JP2012003873A - Base material for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a base material for a lithium secondary battery which has excellent surface smoothness after coating without strike-through of coating liquid of filler particles and resins, high electrolyte retention rate, and excellent heat resistance.SOLUTION: The base material for the lithium secondary battery is characterized by comprising a wet nonwoven fabric containing fibrillation heat resistant fibers and synthetic staple fibers as essential components.

Description

  The present invention relates to a base material for a lithium secondary battery.

  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 battery using an organic electrolyte has attracted attention. This lithium ion battery has a high energy density because an average voltage of about 3.7 V, which is about three times that of an alkaline secondary battery, which is a conventional secondary battery, is obtained. Therefore, a nonaqueous electrolytic solution having sufficient oxidation-reduction resistance is used. 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. There are several possible cases of exposure to fire and other hazards, but overcharging is particularly dangerous.

  In order to prevent overcharging, current non-aqueous secondary batteries are charged at a constant voltage and a constant current, and the battery is equipped with a precise IC (protection circuit). The cost required for this protection circuit is large, and it is a factor that increases the cost of non-aqueous secondary batteries.

  When overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not operate well, and it is difficult to say that it is intrinsically safe. The current non-aqueous secondary battery has a safety circuit / PTC element equipped and a separator with a thermal fuse function for the purpose of destroying the battery safely when overcharged. Has been made. However, even if equipped with the above-mentioned means, depending on the overcharge conditions, the safety during overcharge is not guaranteed, and in fact, non-aqueous secondary battery ignition accidents Is still happening.

  As the separator, a film-like porous film made of a polyolefin such as polyethylene is often used. When the temperature inside the battery reaches around 130 ° C., it melts and closes the micropores, thereby transferring lithium ions. Although there is a thermal fuse function (shutdown function) that cuts off the current and shuts off the current, it is suggested that if the temperature further increases due to some situation, the polyolefin itself melts and short-circuits, causing a thermal runaway. In view of this, a heat-resistant separator that does not melt and shrink even at temperatures close to 200 ° C. has been developed.

  As the heat resistant separator, a polyester nonwoven fabric (for example, see Patent Document 1) is disclosed. In addition, a lithium ion secondary battery using a separator encapsulating a polyester nonwoven fabric and holding an organic polymer that swells in an electrolyte solution (see, for example, Patent Document 2), a resin A having a melting point of 80 to 130 ° C., and electrolysis by heating A separator for an electrochemical element is disclosed which includes a porous film including at least one resin B that swells by absorbing liquid, a porous substrate, and filler particles (see, for example, Patent Document 3). However, all of these separators have large pores in the nonwoven fabric, so that the coating liquid penetrates during coating, and the surface smoothness after coating is poor, and filler particles are filled inside the nonwoven fabric. Further, since the pores inside the nonwoven fabric are closed, there is a problem that the electrolytic solution retention is poor and the internal resistance of the separator is increased.

JP 2003-123728 A JP 2005-293891 A JP 2007-157723 A

  An object of the present invention is to provide a lithium secondary battery that realizes a separator that does not show through the coating liquid containing filler particles or resin, has excellent surface smoothness after coating, has a high electrolyte solution retention ratio, and excellent heat resistance. It is to provide a substrate for use.

  As a result of earnest research to solve the above problems, the inventor has a base for a lithium secondary battery comprising a wet nonwoven fabric containing fibrillated heat-resistant fibers and synthetic short fibers as essential components. I found the material.

  The base material for a lithium secondary battery according to the present invention is a wet nonwoven fabric containing fibrillated heat-resistant fibers and synthetic short fibers as essential components, so that the base material has small pores and is formed relatively uniformly. The Therefore, the back-through of the coating liquid containing the filler particles is suppressed, and the filler particles are intensively laminated on the surface of the base material. Therefore, the surface smoothness after coating is excellent, and there is no useless gap between the electrodes. Hard to occur. Further, since the back-through of the coating liquid is suppressed, the filler particles are not filled inside the base material and do not block the pores inside the base material, so that a separator having a high electrolytic solution retention rate can be obtained. it can. Since it contains fibrillated heat-resistant fibers, it has excellent heat resistance, the lithium secondary battery is internally short-circuited and generates heat, and even if the temperature rises, the separator does not melt or shrink.

  The base material for lithium secondary battery of the present invention (hereinafter sometimes referred to as “base material”) is a base material for impregnating or coating a slurry containing filler particles, and a porous film laminated together. It is a base material for impregnation or coating with a solid electrolyte, a solid electrolyte or a gel electrolyte, and is a precursor sheet for a lithium secondary battery separator. The filler may be either inorganic or organic. Examples of the inorganic filler include inorganic oxides such as alumina, silica, titanium oxide, barium titanate, and zirconium oxide, inorganic nitrides such as aluminum nitride and silicon nitride, aluminum compounds, zeolite, and mica. Examples of the organic filler include polyethylene, polypropylene, polyacrylonitrile, polymethyl methacrylate, polyethylene oxide, polystyrene, polyvinylidene fluoride, ethylene-vinyl monomer copolymer, polyolefin wax, and the like.

The lithium secondary battery in the present invention means a lithium ion battery or a lithium ion polymer battery. Examples of the negative electrode active material of the lithium secondary battery include carbon materials such as graphite and coke, metallic lithium, aluminum, silica, tin, nickel, and an alloy of lithium and lithium, SiO, SnO, Fe Metal oxides such as ₂ O ₃ , WO ₂ , Nb ₂ O ₅ , Li _4/3 Ti _5/3 O ₄ , and nitrides such as Li _0.4 CoN are used. As the positive electrode active material, lithium cobaltate, lithium manganate, lithium nickelate, lithium titanate, lithium nickel manganese oxide, or lithium iron phosphate is used. Further, the lithium iron phosphate may be a composite with one or more metals selected from manganese, chromium, cobalt, copper, nickel, vanadium, molybdenum, titanium, zinc, aluminum, gallium, magnesium, boron, and niobium.

  As an electrolytic solution for a lithium secondary battery, a solution obtained by dissolving a lithium salt in an organic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, dimethoxymethane, or a mixed solvent thereof is used. Examples of the lithium salt include lithium hexafluorophosphate and lithium tetrafluoroborate. As solid electrolyte, what melt | dissolved lithium salt in gel-like polymers, such as polyethyleneglycol, its derivative (s), polymethacrylic acid derivative, polysiloxane, its derivative (s), polyvinylidene fluoride, is used.

  The fibrillated heat-resistant fiber, which is an essential component constituting the base material for the lithium secondary battery of the present invention, includes wholly aromatic polyamide, wholly aromatic polyester, polyimide, polyamideimide, polyetheretherketone, polyphenylene sulfide, polybenzoe. A fibrillated heat-resistant fiber made of imidazole, poly-p-phenylenebenzobisthiazole, poly-p-phenylenebenzobisoxazole, polytetrafluoroethylene, or the like is used. Of these, wholly aromatic polyamides are preferred because of their excellent affinity with the electrolyte. The degree of fibrillation is preferably a Canadian standard freeness of 0 to 400 ml as defined in JIS P8121. If it exceeds 400 ml, the fiber diameter distribution becomes wide, and there are cases where it becomes a textured spot or a thick spot, or the coating liquid may fall through. The length weighted average fiber length of the fibrillated heat resistant fiber is preferably 0.2 to 2.0 mm. If it is less than 0.2 mm, it may fall off from the substrate or the substrate may become fluffy, and if it is longer than 2.0 mm, it may become lumpy.

  Fibrilized heat-resistant fibers consist of refiners, beaters, mills, grinding devices, rotary homogenizers that apply shear force to high-speed rotary blades, cylindrical inner blades that rotate at high speed, and fixed outer blades. High-speed homogenizer that produces a shearing force between them, an ultrasonic crusher that is refined by impact by ultrasonic waves, a pressure difference of at least 3000 psi to the fiber suspension and a small-diameter orifice And by using a high-pressure homogenizer or the like that applies a shearing force or a cutting force to the fiber by causing it to collide and rapidly decelerate.

  Synthetic short fibers which are essential components constituting the base material for lithium secondary battery of the present invention are polyolefin, polyester, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyamide, acrylic, polyvinyl chloride, polyvinylidene chloride, Short fibers made of resins such as polyvinyl ether, polyvinyl ketone, polyether, polyvinyl alcohol, diene, polyurethane, phenol, melamine, furan, urea, aniline, unsaturated polyester, fluorine, silicone, and derivatives thereof, and the above heat-resistant fiber Is mentioned. Synthetic short fibers increase the tensile strength and puncture strength of the substrate.

  The synthetic short fiber may be a fiber (single fiber) made of a single resin or a composite fiber made of two or more kinds of resins. Moreover, the synthetic short fiber contained in the base material for lithium secondary batteries of this invention may be 1 type, and may be used in combination of 2 or more types. Examples of the composite fiber include a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, and a multiple bimetal type.

  The fineness of the synthetic short fiber is preferably 0.007 to 1.1 dtex, and more preferably 0.02 to 0.6 dtex. When the fineness of the synthetic short fiber exceeds 1.1 dtex, the number of fibers in the thickness direction is reduced, so that it is difficult for the coating liquid to break through or to reduce the thickness. Moreover, unevenness | corrugation becomes large and the surface smoothness after coating may be impaired. When the fineness of the synthetic short fiber is less than 0.007 dtex, stable production of the fiber becomes difficult.

  The fiber length of the synthetic short fiber is preferably 1 mm or more and 10 mm or less, and more preferably 1 mm or more and 6 mm or less. If the fiber length exceeds 10 mm, formation may be poor. On the other hand, when the fiber length is less than 1 mm, the mechanical strength of the base material becomes low, and the base material may be damaged during impregnation or coating.

  In the base material for a lithium secondary battery of the present invention, the total content of the fibrillated heat-resistant fiber and the synthetic single fiber is preferably 50 to 100% by mass, more preferably 60 to 100% by mass, and 80 to 100% by mass. Is more preferable. If the total content is less than 50% by mass, the coating liquid may be broken through. The mass ratio of fibrillated heat resistant fiber: synthetic short fiber is preferably 7: 1 to 1:19, more preferably 5: 1 to 3:17, and even more preferably 4: 1 to 1: 5. If the ratio of the synthetic short fibers is more than 1:19, the coating liquid may fall through. If it is less than 7: 1, the puncture strength of the base material becomes weak, and the base material is damaged during handling or coating. There is a case.

  The base material for lithium secondary batteries of the present invention may contain fibers other than fibrillated heat-resistant fibers and synthetic short fibers. Examples thereof include cellulose fibers, pulped and fibrillated cellulose fibers, fibrils made of synthetic resin, pulped products, fibrillated products, and inorganic fibers. Examples of the inorganic fiber include glass, alumina, silica, ceramics, and rock wool. When inorganic fiber is contained, it is preferable because the heat-resistant dimensional stability and puncture strength of the base material are improved. The cellulose fiber may be either natural cellulose or regenerated cellulose.

  4-45 micrometers is preferable, as for the thickness of the base material for lithium secondary batteries of this invention, 6-30 micrometers is more preferable, and 8-20 micrometers is further more preferable. If it exceeds 45 μm, the resistance value of the separator may be high, and if it is less than 4 μm, the strength of the base material becomes too weak and may be damaged during handling or coating of the base material.

Density of the lithium secondary battery base material of the present invention is preferably ^{0.250~0.700g / cm 3, 0.400~0.600g / cm} 3 is more preferable. When the density is less than 0.250 g / cm ³ , the coating liquid may be seen through, and when it exceeds 0.700 g / cm ³ , the resistance value of the separator may be increased.

  The base material for lithium secondary batteries of the present invention is produced by a wet papermaking method. In the wet papermaking method, fibers are dispersed in water to form a uniform papermaking slurry, and this papermaking slurry is rolled up with a papermaking machine to produce a wet nonwoven fabric. Examples of the paper machine include a circular paper machine, a long paper machine, an inclined paper machine, an inclined short paper machine, and a composite paper machine of these. In the process of manufacturing a wet nonwoven fabric, a hydroentanglement process may be performed as necessary. As the wet nonwoven fabric processing, heat treatment, calendar treatment, thermal calendar treatment, or the like may be performed.

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

Examples 1, 4, 5, 6, 7
Slurries 1, 4, 5, 6, and 7 shown in Table 1 were prepared, wet papermaking was performed using a circular paper machine, both surfaces were brought into contact with a metal roll heated to 200 ° C., and heat treatment was performed. Then, the thickness was adjusted to produce substrates of Examples 1, 4, 5, 6, and 7.

Examples 2 and 3
Slurries 2 and 3 shown in Table 1 were prepared, wet papermaking was carried out using a circular paper machine, and then calendered to adjust the thickness to produce substrates of Examples 2 and 3.

(Comparative Example 1)
After preparing slurry 8 shown in Table 1 and wet papermaking using a circular paper machine, both sides were brought into contact with a metal roll heated to 200 ° C., heat treated, and further calendered to adjust thickness, Comparative Example 1 substrate was produced.

(Comparative Examples 2, 4, 5)
Slurries 9, 11, and 12 shown in Table 1 were prepared, wet papermaking was performed using a circular paper machine, and then the thickness was adjusted by calendering to prepare substrates of Comparative Examples 2, 4, and 5.

(Comparative Example 3)
The slurry 10 shown in Table 1 was prepared, and wet papermaking was performed using a circular paper machine. However, since the fibrillated wholly aromatic polyamide fiber had no binding force, a substrate could not be produced.

  In Table 1, A1, A3, and A6 are fibrillated wholly aromatic polyamides, A2 is fibrillated wholly aromatic polyester, A4 is fibrillated polyimide, and A5 is fibrillated polyphenylene sulfide. As shown in Table 1. B1 is a polyester short fiber having a fineness of 0.06 dtex and a fiber length of 3 mm (manufactured by Teijin Fibers, trade name: TP04N). Trade name: TJ04CN), B3 is 0.1 dtex fine fiber, 3 mm long acrylic short fiber (Mitsubishi Rayon, trade name: Bonnell (registered trademark)), B4 is 0.1 dtex fine fiber, 3 mm long nylon 6,6 Short fiber, B5 means polyvinyl alcohol short fiber (manufactured by Kuraray, trade name: VPB107-1x3). C1 means Canadian hemp pulp with a standard freeness of 500 ml.

[Preparation of separator]
Plate boehmite (average particle size: 1 μm, aspect ratio: 10), 1000 g of N-methylpyrrolidone, and 375 g of polyvinylidene fluoride are placed in a container and stirred with a stirrer (trade name: Three-One Motor, Shinto Kagaku Co., Ltd.). The mixture was stirred and dispersed for 1 hour to obtain a uniform slurry. This slurry was applied to the base materials of Examples and Comparative Examples using an applicator and dried with an explosion-proof dryer to obtain a long separator having a boehmite layer with a thickness of 3 μm per side.

[Evaluation]
The following evaluation was performed about the base material of an Example and a comparative example, and the separator produced using this base material, and the result was shown in Table 2.

<Thickness of base material>
The thickness was measured according to JIS C2111, and the average value was calculated.

<Density of base material>
The density was measured according to JIS C2111.

<Back-through>
In the manufacturing process of the separator, ○ when the coating liquid did not penetrate the substrate at all, △, when the back surface did not interfere with the roll of the coating device In the case where the back surface was stuck to the roll and a trouble such as inability to perform smooth coating was observed, the case was evaluated as x.

<Surface smoothness>
About the separator, the thickness of arbitrary 10 places was measured, the standard deviation (micrometer) was computed, and it was set as the parameter | index of surface smoothness. It means that it is excellent in surface smoothness, so that the value of a standard deviation is small.

<Electrolytic solution retention>
The separator was cut to a width of 100 mm × 100 mm and immersed in the electrolyte for 1 minute, then suspended for 1 minute to cut off the excess electrolyte, and the mass W1 of the separator was measured. The value obtained by subtracting the mass W0 of the separator before holding the electrolytic solution from W1 and dividing the value W2 by W0 and multiplying by 100 was defined as the electrolytic solution retention rate (%). As the electrolytic solution, a mixed solution in which 1 mol / l of LiPF ₆ was dissolved was used. The mixed solution is ethylene carbonate and diethyl carbonate in a mass ratio of 3: 7.

<Heat resistance>
The separator was cut into 200 mm width × 200 mm length and left to stand in a constant temperature dryer at 200 ° C. for 3 hours, the shrinkage in the length direction and the width direction was calculated, and the average value is shown in Table 2.

  Since the base materials for lithium secondary batteries of Examples 1 to 7 are made of a wet nonwoven fabric containing fibrillated heat-resistant fibers and synthetic short fibers, the back-through of the coating liquid containing filler particles is suppressed, and the coated surface The surface smoothness was good, the electrolyte solution retention was high, and the heat resistance was excellent.

  On the other hand, the base materials for lithium secondary batteries of Comparative Examples 1 and 2 do not contain fibrillated heat-resistant fibers, but are composed only of synthetic short fibers, so that the coating liquid containing filler particles is broken through. The surface smoothness of the coated surface was poor. Further, the filler particles were filled in the base material, and the pores inside the base material were closed, so that the electrolyte solution retention rate was poor. Since it does not contain fibrillated heat resistant fibers, the heat resistance was poor.

  In Comparative Example 3, an attempt was made to produce a base material for a lithium secondary battery using only fibrillated heat-resistant fibers, but the base material for a lithium secondary battery could not be produced because the fibers had no binding force. .

  The base material for the lithium secondary battery of Comparative Example 4 does not contain synthetic short fibers, and is composed of fibrillated heat resistant fibers and natural pulp. Therefore, the heat resistance is good and the back-through of the coating liquid containing filler particles is suppressed. However, since the base material was too dense, the electrolyte retention rate was poor.

  The base material for the lithium secondary battery of Comparative Example 5 does not contain fibrillated heat-resistant fibers, and is composed of synthetic short fibers and natural pulp. Therefore, the heat resistance is somewhat poor, and filler particles are filled inside the base material. Since the pores inside the substrate were closed, the electrolyte retention rate was poor and the surface smoothness of the coated surface was poor.

  The base material for lithium secondary batteries of this invention can be used conveniently for lithium ion secondary batteries, such as a lithium ion secondary battery and a lithium ion polymer secondary battery.

Claims (2)

  A base material for a lithium secondary battery, comprising a wet nonwoven fabric containing fibrillated heat-resistant fibers and synthetic short fibers as essential components.   The base material for a lithium secondary battery according to claim 1, wherein the fibrillated heat-resistant fiber is a fibrillated wholly aromatic polyamide fiber.