JP6988539B2 - Binder for lithium ion secondary battery, negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

The present invention relates to a binder for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

In recent years, a lithium ion secondary battery that uses a non-aqueous electrolytic solution as a power source for a portable electronic device or the like and moves lithium ions between a positive electrode and a negative electrode to charge and discharge is widely used. In lithium-ion secondary batteries, there is an urgent need to further increase the capacity due to the progress of miniaturization, weight reduction, and multifunctionality of portable electronic devices.

In the negative electrode of a lithium ion secondary battery, graphite (theoretical capacity 372 mAh / g) is widely used as a negative electrode active material. In recent years, university research institutes, manufacturers, and the like have been studying materials that can be alloyed with lithium such as silicon (Si), silicon compounds (for example, SiO), and tin (Sn) as high-capacity negative electrode active materials. .. In particular, since silicon has a high theoretical capacity (theoretical capacity of 4199 mAh / g), it is attracting attention as a negative electrode active material.

As the negative electrode binder used for the negative electrode of the lithium ion secondary battery, for example, it has been proposed to use polyimide (see, for example, Patent Document 1 and Patent Document 2). However, when polyimide is used as a negative electrode binder, there are the following problems.
That is, when polyimide is used as a negative electrode binder, it is applied on a negative electrode current collector in the state of polyamic acid, which is a precursor of polyimide, and then heat-treated at a high temperature to ring-close the polyamic acid to form polyimide. Since the ring closure reaction of the polyamic acid is accompanied by a dehydration reaction, the water generated by the ring closure reaction remains in the negative electrode. Moisture in the negative electrode reacts with the electrolytic solution to generate hydrofluoric acid, which deteriorates the negative electrode active material. Further, when the polyimide is used as the negative electrode binder, the polyimide itself reacts with lithium ions to lower the initial charge / discharge efficiency, so that high charge / discharge efficiency cannot be obtained.

Further, it has been proposed to use a polyamide-imide resin as the negative electrode binder (see, for example, Patent Document 3).

Japanese Patent Application Laid-Open No. 2003-203637 Japanese Unexamined Patent Publication No. 2010-20609 Japanese Unexamined Patent Publication No. 2011-60676

However, when a polyamide-imide resin is used as the negative electrode binder, good charge / discharge cycle characteristics cannot be obtained because the volume change of the negative electrode active material due to charge / discharge cannot be sufficiently suppressed.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a binder for a lithium ion secondary battery capable of suppressing a volume change of a negative electrode active material due to charging and discharging when used as a negative electrode binder. do.
Another object of the present invention is to provide a negative electrode for a lithium ion secondary battery in which the volume change of the negative electrode active material due to charge / discharge is suppressed.
Another object of the present invention is to provide a lithium ion secondary battery having a negative electrode for a lithium ion secondary battery in which the volume change of the negative electrode active material due to charge / discharge is suppressed and having good charge / discharge cycle characteristics. do.

In order to solve the above problems, the present inventor has made extensive studies as shown below.
As a result, it was found that as the negative electrode binder, a binder containing a polyamide-imide resin having a specific range of weight average molecular weight (Mw), number average molecular weight (Mn) and dispersity (Mw / Mn) as a main component may be used. , I came up with the present invention. That is, the present invention relates to the following inventions.

(1) The weight average molecular weight (Mw) is 500,000 to 1,000,000, the number average molecular weight (Mn) is 4,000 to 40,000, and the dispersity (Mw / Mn) is 13 to 250. A binder for a lithium ion secondary battery, which is characterized by containing a polyamide-imide resin as a main component.

(2) It has a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.
The negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer contains 5 to 30% by mass of the binder for the lithium ion secondary battery according to (1).
(3) To conduct lithium ions with a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery according to (2), an insulating separator interposed between the positive electrode and the negative electrode, and lithium ions. A lithium-ion secondary battery characterized by having an electrolyte of.

The binder for a lithium ion secondary battery of the present invention has a weight average molecular weight (Mw) of 500,000 to 1,000,000, a number average molecular weight (Mn) of 4,000 to 40,000, and a degree of dispersion. The main component is a polyamide-imide resin having (Mw / Mn) of 13 to 250. Therefore, by using the binder for the lithium ion secondary battery of the present invention as the negative electrode binder, it is possible to suppress the volume change of the negative electrode active material due to charging and discharging.
Since the negative electrode for a lithium ion secondary battery of the present invention has a negative electrode active material layer containing 5 to 30% by mass of the binder for the lithium ion secondary battery of the present invention, the volume change of the negative electrode active material due to charging and discharging is suppressed. It will be the one that was done.
Since the lithium ion secondary battery of the present invention includes a negative electrode for a lithium ion secondary battery in which the volume change of the negative electrode active material due to charge / discharge is suppressed, it has good charge / discharge cycle characteristics.

It is sectional drawing which showed an example of the lithium ion secondary battery of this invention.

As a result of diligent studies, the present inventor has obtained the following findings.
That is, the weight average molecular weight (Mw) is 500,000 to 1,000,000, the number average molecular weight (Mn) is 4,000 to 40,000, and the dispersity (Mw / Mn) is 13 to 250. It has been found that the volume change of the negative electrode active material due to charging and discharging can be suppressed by using a negative electrode binder containing a certain polyamide-imide resin as a main component.

If the weight average molecular weight (Mw) of the polyamide-imide resin is less than 500,000, the expansion of the negative electrode active material cannot be sufficiently suppressed. Further, even if the weight average molecular weight (Mw) of the polyamide-imide resin is 500,000 or more, if the number average molecular weight (Mn) is less than 4,000, the strength of the negative electrode binder is insufficient. Therefore, the expansion of the negative electrode active material cannot be sufficiently suppressed. Further, when the weight average molecular weight (Mw) of the polyamide-imide resin exceeds 1,000,000 and / or the number average molecular weight (Mn) exceeds 40,000, the viscosity of the negative electrode binder becomes high. Therefore, when a paint containing a negative electrode active material and a negative electrode binder is produced and used to form a negative electrode active material layer, the dispersibility in the paint is lowered. As a result, the concentration of the negative electrode binder in the negative electrode active material layer becomes non-uniform, and the expansion of the negative electrode active material cannot be sufficiently suppressed.
Based on these findings, the present invention was conceived.

Further, the present inventor has a weight average molecular weight (Mw) of 500,000 to 1,000,000, a number average molecular weight (Mn) of 4,000 to 40,000, and a degree of dispersion (Mw / Mn). By using a negative electrode binder containing a polyamide-imide resin having a value of 13 to 250 as a main component, the negative electrode activity is active even when silicon and / or a silicon compound having a large volume change due to charge / discharge is used as the negative electrode active material. It was confirmed that the change in the volume of the substance could be sufficiently suppressed. Therefore, silicon and / or a silicon compound was used as the negative electrode active material, and a polyamide-imide resin having a weight average molecular weight (Mw), a number average molecular weight (Mn), and a dispersity (Mw / Mn) in the above ranges was used as the negative electrode binder. In the lithium ion secondary battery, high capacity and good cycle characteristics can be obtained.

On the other hand, in the conventional lithium ion secondary battery using silicon and / or a silicon compound as the negative electrode active material, the volume change of the negative electrode active material is large with charging and discharging, so that a large amount of stress is applied to the negative electrode due to repeated charging and discharging. Was added. Therefore, the negative electrode active material layer containing the negative electrode active material is likely to be peeled off from the negative electrode current collector forming the negative electrode, and cracks are likely to occur in the negative electrode active material layer. As a result, the conductive path between the negative electrode active material and the negative electrode current collector and / or between the negative electrode active material and the negative electrode active material was blocked, and good cycle characteristics could not be obtained.

Hereinafter, the binder for a lithium ion secondary battery, the negative electrode for a lithium ion secondary battery, and the lithium ion secondary battery of the present invention will be described in detail with reference to the drawings as appropriate. In the drawings used in the following description, the featured portions may be enlarged for convenience in order to make the features of the present invention easy to understand. Therefore, the dimensional ratios of each component shown in the drawings may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

[Lithium-ion secondary battery]
FIG. 1 is a schematic cross-sectional view showing an example of the lithium ion secondary battery of the present invention. The lithium ion secondary battery 100 shown in FIG. 1 includes a laminated body 40, a case 50 for accommodating the laminated body 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminated body 40. Further, although not shown, the case 50 contains an electrolytic solution together with the laminated body 40.

In the laminated body 40, a positive electrode 20 (a positive electrode for a lithium ion secondary battery) and a negative electrode 30 (a negative electrode for a lithium ion secondary battery) are arranged so as to face each other with the separator 10 interposed therebetween. The positive electrode 20 is a plate-shaped (film-shaped) positive electrode current collector 22 provided with a positive electrode active material layer 24. The negative electrode 30 has a negative electrode active material layer 34 provided on a plate-shaped (film-shaped) negative electrode current collector 32.

The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10, respectively. Leads 62 and 60 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively. The ends of the leads 60, 62 extend to the outside of the case 50.
In FIG. 1, the case where only one laminated body 40 is housed in the case 50 is illustrated, but a plurality of laminated bodies 40 may be stacked and housed.

"Negative electrode"
The negative electrode 30 has a negative electrode current collector 32 and a negative electrode active material layer 34 provided on the negative electrode current collector 32.

(Negative electrode current collector)
The negative electrode current collector 32 may be a conductive plate material, and for example, a thin plate made of a metal such as aluminum, copper, or nickel can be used. The negative electrode current collector 32 is preferably not alloyed with lithium, and copper is particularly preferable. The thickness of the negative electrode current collector 32 is preferably 6 to 30 μm.
(Negative electrode active material layer)
The negative electrode active material layer 34 has a negative electrode active material and a negative electrode binder, and has a negative electrode conductive material as needed.

(Negative electrode active material)
As the negative electrode active material, a known negative electrode active material for a lithium ion secondary battery that can occlude and release lithium ions can be used. Specific examples thereof include metallic lithium, carbon materials, metals that combine with lithium, compounds mainly composed of oxides, lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like.
Examples of the carbon material used as the negative electrode active material include graphite (natural graphite, artificial graphite), carbon nanotubes, graphitized carbon, easily graphitized carbon, and low-temperature calcined carbon. Examples of the metal that combines with lithium include aluminum, silicon, tin and the like. Examples of the compound mainly composed of an oxide include silicon oxide (SiO _x (0 <x <2)), tin dioxide and the like.
Among these, it is particularly preferable to use silicon or silicon oxide as the negative electrode active material. Since silicon and silicon oxide are materials having a large theoretical capacity, they contribute to increasing the capacity of the lithium ion secondary battery 100.

(Negative electrode binder)
The negative electrode binder binds the active materials to each other and also binds the active material to the negative electrode current collector 32.
The negative electrode binder has a weight average molecular weight (Mw) of 500,000 to 1,000,000, a number average molecular weight (Mn) of 4,000 to 40,000, and a dispersity (Mw / Mn) of 13 to. The main component is a polyamide-imide resin of 250.
In the negative electrode binder of the present embodiment, the fact that the polyamide-imide resin is the main component means that the content of the polyamide-imide resin in the negative electrode binder is 50% by mass or more. The content of the polyamide-imide resin in the negative electrode binder is preferably 50% by mass or more, more preferably 80% by mass or more, and may be only the polyamide-imide resin.

The weight average molecular weight (Mw) of the polyamide-imide resin is 500,000 or more, and since the expansion of the negative electrode active material can be suppressed more effectively, it is preferably 600,000 or more, preferably 700,000 or more. Is more preferable. Further, the weight average molecular weight (Mw) of the polyamide-imide resin is 1,000,000 or less, and a paint having a viscosity suitable for forming a negative electrode active material layer using a paint containing a negative electrode active material and a negative electrode binder. Is preferably 900,000 or less, and more preferably 800,000 or less.

The number average molecular weight (Mn) of the polyamide-imide resin is 4,000 or more, and since the expansion of the negative electrode active material can be suppressed more effectively, it is preferably 10,000 or more, preferably 20,000 or more. Is more preferable. Further, the number average molecular weight (Mn) of the polyamide-imide resin is 40,000 or less, and a paint having a viscosity suitable for forming a negative electrode active material layer using a paint containing a negative electrode active material and a negative electrode binder can be obtained. Therefore, it is preferably 30,000 or less, and more preferably 25,000 or less.

The polyamide-imide resin has a dispersity (Mw / Mn) of 13 to 250, preferably 20 to 200. When the dispersity (Mw / Mn) of the polyamide-imide resin is 13 to 250, excellent charge / discharge cycle characteristics can be obtained. This is considered to be due to the following reasons. The polyamide-imide resin having a dispersibility of 13 or more sufficiently contains not only high molecular weight polyamide-imide molecules but also low molecular weight polyamide-imide molecules. As a result, the polyamide-imide resin becomes rigid and has excellent flexibility, the stress on the polyamide-imide resin due to the expansion of the negative electrode active material is dispersed, and the tearing of the polyamide-imide resin is suppressed. As a result, the volume expansion of the negative electrode active material can be suppressed for a long period of time, and excellent charge / discharge cycle characteristics can be obtained. Further, when the dispersity (Mw / Mn) of the polyamide-imide resin is 250 or less, the volume expansion of the negative electrode active material can be effectively suppressed while suppressing the tearing of the polyamide-imide resin. When the dispersity (Mw / Mn) exceeds 250, the flexibility of the polyamide-imide resin is further increased, so that the tearing of the polyamide-imide resin is suppressed, but on the other hand, the volume expansion of the negative electrode active material cannot be sufficiently suppressed.

In the present embodiment, the weight average molecular weight (Mw), the number average molecular weight (Mn), and the dispersity (Mw / Mn) of the polyamide-imide resin are values measured by size exclusion chromatography (SEC).
The weight average molecular weight (Mw), number average molecular weight (Mn), and dispersity (Mw / Mn) of the polyamide-imide resin are the reaction temperature, holding time, catalyst type, catalyst addition amount, etc. in the production of the polyamide-imide resin. It can be adjusted by changing the polymerization reaction conditions of.

The polyamide-imide resin, which is the main component of the negative electrode binder of the present embodiment, is a resin having an amide group and an imide group in the molecule, and has a weight average molecular weight (Mw), a number average molecular weight (Mn), and a dispersity (Mw /). Mn) may be within the above range.
As the polyamide-imide resin, for example, an aromatic, aliphatic or alicyclic polyamide-imide resin can be used. Among these polyamide-imide resins, aromatic polyamide-imides are preferable from the viewpoints of strength, elongation, elastic modulus, electrolytic solution resistance, solubility in a solvent, processability, cost and the like.

The polyamide-imide resin contained in the negative electrode binder may be of only one type as long as the weight average molecular weight (Mw), the number average molecular weight (Mn), and the dispersity (Mw / Mn) are within the above ranges. A plurality of different types of polyamide-imide resins may be contained.
When a plurality of polyamide-imide resins are contained in the negative electrode binder, the values measured by size exclusion chromatography (SEC) in a uniformly mixed state are the weight average molecular weight (Mw) of the polyamide-imide resins contained in the negative electrode binder. ), Number average molecular weight (Mn), and dispersity (Mw / Mn).
Therefore, when the negative electrode binder contains a plurality of different types of polyamide-imide resins, any one or more of the weight average molecular weight (Mw), the number average molecular weight (Mn), and the dispersity (Mw / Mn) is within the above range. It may contain a polyamide-imide resin which is not.
When a plurality of polyamide-imide resins are contained in the negative electrode binder, a plurality of types of polyamide-imide resins having different weight average molecular weights, number average molecular weights, and dispersities are prepared, and the polyamide-imide resins are mixed at a predetermined ratio. The weight average molecular weight, the number average molecular weight, and the degree of dispersion of the imide resin mixture may be adjusted.

The polyamide-imide resin can be produced by a known method such as an isocyanate method produced by using an acid component and an isocyanate component, and a diamine method (acid chloride method) produced by using an acid chloride and an amine. The polyamide-imide resin is preferably produced by the isocyanate method from the viewpoint of production cost.
When synthesizing a polyamide-imide resin using the isocyanate method, for example, an acid component, an isocyanate component, a catalyst added as needed, and a solvent are mixed and stirred, and the mixture is heated while continuing stirring. It can be synthesized by a method of raising the temperature to 180 to 220 ° C., holding it for 0 to 12 hours, and then cooling it.

Examples of the acid component used when synthesizing a polyamideimide resin by the isocyanate method include trimellitic acid anhydride, pyromellitic acid, biphenyltetracarboxylic acid, biphenylsulfonetetracarboxylic acid, benzophenonetetracarboxylic acid, and biphenylether. Tetracarboxylic acids such as tetracarboxylic acid, ethylene glycol bistrimethylate, propylene glycol bistrimerite and their anhydrides, oxalic acid, adipic acid, malonic acid, sebacic acid, azelaic acid, dodecanedicarboxylic acid, dicarboxypolybutadiene, di Aliphatic dicarboxylic acids such as carboxypoly (acrylonitrile-butadiene) and dicarboxypoly (styrene-butadiene), 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 4,4'-dicyclohexylmethanedicarboxylic acid, dimer Examples thereof include alicyclic dicarboxylic acids such as acids, terephthalic acids, isophthalic acids, diphenylsulfonic dicarboxylic acids, diphenylether dicarboxylic acids, aromatic dicarboxylic acids such as naphthalenedicarboxylic acids, and trifunctional carboxylic acids such as trimesic acid and cyclohexanetricarboxylic acid. ..
Among these, as the acid component, trimellitic acid anhydride, benzophenone tetracarboxylic acid anhydride, biphenyltetracarboxylic acid anhydride and pyromellitic acid are preferable, and trimellitic acid anhydride is most preferable.

Examples of the isocyanate component used when synthesizing the polyamideimide resin by the isocyanate method include p-phenylenedi isocyanate, m-phenylenedi isocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-diphenyl ether diisocyanate, and diphenylcarbonyl. -3,3'-diisocyanate, 4-isocyanato benzoate 4-isocyanatophenyl, 4,4'-diphenylsulfone diisocyanate, diphenylsulfide-2,4'-diisocyanate, xylylene diisocyanate, tolylene diisocyanate, Examples include aromatic diisocyanates such as trizine diisocyanate, 2,2-bis (4-isocyanatophenyl) hexafluoropropane, and 1-isocyanate-4- [1- (4-isocyanatephenyl) -1-methylethyl] benzene. ..

Among these, as the isocyanate component, p-phenylene diisocyanate, m-phenylenedi isocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-diphenyl ether diisocyanate, xylylene diisocyanate, tolylene diisocyanate, and trizine diisocyanate are preferable. More preferably, it is p-phenylene diisocyanate, m-phenylenedi isocyanate, or 4,4'-diphenylmethane diisocyanate.

Examples of the catalyst used when synthesizing the polyamide imide resin by the isocyanate method include triethylamine, diethylenetriamine, triethylenediamine, amines such as 1,8-diazabicyclo [5.4.0] -7-undecene, and foot. Examples thereof include metal halides such as sodium oxide, potassium fluoride and cesium fluoride, organic bases such as sodium methoxydo and sodium ethoxydo, and inorganic bases of sodium hydroxide and potassium hydroxide.
The amount of the catalyst added is preferably 0 mol% or more and 10 mol% or less, and more preferably 0.1 mol% or more and 5 mol% or less with respect to the acid component.

Examples of the solvent used when producing the polyamide-imide resin by the isocyanate method include N-methyl-2-pyrrolidone, N, N'-dimethylformamide, N, N'-dimethylacetamide, 1,3-dimethyl. -2-imidazolidinone, tetramethylurea, sulfolane, dimethylsulfooxide, γ-butyrolactone, cyclohexanone, cyclopentanone and the like can be mentioned. Among these, it is preferable to use N-methyl-2-pyrrolidone as the solvent.
These solvents may be partially replaced with a hydrocarbon-based organic solvent such as toluene and xylene, an ether-based organic solvent such as diglyme, triglime and tetrahydrofuran, and a ketone-based organic solvent such as methyl ethyl ketone and methyl isobutyl ketone. ..

When the polyamide-imide resin is produced by using the diamine method, the carboxylic acid of the above acid component can be changed to a carboxylic acid chloride, and the isocyanate component can be changed to the corresponding amine.

The negative electrode binder contains a polyamide-imide resin having a specific weight average molecular weight (Mw), number average molecular weight (Mn), and dispersity (Mw / Mn) as a main component, and contains other components as necessary. May be.
As the other component, a component that dissolves in N-methyl-2-pyrrolidone (NMP), which is suitable as a solvent for the polyamide-imide resin, is preferable, and specific examples thereof include polyvinylidene fluoride (PVDF).

(Contents of negative electrode binder)
The negative electrode active material layer 34 contains 5 to 30% by mass of a negative electrode binder (binder for a lithium ion secondary battery). The content of the negative electrode binder is preferably 10 to 25% by mass.
If the content of the negative electrode binder is more than 30% by mass, the negative electrode active material is insufficient, and the lithium ion secondary battery 100 having a sufficient capacity cannot be obtained. If the content of the negative electrode binder is less than 5% by mass, the negative electrode binder is insufficient. Therefore, the bonding force between the negative electrode active materials and the bonding force between the negative electrode active material and the negative electrode current collector 32 cannot be sufficiently obtained, and the volume change of the negative electrode active material due to charging / discharging cannot be sufficiently suppressed. As a result, good charge / discharge cycle characteristics cannot be obtained.

(Negative electrode conductive material)
Examples of the negative electrode conductive material include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, and conductive oxides such as ITO. Among these, it is particularly preferable to use carbon powder such as acetylene black or ethylene black as the negative electrode conductive material.

The lithium ion secondary battery 100 may not contain the negative electrode conductive material if sufficient conductivity can be ensured only by the negative electrode active material.
The composition ratio of the negative electrode conductive material in the negative electrode active material layer 34 is preferably 0% or more and 20% or less in terms of mass ratio.

"Positive electrode"
The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24 provided on the positive electrode current collector 22.
(Positive current collector)
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin plate made of a metal such as aluminum or nickel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 24 has a positive electrode active material and a positive electrode binder, and if necessary, has a positive electrode conductive material.
The positive electrode active material used for the positive electrode active material layer 24 includes storage and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or counter anion of lithium ions and lithium ions (for example, PF ₆ ⁻ ). An electrode active material capable of reversibly advancing doping and dedoping with the electrode can be used.

As the positive electrode active material, for example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the general formula: LiNi _x Co _y Mn _z M _a2 (x + y + z + a = 1, 0 ≦ x <1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ a <1, M is from Al, Mg, Nb, Ti, Cu, Zn, Cr Composite metal oxide represented by one or more selected elements), lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti). , Al, showing one or more elements or VO selected from Zr), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1) , etc. Examples thereof include composite metal oxides, polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene and the like.

(Positive electrode conductive material)
Examples of the positive electrode conductive material include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, and conductive oxides such as ITO. Among these, a carbon material such as carbon black is particularly preferable.
The lithium ion secondary battery 100 may not contain the positive electrode conductive material if sufficient conductivity can be ensured only by the positive electrode active material.

(Positive binder)
As the positive electrode binder, for example, fluororesin, vinylidene fluoride-based fluororubber, or the like can be used. Examples of the fluororesin and vinylidene fluoride-based fluororubber used as the positive electrode binder include the same as those used as the negative electrode binder described above.

The contents of the positive electrode active material, the positive electrode conductive material, and the positive electrode binder contained in the positive electrode active material layer 24 are not particularly limited. The content of the positive electrode active material contained in the positive electrode active material layer 24 is preferably 80% or more and 90% or less in terms of mass ratio. Further, the content of the positive electrode conductive material contained in the positive electrode active material layer 24 is preferably 0.5% or more and 10% or less in terms of mass ratio. The content of the positive electrode binder contained in the positive electrode active material layer 24 is preferably 0.5% or more and 10% or less in terms of mass ratio.

"Separator"
The separator 10 is interposed between the positive electrode 20 and the negative electrode 30. The separator 10 is formed of an electrically insulating porous structure. Examples of the separator 10 include a single layer of a film made of polyolefin, a laminated body, and a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester and polypropylene.

"Electrolytic solution"
As the electrolytic solution, an electrolyte solution containing a lithium salt (an aqueous electrolyte solution, an electrolyte solution using an organic solvent) can be used. As the electrolytic solution, it is preferable to use an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent. Since the non-aqueous electrolyte solution has a higher electrochemical decomposition voltage than, for example, an aqueous electrolyte solution, the withstand voltage during charging can be increased. The non-aqueous electrolyte solution is a solution in which an electrolyte for conducting lithium ions is dissolved in a non-aqueous solvent.

As the non-aqueous solvent, one containing a cyclic carbonate and a chain carbonate can be used.
As the cyclic carbonate, one capable of solvating an electrolyte can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate and the like can be used.
The chain carbonate can reduce the viscosity of the cyclic carbonate. For example, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and the like can be mentioned.

The ratio of cyclic carbonate to chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 in volume.
The non-aqueous solvent may contain methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane and the like.

Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiN (CF ₃ CF). ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB and other lithium salts can be used. One of these lithium salts may be used alone, or two or more thereof may be used in combination. The electrolyte preferably contains _{LiPF 6} from the viewpoint of the degree of ionization.

When dissolving LiPF ₆ in a non-aqueous solvent, it is preferable to adjust the concentration of the electrolyte in the non-aqueous electrolyte solution to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the lithium ion concentration in the non-aqueous electrolyte solution can be sufficiently secured, and a sufficient capacity can be easily obtained at the time of charging / discharging. Further, by suppressing the concentration of the electrolyte to 2.0 mol / L or less, the increase in the viscosity of the non-aqueous electrolyte solution can be suppressed, the mobility of lithium ions can be sufficiently secured, and a sufficient capacity can be easily obtained at the time of charging / discharging.

When LiPF ₆ is mixed with other electrolytes, it is preferable to adjust the lithium ion concentration in the non-aqueous electrolyte solution to 0.5 to 2.0 mol / L. Further, it is more preferable that the lithium ion concentration from LiPF ₆ is contained in an amount of 50 mol% or more of the lithium ion concentration in the non-aqueous electrolyte solution.

"Case"
The case 50 seals the laminate 40 and the electrolytic solution inside the case 50. The case 50 is not particularly limited as long as it can suppress the leakage of the electrolytic solution to the outside and the intrusion of water and the like from the outside into the lithium ion secondary battery 100.

For example, as the case 50, as shown in FIG. 1, a case made of a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. As the metal foil 52, for example, an aluminum foil can be used. As the material of the outer polymer film 54, a polymer having a high melting point, for example, polyethylene terephthalate (PET), polyamide or the like is preferable. As the material of the inner polymer film 54, polyethylene (PE), polypropylene (PP) and the like are preferable.

"Lead"
The leads 60 and 62 are made of a conductive material such as aluminum. The leads 60 and 62 are welded to the positive electrode current collector 22 and the negative electrode current collector 32, respectively, by a known method.

[Manufacturing method of lithium ion secondary battery]
Next, a method for manufacturing the lithium ion secondary battery 100 will be described.
First, the positive electrode 20 and the negative electrode 30 are formed, respectively.
To form the negative electrode 30, a negative electrode active material, a negative electrode binder and a solvent are mixed to prepare a paint. If necessary, a negative electrode conductive material may be further added to the paint. As the solvent, for example, water, N-methyl-2-pyrrolidone, N, N-dimethylformamide and the like can be used.

The method of mixing these components constituting the paint is not particularly limited, and the mixing order is also not particularly limited.
Next, the above paint is applied to the negative electrode current collector 32. The coating method is not particularly limited, and a method usually adopted when manufacturing an electrode can be used. For example, the slit die coat method, the doctor blade method and the like can be mentioned.
As for the positive electrode, the paint prepared in the same manner is applied onto the positive electrode current collector 22.

Subsequently, the solvent is removed from the paints applied on the positive electrode current collector 22 and the negative electrode current collector 32, respectively. By the above steps, the positive electrode active material layer 24 is formed on the positive electrode current collector 22. Further, the negative electrode active material layer 34 is formed on the negative electrode current collector 32.
Next, the positive electrode current collector 22 on which the positive electrode active material layer 24 is formed and the negative electrode current collector 32 on which the negative electrode active material layer 34 is formed are pressed by a roll press device or the like, if necessary.
By the above steps, the positive electrode 20 and the negative electrode 30 are obtained.

Next, the separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30 and inserted into the case 50. Specifically, for example, the positive electrode 20, the negative electrode 30, and the separator 10 are laminated to form a laminated body 40, which is placed in a bag-shaped case 50 prepared in advance.
Finally, the electrolytic solution is injected into the case 50 to seal the entrance of the case 50.
By the above steps, the lithium ion secondary battery 100 is manufactured.
The laminate 40 may be impregnated with the electrolytic solution and then inserted into the case 50 without performing the step of injecting the electrolytic solution into the case 50.

The lithium ion secondary battery of the present embodiment has a weight average molecular weight (Mw) of 500,000 to 1,000,000 and a number average molecular weight (Mn) of 4,000 to 40,000 as a negative electrode binder. , A polyamide-imide resin having a dispersity (Mw / Mn) of 13 to 250 as a main component is used. Therefore, in the lithium ion secondary battery of the present embodiment, the volume change of the negative electrode active material due to charge / discharge is suppressed, and good charge / discharge cycle characteristics can be obtained.

The embodiments of the present invention have been described in detail with reference to the drawings. The configurations and combinations thereof in each of the above-described embodiments are examples, and the configurations can be added, omitted, replaced, and other changes within the scope not deviating from the gist of the present invention.

(Examples 1 to 4, Comparative Examples 1 to 2)
<Synthesis of polyamide-imide resin>
In a 4-necked flask equipped with a quenching tube and a nitrogen gas inlet, 1 mol of trimellitic acid anhydride (manufactured by Wako Pure Chemical Industries, Ltd.) and 1 mol of 4,4'-diphenylmethane diisocyanate (manufactured by Aldrich) were used as catalysts. 0.01 mol of potassium fluoride (manufactured by Genuine Chemical Industries, Ltd.) was charged into N-methyl-2-pyrrolidone (NMP) so that the solid content concentration was 20%, and used as a raw material solution. Next, the raw material solution was stirred at room temperature of 25 ° C. for 1 hour, the temperature was raised to 180 ° C. while continuing stirring, and the temperature was cooled to 25 ° C. without holding the mixture to prepare the polyamide-imide of Example 1.
Further, the polyamide-imide resins of Examples 2 to 4 and Comparative Examples 1 and 2 were prepared in the same manner as in Example 1 except that the holding time after raising the temperature to 180 ° C. was adjusted.

(Examples 5 to 8, Comparative Examples 3 to 4)
The polyamide-imide resin of Example 5 was prepared in the same manner as in Example 1 except that the temperature of the raw material solution after stirring at room temperature for 1 hour was set to 200 ° C.
Further, the polyamide-imide resins of Examples 6 to 8 and Comparative Examples 3 to 4 were prepared in the same manner as in Example 5 except that the holding time after raising the temperature to 200 ° C. was adjusted.

(Examples 9 to 12, Comparative Examples 5 to 6)
The polyamide-imide resin of Example 9 was prepared in the same manner as in Example 1 except that the temperature rise of the raw material solution after stirring at room temperature for 1 hour was set to 220 ° C.
Further, the polyamide-imide resins of Examples 10 to 12 and Comparative Examples 5 to 6 were prepared in the same manner as in Example 9 except that the holding time after raising the temperature to 220 ° C. was adjusted.

(Comparative Examples 7 to 12)
The polyamide-imide resin of Comparative Example 7 was prepared in the same manner as in Example 1 except that the temperature of the raw material solution after stirring at room temperature for 1 hour was set to 150 ° C.
Further, the polyamide-imide resins of Comparative Examples 8 to 12 were produced in the same manner as in Comparative Example 7 except that the holding time after raising the temperature to 150 ° C. was adjusted.

(Comparative Examples 13 to 18)
The polyamide-imide resin of Comparative Example 13 was prepared in the same manner as in Example 1 except that the temperature of the raw material solution after stirring at room temperature for 1 hour was set to 250 ° C.
Further, the polyamide-imide resins of Comparative Examples 14 to 18 were prepared in the same manner as in Comparative Example 13 except that the holding time after raising the temperature to 250 ° C. was adjusted.

(Examples 13 to 16, Comparative Examples 19 and 20)
The polyamide-imide of Example 13 was prepared in the same manner as in Example 1 except that 4,4'-diphenyl ether diisocyanate (manufactured by Aldrich) was used instead of 4,4'-diphenylmethane diisocyanate.
Further, the polyamide-imide resins of Examples 14 to 16 and Comparative Examples 19 and 20 were prepared in the same manner as in Example 13 except that the holding time after raising the temperature to 180 ° C. was adjusted.

For each of the polyamide-imide resins of Examples 1 to 16 and Comparative Examples 1 to 20 thus obtained, the weight average molecular weight (Mw), the number average molecular weight (Mn), and the dispersity (Mw / Mn) are excluded from the size. Measured by chromatography (SEC). The results are shown in Table 1.

Next, the polyamide-imide resins of Examples 1 to 16 and Comparative Examples 1 to 20 were used as the negative electrode binder, respectively, and the lithium ion secondary batteries of Examples 1 to 16 and Comparative Examples 1 to 20 were used by the methods shown below. Was produced.

"Creation of negative electrode"
88% by mass of silicon (Si) as a negative electrode active material, 2% by mass of acetylene black as a negative electrode conductive material, 10% by mass of a negative electrode binder, and N-methyl-2-pyrrolidone (NMP) as a solvent are mixed. , Painted. This paint was applied to a negative electrode current collector made of Cu and dried, and then pressed to form a negative electrode active material layer having a thickness of 13 μm to obtain a negative electrode.

"Creation of positive electrode"
_{LiCoO 2} , which is a positive electrode active material, acetylene black as a positive electrode conductive material, polyvinylidene fluoride (PVDF) as a positive electrode binder, and N-methyl-2-pyrrolidone (NMP), which is a solvent, are mixed to form a paint. Was produced. The mixing ratio of the positive electrode active material, acetylene black, and PVDF in the paint was 96: 2: 2 (mass ratio). This paint was applied to a positive electrode current collector made of Al, dried, and then pressed to form a positive electrode active material layer to obtain a positive electrode.

"Making a lithium-ion secondary battery"
The positive electrode and the negative electrode thus obtained were cut to predetermined dimensions. Then, a lead made of aluminum foil was ultrasonically welded to the positive electrode. In addition, a lead made of nickel foil was ultrasonically welded to the negative electrode.

Next, a polypropylene separator having a thickness of 16 μm was sandwiched between the positive electrode and the negative electrode to form a laminated body. Next, the laminate was placed in a bag-shaped case made of an aluminum laminate film and fixed by thermocompression bonding (heat sealing).

Next, the electrolytic solution was injected into the case, and the entrance of the case was sealed and vacuum-sealed. As the electrolytic solution, a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 3: 7 and _{a non-aqueous electrolyte solution containing lithium hexafluorophosphate (LiPF 6} ) at a concentration of 1 mol / L are used. Using.
Through the above steps, lithium ion secondary batteries of Examples 1 to 16 and Comparative Examples 1 to 20 were obtained.

(Example 17)
78% by mass of silicon (Si) as a negative electrode active material, 2% by mass of acetylene black as a negative electrode conductive material, 20% by mass of a negative electrode binder, and N-methyl-2-pyrrolidone (NMP) as a solvent are mixed. , Painted. A lithium ion secondary battery was obtained in the same manner as in Example 1 except that the negative electrode active material layer was formed using this paint.

(Example 18)
68% by mass of silicon (Si) as a negative electrode active material, 2% by mass of acetylene black as a negative electrode conductive material, 30% by mass of a negative electrode binder, and N-methyl-2-pyrrolidone (NMP) as a solvent are mixed. , Painted. A lithium ion secondary battery was obtained in the same manner as in Example 1 except that the negative electrode active material layer was formed using this paint.

With respect to the lithium ion secondary batteries of Examples 1 to 18 and Comparative Examples 1 to 20 thus produced, the surface morphology of the negative electrode active material layer was observed and evaluated by the following method.
The morphology of the surface of the negative electrode active material layer was observed using an SEM (scanning electron microscope), and the case without aggregates was evaluated as “good”, and the case with aggregates was evaluated as “poor”. The agglomerate means an aggregate of a plurality of negative electrode active materials and auxiliaries to form a coarse particle size.

Further, for the lithium ion secondary batteries of Examples 1 to 18 and Comparative Examples 1 to 20, the "cell expansion rate" and "capacity retention rate" were measured by the following methods. The results are shown in Table 1.

[Cell expansion rate]
Using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.), a lithium-ion secondary battery is charged with a constant current and constant voltage up to 4.2 V at a charge rate of 0.5 C, and up to 2.5 V at a discharge rate of 1 C. 100 cycles of charging and discharging for constant current discharge were performed.
Then, the cell thickness of the lithium ion secondary battery was measured, and the expansion rate was calculated with respect to the cell thickness of the lithium ion secondary battery before charging and discharging.

[Capacity maintenance rate]
Using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.), a lithium-ion secondary battery is charged with a constant current and constant voltage up to 4.2 V at a charge rate of 0.5 C, and up to 2.5 V at a discharge rate of 1 C. 100 cycles of charging and discharging for constant current discharge were performed. Then, the discharge capacity of the 100th cycle with respect to the discharge capacity of the first cycle was calculated as the capacity retention rate.

As shown in Table 1, in the lithium ion secondary batteries of Examples 1 to 18, the surface morphology of the negative electrode active material layer was all “good”. Further, the lithium ion secondary batteries of Examples 1 to 18 have a lower cell expansion rate, a higher capacity retention rate, and better cycle characteristics than the lithium ion secondary batteries of Comparative Examples 1 to 20. rice field.

On the other hand, in the lithium ion secondary batteries of Comparative Examples 1 and 19, the expansion rate of the cell was high because the number average molecular weight (Mn) of the polyamide-imide resin used as the negative electrode binder was low.
In the lithium ion secondary batteries of Comparative Examples 3 and 5, the expansion rate of the cell was high because the polyamide-imide resin used as the negative electrode binder had a low number average molecular weight (Mn) and a high dispersity (Mw / Mn).
Further, in the lithium ion secondary batteries of Comparative Examples 2 and 20, the surface of the negative electrode active material layer is high because the polyamide-imide resin used as the negative electrode binder has a high number average molecular weight (Mn) and a low dispersity (Mw / Mn). The morphology was "poor", the cell expansion rate was high, and the capacity retention rate was low.
In the lithium ion secondary batteries of Comparative Examples 4 and 6, since the polyamide-imide resin used as the negative electrode binder has a high number average molecular weight (Mn), the surface morphology of the negative electrode active material layer becomes “poor” and the expansion rate of the cell becomes high. It was high and the capacity retention rate was low.

In the lithium ion secondary battery of Comparative Example 7, the polyamide-imide resin used as the negative electrode binder has a low weight average molecular weight (Mw) and a low number average molecular weight (Mn), so that the cell expansion rate is high and the capacity retention rate is high. It was low.
In the lithium ion secondary batteries of Comparative Examples 8 and 9, since the weight average molecular weight (Mw) of the polyamide-imide resin used as the negative electrode binder was low, the expansion rate of the cell was high and the capacity retention rate was low.
In the lithium ion secondary batteries of Comparative Examples 10 and 11, the polyamide-imide resin used as the negative electrode binder has a low weight average molecular weight (Mw) and a low dispersity (Mw / Mn), so that the cell expansion rate is high and the capacity is high. The maintenance rate was low.
In the lithium ion secondary battery of Comparative Example 12, the polyamide-imide resin used as the negative electrode binder has a low weight average molecular weight (Mw), a high number average molecular weight (Mn), and a low dispersity (Mw / Mn). The surface morphology of the active material layer became "poor", the expansion rate of the cell was high, and the capacity retention rate was low.

The lithium ion secondary battery of Comparative Example 13 has a high weight average molecular weight (Mw), a low number average molecular weight (Mn), and a high dispersity (Mw / Mn) of the polyamide-imide resin used as the negative electrode binder, and thus has a high negative electrode. The surface morphology of the active material layer became "poor", the expansion rate of the cell was high, and the capacity retention rate was low.
In the lithium ion secondary battery of Comparative Example 14, the polyamide-imide resin used as the negative electrode binder has a high weight average molecular weight (Mw) and a high dispersity (Mw / Mn), so that the surface morphology of the negative electrode active material layer is “poor”. The cell expansion rate was high and the capacity retention rate was low.
Since the lithium ion secondary batteries of Comparative Examples 15 to 17 have a high weight average molecular weight (Mw) of the polyamide-imide resin used as the negative electrode binder, the surface morphology of the negative electrode active material layer becomes “poor” and the expansion rate of the cell becomes high. It was high and the capacity retention rate was low.
In the lithium ion secondary battery of Comparative Example 18, the polyamide-imide resin used as the negative electrode binder has a high weight average molecular weight (Mw) and a high number average molecular weight (Mn), so that the surface morphology of the negative electrode active material layer is “poor”. The expansion rate of the cell was high and the capacity retention rate was low.

10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode current collector, 24 ... Positive electrode active material layer, 30 ... Negative electrode, 32 ... Negative electrode current collector, 34 ... Negative electrode active material layer, 40 ... Laminated body, 50 ... Case, 60 , 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (3)

Polyamide having a weight average molecular weight (Mw) of 500,000 to 1,000,000, a number average molecular weight (Mn) of 4,000 to 40,000, and a dispersity (Mw / Mn) of 13 to 250. A binder for a lithium ion secondary battery, which is characterized by containing an imide resin as a main component. It has a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.
The negative electrode for a lithium ion secondary battery, wherein the negative electrode active material layer contains 5 to 30% by mass of the binder for the lithium ion secondary battery according to claim 1. A positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery according to claim 2, an insulating separator interposed between the positive electrode and the negative electrode, and an electrolyte for conducting lithium ions. A lithium ion secondary battery characterized by being equipped with.

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