by mass or less based on alumina hydrate. the thickener, carboxymethyl cellulose salts der viscosity of less 3000 mPa · sec or more 10000 mPa · sec at 25 ° C. of 1 wt% aqueous solution is, the addition amount of carboxymethyl cellulose salt used as the thickener, alumina water to hydrate, lithium ion battery is less than 3.00 mass% to 0.05 mass% The separator coating liquid, a lithium ion battery separator, characterized by coating, formed by drying the nonwoven fabric.