Lithium-ion secondary battery

Abstract

This lithium-ion secondary battery comprises a negative electrode (30), a positive electrode (20), and an electrolyte between the negative electrode (30) and the positive electrode (20). The negative electrode (30) has a negative electrode active material. The negative electrode active material has composite particles and a carbon material. The composite particles include amorphous carbonaceous particles and amorphous silicon particles having an average primary particle size of 1-50 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 at least partially covered with a Na-containing layer that contains Na. The electrolyte contains 0.1-1.5 mol / L of a lithium salt having imide anions.

Description

Lithium-ion secondary battery

[0001] This disclosure relates to a lithium-ion secondary battery. This application claims priority based on Japanese Patent Application No. 2025-019925 filed in Japan on February 10, 2025, and incorporates its content herein by reference.

[0002] Lithium-ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and notebook computers, and for hybrid cars and the like.

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

[0004] The negative electrode active material containing Si undergoes a large volume expansion during charging. The volume expansion of the negative electrode active material causes a decrease in the cycle characteristics of the battery. When the negative electrode active material expands in volume, for example, cracks may occur in the negative electrode active material, peeling may occur at the interface between the negative electrode active material layer and the current collector, cracks may occur in the SEI (Solid Electrolyte Interface) film, and decomposition of the electrolyte may occur. These reduce the cycle characteristics of the battery.

[0005] For example, Patent Document 1 discloses that cycle characteristics can be improved by using a negative electrode active material obtained by mechanically chemically treating and compounding carbon and silicon. Also, for example, Patent Document 2 discloses that cycle characteristics can be improved by adjusting the porosity of composite particles. Also, for example, Patent Document 3 discloses that by arranging carbonaceous material particles on the surface of composite particles and coating these carbonaceous material particles with amorphous carbon, a lithium-ion secondary battery with a large initial charge-discharge capacity and excellent cycle characteristics can be obtained.

[0006] Japanese Patent Application Laid-Open No. 2000-149927 (A), Japanese Patent Application Laid-Open No. 2005-123175 (A), Japanese Patent Application Laid-Open No. 2008-277232 (A)

[0007] There is a need to improve cycle characteristics, and new methods to improve these characteristics 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 lithium-ion secondary battery according to the first embodiment comprises a negative electrode, a positive electrode, and an electrolyte located between the negative electrode and the positive electrode. The negative electrode has a negative electrode active material. 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 electrolyte contains a lithium salt having an imide anion in a concentration of 0.1 mol / L to 1.5 mol / L.

[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. 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 specimen, pulling the specimen at a speed of 1 mm / min, and measuring the strength at which the specimen breaks. The dumbbell array type test specimen used had a length of 70 mm, a gripping section width of 25 mm at both ends, a constricted section width of 6.25 mm, and a constricted section 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 specimen. If 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 negative electrode active material. The negative electrode active material layer 34 may also contain a binder, conductive additive, dispersion stabilizer, 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 have grown 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 subsequent charge and discharge reactions, reducing the cycle characteristics of the lithium-ion secondary battery 100.

[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 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. Also, 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 composite particles, it is possible to improve the cycle characteristics of lithium-ion secondary batteries while ensuring their capacity. Silicon particles contribute 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 silicon particles undergo depositional changes. The presence of a predetermined amount of carbonaceous particles within the composite particles prevents damage to the composite particles and improves the cycle characteristics of the lithium-ion secondary battery.

[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 ｎ A 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 output of the lithium-ion secondary battery 100 is improved.

[0053] 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.

[0054] 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.

[0055] 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).

[0056] The content rate of the conductive assistant in the negative electrode active material layer 34 is not particularly limited. For example, the content rate of the conductive assistant with respect to the total mass of the negative electrode active material, the organic additive, the conductive assistant, and the 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.

[0057] <Positive Electrode> The positive electrode 20 has, 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.

[0058] [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, stainless steel, etc. Aluminum with a light weight is preferably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0059] [Positive Electrode Active Material Layer] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive assistant and a binder as needed.

[0060] The positive electrode active material includes an electrode active material capable of reversibly proceeding with the occlusion and release of lithium ions, the desorption and insertion (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions.

[0061] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO ２ ), lithium nickel oxide (LiNiO ２ ), lithium manganate (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and the general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２Compounds 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] <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.

[0068] 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.

[0069] <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.

[0070] The electrolyte is, for example, a lithium salt having an imid anion. The imid anion is, for example, (SO4). ２ F) ２ N － (FSI － : bis(fluorosulfonyl)imide anion), (SO ２ CF ３ ) ２ N － (TFSI － : Bis(trifluoromethanesulfonyl)imide anion), (SO ２ C ２ F ５ ) ２ N － (BETI － : Bis(pentafluoroethanesulfonyl)imide anion), (SO ２ F) (SO ２ CF ３ ) N － , (SO ２ CF ３ ) (SO ２C ２ F ５ ) N － The imid anion may be used alone or in combination of two or more types. The electrolyte is, for example, LiFSI or LiTFSI.

[0071] Lithium imide salts suppress the decomposition of the electrolyte during charging and discharging of the lithium-ion secondary battery 100. Furthermore, when the electrolyte contains 0.1 mol / L or more of Li salt containing imide anions, the lone pairs of electrons on the oxygen in the imide salt structure make it easier for the Li salt to swell in the Na-containing coating layer, thereby improving the input / output characteristics and cycle characteristics of the lithium-ion secondary battery 100. In addition, because the element contained in the coating is Na, the packing properties between Na and imide anions are worse than those between Na and Li, which suppresses the reduction in coating durability due to excessive swelling of Li salt in the electrolyte, and improves cycle characteristics.

[0072] The electrolyte may contain lithium salts other than lithium salts having an imid anion. 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 Lithium salts such as LiBOB may be used as the electrolyte.

[0073] The molar concentration ratio of lithium imide salt in the electrolyte is, for example, 0.1 mol / L or more and 1.5 mol / L or less, preferably 0.5 mol / L or more and 1.5 mol / L or less. If the molar concentration of lithium imide salt is too high, the positive electrode current collector 22 may corrode. Also, if the molar concentration of lithium imide salt is too high, the viscosity of the electrolyte will increase, and the impregnation of the electrolyte into the positive electrode 20 and the negative electrode 30 will decrease. If the molar concentration of lithium imide salt is too low, the amount of lithium ions will be insufficient, and the input / output characteristics of the lithium-ion secondary battery 100 will decrease. Also, if the molar concentration of lithium imide salt is 0.1 mol / dm ３ 1.5mol / dm or more ３ The following conditions ensure that the degree of swelling of the electrolyte relative to the Na-containing layer is appropriate. By ensuring that the degree of swelling of the Na-containing layer is appropriate, it is possible to suppress a decrease in the durability of the Na-containing layer and a decrease in the mobility of lithium ions in the Na-containing layer.

[0074] The molar concentration ratio of lithium in the electrolyte can be measured, for example, by gas chromatography, ICP (inductively coupled plasma) emission spectrometry, or ICP mass spectrometry.

[0075] The 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) and propylene carbonate (PC). Examples of linear carbonate compounds include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of cyclic ester compounds include γ-butyrolactone. Examples of linear ester compounds include propyl propionate, ethyl propionate, and ethyl acetate.

[0076] Furthermore, the electrolyte preferably contains 20% to 80% by volume of ester components as the composition of the electrolyte solvent. 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 by 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.

[0077] 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.

[0078] <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.

[0079] 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).

[0080] 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.

[0081] <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.

[0082] "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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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 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 stainless steel is preferred for slurry preparation.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] The electrolyte can be prepared by adding and mixing a lithium salt containing an imido anion with a solvent. The molar concentration ratio in the electrolyte can be adjusted by changing the amount of lithium imido salt added.

[0103] The lithium-ion secondary battery 100 according to this embodiment exhibits excellent cycle characteristics. Although the exact reason is unclear, it is thought that the Na-containing layer suppresses side reactions between the negative electrode active material and the electrolyte, which contributes to the improved cycle characteristics. 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. In addition, the imide salt assists in the formation of the SEI film on the negative electrode surface. As a result, contact between the negative electrode active material and the electrolyte is suppressed, and the decomposition of the electrolyte is inhibited.

[0104] Furthermore, by using an imide lithium salt in a predetermined molar ratio with respect to the negative electrode active material having a Na-containing layer, excessive penetration of the electrolyte into the Na-containing layer can be suppressed, thereby preventing excessive swelling of the Na-containing layer. Excessive swelling of the Na-containing layer reduces its durability. If the Na-containing layer is damaged, the negative electrode active material and the electrolyte come into direct contact, making the electrolyte more susceptible to decomposition reactions.

[0105] Furthermore, the coexistence of silicon-containing composite particles and carbon materials as the negative electrode active material creates a timing difference in the charge-discharge reaction, which is thought to be one of the reasons for the improved cycle characteristics, as it can suppress the generation of lithium dendrites and other elements.

[0106] 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.

[0107] "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.

[0108] Next, a negative electrode slurry was prepared using this negative electrode active material. Carbon black was used as the conductive additive. Polyimide resin was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive additive, and 5 parts by mass of the binder were mixed with N-methyl-2-pyrrolidone to prepare the negative electrode slurry.

[0109] A negative electrode slurry was applied to one surface of the negative electrode current collector and dried. The negative electrode current collector was made of rolled copper foil with a thickness of 8 μm. The tensile strength of the rolled copper foil was 450 MPa, the elongation at break was 5.0%, and the IACS conductivity was 80%. The amount of negative electrode active material supported in the negative electrode active material layer after drying was 2.5 mg / cm³. ２ The negative electrode active material layer was pressurized with a roll press and then fired at over 300°C for 5 hours under a nitrogen atmosphere.

[0110] Furthermore, 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.

[0111] 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 25 mg / cm³. ２ The solvent was removed from the cathode slurry in a drying oven to create a cathode active material layer. The cathode active material layer was pressed with a roll press to produce the cathode.

[0112] Next, the electrolyte was prepared. The solvent for the electrolyte 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, where x = 70. The electrolytic salt of the electrolyte was LiFSI (lithium bis(fluorosulfonyl)imide). The molar concentration of LiFSI was 1.0 mol / L. Additives for power output improvement, gas suppression, cycle characteristic improvement, and safety performance improvement were added to the electrolyte. The viscosity of the electrolyte was 2.6 mPa·s. The ratio of ester components in the electrolyte solvent was 70% by volume.

[0113] (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.

[0114] (Measurement of Capacity Retention Rate after 600 Cycles) The cycle characteristics of a lithium-ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Corporation).

[0115] The battery was charged at a constant current with a charging rate of 1C (the current value at which charging is completed in 1 hour when performing constant current charging at 25°C) until the battery voltage reached 4.2V, and then discharged at a constant current with a discharging rate of 1.0C until the battery voltage reached 2.5V. The discharge capacity after charge and discharge was detected, and the battery capacity Q １ before the cycle test was obtained.

[0116] The battery for which the battery capacity Q １ was obtained above was charged again using the secondary battery charge / discharge test device at a constant current with a charging rate of 1C until the battery voltage reached 4.2V, and then discharged at a constant current with a discharging rate of 1C until the battery voltage reached 2.5V. The above charge and discharge was counted as 1 cycle, and charge and discharge were performed 600 times. Then, the discharge capacity after 600 cycles of charge and discharge was detected, and the battery capacity Q ２ after 600 cycles was obtained. The battery capacity Q １ , Q ２ obtained above were used to obtain the capacity retention rate E after 600 cycles. The capacity retention rate E is obtained by E = Q ２ / Q １ × 100. The capacity retention rate of Example 1 was 82%.

[0117] (Rate Characteristics) Also, the rate characteristics of the lithium-ion secondary battery were obtained. The rate characteristics were measured using a secondary battery charge / discharge test device. The rate characteristics had a voltage range from 4.2V to 2.5V, 1C = 1000 mAh per full cell design capacity, and were evaluated in terms of rate characteristics (%). The rate characteristics are the ratio of the discharge capacity when charging by CCCV (constant current constant voltage charging, termination current value is 0.05C) at a current value of 0.2C and discharging at a current value of 0.2C to the discharge capacity when charging by CCCV (constant current constant voltage charging, termination current value is 0.05C) at a current value of 0.2C and discharging at 2C, and are represented by the following formula (1). (5C Capacity Retention Rate (%)) = (5C Discharge Capacity) / (0.2C Discharge Capacity) × 100... (1) The rate characteristics of Example 1 were 72%.

[0118] "Examples 2-4" In Examples 2-4, the average primary particle size of the silicon particles constituting the composite particles was changed. In Examples 2-4, the ratio of Na to Li in the negative electrode active material was also changed. The ratio of Na to Li was changed by adjusting the amount of Na in the Na-containing layer. 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.

[0119] "Examples 5-8" In Examples 5-8, two or more types of carbon materials were used to constitute the negative electrode active material. In Examples 5-8, the ratio of Na to Li in the negative electrode active material was also changed. In Examples 5-8, the average primary particle size of the silicon particles constituting the composite particles was fixed at 5 nm. 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.

[0120] "Examples 9-12" In Examples 9-12, the weight ratio of carbon material in the negative electrode active material was changed. Also in Examples 9-12, the ratio of Na to Li in the negative electrode active material was changed. In Examples 9-12, the average primary particle size of the silicon particles constituting the composite particles was fixed at 5 nm. In Examples 9-12, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of the lithium-ion secondary batteries were evaluated.

[0121] "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. In Examples 13-18, the average primary particle size of the silicon particles constituting the composite particles was fixed at 5 nm. 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.

[0122] "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. In Example 19, the average primary particle size of the silicon particles constituting the composite particles was fixed at 5 nm. 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.

[0123] "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. In Examples 20-31, the average primary particle diameter of the silicon particles constituting the composite particles was fixed at 5 nm. 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.

[0124] "Examples 32-39" In Examples 32-39, the combination and molar concentration of the electrolytic salts contained in the electrolyte were changed. LiFSI and LiPF were used as the electrolytic salts in the electrolyte. ６ The following were used. In Examples 32 to 39, 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] "Examples 40-41" In Examples 40-41, the type and molar concentration of the electrolytic salt contained in the electrolyte were changed. In Examples 40-41, LiFSI was replaced with LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) among the electrolytic salts. In Example 40, only LiFSI was used as the electrolytic salt, and in Example 41, LiFSI and LiPF ６ In Examples 40 and 41, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and their respective characteristics were evaluated.

[0126] "Examples 42-46" In Examples 42-46, the combination and molar concentration of electrolyte salts contained in the electrolyte were changed to adjust the viscosity of the electrolyte. In Examples 42-46, 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 47-51" In Examples 47-51, the ratio of ester components in the electrolyte solvent was changed. In Examples 47-51, 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] "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. Also in Comparative Examples 1 and 2, the ratio of Na to Li in the negative electrode active material was changed. 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.

[0129] "Comparative Examples 3-5" In Comparative Examples 3-5, the crystallinity of the silicon particles and carbonaceous material constituting the composite particles was changed. In Comparative Examples 3-5, the ratio of Na to Li in the negative electrode active material was also changed. In Comparative Examples 3-5, the average primary particle diameter of the silicon particles constituting the composite particles was fixed at 5 nm. 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.

[0130] "Comparative Example 6" In Comparative Example 6, the negative electrode active material consisted only of composite particles, and no carbon material was mixed in. In Comparative Example 6, the ratio of Na to Li in the negative electrode active material was changed. In Comparative Example 6, the average primary particle size of the silicon particles constituting the composite particles was fixed at 5 nm. 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.

[0131] "Comparative Example 7" In Comparative Example 7, no Na-containing layer was formed. In Comparative Example 7, the average primary particle size of the silicon particles constituting the composite particles was 5 nm. 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.

[0132] "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 contained lithium diisopropylamide. In Comparative Examples 8 and 9, the average primary particle size of the silicon particles constituting the composite particles was 5 nm. 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.

[0133] "Comparative Examples 10 and 11" In Comparative Examples 10 and 11, the molar ratio of the electrolytic salt containing imid anions in the electrolyte was changed. In Comparative Examples 10 and 11, lithium-ion secondary batteries were prepared in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0134] "Comparative Examples 12 and 13" In Comparative Examples 12 and 13, the type and molar ratio of the electrolytic salt containing imide anions in the electrolyte were changed. In Comparative Examples 12 and 13, LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) was used as the electrolytic salt. In Comparative Examples 12 and 13, 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.

[0135] "Comparative Examples 14 and 15" In Comparative Examples 14 and 15, the type and molar ratio of the electrolytic salt contained in the electrolyte were changed. In Comparative Example 14, NaFSI was used as the electrolytic salt, and in Comparative Example 15, NaTFSI was used as the electrolytic salt. In Comparative Examples 14 and 15, lithium-ion secondary batteries were prepared in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0136] "Comparative Example 16" In Comparative Example 16, the type of electrolytic salt contained in the electrolyte was changed. In Comparative Example 14, the electrolytic salt was LiPF ６ LiPF ６ It is not an imide salt. In Comparative Example 16, a lithium-ion secondary battery was prepared in the same manner as in Example 1, and the characteristics of the lithium-ion secondary battery were evaluated.

[0137] The conditions and measurement results for Examples 1 to 39 and Comparative Examples 1 to 16 are summarized in Tables 1 to 8.

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146] Examples 1 to 51 all exhibited superior cycle characteristics compared to Comparative Examples 1 to 16. In Comparative Examples 1 and 2, the average primary particle diameter of the composite particles was inappropriate, which is thought to have caused particle cracking of the composite particles during charging and discharging, resulting in reduced cycle characteristics. In Comparative Examples 3 to 5, some of the composite particles were crystalline, so the volume did not change isotropically during charging and discharging, which is thought to have caused particle cracking of the composite particles during charging and discharging, resulting in reduced cycle characteristics. In Comparative Example 7, there was no Na-containing layer and the negative electrode active material was not coated, which is thought to have caused a side reaction between the negative electrode active material and the electrolyte, resulting in reduced cycle characteristics. In Comparative Examples 8 and 9, although the surface of the negative electrode active material layer was coated, the coating layer did not contain sodium, so the reduction of solvent molecules could not be prevented, and the side reaction of the electrolyte was not suppressed, resulting in reduced cycle characteristics.

[0147] In Comparative Examples 10-13, the molar ratio of lithium imide salt was inappropriate, resulting in an inappropriate degree of swelling of the Na-containing layer, which is thought to have caused damage to the Na-containing layer during the charge-discharge process. In Comparative Examples 14 and 15, the electrolyte contained sodium imide salt, which increased its affinity with the Na-containing layer, resulting in an inappropriate degree of swelling of the Na-containing layer, which is thought to have caused damage to the Na-containing layer during the charge-discharge process. Furthermore, in Comparative Example 16, the electrolytic salt was not an imide salt, which is thought to have resulted in insufficient formation of the SEI film.

[0148] According to the present invention, it is possible to provide a lithium-ion secondary battery that can improve cycle characteristics.

[0149] 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 lithium-ion secondary battery comprising a negative electrode, a positive electrode, and an electrolyte between the negative electrode and the positive electrode, wherein the negative electrode has a negative electrode active material, the negative electrode active material has composite particles and a carbon material, the composite particles have amorphous carbonaceous particles and amorphous silicon particles having an average primary particle diameter of 1 nm or more and 50 nm or less, 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 containing at least a portion of Na, and the electrolyte contains 0.1 mol / L or more and 1.5 mol / L or less of a lithium salt having an imide anion.

2. The 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.

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

4. The lithium-ion secondary battery according to claim 1, wherein the negative electrode has a negative electrode active material layer containing the negative electrode active material, and 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.

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

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

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

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

9. The lithium-ion secondary battery according to claim 1, wherein the electrolyte contains 20% by volume or more and 80% by volume or less of an ester component in the electrolyte solvent.

10. The lithium-ion secondary battery according to claim 1, wherein the viscosity of the electrolyte at 30°C is 2 mPa·s or more and 8 mPa·s or less.

Images