Electronic staining method for resin, distribution state evaluation method, and distribution state evaluating resin and electrode

Abstract

To provide an electronic staining method that enables electronic staining of even a resin like PAA which has an acidic functional group and is easy to elute into a reaction liquid, and can evaluate a distribution state of the resin.SOLUTION: There is provided an electron staining method for evaluating a distribution state of a resin by using an electron staining liquid, wherein the resin has an acidic functional group and the electron staining liquid consists of an inorganic compound and a polar organic solvent. Particularly, it is preferred that the resin is a polyacrylic acid having a carboxyl group, the inorganic compound is a cesium (Cs) compound, and the polar organic solvent is an alcohol.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for electron staining a resin having an acidic functional group and a method for evaluating the distribution state of the electron-stained resin, particularly to a resin used as a binder for a battery electrode, and further to a resin for evaluating the distribution state and an electrode obtained by the electron staining method. [Background technology]

[0002] Electrodes for energy storage devices generally use a resin-based binder (binding agent) alone or together with a thickener to shape a powder mixture (electrode mixture) such as active material particles into a coating film. Materials such as polyvinylidene fluoride (PVDF) and styrene butadiene rubber (SBR) are used as binders, and carboxymethyl cellulose is used as thickeners. In this invention, binders (binding agents) including thickeners are collectively referred to as "binder resins" or simply "resins."

[0003] Furthermore, in recent years, binder resins such as polyacrylic acid (PAA) and copolymers of acrylic acid and polystyrene have been developed for high-capacity electrodes. The adhesion distribution of these resins to the active material of the electrode affects the performance and lifespan of the energy storage device, so evaluating this distribution is an important process.

[0004] Various methods for evaluating the distribution state of binder resins have been investigated. For example, Patent Document 1 discloses that PVDF, which is used as a binder resin, contains fluorine, and that the distribution state of PVDF can be evaluated by observing the fluorine with EDX, EPMA, or the like. This method allows the intensity of the characteristic X-rays of fluorine in PVDF to be analyzed using EPMA, and therefore, is particularly capable of evaluating the distribution state of PVDF without dyeing the PVDF.

[0005] Furthermore, Patent Document 2 discloses an electron staining method using osmium tetroxide (OsO4) as a staining method for observing polymeric materials containing styrene copolymers (styrene rubber, SBR, etc.) under an electron microscope. It is described that osmium tetroxide is effective for staining because it reacts with unsaturated double bonds in polymeric materials.

[0006] Additionally, Patent Document 3 discloses a method of dyeing specific organic materials (binders in electrode plates) such as SBR or polyolefin with bromine or ruthenium, and evaluating the dispersion state of the specific organic material with an EPMA. In particular, it describes that bromine selectively adds to unsaturated double bonds (C=C bonds) in SBR and the like.

[0007] These electron staining methods using osmium (Os) or ruthenium (Ru) are capable of staining resins having unsaturated double bonds, such as SBR.

[0008] However, these evaluation methods using electronic staining stain parts other than the resin (for example, electrode composite parts, etc.).

[0009] Furthermore, in fields other than evaluation technologies for electricity storage devices, for example, in the field of biological samples, staining methods using uranyl acetate and other materials as staining agents have been developed, as disclosed in Non-Patent Document 1. However, most of these methods are based on the principle of negative staining (also called negative staining), in which heavy elements are deposited on the outer shape of the target object, and have the problem that they cannot be applied as staining methods for electricity storage devices, where it is desired to selectively stain only specific resin components from among a wide variety of materials. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-172976 [Patent Document 2] Japanese Patent Application Publication No. 59-72040 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-279508 [Non-patent literature]

[0011] [Non-Patent Document 1] Kota Mayanagi, "Lecture: Single Particle Analysis of Supramolecular Complexes Using Negative Staining Method," Japanese Society of Microscopy, Microscope, Vol. 56, No. 2, pp. 81-86 (2021) Summary of the Invention [Problem to be solved by the invention]

[0012] The above-mentioned conventional electron staining methods have been used to evaluate the distribution state of polyacrylic acids (PAA), which are used as binder resins for high-capacity electrodes. Specifically, because polyacrylic acids (PAA) contain oxygen, when an electrode composite contains oxygen-containing organic substances other than polyacrylic acids (PAA) or when an oxygen-containing electrode material is used, elemental imaging analysis cannot distinguish between polyacrylic acids (PAA) and other oxygen-containing materials. Furthermore, because polyacrylic acids (PAA) lack unsaturated double bonds, electron staining methods using osmium tetroxide cannot be applied. Furthermore, ruthenium tetroxide and the negative staining technique described in the aforementioned non-patent document 1 cannot selectively electron stain polyacrylic acids (PAA).

[0013] Furthermore, a problem specific to polyacrylic acid (PAA) resins is that resins with a high content of carboxyl groups may become water-soluble, and depending on their molecular structure, they may dissolve into the reaction solution during the electron staining process, causing changes in the distribution of the resin. For example, PAA is water-soluble because it has a high content of free acidic functional groups.

[0014] In view of the above problems, the present invention aims to provide an electron staining method that enables electron staining even for resins such as PAA, which have acidic functional groups and are easily eluted into reaction solutions, and that can evaluate the distribution state of the resin. [Means for solving the problem]

[0015] The present inventors have conducted extensive research to solve the above problems and have come to the following findings.

[0016] First, we found that using a polar organic solvent as the reaction solution (staining solution) used in the electron staining method can prevent even water-soluble resins from eluting into the reaction solution.

[0017] Next, we discovered that the cationic atoms bonded to the acidic functional groups of the resin in the reaction solution can be ion-exchanged with other cationic atoms (such as metal ions), and therefore the cationic atoms of inorganic compounds are fixed to the acidic functional groups of the resin, allowing for selective electron staining.

[0018] It was also found that the elution of the resin into the reaction solution could be further suppressed by increasing the concentration of cationic atoms of the inorganic compound present in the reaction solution.

[0019] Furthermore, the present inventors have found that the distribution state of the binder resin can be evaluated by observing a specimen sample containing an electron-stained resin containing an acidic functional group, for example, using a backscattered electron image obtained by an electron microscope and performing elemental imaging analysis.

[0020] The present invention was made based on the above findings and further studies, and the gist of the present invention is as follows. [1] An electronic staining method for evaluating the distribution state of a resin using an electronic staining solution, the resin is a resin having an acidic functional group, The electron staining solution is composed of an inorganic compound and a polar organic solvent. A method for electronically dyeing a resin. [2] The method for electronically staining a resin according to [1] above, wherein the acidic functional group is a carboxy group (-COOH). [3] The method for electronically staining a resin according to [2] above, wherein the resin is a polyacrylic acid (PAA) type. [4] The method for electronically staining a resin according to any one of [1] to [3], wherein the inorganic compound is a cesium (Cs) compound and the polar organic solvent is an alcohol. [5] A method for evaluating the distribution state of a resin, characterized by evaluating the distribution state of an electronically dyed resin by the method for electronically dyeing a resin described in any one of [1] to [4] above. [6] A resin for distribution state evaluation, characterized in that the resin is dyed by the method for electronically staining a resin according to any one of [1] to [4] above. [7] An electrode, wherein the resin for distribution state evaluation according to [6] is a binder for a battery electrode. [Effects of the Invention]

[0021] According to the present invention, in a method for electronically staining a resin, by using an electronic staining solution consisting of an inorganic compound and a polar organic solvent for a resin having an acidic functional group, the distribution state of the resin can be evaluated, which is of great industrial benefit. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is an SEM image of an electrode treated by the electron staining method according to the present invention. [Figure 2] 1 is an SEM image of an electrode that has not been treated with the electron staining method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, the embodiments for carrying out the method for electronically staining a resin and the method for evaluating the distribution state according to the present invention will be described in detail, but the present invention is not limited to these.

[0024] [resin] In the following description, "resin" refers to the "binder resin" of the active material for the electrode, as described above.

[0025] The resins that are the subject of the present invention are resins having acidic functional groups. As will be described later, the electronic staining method according to the present invention is based on the principle of ion exchange, and therefore can be universally applied to any known or arbitrary resins as long as they have acidic functional groups, such as carboxyl groups.

[0026] The carboxyl group (-COOH) is a typical acidic functional group, but other acidic functional groups, such as protons (H + Any functional group that can release a carboxyl group (-SO3H) can be used. Examples include a sulfo group (-SO3H), a phosphate group (-PO4H), and a phenol group (-C6H4OH). The carboxy group also includes a carboxyalkyl group (-RCOOH) such as a carboxymethyl group (-CH2COOH).

[0027] Typical examples of resins having this acidic functional group are polyacrylic acids (PAA), but the resins that are the subject of the present invention are not limited to polyacrylic acids (PAA). Examples of polyacrylic acids (PAA) include polyacrylic acid (PAA), crosslinked PAA, and PAA copolymers, and any one or more of these can be used alone or in combination.

[0028] Polyacrylic acids (PAA) are suitable materials for use as binders in high-capacity electrode materials. For example, in lithium-ion secondary batteries, they are effective as binders between a lithium composite metal oxide, which is a positive electrode active material, and a graphite material, which is a conductive additive, and as binders between a Si-based compound, such as SiO, which is a negative electrode active material, and a conductive additive. Furthermore, polyacrylic acid (PAA) may be a salt of PAA, such as a salt of an alkali metal element, such as sodium polyacrylate, or a salt of an alkaline earth metal element.

[0029] In addition to polyacrylic acid (PAA), there are various other resins with acidic functional groups, such as polystyrene-acrylic acid copolymer and carboxymethyl cellulose (CMC).

[0030] [Staining solution] The electronic staining solution of the present invention comprises an inorganic compound and a polar organic solvent. The "inorganic compound" is an inorganic salt having a cationic atom used in the ion exchange treatment described below. Specifically, it is an element that becomes liberated in a solution to become a cationic atom.

[0031] The criteria for selecting the elements are as follows: (a) The cationic atom binds to the resin. (a) The solubility of the resin decreases after ion exchange. Furthermore, it is preferable that the following indexes be satisfied. (c) It is a heavy atom so that it can be easily detected with an electron microscope. (d) It has absorption lines suitable for elemental mapping processes such as EDX and EPMA. The above (c) is an effective index when observing the distribution using an electron microscope.

[0032] Furthermore, the above (d) is an effective index for evaluating distribution in elemental imaging analysis, and the target atom must be an atom whose characteristic X-rays do not overlap with the characteristic X-rays of carbon (C), oxygen (O), fluorine (F), or silicon (Si). This is because if the characteristic X-rays of the target atom's ion overlap with the characteristic X-rays of carbon or other atoms, the target atom cannot be measured.

[0033] Preferred elements that meet the above criteria are heavy elements from the third period or later in the periodic table. Among these, alkali metal elements (Na, K, Rb, Cs, Fr) or alkaline earth elements (Mg, Ca, Sr, Ba, Ra) from the third period or later are preferred. A particularly preferred element is cesium (Cs). Cs is an element suitable for the staining solution of the present invention because it has high solubility in polar organic solvents and strong bonding with carboxyl groups.

[0034] The concentration of cations for ion exchange in the dyeing solution is preferably high because it can suppress the elution of the resin into the dyeing solution, but is not particularly limited and may be determined appropriately within a range that can suppress the elution of the resin.

[0035] As an example, the cation concentration is preferably set such that the molar amount of the inorganic compound that becomes a cation in the staining solution is 1 to 10 times or more the molar equivalent per carboxy group in the resin (calculated using formula (2) described below). From the viewpoint of more clearly visualizing the distribution using SEM images and elemental imaging, the molar amount of the inorganic compound that becomes a cation is more preferably 100 times or more, and even more preferably 200 times or more the molar equivalent per carboxy group in the resin. In the present invention, from the viewpoint of further suppressing elution of the resin, a higher cation concentration is preferable.

[0036] "Polar organic solvent" refers to a liquid composed of polarized organic molecules. Specific examples include alcohols such as ethanol, isopropyl alcohol, ethylene glycol, and glycerin; ketones such as acetone; ethers such as dimethyl ether and tetrahydrofuran; and carbonates such as propylene carbonate and diethyl carbonate. Other examples include nitrogen-containing organic solvents such as n-methylpyrrolidone, dimethylformamide, and acetonitrile; sulfur-containing organic solvents such as sulfolane and dimethyl sulfoxide; and cyclic esters such as γ-butyrolactone. Among these solvents, alcohols are preferred from the viewpoint of solubility with inorganic compounds, with ethanol being particularly preferred.

[0037] In the present invention, commercially available polar organic solvents can be used, and these may contain water. The polar organic solvent may contain water as long as the effects of the present invention are not achieved, i.e., the resin to be electro-stained is not eluted in the solvent.

[0038] [Electronic staining method] Next, an example of the dyeing process using the electronic dyeing method will be described using PAA as the resin, but the dyeing method is not limited to this.

[0039] (a) Preparation of staining solution The staining solution for PAA contains cesium hydroxide (CsOH) as an inorganic compound and ethanol as a polar organic solvent. In this case, the unit amount of PAA can be calculated using the following formula (1): Amount of PAA unit (mol) = (PAA content in sample (g)) / (molecular weight of PAA polymerization unit (g / mol)) (1) Here, the PAA polymer unit has the following structural formula:

[0040] [ka] In addition, M + =Na + , K. + , etc.

[0041] The cation concentration (the ratio of the molar amount of the inorganic compound that becomes the cation to the molar equivalent per carboxy group in the resin) is calculated from the following formula (2).

[0042] Cs cation concentration (multiplication factor (equivalent)) = Cs content in CsOH reagent (mol) / PAA unit amount (mol) (2) An example of the above preparation method (example of reagent preparation) is shown below. (Example 1) Ethanol: 60 ml, CsOH reagent: 0.982 mmol, PAA unit amount: 0.00482 mmol. In this case, Cs + The concentration is 204 times higher according to the above formula (2). (Example 2) Ethanol: 20 ml, CsOH reagent: 0.327 mmol, PAA unit amount: 0.00161 mmol. In this case, Cs + The concentration is 203 times higher according to the above formula (2).

[0043] (a) Preparation of resin-containing observation samples The positive and negative electrode materials containing PAA were immersed in the dyeing solution described above (A). The time for immersion in the dyeing solution can be appropriately set depending on the target material and resin content, and is, for example, about 1 to 60 minutes. After immersion for the specified time, the samples are removed from the dyeing solution and air-dried, completing the dyeing process.

[0044] During the above immersion, the cation atoms of PAA were converted to Cs by ion exchange. + The atoms are exchanged and immobilized in the resin. The ion exchange reaction is as shown in the following chemical formula 2.

[0045] [ka]

[0046] [Method for evaluating resin distribution] Examples of methods for evaluating the distribution state of the resin include electron microscope observation, elemental imaging analysis, etc. For example, the distribution state is evaluated by observing a resin that has been electron-stained using an electron staining solution with an electron microscope or by elemental imaging analysis.

[0047] Examples of electron microscopes used to observe stained samples include scanning electron microscopes (SEM), transmission electron microscopes (TEM), and scanning transmission electron microscopes (STEM).

[0048] Here, an example of SEM observation of an electrode containing PAA as a binder will be described. First, FIG. 2 is an SEM image of an electrode in which the resin has not been subjected to the electron staining method of the present invention. In this case, the PAA, which is the binder, could not be distinguished from the other electrode material components in terms of contrast. In contrast, FIG. 1 is an SEM image of an electrode in which the resin has been treated with the electron staining method of the present invention. In this case, the PAA contained in the electrode contains Cs +By immobilizing the atoms, the heavy element effect of the Cs atoms made them easily visible (white areas in Figure 1), allowing for selective evaluation of the PAA distribution. Therefore, the use of the electron staining method of the present invention is expected to make a significant contribution to improving and enhancing the performance of electrodes.

[0049] The distribution state can also be evaluated by performing elemental imaging analysis on the electron-stained resin. That is, the distribution of immobilized atoms can be visualized by elemental imaging analysis. Energy dispersive X-ray spectroscopy (EDX), electron probe microanalyzer (EPMA), Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and the like, which are attached to electron microscopes, can be suitably applied. Other elemental imaging analysis methods that do not rely on electron microscopes include X-ray photoelectron spectroscopy (XPS), X-ray fluorescence analysis (XRF), and laser ablation inductively coupled plasma (ICP) mass spectrometry. However, the analytical methods are not limited to those mentioned above. [Example]

[0050] The present invention will be further described below with reference to examples. However, the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0051] Following the previously described method for preparing the staining solution, cesium hydroxide monohydrate (CsOH·HO) was used as the inorganic compound, and ethanol, isopropyl alcohol, and water were used as the solvent. After adjusting the cation concentration of the inorganic compound, the staining solution was prepared and electron staining was performed on a sample of a negative electrode material for lithium-ion secondary batteries (a mixture of graphite and SiO) containing PAA as a binder. After processing, the success of electron staining was confirmed by SEM observation, and the distribution of PAA was evaluated by observing the SEM image data (image analysis).

[0052] The cation concentration was calculated using the above-mentioned formula (2). In this example, the concentration was adjusted to 1 equivalent, 10 equivalents, 100 equivalents, and 200 equivalents.

[0053] Other treatment conditions included a dyeing solution temperature of 22° C. to 24° C. and an immersion time of 1 to 10 minutes. SEM observation was performed in backscattered electron image mode at an accelerating voltage of 0.8 kV and a magnification of 2000x.

[0054] The feasibility of electron staining was confirmed by SEM observation of the electron-stained samples. The evaluation criteria were that if there was a distribution of white shapes on or around the electrode material particles within a specific area (approximately 426 μm × approximately 300 μm) of the SEM image taken in backscattered electron imaging mode, the sample was judged as "passable." This "passable" judgment is indicated by the symbol "o" in Table 1 below. A sample that was not electron-stained (comparative example) was also observed in backscattered electron imaging mode with SEM, and it was confirmed that there was no distribution of white shapes on or around the electrode material particles (this means that the white shapes were visualized by electron staining).

[0055] The conditions and results are shown in Table 1.

[0056] [Table 1]

[0057] For the present invention examples of Test Nos. 1 to 6, the possibility of electron staining was evaluated as ○, and the distribution state could be observed. The electron staining method of the present invention made PAA visible as white particles, allowing visual evaluation of the PAA distribution state. Furthermore, when the cation concentration was 200 equivalents, the PAA distribution was more clearly visible in SEM observation in the backscattered electron mode.

[0058] Figure 1 shows a sample (Test No. 6: Example of the present invention) in which PAA was stained under electron staining conditions with a cation concentration of 200 equivalents. In this sample, PAA is distributed thinly and spread over the electrode material particles and their surroundings, and its dispersibility can be evaluated as being very good. When PAA is distributed thinly and spread out like this, it is preferable to perform electron staining while minimizing resin elution. However, since the resin distribution state is unknown until electron staining is performed and SEM observation is performed, it was found that it is preferable to set the cation concentration high when performing electron staining.

Claims

1. An electronic staining method for evaluating the distribution state of a resin using an electronic staining solution, the resin is a resin having an acidic functional group, The electron staining solution is composed of an inorganic compound and a polar organic solvent. A method for electronically dyeing a resin.

2. 2. The method for electronically dyeing a resin according to claim 1, wherein the acidic functional group is a carboxyl group (--COOH).

3. 3. The method for electronically dyeing a resin according to claim 2, wherein the resin is a polyacrylic acid (PAA) type resin.

4. 4. The method for electronically staining a resin according to claim 1, wherein the inorganic compound is a cesium (Cs) compound, and the polar organic solvent is an alcohol.

5. A method for evaluating a resin distribution state, comprising evaluating a distribution state of a resin that has been electronically dyed by the method for electronically dyeing a resin according to any one of claims 1 to 3.

6. A resin distribution evaluation method, comprising: evaluating the distribution state of a resin that has been electronically dyed by the resin electronic dyeing method according to claim 4.

7. A resin for distribution state evaluation, wherein the resin is dyed by the method for electronically staining a resin according to any one of claims 1 to 3.

8. A resin for distribution state evaluation, wherein the resin is dyed by the method for electronically staining a resin according to claim 4.

9. 8. An electrode, wherein the resin for distribution state evaluation according to claim 7 is a binder for a battery electrode.

10. 9. An electrode, wherein the resin for distribution state evaluation according to claim 8 is a binder for a battery electrode.

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