JP7097601B2 - Coating liquid for electrodes for lithium secondary batteries, manufacturing methods for electrodes for lithium secondary batteries, and electrodes for lithium secondary batteries - Google Patents

Description

The present invention relates to a simple and economical method for manufacturing an electrode for a lithium secondary battery, which is excellent in safety and has good charge / discharge cycle characteristics.

In a lithium secondary battery, scratches or irregularities on the surface of the electrode may cause damage to the electrical insulation of the separator in contact with the electrode. As a result, an electrical internal short circuit may occur.

In order to prevent this internal short circuit, it has been proposed to provide a protective layer made of an insulating porous film on the surface of the electrode active material layer. For example, Patent Documents 1 and 2 describe, as a method for forming an ion-permeable protective layer, after forming a heat-resistant protective layer forming coating film made of polyimide or the like on the electrode surface and before drying the coating film. A method has been proposed in which a porous protective layer is obtained by causing phase separation in a coating film by immersing it in a coagulation bath containing a coagulation liquid such as water or methyl alcohol. However, since the electrode obtained by such a method has low adhesiveness between the active material layer and the porous layer, the effect of preventing short circuit is not always sufficient, and it is improved from the viewpoint of ensuring the safety of the battery. There was a point to be done. In addition, the ion permeability of the porous protective layer was not sufficient, and sufficient cycle characteristics could not be obtained when used as an electrode. Further, in the electrode obtained by the method of causing phase separation using a coagulation liquid such as water or methyl alcohol, the entire active material layer is in contact with the coagulation bath, so that the coagulation liquid may impair the original characteristics of the active material layer. there were. In addition to these problems, this method also has a problem as a manufacturing method from the viewpoint of environmental compatibility because waste liquid containing a coagulating liquid is generated from the coagulation bath.

As a method for solving such a problem, Patent Document 3 describes a method for manufacturing an electrode for a lithium secondary battery, which is a polyamide imide on the surface of an electrode active material layer formed on a metal foil. A coating liquid containing (PAI) and a solvent is applied to form a coating film, and then when the solvent in the coating film is removed, the coating film utilizes the action of the poor solvent remaining in the coating film. A method for causing phase separation within the body to form a heat-resistant porous PAI layer having ion permeability has been disclosed, and it is described that an ether solvent such as tetraglyme or triglime is effective as a poor solvent. ing. By this method, a porous PAI layer having high ion permeability can be formed, but it is necessary to use a large amount of an expensive ether solvent, and there is a point to be improved from the viewpoint of economy.

On the other hand, Patent Document 4 discloses a method for obtaining a porous PAI film from a PAI solution containing an alcohol-based solvent as a poor solvent. In this method, a porous PAI film having a high porosity can be obtained even if the amount of alcohol used as a poor solvent is reduced.

Japanese Patent No. 3371839 Japanese Patent No. 3593345 International Publication No. 2014-106954 Japanese Unexamined Patent Publication No. 2016-22912

However, when the porous PAI layer of the electrode for a lithium secondary battery is formed by using the porous PAI film forming solution described in Patent Document 4, sufficient ion permeability may not be obtained, and lithium may not be obtained. There were some points to be improved in order to obtain good cycle characteristics as electrodes for secondary batteries.

INDUSTRIAL APPLICABILITY According to the present invention, an electrode for a lithium secondary battery in which a porous PAI layer having excellent ion permeability and good cycle characteristics is laminated and integrated with an electrode active material layer can be easily and economically manufactured. It is an object of the present invention to provide a coating liquid and a method for manufacturing an electrode for a lithium secondary battery using the coating liquid.

The present inventors have found that the above-mentioned problems can be solved by specifying a coating liquid composition for forming a porous PAI layer, and have completed the present invention.

The present invention has the following object.

<1> A coating liquid for electrodes for lithium secondary batteries having the following characteristics.
1) The solute consists of PAI.
2) The solvent is a mixed solvent of a good solvent and a poor solvent for the PAI, the good solvent is an amide solvent, and the poor solvent has a higher boiling point than the amide solvent, tripropylene glycol, which is an alcohol solvent. The content of the alcohol-based solvent in the mixed solvent is 5% by mass or more and 30% by mass or less .
3) Contains Fila.
<2> A method for manufacturing an electrode for a lithium secondary battery, wherein the coating liquid is applied to the surface of an electrode active material layer formed on a metal foil to form a coating film, and then a coating film is formed. When the solvent in the coating film is removed, the action of the alcohol-based solvent remaining in the coating film is used to cause phase separation in the coating film to form a porous PAI layer having ion permeability. A method for manufacturing an electrode for a lithium secondary battery.

The electrode for a lithium secondary battery on which a porous PAI layer is formed, which is obtained by using the coating liquid of the present invention, has excellent ion permeability and thus has good cycle characteristics.

Using the coating liquid of the present invention, a porous PAI layer having ion permeability is formed on the surface of the electrode active material layer formed on the metal foil. The electrode for a lithium secondary battery is an electrode constituting a lithium ion secondary battery, and a positive electrode having a positive electrode active material layer bonded to a positive electrode current collector or a negative electrode active material layer bonded to a negative electrode current collector. Refers to the negative electrode. The electrode active material layer is a general term for a positive electrode active material layer and a negative electrode active material layer. From the viewpoint of improving the safety of the lithium secondary battery, it is preferable to form the porous PAI layer on the negative electrode active material layer. By doing so, it is possible to suppress the generation of lithium dendrite (needle-shaped crystals of metallic lithium) on the surface of the negative electrode active material. This lithium dendrite may cause an electrical short circuit.

As the metal foil, a metal foil such as a copper foil, a stainless steel foil, a nickel foil, or an aluminum foil can be used. The metal foil acts as an electrode for a lithium secondary battery, and an aluminum foil is preferably used for the positive electrode and a copper foil is preferably used for the negative electrode. The thickness of these metal foils is preferably 5 to 50 μm, more preferably 9 to 18 μm. The surface of these metal foils may be roughened or rust-proofed to improve the adhesiveness with the active material layer.

The positive electrode active material layer is a layer obtained by binding positive electrode active material particles with a resin binder. As the material used as the positive electrode active material particles, those capable of storing and storing lithium ions are preferable, and those generally used as the positive electrode active material of the power storage element can be mentioned. For example, oxide-based (LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , etc.), composite oxide-based, iron phosphate-based (LiFePO ₄ , Li ₂ FePO ₄ F, etc.), polymer compound-based (polyaniline, polythiophene, etc.). Examples of active material particles such as. Among these, LiCoO ₂ , LiNiO ₂ , and LiFePO ₄ are preferable. In order to reduce the internal resistance of the positive electrode active material layer, a conductive auxiliary agent such as carbon (graphite, carbon black, etc.) particles or metal (silver, copper, nickel, etc.) particles is blended in an amount of about 1 to 30% by mass. May be.

The negative electrode active material layer is a layer obtained by binding negative electrode active material particles with a resin binder. As the material used as the negative electrode active material particles, those capable of storing and storing lithium ions are preferable, and those generally used as the negative electrode active material of the power storage element can be mentioned. For example, active material particles such as graphite, amorphous carbon, silicon-based, and tin-based particles can be mentioned. Among these, graphite particles and silicon-based particles are preferable. Examples of the silicon-based particles include particles such as elemental silicon, a silicon alloy, and a silicon / silicon dioxide composite. Here, the simple substance of silicon means crystalline or amorphous silicon having a purity of 95% by mass or more. In order to reduce the internal resistance of the negative electrode active material layer, a conductive auxiliary agent such as carbon (graphite, carbon black, etc.) particles or metal (silver, copper, nickel, etc.) particles is blended in an amount of about 1 to 30% by mass. May be.

The average particle size of the active material particles and the conductive auxiliary agent is preferably 50 μm or less for both the positive electrode and the negative electrode, and more preferably 10 μm or less. If the average particle size is too small, it will be difficult to bind with a resin binder, so the average particle size is usually 0.1 μm or more, preferably 0.5 μm or more.

The porosity of the electrode active material layer is preferably 5 to 50% by volume for both the positive electrode and the negative electrode, and more preferably 10 to 40% by volume.

The thickness of the electrode active material layer is usually about 20 to 200 μm.

Examples of the resin binder for binding the active material particles include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and styrene / butadiene copolymer. Known resin binders such as coalesced, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, and polyamideimide can be mentioned.

The PAI forming the porous PAI layer is a polymer obtained by performing a polycondensation reaction between a tricarboxylic acid component as a raw material and a diamine component.

The tricarboxylic acid component of PAI is an organic compound having 3 carboxyl groups (including its derivative) and 1 or more aromatic rings per molecule, and at least 2 of the 3 carboxyl groups are carboxyl. The groups are located at positions where they can form an acid anhydride morphology.

Examples of the aromatic tricarboxylic acid component include a benzene tricarboxylic acid component and a naphthalene tricarboxylic acid component.

Specific examples of the benzenetricarboxylic acid component include trimellitic acid, hemmellitic acid, and anhydrides thereof and monochromeides thereof.

Specific examples of the naphthalene tricarboxylic acid component include, for example, 1,2,3-naphthalentricarboxylic acid, 1,6,7-naphthalentricarboxylic acid, 1,4,5-naphthalentricarboxylic acid, and their anhydrides and their monoclonalides. Can be mentioned.

Among the aromatic tricarboxylic acid components, trimellitic acid and trimellitic anhydride chloride (TAC) are preferable.

The tricarboxylic acid component may be used alone or in combination of two or more.

In addition, some of the tricarboxylic acid components are terephthalic acid, isophthalic acid, pyromellitic acid, 3,3', 4,4'-biphenyltetracarboxylic acid, 3,3', 4,4'-benzophenone tetracarboxylic acid. Those substituted with the above components may be used.

The diamine component of PAI is an organic compound having two primary amino groups (including derivatives thereof) and one or more aromatic rings per molecule.

Specific examples of the aromatic diamine component include, for example, 4,4'-diaminodiphenyl ether (DADE), m-phenylenediamine (MDA), p-phenylenediamine, 4,4'-diphenylmethanediamine (DMA), 4,4'. -Diphenyl ether diamine, diphenyl sulfone-4,4'-diamine, diphenyl-4,4'-diamine, o-tridine, 2,4-tolylene diamine, 2,6-tolylenedamine, xylylene diamine, naphthalenediamine, and Examples of these diamine derivatives can be mentioned.

Among the aromatic diamine components, DADE, MDA and DMA are preferable.

The aromatic diamine component may be used alone or in combination of two or more.

PAI usually has a glass transition temperature of 200 ° C. or higher. For the glass transition temperature, the value measured by DSC (differential thermal analysis) is used.

The coating liquid of the present invention contains a mixed solvent consisting of a good solvent and a poor solvent for PAI. Here, the good solvent is a solvent having a solubility in PAI at 25 ° C. of 1% by mass or more, and the poor solvent is a solvent having a solubility in PAI at 25 ° C. of less than 1% by mass. That is.

As the good solvent used in the present invention, it is preferable to use an amide-based solvent. Examples of the amide solvent include N-methyl-2-pyrrolidone (NMP boiling point: 202 ° C.), N, N-dimethylformamide (boiling point: 153 ° C.), and N, N-dimethylacetamide (DMAc boiling point: 166 ° C.). Can be mentioned. The amide-based solvent may be used alone or in combination of two or more. Among these, NMP is preferable.

The poor solvent used in the present invention needs to be an alcohol solvent. As the alcohol solvent, it is preferable to use a solvent having a boiling point of 200 ° C. or higher and a boiling point higher than that of the amide solvent. The boiling point difference is preferably 5 ° C. or higher, more preferably 20 ° C. or higher, and even more preferably 50 ° C. or higher. Examples of the alcohol solvent include diethylene glycol (boiling point: 244 ° C.), triethylene glycol (boiling point: 287 ° C.), dipropylene glycol (boiling point of 232 ° C.), tripropylene glycol (TPG boiling point: 273 ° C.), and diethylene glycol monomethyl. Examples include ether (boiling point: 194 ° C.), tripropylene glycol monomethyl ether (boiling point: 242 ° C.), and triethylene glycol monomethyl ether (boiling point: 249 ° C.). These may be used alone or in combination of two or more. Among these, TPG is preferable.

The content of the alcohol-based solvent in the mixed solvent is preferably 1% by mass or more and less than 50% by mass, and more preferably 5% by mass or more and 30% by mass or less with respect to the total mass of the solvent. By setting the solvent composition as described above, when the coating film obtained from the PAI coating solution is dried and solidified, the phase separation is efficiently performed by the action of the alcohol-based solvent (poor solvent) remaining in the coating film. It can occur and form a porous PAI layer with high porosity. For the PAI solution having such a solvent composition, Patent Document 4 can be referred to. The PAI solution used here is a uniform solution. A uniform solution is a solution that is transparent to visible light.
By using such a uniform solution, a uniform phase separation phenomenon is induced when the coating film is dried. Therefore, for example, a microphase-separated, non-uniform PAI solution as disclosed in Japanese Patent Application Laid-Open No. 2007-269575 is not preferable.

The coating liquid of the present invention needs to mix Fila with the PAI solution. By doing so, good ion permeability of the formed porous PAI layer can be ensured. That is, good ion permeability can be obtained by the synergistic effect of the porous structure formed by the action of the poor solvent and the porous structure formed by the blending of the filler.

The type of filler is not limited, and organic fillers, inorganic fillers and mixtures thereof can be used. Specific examples of the organic filler include styrene, vinyl ketone, acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidyl acrylate, methyl acrylate and the like alone or two or more copolymers, polytetrafluoroethylene, and the like. Examples thereof include powders made of a polymer such as a tetrafluorinated ethylene-6 fluoride propylene copolymer, a tetrafluorinated ethylene-ethylene copolymer, and a fluororesin such as polyvinylidene fluoride. The organic filler can be used alone or in combination of two or more. Specific examples of the inorganic filler include powders made of inorganic substances such as metal oxides, metal nitrides, metal carbides, metal hydroxides, carbonates, and sulfates. Specific examples include powders made of alumina, silica, titanium dioxide, barium sulfate, calcium carbonate and the like. The inorganic filler can be used alone or in combination of two or more. Among these inorganic fillers, alumina powder is preferable from the viewpoint of chemical stability.

The shape of the filler is not limited, and particles such as substantially spherical, plate-shaped, columnar, needle-shaped, whisker-shaped, and fibrous can be used, and substantially spherical particles are preferable. The aspect ratio of the substantially spherical particles (major diameter of the particles / minor diameter of the particles) is preferably 1 or more and 1.5 or less.

The average particle size of the filler is not limited, but is preferably 0.01 μm or more and 2 μm or less. The average particle size can be measured by a measuring device based on the laser diffraction / scattering method.

The surface of the filler may be treated with a surface treatment agent such as a surfactant or a silane coupler.

The amount of the filler compounded is not limited, but is usually 10 to 1000% by mass and preferably 50 to 600% by mass with respect to the PAI solid content.

The PAI solid content concentration in the PAI coating solution is preferably 1 to 50% by mass, more preferably 2 to 30% by mass.

If necessary, known additives such as various surfactants and silane couplers may be added to the PAI coating solution as long as the effects of the present invention are not impaired. Further, if necessary, a polymer other than the imide-based polymer may be added to the PAI coating solution as long as the effect of the present invention is not impaired.

The PAI coating solution obtained as described above is applied to the surface of the electrode active material layer and dried at 100 to 180 ° C. to utilize the action of the poor solvent remaining in the coating film in the coating film. It is possible to cause phase separation in the PAI porous layer having good ion permeability. The thickness of the PAI porous layer is not limited, but is preferably 1 to 20 μm, more preferably 2 to 10 μm.

When applying the PAI coating liquid, a method of continuously applying by roll-to-roll or a method of applying in sheets can be adopted, and any method may be used. As the coating device, a die coater, a multi-layer die coater, a gravure coater, a comma coater, a reverse roll coater, a doctor blade coater and the like can be used.

As described above, the electrode of the present invention can be easily manufactured by a simple process.

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the examples.

The electrodes used in the following examples and comparative examples were obtained as follows.

88 parts by mass of graphite particles (average particle diameter 8 μm), which is a negative electrode active material, 5 parts by mass of carbon black (acetylene black), which is a conductive aid, and 7 parts by mass of PVDF, which is a binder resin, are N-methyl-2-pyrrolidone. It was uniformly dispersed therein to obtain a negative electrode active material dispersion having a solid content concentration of 25% by mass. This dispersion was applied to a copper foil having a thickness of 18 μm, which is a negative electrode current collector, and the obtained coating film was dried at 150 ° C. for 20 minutes and then heat pressed to form a thickness of 100 μm on the copper foil. An electrode (P-1) provided with a negative electrode active material layer was obtained.

The characteristics of the electrodes obtained in the following examples and comparative examples were evaluated by the following methods.

(1) Ion permeability
The electrodes were punched into a circle with a diameter of 16 mm, a separator made of a polyethylene porous membrane and a lithium foil were laminated in order, and these were stored in a stainless steel coin-shaped outer container. An electrolytic solution (solvent: a mixed solvent in which ethylene carbonate and dimethyl carbonate are mixed at a volume ratio of 1: 1; electrolyte: 1MLiPF ₆ ) is injected into this outer container, and the outer container is made of stainless steel via packing. A cell for evaluation was obtained by covering and fixing with a cap and sealing the battery can. Using this cell, the ionic resistivity before and after laminating the porous PAI layer was calculated from the impedance at 25 ° C. and 100 KHz, and the ratio of the ionic resistivity of the post-stacked electrode to the pre-stacked electrode (P-1) (ion). The ion permeability was evaluated by determining the resistivity increase rate).
(2) Cycle characteristics Using the test cell obtained above, the measurement temperature: 30 ° C., voltage range: 0.01 to 2V, charging current and discharging current: 500mA / g-repeatedly under the charging / discharging conditions of the electrode active material layer. The cycle characteristics were evaluated by charging and discharging the battery and determining the ratio of the 20th discharge capacity to the second discharge capacity (discharge capacity retention rate).

<Example 1>
A PAI solution was prepared in accordance with the description of Patent Document 4. That is, the PAI powder (glass transition temperature 280 ° C.) obtained by copolymerizing TAC with DADE and MDA (copolymerization molar ratio: DADE / MDA = 7/3) is composed of NMP and TPG. It was dissolved in a mixed solvent (mass ratio NMP / TPG = 75/25) at 30 ° C. to obtain a uniform PAI solution (L-1) having a solid content concentration of PAI of 13% by mass. The mixed solvent and a commercially available spherical alumina powder (average particle size: 0.2 μm) were added to this solution and mixed with a ball mill to obtain a PAI coating liquid (L-2).
The solid content concentration of L-2 was 25% by mass, and the mass ratio of PAI and alumina was PAI / alumina = 40/60. L-2 was applied to the surface of the electrode (P-1) and dried at 150 ° C. for 20 minutes to obtain an electrode (P-2) on which a porous PAI layer having a thickness of 5 μm was formed. The ion resistance increase rate of P-2 was 15%, and the discharge capacity retention rate was 95% or more.

<Example 2>
A PAI coating liquid (L-3) was obtained in the same manner as in Example 1 except that the mass ratio of PAI and alumina was PAI / alumina = 30/70. L-3 was applied to the surface of the electrode (P-1) and dried at 150 ° C. for 20 minutes to obtain an electrode (P-3) having a porous PAI layer having a thickness of 5 μm. The ion resistance increase rate of P-3 was 12%, and the discharge capacity retention rate was 95% or more.

<Example 3>
A PAI coating liquid (L-4) was obtained in the same manner as in Example 1 except that the mass ratio of PAI and alumina was PAI / alumina = 50/50. L-4 was applied to the surface of the electrode (P-1) and dried at 150 ° C. for 20 minutes to obtain an electrode (P-4) having a porous PAI layer having a thickness of 5 μm. The ion resistance increase rate of P-4 was 18%, and the discharge capacity retention rate was 95% or more.

<Comparative Example 1>
A PAI coating liquid (M-1) was obtained in the same manner as in Example 1 except that only NMP was used as a solvent for dissolving the PAI powder. M-1 was applied to the surface of the electrode (P-1) and dried at 150 ° C. for 20 minutes to obtain an electrode (R-1) having a porous PAI layer having a thickness of 4 μm. The ion resistance increase rate of R-1 was 26%, and the discharge capacity retention rate was 91%.

<Comparative Example 2>
An electrode (R-2) in which a porous PAI layer having a thickness of 5 μm was formed by applying L-1 containing no alumina to the surface of the electrode (P-1) and drying at 150 ° C. for 20 minutes. Got The ion resistance increase rate of R-2 was 31%, and the discharge capacity retention rate was 88%.

As described above, as shown in Examples and Comparative Examples, the electrodes for a lithium secondary battery obtained by using the coating liquid of the present invention have TPG, which is a poor solvent, and alumina, which is a filler, in the porous PAI layer. Since pores can be formed by the synergistic action of the above, it can be seen that good ion permeability of the electrode in which the porous PAI layer is laminated and integrated can be ensured. Further, it can be seen that good cycle characteristics can be obtained by this effect. Further, according to the manufacturing method of the present invention, an electrode having excellent safety can be manufactured by a simple process having high environmental compatibility.

Since the coating liquid of the present invention can be used to form a heat-resistant PAI porous layer having excellent ion permeability on the active material layer of the electrode for a lithium secondary battery, it is excellent in safety and cycle characteristics. It can be suitably used as an electrode for a lithium secondary battery.

Claims (2)

A coating liquid for electrodes for lithium secondary batteries having the following characteristics.
1) The solute consists of polyamide-imide (PAI).
2) The solvent is a mixed solvent of a good solvent and a poor solvent for the PAI, the good solvent is an amide solvent, and the poor solvent has a higher boiling point than the amide solvent, tripropylene glycol, which is an alcohol solvent. The content of the alcohol-based solvent in the mixed solvent is 5% by mass or more and 30% by mass or less .
3) Contains Fila. A method for manufacturing an electrode for a lithium secondary battery, wherein a coating film is formed by applying the coating solution according to claim 1 to the surface of an electrode active material layer formed on a metal foil, and then. When the solvent in the coating film is removed, the action of the alcohol-based solvent remaining in the coating film is used to cause phase separation in the coating film to form a porous PAI layer having ion permeability. A method for manufacturing an electrode for a lithium secondary battery.