Electrode sheet and method for manufacturing the same

Abstract

To provide an electrode which reduces a binder amount required for fixing cathode active material particles, hardly swells with an electrolyte, maintains a space sufficient for Li ion movements and is not deformed even at high temperatures, and a manufacturing method thereof.SOLUTION: The present invention relates to an electrode sheet including: aramid nanofibers of which the average fiber diameter is equal to or less than 100 nm; aramid pulp having a fibril structure; an electrode active material; a conductive assistant agent; and a soluble binder which is soluble in a non-aqueous polar solvent or an electrolytic solution. The electrode active material is held by the aramid nanofibers and the aramid pulp. The content of the aramid nanofibers is 0.05 to 0.9 wt.% with the electrode sheet defined as a reference, the aramid pulp is 0.05 to 0.9 wt.% with the electrode sheet defined as a reference, and the content of the soluble binder is 0.1 to 1.0 wt.% with the electrode sheet defined as a reference.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an electrode sheet and a method for manufacturing the same, and more particularly to an electrode sheet suitable for a positive electrode of a non-aqueous Li-ion secondary battery and a method for manufacturing the same.

Background Art

[0002] Li-ion secondary batteries having high energy density and capacity are expanding their applications to small portable terminals, personal computers, electric vehicles, and the like. The positive electrode of a Li-ion secondary battery is formed by bonding a sheet of an electrode active material such as a Li metal oxide having a high voltage and capacity to a conductive sheet such as an aluminum foil.

[0003] In recent years, high-capacity positive electrode active materials have been developed. However, since these cause performance degradation due to the incorporation of moisture, a non-aqueous polar solvent (such as N-methyl-2-pyrrolidone) or an electrolyte solution (electrolyte) is used as the solvent for forming the slurry for forming the powder sheet. Polyvinylidene fluoride (PVdF) is widely used as a solubility binder that is soluble or swellable in these solvents.

[0004] PVdF is usually dissolved in a non-polar solvent such as N-methyl-2-pyrrolidone (NMP), and appropriately mixed with carbon black (CB) or carbon nanotube (CNT) carbon materials, which are conductive aids, at 2 to 3% by weight with respect to the electrode active material, and coated on a current collector electrode foil to form a sheet.

[0005] Since PVdF is a polymer that is difficult to crystallize, it has a heat distortion temperature (about 90°C) and a melting temperature (about 140 to 170°C). During heat generation due to high-speed charge and discharge of LiB, deformation occurs due to the contraction and expansion of the electrode caused by the movement of Li ions, and as a result, destruction of the conductive network of LiB may occur, which may cause a decrease in capacity. In addition, in the case of abnormal heat generation such as a short circuit, the temperature becomes 200°C or higher, making it difficult to maintain the form of the electrode.

[0006] Assuming that the shape of the positive electrode active material is various but spherical with the same diameter, the porosity in the closest packing is about 27%, but actually, due to the particle size distribution, the porosity is estimated to be 20 - 25%.

[0007] Also, since the specific gravities of the normal positive electrode active material and PVdF are 4 - 5 g / cc and 1.75 g / cc respectively, if 2% by weight of PVdF is used, the space occupancy rate corresponds to about 6% even assuming no swelling, which occupies 1 / 3 - 1 / 4 of the voids. Furthermore, when swollen with the electrolyte, a large space is used.

[0008] Since charge and discharge are carried out by the movement of Li ions in the electrolyte filling the voids, inhibiting the movement of Li ions by the binder leads to an increase in diffusion resistance, which is not preferable. Generally, there is an influence of moisture as a factor that reduces the activity of the positive electrode active material. It is preferable that the electrode is dried at a high temperature, but when dried at a temperature higher than the melting point of PVdF, deformation is a concern.

[0009] In order to solve such problems, the present inventors discovered that an electrode rich in heat resistance can be created by using an aromatic polyamide fibrid as a binder (see Patent Document 1), but in the case of a Li-ion secondary battery, the fixing of the conductive active material was insufficient.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention has been made to solve the above problems, and an object thereof is to reduce the amount of a heat-resistant fiber binder that is insoluble in a non-aqueous polar solvent or an electrolyte solution required for fixing positive electrode active material particles, to be less likely to swell in an electrolytic solution and maintain sufficient space for Li ion movement, and to provide an electrode that does not deform even at high temperatures and a method for manufacturing the same.

Means for Solving the Problems

[0012] As a result of intensive studies, the present inventors have found that by using aramid nanofibers and aramid pulp having a fibril structure as a heat-resistant fiber binder that is insoluble in a non-aqueous polar solvent or an electrolyte solution, it is possible to reduce the amount of binder required for sheeting the positive electrode active material of a Li ion secondary battery, and an electrode sheet with low resistance and little performance degradation due to repeated charge and discharge can be obtained.

[0013] That is, according to the present invention, the problems of the invention are achieved by the following 1 to 6. 1. An electrode sheet containing aramid nanofibers having an average fiber diameter of 100 nm or less, aramid pulp having a fibril structure, an electrode active material, a conductive assistant, and a soluble binder soluble in a non-aqueous polar solvent or an electrolyte solution, wherein the electrode active material is held by the aramid nanofibers and the aramid pulp, the content of the aramid nanofibers is 0.05 to 0.9% by weight based on the electrode sheet, the aramid pulp is 0.05 to 0.9% by weight based on the electrode sheet, and the content of the soluble binder is 0.1 to 1.0% by weight based on the electrode sheet. 2. The electrode sheet according to item 1 above, wherein the aramid nanofibers are made of para-type wholly aromatic polyamide. 3. The electrode sheet according to item 1 above, wherein the aramid pulp is made of para-type wholly aromatic polyamide. 4. The electrode sheet according to any one of items 1 to 3 above, wherein the electrode active material contains a Li metal oxide. 5. The electrode sheet according to any one of the preceding items 1 to 4, which is an electrode sheet used for the positive electrode of a lithium ion secondary battery. 6. A method for producing the electrode sheet according to any one of the preceding items 1 to 5, comprising mixing an aramid nanofiber having an average fiber diameter of 100 nm or less, an aramid pulp having a fibril structure, an electrode active material, a conductive assistant, and a soluble binder soluble in a non-aqueous polar solvent or an electrolyte solution in the non-aqueous polar solvent or the electrolyte solution to form a slurry, coating the slurry on an electrode current collector, and performing a pressing treatment as necessary.

Advantages of the Invention

[0014] The electrode sheet composed of the aramid nanofiber and the aramid pulp of the present invention is excellent in morphological stability even at high temperatures, and a heat-resistant fiber binder that is insoluble in the non-aqueous polar solvent or the electrolyte solution constituting the electrode can reduce the swelling by the electrolyte solution and minimize the surface coating of the electrode active material. Therefore, the industrial effect achieved is remarkable.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0016] According to the present invention, there is provided an electrode sheet containing aramid nanofibers having an average fiber diameter of 100 nm or less, aramid pulp having a fibril structure, an electrode active material, a conductive assistant, and a soluble binder that can be dissolved in a non-aqueous polar solvent or an electrolyte solution, wherein the electrode active material is retained by the aramid nanofibers and the aramid pulp, the content of the aramid nanofibers is 0.05 to 0.9% by weight based on the electrode sheet, the aramid pulp is 0.05 to 0.9% by weight based on the electrode sheet, and the content of the soluble binder is 0.1 to 1.0% by weight based on the electrode sheet.

[0017] Also, according to the manufacturing method of the present invention, an aramid nanofiber having an average fiber diameter of 100 nm or less, an aramid pulp having a fibril structure, an electrode active material, a conductive assistant, and a soluble binder that can be dissolved in a non-aqueous polar solvent or an electrolyte solution are mixed in a non-aqueous polar solvent or an electrolyte solution to form a slurry, which is then coated on an electrode current collector and, if necessary, press-treated to stably manufacture the electrode sheet.

[0018] In addition, by containing 0.1 to 1.0% by weight of a soluble binder that can be dissolved in a non-aqueous polar solvent or an electrolyte, the dropout of nanoparticles such as conductive assistants, which are more likely to drop out from the network structure formed by aramid nanofibers and aramid pulp, is suppressed, and the significant reduction in the total binder amount and the strong network formed by heat-resistant nanofibers suppress deformation at high temperatures, making it possible to obtain an electrode with excellent charge and discharge performance.

[0019] <Aramid (fully aromatic polyamide)> The aramid in the present invention is an all-aromatic polyamide. Examples of all-aromatic polyamides include para-type all-aromatic polyamides (such as poly-p-phenylene terephthalamide, poly-p-benzamide, poly-p-amide hydrazide, poly-p-phenylene terephthalamide-3,4-diphenyl ether terephthalamide, etc.), meta-type all-aromatic polyamides (such as poly-m-phenylene isophthalamide, etc.). Among them, para-type all-aromatic polyamides are preferred because they have high elasticity and high strength. For example, as fibers using para-type all-aromatic polyamides, there are poly-p-phenylene terephthalamide fibers (commercially available products include "Twaron (registered trademark)" manufactured by Teijin Limited, "Kevlar (registered trademark)" manufactured by Toray DuPont Co., Ltd., etc.), and coparaphenylene·3,4'-oxydiphenylene terephthalamide fibers (commercially available products include "Technora (registered trademark)" manufactured by Teijin Limited, etc.). As fibers using meta-type all-aromatic polyamides, there are poly-m-phenylene isophthalamide fibers (commercially available products include "Teijin Conex (registered trademark)" manufactured by Teijin Limited, "Nomex (registered trademark)" manufactured by DuPont, etc.).

[0020] <Aramid nanofiber> The aramid nanofiber in the present invention has an average fiber diameter of 100 nm or less, preferably 50 nm or less, more preferably 25 nm or less. The lower limit of the average fiber diameter is preferably 1 nm or more, more preferably 3 nm or more. Also, it is preferable that the aramid nanofiber does not have fibers with a fiber diameter of 500 nm or more.

[0021] The aspect ratio of the aramid nanofiber in the present invention, represented by fiber length / fiber diameter, is preferably 10 or more and 1,000 or less, more preferably 10 or more and 500 or less, and even more preferably 10 or more and 100 or less. When the aspect ratio is less than 10, it is difficult to develop an entangled structure of the fibers and difficult to form a fine mesh structure. Therefore, it may be difficult to exhibit the expected properties.

[0022] The specific surface area of the aramid nanofiber in the present invention is 50 m 2It is preferably at least / g, more preferably at least 80 m 2 / g. The upper limit of the specific surface area is preferably at most 200 m 2 / g, more preferably at most 150 m 2 / g. If the specific surface area is lower than the above range, a sufficient adhesion area with the electrode active material cannot be ensured, and good binder properties cannot be exhibited in a small amount. Also, if the specific surface area is higher than the above range, it becomes undesirable, such as a decrease in coatability due to thickening of the coating liquid and difficulty in maintaining the dispersion state.

[0023] Further, the aramid nanofibers in the present invention are wholly aromatic polyamides, and are preferably para-type wholly aromatic polyamides. Examples of para-type wholly aromatic polyamides include poly-p-phenylene terephthalamide, poly-p-benzamide, poly-p-amide hydrazide, poly-p-phenylene terephthalamide-3,4-diphenyl ether terephthalamide, etc., and it is preferably poly-p-phenylene terephthalamide fibers having orientation crystallinity (forming domains of a liquid crystal structure in a spinning solution).

[0024] <Production of Aramid Nanofibers> The production of aramid nanofibers used in the present invention is preferably carried out, for example, using para-type wholly aromatic polyamide fibers or para-type wholly aromatic polyamide pulp as raw materials, immersing and swelling the fibers in a highly affinity solvent, and further adding a strongly basic substance to cut the hydrogen bond sites, thereby generating the aramid nanofibers. The para-type wholly aromatic polyamide preferably used in the present invention is a polymer in which one or more divalent aromatic groups are linked by amide bonds. The aromatic groups may have two or more aromatic rings, and the aromatic rings may be directly bonded or bonded via oxygen or sulfur. Further, the hydrogen atoms of the divalent aromatic groups may be substituted with halides, lower alkyl groups, or phenyl groups. As the solvent used in the production of aramid nanofibers, an aprotic polar solvent is preferable, and specifically, dimethyl sulfoxide, dimethylacetamide, N-methyl-2-pyrrolidone, etc. may be mentioned. As the strongly basic substance used in the production of aramid nanofibers, potassium hydroxide, sodium hydroxide, barium hydroxide, calcium hydroxide, etc. may be mentioned.

[0025] Specifically, the production of aramid nanofibers in the present invention can be carried out by immersing para-type wholly aromatic polyamide (for example, poly-p-phenylene terephthalamide) fibers or pulp in dimethyl sulfoxide adjusted to be alkaline.

[0026] Para-type wholly aromatic polyamide (for example, poly-p-phenylene terephthalamide) forms domains of a liquid crystal structure in a spinning solution, and after being discharged from a capillary, the spinning solvent is washed with water to obtain the fibers. After cutting the weak bonds between the liquid crystal domains constituting the fibers by the above method under alkaline conditions, the obtained fibers are released into a highly compatible solvent to obtain highly elastic and high-strength cut fibers. Then, the obtained cut fibers are put into a poor solvent (water, alcohol, acetone, etc.) which is a dispersion solvent to isolate the aramid nanofibers.

[0027] In addition, para-type wholly aromatic polyamide (e.g., poly-p-phenylene terephthalamide) fibers with a diameter of 10 to 20 μm are cut into several millimeters, and para-type wholly aromatic polyamide (e.g., poly-p-phenylene terephthalamide) pulp can be obtained by performing a refiner treatment that applies mutual shearing in water. In such a refiner treatment, the fibers refined from the fiber surface by shear fracture of the liquid crystal interface do not completely separate but are in a branched state. The diameter of the refined fibers is 100 to 1000 nm, and the diameter of the central part of the raw material fibers is several μm. The pulp treated with a refiner can also be made into aramid nanofibers by the above treatment.

[0028] As other methods for refining aramid materials, methods that apply mechanical shearing force rather than the chemical treatment described above can be considered. However, in such methods, only a fibril structure having a fiber diameter on the nano order can be partially obtained. Therefore, since it also includes micro-order structures that are not fibrillated, a homogeneous entanglement structure on the nano order cannot be obtained, and the physical properties expected are not exhibited. In the present invention, it is preferable to form an entanglement structure with uniformly nano-sized fibers rather than partial nano-sizing.

[0029] <Aramid pulp> The aramid pulp in the present invention has a fibril structure (sometimes referred to as fibrid). The aramid pulp having a fibril structure can be obtained, for example, by a known method described in Japanese Patent Application Laid-Open No. 2014-501859. Specifically, for example, methods such as cutting poly-p-phenylene terephthalamide fibers into several millimeters, dispersing them in water, and forming fibrils by rubbing them against each other using a refiner may be used. Alternatively, it is also possible to use a structure obtained by injecting and coagulating a para-type aromatic polyamide solution into a poor solvent such as water.

[0030] In the aramid pulp of the present invention, the fibril structure preferably has a fiber diameter partially exceeding 100 nm, more preferably 300 nm or more. The fibril structure is configured like a trunk portion 2 and a branch portion (fibril portion) 3 as schematically shown in FIG. 4, and it is sufficient if either the trunk portion 2 or the branch portion 3 has a fiber diameter exceeding the above range. Also, the average fiber length is preferably 1000 μm or less. The lower limit of the average fiber length is preferably 100 μm or more, more preferably 500 μm or more.

[0031] The specific surface area of the aramid pulp in the present invention is preferably 10 m 2 / g or more and 40 m 2 / g or less, more preferably 20 m 2 / g or more and 30 m 2 / g or less. Thus, by using aramid pulp having a structure larger than that of aramid nanofibers, it becomes possible to increase the stress transmission distance in the network structure formed by a heat-resistant fiber binder insoluble in a non-aqueous polar solvent or an electrolyte solution.

[0032] <Fibrous binder> In the present invention, it is important to use aramid nanofibers and aramid pulp in combination as the fibrous binder. When only aramid pulp is used as the fibrous binder, as schematically shown in FIG. 4, it is difficult to sufficiently secure the specific surface area necessary for gripping the electrode active material 1, which is not preferable. Also, the coating liquid in which aramid pulp is dispersed has low liquid fluidity because the branch structure of the pulp holds the solvent, and it is difficult to coat it uniformly.

[0033] When only aramid nanofibers are used as the fibrous binder, as schematically shown in FIG. 5, homogeneous coatability and a high specific surface area are ensured. On the other hand, it is difficult to hold the electrode active material 1 having a particle size on the order of several tens of μm, and as a result, it is difficult to suppress the dropout of the electrode active material. This is presumably because the structural size of the aramid nanofibers 4 is small compared to the electrode active material, making it difficult to form a long-distance network connecting the electrode active materials. In addition, the coating liquid in which only the aramid nanofibers 4 are dispersed has high liquid fluidity and is excellent in homogeneous coatability. However, compared with the coating liquid containing aramid pulp, it is likely to cause dripping, and the thick coating coatability is reduced.

[0034] Based on the above, by using aramid pulp and aramid nanofibers in combination as the fibrous binder, as schematically shown in FIG. 6, a long-distance and strong network connecting the electrode active materials 1 is formed by the trunk part 2 and the branch part 3 of the aramid pulp, while the binder performance is expected to be expressed by the high specific surface area derived from the aramid nanofibers 4. In addition, the combined use of aramid pulp and aramid nanofibers is also expected to achieve both homogeneous coatability and thick coating coatability.

[0035] <Electrode active material> The electrode active material in the present invention is a substance capable of releasing and filling ions such as Li by charge and discharge, and is preferably a Li metal oxide. Specifically, cobalt lithium oxide, manganese lithium oxide, NMC (nickel·manganese·cobalt) Li oxide, etc. can be mentioned.

[0036] <Conductive aid> The conductive aid in the present invention is a conductive substance disposed on the surface of the electrode active material and between the electrode active material particles, and is preferably a carbon material from the viewpoint of electrochemical stability. Specifically, it is preferably carbon black (such as acetylene black, ketjen black, etc.), and carbon nanotubes (CNT) or conductive polymers (such as polyacene) may be used in combination.

[0037] The soluble binder in the present invention is a substance that can be dissolved in a non-aqueous polar solvent and is not decomposed by the potential difference generated by the electrode active material. A heat-resistant polymer with little change even at high temperatures is preferable. Specifically, polyvinylidene fluoride (PVdF), polyimide, meta-aramid, etc. can be mentioned, and among them, polyvinylidene fluoride (PVdF) is particularly preferable.

[0038] <Electrode Sheet and Its Manufacture> The electrode sheet in the present invention is composed of the above-mentioned electrode active material, conductive assistant, aramid nanofiber, aramid pulp, and soluble binder. The electrode active material and the conductive assistant are held by the aramid nanofiber, aramid pulp, and soluble binder. The content of the aramid nanofiber is 0.05 to 0.9% by weight based on the electrode sheet, preferably 0.06 to 0.5% by weight, more preferably 0.07 to 0.3% by weight, and even more preferably 0.08 to 0.2% by weight. Also, the content of the aramid pulp is 0.05 to 0.9% by weight based on the electrode sheet, preferably 0.1 to 0.7% by weight, and more preferably 0.2 to 0.5% by weight. Further, the content of the soluble binder is 0.1 to 1.0% by weight based on the electrode sheet, preferably 0.2 to 0.9% by weight, and more preferably 0.3 to 0.8% by weight.

[0039] Aramid nanofibers can form hydrogen bonds with adjacent nanofibers through amide bonds, which are polymer structures, and can sheetify the electrode active material and the conductive assistant.

[0040] Note that if the content of aramid nanofibers and aramid pulp is less than 0.05% by weight, it is not preferable because they cannot be sheetified. Also, if it exceeds 0.9% by weight, the nanofibers and pulp tend to be unevenly distributed, which is not preferable.

[0041] Furthermore, in order to prevent the exfoliation of the conductive assistant due to the rubbing between particles caused by the expansion and contraction of the electrode active material due to the movement of Li ions during charge and discharge, it is preferable to use a small amount of the soluble binder in combination.

[0042] The electrode sheet described above can be manufactured by the following method. That is, for the electrode sheet of the present invention, the weight ratio of aramid nanofiber, aramid pulp, and soluble binder is 0.05 to 0.9: 0.05 to 0.9: 0.1 to 1.0. An electrode active material is added to a conductive auxiliary agent of 4% by weight or less based on the electrode sheet, made into a slurry with a non-aqueous polar solvent, coated on a current collector sheet or film, and dried to obtain it.

[0043] The sheet obtained by the above method is constantly subjected to a pressing process using a heating calendar roll or the like to reduce voids and increase density, increase the electrode active material density per unit volume, and increase the secondary battery capacity. Needless to say, it is required that the electrode sheet is not damaged by heat and pressure.

[0044] More specifically, for example, para-type wholly aromatic polyamide fiber or para-type wholly aromatic polyamide pulp is added with a strongly basic substance under an aprotic polar solvent to form nanofibers, and aramid nanofibers with an average fiber diameter of 100 nm or less obtained, aramid pulp having a fibril structure, an electrode active material, a conductive auxiliary agent, and a soluble binder soluble in a non-aqueous polar solvent or an electrolyte solution are mixed in a non-aqueous polar solvent or an electrolyte solution to form a slurry, which is coated on an electrode current collector and, if necessary, pressed to obtain an electrode sheet.

[0045] By this operation, a sheet with a predetermined thickness and no change in active thickness is obtained. A secondary battery (Li-ion secondary battery) is made by laminating it with a separator and a negative electrode sheet and injecting an electrolyte solution. In addition, the electrode sheet preferably has an electric conductivity of 30×10 -3 S / cm or more, more preferably 33×10 -3 S / cm or more, and even more preferably 35×10 -3It is above S / cm. Also, the electrolyte penetration rate is preferably 230 to 400 seconds, more preferably 250 to 400 seconds, and even more preferably 300 to 400 seconds. Moreover, it is preferable that the single cell performance at both 25°C and 70°C is 95% or more, and more preferably 96% or more.

Examples

[0046] Hereinafter, the present invention will be described in detail with reference to examples and comparative examples, but the scope of the present invention is not limited to the following examples and comparative examples. Each characteristic value in the examples was measured by the following method.

[0047] <Average fiber diameter> Using a scanning electron microscope (manufactured by JEOL Ltd., model: JSM-6330F), the structure of the sample was observed. From the image observed at a magnification setting of 50,000 times, an image area of 1,800 nm to 2,000 nm in width and 1,200 nm to 1,500 nm in height was selected. The selected image area was further divided into 4 equal parts vertically and 4 equal parts horizontally to define a total of 16 grid areas A1-D4. One sample existing within each grid area was selected, and the average value of the fiber diameters of the selected samples measured on the image was adopted as the average fiber diameter (see Figures 2 and 3).

[0048] <Specific surface area measurement> Using a flow-type specific surface area automatic measuring device (FlowSorb manufactured by Shimadzu Corporation), the specific surface area of the target sample was measured. Helium with a known adsorbed occupancy area on the sample surface was adsorbed at liquid nitrogen temperature, and the specific surface area of the sample was determined from the amount of helium gas adsorbed.

[0049] (Coating liquid characteristics) <Homogeneous coatability> The state of the coating liquid (the thickness of the coating liquid immediately after coating) immediately after coating the prepared coating liquid by die coating was visually observed to evaluate whether it could be coated homogeneously without unevenness. ○: Can be coated homogeneously △: There are partial unevenness, but it can be coated ×: Coating unevenness exists throughout, and coating with a die coater is difficult.

[0050] <Thick coating property> After coating the coating liquid on a predetermined coating area, it was left standing for 10 minutes in a horizontal place under normal temperature and normal pressure environment. The prepared coating liquid was coated on a commercially available aluminum foil using a die coater (clearance: 100 μm), and the change over time (area change (area reduction rate)) of the coating liquid applied to the coating area was observed. The evaluation criteria were as follows. ○: The area change (area reduction rate) of the coating area is less than 5%. △: The area change (area reduction rate) of the coating area is 5% or more and less than 20%. ×: The area change (area reduction rate) of the coating area is 20% or more.

[0051] (Electrode coating property) (Deformation during drying) After coating the coating liquid on a predetermined coating area, it was dried for 20 minutes in a horizontal place under a hot plate (100 °C) and normal pressure environment. The prepared coating liquid was coated on a commercially available aluminum foil using a die coater (clearance: 100 μm), and the coating liquid applied to the coating area was observed. The evaluation criteria were as follows. ○: The area change of the coating area is less than 5%. △: The area change of the coating area is 5% or more and less than 10%. ×: The area change of the coating area is 10% or more.

[0052] (Axis winding test) As an index of workability in the process of winding the electrodes of cylindrical and rectangular batteries, an electrode sheet (width 10 mm, length 50 mm) made of a 4 mmφ SUS wire was wound, and the presence or absence of cracks and peeling was evaluated. ○: No cracks or peeling. △: Cracks and peeling occur partially. ×: Cracks and peeling occur overall.

[0053] (Powder shedding) The surface of the prepared electrode sheet was gently rubbed with a wiping cloth (Kimwipe), and it was judged by dirt. ○: No dirt removal △: Slight dirt ×: Dirt

[0054] [Example 1] <Preparation of aramid nanofibers> · Aramid pulp: 10 g (10 g of Twaron 1000 fibers cut to 6 mm by Teijin Aramid Co., Ltd. were boiled and washed in 1000 g of boiling water for 30 minutes, washed with water after cooling, and dried). · Dimethyl sulfoxide (DMSO) >99% 80 g from Tokyo Chemical Industry Co., Ltd., · Potassium hydroxide (KOH) 10 g from Tokyo Ohka Kogyo Co., Ltd. The above three components were put into a planetary mixer and stirred at 70 °C for 2 hours. After stirring, the aramid pulp disappeared in shape, and a red translucent high-viscosity solution was obtained.

[0055] The obtained red translucent solution containing nanofibers was diluted with 3 times the amount of DMSO and gradually added to 20 liters of water while stirring to precipitate the nanofibers of poly-p-phenylene terephthalamide. At this time, the solution was a yellowish-red slurry. The required amount of sulfuric acid was added to this to neutralize KOH, and the poly-p-phenylene terephthalamide nanofibers were collected by filtration.

[0056] Subsequently, washing and stretching dehydration were performed 3 to 5 times using distilled water or ion-exchanged water to remove the solvent and salt. Distilled water was added to the obtained water solid content containing poly-p-phenylene terephthalamide nanofibers so that the solid content concentration became 1%, and a poly-p-phenylene terephthalamide nanofiber-dispersed aqueous solution was obtained using a stone mortar type grinding and kneading machine (Super Mascoloider) manufactured by Masuda Sangyo Co., Ltd. The obtained poly-p-phenylene terephthalamide nanofiber dispersion was diluted with isopropyl alcohol, dropped onto a preparate, and dried, and it was confirmed that the average fiber diameter was 20 nm (see Figures 1 and 2).

[0057] 0.25 parts by mass of the above aramid nanofibers and microfibrillated aramid pulp (Twaron (registered trademark): 1094 pulp manufactured by Teijin Limited) treated with a high-pressure homogenizer. Specific surface area: 21 m 2 / g) 0.25 parts by mass, and 0.5 parts by mass of PVdF (manufactured by Kuraray or Nippon Zeon) were dispersed in NMP, and a positive electrode active material (Ni:Mn:Co = 1:1:1) LiO 2 95 parts by mass, 4 parts by mass of acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) were mixed so that the solid content concentration became 70%, and dispersed at 2000 rpm for 5 minutes using a thin-film swirling high-speed mixer (Filmix) to prepare an electrode slurry.

[0058] A film was formed on a commercially available aluminum foil using a die coat, and dried on a hot plate (100 °C) for 20 minutes. The dried electrode sheet was pressed using a metal roll calender (treatment temperature: 60 °C, treatment line pressure: 2 kN / cm) to obtain a coated electrode sheet. The obtained electrode sheet was punched into a predetermined size of 50 mm × 50 mm and dried at 170 °C for 10 hours to obtain an evaluation electrode. The obtained evaluation electrode sheet was evaluated according to the above-described evaluation method.

[0059] [Example 2] 0.25 parts by mass of aramid nanofibers and an aramid jet coagulum (obtained by injecting and coagulating an aramid polymer solution in a poor solvent. Specific surface area: 26 m 2 / g), 0.5 parts by mass of PVdF, 4 parts by mass of acetylene black, and 95 parts by mass of the positive electrode active material were used. An electrode sheet was prepared and evaluated in the same manner as in Example 1 except for the changes.

[0060] [Comparative Example 1] An electrode sheet was prepared and evaluated in the same manner as in Example 1 except that 1.0 part by mass of PVdF, 4 parts by mass of acetylene black, and 95 parts by mass of the positive electrode active material were changed.

[0061] [Comparative Example 2] An electrode sheet was prepared and evaluated in the same manner as in Example 1, except that the amount was changed to 0.5 parts by mass of aramid nanofibers, 0.5 parts by mass of PVdF, 4 parts by mass of acetylene black, and 95 parts by mass of the positive electrode active material.

[0062] [Comparative Example 3] An electrode sheet was prepared and evaluated in the same manner as in Example 1, except that the amount was changed to 0.5 parts by mass of microfibrillated aramid pulp, 0.5 parts by mass of PVdF, 4 parts by mass of acetylene black, and 95 parts by mass of the positive electrode active material.

[0063] [Comparative Example 4] An electrode sheet was prepared and evaluated in the same manner as in Example 1, except that the amount was changed to 0.5 parts by mass of aramid jet coagulum, 0.5 parts by mass of PVdF, 4 parts by mass of acetylene black, and 95 parts by mass of the positive electrode active material.

[0064]

Table 1

Industrial Applicability

[0065] The electrode sheet of the present invention is an electrode sheet suitable for a positive electrode of a non-aqueous Li-ion secondary battery.

Explanation of Symbols

[0066] 1 Positive electrode active material 2 Aramid pulp (dry part) 3 Aramid pulp (fibril part) 4 Aramid nanofibers

Claims

1. An electrode sheet comprising: aramid nanofibers having an average fiber diameter of 100 nm or less; aramid pulp having a fibril structure; an electrode active material; a conductive assistant; and a soluble binder that is soluble in a non-aqueous polar solvent or an electrolyte solution, An electrode sheet, characterized in that an electrode active material is held by aramid nanofibers and aramid pulp, the content of the aramid nanofibers is 0.05 to 0.9% by weight based on the electrode sheet, the content of the aramid pulp is 0.05 to 0.9% by weight based on the electrode sheet, and the content of the soluble binder is 0.1 to 1.0% by weight based on the electrode sheet.

2. The electrode sheet according to claim 1 , wherein the aramid nanofibers are made of para-type wholly aromatic polyamide.

3. 2. The electrode sheet according to claim 1, wherein the aramid pulp comprises a para-type wholly aromatic polyamide.

4. The electrode sheet according to any one of claims 1 to 3, wherein the electrode active material contains Li metal oxide.

5. The electrode sheet according to any one of claims 1 to 4, which is an electrode sheet used for a positive electrode of a Li-ion secondary battery.

6. 6. A method for producing an electrode sheet according to any one of claims 1 to 5, comprising the steps of mixing aramid nanofibers having an average fiber diameter of 100 nm or less, aramid pulp having a fibril structure, an electrode active material, a conductive assistant, and a soluble binder that is soluble in a non-aqueous polar solvent or an electrolyte solution with a non-aqueous polar solvent or an electrolyte solution to form a slurry, and coating the slurry on an electrode current collector.

Images