Negative electrode for lithium-ion secondary battery and lithium-ion secondary battery
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2026-01-30
- Publication Date
- 2026-08-13
Smart Images
Figure JP2026003402_13082026_PF_FP_ABST
Abstract
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. 2025-019916, 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 characteristics. 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 can cause the conductive path to break. Lithium-ion secondary batteries with broken conductive paths experience a decrease in output characteristics and reduced cycle characteristics.
[0005] For example, Patent Document 1 discloses a metal foil with a carrier foil attached, which is a metal foil with a carrier foil laminated on it via a bonding interface layer, as a current collector that can withstand stress loading due to volume changes of the active material even if it is thin.
[0006] Japanese Patent Application Publication No. 2005-353384 (A)
[0007] Further improvements in output characteristics are needed.
[0008] This disclosure has been made in view of the above-mentioned issues and aims to provide a negative electrode for lithium-ion secondary batteries and a lithium-ion secondary battery with excellent output 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 current collecting layer and a carbon coating layer. The carbon coating layer is between the current collecting layer and the negative electrode active material layer. The thickness of the carbon coating layer is 0.2 μm or more and 2.0 μm 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 20 N / m or more and 196 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 current collecting layer 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 IACS conductivity of the negative electrode current collector may be 60% or more and 99% or less.
[0014] (5) 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.
[0015] P (6) 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.
[0016] (7) 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.
[0017] The lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to the above aspect has excellent output characteristics.
[0018] This is a schematic diagram of a lithium-ion secondary battery according to the first embodiment. This is a cross-sectional view of the current collector of the lithium-ion secondary battery according to the first embodiment.
[0019] 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.
[0020] "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.
[0021] (Power generation element) The power generation element 40 comprises a separator 10, a positive electrode 20, and a negative electrode 30.
[0022] <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.
[0023] [Negative electrode current collector] Figure 2 is a cross-sectional view of a negative electrode current collector 32 according to the first embodiment. The negative electrode current collector 32 has a current collecting layer 321 and a carbon coating layer 322.
[0024] The current collector layer 321 is, for example, a conductive plate material. The current collector layer 321 is, for example, a thin metal sheet such as aluminum, copper, nickel, titanium, or stainless steel. The current collector layer 321 preferably contains copper. The current collector layer 321 is, for example, a copper-tin alloy (Sn-Cu alloy), a beryllium copper alloy (Be-Cu alloy), or electrolytic copper. The current collector layer 321 may also be rolled foil or electrolytic foil.
[0025] The thickness of the current collector layer 321 is, for example, 2 μm or more and 20 μm or less. Preferably, the thickness of the current collector layer 321 is, for example, 4.5 μm or more and 16 μm or less. The thickness of the current collector layer 321 is, for example, the average value of the thickness measured at 10 different points in the plane on which the current collector layer 321 extends. If the thickness of the current collector layer 321 is too thin, it can cause the negative electrode current collector 32 to break. If the thickness of the current collector layer 321 is too thick, the energy density of the lithium-ion secondary battery 100 will decrease. This is because the current collector layer 321 does not directly contribute to the charge-discharge reaction.
[0026] The carbon coating layer 322 contains carbon. The carbon coating layer 322 is formed by coating the surface of the current collector layer 321 with a carbon-containing material. The carbon coating layer 322 may be made of, for example, carbon. Alternatively, the carbon coating layer 322 may be an organic material that contains carbon and is conductive. Examples of materials for the carbon coating layer 322 include carbon black, graphite, and carbon nanotubes.
[0027] The carbon coating layer 322 is in contact with at least one surface of the current collector layer 321. The carbon coating layer 322 is located between the current collector layer 321 and the negative electrode active material layer 34. The carbon coating layer 322 may be formed on each of the two sides of the current collector layer 321. For example, if the negative electrode active material layer 34 is on both sides of the negative electrode current collector 32, it is preferable that the carbon coating layer 322 is on both sides of the current collector layer 321.
[0028] The carbon coating layer 322 functions as a buffer layer when the volume of the negative electrode active material in the negative electrode active material layer 34 changes due to charging and discharging. The carbon coating layer 322 also enhances the adhesion between the negative electrode active material layer 34 and the current collector layer 321. It is believed that the complex interlocking of the interface between the negative electrode active material layer 34 and the carbon coating layer 322 enhances the adhesion between the negative electrode active material layer 34 and the current collector layer 321 through an anchoring effect. Furthermore, since the carbon coating layer 322 is conductive, it can suppress the increase in interfacial resistance between the negative electrode active material layer 34 and the current collector layer 321.
[0029] The thickness of the carbon coating layer 322 is, for example, 0.2 μm or more and 2.0 μm or less. Preferably, the thickness of the carbon coating layer 322 is 0.5 μm or more and 2.0 μm or less, and preferably 1.0 μm or more and 1.5 μm or less. The thickness of the carbon coating layer 322 is, for example, the average value of the thickness measured at 10 different points in the plane on which the carbon coating layer 322 extends. The thickness at each measurement point can be measured, for example, from an image taken of the cross-section of the current collector 32 with a scanning electron microscope.
[0030] If the carbon coating layer 322 is too thin, the effect of suppressing the anchoring effect and the increase in interfacial resistance may not be sufficiently obtained. If the carbon coating layer 322 is too thick, the thickness of the electrode itself increases, the distance required to transmit electrons increases, resulting in high resistance, and the effect may not be sufficiently obtained.
[0031] 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.
[0032] 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 diaminodiphenylmethane, 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.
[0033] [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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] The composite containing silicon is a composite of silicon and a conductive material. The conductive material is, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, etc. For example, a silicon-carbon composite material (Si-C) is an example of the composite. The composite containing silicon may be one in which at least a part of the surface of silicon particles is coated with a conductive material. 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 produced, for example, by a mechanical alloying method, a chemical vapor deposition method, a wet method, a method of carbonizing by thermally decomposing a polymer after coating the polymer, etc.
[0038] 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. When the specific surface area is small, it becomes difficult for Li ions to intercalate and desorb between the negative electrode active materials. When the specific surface area is large, a large amount of binder is required for polarization, and the capacity per unit volume becomes small.
[0039] 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. The fact that the negative electrode active material layer 34 has diaminodiphenylmethane can be specified by a thermal decomposition gas chromatography mass spectrometry (GC-MS) method.
[0040] Furthermore, the hydrogen atoms in the N-H bonds at the ends of the diaminodiphenylmethane form hydrogen bonds with the silicon particles. The diaminodiphenylmethane that has hydrogen-bonded with the silicon particles affects the volume change of the silicon particles during charging and discharging. Since the diaminodiphenylmethane interacts with the surface functional groups of the carbon material in the carbon coating layer 322 through hydrogen bonds and van der Waals forces, the adhesion at the interface between the negative electrode active material layer 34 and the negative electrode current collector 32 can be further improved. As a result, the negative electrode active material layer 34 can perform a uniform reaction, reducing electronic resistance and improving output characteristics.
[0041] Furthermore, the organic additive assists in the bonding between the negative electrode active material and the binder. The bonding between the negative electrode active material and the binder via the organic additive increases the adhesion between the negative electrode active material and the binder, improving the output characteristics of the lithium-ion secondary battery 100.
[0042] 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.
[0043] 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 1% by mass or more and 15% by mass or less, preferably 4% by mass or more and 10% by mass or less.
[0044] 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.
[0045] 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 20% by mass or less, preferably 3% by mass or more and 16% by mass or less, and more preferably 5% by mass or more and 14% 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.
[0046] 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.
[0047] 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 power output characteristics.
[0048] Furthermore, 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. In addition, 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.
[0049] 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.
[0050] The peeling strength when peeling the negative electrode active material layer 34 from the negative electrode current collector 32 is preferably, for example, 20 N / m or more and 196 N / m or less, and more preferably 55 N / m or more and 87 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 volume changes of the negative electrode active material layer 34 during charging and discharging.
[0051] <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.
[0052] [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.
[0053] [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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] <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.
[0062] 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.
[0063] <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.
[0064] 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.
[0065] 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.
[0066] <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.
[0067] 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).
[0068] 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.
[0069] <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.
[0070] "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.
[0071] First, the negative electrode 30 is manufactured. The negative electrode 30 is manufactured by sequentially performing steps such as a negative electrode current collector preparation step, a slurry manufacturing step, an electrode coating step, a drying step, and a rolling step.
[0072] In the negative electrode current collector preparation step, a carbon coating layer 322 is formed on the surface of the current collector layer 321. A commercially available current collector foil can be used for the current collector layer 321. The method for forming the carbon coating layer 322 is not particularly limited. For example, the method for forming the carbon coating layer 322 is preferably one that allows for uniform application of the carbon coating layer 322, and it is preferable to use methods such as gravure coating or die coating. The carbon coating layer 322 can be produced by uniformly coating the coating film onto the current collector foil using the prepared paint and allowing it to dry thoroughly. The thickness of the carbon coating layer 322 can be controlled, for example, by the number of lines and rotations of the gravure printer in the case of gravure coating. It can also be adjusted by gap control or selection of carbon type.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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.
[0080] The lithium-ion secondary battery 100 according to this embodiment exhibits excellent output characteristics. Although the reason for this is not clear, it is thought that the carbon coating layer 322 enhances the adhesion between the negative electrode active material layer 34 and the negative electrode current collector 32, thereby reducing the interfacial resistance between the negative electrode active material layer 34 and the negative electrode current collector 32. Furthermore, the carbon coating layer 322 interacts with the organic additives added to the negative electrode active material layer 34 through hydrogen bonding and van der Waals forces, further enhancing the adhesion at the interface between the negative electrode active material layer 34 and the negative electrode current collector 32. Increased adhesion at these interfaces allows for a more uniform reaction in the negative electrode active material layer 34, leading to a reduction in electronic resistance. In addition, the organic additives interact with various components within the negative electrode active material layer 34, enhancing the adhesion between the negative electrode active materials themselves and between the negative electrode active materials and the negative electrode current collector 32.
[0081] 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.
[0082] "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.
[0083] 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 then pressed with a roll press to produce the cathode.
[0084] Next, a rolled copper foil with a thickness of 6 μm was prepared as the current collector layer. Then, a carbon coating layer was formed on both sides of the current collector layer using a gravure coater. The thickness of each carbon coating layer was 1.0 μm. The IACS conductivity of the current collector layer was 92%.
[0085] Next, the negative electrode slurry was applied to both sides of the electrolytic copper foil, which served as the negative electrode current collector, and dried. The negative electrode slurry was prepared by mixing the negative electrode active material, organic additives, conductive additives, a binder, and a solvent. The negative electrode active material was a silicon-carbon composite material (Si-C) made by compounding silicon and a carbonaceous material. The organic additive was 4,4'-diaminodiphenylmethane. The binder was polyacrylic acid. The conductive additives were carbon black and single-walled carbon nanotubes. The mass ratio of carbon black to single-walled carbon nanotubes was 80:1. Water was used as the solvent.
[0086] A negative electrode slurry was prepared by mixing 83 parts by mass of negative electrode active material, 1 part by mass of organic additive, 6 parts by mass of conductive additive, 10 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 or higher 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 = 83% by mass:1% by mass:6% by mass:10% by mass.
[0087] 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.
[0088] (Fabrication of Lithium-ion Secondary Battery for Evaluation) A unit was fabricated by stacking the fabricated negative electrode and positive electrode 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. Ten of these units were stacked 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 except for 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.
[0089] (Output Characteristics Evaluation) The output characteristics of the fabricated lithium-ion secondary battery were measured using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.) at an environment of 25°C. The output characteristics were evaluated using the 4C capacity retention rate (%), with a voltage range from 4.2V to 3.0V and 1C = 3500mAh per full cell design capacity. The 4C capacity retention rate is the ratio of the discharge capacity at 4C constant current discharge to the discharge capacity at 0.2C, based on the discharge capacity at 0.2C constant current discharge, and is expressed by the following formula (1).
[0090] (Output characteristics (%)) = (Discharge capacity at 5C constant current discharge) / (Discharge capacity at 0.2C constant current discharge) × 100 ... (1)
[0091] A higher output characteristic indicates better rapid charging characteristics for the lithium-ion secondary battery. The cycle output characteristic of Example 1 was 92.8%.
[0092] "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 output characteristics were determined under the same conditions as in Example 1.
[0093] "Examples 3-6" Examples 3-6 differ from Example 1 in that the thickness of the carbon coating layer in the negative electrode current collector is changed. Other conditions are the same as in Example 1, and the output characteristics were determined.
[0094] "Examples 7-13" Examples 7-13 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. Output characteristics were determined under the same conditions as in Example 1.
[0095] "Examples 14-17" Examples 14-17 differ from Example 1 in that the thickness of the current collection layer of the negative electrode current collector was changed. Other conditions were the same as in Example 1, and the output characteristics were determined.
[0096] "Example 18" Example 18 differs from Example 1 in that the conductive additive used in the negative electrode active material layer is changed. Other conditions were the same as in Example 1, and the output characteristics were determined. In Example 18, carbon black and multi-walled carbon nanotubes were used as conductive additives. The mass ratio of carbon black to multi-walled carbon nanotubes was 9:1.
[0097] "Comparative Examples 1 and 2" Comparative Examples 1 and 2 differ from Example 1 in that the thickness of the carbon coating layer in the negative electrode current collector is changed. The output characteristics were determined under the same conditions as in Example 1.
[0098] "Comparative Example 3" Comparative Example 3 differs from Example 1 in that no organic additives were added to the negative electrode active material layer. The output characteristics were determined under the same conditions as in Example 1.
[0099] "Comparative Example 4" Comparative Example 4 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 4, p-xylylenediamine was used as the organic additive. The output characteristics were determined under the same conditions as in Example 1.
[0100] The conditions and measurement results for Examples 1 to 18 and Comparative Examples 1 to 4 are summarized in Table 1.
[0101]
[0102] Examples 1 to 18 all exhibited superior output characteristics compared to Comparative Examples 1 to 4. Comparative Examples 1 and 2 had excessively thick carbon coating layers, resulting in high resistance and reduced output characteristics. Comparative Examples 3 and 4 lacked diaminodiphenylmethane, preventing the development of hydrogen bonding and van der Waals forces with the carbon coating layer. As a result, electronic resistance was not reduced, leading to reduced output characteristics.
[0103] 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 output characteristics.
[0104] 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 321 Current collector layer 322 Carbon coating layer 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 comprises a current collector layer and a carbon coating layer, the carbon coating layer is located between the current collector layer and the negative electrode active material layer, the thickness of the carbon coating layer is 0.2 μm or more and 2.0 μm or less, and the negative electrode active material layer comprises a silicon-containing negative electrode active material 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 20 N / m or more and 196 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 current collector layer 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 IACS conductivity of the negative electrode current collector is 60% or more and 99% or less.
5. 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.
6. 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.
7. 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.