Negative electrode active material paste for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide a lithium ion secondary battery having excellent cycle characteristics.SOLUTION: A lithium ion secondary battery has polyimide containing a repeating unit represented by the chemical formula (1), and in the chemical formula (1), A includes at least one of a tetravalent group obtained by removing the four hydrogens at the connecting points from an aromatic compound and a tetravalent group obtained by removing the four hydrogens at the connection points from an alicyclic compound, and B includes a divalent group obtained by removing the two hydrogens at the connection points from a norbornane-containing skeleton, and a divalent group obtained by removing two hydrogens at the connecting points from a skeleton containing a sulfonic acid group.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention , Li Negative electrode for lithium-ion secondary battery active material The present invention relates to a paste and a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are also widely used as a power source for mobile devices such as mobile phones and laptop computers, as well as hybrid cars.

[0003] The capacity of a lithium-ion secondary battery depends mainly on the active material of the electrode. Graphite is generally used as the negative electrode active material, but there is a demand for a negative electrode active material with higher capacity. Therefore, silicon (Si), which has a theoretical capacity much larger than that of graphite (372 mAh / g), has attracted attention.

[0004] Negative electrode active materials containing Si undergo significant volume expansion during charging. This volume expansion of the negative electrode active material causes a decrease in the cycle characteristics of the battery. When the negative electrode active material expands in volume, for example, the conductive paths between the negative electrode active materials are severed, peeling occurs at the interface between the negative electrode active material layer and the current collector, cracks occur in the SEI (Solid Electrolyte Interphase) coating, and electrolyte decomposition occurs. These all reduce the cycle characteristics of the battery.

[0005] In order to improve the cycle characteristics of the battery, a binder is used in the negative electrode active material layer. For example, Patent Document 1 discloses a lithium ion secondary battery that uses polyimide as a binder. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-242405 Summary of the Invention [Problem to be solved by the invention]

[0007] Further improvement in cycle characteristics is required.

[0008] The present disclosure has been made in view of the above problems, and aims to provide a configuration that can improve the cycle characteristics of a lithium ion secondary battery. [Means for solving the problem]

[0009] In order to solve the above problems, the following means are provided.

[0010] (1) A lithium ion secondary battery according to a first embodiment has a polyimide containing a repeating unit represented by the following chemical formula (1): A contains at least one of a tetravalent group obtained by removing four hydrogen atoms from an aromatic compound at connecting positions and a tetravalent group obtained by removing four hydrogen atoms from an alicyclic compound at connecting positions. B contains a divalent group obtained by removing two hydrogen atoms from a norbornane-containing skeleton and a divalent group obtained by removing two hydrogen atoms from a sulfonic acid group-containing skeleton. Each B in the repeating unit is a divalent group obtained by removing two hydrogen atoms from a norbornane-containing skeleton at connecting positions, or a divalent group obtained by removing two hydrogen atoms from a sulfonic acid group-containing skeleton at connecting positions.

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[0012] (2) In the lithium ion secondary battery according to the above aspect, the polyimide may have 50 mol % or more and 95 mol % or less of the B in the chemical formula (1) being a divalent group in which two hydrogen atoms at connecting positions have been removed from a skeleton containing a sulfonic acid group.

[0013] (3) A negative electrode active material paste for a lithium ion secondary battery according to a second aspect includes the binder for a lithium ion secondary battery according to the above aspect and a negative electrode active material containing silicon or a silicon compound.

[0014] (4) A lithium ion secondary battery according to a third aspect includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte solution, and the negative electrode contains silicon or a silicon compound and the binder for a lithium ion secondary battery according to the above aspect. [Effects of the Invention]

[0015] The lithium ion secondary battery according to the above embodiment has excellent cycle characteristics. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate changes can be made within the scope of the present invention.

[0018] "Lithium-ion secondary battery" FIG. 1 is a schematic diagram of a lithium-ion secondary battery according to a first embodiment. The lithium-ion secondary battery 100 shown in FIG. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of connected terminals 60, 62. The non-aqueous electrolyte is accommodated in the exterior body 50. Although FIG. 1 illustrates a case where one power generating element 40 is provided within the exterior body 50, multiple power generating elements 40 may be stacked.

[0019] (power generating element) The power generating element 40 includes a separator 10 , a positive electrode 20 , and a negative electrode 30 .

[0020] <Positive electrode> The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[0021] [Positive electrode current collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0022] [Cathode active material layer] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive additive and a binder as necessary.

[0023] The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions with counter anions.

[0024] The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), lithium manganese oxide (LiMnO), lithium manganese spinel (LiMnO), and lithium manganese oxides represented by the general formula: LiNi x Co y Mn z M aCompounds of O2 (where in the general formula x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compounds (LiV2O5), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material may be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0025] The positive electrode active material may also be a lithium-free material. Lithium-free materials include, for example, FeF3, conjugated polymers containing organic conductive substances, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. The lithium-free material may use only one of the materials, or may be used in combination of multiple materials. When the positive electrode active material is a lithium-free material, for example, discharge is first performed. Lithium is inserted into the positive electrode active material by discharge. In addition, the lithium-free material of the positive electrode active material may be pre-doped with lithium chemically or electrochemically.

[0026] The conductive assistant enhances the electron conductivity between the positive electrode active materials. The conductive assistant is, for example, carbon powder, carbon nanotubes, carbon materials, metal fine powder, a mixture of carbon materials and metal fine powder, or a conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, etc. The metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, etc.

[0027] The content rate of the conductive assistant in the positive electrode active material layer 24 is not particularly limited. For example, the content rate of the conductive assistant with respect to the total mass of the positive electrode active material, the conductive assistant, and the binder is 0.5 mass% or more and 20 mass% or less, preferably 1 mass% or more and 5 mass% or less.

[0028] The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Known binders can be used. Alternatively, the binder may be the same as that used in the negative electrode active material layer 34, which will be described later. The binder is preferably one that is insoluble in the electrolyte, has oxidation resistance, and is adhesive. Examples of the binder include fluororesin. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

[0029] The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content relative to the total mass of the positive electrode active material, conductive additive, and binder is 1% by mass or more and 15% by mass or less, and preferably 1.5% by mass or more and 5% by mass or less. If the binder content is low, the adhesive strength of the positive electrode 20 will be weakened. If the binder content is high, the binder will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0030] <Negative electrode> The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[0031] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[0032] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer 34 may contain a conductive additive as needed.

[0033] The negative electrode active material includes silicon or a silicon compound. The silicon compound is, for example, a silicon alloy or silicon oxide. For example, the silicon or silicon compound may be crystalline or amorphous. Amorphous silicon or silicon compound can be produced by a melt-spun method, a gas atomization method, or the like.

[0034] Silicon alloys are represented by XnSi. X is a cation. Examples of X include Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, and K. n satisfies 0≦n≦0.5. Silicon oxide is represented by SiO x where x satisfies, for example, 0.8≦x≦2. Silicon oxide may consist of only SiO2, may consist of only SiO, or may be a mixture of SiO and SiO2. Silicon oxide may also have some oxygen deficiencies.

[0035] The negative electrode active material may be a composite of silicon or a silicon compound. The composite is a silicon or silicon compound particle in which at least a portion of the surface is coated with a conductive material. The conductive material may be, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or the like. For example, a silicon-carbon composite material (Si-C) is an example of the composite. The amount of the conductive material coated on the silicon or silicon compound particle is, for example, 0.01% by mass or more and 30% by mass or less, and preferably 0.1% by mass or more and 20% by mass or less, based on the total mass of the composite. The composite can be produced, for example, by mechanical alloying, chemical vapor deposition, a wet process, or a method in which a polymer is coated and then thermally decomposed to carbonize the polymer.

[0036] The specific surface area of ​​the negative electrode active material determined by the BET method is, for example, 0.5 m 2 / g or more 100m 2 / g or less, preferably 1.0m 2 / g or more 20m 2 / g or less. If the specific surface area is small, it becomes difficult for Li ions to be inserted and removed between the negative electrode active material. If the specific surface area is large, a large amount of binder is required to form the electrode, resulting in a small capacity per unit volume.

[0037] The binder binds together the negative electrode active material in the negative electrode active material layer 34. The binder has a polyimide containing a repeating unit represented by the following chemical formula (1).

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[0039] In chemical formula (1), A includes at least one of a tetravalent group obtained by removing four hydrogen atoms at the connection points from an aromatic compound and a tetravalent group obtained by removing four hydrogen atoms at the connection points from an alicyclic compound. A may be a group represented by the same chemical formula for all of the repeating units of the polyimide, or may include groups represented by different chemical formulas. Specific examples of A are listed below. The following chemical formulae (A-1), (A-6), (A-7), (A-8), and (A-9) are examples of tetravalent groups obtained by removing four hydrogen atoms at the connection points from an aromatic compound. The following chemical formulae (A-2), (A-3), (A-4), (A-5), and (A-10) are examples of tetravalent groups obtained by removing four hydrogen atoms at the connection points from an alicyclic compound. A is, for example, a group having a cyclic skeleton.

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[0050] In chemical formula (1), B includes a divalent group obtained by removing two hydrogen atoms at the connection points from a skeleton containing norbornane, and a divalent group obtained by removing two hydrogen atoms at the connection points from a skeleton containing sulfonic acid groups. For example, some of the Bs in the repeating structure of chemical formula (1) are divalent groups obtained by removing two hydrogen atoms at the connection points from a skeleton containing norbornane, and some of the remaining Bs are divalent groups obtained by removing two hydrogen atoms at the connection points from a skeleton containing sulfonic acid groups. Each B in the repeating unit is a divalent group obtained by removing two hydrogen atoms at the connection points from a skeleton containing norbornane, or a divalent group obtained by removing two hydrogen atoms at the connection points from a skeleton containing sulfonic acid groups.

[0051] The norbornane-containing skeleton increases the molecular weight of the binder, making it stronger. The sulfonic acid group-containing skeleton binds to the negative electrode active material, increasing the binder's binding strength. This is thought to be because the sulfonic acid group has strong polarity and electrostatically bonds or hydrogen bonds with OH groups and other groups attached to the surface of the negative electrode active material.

[0052] The following chemical formulas (B-1) and (B-2) are examples of divalent groups in which two hydrogen atoms at the connection points have been removed from a skeleton containing norbornane. The following chemical formulas (B-3) to (B-11) are divalent groups in which two hydrogen atoms at the connection points have been removed from a skeleton containing sulfonic acid groups. The skeleton containing sulfonic acid groups is, for example, an aromatic compound.

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[0064] It is preferable that 50 mol % or more and 95 mol % or less of B are divalent groups (for example, (B-3) to (B-11) above) in which two hydrogen atoms at the connection sites have been removed from a skeleton containing sulfonic acid groups. When the binder satisfies the above range, the toughness and binding ability of the binder are increased, and the cycle characteristics of the lithium ion secondary battery 100 are particularly improved. Since the binder has sulfonic acid groups, it is water-soluble and can be extracted from the produced electrode. The structure of the binder can be detected by analyzing the binder extracted from the electrode.

[0065] The binder content in the negative electrode active material layer 34 is not particularly limited. For example, the binder content relative to the total mass of the negative electrode active material, conductive additive, and binder is 1% by mass or more and 20% by mass or less, and preferably 3% by mass or more and 15% by mass or less. If the binder content is low, the adhesive strength of the negative electrode 30 will be weakened. If the binder content is high, the binder will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0066] The conductive additive in the negative electrode active material layer 34 enhances the electronic conductivity between the negative electrode active materials. The conductive additive may be the same as that in the positive electrode active material layer 24.

[0067] There are no particular limitations on the content of the conductive additive in the negative electrode active material layer 34. For example, the content of the conductive additive relative to the total mass of the negative electrode active material, conductive additive, and binder is 5% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 12% by mass or less.

[0068] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents short-circuiting between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0069] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. The separator 10 may be a fibrous nonwoven fabric made of at least one material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. Examples of the solid electrolyte include a polymer solid electrolyte, an oxide-based solid electrolyte, and a sulfide-based solid electrolyte. The separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is formed by coating the surface of the above-mentioned film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0070] <Electrolyte> The electrolytic solution is sealed in the exterior body 50 and impregnates the power generating element 40. The non-aqueous electrolytic solution includes, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0071] The electrolyte may be a known electrolyte, and may contain, for example, a non-aqueous solvent and an electrolyte salt.

[0072] The electrolytic salt is, for example, a lithium salt. Examples of the electrolyte include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, and LiN(FSO2)2. One type of lithium salt may be used alone, or two or more types may be used in combination. From the viewpoint of the degree of ionization, it is preferable that the electrolyte contains LiPF6.

[0073] The non-aqueous solvent is, for example, an aprotic organic solvent, such as a cyclic carbonate, a chain carbonate, an ether, or a mixture thereof.

[0074] The cyclic carbonate solvates the electrolyte. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and fluoroethylene carbonate. The cyclic carbonate preferably contains at least fluoroethylene carbonate. Fluoroethylene carbonate (FEC) has a high oxidation-reduction potential and is easily reduced and decomposed. Reductive decomposition of a portion of the fluoroethylene carbonate (FEC) makes the electrolyte and remaining solvent in the electrolyte solution less susceptible to decomposition. Furthermore, fluoroethylene carbonate (FEC) forms a thin, stable coating (SEI coating) on ​​the entire surface of the negative electrode active material during the initial use of the lithium-ion secondary battery. The SEI coating prevents direct contact between the negative electrode active material and the electrolyte solution, preventing decomposition of the electrolyte solution.

[0075] The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent may also contain methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, and 1,2-diethoxyethane.

[0076] <Exterior body> The exterior body 50 seals the power generating element 40 and the non-aqueous electrolyte solution inside. The exterior body 50 prevents the non-aqueous electrolyte solution from leaking to the outside and prevents moisture and the like from entering the lithium-ion secondary battery 100 from the outside.

[0077] 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0078] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the inner and outer resin layers 54 may be different. For example, the outer material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film may be made of polyethylene (PE), polypropylene (PP), or the like.

[0079] <Terminal> Terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screw fastening. It is preferable to protect the terminals 60 and 62 with insulating tape to prevent short circuits.

[0080] "Method of manufacturing lithium-ion secondary batteries" The lithium ion secondary battery 100 is fabricated by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50. An example of a method for manufacturing the lithium ion secondary battery 100 will be described below.

[0081] The negative electrode 30 is produced, for example, by sequentially carrying out a slurry production step, an electrode application step, a drying step, and a rolling step.

[0082] The slurry preparation step is a step of preparing a slurry by mixing a negative electrode active material (silicon or silicon compound), a binder, a conductive additive, and a solvent. The binder is as described above. A precursor of the polyimide represented by the above-described chemical formula (1), i.e., polyamic acid before imide ring formation, may be added to the slurry. Examples of the solvent include water and N-methyl-2-pyrrolidone. The composition ratio of the negative electrode active material, conductive additive, and binder is preferably 70 wt% to 100 wt%: 0 wt% to 10 wt%: 0 wt% to 20 wt% by mass. The mass ratio of these is adjusted so that the total is 100 wt%.

[0083] The negative electrode active material may be a composite obtained by mixing active material particles and a conductive material while applying shearing force. When the active material particles are mixed under shearing force to a degree that does not alter their properties, the surfaces of the active material particles are coated with the conductive material. The particle size of the negative electrode active material can be adjusted by adjusting the degree of mixing. The negative electrode active material may also be sieved after preparation to make the particle size uniform.

[0084] The electrode coating step is a step of coating a slurry on the surface of the negative electrode current collector 32. There are no particular limitations on the method of coating the slurry. For example, a slit die coating method or a doctor blade method can be used as the method of coating the slurry.

[0085] The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere at 80°C to 350°C. The drying step may also cause a ring-closing reaction of the precursor polyamic acid. In this case, the drying step is preferably performed at a temperature of 200°C to 350°C. By drying the slurry or causing the ring-closing reaction to proceed, the negative electrode active material layer 34 is formed on the negative electrode current collector 32.

[0086] The rolling step is performed as necessary. The rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling step is performed using, for example, a roll press device.

[0087] The positive electrode 20 can be produced by the same procedure as that for the negative electrode 30. The separator 10 and the outer casing 50 can be commercially available products.

[0088] Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

[0089] Finally, the power generating element 40 is sealed in the exterior body 50. A non-aqueous electrolyte solution is poured into the exterior body 50. After the non-aqueous electrolyte solution is poured, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte solution is impregnated into the power generating element 40. The exterior body 50 is sealed by applying heat, etc., to obtain the lithium-ion secondary battery 100. Note that instead of pouring the electrolyte solution into the exterior body 50, the power generating element 40 may be impregnated with the electrolyte solution.

[0090] The lithium-ion secondary battery 100 according to the first embodiment has excellent cycle characteristics. This is because the binder contained in the negative electrode active material layer 34 is strong and has a high binding property with the negative electrode active material, such as silicon. The binder according to this embodiment has a skeleton portion containing norbornane, which gives it a large molecular weight and strength. Furthermore, the binder according to this embodiment has a skeleton portion containing a sulfonic acid group, which gives it a strong binding property with the negative electrode active material.

[0091] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention. [Example]

[0092] "Example 1" One surface of a 15 μm thick aluminum foil was coated with a positive electrode slurry prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0093] The positive electrode active material is Lix CoO2 was used. Acetylene black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive additive, 2 parts by mass of the binder, and 70 parts by mass of the solvent were mixed to prepare a positive electrode slurry. The amount of the positive electrode active material supported in the positive electrode active material layer after drying was 25 mg / cm. 2 The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer, which was then pressed with a roll press to prepare a positive electrode.

[0094] Next, one surface of a 10 μm-thick copper foil was coated with a negative electrode slurry prepared by mixing a negative electrode active material, a conductive additive, a binder precursor, and a solvent.

[0095] The negative electrode active material was silicon with a particle size of 3 μm. Carbon black was used as the conductive additive. The binder was such that A in the above-mentioned chemical formula (1) was represented by chemical formula (A-1), and B was represented by chemical formula (B-1) and chemical formula (B-3). B in the chemical formula (1) consisted of 3 mol% of chemical formula (B-1) and 97 mol% of chemical formula (B-3). The contents of A and B in the binder were determined by extracting and analyzing the binder from the prepared electrode. The binder has sulfonic acid groups, is highly water-soluble, and can be easily extracted from the electrode. A precursor of the polyimide represented by the above-mentioned chemical formula (1), i.e., polyamic acid before imide ring formation, was added to the slurry.

[0096] N-methyl-2-pyrrolidone was used as the solvent. 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive additive, and 5 parts by mass of the binder were mixed with N-methyl-2-pyrrolidone to prepare a negative electrode slurry. After drying, the amount of the negative electrode active material supported in the negative electrode active material layer was 2.5 mg / cm. 2 The solvent was removed from the negative electrode slurry in a drying furnace to prepare a negative electrode active material layer. The negative electrode active material layer was pressed with a roll press and then thermally baked in a nitrogen atmosphere at 300°C or higher for 5 hours.

[0097] Next, an electrolyte solution was prepared. The solvent was a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a mass ratio of 11:89. LiPF6 was used as the electrolyte salt. The concentration of LiPF6 was 1 mol / L.

[0098] (Fabrication of lithium-ion secondary batteries for evaluation) The prepared negative and positive electrodes were laminated with a separator (porous polyethylene sheet) interposed between them so that the positive and negative active material layers faced each other, to obtain a laminate. A nickel negative electrode lead was attached to the negative electrode of the laminate. An aluminum positive electrode lead was attached to the positive electrode of the laminate. The positive and negative electrode leads were welded using an ultrasonic welder. This laminate was inserted into an exterior body made of aluminum laminate film and heat-sealed except for one peripheral location to form a closed opening. Finally, the above-mentioned electrolyte solution was injected into the exterior body, and the remaining location was heat-sealed while reducing the pressure using a vacuum sealer, to produce a lithium-ion secondary battery.

[0099] (Measurement of capacity retention rate after 500 cycles) The cycle characteristics of the lithium ion secondary battery were measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation).

[0100] The battery was charged at a constant current of 0.5 C (the current value at which charging is completed in 1 hour when constant current charging is performed at 25°C) until the battery voltage reached 4.2 V, and then discharged at a constant current of 1.0 C until the battery voltage reached 2.5 V. The discharge capacity after charging and discharging was detected to determine the battery capacity Q1 before the cycle test.

[0101] The battery whose battery capacity Q1 was calculated above was again charged using a secondary battery charge / discharge tester at a constant current charge rate of 0.5 C until the battery voltage reached 4.2 V, and then discharged at a constant current discharge rate of 0.5 C until the battery voltage reached 2.5 V. This charge / discharge cycle was counted as one cycle, and 500 charge / discharge cycles were performed. The discharge capacity after 500 charge / discharge cycles was then measured, and the battery capacity Q2 after 500 cycles was calculated.

[0102] The capacity retention rate E after 500 cycles was calculated from the capacities Q1 and Q2 calculated above. The capacity retention rate E is calculated by E=Q2 / Q1×100. The capacity retention rate of Example 1 was 67%.

[0103] "Examples 2 to 7" Examples 2 to 7 differ from Example 1 in that the ratio of the chemical formula (B-1) and the chemical formula (B-3) constituting B in the chemical formula (1) was changed. The other conditions were the same as in Example 1, and the capacity retention rate was determined. The results are summarized in Table 1.

[0104] "Examples 8 to 15" Examples 8 to 15 differ from Example 4 in that the chemical formula (B-3) of B in chemical formula (1) was changed to any of the chemical formulas (B-4) to (B-11). The capacity retention rate was determined under the same conditions as in Example 4. The results are summarized in Table 1.

[0105] "Examples 16 to 24" Examples 16 to 24 differ from Example 4 in that the chemical formula (A-1) of A in chemical formula (1) was changed to any of chemical formulas (A-2) to (A-10). The other conditions were the same as in Example 4, and the capacity retention rate was determined. The results are summarized in Table 1.

[0106] "Comparative Example 1, Comparative Example 2" Comparative Examples 1 and 2 differ from Example 1 in that B in chemical formula (1) consists solely of chemical formula (B-1) or chemical formula (B-3). The binder of Comparative Example 1 does not contain a divalent group in which two hydrogen atoms at the connection points have been removed from a skeleton containing sulfonic acid groups. The binder of Comparative Example 2 does not contain a divalent group in which two hydrogen atoms at the connection points have been removed from a skeleton containing norbornane.

[0107] The results of Examples 1 to 24 and Comparative Examples 1 and 2 are summarized in Table 1 below. N is a divalent group in which two hydrogen atoms at the connection points are removed from a skeleton containing norbornane, and B S is a divalent group in which two hydrogen atoms at the connection points are removed from a skeleton containing sulfonic acid groups. The weight-average molecular weight of each binder was also determined, and the results are also summarized in Table 1.

[0108] [Table 1]

[0109] All of Examples 1 to 24 had a higher capacity retention rate than Comparative Examples 1 and 2. That is, the lithium ion secondary batteries according to Examples 1 to 24, in which the negative electrode active material layer contained a predetermined binder, had excellent cycle characteristics. [Explanation of symbols]

[0110] 10 Separator 20 positive electrode 22 Positive electrode current collector 24 Cathode active material layer 30 negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 terminals 100 Lithium-ion secondary battery

Claims

1. A binder for a lithium ion secondary battery, a negative electrode active material containing silicon or a silicon compound, The binder for a lithium ion secondary battery has a polyimide containing a repeating unit represented by chemical formula (1), In the chemical formula (1), A includes at least one of a tetravalent group obtained by removing four hydrogen atoms at connection positions from an aromatic compound and a tetravalent group obtained by removing four hydrogen atoms at connection positions from an alicyclic compound, B includes a divalent group obtained by removing two hydrogen atoms at connection positions from a skeleton containing norbornane, and a divalent group obtained by removing two hydrogen atoms at connection positions from a skeleton having a sulfonic acid group bonded to an aromatic ring, A negative electrode active material paste for a lithium ion secondary battery, wherein each B in the repeating unit is a divalent group obtained by removing two hydrogen atoms at connection points from a skeleton containing norbornane, or a divalent group obtained by removing two hydrogen atoms at connection points from a skeleton in which a sulfonic acid group is bonded to an aromatic ring. 【Chemistry 1】

2. 2. The negative electrode active material paste for a lithium ion secondary battery according to claim 1, wherein 50 mol % to 95 mol % of the B in the chemical formula (1) of the polyimide are divalent groups in which two hydrogen atoms at connecting positions are removed from a skeleton in which sulfonic acid groups are bonded to aromatic rings.

3. a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte; the negative electrode contains silicon or a silicon compound and a binder for a lithium ion secondary battery, The binder for a lithium ion secondary battery has a polyimide containing a repeating unit represented by chemical formula (1), In the chemical formula (1), A includes at least one of a tetravalent group obtained by removing four hydrogen atoms at connection positions from an aromatic compound and a tetravalent group obtained by removing four hydrogen atoms at connection positions from an alicyclic compound, B includes a divalent group obtained by removing two hydrogen atoms at connection positions from a skeleton containing norbornane, and a divalent group obtained by removing two hydrogen atoms at connection positions from a skeleton having a sulfonic acid group bonded to an aromatic ring, A lithium ion secondary battery, wherein each B in the repeating unit is a divalent group obtained by removing two hydrogen atoms at connection points from a skeleton containing norbornane, or a divalent group obtained by removing two hydrogen atoms at connection points from a skeleton in which a sulfonic acid group is bonded to an aromatic ring. 【Chemistry 2】

4. The lithium ion secondary battery according to claim 3, wherein 50 mol % to 95 mol % of the B in the chemical formula (1) of the polyimide is a divalent group in which two hydrogen atoms at the connection points have been removed from a skeleton in which sulfonic acid groups are bonded to aromatic rings.

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