JP2015105906A - Iron detection method, carbon material-containing slurry managed by detection method, and lithium ion battery manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a detection method for sensitively and efficiently detecting iron in a carbon material in use.SOLUTION: A detection method includes: closely attaching filter paper containing a coloration liquid onto a glass plate so as to prevent mixture of bubbles; spraying an appropriate amount of a carbon material on the filter paper; providing another glass plate on the filter paper to interpose the filter paper between the glass plates and, at the same time, confirming whether a color of the filter paper changes to blue; and thereby detecting whether iron is present. At this time, the glass plates are used since the color change of the filter paper can be confirmed while the filter paper is interposed between the glass plates. The plates are, however, not limited to the glass plates as long as the plates do not contain iron. An iron ionization agent contained in the coloration liquid is not limited to a specific one as long as the agent can ionize iron. Potassium chloride, sodium chloride, or hydrochloric acid is particularly suitable as the iron ionization agent.

Description

  The present invention relates to a carbon material-dispersed slurry, and more particularly to a non-aqueous carbon material slurry for an electrode slurry for a lithium ion secondary battery positive electrode plate and a lithium ion secondary battery using the same.

Lithium ion batteries have features such as high energy density and long life, so home appliances such as video cameras, portable electronic devices such as notebook computers and mobile phones, and power tools such as power tools In recent years, it has been applied to large batteries mounted on electric vehicles (EV) and hybrid electric vehicles (HEV).
A lithium ion battery is a secondary battery that has a structure in which lithium is melted as ions from the positive electrode during charging, moves to the negative electrode and is stored, and reversely, lithium ions return from the negative electrode to the positive electrode during discharge. It is known to be caused by the potential of the active material.

This type of lithium ion battery is composed of a positive electrode, a negative electrode, and an ion conductive layer sandwiched between the two electrodes. The ion conductive layer includes a separator made of a porous film such as polyethylene or polypropylene, and a nonaqueous electrolytic cell. The one filled with liquid is used.
The separator is a member that plays a role of isolating the positive electrode and the negative electrode in the battery and holding the electrolytic solution to ensure ion conductivity between the positive electrode and the negative electrode. For example, as a separator of a lithium ion battery, an electrochemically inactive porous body (including a porous film) that can form a lithium ion conduction path between electrodes by holding an electrolyte in pores. In general, a microporous polyolefin film made of polyethylene or polypropylene is used.

As the negative electrode active material constituting the negative electrode, a material capable of reversibly intercalating lithium in an ionic state is used. As the negative electrode active material, a carbon material, a silicon oxide compound, lithium titanate, a tin alloy, or a mixture of these negative electrode materials is mainly used. For example, graphitic carbon material, pitch coke, fibrous carbon Etc. are known.
The positive electrode active material constituting the positive electrode includes layered rock salt type lithium metal oxides such as LiCoO2, LiNixMnyCo1-x-yO2, LiNiO2, and manganese-based spinel type lithium metal oxides such as LiMn2O4 and LiNi0.5Mn0.5O4. An olivine type such as LiFePO4 is known.

  By the way, the lithium ion battery has a problem that dendrite grows in the negative electrode and causes a short circuit between the electrodes as the number of charge / discharge cycles increases. There are various causes for such dendrite growth, but it is said that foreign substances such as iron contained in the positive electrode active material contribute. That is, it is said that foreign matters such as iron and stainless steel mixed in the positive electrode material are eluted while charging and discharging are repeated, and are deposited on the negative electrode to generate dendrites. It is considered that iron is mixed into the positive electrode material due to various causes. In particular, in the production of carbon materials such as carbon black, a combustion furnace made of metal such as iron or stainless steel is used, so it is almost impossible to avoid mixing iron in the carbon material production process. it is conceivable that.

Therefore, conventionally, in order to remove iron or the like from the material, a method has been proposed in which the material is passed through a magnetic field having a predetermined intensity so as to remove iron (see Patent Documents 1 and 2).
In addition, for the management of foreign materials such as iron in the slurry containing carbon materials and carbon materials, the amount of foreign materials removed by the magnetic field is measured and the semi-finished product in the process is decomposed with nitric acid, hydrochloric acid, aqua regia, etc. It is common to quantify iron content using an X-ray fluorescence analyzer, an atomic absorption spectrometer, or an inductively coupled plasma emission spectrometer (Patent Document 3, for an analysis method using an atomic absorption spectrometer or an inductively coupled plasma emission spectrometer) (See Patent Document 4 and Patent Document 5 for analysis methods using a fluorescent X-ray analyzer).

JP 2003-1119026 A JP 2003-1119029 A

JP 2010-078381 A JP 2000-310586 A JP 2008-157752 A

  As described above, measures have been taken to prevent the entry of foreign substances such as iron into the materials used in lithium ion batteries, but the process management still uses X-ray fluorescence analyzers, atomic absorption spectrometers, and inductive coupling. No effective method other than a detection device such as a plasma emission spectrometer has been proposed. Here, in order to measure iron content with a fluorescent X-ray analyzer, an atomic absorption spectrometer, or an inductively coupled plasma emission spectrometer, it is necessary to first dissolve the sample with an acid or to melt the alkali. This process takes a lot of time. There is a problem in adopting it as a process inspection. In addition, the analysis devices of the fluorescent X-ray analyzer, the atomic absorption spectrometer, and the inductively coupled plasma emission spectrometer are very expensive, and these analysis methods are difficult to adopt as process inspections even considering cost effectiveness. It is.

  The present invention establishes a detection method that can detect iron quickly and inexpensively so that it can be introduced as a process inspection instead of an analysis method using an X-ray fluorescence spectrometer, an atomic absorption spectrometer, or an inductively coupled plasma emission spectrometer. Let it be an issue.

The inventors of the present invention have tried to perform process management using a color reaction, and as a result, have found that the detection can be performed sensitively by a simple method, and have reached the present invention. That is, the present invention
(Chemical formula 1)
Reaction formula: Fe2 + K3 [Fe (CN) 6] = KFeIIIFeII (CN) 6 ↓ + 3K +

This is an application of the fact that potassium ferricyanide reacts with divalent iron ions to form a blue precipitate, and the iron content in the carbon material mixture is converted to iron by using chemicals or electrochemical methods. This is a detection method in which iron is detected using a colored solution after ionization. Moreover, it is the manufacturing method of the lithium ion battery managed using such a detection method of this invention, and can obtain the lithium ion battery which suppressed the growth of dentlite efficiently.

That is, the present invention
(1) A method for detecting iron content in a carbon material, wherein the carbon material is placed on a filter paper containing a coloring solution and sandwiched between plates,
(2) A method for detecting iron content in a carbon material-containing slurry, wherein the carbon material-containing slurry is coated on a substrate and coated on a carbon material-containing material coated surface of a test piece, and covered with a filter paper containing a coloring solution, A detection method characterized by being sandwiched between plates,
(3) A method for detecting iron content in a carbon material-containing slurry, wherein a test piece obtained by applying and drying a carbon material-containing slurry on a substrate is electrochemically dissolved in a solvent, and the solvent is used as a color solution. A detection method characterized by mixing,
(4) A method for detecting iron content in a carbon material-containing slurry, comprising mixing a carbon material-containing slurry and an iron ionizing agent, centrifuging, and then mixing a supernatant solution and a color solution. Detection method,
(5) The detection method according to any one of (1) to (4) above, wherein the color developing solution comprises (i) an ionizing agent for iron and (ii) potassium ferricyanide and / or potassium ferrocyanide. A detection method characterized by
(6) A method for producing a carbon material-containing slurry comprising a step of managing the iron content using the detection method according to any one of (1) to (5) above,
(7) A method for detecting iron content in a carbon material-containing coating film, wherein the filter paper containing the color liquid is brought into contact with the carbon material-containing coating film and sandwiched between plates,
(8) A method for producing a lithium ion battery, comprising the step of managing the iron content using the detection method of (7) above,
It is in.

  According to the present invention, it becomes possible to easily and sharply detect the iron content in the carbon material, and can be easily introduced into the process management of the production of the carbon material and the battery using the carbon material, and thus It becomes possible to efficiently manufacture a battery in which the generation of dendrites is suppressed.

FIG. 1 is a flowchart showing a manufacturing process management method of a lithium ion battery using the method of the present invention. FIG. 2 is a photograph showing the iron detection limit. FIG. 3 is a photograph showing the state of blue discoloration of the filter paper obtained by the filter paper method 1 of Example 1. FIG. 4 is a photograph showing the blue discoloration state of the filter paper obtained by the filter paper method 2 of Example 2. FIG. 5 is a photograph showing the blue discoloration state of the filter paper obtained by the filter paper method 2 of Example 3. FIG. 6 is a photograph showing the results of the electrochemical dissolution method of Example 4. FIG. 7 is a photograph showing the results of the electrochemical dissolution method of Example 5. FIG. 8 is a photograph showing the results of Comparative Example 1. FIG. 9 is a photograph showing the results of Comparative Example 2. FIG. 10 is a photograph showing the results of Comparative Example 3 and Example 6. FIG. 11 is a schematic view showing a flag-type electrode. FIG. 12 is a schematic view showing an electrochemical cell. In the electrochemical cell of FIG. 12, 1 is a Teflon cap, 2 is a glass beaker, 3 is a counter electrode (platinum), 4 is a working electrode (sample for electrochemical dissolution method), 5 is a reference electrode (silver), and 6 is stainless steel. A wire, 7 is a crimp terminal, 8 is a stainless bolt nut, and 9 is an electrolyte for a lithium ion battery.

Hereinafter, the present invention will be specifically described.
[Carbon material]
The carbon material that is the subject of the present invention is not particularly limited.
Examples thereof include carbon materials such as furnace black, lamp black, channel black, acetylene black, thermal black, ketjen black, natural graphite, artificial graphite, carbon nanotube, and carbon fiber. Powder, granular shape, needle shape and other shapes are not limited.

[Carbon material-containing slurry]
It is a liquid containing at least a carbon material and a solvent. In addition, there is no problem even if it contains an active material, a binder, a dispersant, and an additive. For example, those described in Patent Document 1 and Patent Document 2 can also be used.

[Detection method of the present invention]
The present invention provides (i) a method for detecting a carbon material, (ii) a method for detecting a slurry containing a carbon material, and (iii) a method for detecting a coating film containing a carbon material. (i) the following “filter paper method 1”, (ii) the following “filter paper method 2” or “centrifugation”, and (iii) the following “filter paper method 2” or “electrochemical dissolution method” Can be used.
In either method, an iron ionization solution is prepared using an iron ionizing agent, and a liquid containing this and at least one of potassium ferricyanide or potassium ferrocyanide is used as a coloration liquid. The iron in the carbon material is detected by contacting with the material.

[Ionizing agent for iron]
The iron ionizing agent is not particularly limited as long as it is a material that ionizes iron, but potassium chloride, sodium chloride, and hydrochloric acid are suitable.

[Ionizing agent solution for iron]
It is a solution containing an iron ionizing agent.
An example of an iron ionization solution is shown below.
A glass bottle containing 97% by weight of ion-exchanged water and 3% by weight of potassium chloride was tightly stoppered and stirred thoroughly with a touch mixer to confirm that there was no undissolved material, and an iron ionization agent solution was obtained.

[Coloring liquid]
It is a solution containing an iron ionizing agent and potassium ferricyanide or potassium ferrocyanide.
Below, an example of a coloring liquid is shown.
A glass bottle containing 95% by weight of ion-exchanged water and 2% by weight of potassium ferricyanide and 3% by weight of potassium chloride, tightly plugged and thoroughly stirred with a touch mixer to make sure there is no undissolved product.

[Reagent used]
In the examples described later, the following reagents were used.
Potassium ferricyanide K3 [Fe (CN) 6] reagent grade Nacalai Tesque potassium chloride KCl reagent grade Nacalai Tesque

[Filter paper method 1]
Adhere the filter paper soaked in color solution on the glass plate so as not to enter air bubbles, spray an appropriate amount of carbon material on the filter paper, and sandwich it with the glass plate from that time. This is a detection method for detecting the presence or absence of iron by confirming that the color changes.
At this time, the glass plate is used because it can be confirmed in a state where the discoloration of the filter paper is sandwiched, but the plate is not limited to the glass plate as long as the plate does not contain iron.
The coloring solution is not particularly limited as long as the iron ionizing agent can ionize iron, but potassium chloride, sodium chloride and hydrochloric acid are particularly suitable.

[Filter paper method 2]
A carbon material-containing slurry is applied and dried on a glass plate to form a carbon material-containing coating film. At this time, if it is difficult to form a coating film, a binder component may be mixed to form a coating film. This is a detection method for detecting the presence or absence of iron by adhering the filter paper onto the coating film so that bubbles do not enter, and sandwiching the filter paper from above with a glass plate, and confirming that the filter paper turns blue at that time.

[Electrochemical dissolution method]
A carbon material-containing slurry is applied and dried on an aluminum foil to form a carbon material-containing coating film. A carbon material-containing coating film is formed by assembling an electrochemical cell with a carbon material-containing coating film as a working electrode, a counter electrode as a platinum electrode, a reference electrode as silver, and an electrolyte for a lithium ion battery, and passing an electric current through the electrochemical cell. The iron contained in the product is eluted. Thereafter, the electrolytic solution for a lithium ion battery is taken out, mixed with a color developing solution, and the presence or absence of iron is detected by confirming that the mixed solution turns blue.
The electrolyte for a lithium ion battery is not particularly limited as long as it is an electrolyte that can be used for a lithium ion battery.
Since the coloring solution is already iron ions, no iron ionizing agent is required, but there is no problem even if it exists.
There are no particular limitations on the way the current flows as long as iron can be dissolved in the solvent.
Although the counter electrode is a platinum electrode and the reference electrode is silver, the counter electrode and the reference electrode are metallic lithium if the conditions are similar to those of a lithium ion battery.

[Centrifuge]
By thoroughly mixing the carbon-containing slurry and the iron ionizing agent, separating the solvent and the carbon material-containing material with a centrifuge, mixing the solvent and the color liquid, and confirming that the liquid mixture turns blue. This is a detection method for detecting the presence or absence of iron.

〔Detection sensitivity〕
As will be apparent from the examples described later, when 1 g of the coloring solution is mixed with 2 g of a ferric chloride solution containing 1 ppm of iron ions, a comparison is made with a reference solution in which 1 g of the coloring solution is mixed with 2 g of ion-exchanged water. It was confirmed that the color changed to blue. The sensitivity of 1 ppm iron ion was confirmed.
On the other hand, the detection sensitivity of ICP and XPS is 1 ppm, and it was surprisingly found that it is equivalent to ICP and XPS, and iron can be confirmed at a low cost in a short time.

[Manufacturing Process Management Method for Lithium Ion Battery Using the Method of the Present Invention]
The method for producing a carbon material-containing slurry using the detection method of the present invention or the method for producing a lithium ion battery is not particularly limited, and the above-described method for detecting iron content in a carbon material, containing a carbon material Using any one or more of the methods for detecting the iron content in the slurry, a carbon material-containing slurry including a step of managing the iron content and a lithium ion battery using the carbon material-containing slurry can be produced. For example, as shown in FIG. 1, the filter paper method 1 is applied to the carbon material, the filter paper method 2 is applied to the carbon material-containing slurry, or the filter paper method 2 is applied to the electrode plate formed using the carbon material slurry. It is preferred to apply an electrochemical dissolution method. One or more of these may be employed.

[Production of carbon material-containing slurry]
Although an example in the case of utilizing for manufacture of a lithium ion battery is given, the process in the middle will not be limited if the carbon material containing slurry of the target lithium ion battery is obtained.

  Hereinafter, the present invention will be described more specifically with reference to examples.

Carbon fiber ("Denka black powder" (trade name; carbon black manufactured by Denki Kagaku Kogyo Co., Ltd.)) and iron powder (manufactured by Nacalai Tesque Co., Ltd.) 0.1 g are sufficiently mixed with "filter paper method 1" A filter paper (5C) containing a coloring solution on a glass plate (15 cm × 15 cm) so that no air bubbles enter, and 1 g of the filter paper method 1 sample is sprayed on the filter paper, From there, it was sandwiched between glass plates (15 cm × 15 cm), and the presence or absence of discoloration of the filter paper to blue was confirmed.
More specifically, the following operation was performed.
-Put 0.1 g of iron powder in 1 g of carbon material in a polyethylene bag and seal it.
・ Shake a polyethylene bag containing carbon material and iron powder well. ⇒ Place the filter paper on the sample glass plate (15cm x 15cm) for the filter paper method 1.
・ Wet the filter paper with the colored solution.
・ Spread the sample for filter paper method 1 on the filter paper.
・ Place a glass plate on top of it.
-Check for any discoloration of the filter paper to blue.

  Carbon material ("acetylene black powder form" (trade name; manufactured by Denki Kagaku Kogyo Co., Ltd.) acetylene black 1 g, iron powder (manufactured by Nacalai Tesque Co., Ltd.) 0.1 g and solvent (N-methyl-2-pyrrolidone Mitsubishi Chemical Co., Ltd. (14g) and PVDF solution ("KF Polymer # 7208" (trade name, manufactured by Kureha Co., Ltd.)) (0.5g) "Awatori Netaro" (trade name, manufactured by Shinky Co., Ltd.) The sample was stirred for 5 minutes with a “mixer” to obtain a sample for the filter paper method 2. A sample for “filter paper method 2” was applied on a PET film with an applicator of 10 mil, and dried at 100 ° C. for 20 minutes with a warm air dryer (“ST-120”, trade name, manufactured by Espec Corp.) to form a coating film. A filter paper containing a color developing solution was brought into close contact with this coating film so as not to contain air bubbles, and was further sandwiched by a glass plate from above to confirm whether the filter paper had changed to blue.

  20 g of carbon material (“acetylene black powder” (trade name, acetylene black manufactured by Denki Kagaku Kogyo Co., Ltd.)) and 2 g of iron powder (manufactured by Nacalai Tesque Co., Ltd.) and a dispersant (“PVP K-15” trade name, 2 g of ISP (manufactured by ISP), 76 g of solvent (manufactured by N-methyl-2-pyrrolidone, Mitsubishi Chemical Corporation) and 0.5 mm zirconia beads (trade name “YTZ”, manufactured by Nikkato Co., Ltd.) For 2 hours to prepare a dispersion. 5 g of PVDF solution (manufactured by Kureha, KF Polymer # 7208, Kureha) was stirred with 5 g of the prepared dispersion with “Awatori Nertaro” (trade name, made by Shinky Co., Ltd., a rotation / revolution mixer), and a sample for “filter paper method 2” It was. A sample for “filter paper method 2” was applied on a glass plate at a spin coater of 1000 rpm × 10 seconds. Dry in a hot air dryer at 100 ° C. for 20 minutes to form a coating film. A filter paper was closely attached to the coating film so that no bubbles were introduced, and was further sandwiched by a glass plate from above to confirm whether the filter paper had changed to blue.

A carbon material-containing slurry was applied and dried on a flag-type aluminum foil having a size of 1 cm × 1 cm to form a carbon material-containing coating film. Using a coating film containing a carbon material as a working electrode, a counter electrode as a platinum electrode, a reference electrode as silver, and an electrolyte for a lithium ion battery (1M LiPF6 (EC: DEC = 1: 1) manufactured by Kishida Chemical Co., Ltd.) 10 g Assembly and energization of the electrochemical cell. 2 g of the electrolyte solution for lithium ion batteries was taken out into a test tube and mixed with 1 g of a color developing solution, and the presence or absence of a color change of the mixed solution to blue was confirmed.
POTENTOSTAT / GALVANOSTAT (HA-151 Hokuto Denko Co., Ltd.) was used as the energization device. An outline of the electrochemical cell is shown in FIG. The electrode is a flag electrode schematically shown in FIG. The energization condition is 1 mA / s × 30 minutes.
Details including the production of the carbon material-containing slurry and the production of the coating film are as follows.
20 g of carbon material + 2 g of iron powder + 2 g of dispersant + 76 g of solvent + 300 g of 0.5 mm zirconia beads are put in a plastic container.
-Disperse for 2 hours with a disperser (paint shaker). ⇒ Slurry containing carbon (1)
・ 96 g of active material + 10 g of PVDF solution + 10 g of the carbon-containing slurry (1) is stirred for 5 minutes with Nertaro. ⇒ Slurries containing carbon material (2)
-Apply carbon material-containing slurry (2) onto flag-type aluminum foil by dipping.
-The coated flag-shaped aluminum foil is dried with a hot air dryer under drying conditions of 100 ° C for 20 minutes. ⇒Assemble the sample and electrochemical cell for electrochemical dissolution method.
Working electrode: Sample for electrochemical dissolution method, counter electrode: platinum, reference electrode: silver lithium ion battery electrolyte (1M LiPF6 (EC: DEC = 1: 1), manufactured by Kishida Chemical Co., Ltd.) 10 g
-Energize the electrochemical cell.
Current-carrying device: POTENTOSTAT / GALVANOSTAT (HA-151, manufactured by Hokuto Denko)
Current-carrying conditions: 1 mA / s × 30 minutes. A test tube is mixed with 2 g of an electrolyte solution for a lithium ion battery and 1 g of a color solution.
-Check for any discoloration of the mixture to blue.

The same operation as in Example 4 was performed except that the energization device and energization conditions were as follows.
Current-carrying device: POTENTISTAT / GALVANOSTAT (VERSASSTAT4 manufactured by Toyo Technica)
Energizing condition: 10 mV / s x 3 cycles 0V to 1.5V
Details including the production of the carbon material-containing slurry and the production of the coating film are as follows.
20 g of carbon material + 2 g of iron powder + 2 g of dispersant + 76 g of solvent + 300 g of 0.5 mm zirconia beads are put in a plastic container.
-Disperse for 2 hours with a disperser (paint shaker). ⇒ Slurry containing carbon material (1)
・ 96 g of active material + 10 g of PVDF solution + 10 g of the slurry containing carbon material (1) are stirred for 5 minutes with Nertaro. ⇒ Slurries containing carbon material (2)
-Apply carbon material-containing slurry (2) onto flag-type aluminum foil by dipping.
-The coated flag-shaped aluminum foil is dried with a hot air dryer under drying conditions of 100 ° C for 20 minutes. ⇒Assemble the sample and electrochemical cell for electrochemical dissolution method.
Working electrode: Sample for electrochemical dissolution method, counter electrode: platinum, reference electrode: silver lithium ion battery electrolyte (1M LiPF6 (EC: DEC = 1: 1), manufactured by Kishida Chemical Co., Ltd.) 10 g
-Energize the electrochemical cell.
Current-carrying device: POTENTISTAT / GALVANOSTAT (VERSASSTAT4 manufactured by Toyo Technica)
Energizing condition: 10 mV / s x 3 cycles 0V to 1.5V
-Mix 2g of electrolyte solution for lithium ion batteries and 1g of colored solution in a test tube.
-Check for any discoloration of the mixture to blue.

40 g of carbon-containing slurry and 10 g of iron ionization agent solution 10 g of iron powder are mixed with “Awatori Netaro” (model number AR-100, manufactured by Shinky Co., Ltd.), and the resulting mixture 10 g is centrifuged. It was separated into a solvent and a precipitate by Tommy Seiko Co., Ltd. 2 g of the separated solvent and 1 g of the color developing solution were mixed, and the presence or absence of the color change of the mixed solution to blue was confirmed.
More specifically, it was performed as follows.
-Stir 40 g of carbon-containing slurry + 0.4 g of iron powder + 20 g of 0.5 N hydrochloric acid with Nertaro (AR-100) for 1 minute.
-The mixed solution is centrifuged at 10,000 rpm x 5 minutes by a centrifuge (GRX-220, manufactured by Tommy Seiko Co., Ltd.).
-Filter the supernatant with a membrane filter (PTFE type pore size 1.00 μm).
-Mix 2g of filtrate and 1g of colored solution.
-Check for any discoloration of the mixture to blue.

Comparative Example 1

  A filter paper (5C) containing a color developing solution is closely attached to a glass plate (15 cm × 15 cm) so that air bubbles do not enter, and the filter paper is colored blue by being sandwiched by the glass plate (15 cm × 15 cm). The presence or absence was confirmed.

Comparative Example 2

  2 g of an electrolyte for a lithium ion battery (1M LiPF6 (EC: DEC = 1: 1) manufactured by Kishida Chemical Co., Ltd.) and 1 g of a colored solution were mixed, and the presence or absence of a color change of the mixed solution to blue was confirmed.

Comparative Example 3

  2 g of 0.5 N hydrochloric acid and 1 g of the color developing solution were mixed, and the presence or absence of discoloration of the mixed solution to blue was confirmed.

[Results of Examples 1 to 6 and Comparative Examples 1 to 3]
Table 1 shows the presence or absence of coloration. It has been found that the iron content in the carbon material can be easily detected by the method of the present invention.

FIGS. 3 to 5 show photographs of filter papers having a blue discolored portion obtained by “filter paper method 1” and “filter paper method 2” in Examples 1-3. In both cases, it turned out that the carbon material turned blue due to the iron content and was detectable.
FIGS. 6 and 7 show the results of the electrochemical dissolution method in Examples 4 and 5. FIG. It turned out to be blue and detectable by the iron content in the carbon material.
FIG. 8 shows the result of Comparative Example 1. There was no blue discoloration part, it turned out that the blue discoloration of the result of the "filter paper method" of Examples 1-3 originates in a carbon material, and it turned out that the iron content in a carbon material can be detected by this method.
FIG. 9 shows the result of Comparative Example 2. There was no blue discoloration part, and the blue discoloration of Examples 4 and 5 was derived from the carbon material, and it was found that the iron content in the carbon material can be detected by this method.

[Results of Example 6 and Comparative Example 3]
The results of Example 6 and Comparative Example 3 are shown in FIG. In Example 6, it showed darker green than Comparative Example 3, and it turned out that it has changed into blue by presence of iron content.

  According to the present invention, the iron content in the carbon material can be detected sharply and easily, and the iron content can be easily managed by incorporating it into the process management as the battery material, and the formation of dendrites can be prevented.

Electrochemical cell 1: Teflon cap 2: Glass beaker 3: Counter electrode (platinum)
4: Working electrode (sample for electrochemical dissolution method)
5: Reference electrode (silver)
6: Stainless wire 7: Crimp terminal 8: Stainless steel bolt nut 9: Electrolyte for lithium ion battery

Claims (8)

A method for detecting iron content in a carbon material, wherein the carbon material is placed on a filter paper containing a coloring solution and sandwiched between plates. A method for detecting iron content in a slurry containing carbon material, wherein the carbon material-containing slurry is coated on a substrate and covered with a carbon paper-containing material coated surface of a test piece and covered with a filter paper containing a coloring solution, and sandwiched between plates. A detection method characterized by the above. A method for detecting iron content in a slurry containing carbon material, wherein a test piece obtained by applying and drying a slurry containing carbon material on a substrate is electrochemically dissolved in a solvent, and the solvent is mixed with a color solution. A detection method characterized by the above. A method for detecting iron content in a carbon material-containing slurry, comprising mixing a carbon material-containing slurry and an iron ionizing agent, centrifuging, and then mixing a supernatant solution and a color solution. . The detection method according to any one of claims 1 to 4, wherein the color solution is a solution containing (i) an ionizing agent for iron and (ii) potassium ferricyanide and / or potassium ferrocyanide. Feature detection method. The manufacturing method of the carbon material containing slurry including the process of managing iron content using the detection method in any one of Claim 1 to 5. A method for detecting iron content in a carbon material-containing coating film, wherein the filter paper containing a color developing solution is brought into contact with the carbon material-containing coating film and sandwiched between plates. The manufacturing method of a lithium ion battery including the process of managing iron content using the detection method of Claim 7.

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