Positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

This positive electrode for a lithium-ion secondary battery comprises a positive electrode current collector and a positive electrode active material layer in contact with the positive electrode current collector. The positive electrode active material layer has a positive electrode active material and an aqueous binder. In an image obtained by photographing, with a scanning electron microscope, a peeled surface obtained by peeling a surface layer of a first surface of the positive electrode active material layer on the side opposite to the surface in contact with the positive electrode current collector, the proportion of the positive electrode active material is 40-80%, inclusive.

Description

Positive electrode for lithium-ion secondary batteries and lithium-ion secondary batteries

[0001] This disclosure relates to a positive electrode for a lithium-ion secondary battery and a lithium-ion secondary battery. This application claims priority under Japanese Patent Application No. 2025-042086, filed in Japan on March 17, 2025, the contents of which are incorporated herein by reference.

[0002] Non-aqueous electrolyte secondary batteries are widely used as power sources for mobile devices such as cell phones and laptops, as well as hybrid cars. For example, lithium-ion secondary batteries are one type of non-aqueous electrolyte secondary battery.

[0003] One of the characteristics required of lithium-ion secondary batteries is the rate characteristic. The rate characteristic is one of the indicators that shows the performance of rapid charging and discharging. The rate characteristic is the ratio of the battery capacity when charged and discharged with a high current to the battery capacity when charged and discharged with a low current. For example, Patent Document 1 discloses that the rate characteristic of a battery can be improved by combining a positive electrode active material having a coating layer made of a resin composition with an uncoated positive electrode active material without a coating layer.

[0004] Japanese Patent Publication No. 2020-119893

[0005] In recent years, due to environmental regulations and other factors, there has been a demand for the development of lithium-ion secondary batteries with a low environmental impact. For example, some organofluorine compounds may be subject to restrictions on their use. Under these circumstances, the use of aqueous binders in lithium-ion secondary batteries is being investigated. Aqueous binders have a low environmental impact because their solvent is water. On the other hand, using aqueous binders may result in insufficient rate characteristics being obtained in lithium-ion secondary batteries.

[0006] This disclosure is made in view of the above-mentioned issues and aims to provide a positive electrode for lithium-ion secondary batteries that can realize lithium-ion secondary batteries with excellent rate characteristics.

[0007] To solve the above problems, the following means are provided.

[0008] The positive electrode for a lithium-ion secondary battery according to the first embodiment comprises a positive electrode current collector and a positive electrode active material layer in contact with the positive electrode current collector. The positive electrode active material layer comprises a positive electrode active material and a water-based binder. In an image taken with a scanning electron microscope of the peeled surface obtained by peeling off the surface layer of the first surface of the positive electrode active material layer opposite to the surface in contact with the positive electrode current collector, the proportion occupied by the positive electrode active material is 40% or more and 80% or less.

[0009] Using the positive electrode for lithium-ion secondary batteries according to the above embodiment improves the rate characteristics of the lithium-ion secondary battery.

[0010] This is a schematic diagram of a lithium-ion secondary battery according to the first embodiment. This is an image showing an analysis of the peeled surface of the positive electrode active material layer of the lithium-ion secondary battery according to the first embodiment.

[0011] 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.

[0012] "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.

[0013] (Power generation element) The power generation element 40 comprises a separator 10, a positive electrode 20, and a negative electrode 30.

[0014] <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. Although the case where the positive electrode 20 has a two-layer structure of the positive electrode current collector 22 and the positive electrode active material layer 24 is illustrated herein, the positive electrode 20 may be a single layer in which the conductor constituting the positive electrode current collector 22 and the positive electrode active material constituting the positive electrode active material layer 24 are mixed, or may consist only of the positive electrode active material layer 24 containing the positive electrode active material.

[0015] [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 of aluminum, copper, nickel, titanium, stainless steel, or the like. Light-weight aluminum is suitably used for the positive electrode current collector 22.

[0016] [Positive Electrode Active Material Layer] The positive electrode active material layer 24 includes, for example, a positive electrode active material and an aqueous binder. The positive electrode active material layer 24 may optionally contain a conductive auxiliary agent.

[0017] The positive electrode active material includes an electrode active material capable of reversibly promoting occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions.

[0018] The positive electrode active material is, for example, a composite metal oxide. The positive electrode active material may be, for example, lithium iron phosphate (LiFePO ４ ). The composite metal oxide includes, for example, lithium cobaltate (LiCoO ２ ), lithium nickelate (LiNiO ２ ), lithium manganate (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and a compound represented by the general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ (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), a lithium vanadium compound (LiV ２ O５ ), olivine-type LiMPO ４ (wherein 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) may also be used. 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.

[0019] At least a part of the positive electrode active material may be formed with a predetermined coating film (surface treatment film). The coating film contributes to hydrophobization of the positive electrode active material. The coating film is attached to the surface of the positive electrode active material by performing plasma discharge on a gas containing a hydrocarbon compound described later. The surface treatment film contains carbon.

[0020] The carbon content on the surface of the positive electrode active material may be, for example, 100 ppm or more and 400 ppm or less. The carbon content can be measured by combusting the positive electrode active material including the surface treatment film and performing mass spectrometry on the sublimated gas. The carbon content on the surface of this positive electrode active material may be derived from the surface treatment film, for example. The positive electrode active material for measuring the surface carbon content may be one exposed on a peeled surface obtained by peeling the surface layer of the first surface of the positive electrode active material layer. Alternatively, the positive electrode active material may be extracted from the positive electrode active material layer and the surface thereof may be analyzed.

[0021] An aqueous binder is generally a binder soluble in water. The binder improves the adhesion between the positive electrode active materials and between the positive electrode active material layer 24 and the positive electrode current collector 22. The aqueous binder may be, for example, one or more selected from the group consisting of acrylic binders such as polymethyl methacrylate (PMMA), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyimide (PI), polytetrafluoroethylene (PTFE), and polyacrylic acid (PAA). A plurality of types of aqueous binders may be mixed and used.

[0022] Water-based binders tend to adsorb to hydrophilic positive electrode active materials. This is a problem specific to water-based binders and does not occur with solvent-based binders such as polyvinylidene fluoride (PVdF). By making the surface of the positive electrode active material hydrophobic, it is possible to suppress the adhesion of excess water-based binder to the surface of the positive electrode active material.

[0023] Conductive additives include, for example, carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Carbon powders include, for example, carbon black, acetylene black, and Ketjen black. Metal powders include, for example, powders of copper, nickel, stainless steel, and iron. Conductive additives may be in the form of powders or fibers.

[0024] The weight ratio of the positive electrode active material in the positive electrode active material layer 24 is, for example, 94 wt% or more and 96 wt% or less. The weight ratio of the aqueous binder in the positive electrode active material layer 24 is, for example, greater than 0 wt% and 4 wt% or less. The weight ratio of the aqueous binder in the positive electrode active material layer 24 may be, for example, 2 wt% or more and 4 wt% or less, or 2 wt% or more and 3 wt% or less. If there are multiple types of aqueous binders, it is the total weight ratio. For example, if there are two types of aqueous binders, the weight ratio of the first aqueous binder in the positive electrode active material layer 24 may be greater than 0 wt% and 2 wt% or less, and the weight ratio of the second aqueous binder in the positive electrode active material layer 24 may be greater than 0 wt% and 2 wt% or less. The weight ratio of the conductive additive in the positive electrode active material layer 24 is, for example, 0 wt% or more and 2 wt% or less. The total weight ratio of the positive electrode active material, water-based binder, and conductive additive in the positive electrode active material layer 24 is 100 wt% or less.

[0025] If the weight ratio of the water-based binder is low, the adhesion between the positive electrode active materials and between the positive electrode active material layer 24 and the positive electrode current collector 22 will decrease. If the weight ratio of the water-based binder is low, delamination may occur at the interface between the positive electrode active material layer 24 and the positive electrode current collector 22, for example. If the weight ratio of the water-based binder is too high, the water-based binder will adhere to the surface of the positive electrode active material, and the binder will hinder the conduction of lithium ions. In particular, during high-rate charging and discharging with a large current applied, the rapid movement of lithium ions is required, so the effect of the binder becomes significant.

[0026] In the positive electrode active material layer 24, the proportion of positive electrode active material in an image taken with a scanning electron microscope of the peeled surface obtained by peeling off the surface of the first surface is 40% to 80%. The proportion of positive electrode active material in the captured image may be 45% to 65%, or 60% to 65%.

[0027] The first surface is the surface of the positive electrode active material layer 24 opposite to the surface in contact with the positive electrode current collector 22. The peeled surface is the internal surface of the positive electrode active material layer 24 after the surface layer has been peeled off from the first surface. The peeled surface is, for example, the surface obtained by pressing adhesive tape against the first surface and peeling off the surface layer together with the adhesive tape. The adhesive tape is, for example, cellophane tape. During the rolling process in manufacturing, binder components may be exposed, and an excess of binder may be observed on the first surface of the positive electrode active material layer 24. By peeling off the surface layer of the first surface, the internal state of the positive electrode active material layer 24 where the actual reaction is occurring can be understood.

[0028] The delamination surface is evaluated using the following procedure. First, the delamination surface is photographed using a scanning electron microscope. The image of the delamination surface is taken at a magnification of, for example, 1000x. The captured image is analyzed and binarized by brightness. The brightness of the positive electrode active material and the aqueous binder differs in the captured image. Since the brightness in the captured image has a first peak due to the positive electrode active material and a second peak due to the aqueous binder, the image is binarized using the midpoint between the brightness of the first peak and the brightness of the second peak as the threshold. Figure 2 shows the image of the delamination surface after binarization. The white areas are the positive electrode active material, and the black areas are the aqueous binder. By analyzing the binarized image and determining the area ratio of the white areas, the proportion of the captured image occupied by the positive electrode active material can be determined.

[0029] The same process is performed at 10 different points, and the proportion of positive electrode active material in each captured image is determined. Then, by calculating the average of these proportions, the proportion of positive electrode active material in the delamination surface can be determined.

[0030] The binarization process and the analysis of the proportion of the positive electrode active material may be performed automatically using AI (Artificial Intelligence) analysis techniques.

[0031] Even when the positive electrode active material layer 24 contains a conductive additive, the conductive additive is recognized as a low-brightness area in the captured image. That is, since the conductive additive does not belong to the white areas, the proportion occupied by the positive electrode active material in the captured image can be determined. Furthermore, to determine the proportion occupied by the water-based binder in the black areas, the ratio of the binder to the conductive additive can be determined from the weight ratio, and the proportion occupied by the water-based binder can be determined by dividing the proportion of the areas identified as black in the binarization process by the weight ratio.

[0032] In the captured image, if the proportion of positive electrode active material is between 40% and 80%, it means that this amount of positive electrode active material is exposed in the captured image. If the surface of the positive electrode active material is covered with a water-based binder, the water-based binder inhibits lithium conduction. In particular, during high-rate charging and discharging with a large current applied, rapid movement of lithium ions is required, so the effect of the binder becomes significant. If the surface of the positive electrode active material is sufficiently exposed, the movement of lithium ions to the positive electrode active material becomes smoother, and high battery capacity can be achieved even at high rates. Also, if the proportion of positive electrode active material in the captured image is too high, it means that there is little binder responsible for bonding within the positive electrode active material layer 24, which can lead to delamination of the positive electrode active material layer 24.

[0033] Generally, positive electrode active materials are hydrophilic and readily adsorb water-based binders. In the case of solvent-based binders, it is easy to satisfy the condition that the proportion of positive electrode active material in the captured image is between 40% and 80%, but in the case of water-based binders, this can be achieved by modifying the surface of the positive electrode active material.

[0034] Furthermore, the positive electrode 20 according to this embodiment may be one in which no delamination of the positive electrode active material layer 24 from the positive electrode current collector 22 is observed in a mandrel test using a mandrel with a diameter of 1.0 mm. The mandrel test is a bending test. The mandrel test is performed, for example, in accordance with JIS K 5600-5-1. The positive electrode 20 is placed perpendicular to a cylindrical mandrel with a diameter of 1.0 mm, and the positive electrode 20 is bent around the mandrel. The mandrel test is performed under the conditions of 1 N and 1 minute. Whether or not delamination is observed in the mandrel test is determined by observing the presence or absence of cracks on the electrode surface. If cracks occur and the base material of the current collector foil is exposed, it is determined that delamination has occurred.

[0035] If no delamination occurs in the mandrel test, it means that there is strong adhesion between the positive electrode current collector 22 and the positive electrode active material layer 24 of the lithium-ion secondary battery. A lithium-ion secondary battery that does not meet the mandrel test requirements may experience delamination between the positive electrode current collector 22 and the positive electrode active material layer 24 due to, for example, an unexpected external force applied during use.

[0036] Increasing the weight ratio of the aqueous binder within the positive electrode active material layer 24 increases the likelihood of passing the mandrel test, but there is a risk that the aqueous binder will adhere to the surface of the positive electrode active material, preventing the acquisition of sufficient rate characteristics.

[0037] <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. Here, we illustrate the case where the negative electrode 30 has a two-layer configuration 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.

[0038] [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. The negative electrode current collector 32 may also be in the form of powder, foil, punched, or expanded.

[0039] [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.

[0040] The negative electrode active material is not particularly limited, as long as it can reversibly carry out the intercalation and release of lithium ions, and the insertion and deintercalation of lithium ions. Known negative electrode active materials can be used. Examples of negative electrode active materials include graphite, silicon, tin, and lithium titanate (Li ４ Ti ５ O １２ It may include one or more of the following selected from:

[0041] 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. Preferably, the binder does not dissolve in the electrolyte, is oxidation-resistant, and has adhesive properties. The binder may be a solvent-based binder or a water-based binder. A water-based binder is preferred when used in combination with the positive electrode 20. 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, or mixtures thereof. Other binders may include cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc. Cellulose may be, for example, carboxymethylcellulose (CMC).

[0042] The same conductive additive as that used for the positive electrode 20 can be used.

[0043] The weight ratio of the negative electrode active material in the negative electrode active material layer 34 is, for example, 94 wt% or more and 96 wt% or less. The weight ratio of the binder in the negative electrode active material layer 34 is, for example, 0 wt% or more and 4 wt% or less. The weight ratio of the binder in the negative electrode active material layer 34 may also be, for example, 2 wt% or more and 4 wt%, or 2 wt% or more and 3 wt% or less. The weight ratio of the conductive additive in the negative electrode active material layer 34 is, for example, 0 wt% or more and 2 wt% or less. The total weight ratio of the negative electrode active material, system binder, and conductive additive in the negative electrode active material layer 34 is 100 wt% or less.

[0044] <Separator> The separator 10 is located between the positive electrode 20 and the negative electrode 30. The separator 10 isolates adjacent positive electrodes 20 and negative electrodes 30, preventing a short circuit between them. The separator 10 extends in plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0045] 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.

[0046] <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.

[0047] (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.

[0048] 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).

[0049] 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.

[0050] (Electrolyte) The electrolyte is sealed inside the outer casing 50 and impregnates the power generation element 40. If the separator 10 is a solid electrolyte, it does not need to contain an electrolyte. A known electrolyte can be used. The electrolyte includes, for example, a non-aqueous solvent and an electrolyte.

[0051] 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 SO３ 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.

[0052] The non-aqueous solvent is not particularly limited as long as it is a solvent commonly used in lithium-ion secondary batteries. The solvent may include, for example, a cyclic carbonate compound, a linear carbonate compound, a cyclic ester compound, or a linear ester compound. The solvent may also contain a mixture of these in any proportion. Examples of cyclic carbonate compounds include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, vinylene carbonate, etc. Examples of linear carbonate compounds include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of cyclic ester compounds include γ-butyrolactone, etc. Examples of linear ester compounds include propyl propionate, ethyl propionate, ethyl acetate, etc.

[0053] "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 these components.

[0054] The positive electrode 20 can be manufactured by applying a positive electrode slurry to the surface of the positive electrode current collector 22 and drying it. The positive electrode slurry is a mixture of positive electrode active material, a water-based binder, and a conductive additive dispersed in water. The mixing ratio of positive electrode active material, water-based binder, and conductive additive in the positive electrode slurry is set to match the weight ratio of the positive electrode active material after manufacturing. If the proportion of water-based binder is sufficiently small, the occupancy rate of positive electrode active material near the first surface of the positive electrode active material layer will be high.

[0055] Furthermore, the positive electrode active material dispersed in the positive electrode slurry may have at least a portion of its hydrophilic surface modified to be hydrophobic. Surface modification increases the occupancy rate of the positive electrode active material near the first surface of the positive electrode active material layer. When modifying the surface of the positive electrode active material to be hydrophobic, for example, plasma vapor polymerization can be used. In plasma vapor polymerization, plasma is generated in a space containing a reaction gas, and the reaction product is deposited on the surface of a sample placed in the space. The reaction gas is, for example, a hydrocarbon compound, such as acetone or toluene. The positive electrode active material is placed between opposing electrodes, and a high-frequency voltage is applied between the opposing electrodes, causing the plasma-generated reaction gas to adhere to the surface of the positive electrode active material. By performing this treatment, the surface of the positive electrode active material can be modified to be hydrophobic. The pressure during surface modification varies depending on the proportion of the binder, but for example, it may be 30 Pa or more and 70 Pa or less, and the surface modification treatment time may be 90 minutes or more and 150 minutes or less.

[0056] There are no particular restrictions on the method of applying the positive electrode slurry to the positive electrode current collector 22. For example, the slit die coating method and the doctor blade method can be used. When the positive electrode slurry is dried, the solvent is removed from the positive electrode slurry. For example, the positive electrode current collector 22 coated with the positive electrode slurry is dried in an atmosphere between 80°C and 350°C. As the positive electrode slurry dries, it becomes a positive electrode active material layer 24. The positive electrode active material layer 24 after drying may be further rolled.

[0057] The negative electrode 30 can be manufactured in the same way as the positive electrode 20 by applying a negative electrode slurry to the surface of the negative electrode current collector and drying it. The separator 10 and the outer casing 50 can be commercially available.

[0058] 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.

[0059] 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.

[0060] The lithium-ion secondary battery according to this embodiment exhibits excellent rate characteristics. This is thought to be because the proportion of binder adhering to the surface of the positive electrode active material is low, and the conduction of lithium ions is not easily inhibited.

[0061] 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.

[0062] "Example 1" First, a positive electrode slurry was prepared. Surface-modified lithium cobalt oxide (LCO) was used as the positive electrode active material. Surface modification was performed using toluene as the reaction gas species, under the conditions of a pressure of 30 Pa, an output of 50 W, a processing time of 150 minutes, and an active material powder amount of 150 g. The carbon content on the surface of the positive electrode active material after surface modification was measured to be 328 ppm.

[0063] Carboxymethylcellulose (CMC) was used as the binder. Carbon black was used as the conductive additive. Using water as the dispersion medium, the materials were mixed in the following order: positive electrode active material:binder:conductive additive = 96 wt%:2 wt%:2 wt% to prepare a positive electrode slurry.

[0064] Next, the positive electrode slurry was applied to one surface of the positive electrode current collector. The positive electrode current collector was made of carbon-coated aluminum foil with a thickness of 22 μm. The positive electrode slurry was dried to create a positive electrode active material layer. The positive electrode active material layer was then pressed with a roll press to create the positive electrode.

[0065] The surface of the fabricated positive electrode active material layer was peeled off with cellophane tape, and the peeled surface was photographed with a scanning electron microscope. The images were binarized using an AI analysis device, and the proportion of positive electrode active material on the peeled surface was determined. In Example 1, the proportion of positive electrode active material on the peeled surface was 65%.

[0066] Furthermore, a mandrel test was performed using a positive electrode manufactured under the same conditions. In the mandrel test, no peeling of the positive electrode active material layer was observed in the positive electrode of Example 1.

[0067] The negative electrode used a 0.5 mm thick, Φ16 Li foil. In other words, Example 1 was a half-cell used to evaluate the configuration of the positive electrode.

[0068] Next, the electrolyte was prepared. The solvent for the electrolyte was ethylene carbonate (EC):diethyl carbonate (DEC) = 30% by volume:70% by volume. The electrolyte was LiPF4. ６ LiPF was used. ６ The concentration was set to 1 mol / L.

[0069] (Fabrication of Lithium-ion Secondary Batteries for Evaluation) The lithium-ion secondary batteries for evaluation were made using a 2032 type coin cell. The following components were stacked from bottom to top: case, positive electrode punched out to Φ13 mm (with the current collector foil facing downwards), separator (porous polyethylene sheet), gasket, negative electrode Li foil, 1.0 mm thick spacer (made of SUS), wave washer (made of SUS), and cap. Finally, the cells were crimped using a coin cell crimping tool. At this time, 20 μL of electrolyte was injected between the case and the positive electrode, between the positive electrode and the separator, between the separator and the negative electrode Li foil, and between the negative electrode Li foil and the spacer.

[0070] (Rate Characteristics) The rate characteristics of lithium-ion secondary batteries were determined. The rate characteristics were measured using a secondary battery charge / discharge test apparatus. The rate characteristics were evaluated as a percentage (%) with a voltage range from 4.5V to 2.5V, and with 1C = 3.58mAh per half-cell design capacity. The rate characteristics are the ratio of the discharge capacity when CCCV charging (constant current constant voltage charging, termination current value 0.05C) at a current value of 0.2C and discharging at a current value of 0.2C to the discharge capacity when CCCV charging (constant current constant voltage charging, termination current value 0.05C) at a current value of 0.2C and discharging at a current value of 0.2C, and are expressed by the following formula (1): (5C capacity retention rate (%)) = (5C discharge capacity) / (0.2C discharge capacity) × 100 ... (1)

[0071] "Example 2" Example 2 differs from Example 1 in that the conditions for surface modification of the positive electrode active material were changed. Specifically, the toluene pressure and output were kept the same, but the processing time was changed to 90 minutes. Other than that, scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed in the same manner as in Example 1.

[0072] "Example 3" Example 3 differs from Example 2 in that the mixing ratio of the positive electrode active material, binder, and conductive additive was changed. Specifically, the ratio of positive electrode active material:binder:conductive additive was set to 94 wt%:4 wt%:2 wt%. Other aspects were the same as in Example 1, and scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed.

[0073] "Examples 4-7" Examples 4-7 differ from Example 1 in that the surface treatment conditions for the positive electrode active material were changed and the type of binder was changed. In Examples 4-7, the pressure during surface modification of the positive electrode active material was set to 70 Pa and the treatment time was set to 150 minutes. Examples 4 and 5 used polymethyl methacrylate (PMMA) as the binder. Example 5 differs from Example 4 in that the ratio of positive electrode active material:binder:conductive additive was 94 wt%:4 wt%:2 wt%. Example 6 used polyimide (PI) as the binder. Example 7 used polymethyl methacrylate (PMMA) and carboxymethylcellulose (CMC) as binders. In Example 5, polymethyl methacrylate (PMMA) and carboxymethylcellulose (CMC) were mixed in the positive electrode slurry at a weight ratio of 1% each. Other aspects were carried out in the same manner as in Example 1, including image analysis using a scanning electron microscope, mandrel testing, and rate characteristic evaluation.

[0074] "Example 8" Example 8 differs from Example 1 in that the weight ratio of the positive electrode active material to the binder mixed in the positive electrode slurry was changed, and the surface treatment conditions for the positive electrode active material were also changed. In Example 8, the pressure during surface modification of the positive electrode active material was set to 70 Pa, and the treatment time was changed to 90 minutes. Other than that, scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed in the same manner as in Example 1.

[0075] Examples 9 and 10 differ from Example 1 in that the type of binder was changed. Example 9 used styrene-butadiene rubber (SBR) as the binder. Example 10 used polyacrylic acid (PAA) as the binder. In other respects, scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed in the same manner as in Example 1.

[0076] "Examples 11-13" Examples 11-13 differ from Examples 1, 4, and 10 in that the surface of the positive electrode active material was not modified, and the mixing ratio of the positive electrode active material, binder, and conductive additive was changed. The mixing ratio of the positive electrode active material, binder, and conductive additive was set to positive electrode active material:binder:conductive additive = 94 wt%:1 wt%:5 wt%. Other aspects were the same as in Example 1, and scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed.

[0077] "Comparative Example 1" Comparative Example 1 differs from Comparative Example 1 in that the surface treatment conditions for the positive electrode active material were changed, and the weight ratio of the positive electrode active material to the binder mixed in the positive electrode slurry was changed. In Comparative Example 1, the pressure during surface modification of the positive electrode active material was set to 70 Pa, and the processing time was changed to 90 minutes.

[0078] "Comparative Example 2" Comparative Example 2 differs from Example 1 in that no surface modification was performed on the positive electrode active material mixed into the positive electrode slurry. In other respects, scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed in the same manner as in Example 1.

[0079] "Comparative Examples 3-6" Comparative Examples 3-6 differ from Comparative Example 1 in that the surface of the positive electrode active material mixed into the positive electrode slurry was not modified, but the type of binder mixed into the positive electrode slurry and the weight ratio of the positive electrode active material to the binder were changed. In Comparative Example 3, polymethyl methacrylate (PMMA) was used as the binder, and the positive electrode slurry was prepared by mixing the positive electrode active material:binder:conductive additive in a ratio of 94 wt%:4 wt%:2 wt%. In Comparative Example 4, a composite of polyethylenedioxythiophene and polystyrene sulfonic acid (PEDOT-PSS) was used as the binder, and the positive electrode slurry was prepared by mixing the positive electrode active material:binder:conductive additive in a ratio of 96 wt%:2 wt%:2 wt%. In Comparative Example 5, styrene-butadiene rubber was used as the binder, and a cathode slurry was prepared by mixing the cathode active material, binder, and conductive additive in a ratio of 96 wt%: 2 wt%: 2 wt%. In Comparative Example 6, polyacrylic acid was used as the binder, and a cathode slurry was prepared by mixing the cathode active material, binder, and conductive additive in a ratio of 96 wt%: 2 wt%: 2 wt%. In other respects, scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed in the same manner as in Example 1.

[0080] "Reference Example 1" Reference Example 1 differs from Example 1 in that the surface of the positive electrode active material mixed into the positive electrode slurry was not modified, and the binder was changed to a solvent-based binder, polyvinylidene fluoride. In other respects, scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed in the same manner as in Example 1.

[0081] "Reference Example 2" Reference Example 2 differs from Example 2 in that the surface of the positive electrode active material mixed into the positive electrode slurry was not modified, and the binder was changed to polyvinylidene fluoride, which is a solvent-based binder. In other respects, scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed in the same manner as in Example 1.

[0082] "Reference Examples 3 and 4" Reference Examples 3 and 4 differ from Reference Example 1 in that the weight ratio of the positive electrode active material and binder mixed into the positive electrode slurry was changed. In Reference Example 3, the positive electrode slurry was prepared by mixing the positive electrode active material, binder, and conductive additive in a ratio of 97 wt%, 1 wt%, and 2 wt%. In Reference Example 4, the positive electrode slurry was prepared by mixing the positive electrode active material, binder, and conductive additive in a ratio of 93 wt%, 3 wt%, and 4 wt%. Other aspects were the same as in Example 1, and scanning electron microscope image analysis, mandrel testing, and rate characteristic evaluation were performed.

[0083] The results for Examples 1-8, Comparative Examples 1-6, and Reference Examples 1-4 are summarized in Table 1 below.

[0084]

[0085] Examples 1-8 exhibited excellent rate characteristics. Furthermore, Examples 1-5, 7, and 8 also showed good results in the mandrel test. Comparative Example 1 had a low proportion of positive electrode active material on the peeled surface, resulting in insufficient rate characteristics. The reason for the low proportion of positive electrode active material on the peeled surface in Comparative Example 1 is thought to be due to differences in surface modification conditions compared to Example 1. The difference in surface modification conditions resulted in differences in the amount of carbon on the active material surface; Comparative Example 1 had less carbon on its active material surface compared to Examples 1 and 2. As a result, the positive electrode active material was unable to adequately repel the water-based binder, leading to a low positive electrode active material occupancy rate. Comparative Example 2, lacking surface modification, had a large amount of binder adhering to the positive electrode active material, resulting in a low positive electrode active material occupancy rate on the peeled surface. In Comparative Example 2, the binder inhibited lithium ion movement, resulting in poor rate characteristics. Similar trends were observed in Comparative Examples 3-6, which used different binder types.

[0086] Furthermore, Reference Examples 1 to 4 show the results using solvent-based binders. Examples 1 and 2 showed a positive electrode active material occupancy rate equivalent to that of Reference Examples 1 and 2, confirming that surface modification of the positive electrode active material can achieve performance approaching that of solvent-based binders, even with water-based binders.

[0087] 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 positive electrode for a lithium-ion secondary battery, comprising a positive electrode current collector and a positive electrode active material layer in contact with the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode active material and a water-based binder, and in an image taken with a scanning electron microscope of the peeled surface obtained by peeling off the surface layer of the first surface of the positive electrode active material layer opposite to the surface in contact with the positive electrode current collector, the proportion of the positive electrode active material is 40% or more and 80% or less.

2. In a mandrel test using a mandrel with a diameter of 1.0 mm, no delamination of the positive electrode active material layer from the positive electrode current collector is observed, as described in claim 1.

3. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the positive electrode active material layer further comprises a conductive additive, the weight ratio of the positive electrode active material in the positive electrode active material layer is 94 wt% or more and 96 wt% or less, the weight ratio of the aqueous binder in the positive electrode active material layer is greater than 0 wt% and 4 wt% or less, the weight ratio of the conductive additive in the positive electrode active material layer is 0 wt% or more and 2 wt% or less, and the total mass ratio of the positive electrode active material, the aqueous binder and the conductive additive in the positive electrode active material layer is 100 wt% or less.

4. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the aqueous binder is one or more selected from the group consisting of polymethyl methacrylate, carboxymethylcellulose, styrene-butadiene rubber, polyimide, polytetrafluoroethylene, and polyacrylic acid.

5. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the positive electrode active material comprises a surface treatment film containing carbon.

6. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the carbon content on the surface of the positive electrode active material is 100 ppm or more and 400 ppm or less.

7. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the peeling surface is the surface obtained by pressing an adhesive tape against the first surface and peeling off the surface layer together with the adhesive tape.

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

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