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 active material and a binder. The negative electrode active material has composite particles and a carbon material. The composite particles include amorphous carbonaceous particles and amorphous silicon particles. The carbon material is at least one selected from the group consisting of graphite, hard carbon, and soft carbon. The negative electrode active material is at least partially covered with a Na-containing layer containing Na. The binder contains a polymer having a polyacrylic acid backbone that has a weight-average molecular weight of 50,000-3,000,000. The substitution rate of the carboxy group of the polymer having a polyacrylic acid backbone with a sodium salt or a lithium salt is 10-49%.

Description

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

[0001] This disclosure relates to a negative electrode for lithium-ion secondary batteries and lithium-ion secondary batteries. This application claims priority under Japanese Patent Application No. 2025-019939, 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 Si undergoes significant volume expansion during charging. This volume expansion of the negative electrode active material can cause a decrease in the battery's cycle characteristics. When the negative electrode active material expands in volume, it can lead to cracks in the negative electrode active material, delamination at the interface between the negative electrode active material layer and the current collector, cracks in the SEI (Solid Electrolyte Interphase) coating, and decomposition of the electrolyte, among other things.

[0005] Various studies have been conducted to improve the characteristics of lithium-ion secondary batteries that have silicon as the negative electrode. For example, Patent Document 1 describes using a predetermined electrolyte. For example, Patent Document 2 describes a negative electrode active material that is a composite of carbon and silicon by mechanochemical treatment. Patent Document 3 describes a binder using an acrylic polymer.

[0006] Japanese Patent Publication No. 2015-534254 (A) Japanese Patent Publication No. 2000-149927 (A) Japanese Patent No. 6581568 (B)

[0007] One of the characteristics required of lithium-ion secondary batteries is their cycle performance. For example, Patent Documents 1 to 3 describe methods for reducing cycle performance. However, there are cases where a specific method cannot be selected due to various factors, and new methods for improving cycle performance are being investigated.

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

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

[0010] A negative electrode for a lithium-ion secondary battery according to the first embodiment comprises a negative electrode active material and a binder. The negative electrode active material comprises composite particles and a carbon material. The composite particles comprise amorphous carbonaceous particles and amorphous silicon particles having an average primary particle diameter of 1 nm to 50 nm. The carbon material is at least one selected from the group consisting of graphite, hard carbon, and soft carbon. The negative electrode active material is coated with a Na-containing layer that contains at least a portion of Na. The binder contains a polymer having a polyacrylic acid backbone with a weight-average molecular weight of 50,000 to 3,000,000. The substitution rate of the carboxyl groups of the polymer having a polyacrylic acid backbone with sodium or lithium salts is 10% to 49%.

[0011] The lithium-ion secondary battery according to the above embodiment has excellent cycle characteristics.

[0012] This is a schematic diagram of a lithium-ion secondary battery according to the first embodiment.

[0013] 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 of the actual components. The materials, dimensions, etc., exemplified in the following description are examples only, and this disclosure is not limited to them. It is possible to modify and implement these examples as appropriate without altering the essence of the disclosure.

[0014] "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.

[0015] (Power generation element) The power generation element 40 comprises a separator 10, a positive electrode 20, and a negative electrode 30.

[0016] <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. Here, we illustrate the case where the negative electrode 30 has a two-layer structure consisting of a negative electrode current collector 32 and a negative electrode active material layer 34, but the negative electrode 30 may also be a single layer in which the conductor constituting the negative electrode current collector 32 and the negative electrode active material constituting the negative electrode active material layer 34 are mixed, or it may consist only of a negative electrode active material layer 34 containing the negative electrode active material.

[0017] [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 sheet such as aluminum, copper, nickel, titanium, or stainless steel. The negative electrode current collector 32 preferably contains copper, for example. The negative electrode current collector 32 may also be, for example, rolled copper foil or electrolytic copper foil.

[0018] The thickness of the negative electrode current collector 32 is, for example, 10 μm or less. Preferably, the thickness of the negative electrode current collector 32 is, for example, 2.0 μm or more and 10 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 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. If the thickness of the negative electrode current collector 32 is too thin, it can cause the negative electrode current collector 32 to break.

[0019] The breaking strength of the negative electrode current collector 32 is, for example, 200 MPa to 900 MPa, and preferably 450 MPa to 900 MPa. The breaking strength of the negative electrode current collector 32 is measured using a tensile testing machine. The breaking strength is determined by preparing a dumbbell array type test piece, pulling the test piece at a speed of 1 mm / min, and measuring the strength at which the test piece breaks. The dumbbell array type test piece used had a length of 70 mm, a gripping width of 25 mm at both ends, a constricted width of 6.25 mm, and a constricted length of 40 mm. The load cell was set to 100 N. The strength is the value obtained by dividing the tensile load value by the cross-sectional area of ​​the test piece. When the breaking strength is within this range, even if the volume of the negative electrode active material layer 34 changes during charging and discharging, wrinkles are less likely to occur in the negative electrode current collector 32, and interfacial delamination between the negative electrode current collector 32 and the negative electrode active material layer 34 can be prevented. Furthermore, by having a fracture strength above a certain value, the risk of the negative electrode current collector 32 fracturing can be reduced even if the negative electrode active material becomes trapped in the negative electrode current collector 32 during rolling. When the negative electrode active material becomes trapped in the negative electrode current collector 32, the anchoring effect increases the adhesion between the negative electrode current collector 32 and the negative electrode active material layer 34.

[0020] The elongation at break of the negative electrode current collector 32 is, for example, 2.0% to 7.0%. 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 test specimen, pulling the specimen at a speed of 1 mm / min, and measuring the elongation of the specimen when it breaks. The test specimen used to measure the elongation at break is the same as the test specimen used to measure the breaking strength. For example, the length of the test specimen at break is L, and the length of the test specimen before testing is L. ０In 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, even if the volume of the negative electrode active material layer 34 changes during charging and discharging, the negative electrode current collector 32 can follow the volume change of the negative electrode active material layer 34, and interfacial delamination between the negative electrode current collector 32 and the negative electrode active material layer 34 can be prevented.

[0021] The IACS (International annealed copper standard) conductivity of the negative electrode current collector 32 is, for example, 60% or more. The IACS conductivity of the negative electrode current collector 32 is, for example, 100% or less. 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 and discharge characteristics of the lithium-ion secondary battery 100 deteriorate.

[0022] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer 34 may also contain conductive additives, dispersion stabilizers, etc., as needed.

[0023] The negative electrode active material contains silicon-containing composite particles and carbon material. Within the negative electrode active material layer 34, there are two types of negative electrode active materials: silicon-containing composite particles and carbon material. The carbon material reacts later during charging than the silicon-containing composite particles. By having the charge and discharge reactions proceed at different timings for the two types of negative electrode active materials, the generation of lithium dendrites can be suppressed. Lithium dendrites are lithium crystals that grow in a tree-like structure. If the trunk of a lithium dendrite breaks, the lithium contained in the lithium dendrite can no longer contribute to the subsequent charge and discharge reaction, reducing the cycle characteristics of the lithium-ion secondary battery 100. Lithium dendrites are also more likely to form during fast charging. By suppressing lithium dendrites, the rate characteristics of the lithium-ion secondary battery are improved.

[0024] The carbon material is at least one selected from the group consisting of graphite, hard carbon, and soft carbon. The carbon material is not limited to one type; two or more types may be used. The carbon material has a low reaction potential and reacts later during charging than the silicon-containing composite particles.

[0025] The weight ratio of carbon material in the negative electrode active material layer 34 is, for example, 50% by weight or more and 95% by weight or less, preferably 60% by weight or more and 95% by weight or less, and preferably 80% by weight or more and 95% by weight or less. The higher the weight ratio of carbon material in the negative electrode active material layer 34, the more effectively lithium dendrite formation can be suppressed.

[0026] The particle size of the carbon material is, for example, 4.0 μm or more and 25.0 μm or less, preferably 6.0 μm or more and 23.0 μm or less, and more preferably 7 μm or more and 22 μm or less.

[0027] If carbon material is available in particulate form, the median diameter (D50) can be determined as the average particle diameter using a particle size distribution analyzer (e.g., Malvern Panalytical). When using a particle size distribution analyzer, for example, the average particle diameter of 50,000 particles is calculated. If the carbon material is inside an electrode and separation of the carbon material is difficult, the average particle diameter can be determined using at least 100 carbon material particles that can be identified from cross-sectional images. The average particle diameter measured using a particle size distribution analyzer and the average particle diameter determined from cross-sectional images do not deviate significantly and generally agree.

[0028] First, a contrast threshold is set, and carbon materials are extracted from the image. Then, the diameter of at least 100 extracted carbon materials is determined. The frequency of each determined carbon material diameter is graphed, and the mode is taken as the average particle diameter. If the shape of the carbon material is irregular, the diameter of the long axis is used to calculate the average particle diameter.

[0029] The composite particle contains multiple carbonaceous particles and multiple silicon particles. The composite particle may also contain, for example, hydrogen, oxygen, nitrogen, etc., in addition to carbonaceous particles and silicon particles. Within the composite particle, silicon particles bond with adjacent silicon particles, forming a network. The carbonaceous particles are mixed in, filling the gaps in this network.

[0030] Composite particles are secondary particles formed by the aggregation of multiple carbonaceous particles and multiple silicon particles. The average secondary particle diameter of composite particles is, for example, 1 μm to 10 μm, preferably 2 μm to 8 μm, and more preferably 3 μm to 7 μm. The average secondary particle diameter of composite particles can be measured using the same procedure as for the particle diameter of carbon materials.

[0031] If the average secondary particle diameter of the composite particles is within the above range, the cycle characteristics of the lithium-ion secondary battery 100 will improve. If the composite particles are too small, it becomes difficult to ensure sufficient strength and conductivity when forming the negative electrode active material layer, and the amount of binder and conductive additive used will increase. Since the binder and conductive additive are not active materials that perform charging and discharging, an increase in their relative abundance in the negative electrode active material layer will reduce the capacity of the lithium-ion secondary battery 100. Furthermore, if the composite particles are too large, they may break due to expansion and contraction, increasing the risk of side reactions such as electrolyte decomposition occurring on the newly formed surface resulting from the breakage.

[0032] The molar ratio of silicon particles in the composite particles is, for example, 15 mol% to 40 mol%, preferably 20 mol% to 30 mol%. The weight of silicon in the composite particles can be measured by ICP (inductively coupled plasma) emission spectroscopy or the like. Alternatively, the weight of silicon in the composite particles may be calculated by determining the ratio of elements other than silicon (e.g., oxygen, nitrogen, hydrogen, carbon) using oxygen-nitrogen-hydrogen analysis (ONH analysis) and carbon-sulfur analysis (C-S analysis), and then calculating the silicon weight based on that ratio. The molar ratio of silicon particles can be determined from the weight of silicon.

[0033] The molar ratio of carbonaceous particles in the composite particles is, for example, 50 mol% to 80 mol%, preferably 60 mol% to 70 mol%. The weight ratio of carbonaceous particles can be measured by methods such as high-frequency induction heating combustion-infrared absorption spectroscopy. The molar ratio of carbonaceous particles can be determined from the weight of the carbonaceous particles.

[0034] By controlling the molar ratio of silicon particles and carbonaceous particles in the composite particle, it is possible to improve the cycle characteristics of a lithium-ion secondary battery while ensuring its capacity. Silicon particles are the part that contributes most to charging and discharging, and the molar ratio of silicon particles affects the capacity of the lithium-ion secondary battery. Carbonaceous particles function as a buffer when the silicon particles change volume. The presence of a predetermined amount of carbonaceous particles within the composite particle prevents damage to the composite particle and improves the cycle characteristics of the lithium-ion secondary battery 100.

[0035] The molar ratio of components other than silicon particles and carbonaceous particles (e.g., hydrogen, oxygen, nitrogen) in the composite particles is preferably 15% or less.

[0036] Carbonaceous particles and silicon particles are both amorphous. Lithium ions penetrate along the orientation direction of the crystal. Therefore, the pathways through which lithium ions can penetrate are limited in crystals. In contrast, amorphous carbonaceous particles and silicon particles do not have a fixed crystal orientation, and the direction in which lithium ions can penetrate is not limited. If the carbonaceous particles and silicon particles constituting the composite particle are amorphous, the diffusion of lithium ions in the composite particle becomes uniform, and local volume changes in the composite particle can be suppressed. Whether carbonaceous particles and silicon particles are amorphous or not can be determined using X-ray diffraction. If the X-ray diffraction result does not have a predetermined plane peak, it can be determined that they are amorphous.

[0037] The silicon particles can be elemental silicon or silicon oxide (SiO₂) ｘ : For example, x satisfies 0.8 ≤ x ≤ 2. ) Or X ｎA silicon alloy represented by Si may also be used. 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.

[0038] The average primary particle diameter of silicon particles is, for example, between 1 nm and 50 nm, preferably between 3 nm and 30 nm. The average primary particle diameter of silicon particles can be determined from a cross-sectional image of the composite particles. The cross-sectional image can be measured using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). For example, using a scanning electron microscope JSM-7600 (manufactured by JEOL Ltd.), the composite particles can be observed at a magnification of 100,000x, and the average primary particle diameter can be measured by performing image processing on the captured image. The average primary particle diameter of silicon particles determined by image processing is approximately the same as the particle diameter of silicon particles prepared before manufacturing.

[0039] The average primary particle diameter is determined using the image processing software HALCON (registered trademark, manufactured by MVTec Software GmbH). This software recognizes particles in the captured image and removes particles whose entirety is not captured at the edges of the observation field. When measuring the average primary particle diameter, the connecting parts between particles are excluded. For example, the parts connecting silicon particles, which can be seen in the same way as silicon particles, are excluded, and only silicon particles are extracted. For example, parts with a width of 1 nm or less from the white areas visible in the image are removed. Then, for each extracted particle, the shortest width (shortest diameter of the circumscribed circle of the particle) is measured, and the particle diameter is converted from the shortest width. This measurement is performed for 200 particles, and the number-based cumulative particle size distribution is obtained, from which the average primary particle diameter can be calculated.

[0040] If the average primary particle diameter of the silicon particles is within the above range, the increase in the film resistance of the electrolyte due to side reactions caused by contact between the silicon particles and the electrolyte can be suppressed. Furthermore, if the average primary particle diameter of the silicon particles is within the above range, damage to the silicon particles due to expansion and contraction during charging and discharging can be suppressed.

[0041] The carbonaceous particles are compounded with silicon particles. Examples of carbonaceous particles include carbides produced after calcining graphite, graphene, pitches, and resins. Two or more types of carbonaceous particles may be used.

[0042] Pitches may include coal-based pitches, petroleum-based pitches, or synthetic pitches, such as coal tar, tar light oil, tar intermediate oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, heavy oil, coke, low molecular weight heavy oil, and their derivatives.

[0043] Resins include, for example, thermoplastic resins such as polyvinyl alcohol, phenolic resins, epoxy resins, melamine resins, urea resins, aniline resins, cyanate resins, furan resins, ketone resins, unsaturated polyester resins, urethane resins, and modified versions thereof. Phenolic resins include, for example, novolac-type phenolic resins and resol-type phenolic resins. Epoxy resins include, for example, bisphenol-type epoxy resins and novolac-type epoxy resins. Resins include, for example, polyethylene, polystyrene, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, polyethylene terephthalate, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetheretherketone, and polyvinyl chloride.

[0044] The carbonaceous particles preferably contain graphite or graphene. The sp of graphite or graphene ２ The bond has high electronic conductivity and ensures a uniform supply of lithium ions to the silicon particles.

[0045] The weight ratio of composite particles in the negative electrode active material layer 34 is, for example, 5% by weight or more and 50% by weight or less, preferably 5% by weight or more and 40% by weight or less, and more preferably 5% by weight or more and 20% by weight or less. A higher weight ratio of composite particles in the negative electrode active material layer 34 results in a larger capacity for the lithium-ion secondary battery 100.

[0046] The surface of the negative electrode active material is coated with a Na-containing layer, at least partially. The surface of the negative electrode active material may be the surface of a carbon material or the surface of composite particles. For example, composite particles and carbon materials are coated with a Na-containing layer, at least partially containing Na. The Na-containing layer can be confirmed by energy dispersive X-ray spectroscopy (EDX).

[0047] The sodium-containing layer suppresses side reactions between the negative electrode active material and the electrolyte. Because the sodium-containing layer contains sodium, it interacts with solvents in the electrolyte such as ethylene carbonate and dimethyl carbonate, forming stable complexes such as sodium-carbonate complexes, thereby suppressing the decomposition of these solvents. By preventing the reduction of solvent molecules, sodium improves the chemical stability of the electrolyte and suppresses side reactions.

[0048] The Na-containing layer is, for example, an organic substance containing Na. Preferably, the Na-containing layer is an organic substance having an aromatic ring, and more preferably, an organic substance having a phenyl group. Organic substances having an aromatic ring exhibit excellent toughness. The Na-containing layer, with its excellent toughness, can follow the volume changes of the negative electrode active material and continuously suppress side reactions between the negative electrode active material and the electrolyte. Examples of Na-containing layers include sodium benzenesulfonate, sodium phenyl acetate, sodium phenylphosphonate, sodium phenyl sulfide, and sodium phenylalanine.

[0049] The coverage rate of the Na-containing layer on the surface of the negative electrode active material is, for example, 10% or more. Preferably, the coverage rate of the Na-containing layer on the surface of the negative electrode active material is, for example, 20% to 95%, and more preferably 30% to 80%. The coverage rate can be determined by observation using cross-sectional SEM and EDX, or by XPS. For example, the coverage rate of the Na-containing layer on 100 negative electrode active materials is measured in a cross-sectional SEM image, and the average is calculated. The coverage rate can then be determined as the average value of 100 samples.

[0050] The thickness of the Na-containing layer is preferably, for example, 10 nm to 1000 nm, and more preferably 30 nm to 500 nm.

[0051] The ratio of Na to Li content (Na / Li) in the negative electrode active material is, for example, 0.01 to 5.0, preferably 0.3 to 4.5. The Na and Li content in the negative electrode active material is determined by disassembling a fully discharged battery, cleaning the electrodes with an organic solvent such as dimethyl carbonate, and then analyzing the negative electrode active material using ICP or XPS. Since the Li content fluctuates in the charged state, the analysis is performed using negative electrode active material in a discharged state. When the Na to Li ratio is within this range, side reactions between the electrolyte and the negative electrode active material can be suppressed while improving the conductivity characteristics of lithium ions.

[0052] Furthermore, it is preferable that the negative electrode active material does not contain sodium internally. Here, "internally" refers to the area inside the line connecting the center of the circumscribed circle of the negative electrode active material (which may be a carbon material or composite particles) as confirmed by cross-sectional SEM, and the midpoint of the line connecting the outer shape of the negative electrode active material. Also, "sodium-free" means that it is below the detection limit in energy-dispersive X-ray spectroscopy (SEM-EDX). Sodium internally in the negative electrode active material can inhibit the conduction of lithium ions. By not having sodium internally in the negative electrode active material, the rate characteristics of the lithium-ion secondary battery 100 are improved.

[0053] The binder binds the negative electrode active materials together and the negative electrode active materials together with the negative electrode current collector. The binder is a polymer having a polyacrylic acid backbone with a weight-average molecular weight of 50,000 to 3,000,000. Preferably, the weight-average molecular weight of the polymer is 500,000 to 2,000,000. The binder weight-average molecular weight can be determined by disassembling the battery, removing the negative electrode, extracting the binder components with pure water, and then measuring it by size exclusion chromatography (SEC).

[0054] The binder binds to the negative electrode active material through the carboxy groups of the polyacrylic acid backbone. If the weight-average molecular weight of the binder is 50,000 or more, there are sufficient binding points in the binder to bind to the negative electrode active material, and the negative electrode active material and the binder are firmly bound. As a result, intramolecular crosslinking of the binder or intermolecular crosslinking between binders is suppressed, and aggregation of the binder is suppressed. Since the binder cannot occlude lithium, lithium dendrites are likely to occur at the aggregation sites of the binder. By preventing aggregation of the binder, the rate characteristics of the lithium ion secondary battery 100 can be improved. Also, if the weight-average molecular weight of the binder is 3,000,000 or less, intramolecular crosslinking of the binder can be suppressed and aggregation of the binder can be prevented.

[0055] A part of the carboxy groups of the polymer having a polyacrylic acid backbone is substituted with a sodium salt or a lithium salt. The polymer having a polyacrylic acid backbone is represented by [-CH ２ -CH(COOH)-] ｎ -[-CH ２ -CH(COOX)-] ｍ In this, a part of this COOH is substituted with Na or Li. For example, a polymer in which a part of the carboxy groups of the polymer having a polyacrylic acid backbone is substituted with a lithium salt is [-CH ２ -CH(COOH)-] ａ -[-CH ２ -CH(COOLi)-] ｂ -[-CH ２ -CH(COOX)-] ｍIt can be expressed as follows: n, m, a, and b are natural numbers, and a + b = n. If sodium salt substitution occurs, Li is replaced with Na. Here, X in the polyacrylic acid skeleton is a preferred substituent that can improve rate properties, and preferred substituents include alkyl groups having 15 or fewer carbon atoms such as methyl, ethyl, propyl, and butyl groups; fluorinated alkyl groups having 15 or fewer carbon atoms such as trifluoromethyl, pentafluoroethyl, and 2,2,3,3,3-pentafluoropropyl groups; aromatic ring-containing groups such as phenyl, mephenylmethyl, 4-methylbenzonitrile, naphthyl, naphthylmethyl, pyrenyl, and pyrenylmethyl groups; and organic radical-containing groups such as 2,2,6,6-tetramethylpiperidine 1-oxyl groups.

[0056] The salt substitution rate of carboxyl groups is between 10% and 49%. The salt substitution rate is the ratio of salt-substituted carboxyl groups to the total number of carboxyl groups and salt-substituted carboxyl groups in the polymer. Although not particularly limited, the salt substitution rate of carboxyl groups may be 20% or more, or 30% or more. Also, although not particularly limited, the salt substitution rate of carboxyl groups may be 45% or less, or 40% or less.

[0057] Since carboxyl groups serve as binding sites with the negative electrode active material, leaving some of the carboxyl groups of the polymer unsubstituted with salt can improve the binding properties between the negative electrode active material and the binder via the carboxyl groups. By improving the binding properties between the binder and the negative electrode active material, even when the negative electrode active material expands and contracts during charging and discharging, the binder can follow, preventing the disruption of conductive paths between negative electrode active materials and between the negative electrode active material and the negative electrode current collector. Furthermore, substituting some of the carboxyl groups of the polymer with salt can suppress intramolecular crosslinking. Intramolecular crosslinking of the polymer causes gelation, reducing the binder's function. As described above, sufficient flexibility and binding properties of the binder improve the cycle characteristics of the lithium-ion secondary battery 100. The proportion of salt-substituted carboxyl groups in the polymer can be determined by the amount of alkali consumed for hydrolysis of copolymer units or by compositional analysis using nuclear magnetic resonance (NMR).

[0058] 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.

[0059] 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 powders. When the conductive additive is a carbon material, the carbon material as a negative electrode active material and the carbon material as a conductive additive can be distinguished, for example, by the D-band and G-band ratios of Raman spectroscopy or TG-DTA (differential thermal and thermogravimetric simultaneous measurement).

[0060] The conductive additive in the negative electrode active material layer 34 preferably contains acetylene black and carbon nanotubes. The carbon nanotubes preferably contain both single-walled carbon nanotubes and multi-walled carbon nanotubes. The ratio of single-walled carbon nanotubes to carbon nanotubes is preferably 0.1% to 2.0%.

[0061] Acetylene black has high conductivity and homogenizes the current density within the negative electrode active material layer 34. Carbon nanotubes have high mechanical strength. Carbon nanotubes not only serve as a framework supporting the negative electrode active material layer 34 but also function as conductive paths between the negative electrode active materials. Multiwall carbon nanotubes are particularly useful as conductive paths between negative electrode active materials, while singlewall carbon nanotubes coat the negative electrode active materials and improve their conductivity.

[0062] 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.

[0063] <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.

[0064] [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.

[0065] [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.

[0066] 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.

[0067] 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 ２O 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.

[0068] 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.

[0069] 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. Preferably, the conductive additive contains, for example, acetylene black and carbon nanotubes.

[0070] 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.

[0071] 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. Furthermore, the binder is not limited to the one used in the negative electrode active material layer 34, but any known binder can be used.

[0072] 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.

[0073] <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.

[0074] 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.

[0075] <Electrolyte> The electrolyte is sealed inside the outer casing 50 and impregnates the power generation element 40. A known electrolyte can be used. The electrolyte includes, for example, a non-aqueous solvent and an electrolyte.

[0076] The electrolyte is, for example, a lithium salt. The electrolyte is, for example, LiPF ６ LiClO ４ LiBF ４ LiCF ３ SO ３ LiCF ３ CF ２ SO ３ LiC (CF ３ SO ２ ) ３ ,LiN(CF ３ SO ２ ) ２ ,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.

[0077] The electrolyte may be, for example, a lithium salt having an imid anion. The imid anion may be, for example, (SO4). ２ F) ２ N － (FSI － : bis(fluorosulfonyl)imide anion), (SO ２ CF ３ ) ２ N － (TFSI － : Bis(trifluoromethanesulfonyl)imide anion), (SO ２C ２ F ５ ) ２ N － (BETI － : Bis(pentafluoroethanesulfonyl)imide anion), (SO ２ F)(SO ２ CF ３ )N － , (SO ２ CF ３ )(SO ２ C ２ F ５ )N － It is. The imide anion may be used alone or in combination of two or more. The electrolyte is, for example, LiFSI, LiTFSI.

[0078] The imide lithium salt suppresses the decomposition of the electrolyte during charge and discharge of the lithium ion secondary battery 100. Further, since the imide salt is a lithium imide salt, it is possible to prevent the Na-containing layer from swelling excessively due to the electrolyte. It is considered that the affinity between the Na-containing layer containing Na and the imide lithium salt containing Li is lower than that in the case of the same elements, and the electrolyte containing the imide lithium salt is suppressed from excessively penetrating into the Na-containing layer.

[0079] The solvent is not particularly limited as long as it is a solvent generally used in lithium ion secondary batteries. The solvent includes, for example, any of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may contain these in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), etc. Examples of the chain carbonate compound include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of the cyclic ester compound include γ-butyrolactone, etc. Examples of the chain ester compound include propyl propionate, ethyl propionate, ethyl acetate, etc.

[0080] Furthermore, the electrolyte preferably contains 20% to 80% by volume of ester components as part of the electrolyte solvent composition. The volume ratio of ester components in the electrolyte solvent can be measured, for example, by diluting the electrolyte recovered from the lithium-ion secondary battery 100 by centrifugation with a standard solvent and measuring it using a method called GC-MS (gas chromatography-mass spectrometry). On the other hand, we believe that the change in the volume ratio may be causing a change in the dielectric constant of the mixed solvent contained in the electrolyte.

[0081] Furthermore, the viscosity of the electrolyte is preferably between 2 mPa·s and 8 mPa·s at 30°C. When the viscosity of the electrolyte is within this range, it easily penetrates the positive and negative electrodes. For example, even if a void is created inside the negative electrode due to expansion and contraction, the electrolyte penetrates into the void. As a result, localized depletion of the electrolyte is suppressed, and the cycle characteristics of the lithium-ion secondary battery 100 are improved. The viscosity of the electrolyte can be measured, for example, by using a rotational viscometer in a 30°C environment with the electrolyte recovered from the lithium-ion secondary battery 100 by centrifugation.

[0082] <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.

[0083] 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).

[0084] 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.

[0085] <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.

[0086] "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.

[0087] First, the negative electrode active material is prepared. The negative electrode active material can be produced by performing a composite particle fabrication process, a mixing process, and a coating layer fabrication process.

[0088] In the composite particle manufacturing process, silicon particles and a carbon source are mixed in an organic solvent. The molar ratio of silicon particles to carbonaceous particles in the composite particle can be adjusted by controlling the mixing ratio of silicon particles to the carbon source.

[0089] The carbon source may be graphite, graphene, pitches, resins, etc. The pitches and resins mentioned above can be used. Preferably, the carbon source is at least one selected from the group consisting of graphite, graphene, novolac-type phenolic resin, resol-type phenolic resin, coal-based pitch, and petroleum-based pitch. Two or more types of carbon sources may be used.

[0090] Organic solvents include methanol, ethanol, and tetrahydrofuran. Dispersants may be added to the organic solvent. Adding a dispersant allows carbonaceous particles to uniformly coat the silicon particles during compounding. Such composite particles have excellent electronic conductivity and are less likely to undergo side reactions with the electrolyte during charging and discharging.

[0091] Next, the mixed mixture is dried. Drying removes the organic solvent from the mixture, yielding a powder. The drying method is not particularly limited, but for example, spray drying is used.

[0092] Next, the dried powder is subjected to heat treatment. During the heat treatment process, the resin or resin composition, which serves as the carbon source, undergoes incomplete combustion and carbonization, resulting in the formation of carbonaceous particles. This creates a composite of silicon particles and carbonaceous particles.

[0093] The heat treatment is preferably carried out at a heat treatment temperature of 350 to 1200°C for the mixture. If the heat treatment temperature is too low, the carbon source will not be sufficiently carbonized, and an insufficiently carbonized carbon source can cause lithium to be trapped during charging and discharging. When lithium is trapped, the initial efficiency of the lithium-ion secondary battery decreases. If the heat treatment temperature is too high, the silicon particles and carbonaceous particles react, and an excess of silicon carbide is produced. Among silicon compounds, silicon carbide has a small contribution to charging and discharging, and it reduces the conductivity of lithium ions, causing a decrease in the discharge capacity of the lithium-ion secondary battery.

[0094] Furthermore, the heat treatment time is preferably between 1 hour and 72 hours. The heat treatment atmosphere is preferably a reducing atmosphere such as a nitrogen atmosphere or an argon atmosphere.

[0095] Next, the composite particles may be irradiated with a laser or electron beam to perform a thermal welding treatment between the silicon particles and carbonaceous particles. By performing the thermal welding treatment, a network structure is formed in which the silicon particles are connected.

[0096] In the mixing process, composite particles and carbon material are mixed. The carbon material is at least one selected from the group consisting of graphite, hard carbon, and soft carbon. Commercially available carbon materials can be used. By changing the mixing ratio of composite particles and carbon material, the weight ratio of composite particles and carbon material in the negative electrode active material can be adjusted.

[0097] In the coating layer fabrication process, a Na-containing layer is formed on the surface of the negative electrode active material. The Na-containing layer can be formed by chemical vapor deposition (CVD) or electrostatic adsorption.

[0098] Next, the negative electrode 30 is manufactured using this negative electrode active material. The negative electrode 30 is manufactured, for example, by sequentially performing a slurry manufacturing step, an electrode coating step, a drying step, and a rolling step.

[0099] In the slurry preparation process, a slurry is created by mixing the negative electrode active material, binder, conductive additive, and solvent. The solvent is, for example, water or N-methyl-2-pyrrolidone. The binder is a polymer having a polyacrylic acid backbone with a weight-average molecular weight of 50,000 to 3,000,000. Between 10% and 49% of the carboxyl groups of the polymer having the polyacrylic acid backbone are substituted with sodium or lithium salts. For example, NaOH, Na ２ CO ３ LiOH, Li ２ CO ３ By adding aqueous solutions such as those mentioned above to a polymer solution and stirring, some of the carboxyl groups of the polymer can be replaced. The composition ratio of the negative electrode active material, conductive material, and binder is preferably 70 wt% to 100 wt%: 0 wt% to 10 wt%: 0 wt% to 20 wt% by mass ratio. These mass ratios are adjusted so that the total is 100 wt%. A metal container such as SUS is preferred for slurry preparation.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] Finally, the power generation element 40 is sealed in the casing 50. The electrolyte is injected into the casing 50. After injecting the electrolyte, the 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.

[0106] The lithium-ion secondary battery 100 according to this embodiment exhibits excellent cycle characteristics. Although the reason for this is not entirely clear, it is thought that the carboxyl groups of the polyacrylic acid binder preferentially adsorb to the Na in the Na-containing layer, causing the binder to penetrate the outermost layer of the Na-containing layer and strengthen the bond between the Na-containing layer and the binder. This strengthens the bond between the Na-containing layer and the binder, thereby suppressing electrode collapse even if the composite particles containing silicon particles expand and contract.

[0107] Furthermore, by setting the weight-average molecular weight and salt substitution rate of the polymer containing polyacrylic acid within a predetermined range, the flexibility of the binder can be ensured while improving the adhesion between the negative electrode active materials and between the negative electrode active materials and the negative electrode current collector. As the binder follows the volume change of the negative electrode during charging and discharging, the cycle characteristics of the lithium-ion secondary battery 100 are improved.

[0108] Furthermore, the sodium in the Na-containing layer interacts with solvents such as ethylene carbonate and dimethyl carbonate to form stable complexes such as sodium-carbonate complexes. When contact between the negative electrode active material and the electrolyte is suppressed, and the decomposition of the electrolyte is suppressed, the cycle characteristics of the lithium-ion secondary battery 100 are improved.

[0109] Furthermore, the coexistence of silicon-containing composite particles and carbon material as the negative electrode active material creates a timing difference in the charge-discharge reaction, which can suppress the generation of lithium dendrites and other elements. This is considered to be one of the factors contributing to the improved cycle characteristics of the lithium-ion secondary battery 100.

[0110] 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.

[0111] "Example 1" First, composite particles were prepared by mixing silicon particles with an average primary particle diameter of 1 nm with a carbon source and firing them. After preparing the composite particles, a laser-based heat welding treatment was performed. Next, the composite particles and carbon material were mixed in a ratio of composite particles:carbon material = 10% by mass:90% by mass. Graphite was used as the carbon material. Next, an organic substance containing Na was attached to the mixture of composite particles and carbon material using electrostatic adsorption. Sodium benzenesulfonate was used as the organic substance containing Na. The ratio of Na to Li (Na / Li) in the prepared negative electrode active material was 0.76. The prepared negative electrode active material has not undergone charge-discharge treatment and corresponds to the negative electrode active material in a discharged state.

[0112] Next, a negative electrode slurry was prepared using this negative electrode active material. The conductive additive was a mixture of acetylene black and carbon nanotubes. The ratio of single-walled carbon nanotubes in the carbon nanotubes was 1.0%. The binder was a polymer with a polyacrylic acid backbone and a weight-average molecular weight of 500,000, with 30% of the carboxyl groups replaced with sodium salt. Pure water was used as the solvent. 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive additive, and 5 parts by mass of the binder were mixed with pure water to prepare the negative electrode slurry.

[0113] A negative electrode slurry was applied to one side of a negative electrode current collector and dried. The negative electrode current collector was a rolled copper foil with a thickness of 8 μm. The breaking strength of the rolled copper foil was 450 MPa, the elongation at break was 5.0%, and the IACS conductivity was 80%. The loading amount of the negative electrode active material in the dried negative electrode active material layer was 2.5 mg / cm ２ After the negative electrode active material layer was pressed by a roll press, it was fired at 300 °C or higher for 5 hours in a nitrogen atmosphere.

[0114] A positive electrode slurry was also applied to one side of a positive electrode current collector. The positive electrode current collector was an aluminum foil with a thickness of 15 μm. The positive electrode slurry was prepared by mixing a positive electrode active material, a conductive assistant, a binder, and a solvent.

[0115] Lithium cobaltate was used as the positive electrode active material. Acetylene black was used as the conductive assistant. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive assistant, 2 parts by mass of the binder, and 70 parts by mass of the solvent were mixed to prepare a positive electrode slurry. The loading amount of the positive electrode active material in the dried positive electrode active material layer was 25 mg / cm ２ The solvent was removed from the positive electrode slurry in a drying furnace to form a positive electrode active material layer. The positive electrode active material layer was pressed by a roll press to produce a positive electrode.

[0116] Next, an electrolytic solution was prepared. When the solvent of the electrolytic solution was fluoroethylene carbonate (FEC): ethylene carbonate (EC): diethyl carbonate (DEC): propyl propionate (PP) = 10% by volume: 10% by volume: 80 - x% by volume: x% by volume, x = 70. The electrolyte of the electrolytic solution was LiFSI (lithium bis(fluorosulfonyl)imide). The molar concentration of LiFSI was 1.0 mol / L. Additives for improving output, gas suppression additives, cycle characteristic improvement additives, safety performance improvement additives, etc. were added to the electrolytic solution. The viscosity of the electrolytic solution was 2.6 mPa·s. The ratio of the ester component in the electrolytic solution solvent was 70% by volume.

[0117] (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.

[0118] (Measurement of capacity retention rate after 300 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.).

[0119] The battery was charged at a constant current charging rate of 1C (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.

[0120] 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 1C until the battery voltage reached 4.2V, and then discharged with a constant current discharge rate of 1C 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 The following was calculated: The battery 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 It can be calculated by multiplying by 100. The volume retention rate in Example 1 was 84%.

[0121] "Examples 2-4" In Examples 2-4, the average primary particle size of the silicon particles constituting the composite particles was changed. Other changes from Example 1 are summarized in the table below. In Examples 2-4, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0122] "Examples 5-8" In Examples 5-8, two or more types of carbon materials were used to constitute the negative electrode active material. Other changes from Example 1 are summarized in the table below. In Examples 5-8, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0123] "Examples 9-12" In Examples 9-12, the weight ratio of carbon material in the negative electrode active material was changed. Other changes from Example 1 are summarized in the table below. In Examples 9-12, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0124] "Examples 13-18" In Examples 13-18, the ratio of Na to Li in the negative electrode active material was changed. The ratio of Na to Li was adjusted by changing the amount of Na-containing layer attached. Other changes from Example 1 are summarized in the table below. In Examples 13-18, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0125] "Example 19" In Example 19, the material constituting the Na-containing layer was changed. In Example 19, sodium carbonate was used for the Na-containing layer. Sodium carbonate is an inorganic substance. Other changes from Example 1 are summarized in the table below. In Example 19, a lithium-ion secondary battery was fabricated in the same manner as in Example 1, and the characteristics of the lithium-ion secondary battery were evaluated.

[0126] "Examples 20-31" In Examples 20-31, the characteristics of the negative electrode current collector were modified. The characteristics of the negative electrode current collector were modified by changing the thickness, heat treatment conditions, etc. Other changes from Example 1 are summarized in the table below. In Examples 20-31, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0127] "Examples 32-50" In Examples 32-50, the binder contained in the negative electrode was changed. Other changes from Example 1 are summarized in the table below. In Examples 32-50, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0128] "Examples 51-58" In Examples 51-58, the combination and molar concentration of the electrolytic salts contained in the electrolyte were changed. In Examples 51-58, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0129] "Examples 59-60" In Examples 59-60, the type and molar concentration of the electrolytic salt contained in the electrolyte were changed. In Examples 59-60, LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) was used as the electrolytic salt. In Examples 59-60, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0130] "Examples 61-65" In Examples 61-65, the combination and molar concentration of electrolyte salts contained in the electrolyte were changed to adjust the viscosity of the electrolyte. In Examples 61-65, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0131] "Examples 66-70" In Examples 66-70, the ratio of ester components in the electrolyte solvent was changed. In Examples 66-70, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0132] "Comparative Examples 1 and 2" In Comparative Examples 1 and 2, the average primary particle size of the silicon particles constituting the composite particles was changed. Other changes from Example 1 are summarized in the table below. In Comparative Examples 1 and 2, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0133] "Comparative Examples 3-5" In Comparative Examples 3-5, the crystallinity of the silicon particles and carbonaceous material constituting the composite particles was changed. Other changes from Example 1 are summarized in the table below. In Comparative Examples 3-5, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0134] "Comparative Example 6" In Comparative Example 6, the negative electrode active material consisted only of composite particles, and no carbon material was mixed in. Other changes from Example 1 are summarized in the table below. In Comparative Example 6, a lithium-ion secondary battery was fabricated in the same manner as in Example 1, and the characteristics of the lithium-ion secondary battery were evaluated.

[0135] "Comparative Example 7" In Comparative Example 7, no Na-containing layer was formed. Other changes from Example 1 are summarized in the table below. In Comparative Example 7, a lithium-ion secondary battery was fabricated in the same manner as in Example 1, and the characteristics of the lithium-ion secondary battery were evaluated.

[0136] "Comparative Examples 8 and 9" In Comparative Examples 8 and 9, the Na-containing layer was replaced with a different coating layer. Comparative Example 8 used a carbon coating layer, and Comparative Example 9 used a lithium-containing organic layer. The lithium-containing organic layer contains lithium diisopropylamide. Other changes from Example 1 are summarized in the table below. In Comparative Examples 8 and 9, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0137] "Comparative Examples 10-17" In Comparative Examples 10-17, the binder contained in the negative electrode was changed. Other changes from Example 1 are summarized in the table below. In Comparative Examples 10-17, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0138] The conditions and measurement results for Examples 1 to 39 and Comparative Examples 1 to 16 are summarized in Tables 1 to 10. In the tables, substituted ions indicate ions that have been substituted with a carboxyl group.

[0139]

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148]

[0149] Examples 1 to 70 all exhibited superior cycle characteristics compared to Comparative Examples 1 to 17. This is thought to be because, by using predetermined materials and configurations for each element constituting the negative electrode, the binder was able to follow the volume change of the negative electrode active material during charging and discharging, and the side reactions between the electrolyte and the negative electrode active material were suppressed by the Na-containing layer.

[0150] According to the present invention, it is possible to provide a lithium-ion secondary battery that can improve cycle characteristics.

[0151] 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 active material and a binder, wherein the negative electrode active material comprises composite particles and a carbon material, the composite particles comprising amorphous carbonaceous particles and amorphous silicon particles having an average primary particle diameter of 1 nm to 50 nm, the carbon material being at least one selected from the group consisting of graphite, hard carbon, and soft carbon, the negative electrode active material being coated with a Na-containing layer containing at least a portion of Na, the binder comprising a polymer having a polyacrylic acid backbone with a weight-average molecular weight of 50,000 to 3,000,000, and the substitution rate of the carboxyl groups of the polymer having a polyacrylic acid backbone with sodium or lithium salts being 10% to 49%.

2. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the weight-average molecular weight of the polymer having the polyacrylic acid skeleton is 500,000 or more and 2,000,000 or less.

3. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein, in the discharge state, the ratio (Na / Li) of the amount of Na contained in the negative electrode active material to the amount of Li contained in the negative electrode active material is 0.01 or more and 5.0 or less.

4. The anode for a lithium-ion secondary battery according to claim 1, wherein the Na-containing layer contains an organic compound including an aromatic ring.

5. The negative electrode for a lithium-ion secondary battery according to claim 1, comprising a negative electrode active material layer containing the negative electrode active material and the binder, wherein the weight ratio of the carbon material in the negative electrode active material layer is 50% by weight or more and 95% by weight or less.

6. The negative electrode for a lithium-ion secondary battery according to claim 1, further comprising a conductive additive, wherein the conductive additive comprises acetylene black and carbon nanotubes, and the carbon nanotubes comprises single-walled carbon nanotubes and multi-walled carbon nanotubes.

7. The negative electrode for a lithium-ion secondary battery according to claim 1, comprising a negative electrode active material layer containing the negative electrode active material and the binder, and a negative electrode current collector in contact with the negative electrode active material layer, wherein the thickness of the negative electrode current collector is 10 μm or less.

8. The negative electrode for a lithium-ion secondary battery according to claim 1, comprising a negative electrode active material layer containing the negative electrode active material and the binder, and a negative electrode current collector in contact with the negative electrode active material layer, wherein the tensile strength of the negative electrode current collector is 200 MPa or more and 900 MPa or less.

9. The negative electrode for a lithium-ion secondary battery according to claim 1, comprising a negative electrode active material layer containing the negative electrode active material and the binder, and a negative electrode current collector in contact with the negative electrode active material layer, wherein the elongation at break of the negative electrode current collector is 2% or more and 7% or less.

10. The negative electrode for a lithium-ion secondary battery according to claim 1, comprising a negative electrode active material layer containing the negative electrode active material and the binder, and a negative electrode current collector in contact with the negative electrode active material layer, wherein the IACS conductivity of the negative electrode current collector is 60% or more.

11. A lithium-ion secondary battery further comprising a negative electrode for a lithium-ion secondary battery as described in claim 1, a positive electrode, and an electrolyte located between the negative electrode for a lithium-ion secondary battery and the positive electrode.

12. The lithium-ion secondary battery according to claim 11, wherein the electrolyte contains 20% by volume or more and 80% by volume or less of an ester component.

13. The lithium-ion secondary battery according to claim 11, wherein the viscosity of the electrolyte at 30°C is 2 mPa·s or more and 8 mPa·s or less.

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