JP7097410B2 - A method for producing coated negative electrode active material particles for a lithium ion battery, a negative electrode for a lithium ion battery, a lithium ion battery, and a coated negative electrode active material particle for a lithium ion battery. - Google Patents

Description

The present invention relates to a coated negative electrode active material particle for a lithium ion battery, a negative electrode for a lithium ion battery, a lithium ion battery, and a method for producing a coated negative electrode active material particle for a lithium ion battery.

Lithium-ion batteries have been widely used in various applications in recent years as secondary batteries that can achieve high energy density and high output density.

As a method for manufacturing a lithium ion battery, a method in which a slurry in which an electrode active material is mixed with a binder and a solvent is applied onto a base material, the solvent is removed, and then the battery is compressed can be mentioned.

However, in such a method, it is necessary to reliably dry the refractory solvent to a level that does not affect the battery performance, and therefore it is necessary to design a large-sized drying furnace. Therefore, there is a problem that it takes a lot of time and energy to remove the solvent.
Further, since the solvent is generally a non-aqueous electrolytic solution, it is difficult to control the manufacturing cost because a solvent recovery mechanism is required from the viewpoint of preventing air pollution.

On the other hand, for example, an electrode for a lithium ion battery is formed by using a coated active material particle in which at least a part of the surface of the active material particle is covered with a coating agent containing a coating resin and a conductive auxiliary agent without undergoing a solvent removal step. A method for producing the particles is known (see Patent Document 1).

In such a method, the drying equipment can be designed compactly, and not only the energy required for the conventional solvent removal can be suppressed, but also the land for manufacturing the lithium ion battery can be reduced.
Further, since the amount of the solvent used when producing the lithium ion battery can be reduced, the solvent recovery cost can be reduced, and the load on the surrounding environment can be reduced.

Japanese Unexamined Patent Publication No. 2016-189325

However, the electrode for a lithium ion battery manufactured by using the above-mentioned coated active material particles may not have sufficient mechanical strength, and as a result, the electrode for the lithium ion battery may be damaged in the manufacturing process of the lithium ion battery, or the electrode for the lithium ion battery may be damaged. The cycle characteristics of the lithium-ion battery may deteriorate, and there is room for improvement.

Therefore, according to the present invention, a negative electrode for a lithium ion battery having excellent mechanical strength can be manufactured, and even if a lithium ion battery is manufactured without going through a solvent removal step, a lithium ion battery having excellent cycle characteristics can be manufactured. It is an object of the present invention to provide a coated negative electrode active material particle which can be used.

As a result of diligent studies to solve the above problems, the present inventors have found that the coating layer constituting the coated negative electrode active material particles contains a specific polymer compound and the compound (A), whereby the above compound (A). ) Acts as a plasticizer for the polymer compound, imparting excellent elasticity to the coating layer and improving the adhesiveness between the coated negative electrode active material particles. Therefore, by using the coated negative electrode active material particles. , It has been found that a negative electrode for a lithium ion battery having excellent mechanical strength can be formed. Further, in the lithium ion battery formed by using the negative electrode for a lithium ion battery, the present inventors have improved the adhesiveness between the coated negative electrode active material particles in the negative electrode for a lithium ion battery, and the lithium ion battery is filled. We have found that the cycle characteristics are improved because the structure of the negative electrode for a lithium ion battery can be maintained even if the battery is discharged, and the present invention has been reached.

That is, in the present invention, at least a part of the surface of the negative electrode active material particles is coated with a coating layer containing the polymer compound and the compound (A), and the polymer compound constitutes (meth) acrylic acid. It is a polymer as a polymer, and the weight ratio of (meth) acrylic acid in the polymer is 70 to 95% by weight based on the weight of the polymer, and the compound (A) is tetrahydrothiophene 1,1. -Coated negative polymer particles for lithium ion batteries, which is at least one selected from the group consisting of dioxide, ethylene carbonate and vinylene carbonate; negative polymer for lithium ion batteries having the coated negative polymer particles; negative polymer for lithium ion batteries. A mixing step of mixing the polymer compound and the solution in which the compound (A) is dissolved in an organic solvent and the negative electrode active material particles, and a distillation step of distilling off the organic solvent after the mixing step. The above-mentioned polymer compound is a polymer having (meth) acrylic acid as a constituent monomer, and the weight ratio of (meth) acrylic acid in the above-mentioned polymer is 70 to 95 based on the weight of the above-mentioned polymer. In a method for producing a coated negative polymer particle for a lithium ion battery, which is by weight% and the compound (A) is at least one selected from the group consisting of tetrahydrothiophene 1,1-dioxide, ethylene carbonate and vinylene carbonate. be.

According to the present invention, a negative electrode for a lithium ion battery having excellent mechanical strength can be manufactured, and even if a lithium ion battery is manufactured without going through a solvent removal step, a lithium ion battery having excellent cycle characteristics can be manufactured. It is possible to provide coated negative electrode active material particles which can be used.

<Coated negative electrode active material particles for lithium-ion batteries>
In the coated negative electrode active material particles for a lithium ion battery of the present invention, at least a part of the surface of the negative electrode active material particles is coated with a coating layer containing the polymer compound and the compound (A). It is a polymer containing (meth) acrylic acid as a constituent monomer, and the weight ratio of (meth) acrylic acid in the polymer is 70 to 95% by weight based on the weight of the polymer, and the compound (A). ) Is at least one selected from the group consisting of tetrahydrothiophene 1,1-dioxide, ethylene carbonate and vinylene carbonate.

(Negative electrode active material particles)
Examples of the negative electrode active material particles include carbon-based materials [graphite, non-graphitizable carbon, amorphous carbon, fired resin (for example, phenol resin, furan resin, etc., carbonized), cokes (for example, pitch coke, etc.). Needle coke and petroleum coke etc.) and carbon fiber], silicon-based materials [silicon, silicon oxide (SiOx), silicon-carbon composite (carbon particles whose surface is coated with silicon and / or silicon carbide, silicon particles or The surface of silicon oxide particles coated with carbon and / or silicon carbide, silicon carbide, etc.) and silicon alloys (silicon-aluminum alloy, silicon-lithium alloy, silicon-nickel alloy, silicon-iron alloy, silicon-titanium alloy, etc. Silicon-manganese alloy, silicon-copper alloy, silicon-tin alloy, etc.)], conductive polymers (eg, polyacetylene and polypyrrole, etc.), metals (tin, aluminum, zirconium, titanium, etc.), metal oxides (titanium oxide) And lithium-titanium oxides, etc.), metal alloys (for example, lithium-tin alloys, lithium-aluminum alloys, lithium-aluminum-manganese alloys, etc.) and the like, and mixtures of these with carbon-based materials.
These negative electrode active material particles may be used alone or in combination of two or more.

The volume average particle diameter of the negative electrode active material particles is preferably 0.01 to 100 μm, more preferably 0.1 to 20 μm, and even more preferably 2 to 10 μm from the viewpoint of the electrical characteristics of the battery.

(Coating layer)
The coating layer contains a polymer compound and the compound (A), and the polymer compound is a polymer having (meth) acrylic acid as a constituent monomer, and the weight ratio of the (meth) acrylic acid in the polymer is It is 70 to 95% by weight based on the weight of the polymer, and the compound (A) is at least one selected from the group consisting of tetrahydrothiophene 1,1-dioxide, ethylene carbonate and vinylene carbonate.
When the coating layer constituting the coated negative electrode active material particles contains a specific polymer compound and the compound (A), the compound (A) acts as a plasticizer for the polymer compound, and the coating layer has excellent elasticity. And can improve the adhesiveness between the coated negative electrode active material particles. Then, by using such coated negative electrode active material particles, it is possible to form a negative electrode for a lithium ion battery having excellent mechanical strength.

The polymer compound is a polymer containing (meth) acrylic acid as a constituent monomer, and the weight ratio of (meth) acrylic acid in the polymer is 70 to 95% by weight based on the weight of the polymer.
From the viewpoint of controlling the degree of swelling of the polymer compound into the compound (A) and the electrolytic solution, the weight ratio of (meth) acrylic acid is preferably 80 to 92% by weight based on the weight of the polymer.

The polymer compound is a polymer having a vinyl monomer (b) as a constituent monomer from the viewpoint of improving the adhesiveness between the coating active material particles, and is represented by the following general formula (1) as the vinyl monomer (b). It is preferable to contain the vinyl monomer (b1) to be added.
CH ₂ = C (R ¹ ) COOR ² (1)
[In the general formula (1), R ¹ is a hydrogen atom or a methyl group, and R ² is an alkyl group having 1 to 12 carbon atoms. ]

The alkyl group having 1 to 12 carbon atoms in ^R2 may be a linear alkyl group or a branched alkyl group.
The linear alkyl group of the alkyl group having 1 to 12 carbon atoms includes a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group and a dodecyl group. The group is mentioned.
Branched alkyl groups of alkyl groups having 1 to 12 carbon atoms include 1-methylpropyl group (sec-butyl group), 2-methylpropyl group, 1,1-dimethylethyl group (tert-butyl group), and 1-methylbutyl group. Group, 1,1-dimethylpropyl group, 1,2-dimethylpropyl group, 2,2-dimethylpropyl group (neopentyl group), 1-methylpentyl group, 2-methylpentyl group, 3-methylpentyl group, 4- Methylpentyl group, 1,1-dimethylbutyl group, 1,2-dimethylbutyl group, 1,3-dimethylbutyl group, 2,2-dimethylbutyl group, 2,3-dimethylbutyl group, 1-ethylbutyl group, 2 -Ethylbutyl group, 1-methylhexyl group, 2-methylhexyl group, 2-methylhexyl group, 4-methylhexyl group, 5-methylhexyl group, 1-ethylpentyl group, 2-ethylpentyl group, 3-ethylpentyl group Group, 1,1-dimethylpentyl group, 1,2-dimethylpentyl group, 1,3-dimethylpentyl group, 2,2-dimethylpentyl group, 2,3-dimethylpentyl group, 2-ethylpentyl group, 1- Methylheptyl group, 2-methylheptyl group, 3-methylheptyl group, 4-methylheptyl group, 5-methylheptyl group, 6-methylheptyl group, 1,1-dimethylhexyl group, 1,2-dimethylhexyl group, 1,3-dimethylhexyl group, 1,4-dimethylhexyl group, 1,5-dimethylhexyl group, 1-ethylhexyl group, 2-ethylhexyl group, 1-methyloctyl group, 2-methyloctyl group, 3-methyloctyl Group, 4-methyloctyl group, 5-methyloctyl group, 6-methyloctyl group, 7-methyloctyl group, 1,1-dimethylheptyl group, 1,2-dimethylheptyl group, 1,3-dimethylheptyl group, 1,4-dimethylheptyl group, 1,5-dimethylheptyl group, 1,6-dimethylheptyl group, 1-ethylheptyl group, 2-ethylheptyl group, 1-methylnonyl group, 2-methylnonyl group, 3-methylnonyl group , 4-Methylnonyl group, 5-Methylnonyl group, 6-Methylnonyl group, 7-Methylnonyl group, 8-Methylnonyl group, 1,1-dimethyloctyl group, 1,2-dimethyloctyl group, 1,3-dimethyloctyl group, 1,4-Dimethyloctyl group, 1,5-dimethyloctyl group, 1,6-dimethyloctyl group, 1,7-dimethyloctyl group, 1-ethyloctyl group, 2-ethyloctyl group, 1-methyldecyl group, 2 -Methyldecyl Group, 3-methyldecyl group, 4-methyldecyl group, 5-methyldecyl group, 6-methyldecyl group, 7-methyldecyl group, 8-methyldecyl group, 9-methyldecyl group, 1,1-dimethylnonyl group, 1,2-dimethyl Nonyl group, 1,3-dimethylnonyl group, 1,4-dimethylnonyl group, 1,5-dimethylnonyl group, 1,6-dimethylnonyl group, 1,7-dimethylnonyl group, 1,8-dimethylnonyl group , 1-ethylnonyl group, 2-ethylnonyl group, 1-methylundecyl group, 2-methylundecyl group, 3-methylundesyl group, 4-methylundecyl group, 5-methylundecyl group, 6-methylundecyl group Decyl group, 7-methylundecyl group, 8-methylundecyl group, 9-methylundecyl group, 10-methylundecyl group, 1,1-dimethyldecyl group, 1,2-dimethyldecyl group, 1,3 -Dimethyldecyl group, 1,4-dimethyldecyl group, 1,5-dimethyldecyl group, 1,6-dimethyldecyl group, 1,7-dimethyldecyl group, 1,8-dimethyldecyl group, 1,9-dimethyl Examples thereof include a decyl group, a 1-ethyl decyl group and a 2-ethyl decyl group.

The vinyl monomer (b1) contains methyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, butyl acrylate (iso), from the viewpoint of improving flexibility when the polymer compound swells with respect to the compound (A). Alternatively, it is preferably 2-ethylhexyl acrylate.

In the polymer compound, the weight ratio of the vinyl monomer (b) is preferably 5 to 30% by weight, more preferably 10 to 15% by weight, based on the weight of the polymer.

The polymer compound may contain other monomers as long as the effects of the present invention are not impaired.
As the other monomer, for example, the monomer used for the active material coating resin in JP-A-2017-054703 and International Publication No. 2015-005117 can be appropriately selected and used.

Examples of the polymer compound include known polymerization initiators {azo-based initiators [2,2'-azobis (2-methylpropionitrile), 2,2'-azobis (2,4-dimethylvaleronitrile), 2 , 2'-azobis (2-methylbutyronitrile), 2,2'-azobis (2-methylpropionamidine) dihydrochloride, etc.], Peroxide-based initiators (benzoyl peroxide, di-t-butyl peroxide) , Lauryl peroxide, etc.)} can be produced by a known polymerization method (lumpy polymerization, solution polymerization, emulsification polymerization, suspension polymerization, etc.).
The amount of the polymerization initiator used is preferably 0.01 to 5% by weight, more preferably 0.05 to 2% by weight, based on the total weight of the monomer, from the viewpoint of adjusting the weight average molecular weight to a preferable range. %, More preferably 0.1 to 1.5% by weight, and the polymerization temperature and the polymerization time are adjusted according to the type of the polymerization initiator and the like, but the polymerization temperature is preferably −5 to 150 ° C. (more). The reaction time is preferably 0.1 to 50 hours (more preferably 2 to 24 hours).

Solvents used in the case of solution polymerization include, for example, esters (2 to 8 carbon atoms, such as ethyl acetate and butyl acetate), alcohols (1 to 8 carbon atoms, such as methanol, ethanol and octanol), and hydrocarbons (carbon atoms). Examples include 4-8, such as n-butane, cyclohexane and toluene), amides (eg, N, N-dimethylformamide (hereinafter abbreviated as DMF)) and ketones (3-9 carbon atoms, eg methylethylketone), weight average. From the viewpoint of adjusting the molecular weight to a preferable range, the amount used is preferably 5 to 900% by weight, more preferably 10 to 400% by weight, still more preferably 30 to 300% by weight based on the total weight of the monomers. The monomer concentration is preferably 10 to 95% by weight, more preferably 20 to 90% by weight, and even more preferably 30 to 80% by weight.

Examples of the dispersion medium in emulsification polymerization and suspension polymerization include water, alcohol (for example, ethanol), ester (for example, ethyl propionate), light naphtha and the like, and examples of the emulsifier include higher fatty acid (10 to 24 carbon atoms) metal salt. (For example, sodium oleate and sodium stearate), higher alcohol (10 to 24 carbon atoms) sulfate ester metal salt (for example, sodium lauryl sulfate), tetramethyldecine ethoxylated tetramethyldecine diol, sodium sulfoethyl methacrylate, dimethylaminomethyl methacrylate, etc. Can be mentioned. Further, polyvinyl alcohol, polyvinylpyrrolidone and the like may be added as stabilizers.
The monomer concentration of the solution or dispersion is preferably 5 to 95% by weight, more preferably 10 to 90% by weight, still more preferably 15 to 85% by weight, and the amount of the polymerization initiator used is the monomer. Based on the total weight, it is preferably 0.01 to 5% by weight, more preferably 0.05 to 2% by weight.
For polymerization, known chain transfer agents such as mercapto compounds (dodecyl mercaptan, n-butyl mercaptan, etc.) and / or halogenated hydrocarbons (carbon tetrachloride, carbon tetrabromide, benzyl chloride, etc.) can be used. ..

The polymer compound is a cross-linking agent (A') having a reactive functional group that reacts the polymer compound with a carboxyl group {preferably a polyepoxy compound (a'1) [polyglycidyl ether (bisphenol A diglycidyl ether, propylene). Glycoldiglycidyl ether and glycerin triglycidyl ether, etc.) and polyglycidylamine (N, N-diglycidylaniline and 1,3-bis (N, N-diglycidylaminomethyl)), etc.] and / or polyol compound (a' 2) It may be a crosslinked polymer crosslinked with (ethylene glycol or the like)}.

Examples of the method for cross-linking the polymer compound using the cross-linking agent (A') include a method in which the negative electrode active material particles are coated with the polymer compound and then cross-linked. Specifically, after the coated active material particles are produced by mixing the negative electrode active material particles and the resin solution containing the polymer compound and removing the solvent, the coated active material particles are mixed with the solution containing the cross-linking agent (A'). A method of causing a desolving reaction and a cross-linking reaction by mixing the particles with and heating the particles to cause a reaction in which the polymer compound is cross-linked by the cross-linking agent (A') on the surface of the negative electrode active material particles can be mentioned.
The heating temperature is adjusted according to the type of the cross-linking agent, but is preferably 70 ° C. or higher when the polyepoxy compound (a'1) is used as the cross-linking agent, and is preferably 70 ° C. or higher when the polyol compound (a'2) is used. It is preferably 120 ° C. or higher.

The polymer compound preferably has a swelling degree of 150 to 400% by weight, more preferably 180 to 220% by weight, based on the compound (A).
Further, from the viewpoint of maintaining conductivity in the negative electrode active material layer in the lithium ion battery, the polymer compound more preferably has a swelling degree of 1 to 30% by weight with respect to the electrolytic solution described later, and 5 to 10% by weight. Is more preferable.
When the polymer compound has such a property, excellent elasticity can be imparted to the coating layer.
The degree of swelling with respect to the compound (A) in this paragraph means the degree of swelling with respect to the compound (A) used when producing the coated negative electrode active material particles for a lithium ion battery described later. Further, the degree of swelling with respect to the electrolytic solution in this paragraph means the degree of swelling with respect to the electrolytic solution used when manufacturing the negative electrode for a lithium ion battery described later.

The polymer compound preferably has a swelling degree of 150 to 250% by weight, more preferably 180 to 220% by weight, based on ethylene carbonate.
The polymer compound is an electrolytic solution prepared by dissolving 10 parts by weight of LiFSI [LiN (FSO ₂ ) ₂ ] in a mixed solvent of 3.5 parts by weight of ethylene carbonate (EC) and 5 parts by weight of propylene carbonate (PC). The degree of swelling is more preferably 1 to 20% by weight, and more preferably 5 to 10% by weight.

The degree of swelling can be measured by, for example, the following method.
The polymer compound is roughly crushed with a hammer and additionally crushed with a coffee mill to make a powder. Further, additional pulverization is performed using an agate mortar to make the polymer compound into a fine powder. Next, using a heat press machine in which the heating block was heated to a temperature at which the polymer compound can be molded (for example, 110 ° C.), a 0.1 mm Teflon (registered trademark) sheet was coated with a mold release agent (10 × 40). A metal frame of × 0.2 mm is placed, a powdery polymer compound is spread in the metal frame, covered with a Teflon (registered trademark) sheet, and pressed at a pressure of 1 MPa for 60 seconds. After pressing, the powdered polymer compound is further spread in the metal frame, and the operation of pressing at a pressure of 1 MPa for 60 seconds is repeated until there are no opaque parts or bubbles in the metal frame, and the metal frame is removed from the metal frame. And get a test piece. This test piece is immersed in a solvent (compound (A) or electrolytic solution) at 50 ° C. for 3 days to bring it into a saturated liquid absorbing state.
After that, the degree of swelling can be obtained by the following formula from the change in weight of the test piece before and after absorbing the liquid.
Swelling degree [% by weight] = [(weight of test piece after liquid absorption-weight of test piece before liquid absorption) / weight of test piece before liquid absorption] × 100

The preferable lower limit of the weight average molecular weight of the polymer compound is 3,000, the more preferable lower limit is 5,000, and the further preferable lower limit is 7,000. On the other hand, the preferable upper limit of the weight average molecular weight of the polymer compound is 100,000, and the more preferable upper limit is 70,000.

The weight average molecular weight of the polymer compound can be determined by gel permeation chromatography (hereinafter abbreviated as GPC) measurement under the following conditions.
Equipment: Alliance GPC V2000 (manufactured by Waters)
Solvents: orthodichlorobenzene, DMF, THF
Standard substance: Polystyrene Sample concentration: 3 mg / ml
Column stationary phase: PLgel 10 μm, MIXED-B 2 in series (manufactured by Polymer Laboratories)
Column temperature: 135 ° C

In the coated negative electrode active material particles for lithium ion batteries of the present invention, the weight ratio of the polymer compound is 1 to 7% by weight based on the weight of the coated negative electrode active material particles for lithium ion batteries from the viewpoint of electrical resistance and energy density. It is preferably 2 to 6% by weight, and more preferably 2 to 6% by weight.

Compound (A) is at least one selected from the group consisting of tetrahydrothiophene 1,1-dioxide, ethylene carbonate and vinylene carbonate.
When the coating layer contains the compound (A), the coating layer is in a state of being swollen by the compound (A), imparting excellent elasticity to the coating layer and improving the adhesiveness between the coated negative electrode active material particles. Can be done. Further, even when the coated negative electrode active material particles are immersed in the electrolytic solution, the coating layer partially maintains the compound (A), and the adhesiveness between the coated negative electrode active material particles is also maintained, so that the mechanical strength is maintained. It is considered that an excellent negative electrode for a lithium ion battery can be obtained.

From the viewpoint of preferably improving the cycle characteristics of the lithium ion battery, the compound (A) is preferably a combination of ethylene carbonate and vinylene carbonate, or a combination of tetrahydrothiophene 1,1-dioxide and vinylene carbonate.

When the compound (A) contains vinylene carbonate, the vinylene carbonate is preferably 10% by weight or less based on the weight of the compound (A) from the viewpoint of preferably forming the SEI film and improving the cycle characteristics. ..

In the coated negative electrode active material particles for lithium ion batteries of the present invention, the weight ratio of the compound (A) is 0.5 based on the weight of the coated negative electrode active material particles for lithium ion batteries from the viewpoint of imparting elasticity to the coating layer. It is preferably ~ 14% by weight, more preferably 1-2% by weight.

The coating layer preferably contains a conductive auxiliary agent from the viewpoint of internal resistance and the like.
The conductive auxiliary agent is preferably selected from materials having conductivity.
Preferred conductive aids are metals [aluminum, stainless steel (SUS), silver, gold, copper and titanium, etc.], carbon [graphite and carbon black (acetylene black, ketjen black, furnace black, channel black, thermal lamps, etc.]. Black, carbon nanofibers, etc.)], and mixtures thereof, etc. may be mentioned.
These conductive aids may be used alone or in combination of two or more. Further, it may be used as these alloys or metal oxides.
Among them, from the viewpoint of electrical stability, aluminum, stainless steel, carbon, silver, gold, copper, titanium and mixtures thereof are more preferable, and silver, gold, aluminum, stainless steel and carbon are particularly preferable. It is preferably carbon.
Further, as these conductive auxiliaries, a conductive material [preferably a metal one among the above-mentioned conductive auxiliaries] may be coated around a particle-based ceramic material or a resin material by plating or the like.

The shape (form) of the conductive auxiliary agent is not limited to the particle form, and may be a form other than the particle form, and is a form practically used as a so-called filler-based conductive auxiliary agent such as carbon nanofibers and carbon nanotubes. You may.

The average particle size of the conductive auxiliary agent is not particularly limited, but is preferably about 0.01 to 10 μm from the viewpoint of the electrical characteristics of the battery.
In the present specification, the "particle diameter of the conductive auxiliary agent" means the maximum distance L among the distances between arbitrary two points on the contour line of the conductive auxiliary agent. As the value of the "average particle size", the average value of the particle size of the particles observed in several to several tens of fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The calculated value shall be adopted.

The ratio of the polymer compound to the conductive auxiliary agent is not particularly limited, but from the viewpoint of the internal resistance of the battery and the like, the polymer compound (resin solid content weight): conductive auxiliary agent is 1: 0.01 in terms of weight ratio. It is preferably from 1:50 to 1: 0.2 to 1: 3.0, and more preferably from 1: 0.2 to 1: 3.0.

(Coated negative electrode active material particles)
In the coated negative electrode active material particles for a lithium ion battery of the present invention, at least a part of the surface of the negative electrode active material particles is coated with a coating layer containing the polymer compound and the compound (A).
From the viewpoint of cycle characteristics, the negative electrode active material particles preferably have a coverage of 30 to 95% obtained by the following formula.
Coverage (%) = {1- [BET specific surface area of coated negative electrode active material particles / (BET specific surface area of negative electrode active material particles × weight ratio of negative electrode active material particles contained in coated negative electrode active material particles + conductive auxiliary agent BET specific surface area × weight ratio of conductive auxiliary agent contained in coated negative electrode active material particles)]} × 100

<Manufacturing method of coated negative electrode active material particles for lithium ion batteries>
The method for producing coated negative electrode active material particles for a lithium ion battery of the present invention comprises a mixing step of mixing a solution in which a polymer compound and the compound (A) are dissolved in an organic solvent and negative electrode active material particles, and after the above mixing step. Including the distillation step of distilling off the organic solvent, the polymer compound is a polymer having (meth) acrylic acid as a constituent monomer, and the weight ratio of (meth) acrylic acid in the polymer is It is 70 to 95% by weight based on the weight of the polymer, and the compound (A) is at least one selected from the group consisting of tetrahydrothiophene 1,1-dioxide, ethylene carbonate and vinylene carbonate. It is a feature.

(Mixing process)
The method for producing coated negative electrode active material particles for a lithium ion battery of the present invention includes a mixing step of mixing a solution in which the polymer compound and the compound (A) are dissolved in an organic solvent and the negative electrode active material particles.

In the method for producing coated negative electrode active material particles for a lithium ion battery of the present invention, the material described in the above-mentioned coated negative electrode active material particles for a lithium ion battery of the present invention can be appropriately selected and used.
Further, the organic solvent is not particularly limited as long as it can dissolve the polymer compound and the compound (A), and for example, those exemplified as the solvent used in the case of the above-mentioned solution polymerization can be used. ..

The method for mixing the polymer compound and the solution in which the compound (A) is dissolved in the organic solvent and the negative electrode active material particles is not particularly limited, and a known method can be used.
For example, in a state where the negative electrode active material particles are placed in a universal mixer and stirred at 30 to 500 rpm, a solution in which the polymer compound and the compound (A) are dissolved in an organic solvent is dropped and mixed over 1 to 90 minutes, and it is necessary. Depending on the method, a method of mixing the conductive auxiliary agent and the like can be mentioned.

In the mixing step, the blending ratio of each component is not particularly limited, but for example, the negative weight active material particles are 79.5 to 99.5% by weight, the polymer compound is 1 to 7% by weight, and the compound (weight ratio in solid content). It is preferable that A) is blended so as to be 0.5 to 14% by weight.
Further, the conductive auxiliary agent is preferably blended so that the polymer compound (resin solid content weight): conductive auxiliary agent has a weight ratio of 1: 0.01 to 1:50.

(Distilling process)
The method for producing coated negative electrode active material particles for a lithium ion battery of the present invention includes a distillation step of distilling off an organic solvent after a mixing step.

The distillation step is not particularly limited, and a known method can be used.
For example, the mixed composition obtained in the mixing step is heated to 50 to 200 ° C. while stirring, reduced to 0.007 to 0.04 MPa, and then held for 10 to 150 minutes to distill off the organic solvent. A method or the like can be used.
The distillation step can be operated in a compact facility because the amount of the organic solvent to be distilled off is smaller than that in the solvent removal step when manufacturing a conventional lithium ion battery.

<Negative electrode for lithium-ion batteries>
The negative electrode for a lithium ion battery of the present invention is characterized by having the coated negative electrode active material particles of the present invention described above.
The negative electrode for a lithium ion battery of the present invention preferably includes a negative electrode active material layer containing coated negative electrode active material particles, an electrolytic solution containing an electrolyte and a solvent, and a negative electrode current collector.

(Negative electrode active material layer)
In the negative electrode active material layer, the weight ratio of the coated negative electrode active material particles for a lithium ion battery of the present invention is preferably 40 to 95% by weight, preferably 60 to 90% by weight, based on the weight of the negative electrode active material layer. Is more preferable.

As the electrolyte, an electrolyte used in a known electrolytic solution can be used, for example, a lithium salt of an inorganic anion such as LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ and LiN (FSO ₂ ) ₂ , LiN. Examples thereof include lithium salts of organic anions such as (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₃ . Of these, LiN (FSO ₂ ) ₂ is preferable from the viewpoint of battery output and charge / discharge cycle characteristics.

As the solvent, a non-aqueous solvent used in a known electrolytic solution can be used, and for example, a lactone compound, a cyclic or chain carbonate ester, a chain carboxylic acid ester, a cyclic or chain ether, a phosphoric acid ester, or a nitrile compound can be used. , Amid compounds, sulfones, sulfolanes and mixtures thereof can be used.

Examples of the lactone compound include a 5-membered ring (γ-butyrolactone, γ-valerolactone, etc.) and a 6-membered ring (δ-valerolactone, etc.) lactone compound.

Examples of the cyclic carbonic acid ester include propylene carbonate, ethylene carbonate (EC) and butylene carbonate (BC).
Examples of the chain carbonate ester include dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl-n-propyl carbonate, ethyl-n-propyl carbonate, di-n-propyl carbonate and the like. ..

Examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, propyl acetate, methyl propionate and the like.

Examples of the cyclic ether include tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,4-dioxane and the like. Examples of the chain ether include dimethoxymethane and 1,2-dimethoxyethane.

Examples of the phosphoric acid ester include trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, tributyl phosphate, tri (trifluoromethyl) phosphate, and tri (trichloromethyl) phosphate. Tri (trifluoroethyl) phosphate, Tri (triperfluoroethyl) phosphate, 2-ethoxy-1,3,2-dioxaphosphoran-2-one, 2-trifluoroethoxy-1,3,2- Examples thereof include dioxaphosphoran-2-one and 2-methoxyethoxy-1,3,2-dioxaphosphoran-2-one.

Examples of the nitrile compound include acetonitrile and the like. Examples of the amide compound include DMF and the like. Examples of the sulfone include dimethyl sulfone and diethyl sulfone.

One of these solvents may be used alone, or two or more of them may be used in combination.

The concentration of the electrolyte in the electrolytic solution is preferably 1.2 to 5.0 mol / L, more preferably 1.5 to 4.5 mol / L, and 1.8 to 4.0 mol / L. It is more preferably present, and particularly preferably 2.0 to 3.5 mol / L.
Since such an electrolytic solution has an appropriate viscosity, a liquid film can be formed between the coated negative electrode active material particles, and the coated negative electrode active material particles have a lubricating effect (position adjusting ability of the coated negative electrode active material particles). Can be granted.

The negative electrode active material layer may further contain a conductive auxiliary agent in addition to the conductive auxiliary agent contained in the coating layer of the coated negative electrode active material particles described above, if necessary. The conductive auxiliary agent contained in the coating layer as needed is integrated with the coated negative electrode active material particles, whereas the conductive auxiliary agent contained in the negative electrode active material layer is contained separately from the coated negative electrode active material particles. Can be distinguished by.
As the conductive auxiliary agent that may be contained in the negative electrode active material layer, those described in <Coated negative electrode active material particles for lithium ion battery> can be used.

When the negative electrode active material layer contains a conductive auxiliary agent, the total content of the conductive auxiliary agent contained in the negative electrode active material layer and the conductive auxiliary agent contained in the coating layer is the weight obtained by removing the electrolytic solution from the negative electrode active material layer. It is preferably less than 4% by weight, more preferably less than 3% by weight. On the other hand, the total content of the conductive auxiliary agent contained in the negative electrode active material layer and the conductive auxiliary agent contained in the coating layer is 2.5% by weight or more based on the weight of the negative electrode active material layer excluding the electrolytic solution. It is preferable to have.

The negative electrode active material layer preferably does not contain a binder.
In the present specification, the binder means an agent that cannot reversibly fix the coated negative electrode active material particles to each other and the coated negative electrode active material particles to the current collector, and includes starch, polyvinylidene fluoride, and the like. Examples thereof include known solvent-drying binders for lithium ion batteries such as polyvinyl alcohol, carboxymethyl cellulose, polyvinylpyrrolidone, tetrafluoroethylene, styrene-butadiene rubber, polyethylene and polypropylene.
These binders are used by dissolving or dispersing in a solvent, and solidify by volatilizing and distilling off the solvent to irreversibly separate the coated negative electrode active material particles and the coated negative electrode active material particles from the current collector. It is fixed to.

The negative electrode active material layer may contain an adhesive resin.
The adhesive resin means a resin that does not solidify even when the solvent component is volatilized and dried, and is a material different from the binder and is distinguished.
Further, while the coating layer constituting the coated negative electrode active material particles is fixed to the surface of the negative electrode active material particles, the adhesive resin reversibly fixes the surfaces of the negative electrode active material particles to each other. The adhesive resin can be easily separated from the surface of the negative electrode active material particles, but the coating layer cannot be easily separated. Therefore, the coating layer and the adhesive resin are different materials.

The adhesive resin contains at least one low Tg monomer selected from the group consisting of vinyl acetate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, butyl acrylate and butyl methacrylate as an essential constituent monomer and the above low Tg monomer. Examples thereof include polymers in which the total weight ratio of the above is 45% by weight or more based on the total weight of the constituent monomers.
When the adhesive resin is used, it is preferable to use 0.01 to 10% by weight of the adhesive resin with respect to the total weight of the negative electrode active material particles.

The thickness of the negative electrode active material layer is preferably 150 to 600 μm, more preferably 200 to 450 μm from the viewpoint of battery performance.

(Negative electrode current collector)
It is preferable that the negative electrode for a lithium ion battery includes a negative electrode current collector, and the negative electrode active material layer is provided on the surface of the current collector.

Examples of the material constituting the negative electrode current collector include metal materials such as copper, aluminum, titanium, stainless steel, nickel and alloys thereof, as well as calcined carbon, a conductive polymer material, and conductive glass.
The shape of the negative electrode current collector is not particularly limited, and may be a sheet-shaped current collector made of the above-mentioned material and a deposited layer made of fine particles made of the above-mentioned material.

It is preferable that the negative electrode for a lithium ion battery of the present invention includes a resin current collector made of a conductive polymer material, and a negative electrode active material layer is provided on the surface of the resin current collector.

As the conductive polymer material constituting the resin current collector, for example, a resin to which a conductive material is added can be used.
As the conductive material constituting the conductive polymer material, the same material as the conductive auxiliary agent which is an arbitrary component of the coating layer can be preferably used.
Resins constituting the conductive polymer material include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polycycloolefin (PCO), polyethylene terephthalate (PET), polyether nitrile (PEN), and poly. Tetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVdF), epoxy resin, silicone resin or a mixture thereof. And so on.
From the viewpoint of electrical stability, polyethylene (PE), polypropylene (PP), polymethylpentene (PMP) and polycycloolefin (PCO) are preferable, and polyethylene (PE), polypropylene (PP) and polymethylpentene are more preferable. (PMP).
The resin current collector can be obtained by a known method described in JP-A-2012-150905 and International Publication No. 2015/005116.

The thickness of the negative electrode current collector is not particularly limited, but is preferably 5 to 150 μm.

The negative electrode for a lithium ion battery of the present invention is, for example, a negative electrode current collector of a powder (negative electrode precursor) in which coated negative electrode active material particles swollen with the compound (A) of the present invention and, if necessary, a conductive auxiliary agent and the like are mixed. It can be produced by applying it to a body and pressing it with a press to form a negative electrode active material layer, and then injecting an electrolytic solution.
Further, the negative electrode precursor may be applied and pressed on the release film to form a negative electrode active material layer, the negative electrode active material layer may be transferred to the negative electrode current collector, and then the electrolytic solution may be injected. In the negative electrode for a lithium ion battery of the present invention, the structure of the negative electrode active material layer can be maintained even if the electrolytic solution is injected because the coated negative electrode active material particles have excellent adhesion to each other, so that the mechanical strength is excellent. At the same time, it has excellent cycle characteristics.
In manufacturing the negative electrode for a lithium ion battery of the present invention, an unnecessary solvent is not used, so that it is not necessary to use a large-sized drying furnace or solvent recovery mechanism required in the conventional solvent removal step.

<Lithium-ion battery>
The lithium ion battery of the present invention has a negative electrode for a lithium ion battery of the present invention.
The lithium ion battery of the present invention includes a negative electrode for a lithium ion battery of the present invention, a separator, and a positive electrode.

(Separator)
As the separator, a porous film made of polyethylene or polypropylene, a laminated film of a porous polyethylene film and a porous polypropylene, a non-woven fabric made of synthetic fibers (polyester fiber, aramid fiber, etc.) or glass fiber, etc., and silica on the surface thereof. , Known separators for lithium ion batteries such as those to which ceramic fine particles such as alumina and titania are attached can be mentioned.

(Positive electrode)
The positive electrode preferably includes a positive electrode active material layer and a positive electrode current collector.

The positive electrode active material layer contains positive electrode active material particles.
The positive electrode active material particles include a composite oxide of lithium and a transition metal {composite oxide having one kind of transition metal (LiCoO ₂ , LiNiO ₂ , LiAlMnO ₄ , LiMnO ₂ , _{LiMn2O 4} , etc.), a transition metal element. (For example, LiFeMnO ₄ , LiNi _1-x Co _x O ₂ , LiMn _{1-y Coy} O ₂ , LiNi _1/3 Co _1/3 Al _1/3 O ₂ and LiNi _0.8 Co _0.15 Al _0.05 O ₂ ) and composite oxides containing three or more kinds of metal elements [for example, LiM _a M'b _M''c O ₂ (M, _M'and M'' are different transition metals. It is an element and satisfies a + b + c = 1. For example, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) etc.], etc.}, lithium-containing transition metal phosphates (for example, LiFePO ₄ , LiCoPO ₄ , LiMnPO ₄ and LiNiPO). ₄ ), transition metal oxides (eg MnO ₂ and V ₂ O ₅ ), transition metal sulfides (eg MoS ₂ and TiS ₂ ) and conductive polymers (eg polyaniline, polypyrrole, polythiophene, polyacetylene and poly-p-phenylene). And polyvinylcarbazole) and the like, and two or more kinds may be used in combination.
The lithium-containing transition metal phosphate may be obtained by substituting a part of the transition metal site with another transition metal.

The volume average particle size of the positive electrode active material particles is preferably 0.01 to 100 μm, more preferably 0.1 to 35 μm, and further preferably 2 to 30 μm from the viewpoint of the electrical characteristics of the battery. preferable.

The positive electrode active material particles may be coated positive electrode active material particles in which at least a part of the surface thereof is coated with a coating layer containing a polymer compound.
When the periphery of the positive electrode active material particles is covered with a coating layer, the volume change of the positive electrode is alleviated, and the expansion of the positive electrode can be suppressed.

As the coating layer, the same coating layer as that described in the above-mentioned coated negative electrode active material particles for a lithium ion battery of the present invention can be preferably used.

The positive electrode active material layer preferably does not contain a binder.
The binder means the one described in the negative electrode.

The positive electrode active material layer may contain an adhesive resin.
As the adhesive resin, the same adhesive resin as the adhesive resin which is an optional component of the negative electrode active material layer can be preferably used.

The positive electrode active material layer may contain a conductive auxiliary agent.
As the conductive auxiliary agent, a conductive material similar to the conductive filler contained in the negative electrode active material layer can be preferably used.
The weight ratio of the conductive auxiliary agent in the positive electrode active material layer is preferably 2 to 10% by weight.

The positive electrode active material layer may contain an electrolytic solution.
As the electrolytic solution, the one described in the negative electrode active material layer can be appropriately selected and used.

The thickness of the positive electrode active material layer is not particularly limited, but is preferably 150 to 600 μm, more preferably 200 to 450 μm from the viewpoint of battery performance.

The positive electrode current collector is a resin current collector composed of a known metal current collector and a conductive resin composition containing a conductive material and a resin (Japanese Patent Laid-Open No. 2012-150905 and International Publication No. 2015-005116). The resin collector described in No. etc.) can be used.
The positive electrode current collector is preferably a resin current collector from the viewpoint of battery characteristics and the like.
The thickness of the positive electrode current collector is not particularly limited, but is preferably 5 to 150 μm.

The positive electrode can be produced, for example, by applying a mixture containing positive electrode active material particles and an electrolytic solution to the surface of a positive electrode current collector or a base material to remove excess electrolytic solution.
When the positive electrode active material layer is formed on the surface of the base material, the positive electrode active material layer may be combined with the positive electrode current collector by a method such as transfer.
If necessary, the mixture may contain a conductive auxiliary agent, an adhesive resin, or the like.

(Manufacturing method of lithium-ion battery)
The lithium ion battery of the present invention can be manufactured, for example, by stacking a positive electrode, a separator, and a negative electrode for a lithium ion battery of the present invention in this order, and then injecting an electrolytic solution as needed.

Next, the present invention will be specifically described with reference to Examples, but the present invention is not limited to the Examples as long as it does not deviate from the gist of the present invention. Unless otherwise specified, parts mean parts by weight and% means parts by weight.

<Manufacturing of polymer compounds>
The monomers used to prepare the polymer compound are as follows.
AA: Acrylic acid MAA: Methacrylic acid MMA: Methyl methacrylate BMA: Butyl methacrylate EHMA: 2-ethylhexyl methacrylate PCMA: (2-oxo-1,3-dioxolan-4-yl) methyl acrylate

(Preparation of Polymer Compound 1)
150 parts of DMF was placed in a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel and a nitrogen gas introduction tube, and the temperature was raised to 75 ° C. Next, a monomer composition in which 92 parts of acrylic acid and 8 parts of methyl methacrylate were dissolved in 50 parts of DMF, and 0.1 parts and 2,2'-azobis (2,4-dimethylvaleronitrile) of 2,2'-azobis (2,4-dimethylvaleronitrile). -Initiator solution in which 0.4 part of azobis (2-methylbutyronitrile) is dissolved in 30 parts of DMF is continuously added dropwise over 2 hours with a dropping funnel while blowing nitrogen into a four-necked flask. And radical polymerization was carried out. After completion of the dropping, the temperature was raised to 80 ° C. and the reaction was continued for 2 hours. Then, the temperature was raised to 85 ° C. and the reaction was continued for 2 hours, and the temperature was further raised to 95 ° C. and the reaction was continued for 1 hour to obtain a copolymer solution having a resin concentration of 30%. The obtained copolymer solution was transferred to a Teflon (registered trademark) vat and dried under reduced pressure at 100 ° C. and 0.01 MPa for 3 hours, and DMF was distilled off to obtain a copolymer. This copolymer was roughly pulverized with a hammer and then additionally pulverized with a coffee mill [Force Mill, manufactured by Osaka Chemical Co., Ltd.] to prepare a powdery polymer compound 1.

(Degree of swelling with respect to ethylene carbonate)
Ethylene carbonate (EC) was prepared as compound (A).
The polymer compound 1 was further pulverized using an agate mortar to form a fine powder. Next, a 10 × 40 × 0.2 mm metal frame coated with a mold release agent was placed on a 0.1 mm Teflon (registered trademark) sheet, and a powdery polymer compound was spread in the metal frame to form Teflon (Teflon). It was covered with a sheet (registered trademark) and placed in the center of the table on a tabletop test press machine [SA-302, manufactured by Tester Sangyo Co., Ltd.] in which the temperature of the upper table was adjusted to 110 ° C and the temperature of the lower table was adjusted to 110 ° C. Pressing was performed at a pressure of 1 MPa for 60 seconds. After pressing, the powdered polymer compound is further spread in the metal frame, and the operation of pressing at a pressure of 1 MPa for 60 seconds is repeated until there are no opaque parts or bubbles in the metal frame, and the metal frame is removed from the metal frame. I got a test piece. This test piece was immersed in compound (A) at 50 ° C. for 3 days to obtain a saturated liquid absorbing state.
Then, the degree of swelling was determined by the following formula from the change in weight of the test piece before and after absorbing the liquid. The results are shown in Table 1.
Swelling degree [% by weight] = [(weight of test piece after liquid absorption-weight of test piece before liquid absorption) / weight of test piece before liquid absorption] × 100

(Degree of swelling with respect to electrolytic solution A)
An electrolytic solution A was prepared by dissolving 10 parts of LiFSI [LiN (FSO ₂ ) ₂ ] in a mixed solvent of 3.5 parts of ethylene carbonate (EC) and 5 parts of propylene carbonate (PC).
The polymer compound 1 was further pulverized using an agate mortar to form a fine powder. Next, a 10 × 40 × 0.2 mm metal frame coated with a mold release agent was placed on a 0.1 mm Teflon (registered trademark) sheet, and a powdery polymer compound was spread in the metal frame to form Teflon (Teflon). It was covered with a sheet (registered trademark) and placed in the center of the table on a tabletop test press machine [SA-302, manufactured by Tester Sangyo Co., Ltd.] in which the temperature of the upper table was adjusted to 110 ° C and the temperature of the lower table was adjusted to 110 ° C. Pressing was performed at a pressure of 1 MPa for 60 seconds. After pressing, the powdered polymer compound is further spread in the metal frame, and the operation of pressing at a pressure of 1 MPa for 60 seconds is repeated until there are no opaque parts or bubbles in the metal frame, and the metal frame is removed from the metal frame. I got a test piece. This test piece was immersed in the electrolytic solution A at 50 ° C. for 3 days to obtain a saturated liquid absorbing state.
Then, the degree of swelling was determined by the following formula from the change in weight of the test piece before and after absorbing the liquid. The results are shown in Table 1.
Swelling degree [% by weight] = [(weight of test piece after liquid absorption-weight of test piece before liquid absorption) / weight of test piece before liquid absorption] × 100

(Preparation of polymer compounds 2 to 6)
Polymer compounds 2 to 6 were prepared in the same manner as in the preparation of polymer compound 1, except that the compounding ratio (% by weight) of the monomer composition was changed as shown in Table 1.
Further, the degree of swelling with respect to ethylene carbonate and the electrolytic solution A was measured in the same manner as in the polymer compound 1. The results are shown in Table 1.

(Degree of swelling with respect to compound (A))
The compound (A) having the compounding ratio (% by weight) shown in Table 2 was prepared.
The swelling degree of each of the polymer compounds 1 to 6 (A) was measured in the same manner as in the measurement of the swelling degree with respect to ethylene carbonate. The results are shown in Table 2.
Each polymer compound and the compound (A) used for the measurement correspond to the polymer compound and the compound (A) used for producing the coated negative electrode active material particles for each lithium ion battery described later.

(Degree of swelling with respect to electrolytic solution)
An electrolytic solution having the blending ratio (% by weight) shown in Table 3 was prepared.
The swelling degree of each of the polymer compounds 1 to 6 was measured in the same manner as in the measurement of the swelling degree with respect to the electrolytic solution A. The results are shown in Table 3.
The polymer compound and the electrolytic solution used for the measurement thereof are the polymer compound and the electrolytic solution used when producing the negative electrode for each lithium ion battery described later (the compound (A) shown in Table 4 is also an electrolytic solution). (Calculated as part of) corresponds to.

<Preparation of coated negative electrode active material particles>
The materials used to prepare the coated negative electrode active material particles are as follows.
(Negative electrode active material particles)
HC: Hard carbon (Product name Carbotron (registered trademark) PS (F), manufactured by Kureha Battery Materials Japan Co., Ltd.)
(Compound (A))
EC: Ethylene carbonate Sf: Tetrahydrothiophene 1,1-dioxide VC: Vinylene carbonate (conductive aid)
AB: Acetylene Black [Denka Black (registered trademark) manufactured by Denka Co., Ltd.]

(Preparation of coated negative electrode active material particles 1)
A polymer compound solution obtained by dissolving the polymer compound 1 and ethylene carbonate (EC) in methanol at a concentration of 5.0% by weight was prepared.
The negative electrode active material particles were placed in a universal mixer high-speed mixer FS25 [manufactured by EarthTechnica Co., Ltd.], and the polymer compound solution was added dropwise over 2 minutes while stirring at room temperature at 720 rpm, and the mixture was further stirred for 5 minutes.
Then, in a stirred state, 3 parts of acetylene black (AB), which is a conductive auxiliary agent, was divided and added in 2 minutes, and stirring was continued for 30 minutes.
Then, the pressure was reduced to 0.01 MPa while maintaining the stirring, then the temperature was raised to 80 ° C. while maintaining the stirring and the degree of pressure reduction, and the stirring, the degree of pressure reduction and the temperature were maintained for 3 hours to distill off the volatile components. ..
The obtained powder was classified by a sieve having an opening of 200 μm to prepare coated negative electrode active material particles 1.
Table 4 shows the compounding ratio (% by weight) of each material used for producing the coated negative electrode active material particles 1.

(Measurement of angle of repose)
A glass funnel (length of funnel foot: 50 mm, inner diameter: 4 mm) was horizontally installed so that the tip of the funnel was located 10 cm above the surface of the horizontally installed metal flat plate.
A coated negative electrode active material particle 1 having an apparent volume of 15 ml was supplied to the funnel using a spoon having a capacity of 15 ml, and a conical laminate was formed on a metal flat plate by the coated negative electrode active material particles 1 dropped from the funnel.
Under the condition of 20 ° C., the angle formed by the portion corresponding to the generatrix of the cone formed by the laminated body and the surface of the metal flat plate was measured using a three-dimensional shape measuring instrument VR-3200 (manufactured by KEYENCE CORPORATION).
The angle was set at a place where the bottom surface of the cone was divided into 8 equal parts by 45 degrees, and the average value was taken as the angle of repose (°) of the coated negative electrode active material particles 1. The results are shown in Table 4.
The angle of repose is an index showing the surface state of the coated negative electrode active material particles 1, and the larger the angle of repose, the more the coating layer is swollen by the compound (A).

(Preparation of coated negative electrode active material particles 2 to 10)
The compounding ratio (% by weight) of each material was changed as shown in Table 4, and DMF was used as a solvent instead of methanol in the coated negative electrode active material particles 3 and 5, and the coated negative electrode active material particles 6 were used. The coated negative electrode active material was prepared in the same manner as in the production of the coated negative electrode active material particles 1, except that tetrahydrofuran was used as the solvent instead of methanol and the drying temperature was changed to 140 ° C. for the coated negative electrode active material particles 3 and 5. Particles 2 to 10 were prepared.
In addition, the angle of repose (°) was measured in the same manner as in the coated negative electrode active material particles 1. The results are shown in Table 4.
In the production of the particles shown as the coated negative electrode active material particles 7, the coating layer is not formed because the polymer compound is not blended, and it cannot be said that the particles are strictly coated negative electrode active material particles, but for convenience. It is shown as coated negative electrode active material particles 7.

<Preparation of electrode precursor>
The materials used to prepare the negative electrode active material layer are as follows.
(Transport medium)
EC: Ethylene carbonate (binder)
SBR: Styrene-butadiene rubber (product name BM-400B, manufactured by Nippon Zeon Corporation)
(Conductive aid)
CNF: Carbon nanofiber (product name VGCF-H, manufactured by Showa Denko KK)

(Preparation of negative electrode precursor 1)
A conductive auxiliary agent was added to the coated negative electrode active material particles 1 and mixed. Then, it was pressed at a pressure of 1.4 MPa for about 10 seconds to prepare a negative electrode precursor 1 having a thickness of 350 μm.
Table 5 shows the compounding ratio (% by weight) of each material used for producing the negative electrode precursor 1.

(Measurement of breaking stress)
The prepared negative electrode precursor 1 was measured with reference to the method A described in JIS K7074: 1988. A three-point bending jig was installed in an autograph [AGS-X10kN] manufactured by Shimadzu Corporation, and the negative electrode precursor 1 molded to 100 × 15 mm was allowed to stand on a slit having a distance between fulcrums of 80 mm. The test was carried out at a test speed of 1 mm / min using a 50 N load cell. The breaking stress was analyzed by the autograph dedicated software TRAPEZIUM X with the point where the stress dropped sharply in the graph as the breaking point.

(Preparation of negative electrode precursors 2 to 7 and 9 to 11)
Negative electrode precursors 2 to 7 and 9 to 11 were produced in the same manner as in the production of the negative electrode precursor 1, except that the compounding ratio (% by weight) of each material was changed as shown in Table 5.
Further, the breaking stress was measured in the same manner as in the negative electrode precursor 1. The results are shown in Table 5.

(Preparation of Negative Electrode Precursor 8)
95 parts of coated negative electrode active material particles 8, 20 parts of SBR (solid content 40% by weight), 1 part of CNF, and 10 parts of ion-exchanged water are mixed and kneaded by planetary stirring type {Awatori Rentaro [manufactured by Shinky Co., Ltd.] } And mixed at 2000 rpm for 5 minutes to obtain a negative electrode precursor slurry. The negative electrode precursor slurry was applied onto the copper foil and dried in a normal air dryer at 100 ° C. for 1 hour, and further dried in a vacuum dryer at 100 ° C. with a reduced pressure of −0.1 MPa (gauge pressure) for another 3 hours. After that, it was pressed at a pressure of 1,4 MPa for about 10 seconds to prepare a negative electrode precursor 8.

<Manufacturing negative electrodes for lithium-ion batteries>
The materials used to make the lithium-ion battery are as follows.
(Electrolytic solution)
LiFSI: LiN (FSO ₂ ) ₂
PC: Propylene carbonate EC: Ethylene carbonate Sf: Tetrahydrothiophene 1,1-dioxide (negative electrode current collector)
Copper foil (thickness: 20 μm)

(Example 1)
(Manufacturing of Negative Electrode 1 for Lithium Ion Battery)
100 parts of the obtained negative electrode precursor 1 was laminated on one side of the negative electrode current collector. Then, an electrolytic solution prepared by dissolving 10 parts of LiFSI in a mixed solvent of 3.5 parts of EC and 5 parts of PC was injected to form a negative electrode active material layer 1 (thickness 350 μm), and the thickness was 370 μm. The negative electrode 1 for a lithium ion battery according to No. 1 was manufactured.
Table 6 shows the compounding ratio (part by weight) of the negative electrode precursor 1 and the electrolytic solution used in the production of the lithium ion battery 1.

(Measurement of breaking stress)
The breaking stress of the negative electrode 1 for a lithium ion battery was measured with reference to the method A described in JIS K7074: 1988. A three-point bending jig was installed in an autograph [AGS-X10kN] manufactured by Shimadzu Corporation, and a negative electrode active material layer 1 for a lithium-ion battery molded into 100 x 15 mm was used as a fulcrum on a liquid-absorbent paper as a base material. It was allowed to stand on a slit having a distance of 80 mm (after injecting the electrolytic solution, it was allowed to stand for 12 hours in an atmosphere with a dew point of −40 ° C. and a room temperature of 20 ° C.). The test was carried out at a test speed of 1 mm / min using a 50 N load cell. The breaking stress was analyzed by the autograph dedicated software TRAPEZIUM X with the point where the stress dropped sharply in the graph as the breaking point.

(Shape retention evaluation)
In the shape retention evaluation of the negative electrode 1 for a lithium ion battery, the negative electrode 1 for a lithium ion battery was observed for 1 minute when the electrolytic solution was injected, and the evaluation was made according to the following criteria.
〇: No change △: A part of the electrolyte was defective after injection. ×: The electrolyte collapsed due to an impact immediately after injection.

(Creation of positive electrode)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ [Made by Toda Kogyo, volume average particle diameter (D50 particle size) 6.5 μm, expressed as NCA] 94 parts of universal mixer high speed mixer The mixture was placed in FS25 [manufactured by Artecnica] and stirred at 720 rpm at room temperature, and a 5% toluene solution of the polymer compound 6 was added dropwise over 2 minutes so as to have a solid content of 3 parts, and the mixture was further stirred for 5 minutes.
Then, in a stirred state, 3 parts of acetylene black [Denka Black (registered trademark) manufactured by Denka Co., Ltd.] was added as a conductive auxiliary agent in 2 minutes while being divided, and stirring was continued for 30 minutes. Then, the pressure was reduced to 0.01 MPa while maintaining the stirring, then the temperature was raised to 150 ° C. while maintaining the stirring and the degree of decompression, and the stirring, the degree of decompression and the temperature were maintained for 8 hours to distill off the volatile components. .. The obtained powder was classified with a sieve having an opening of 212 μm to obtain coated positive electrode active material particles. 1 part of CNF was added to 99 parts of the coated positive electrode active material particles and mixed. Then, it was pressed at a pressure of 1.4 MPa for about 10 seconds to prepare a positive electrode precursor having a thickness of 280 μm. The electrolytic solutions shown in Table 6 were injected into 100 parts by weight of the positive electrode precursor placed on the aluminum current collector foil to obtain a positive electrode for a lithium ion battery.

(Cycle test)
A test lithium-ion battery was produced by combining the negative electrode 1 for a lithium-ion battery with a positive electrode for a lithium-ion battery prepared at the opposite electrode via a separator (# 3501 manufactured by Cellguard).
For the manufactured test lithium-ion battery, the DC resistance value (first DCR) in the first cycle and the DC resistance value (100 cycle DCR) after 100 cycles were measured.
The initial DCR was calculated from the voltage drop for 10 seconds from the start of the discharge in the first cycle, and the 100-cycle DCR was calculated from the voltage drop for 10 seconds from the start of the discharge in the 100th cycle. The results are shown in Table 6.

The battery capacity at the time of the first charge (initial discharge capacity) at the time of the cycle test and the battery capacity at the time of the 100th cycle charge (discharge capacity after 100 cycles) were measured. The discharge capacity retention rate was calculated from the following formula. The results are shown in Table 6. The larger the value, the less the deterioration of the battery.
Discharge capacity retention rate (%) = (100th cycle discharge capacity / 1st cycle discharge capacity) x 100

(Examples 2 to 6, Comparative Examples 1 to 5)
Negative electrodes 2 to 11 for lithium ion batteries were prepared in the same manner as the negative electrode 1 for lithium ion batteries, except that the negative electrode precursor and the electrolytic solution were changed to the blending ratios (parts by weight) shown in Table 6. Measurements and evaluations were performed.

Coating Negative Electrode In Examples 1 to 6 in which the coating layer constituting the active material particles contains a specific polymer compound and the compound (A), the negative electrode for a lithium ion battery having excellent mechanical strength and excellent cycle characteristics is used. It was confirmed that it could be obtained.
On the other hand, in Comparative Example 1 in which the polymer constituting the coating layer does not contain a predetermined amount of (meth) acrylic acid, the degree of swelling with respect to the electrolytic solution is too high, so that the conductive auxiliary agent contained in the coating layer is active as the coating negative electrode. It is considered that the cycle characteristics deteriorated due to the separation from the material particles and the breakage of the conductive path.
Further, in Comparative Example 2 in which the polymer constituting the coating layer does not contain a predetermined amount of (meth) acrylic acid, the polymer does not swell with respect to the compound (A), and the adhesiveness between the coated negative electrode active material particles is not sufficient. Therefore, it is considered that the conductive path was cut and the cycle characteristics deteriorated.
Further, in Comparative Example 3 in which the coating layer does not contain the compound (A), the adhesiveness between the coated negative electrode active material particles is insufficient, the mechanical strength of the negative electrode for a lithium ion battery is insufficient, and over time. It is probable that the conductive path was cut and the cycle characteristics deteriorated.
Further, in Comparative Example 4 having no coating layer, it is considered that the shape retention evaluation of the negative electrode for the lithium ion battery was insufficient, and the conductive path was cut off with the passage of time, and the cycle characteristics deteriorated.
Further, in Comparative Example 5 in which the coating layer did not contain the compound (A) and ethylene carbonate was added as a transport medium, in the negative electrode precursor 9, EC was solidified by the press when producing the negative electrode precursor 9 and mechanically. Although the strength was high, the injection of the electrolytic solution when manufacturing the lithium-ion electrode melted the solidified EC and eliminated the adhesive points, resulting in insufficient mechanical strength of the negative electrode for the lithium-ion battery, resulting in time. It is probable that the conductive path was cut and the cycle characteristics deteriorated with the progress of.

The lithium ion battery using the coated negative electrode active material particles for a lithium ion battery of the present invention is particularly useful as a lithium ion battery used for mobile phones, personal computers, hybrid vehicles, and electric vehicles.

Claims (6)

At least a part of the surface of the negative electrode active material particles is coated with a coating layer containing the polymer compound and the compound (A).
The polymer compound is a polymer containing (meth) acrylic acid as a constituent monomer, and the weight ratio of (meth) acrylic acid in the polymer is 70 to 95% by weight based on the weight of the polymer. can be,
The coated negative electrode active material particles for a lithium ion battery, wherein the compound (A) is at least one selected from the group consisting of tetrahydrothiophene 1,1-dioxide, ethylene carbonate and vinylene carbonate. The weight ratio of the polymer compound contained in the coated negative electrode active material particles for a lithium ion battery is 1 to 7% by weight based on the weight of the coated negative electrode active material particles for a lithium ion battery.
Claim 1 in which the weight ratio of the compound (A) contained in the coated negative electrode active material particles for a lithium ion battery is 0.5 to 14% by weight based on the weight of the coated negative electrode active material particles for a lithium ion battery. The coated negative electrode active material particles for a lithium ion battery according to. Claim 1 in which the polymer compound is a polymer having a vinyl monomer (b) as a constituent monomer and contains the vinyl monomer (b1) represented by the following general formula (1) as the vinyl monomer (b). Or the coated negative electrode active material particles for a lithium ion battery according to 2.
CH ₂ = C (R ¹ ) COOR ² (1)
[In the general formula (1), R ¹ is a hydrogen atom or a methyl group, and R ² is an alkyl group having 1 to 12 carbon atoms. ] The negative electrode for a lithium ion battery having the coated negative electrode active material particles according to any one of claims 1 to 3. The lithium ion battery having the negative electrode for the lithium ion battery according to claim 4. It includes a mixing step of mixing a solution in which the polymer compound and the compound (A) are dissolved in an organic solvent and negative electrode active material particles, and a distillation step of distilling off the organic solvent after the mixing step.
The polymer compound is a polymer containing (meth) acrylic acid as a constituent monomer, and the weight ratio of (meth) acrylic acid in the polymer is 70 to 95% by weight based on the weight of the polymer. can be,
A method for producing coated negative electrode active material particles for a lithium ion battery, wherein the compound (A) is at least one selected from the group consisting of tetrahydrothiophene 1,1-dioxide, ethylene carbonate and vinylene carbonate.