JP7690349B2 - Electrode sheet and its manufacturing method - Google Patents

Description

The present invention relates to an electrode sheet and a manufacturing method thereof, and in particular to an electrode sheet suitable for a positive electrode for a non-aqueous Li-ion secondary battery and a manufacturing method thereof.

Li-ion secondary batteries, which have high energy density and capacity, are being used in a wide range of applications, including small portable terminals, personal computers, and electric vehicles.
The positive electrode of a Li-ion secondary battery is made by laminating a sheet of electrode active material such as Li metal oxide, which has high voltage and capacity, to a conductive sheet such as aluminum foil.

In recent years, high-capacity positive electrode active materials have been developed. However, the inclusion of moisture in these materials can cause a decrease in performance. For this reason, 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 used to form the powder sheet.
Polyvinylidene fluoride (PVdF) is widely used as a soluble binder that is soluble or swellable in these solvents.

PVdF is usually dissolved in a non-polar solvent such as N-methyl-2-pyrrolidone (NMP), and mixed at 2-3% by weight of the electrode active material with an appropriate amount of conductive additive carbon black (CB) or carbon nanotube (CNT) carbon material, which is then coated onto a collector electrode foil to form a sheet.

PVdF is a polymer that does not easily crystallize, and has a heat distortion temperature (approximately 90°C) and melting temperature (approximately 140-170°C). When heat is generated by rapid charging and discharging of LiB, the movement of Li ions causes the electrodes to shrink and expand, resulting in deformation, which can lead to the destruction of the LiB's conductive network and a decrease in capacity. In addition, abnormal heating such as a short circuit can reach temperatures of over 200°C, making it difficult to maintain the shape of the electrode.

Although the shape of the positive electrode active material varies, assuming that they are spheres of the same diameter, the porosity in close packing is approximately 27%, but in reality, since there is a particle size distribution, the porosity is estimated to be 20-25%.

In addition, since the specific gravities of normal positive electrode active materials and PVdF are 4 to 5 g/cc and 1.75 g/cc, respectively, if 2 wt % of PVdF is used, the space occupancy rate is equivalent to about 6% even assuming no swelling, which occupies 1/3 to 1/4 of the voids.
Furthermore, when it swells with the electrolyte, it uses a large space.

Since charging and discharging are performed by the movement of Li ions through the electrolyte filling the voids, it is not preferable to inhibit the movement of Li ions by the binder, as this leads to an increase in diffusion resistance.
Generally, moisture is a factor that reduces the activity of positive electrode active materials, and it is preferable to dry the electrode at high temperatures. However, there is a concern that the electrode may be deformed if it is dried at a temperature higher than the melting point of PVdF.

To solve this problem, the inventors discovered that it was possible to create a heat-resistant electrode by using aromatic polyamide fibrids as a binder (see Patent Document 1), but in the case of Li-ion secondary batteries, the conductive active material was not sufficiently fixed.

JP 2007-158163 A

The present invention has been made to solve the above problems, and its purpose is to provide an electrode and a method for manufacturing the same that reduces the amount of heat-resistant fiber binder that is insoluble in non-aqueous polar solvents or electrolyte solutions, which is necessary to fix the positive electrode active material particles, and that is not easily swollen by electrolyte solutions, maintains sufficient space for Li ion movement, and does not deform even at high temperatures.

As a result of intensive research, the 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 non-aqueous polar solvents or electrolyte solutions, it is possible to reduce the amount of binder required to form a sheet from the positive electrode active material of a Li-ion secondary battery, and to obtain an electrode sheet that has low resistance and little performance degradation due to repeated charging and discharging.

That is, according to the present invention, the objects 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 that can be dissolved 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 the above item 1, wherein the aramid nanofibers are made of para-type wholly aromatic polyamide.
3. The electrode sheet according to the above paragraph 1, wherein the aramid pulp comprises a para-type wholly aromatic polyamide.
4. The electrode sheet according to any one of the above items 1 to 3, wherein the electrode active material contains Li metal oxide.
5. The electrode sheet according to any one of items 1 to 4 above, wherein the electrode sheet is an electrode sheet used for a positive electrode of a Li-ion secondary battery.
6. A method for producing an electrode sheet according to any one of items 1 to 5 above, comprising 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 can be dissolved in a non-aqueous polar solvent or an electrolyte solution with a non-aqueous polar solvent or electrolyte solution to form a slurry, which is then coated on an electrode current collector, and if necessary, subjected to a pressing treatment.

The electrode sheet made of the aramid nanofibers and aramid pulp of the present invention has excellent shape stability even at high temperatures, and the heat-resistant fiber binder that composes the electrode is insoluble in the non-aqueous polar solvent or electrolyte solution, so it is less likely to swell with the electrolyte and the surface coverage of the electrode active material can be reduced, resulting in exceptional industrial effects.

1 is an example showing a structural observation image of the aramid nanofiber of the present invention. FIG. 2 is an example of a structural observation image enlarged from FIG. 1 for calculating the average fiber diameter. FIG. 2 is an explanatory diagram regarding a method for calculating an average fiber diameter. FIG. 1 is a schematic diagram of an electrode sheet using only aramid pulp as a binder. FIG. 1 is a schematic diagram of an electrode sheet using only aramid nanofibers as a binder. FIG. 1 is a schematic diagram of an electrode sheet using aramid pulp and aramid nanofibers together as a binder.

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, characterized in that the electrode active material is held by the aramid nanofibers and aramid pulp, the content of the aramid nanofibers is 0.05 to 0.9 wt % based on the electrode sheet, the content of the aramid pulp is 0.05 to 0.9 wt % based on the electrode sheet, and the content of the soluble binder is 0.1 to 1.0 wt % based on the electrode sheet.

In addition, according to the manufacturing method of the present invention, 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 are mixed with a non-aqueous polar solvent or an electrolyte solution to form a slurry, which is then coated onto an electrode current collector and pressed as necessary, thereby making it possible to stably manufacture an electrode sheet.

In addition, by including 0.1 to 1.0% by weight of a soluble binder that can be dissolved in a non-aqueous polar solvent or electrolyte, the loss of nanoparticles such as conductive assistants, which tend to fall off due to the network structure formed by the aramid nanofibers and aramid pulp, is suppressed. The significant reduction in the total binder amount and the strong network of the heat-resistant nanofibers suppress deformation at high temperatures, making it possible to obtain an electrode with excellent charge/discharge performance.

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

<Aramid nanofiber>
The aramid nanofibers in the present invention have an average fiber diameter of 100 nm or less, preferably 50 nm or less, and more preferably 25 nm or less. The lower limit of the average fiber diameter is preferably 1 nm or more, and more preferably 3 nm or more. In addition, it is preferable that the aramid nanofibers do not have a fiber diameter of 500 nm or more.

The aramid nanofibers of the present invention preferably have an aspect ratio, expressed as fiber length/fiber diameter, of 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. If the aspect ratio is less than 10, it is difficult for the fibers to form an entangled structure and a fine knit structure. This may make it difficult to achieve the expected characteristics.

The specific surface area of the aramid nanofiber in the present invention is preferably 50 m ² /g or more, more preferably 80 m ² /g or more. The upper limit of the specific surface area is preferably 200 m ² /g or less, more preferably 150 m ² /g or less. If the specific surface area is lower than the above range, a sufficient adhesion area with the electrode active material is not ensured, and good binder properties are not achieved even with a small amount. In addition, if the specific surface area is higher than the above range, the coating liquid thickens, which is undesirable because of a decrease in coatability and difficulty in maintaining the dispersed state.

The aramid nanofibers in the present invention are fully aromatic polyamides, and preferably para-type fully aromatic polyamides. Preferred examples of para-type fully aromatic polyamides include poly-p-phenylene terephthalamide, poly-p-benzamide, poly-p-amide hydrazide, and poly-p-phenylene terephthalamide-3,4-diphenyl ether terephthalamide, and are preferably poly-p-phenylene terephthalamide fibers having oriented crystallinity (forming domains of a liquid crystal structure in the spinning solution).

<Production of aramid nanofibers>
The aramid nanofibers used in the present invention are preferably produced by, for example, using para-type wholly aromatic polyamide fibers or para-type wholly aromatic polyamide pulp as a raw material, immersing and swelling the fibers in a solvent with high affinity, and further adding a strongly basic substance to break the hydrogen bonds, resulting in the production of the aramid nanofibers. The para-type wholly aromatic polyamides that can be preferably used in the present invention are polymers in which one or more divalent aromatic groups are linked by amide bonds, and the aromatic groups may have two or more aromatic rings, and the aromatic rings may be directly bonded or may be bonded via oxygen or sulfur. In addition, the hydrogen atoms of the divalent aromatic groups may be substituted with halides, lower alkyl groups, or phenyl groups. In addition, the solvent used in producing the aramid nanofibers is preferably an aprotic polar solvent, and specific examples thereof include dimethyl sulfoxide, dimethylacetamide, and N-methyl-2-pyrrolidone. Examples of the strongly basic substances used in producing the aramid nanofibers include potassium hydroxide, sodium hydroxide, barium hydroxide, and calcium hydroxide.

Specifically, the aramid nanofibers of the present invention can be produced by immersing para-type wholly aromatic polyamide (e.g., poly-p-phenylene terephthalamide) fibers or pulp in alkaline dimethyl sulfoxide.

Para-type wholly aromatic polyamide (e.g., poly-p-phenylene terephthalamide) is obtained by forming domains of a liquid crystal structure in a spinning solution, discharging the solution from a capillary, and then washing off the spinning solvent with water. The weak bonds between the liquid crystal domains that make up the obtained fiber are cut under alkaline conditions using the above-mentioned method, and the obtained fiber is then liberated in a highly compatible solvent to obtain cut fibers with high elasticity and strength. The cut fibers obtained are then placed in a poor solvent (water, alcohol, acetone, etc.) that is a dispersion solvent, making it possible to isolate aramid nanofibers.

Para-type wholly aromatic polyamide (e.g., poly-p-phenylene terephthalamide) pulp can be obtained by cutting para-type wholly aromatic polyamide (e.g., poly-p-phenylene terephthalamide) fibers with a diameter of 10 to 20 μm into pieces of several mm and subjecting them to a refiner treatment in which shear is applied to the fibers in water. In this refiner treatment, the fibers that have been refined from the fiber surface due to shear fracture at the liquid crystal interface do not separate completely but are in a branched state, and the diameter of the fine fibers is 100 to 1000 nm, and the diameter of the center of the raw fiber is several μm. Refined pulp can also be made into aramid nanofibers by the above treatment.

As another method for micronizing aramid materials, a method of applying mechanical shear force rather than the chemical treatment described above is also possible, but this method only partially obtains a fibril structure with a fiber diameter on the nano-order. Therefore, since it also contains non-fibrillated micro-order structures, a homogeneous entangled structure on the nano-order is not obtained and the expected physical properties are not expressed. In the present invention, it is preferable to form an entangled structure from homogeneously nano-sized fibers rather than partial nano-ization.

<Aramid pulp>
The aramid pulp in the present invention has a fibril structure (sometimes referred to as fibrids). Aramid pulp having a fibril structure can be obtained by a known method, for example, as described in JP-A-2014-501859. Specifically, for example, a method may be used in which poly-p-phenylene terephthalamide fibers are cut to several millimeters, dispersed in water, and rubbed against each other using a refiner to form fibrils. Alternatively, it is also possible to use a structure in which a para-type aromatic polyamide solution is sprayed and solidified into a poor solvent such as water.

In the present invention, the fibril structure of the aramid pulp preferably has a fiber diameter partially exceeding 100 nm, and more preferably 300 nm or more. As shown in FIG. 4, the fibril structure is composed of a trunk 2 and a branch (fibril portion) 3, and it is sufficient that either the trunk 2 or the branch 3 has a fiber diameter exceeding the above range. In addition, it is preferable that the average fiber length is 1000 μm or less. The lower limit of the average fiber length is 100 μm or more, and preferably 500 μm or more.

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, and more preferably 20 m ² /g or more and 30 m ² /g or less.
In this way, by using aramid pulp having a structure larger than that of aramid nanofibers, it is possible to increase the stress transmission distance in a network structure formed with a heat-resistant fiber binder that is insoluble in a non-aqueous polar solvent or an electrolyte solution.

<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, it is difficult to secure a sufficient specific surface area required for holding the electrode active material 1, as shown in Fig. 4. In addition, the branched structure of the pulp entraps the solvent in the coating liquid in which the aramid pulp is dispersed, which reduces the liquid fluidity and makes it difficult to apply the liquid uniformly.

When only aramid nanofibers are used as the fibrous binder, uniform coating properties and a high specific surface area are ensured, as shown diagrammatically in Figure 5. On the other hand, it is difficult to hold the electrode active material 1, which has a particle size on the order of several tens of μm, and as a result, it is difficult to prevent the electrode active material from falling off. This is presumably because the structural size of the aramid nanofibers 4 is smaller than that of the electrode active material, making it difficult to form a long-distance network connecting the electrode active materials. In addition, a coating liquid in which only aramid nanofibers 4 are dispersed has high liquid fluidity and excellent uniform coating properties, but is more prone to dripping than a coating liquid containing aramid pulp, and thick coating properties are reduced.

In light of the above, by using aramid pulp and aramid nanofibers together as a fibrous binder, as shown diagrammatically in Figure 6, it is expected that a long-distance, strong network connecting electrode active materials 1 will be formed by the trunk 2 and branch 3 of the aramid pulp, while binder performance will be expressed due to the high specific surface area derived from the aramid nanofibers 4. In addition, it is expected that the combined use of aramid pulp and aramid nanofibers will achieve both uniform coating properties and thick coating properties.

<Electrode active material>
The electrode active material in the present invention is a material capable of releasing and filling ions such as Li by charging and discharging, and is preferably a Li metal oxide. Specific examples thereof include cobalt lithium oxide, manganese lithium oxide, and NMC (nickel manganese cobalt) Li oxide.

<Conductive assistant>
The conductive assistant in the present invention is a conductive material disposed on the surface of the electrode active material and between the particles of the electrode active material, and is preferably a carbon material from the viewpoint of electrochemical stability. Specifically, carbon black (acetylene black, ketjen black, etc.) is preferable, and carbon nanotubes (CNTs) or conductive polymers (polyacene, etc.) may be used in combination.

The soluble binder in the present invention is a substance that is soluble in a non-aqueous polar solvent and is not decomposed by the potential difference generated by the electrode active material. A heat-resistant polymer that changes little even at high temperatures is preferable. Specific examples include polyvinylidene fluoride (PVdF), polyimide, meta-aramid, etc., and among these, polyvinylidene fluoride (PVdF) is particularly preferable.

<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, and the electrode active material and conductive assistant are held by the aramid nanofiber, aramid pulp, and soluble binder, and the content of the aramid nanofiber is 0.05 to 0.9 wt% based on the electrode sheet, preferably 0.06 to 0.5 wt%, more preferably 0.07 to 0.3 wt%, and even more preferably 0.08 to 0.2 wt%. The content of the aramid pulp is 0.05 to 0.9 wt% based on the electrode sheet, preferably 0.1 to 0.7 wt%, and more preferably 0.2 to 0.5 wt%. The content of the soluble binder is 0.1 to 1.0 wt%, preferably 0.2 to 0.9 wt%, and more preferably 0.3 to 0.8 wt% based on the electrode sheet.

Aramid nanofibers form hydrogen bonds between adjacent nanofibers through amide bonds, which are a polymer structure, making it possible to form sheets of electrode active materials and conductive assistants.

If the aramid nanofibers and aramid pulp are less than 0.05% by weight, it is not preferable because the sheet cannot be made. If the aramid nanofibers and aramid pulp are more than 0.9% by weight, the nanofibers and pulp tend to be unevenly distributed, which is also not preferable.

Furthermore, the movement of Li ions during charging and discharging causes the electrode active material to expand and contract, resulting in friction between particles, which in turn causes the conductive assistant to fall off. To prevent this, it is preferable to use a small amount of a soluble binder in combination.

The electrode sheet described above can be manufactured by the following method. That is, the electrode sheet of the present invention has a weight ratio of aramid nanofiber, aramid pulp, and soluble binder of 0.05-0.9:0.05-0.9:0.1-1.0, and is obtained by adding an electrode active material to a conductive assistant of 4% by weight or less based on the electrode sheet, forming a slurry with a non-aqueous polar solvent, coating the slurry on a current collector sheet or film, and drying the slurry.

The sheet obtained by the above method is routinely pressed using a heated calendar roll or the like to reduce voids, increase density, increase the electrode active material density per volume, and increase the secondary battery capacity, and it goes without saying that it is required that the electrode sheet not be destroyed by heat and pressure.

More specifically, for example, a para-type wholly aromatic polyamide fiber or a para-type wholly aromatic polyamide pulp is nano-fiberized by adding a strong basic substance in an aprotic polar solvent to obtain aramid nanofibers having an average fiber diameter of 100 nm or less, and 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 with a non-aqueous polar solvent or an electrolyte solution to form a slurry, which is then coated on an electrode current collector and pressed as necessary to obtain an electrode sheet.

This operation produces a sheet of a specified thickness with no change in active thickness. A secondary battery (Li-ion secondary battery) is produced by laminating it with a separator and a negative electrode sheet and injecting an electrolyte.
The electrode sheet preferably has an electrical conductivity of 30×10 ⁻³ S/cm or more, more preferably 33×10 ⁻³ S/cm or more, and even more preferably 35×10 ⁻³ S/cm or more. The electrolyte permeation speed is preferably 230 to 400 sec, more preferably 250 to 400 sec, and even more preferably 300 to 400 sec. The unit cell performances at both 25° C. and 70° C. are preferably 95% or more, and more preferably 96% or more.

The present invention will be described in detail below with reference to examples and comparative examples, but the scope of the present invention is not limited to the following examples and comparative examples. The characteristic values in the examples were measured by the following methods.

<Average fiber diameter>
The structure of the sample was observed using a scanning electron microscope (manufactured by JEOL Ltd., plate: JSM-6330F). From an image observed at a magnification setting of 50,000 times, an image region of 1,800 nm to 2,000 nm in width and 1,200 nm to 1,500 nm in length was selected, and the image region was further divided vertically into 4 and horizontally into 4 to define a total of 16 grid regions A1-D4. One sample present in each grid region 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).

<Specific surface area measurement>
The specific surface area of the target sample was measured using a flow type automatic specific surface area measuring device (Flowsorb, manufactured by Shimadzu Corporation). Helium with a known adsorption occupancy area was adsorbed onto the sample surface at liquid nitrogen temperature, and the specific surface area of the sample was calculated from the amount of helium gas adsorbed.

(Coating Fluid Characteristics)
<Uniform coating ability>
The state of the coating liquid immediately after the prepared coating liquid was applied by die coating (thickness of the coating liquid immediately after application) was visually observed to evaluate whether the coating liquid could be applied evenly and uniformly.
○: Uniform coating possible △: Coating possible, although there are some partial unevenness ×: Coating unevenness exists over the entire surface, making coating with a die coat difficult

<Thick coating properties>
After the coating liquid was applied to a predetermined coating area, it was left to stand for 10 minutes on a horizontal surface under normal temperature and pressure conditions, and the prepared coating liquid was applied to 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 coated area is less than 5%. △: The area change (area reduction rate) of the coated area is 5% or more and less than 20%. ×: The area change (area reduction rate) of the coated area is 20% or more.

(Electrode Coating Properties)
<Deformation during drying>
After the coating liquid was applied to a predetermined coating area, it was dried for 20 minutes on a horizontal surface under a hot plate (100°C) and normal pressure environment, and the prepared coating liquid was applied to 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 change in area of the coated area is less than 5% △: The change in area of the coated area is 5% or more but less than 10% ×: The change in area of the coated area is 10% or more

<Axle winding test>
As an index of workability in the process of winding electrodes for cylindrical and prismatic batteries, the prepared electrode sheet (width 10 mm, length 50 mm) was wound around a 4 mmφ SUS wire, 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

<Powder Fall>
The surface of the prepared electrode sheet was gently rubbed with a wiping cloth (Kimwipe) and the degree of staining was evaluated.
○: No dirt removal △: Slight dirt removal ×: Stain removal

[Example 1]
<Preparation of aramid nanofibers>
Aramid pulp: 10 g (10 g of fibers obtained by cutting Twaron 1000 manufactured by Teijin Aramid Co., Ltd. into 6 mm pieces were washed by boiling in 1000 g of boiling water for 30 minutes, cooled, washed with water and dried).
Tokyo Chemical Industry Co., Ltd. Dimethyl sulfoxide (DMSO) > 99% 80g,
・Tokyo Ohka Kogyo Co., Ltd. Potassium hydroxide (KOH) 10g
The above three components were charged into a planetary mixer and stirred for 2 hours at 70° C. After stirring, the aramid pulp lost its shape, and a red semi-transparent highly viscous solution was obtained.

The resulting red translucent solution containing nanofibers was diluted with three times the amount of DMSO and gradually poured into 20 liters of water while stirring, causing poly-p-phenylene terephthalamide nanofibers to precipitate. At this point, the solution was a yellow-red slurry, to which sulfuric acid was added in an amount required for KOH neutralization, and the poly-p-phenylene terephthalamide nanofibers were collected by filtration.

Then, the solvent and salt were removed by washing 3 to 5 times using distilled water or ion-exchanged water and stretching and dehydrating. Distilled water was added to the obtained aqueous solid containing poly-p-phenylene terephthalamide nanofibers so that the solid concentration was 1%, and an aqueous dispersion of poly-p-phenylene terephthalamide nanofibers was obtained using a stone mill-type grinding and kneading machine (Supermasscolloider) manufactured by Masuko Sangyo Co., Ltd. The obtained poly-p-phenylene terephthalamide nanofiber dispersion was diluted with isopropyl alcohol, dropped onto a preparation, and dried, and it was confirmed that the average fiber diameter was 20 nm (see Figures 1 and 2).

0.25 parts by mass of the above-mentioned aramid nanofibers and finely divided aramid pulp (Twaron (registered trademark): 1094 pulp manufactured by Teijin Limited) were treated with a high-pressure homogenizer. 0.25 parts by mass of the above-mentioned aramid nanofibers (specific surface area: 21 ^m2 /g) and 0.5 parts by mass of PVdF (manufactured by Kureha or Zeon Corporation) were dispersed in NMP, and 95 parts by mass of positive electrode active material (Ni:Mn:Co = 1:1:1) _LiO2 and 4 parts by mass of acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) were mixed into the solution to a solid concentration of 70%, and the mixture was dispersed at 2000 rpm for 5 minutes using a thin film swirling high-speed mixer (Filmix) to prepare an electrode slurry.

A film was formed on a commercially available aluminum foil using a die coater, and then dried on a hot plate (100° C.) for 20 minutes.
The dried electrode sheet was subjected to a press treatment with a metal roll calendar (treatment temperature: 60° C., treatment line pressure: 2 kN/cm) to obtain a coated electrode sheet.
The obtained electrode sheet was punched out to a predetermined size of 50 mm x 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 evaluation method described above.

[Example 2]
An electrode sheet was prepared and evaluated in the same manner as in Example 1, except that the ingredients were changed to 0.25 parts by mass of aramid nanofiber, aramid injection coagulated material (aramid polymer solution injected and coagulated into a poor solvent; specific surface area: 26 ^m2 /g), 0.5 parts by mass of PVdF, 4 parts by mass of acetylene black, and 95 parts by mass of positive electrode active material.

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

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

[Comparative Example 3]
An electrode sheet was prepared and evaluated in the same manner as in Example 1, except that the amounts of finely divided 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 were changed to 0.5 parts by mass.

[Comparative Example 4]
An electrode sheet was produced and evaluated in the same manner as in Example 1, except that the components were changed to 0.5 parts by mass of aramid jet coagulate, 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.

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

1 Electrode active material 2 Aramid pulp (stem portion)
3 Aramid pulp (fibril part)
4. Aramid nanofiber

Claims (6)

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. The electrode sheet according to claim 1, wherein the aramid nanofibers are made of para-type wholly aromatic polyamide. The electrode sheet according to claim 1, wherein the aramid pulp is made of para-type wholly aromatic polyamide. An electrode sheet according to any one of claims 1 to 3, in which the electrode active material contains Li metal oxide. 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. 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.