Negative electrode for lithium-ion secondary battery and lithium-ion secondary battery

Abstract

This negative electrode for a lithium-ion secondary battery comprises a negative electrode current collector (32) and a negative electrode active material layer (34) that is in contact with at least one surface of the negative electrode current collector (32). The negative electrode current collector (32) has a tensile strength of 400-805 MPa. The negative electrode active material layer (34) has a negative electrode active material containing silicon, and an organic additive containing diaminodiphenylmethane in the structure thereof.

Description

Negative electrode for lithium-ion secondary battery and lithium-ion secondary battery

[0001] This invention relates to a negative electrode for a lithium-ion secondary battery and a lithium-ion secondary battery. This application claims priority based on Japanese Patent Application No. 2025-019910, filed in Japan on February 10, 2025, the contents of which are incorporated herein by reference.

[0002] Lithium-ion rechargeable batteries are widely used as power sources for mobile devices such as cell phones and laptops, as well as hybrid cars.

[0003] The capacity of lithium-ion secondary batteries primarily depends on the active material of the electrodes. While graphite is commonly used as the negative electrode active material, there is a demand for negative electrode active materials with higher capacities. Therefore, silicon (Si), which has a theoretical capacity far greater than that of graphite (372 mAh / g), is attracting attention.

[0004] The negative electrode active material containing silicon undergoes significant volume expansion during charging. This volume expansion of the negative electrode active material causes a decrease in the battery's cycle performance. When the negative electrode active material expands in volume, for example, cracks may form in the negative electrode active material, delamination may occur at the interface between the negative electrode active material layer and the current collector, or cracks may form in the SEI (Solid Electrolyte Interphase) coating, leading to electrolyte decomposition, etc. These factors reduce the battery's cycle performance.

[0005] For example, Patent Document 1 discloses a state in which multiple particles are linked together via polyimide, polyamide, and polyamideimide in the negative electrode active material layer. Patent Document 1 also discloses that this linked state mitigates the expansion and contraction of the negative electrode active material, thereby improving the charge-discharge cycle characteristics of the battery.

[0006] Japanese International Publication No. 2012 / 081534 (A)

[0007] Further improvements in cycle characteristics are needed.

[0008] This disclosure is made in view of the above-mentioned problems and aims to provide a negative electrode for lithium-ion secondary batteries and a lithium-ion secondary battery that can improve cycle characteristics.

[0009] To solve the above problems, the following means are provided.

[0010] (1) The negative electrode for a lithium-ion secondary battery according to the first aspect has a negative electrode current collector and a negative electrode active material layer in contact with at least one surface of the negative electrode current collector. The negative electrode current collector has a tensile strength of 400 MPa or more and 805 MPa or less. The negative electrode active material layer has a negative electrode active material containing silicon and an organic additive containing diaminodiphenylmethane in its structure.

[0011] (2) In the negative electrode for a lithium-ion secondary battery according to the above aspect, the peeling strength when peeling the negative electrode active material layer from the negative electrode current collector may be 16 N / m or more and 144 N / m or less.

[0012] (3) In the negative electrode for a lithium-ion secondary battery according to the above aspect, the thickness of the negative electrode current collector may be 2 μm or more and 20 μm or less.

[0013] (4) In the negative electrode for a lithium-ion secondary battery according to the above aspect, the elongation at break of the negative electrode current collector may be 1% or more and 7% or less.

[0014] (5) In the negative electrode for a lithium-ion secondary battery according to the above aspect, the IACS conductivity of the negative electrode current collector may be 60% or more and 99% or less.

[0015] (6) In the negative electrode for a lithium-ion secondary battery according to the above aspect, the negative electrode active material layer further includes a conductive assistant, and the conductive assistant may include single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0016] (7) In the negative electrode for a lithium-ion secondary battery according to the above aspect, a rust inhibitor having a benzene ring in its structure may be applied to the negative electrode current collector.

[0017] (8) The lithium-ion secondary battery according to the second aspect includes the negative electrode for a lithium-ion secondary battery according to the above aspect, a positive electrode, and an electrolyte, and the electrolyte is between the negative electrode for a lithium-ion secondary battery and the positive electrode.

[0018] A lithium-ion secondary battery using the negative electrode for a lithium-ion secondary battery according to the above embodiment exhibits excellent cycle characteristics.

[0019] This is a schematic diagram of a lithium-ion secondary battery according to the first embodiment.

[0020] The embodiments will be described in detail below, with reference to the figures as appropriate. The drawings used in the following description may be enlarged for convenience to clearly illustrate the features, and the dimensional ratios of each component may differ from those in reality. The materials, dimensions, etc., exemplified in the following description are examples only, and the present invention is not limited to them. It can be implemented with appropriate modifications without changing the essence of the invention.

[0021] "Lithium-ion secondary battery" Figure 1 is a schematic diagram of a lithium-ion secondary battery according to the first embodiment. The lithium-ion secondary battery 100 shown in Figure 1 comprises a power generation element 40, an outer casing 50, and a non-aqueous electrolyte (not shown). The outer casing 50 covers the periphery of the power generation element 40. The power generation element 40 is connected to the outside by a pair of terminals 60 and 62 connected to the power generation element 40. The non-aqueous electrolyte is contained within the outer casing 50. In Figure 1, a case in which there is one power generation element 40 inside the outer casing 50 is illustrated, but multiple power generation elements 40 may be stacked. The lithium-ion secondary battery 100 may also be cylindrical, prismatic, laminated, button-shaped, etc.

[0022] (Power generation element) The power generation element 40 comprises a separator 10, a positive electrode 20, and a negative electrode 30.

[0023] <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 in contact with at least one surface of the negative electrode current collector 32. The negative electrode 30 is an example of a negative electrode for a lithium-ion secondary battery.

[0024] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 is, for example, a thin metal plate such as aluminum, copper, nickel, titanium, or stainless steel. The negative electrode current collector 32 preferably contains, for example, a copper alloy. Examples of copper alloys include copper-tin alloy (Sn-Cu alloy) and beryllium copper alloy (Be-Cu alloy).

[0025] The tensile strength of the negative electrode current collector 32 is 400 MPa or more and 805 MPa or less. Preferably, the tensile strength of the negative electrode current collector 32 is 400 MPa or more and 700 MPa or less, more preferably 400 MPa or more and 600 MPa or less, and even more preferably 450 MPa or more and 550 MPa or less. The tensile strength of the negative electrode current collector 32 is measured using a tensile testing machine. The tensile strength is determined by preparing a dumbbell-shaped test piece, pulling the test piece at a speed of 0.5 mm / min, and measuring the strength at which the test piece breaks. A dumbbell array-type test piece with a length of 70 mm, a gripping width of 15 mm at both ends, a constricted width of 6.25 mm, and a constricted length of 40 mm was used. The strength is the value obtained by dividing the tensile load value by the cross-sectional area of ​​the test piece.

[0026] When the tensile strength of the negative electrode current collector 32 is within this range, it is possible to suppress wrinkle formation in the negative electrode 30 due to the charge-discharge reaction and the expansion of the entire negative electrode 30 in the in-plane direction during charge-discharge. Furthermore, the tensile strength of the negative electrode current collector 32 is set to match the expansion and contraction of the negative electrode active material in the negative electrode active material layer 34 during charge-discharge. When a negative electrode current collector 32 with a tensile strength within the above range is used for a given negative electrode active material layer 34, delamination between the negative electrode current collector 32 and the negative electrode active material layer 34 is less likely to occur, improving the cycle characteristics of the lithium-ion secondary battery.

[0027] The thickness of the negative electrode current collector 32 is, for example, 2 μm or more and 20 μm or less. Preferably, the thickness of the negative electrode current collector 32 is, for example, 4.5 μm or more and 16 μm or less. The thickness of the negative electrode current collector 32 is, for example, the average value of the thickness measured at 10 different points in the plane on which the negative electrode current collector 32 extends. The thickness of the negative electrode current collector 32 is a parameter that affects the tensile strength of the negative electrode current collector 32. If the thickness of the negative electrode current collector 32 is too thin, it can cause the negative electrode current collector 32 to break. If the thickness of the negative electrode current collector 32 is too thick, the energy density of the lithium-ion secondary battery 100 decreases. This is because the negative electrode current collector 32 does not directly contribute to the charge-discharge reaction.

[0028] The elongation at break of the negative electrode current collector 32 is, for example, 1.0% to 7.0%. Preferably, the elongation at break of the negative electrode current collector 32 is, for example, 1.8% to 3.9%. The elongation at break of the negative electrode current collector 32 is measured using a tensile testing machine. The elongation at break is determined by preparing a dumbbell-shaped test specimen, pulling the specimen at a speed of 0.5 mm / min, and measuring the elongation of the specimen when it breaks. The shape of the dumbbell-shaped test specimen is the same as that of the test specimen used for measuring tensile strength. For example, the length of the test specimen at break is L, and the length of the test specimen before testing is L. ０ In this case, the elongation at break is "elongation at break" = 100 × (L - L ０ ) / L ０ However, this is required. If the elongation at break is within this range, the negative electrode current collector 32 is less likely to break even if the volume of the negative electrode active material layer 34 changes during charging and discharging.

[0029] The IACS (International Annealed Copper Standard) conductivity of the negative electrode current collector 32 is, for example, 60% to 99%. The IACS conductivity is equal to the conductivity of annealed standard soft copper (5.8 × 10⁻⁶). ７ This is the conductivity with S / m set to 100%. If the conductivity of the negative electrode current collector 32 is low, the efficient output of electrons generated in the negative electrode active material layer 34 to the outside is inhibited, and the charge-discharge characteristics of the lithium-ion secondary battery 100 deteriorate. Although not particularly limited, the upper limit of the IACS conductivity of the negative electrode current collector 32 may be 95% or less, or 93% or less. Also, although not particularly limited, the lower limit of the IACS conductivity of the negative electrode current collector 32 may be 65% or more, or 75% or more.

[0030] The surface of the negative electrode current collector 32 may be coated with a rust inhibitor having a benzene ring in its structure. Between the negative electrode current collector 32 and the negative electrode active material layer 34, there may be, for example, a rust inhibitor having a benzene ring in its structure. An example of a rust inhibitor having a benzene ring in its structure is benzotriazole. The benzene ring of the rust inhibitor may be bonded to the benzene ring of dianodiphenylmethane, which will be described later, by a π-π bond. When these benzene rings bond to each other, the bond between the negative electrode current collector 32 and the negative electrode active material layer 34 becomes stronger, and separation of the negative electrode current collector 32 and the negative electrode active material layer 34 due to volume changes during charging and discharging can be suppressed.

[0031] [Negative electrode active material layer] The negative electrode active material layer 34 contains negative electrode active material and organic additives. The negative electrode active material layer 34 may also contain a binder, conductive additive, etc., as needed.

[0032] The negative electrode active material contains silicon. The silicon may be pure silicon, silicon oxide, a silicon compound, or a silicon-containing composite. Pure silicon, silicon oxide, silicon compounds, and silicon-containing composites may be crystalline, amorphous, or a mixture of crystalline material dispersed within an amorphous material. Amorphous silicon is often used as the negative electrode active material, and amorphous silicon can be produced by methods such as the melt-spun method or gas atomization method.

[0033] Silicon oxide is SiO ｘ It is expressed as follows: x satisfies, for example, 0.8 ≤ x ≤ 2. Silicon oxide is SiO ２ It may consist of only SiO, or only SiO and SiO ２ A mixture of these may also be used. Furthermore, silicon oxide may have some oxygen missing.

[0034] Silicon alloys are X ｎ It is represented by Si. X is a cation. X can be, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K, etc. n satisfies 0 ≤ n ≤ 0.5.

[0035] A silicon-containing composite is a composite of silicon and a conductive material. Examples of conductive materials include carbon materials, Al, Ti, Fe, Ni, Cu, Zn, Ag, and Sn. For example, silicon-carbon composite material (Si-C) is one example of a composite. A silicon-containing composite may also have a conductive material coating on at least a portion of the surface of silicon particles. The mass ratio of the conductive material to the total mass of the composite is, for example, 0.01% by mass or more and 30% by mass or less, preferably 0.1% by mass or more and 20% by mass or less. The composite can be manufactured by methods such as mechanical alloying, chemical vapor deposition, wet processes, or a method in which the 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 and 100 m ２ / g or less, preferably 1.0 m ２ / g or more and 20 m ２ / g or less. If the specific surface area is small, it becomes difficult for Li ions to intercalate and desorb between the negative electrode active materials. If the specific surface area is large, a large amount of binder is required for polarization, and the capacity per unit volume becomes small.

[0037] The organic additive is an organic compound containing diaminodiphenylmethane in its structure. The organic compound is, for example, 4,4'-diaminodiphenylmethane. Diaminodiphenylmethane has a plurality of benzene rings and is an organic substance with excellent toughness. When the negative electrode active material layer 34 contains an additive with excellent toughness, the strength of the entire negative electrode active material layer 34 increases, and the cycle characteristics of the lithium ion secondary battery 100 are improved.

[0038] Also, the hydrogen of the N-H bond at the end of diaminodiphenylmethane forms a hydrogen bond with the silicon particles. Diaminodiphenylmethane hydrogen-bonded to the silicon particles affects the volume change of the silicon particles during charge and discharge. By adjusting the tensile strength of the negative electrode current collector 32 to match the volume change of the silicon particles in the state where diaminodiphenylmethane is bonded, the peeling between the negative electrode current collector 32 and the negative electrode active material layer 34 can be suppressed, and the cycle characteristics of the lithium ion secondary battery 100 are improved.

[0039] Also, the organic additive assists the bonding between the negative electrode active material and the binder. By bonding the negative electrode active material and the binder through the organic additive, the adhesion between the negative electrode active material and the binder is enhanced, and the cycle characteristics of the lithium ion secondary battery 100 are improved.

[0040] Furthermore, organic additives enhance the dispersibility of carbon nanotubes within the negative electrode active material layer 34. Carbon nanotubes tend to aggregate, and aggregation can occur in the slurry used to prepare the negative electrode active material layer 34. Organic additives also function as dispersants in the slurry, improving the dispersibility of carbon nanotubes. A negative electrode active material layer 34 in which carbon nanotubes are uniformly dispersed is stable, as localized reactions during charging and discharging are suppressed.

[0041] Furthermore, if the negative electrode current collector 32 contains a rust inhibitor having a benzene ring, diaminodiphenylmethane forms hydrogen bonds with the silicon particles and also forms π-π bonds with the rust inhibitor. In other words, the organic additive enhances the adhesion between the negative electrode active material and the negative electrode current collector 32 and suppresses peeling between the negative electrode current collector 32 and the negative electrode active material layer 34.

[0042] The content of organic additives in the negative electrode active material layer 34 is not particularly limited. For example, the content of organic additives relative to the total mass of the negative electrode active material, organic additives, conductive additives, and binder is 0.1% by mass or more and 8.0% by mass or less, preferably 1.0% by mass or more and 5.0% by mass or less.

[0043] The binder binds the negative electrode active materials together and the negative electrode active materials together with the negative electrode current collector. Known binders can be used. The binder is a compound different from organic additives. Preferably, the binder does not dissolve in the electrolyte, is oxidation-resistant, and has adhesive properties. Examples of binders include fluororesins. Examples of binders 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, polypropylene (PP) or polyethylene (PE) grafted with maleic anhydride, and mixtures thereof. Other binders may include cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc. For example, carboxymethylcellulose (CMC) may be used instead of cellulose.

[0044] 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, organic additives, conductive additives, and binder is 1% by mass or more and 15% by mass or less, preferably 4% by mass or more and 10% 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 is electrochemically inert and does not contribute to the discharge capacity, so the energy density of the lithium-ion secondary battery 100 will be low.

[0045] The conductive additive in the negative electrode active material layer 34 enhances the electronic conductivity between the negative electrode active materials. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include copper, nickel, stainless steel, and iron. Preferably, the conductive additive in the negative electrode active material layer 34 contains single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0046] Single-walled carbon nanotubes or multi-walled carbon nanotubes have high mechanical strength. Single-walled carbon nanotubes or multi-walled carbon nanotubes also play a role in supporting the negative electrode active material layer 34. As a result, lithium-ion secondary battery 100 containing single-walled carbon nanotubes or multi-walled carbon nanotubes as conductive additives for the negative electrode active material layer 34 has excellent cycle characteristics.

[0047] As described above, the organic additives contained in the negative electrode active material layer 34 also function as dispersants for single-walled carbon nanotubes or multi-walled carbon nanotubes. The negative electrode active material layer 34 containing organic additives has high dispersibility for single-walled carbon nanotubes or multi-walled carbon nanotubes. When single-walled carbon nanotubes or multi-walled carbon nanotubes are uniformly dispersed within the negative electrode active material layer 34, electron input and output become smoother, improving the output characteristics of the lithium-ion secondary battery 100. Furthermore, the negative electrode active material layer 34 in which single-walled carbon nanotubes or multi-walled carbon nanotubes are uniformly supported by the single-walled carbon nanotubes or multi-walled carbon nanotubes, resulting in small volume changes during charging and discharging.

[0048] The content of the conductive additive in the negative electrode active material layer 34 is not particularly limited. For example, the content of the conductive additive relative to the total mass of the negative electrode active material, organic additives, conductive additive, and binder is 5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 12% by mass or less.

[0049] The peeling strength when peeling the negative electrode active material layer 34 from the negative electrode current collector 32 is preferably, for example, 16 N / m or more and 144 N / m or less. If the adhesion between the negative electrode active material layer 34 and the negative electrode current collector 32 is high, it is possible to suppress the peeling of the negative electrode active material layer 34 and the negative electrode current collector 32 due to the volume change of the negative electrode active material layer 34 during charging and discharging. Although not particularly limited, the upper limit of the peeling strength when peeling the negative electrode active material layer 34 from the negative electrode current collector 32 may be 115 N / m or less, or 100 N / m or less. Also, although not particularly limited, the lower limit of the peeling strength when peeling the negative electrode active material layer 34 from the negative electrode current collector 32 may be 43 N / m or more, or 75 N / m or more.

[0050] <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.

[0051] [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 such as aluminum, copper, nickel, titanium, or stainless steel. Lightweight aluminum is suitably 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.

[0052] [Positive electrode active material layer] The positive electrode active material layer 24 includes, for example, a positive electrode active material. The positive electrode active material layer 24 may also include a conductive additive and a binder as needed.

[0053] The positive electrode active material includes an electrode active material capable of reversibly carrying out intercalation (intercalation) of lithium ions, or doping and dedoping of lithium ions and counteranions.

[0054] The positive electrode active material is, for example, a composite metal oxide. A composite metal oxide is, for example, lithium cobalt oxide (LiCoO2). ２ ), lithium nickelate (LiNiO ２ ), lithium manganese (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ Compounds of (in the general formula x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compounds (LiV ２ O ５ ), olivine-type LiMPO ４ (However, M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li ４ Ti ５ O １２ ), LiNi ｘ Co ｙ Al ｚ O ２(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.

[0055] The positive electrode active material may be a lithium-free material. Examples of lithium-free materials include FeF ３ These include conjugated polymers containing organic conductive materials, Schevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. Lithium-free materials may be used individually or in combination. If the positive electrode active material is lithium-free, for example, a discharge is performed first. Lithium is inserted into the positive electrode active material by the discharge. Alternatively, lithium may be pre-doped chemically or electrochemically into lithium-free positive electrode active materials.

[0056] Conductive additives enhance the electronic conductivity between positive electrode active materials. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include copper, nickel, stainless steel, and iron powders.

[0057] The content of the conductive additive in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive additive relative to the total mass of the positive electrode active material, conductive additive, and binder is 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 5% by mass or less.

[0058] The binder in the positive electrode active material layer 24 binds the positive electrode active materials together. The binder may be the same as the one used in the negative electrode active material layer 34.

[0059] 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, 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 is electrochemically inert and does not contribute to the discharge capacity, so the energy density of the lithium-ion secondary battery 100 will be low.

[0060] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30 and prevents a short circuit between them. The separator 10 spreads in plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0061] The separator 10 may have, for example, an electrically insulating porous structure. The separator 10 may be, for example, a single layer or laminate of a polyolefin film. The separator 10 may also be a stretched film of a mixture of polyethylene or polypropylene. The separator 10 may also be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may also be, for example, a solid electrolyte. The solid electrolyte may be, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 may also be an inorganic coated separator. An inorganic coated separator is obtained by coating the surface of the above film with a mixture of resin such as PVDF or CMC and inorganic substances such as alumina or silica. Inorganic coated separators have excellent heat resistance and suppress the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0062] <Electrolyte> The electrolyte is sealed inside the outer casing 50 and impregnates the power generation element 40. If the separator 10 is a solid electrolyte, it does not need to contain an electrolyte. A known electrolyte can be used. The electrolyte includes, for example, a non-aqueous solvent and an electrolyte.

[0063] The electrolyte is, for example, a lithium salt. The electrolyte is, for example, LiPF６ 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 ,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, LiN(FSO ２ ) ２ These are examples. A single lithium salt may be used alone, or two or more may be used in combination. From the viewpoint of degree of ionization, the electrolyte is LiPF ６ It is preferable that it contains [the specified element]. The concentration of the electrolyte is, for example, 0.8 mol / L or more and 5.0 mol / L or less.

[0064] The non-aqueous solvent is not particularly limited as long as it is a solvent commonly used in lithium-ion secondary batteries. The solvent may include, for example, a cyclic carbonate compound, a linear carbonate compound, a cyclic ester compound, or a linear ester compound. The solvent may also contain a mixture of these in any proportion. Examples of cyclic carbonate compounds include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, vinylene carbonate, etc. Examples of linear carbonate compounds include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of cyclic ester compounds include γ-butyrolactone, etc. Examples of linear ester compounds include propyl propionate, ethyl propionate, ethyl acetate, etc.

[0065] <Outer casing> The outer casing 50 seals the power generation element 40 and the non-aqueous electrolyte inside. The outer casing 50 prevents leakage of the non-aqueous electrolyte to the outside and prevents moisture and other substances from entering the lithium-ion secondary battery 100 from the outside.

[0066] The outer casing 50, as shown in Figure 1 for example, has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The outer casing 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0067] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used for the resin layer 54. The materials constituting the resin layer 54 may differ between the inside and outside. For example, a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), can be used as the outer material, while polyethylene (PE) or polypropylene (PP) can be used as the material for the inner polymer film.

[0068] <Terminals> Terminals 60 and 62 are connected to the negative electrode 30 and the positive electrode 20, respectively. Terminal 62, connected to the positive electrode 20, is the positive terminal, and terminal 60, connected to the negative electrode 30, is the negative terminal. Terminals 60 and 62 are responsible for electrical connections to the outside. Terminals 60 and 62 are made of conductive materials such as aluminum, nickel, and copper. The connection method may be welding or screw fastening. It is preferable to protect terminals 60 and 62 with insulating tape to prevent short circuits.

[0069] "Method for Manufacturing a Lithium-Ion Secondary Battery" A lithium-ion secondary battery 100 is manufactured by preparing a negative electrode 30, a positive electrode 20, a separator 10, an electrolyte, and an outer casing 50, and assembling them. An example of the manufacturing method for a lithium-ion secondary battery 100 is described below.

[0070] First, the negative electrode 30 is manufactured. The negative electrode 30 is manufactured by sequentially performing a slurry manufacturing process, an electrode coating process, a drying process, and a rolling process.

[0071] In the slurry preparation process, the negative electrode active material, organic additives, binder, conductive additive, and solvent are mixed to create a slurry. The organic additives also function as dispersion stabilizers, thus suppressing the aggregation of the negative electrode active material and conductive additive. Examples of solvents include water and N-methyl-2-pyrrolidone.

[0072] Next, the electrode coating process is performed. The electrode coating process involves applying a slurry to the surface of the negative electrode current collector 32. There are no particular restrictions on the method of applying the slurry. For example, the slit die coating method and the doctor blade method can be used as slurry coating methods.

[0073] Next, a drying process is performed. The drying process is a process of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with slurry is dried in an atmosphere between 80°C and 350°C.

[0074] The rolling process is performed as needed. The rolling process involves applying pressure to the negative electrode active material layer 34 to adjust its density. The rolling process is performed, for example, using a roll press or the like.

[0075] The positive electrode 20 can be manufactured using the same procedure as the negative electrode 30. The separator 10 and the outer casing 50 can be commercially available.

[0076] Next, the positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to create a power generation element 40. If the power generation element 40 is a wound body, the positive electrode 20, negative electrode 30, and separator 10 are wound around one end of each as an axis.

[0077] Finally, the power generation element 40 is sealed in the casing 50. The non-aqueous electrolyte is injected into the casing 50. After injecting the non-aqueous electrolyte, the non-aqueous electrolyte is impregnated into the power generation element 40 by applying reduced pressure, heating, etc. By sealing the casing 50 with heat, etc., a lithium-ion secondary battery 100 is obtained. Alternatively, instead of injecting the electrolyte into the casing 50, the power generation element 40 may be impregnated in the electrolyte.

[0078] The lithium-ion secondary battery 100 according to this embodiment exhibits excellent cycle characteristics. Although the reason for this is not clear, it is thought that the volume change of the negative electrode active material to which the organic additive is bound during charging and discharging, and the tensile strength of the negative electrode current collector 32 fit well together, resulting in increased mechanical strength of the negative electrode 30. The organic additive interacts with various components within the negative electrode active material layer 34, improving the adhesion between the negative electrode active materials themselves and between the negative electrode active material and the negative electrode current collector 32.

[0079] Although an example of this embodiment has been described in detail above with reference to the drawings, the configurations and their combinations in this embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of this disclosure.

[0080] "Example 1" A positive electrode slurry was applied to one surface of the positive electrode current collector. The positive electrode current collector was made of 15 μm thick aluminum foil. The positive electrode slurry was prepared by mixing the positive electrode active material, a conductive additive, a binder, and a solvent.

[0081] Lithium cobalt oxide was used as the positive electrode active material. 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. A positive electrode slurry was prepared by mixing 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. The amount of positive electrode active material supported in the positive electrode active material layer after drying was 20 mg / cm³. ２ The solvent was removed from the cathode slurry in a drying oven to create a cathode active material layer. The cathode active material layer was pressed with a roll press to produce the cathode.

[0082] Furthermore, a negative electrode slurry was applied to one surface of the negative electrode current collector and dried. The negative electrode current collector was a 6 μm thick Sn-Cu foil. The Sn-Cu foil used was heat-treated at 110°C. The tensile strength of the Sn-Cu foil was 513 MPa, the elongation at break was 3.0%, and the IACS conductivity was 83%. The negative electrode slurry was prepared by mixing the negative electrode active material, organic additives, conductive additives, binder, and solvent.

[0083] The negative electrode active material was a silicon-carbon composite material (Si-C) formed by combining silicon and a carbonaceous material. 4,4'-diaminodiphenylmethane was used as the organic additive. Polyacrylic acid was used as the binder. Carbon black and single-walled carbon nanotubes were used as the conductive additives. The mass ratio of carbon black to single-walled carbon nanotubes was 80:1. Water was used as the solvent.

[0084] A negative electrode slurry was prepared by mixing 85 parts by mass of negative electrode active material, 1 part by mass of organic additive, 6 parts by mass of conductive additive, 8 parts by mass of binder, and 40 parts by mass of solvent. The solvent was removed from the negative electrode slurry in a drying oven to prepare a negative electrode active material layer. Subsequently, the negative electrode active material layer was pressurized with a roll press and then fired at 120°C for 12 hours under a vacuum atmosphere. The mass ratio of each component in the negative electrode active material layer was negative electrode active material:organic additive:conductive additive:binder = 85% by mass:1% by mass:6% by mass:8% by mass.

[0085] Next, the electrolyte was prepared. The solvent for the electrolyte was fluoroethylene carbonate (FEC): ethylene carbonate (EC): diethyl carbonate (DEC) = 10% by volume: 20% by volume: 70% by volume. In addition, additives for improving output, gas suppression, cycle characteristic improvement, and safety performance improvement were added to the electrolyte. The electrolytic salt was LiPF ６ LiPF was used. ６ The concentration was set to 1 mol / L.

[0086] (Fabrication of Lithium-ion Secondary Battery for Evaluation) The fabricated negative electrode and positive electrode were stacked with a separator (porous polyethylene sheet) in between, so that the positive electrode active material layer and the negative electrode active material layer 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 electrode lead and negative electrode lead were welded together using an ultrasonic welding machine. This laminate was inserted into an aluminum laminate film enclosure and a closed section was formed by heat sealing all but one corner around the perimeter. Finally, after injecting the electrolyte into the enclosure, the remaining corner was sealed by heat sealing under reduced pressure using a vacuum sealing machine to fabricate a lithium-ion secondary battery.

[0087] (Measurement of capacity retention rate after 200 cycles) The cycle characteristics of the lithium-ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.).

[0088] The battery was charged at a constant current charge rate of 0.5C (the current value at which charging is completed in one hour when constant current charging is performed at 25°C) until the battery voltage reached 4.2V, and then discharged at a constant current discharge rate of 1.0C until the battery voltage reached 2.5V. The discharge capacity after the completion of charging and discharging was detected, and the battery capacity Q before the cycle test was determined. １ They sought it.

[0089] The above battery capacity Q １ The battery whose capacity was determined was then again charged using a secondary battery charge / discharge test device with a constant current charge rate of 0.5C until the battery voltage reached 4.2V, and then discharged with a constant current discharge rate of 1.0C until the battery voltage reached 2.5V. The above charge / discharge was counted as one cycle, and 200 charge / discharge cycles were performed. After that, the discharge capacity after 200 charge / discharge cycles was detected, and the battery capacity Q after 200 cycles was determined. ２ They sought it.

[0090] The capacity Q calculated above １ Q ２ From this, the capacity retention rate E after 200 cycles was calculated. The capacity retention rate E is given by E = Q ２ / Q １ This is calculated by multiplying by 100 and is called the cycle characteristic. The cycle characteristic of Example 1 was 88.8%.

[0091] "Example 2" Example 2 differs from Example 1 in that the negative electrode active material is changed to a mixture of silicon-carbon composite material (Si-C) and graphite. The mass ratio of silicon-carbon composite material (Si-C) to graphite in the negative electrode active material was set to 10% by mass:90% by mass. In addition, the mass ratio of the binder in the negative electrode active material layer was changed so that the mass ratio of each component in the negative electrode active material layer was negative electrode active material:organic additive:conductive additive:binder = 89% by mass:1% by mass:6% by mass:4% by mass. The cycle characteristics were determined under the same conditions as in Example 1.

[0092] "Examples 3-7" Examples 3-7 differ from Example 1 in that the heat treatment temperature of the Sn-Cu foil, which is the negative electrode current collector, is changed. The heat treatment temperature of the negative electrode current collector affects the tensile strength, elongation at break, and IACS conductivity of the negative electrode current collector. The cycle characteristics were determined under the same conditions as in Example 1.

[0093] "Examples 8-15" Examples 8-15 differ from Example 1 in that the mass ratio of the binder in the negative electrode active material layer is changed. The mass ratio of the binder affects the peel strength when the negative electrode active material layer is peeled from the negative electrode current collector. The cycle characteristics were determined under the same conditions as in Example 1.

[0094] "Examples 16-19" Examples 16-19 differ from Example 1 in that the thickness of the negative electrode current collector was changed. The thickness of the negative electrode current collector affects the tensile strength and elongation at break of the negative electrode current collector. The cycle characteristics were determined under the same conditions as in Example 1.

[0095] "Examples 20-22" Examples 20-22 differ from Example 1 in that the material type of the negative electrode current collector was changed. The cycle characteristics were determined under the same conditions as in Example 1.

[0096] "Examples 23 and 24" Examples 23 and 24 differ from Example 1 in that the conductive additive used in the negative electrode active material layer was changed. The cycle characteristics were determined under the same conditions as in Example 1. Example 23 used carbon black and multi-walled carbon nanotubes as conductive additives. The mass ratio of carbon black to multi-walled carbon nanotubes was 9:1. Example 24 used only carbon black as the conductive additive.

[0097] "Comparative Examples 1 and 2" Comparative Example 1 differs from Example 1 in that the heat treatment temperature of the Sn-Cu foil, which is the negative electrode current collector, is changed. The heat treatment temperature of the negative electrode current collector affects the tensile strength, elongation at break, and IACS conductivity of the negative electrode current collector. Comparative Examples 1 and 2 have low tensile strength of the negative electrode current collector. The cycle characteristics were determined under the same conditions as Example 1.

[0098] "Comparative Example 3" Comparative Example 3 differs from Example 1 in that the material type of the negative electrode current collector is changed. Comparative Example 3 has a higher tensile strength of the negative electrode current collector. The cycle characteristics were determined under the same conditions as in Example 1.

[0099] "Comparative Example 4" Comparative Example 4 differs from Example 1 in that no organic additives were added to the negative electrode active material layer. The cycle characteristics were determined under the same conditions as in Example 1.

[0100] "Comparative Example 5" Comparative Example 5 differs from Example 1 in that the material type of the negative electrode current collector is changed and no organic additives are added to the negative electrode active material layer. Comparative Example 5 has a lower tensile strength of the negative electrode current collector. The cycle characteristics were determined under the same conditions as in Example 1.

[0101] "Comparative Example 6" Comparative Example 6 differs from Example 1 in that the additive added to the organic additive in the negative electrode active material layer was changed. In Comparative Example 6, p-xylylenediamine was used as the organic additive. The cycle characteristics were determined under the same conditions as in Example 1.

[0102] The conditions and measurement results for Examples 1-24 and Comparative Examples 1-6 are summarized in Tables 1 and 2.

[0103]

[0104]

[0105] Examples 1 to 24 all exhibited superior cycle characteristics compared to Comparative Examples 1 to 6. This is thought to be because the tensile strength of the negative electrode current collector and the volume change of the negative electrode active material bonded with the organic additive fit well together, reducing the disruption of the conductive path within the negative electrode due to the volume change of the negative electrode active material layer.

[0106] According to the present invention, it is possible to provide a negative electrode for lithium-ion secondary batteries and a lithium-ion secondary battery with excellent cycle characteristics.

[0107] 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 Power generation element 50 Outer casing 52 Metal foil 54 Resin layer 60, 62 Terminals 100 Lithium-ion secondary battery

Claims

1. A negative electrode for a lithium-ion secondary battery, comprising a negative electrode current collector and a negative electrode active material layer in contact with at least one surface of the negative electrode current collector, wherein the negative electrode current collector has a tensile strength of 400 MPa or more and 805 MPa or less, and the negative electrode active material layer comprises a negative electrode active material containing silicon and an organic additive containing diaminodiphenylmethane in its structure.

2. The peel strength when peeling the negative electrode active material layer from the negative electrode current collector is 16 N / m or more and 144 N / m or less, the negative electrode for a lithium-ion secondary battery according to claim 1.

3. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the thickness of the negative electrode current collector is 2 μm or more and 20 μm or less.

4. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the elongation at break of the negative electrode current collector is 1% or more and 7% or less.

5. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the IACS conductivity of the negative electrode current collector is 60% or more and 99% or less.

6. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the negative electrode active material layer further comprises a conductive additive, and the conductive additive comprises a single-walled carbon nanotube or a multi-walled carbon nanotube.

7. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the negative electrode current collector is coated with a rust inhibitor having a benzene ring in its structure.

8. A lithium-ion secondary battery comprising a negative electrode for a lithium-ion secondary battery, a positive electrode, and an electrolyte, wherein the electrolyte is located between the negative electrode and the positive electrode for the lithium-ion secondary battery.

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