Method for producing positive electrode slurry and method for producing positive electrode

Abstract

This method for producing a positive electrode slurry comprises: a step wherein an aqueous solution, in which a smectite is dissolved, is mixed with a solvent containing N-methyl-2-pyrrolidone (NMP), thereby preparing an aqueous smectite-NMP solution; a step wherein the aqueous smectite-NMP solution is subjected to thermal dehydration, thereby obtaining a smectite-containing solid content lump; and a step wherein the smectite-containing solid content lump, a positive electrode active material and the solvent containing N-methyl-2-pyrrolidone (NMP) are mixed with each other, thereby preparing a slurry.

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a positive electrode slurry and a method for producing a positive electrode. [Background technology]

[0002] An electrode used in a lithium-ion secondary battery includes a current collector and an active material layer formed on the current collector, and is fabricated by coating a slurry containing the active material onto the current collector.

[0003] For example, Patent Documents 1 to 5 disclose techniques for coating a positive electrode current collector with a positive electrode slurry containing a positive electrode active material and smectite. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-27768 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-170881 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-32776 [Patent Document 4] Japanese Patent Application Laid-Open No. 2008-71757 [Patent Document 5] Japanese Patent Application Laid-Open No. 2001-266855 Summary of the Invention

[0005] After supplying the positive electrode slurry to the positive electrode current collector and coating it to a desired length, stopping the supply of the positive electrode slurry to the positive electrode current collector results in stringy traces of the slurry at the end of the coating. These stringy traces deviate from the designed thickness of the positive electrode, which may lead to a decrease in battery capacity. Furthermore, the positive electrode lead is welded to the uncoated portion of the positive electrode current collector that has not been coated with the positive electrode slurry. However, if the stringy traces at the end of the coated portion are long, it may be difficult to weld the positive electrode lead to the uncoated portion.

[0006] A method for producing a positive electrode slurry according to one embodiment of the present disclosure includes the steps of: mixing an aqueous solution of smectite dissolved therein with an N-methyl-2-pyrrolidone (NMP)-containing solvent to prepare a smectite-NMP aqueous solution; heating and dehydrating the smectite-NMP aqueous solution to obtain a smectite-containing solid mass; and mixing the smectite-containing solid mass, a positive electrode active material, and an N-methyl-2-pyrrolidone (NMP)-containing solvent to prepare a slurry.

[0007] A method for producing a positive electrode according to one embodiment of the present disclosure includes a step of applying the positive electrode slurry obtained by the above-described production method to a positive electrode current collector.

[0008] According to the method for producing a positive electrode slurry according to one aspect of the present disclosure, it is possible to suppress stringiness at the end of the coating. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing an example of the configuration of a coating device according to this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing a state in which the coating slurry is applied onto the current collector. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0011] Fig. 1 is a schematic diagram showing an example of the configuration of a coating device according to this embodiment. The coating device 1 shown in Fig. 1 includes a stirrer 10 as a stirring means, a liquid feeder 14, and a coating head 16 and intermittent mechanism 18 as a coating means. Specific operations of the coating device 1 will be described later. The coating device 1 can produce, for example, a positive electrode in which a coated portion 26 in which the positive electrode slurry is coated on a positive electrode current collector 34 and an uncoated portion 28 in which the positive electrode slurry is not coated are formed.

[0012] The agitator 10 agitates the cathode slurry, which will be described later. Any known agitator can be used as the agitator 10, and examples of such a agitator include a magnetic stirrer, a three-one motor, a homogenizer, a media mill, a colloid mill, a homomixer, a homodisper, a planetary mixer, an in-line mixer, and a pipeline mixer.

[0013] The liquid delivery device 14 delivers the positive electrode slurry to the coating head 16, and is, for example, a pump.

[0014] The coating head 16 typically uses a type called a slot die, which ejects the cathode slurry from a slit of a fixed width. The slit is typically created by sandwiching a plate called a shim of the required thickness between the upstream head and the downstream head. Also, a liquid reservoir called a manifold is typically provided on the upstream head side, and coating is typically performed by ejecting the coating slurry through this.

[0015] Although the intermittent mechanism 18 has been described as an example of a coating liquid suction system, the present invention is not limited to this system. For example, the intermittent mechanism 18 intermittently suctions the positive electrode slurry supplied to the coating head 16, momentarily creating a negative pressure inside the coating head 16. This causes the positive electrode slurry to be intermittently discharged from the coating head 16, forming coated areas 26 and uncoated areas 28 on the positive electrode current collector 34. The positive electrode current collector 34 may be, for example, a foil of a metal such as aluminum that is stable within the potential range of the positive electrode, or a film having such a metal disposed on its surface.

[0016] An example of the operation of the coating device 1 according to this embodiment will be described.

[0017] Positive electrode slurry is introduced into agitator 10, and the positive electrode slurry is agitated by agitator 10. The positive electrode slurry in agitator 10 is supplied by liquid feeder 14 to coating head 16 via flow path 30b. Then, roll 32 is rotated at a set rotation speed, and a strip-shaped positive electrode current collector 34 is transported in the X direction, while positive electrode slurry is discharged from coating head 16 to positive electrode current collector 34 at a set discharge rate. In addition, intermittent mechanism 18 is periodically activated to stop the discharge of positive electrode slurry from coating head 16. In this manner, the positive electrode slurry is intermittently coated onto positive electrode current collector 34, forming coated portions 26 and uncoated portions 28.

[0018] The positive electrode having the coated portion 26 and the uncoated portion 28 formed on the positive electrode current collector 34 is dried and rolled as necessary.

[0019] 2 is a schematic diagram showing a state in which the positive electrode slurry has been coated onto a positive electrode current collector. As shown in FIG. 2, the positive electrode slurry is intermittently coated onto a positive electrode current collector 34, thereby forming a plurality of coated areas 26 and uncoated areas 28 alternately on the positive electrode current collector 34. The coated areas 26 refer to the region from a coating start point P1 to a coating end point P2. The uncoated areas 28 are later used for welding leads or cutting the electrodes into individual batteries.

[0020] Normally, even when the discharge of the positive electrode slurry from the coating head 16 stops, the surface tension of the positive electrode slurry causes the coating slurry to spread from the coating head 16, so that the coating slurry spreads from the coating end portion P2 toward the uncoated portion 28 along the conveyance direction X, easily leaving stringy marks 36. If the length L of the stringy marks 36 is large, this may cause poor lead welding in the uncoated portion 28 or a decrease in battery capacity.

[0021] However, by using a positive electrode slurry obtained by the method for producing a positive electrode slurry of the present embodiment described below, stringiness at the coating end P2 is suppressed, and the length L of the stringy trace 36 is shortened.

[0022] The method for producing the positive electrode slurry of this embodiment will be described below.

[0023] The method for producing a positive electrode slurry of this embodiment includes a step of mixing an aqueous solution in which smectite has been dissolved with an N-methyl-2-pyrrolidone (NMP)-containing solvent to prepare a smectite-NMP aqueous solution (aqueous solution preparation step), a step of heating and dehydrating the smectite-NMP aqueous solution to obtain a smectite-containing solid mass (solid mass generation step), and a step of mixing the smectite-containing solid mass, a positive electrode active material, and an N-methyl-2-pyrrolidone (NMP)-containing solvent to prepare a slurry (slurry preparation step).

[0024] (Aqueous solution preparation process) The smectite used in the aqueous solution preparation step is a swelling clay mineral, such as montmorillonite, beidellite, saponite, nontronite, and hectorite. Among these, hectorite is preferred. The concentration of smectite in the aqueous solution is not particularly limited, but is preferably 5% by mass or more and 20% by mass or less. Water is typically used as the medium for dissolving smectite, but the medium may also contain water-soluble organic media such as methanol, ethanol, isopropanol, butanol, acetone, methyl ethyl ketone, and tetrahydrofuran.

[0025] The N-methyl-2-pyrrolidone (NMP)-containing solvent may be a single N-methyl-2-pyrrolidone (NMP) solvent or a mixed solvent with other solvents. Examples of mixed solvents include a mixed solution of NMP and an ester-based solvent, or a mixed solution of NMP and a glyme-based solvent. Examples of ester-based solvents include ethyl acetate, n-butyl acetate, butyl cellosolve acetate, and butyl carbitol acetate. Examples of glyme-based solvents include diglyme, triglyme, and tetraglyme.

[0026] The aqueous solution in which smectite is dissolved and the N-methyl-2-pyrrolidone-containing solvent are preferably mixed in a volume ratio ranging from 1:0.5 to 1:1.5, for example.

[0027] (Solid agglomeration process) The temperature at which the smectite-NMP aqueous solution is heated and dehydrated may be any temperature that evaporates water from the smectite-NMP aqueous solution but does not evaporate NMP. For example, the temperature is preferably 80°C or higher, more preferably 100°C or higher, and further preferably 130°C or lower, but not exceeding 150°C.

[0028] The thermal dehydration is preferably carried out until the moisture content in the resulting smectite-containing solid mass reaches 0%, but may be stopped at a high moisture content (e.g., 5% or more and 10% or less). The thermal dehydration may be carried out by placing the smectite-NMP aqueous solution in a pressure-resistant vessel under reduced pressure.

[0029] After thermal dehydration, the smectite solid mass changes from a gel or jelly state to a mass with a dry surface depending on the heating time. To completely remove the moisture content, it is preferable to dry it in a sealed container such as a desiccator in a dry atmosphere for 12 hours or more. In particular, if thermal dehydration is stopped while moisture remains, it is preferable to dry it for 24 hours or more in a -20°C dew point atmosphere.

[0030] (Slurry preparation process) In the slurry preparation process, the smectite-containing solid mass dissolves in the N-methyl-2-pyrrolidone-containing solvent. This is thought to be due to the water in the smectite being replaced with NMP by thermal dehydration. Because smectite has excellent thixotropy, the positive electrode slurry in which the smectite-containing solid mass is dissolved in the NMP solvent is imparted with thixotropy. By applying the thixotropic positive electrode slurry to the positive electrode current collector 34, stringiness at the coating end portion P2 is suppressed and the length L of the stringiness mark 36 is shortened. Note that in conventional positive electrode slurries containing smectite, the smectite does not dissolve in the NMP solvent, and therefore the positive electrode slurry is not imparted with thixotropy.

[0031] The concentration of the smectite-containing solid mass in the N-methyl-2-pyrrolidone-containing solvent is preferably, for example, 10% by mass or more.

[0032] In the slurry preparation step, an N-methyl-2-pyrrolidone-containing solvent, a smectite-containing solid mass, and a positive electrode active material are mixed, and other additives such as a binder and a conductive material may be added as necessary. As described above, the N-methyl-2-pyrrolidone-containing solvent may be a single solvent of NMP or a mixed solvent with other solvents.

[0033] Examples of the positive electrode active material include lithium transition metal composite oxides. Specific examples include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel manganese composite oxide, and lithium nickel cobalt composite oxide. Furthermore, these lithium transition metal composite oxides may contain, for example, Al, Ti, Zr, Nb, B, W, Mg, or Mo.

[0034] Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), and polyvinyl alcohol (PVA).

[0035] Examples of conductive materials include carbon black, acetylene black, and ketjen black.

[0036] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0037] Example 1 Smectite SWN (Kunimine Industries Co., Ltd.) was dissolved in water to prepare a 10% by mass smectite-containing aqueous solution. N-methyl-2-pyrrolidone alone was added to the solution to prepare a 5% by mass smectite-NMP aqueous solution. A beaker containing the 5% by mass smectite-NMP aqueous solution was placed on a 100°C hot plate. Once the solution solidified into a jelly, heating was stopped and the mixture was left in a dry atmosphere for 24 hours to obtain a smectite-containing solid mass.

[0038] The obtained smectite-containing solid mass was mixed with a lithium transition metal composite oxide as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder in a mass ratio of 1:100:2:1, and then an appropriate amount of N-methyl-2-pyrrolidone as a sole solvent was added to obtain a positive electrode slurry containing 0.1 wt % smectite relative to the slurry solid concentration.

[0039] As shown in Figure 1, while rotating the roll and transporting the aluminum foil serving as a positive electrode current collector in the transport direction X, the positive electrode slurry was intermittently discharged from the coating head to form coated and uncoated areas on the aluminum foil. The coating conditions were set to a coating speed of 40 m / min and a thickness of 150 ± 10 µm for the coating slurry. Coating was performed so that the length of the coated area in the transport direction X was 600 mm and the length of the uncoated area in the transport direction X was 30 mm.

[0040] <Example 2> The obtained smectite-containing solid mass was mixed with a lithium transition metal composite oxide as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder in a mass ratio of 1:200:3:2 (positive electrode slurry containing 0.05 wt % smectite added relative to the slurry solid concentration), and the mixture was coated in the same manner as in Example 1.

[0041] <Comparative Example 1> Coating was carried out in the same manner as in Example 1, except that a positive electrode slurry containing no smectite-containing solid mass was used.

[0042] <Comparative Example 2> Coating was performed in the same manner as in Example 1, except that smectite that had not been heat-dehydrated (Sumecton-SWN, manufactured by Kunimine Industries Co., Ltd.), a lithium transition metal composite oxide as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder were mixed in a mass ratio of 1:1000:10:5 (positive electrode slurry containing 0.1 wt % smectite added relative to the slurry solid content).

[0043] <Comparative Example 3> Coating was performed in the same manner as in Example 1, except that smectite that had not been heat-dehydrated (Sumecton-SWN, manufactured by Kunimine Industries Co., Ltd.), a lithium transition metal composite oxide as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder were mixed in a mass ratio of 1:2000:20:10 (positive electrode slurry containing 0.05 wt % smectite added relative to the slurry solid content).

[0044] The length of stringy marks at the coating end terminal was determined for Examples 1 and 2 and Comparative Examples 1 to 3. Specifically, three coated areas were randomly selected from the multiple coated areas formed intermittently, and the lengths of stringy marks at the coating end terminals in the three selected coated areas were measured and the average value was determined, which was designated as the stringy mark length L. The length of stringy marks at the coating end terminals in the three coated areas is the average length of the multiple stringy marks formed at each coating end terminal.

[0045] When the length L of the stringy trace in Comparative Example 1 is set to 100, and the length L of the stringy trace in Examples 1 and 2 and Comparative Examples 2 and 3 is expressed as a relative value, it was 75 in Example 1, 82 in Example 2, and 95 in both Comparative Examples 2 and 3. That is, stringy traces at the coating end can be suppressed by preparing a positive electrode slurry using a smectite-containing solid mass obtained by heating and dehydrating a smectite-NMP aqueous solution obtained by mixing an aqueous solution in which smectite has been dissolved with an N-methyl-2-pyrrolidone (NMP)-containing solvent. [Explanation of symbols]

[0046] 1 Coating device 10 Mixer 14 Liquid delivery device 16 Coating head 18 Intermittent mechanism 26 Coating Department 28 Uncoated area 30b flow path 32 rolls 34 Positive electrode current collector 36 Thread marks P1 Coating start point P2 Coating end X conveying direction

Claims

1. a step of mixing an aqueous solution of smectite dissolved therein with a solvent containing N-methyl-2-pyrrolidone (NMP) to prepare a smectite-NMP aqueous solution; a step of heating and dehydrating the smectite-NMP aqueous solution to obtain a smectite-containing solid mass; and mixing the smectite-containing solid mass, a positive electrode active material, and a solvent containing N-methyl-2-pyrrolidone (NMP) to prepare a slurry.

2. 2. The method for producing a positive electrode slurry according to claim 1, wherein a concentration of the smectite in the aqueous solution in which the smectite is dissolved is 5% by mass or more and 20% by mass or less.

3. 3. The method for producing a positive electrode slurry according to claim 1, wherein a mixing ratio of the aqueous solution in which the smectite is dissolved to the N-methyl-2-pyrrolidone (NMP)-containing solvent is in a range of 1:0.5 to 1:1.5 by volume.

4. 4. The method for producing a positive electrode slurry according to claim 1, wherein the smectite-NMP aqueous solution is heated and dehydrated until the moisture content of the smectite-containing solid mass is 5% or more and 10% or less.

5. 4. The method for producing a positive electrode slurry according to claim 1, wherein the smectite-NMP aqueous solution is heated and dehydrated until the moisture content of the smectite-containing solid mass is 0%.

6. A method for producing a positive electrode slurry described in any one of claims 1 to 5, wherein in the step of preparing the slurry, the concentration of the smectite-containing solid mass in the N-methyl-2-pyrrolidone (NMP)-containing solvent is 10 mass% or more.

7. A method for producing a positive electrode, comprising the step of applying the positive electrode slurry obtained by the production method according to any one of claims 1 to 6 to a positive electrode current collector.

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