JP7618467B2 - Electrode, lithium ion secondary battery, and method for manufacturing electrode - Google Patents

Description

The present invention relates to an electrode, a lithium ion secondary battery, and a method for manufacturing an electrode.

Lithium-ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptops, as well as hybrid cars.

Electrodes for lithium-ion secondary batteries are produced by drying a coating applied to a current collector, as described in, for example, Patent Document 1. Attempts have been made to speed up the drying process in order to improve production efficiency.

JP 2019-169286 A

The drying conditions of the coating film affect the performance of the electrode. One such performance is the rate characteristic. The rate characteristic is the charge/discharge characteristic when charging/discharging at high speed. However, it is not clear what conditions the coating film must satisfy after drying in order to have good rate characteristics. As a result, it has not been possible to obtain a sufficient number of electrodes with excellent rate characteristics. It has also not been possible to determine the manufacturing conditions for electrodes with excellent rate characteristics.

This disclosure has been made in consideration of the above problems, and aims to provide an electrode, a lithium-ion secondary battery, and a method for manufacturing an electrode that have excellent rate characteristics.

The present inventors have found parameters that affect rate characteristics, and have discovered that by designing these parameters, it is possible to obtain an electrode and a lithium ion secondary battery that are excellent in rate characteristics.
That is, in order to solve the above problems, the following means are provided.

(1) The electrode according to the first aspect comprises a current collector and an active material layer in contact with at least one surface of the current collector, the active material layer containing an active material, a conductive assistant, and a binder, and in a first region of the active material layer within 16.7% of the total thickness of the active material layer in a thickness direction from the surface of the current collector, a first ratio obtained by dividing the abundance ratio of the conductive assistant by the abundance ratio of the binder, which is determined by performing a glow discharge spectroscopy analysis on the active material layer, is 94% or more of the overall average.

(2) In the electrode according to the above aspect, the first ratio in the first region may be 130% or less of the overall average.

(3) In the electrode according to the above aspect, the difference between the first ratio in the first region and the first ratio in a region within 16.7% of the total thickness of the active material layer in the thickness direction from the surface of the active material layer opposite the current collector may be 25% or less.

(4) In the electrode according to the above aspect, the elemental ratio of carbon element to fluorine element in the binder may be 0.5 or more and 2.1 or less.

(5) The lithium ion secondary battery according to the second aspect has an electrode according to the above aspect.

(6) The manufacturing method of the lithium ion secondary battery according to the third aspect includes a conditioning step and an actual drying step, and the conditioning step includes a first step of drying a coating film containing an active material, a conductive assistant, and a binder applied to a current collector under a plurality of conditions in which at least one of the conditions of the conveying speed, the drying temperature, and the drying time is changed, and a second step of analyzing each of the active material layers obtained by drying the coating film in the first step, and determining a first condition in which a first ratio obtained by dividing the abundance ratio of the conductive assistant obtained by performing a glow discharge spectroscopy analysis on the active material layer in a first region within 16.7% of the total thickness of the active material layer in the thickness direction from the surface of the current collector of the active material layer by the abundance ratio of the binder is 94% or more of the overall average, and the actual drying step is performed under the first condition.

The electrode and lithium ion secondary battery according to the above aspect have excellent rate characteristics. Furthermore, the manufacturing method of the electrode according to the above aspect can produce an electrode with excellent rate characteristics.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment; FIG. 2 is a cross-sectional view of a positive electrode according to the first embodiment.

The following describes the embodiments in detail, with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of clarity, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them. Appropriate modifications may be made without departing from the spirit of the invention.

"Lithium-ion secondary battery"
Fig. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. The lithium ion secondary battery 100 shown in Fig. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of connected terminals 60, 62. The non-aqueous electrolyte is contained within the exterior body 50.

(Power generation element)
The power generating element 40 includes a positive electrode 20 , a negative electrode 30 , and a separator 10 .

<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.

[Positive electrode current collector]
The positive electrode collector 22 is, for example, a conductive plate material. The positive electrode collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, etc. The average thickness of the positive electrode collector 22 is, for example, 10 μm or more and 30 μm or less.

[Positive electrode active material layer]
The positive electrode active material layer 24 includes, for example, a positive electrode active material, a conductive assistant, and a binder.

The positive electrode active material can be an electrode active material that can reversibly absorb and release lithium ions, remove and insert lithium ions (intercalation), or dope and dedope lithium ions and counter anions.

The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a compound of the general formula: LiNi _x Co _y Mn _z M _a O ₂ (wherein x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithium vanadium compound (LiV ₂ O ₅ ), an olivine-type LiMPO ₄ (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and a lithium titanate (Li _{4 Ti5O12} ) _{, LiNixCoyAlzO2} (0.9<x+y+ _{z <} 1.1). The positive electrode active material may be an organic material. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene _.

The conductive assistant enhances the electronic conductivity between active materials. Examples of the conductive assistant include carbon powders such as carbon black and Ketjen black, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. The conductive assistant is preferably a carbon material such as carbon black or Ketjen black.

The binder binds the active materials together. Any known binder can be used. The binder is, for example, a fluororesin. Examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc.

In addition to the above, the binder may be, for example, vinylidene fluoride-based fluororubber such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber). The binder may also be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc.

Figure 2 is an enlarged cross-sectional view of a characteristic portion of the positive electrode 20 according to this embodiment. The positive electrode active material layer 24 is on the positive electrode current collector 22. The positive electrode active material layer 24 can be divided into a plurality of regions in the thickness direction. The positive electrode active material layer 24 shown in Figure 2 is divided into six regions in the thickness direction, each of which is 16.7% of the total thickness. These regions are referred to as the first region A1, the second region A2, the third region A3, the fourth region A4, the fifth region A5, and the sixth region A6, in that order from the positive electrode current collector 22.

The first ratio of the first region A1 is 94% or more of the overall average. It is preferable that the first ratio of the first region A1 is 130% or less of the overall average, and more preferably 107% or less.

The first ratio is the ratio of the conductive additive present in a specific region of the positive electrode active material layer 24 divided by the binder present. The first ratio is determined using glow discharge spectroscopy. A specific method for determining the first ratio will be described below.

First, glow discharge spectroscopy is performed on the positive electrode active material layer 24. Glow discharge spectroscopy is a method for measuring the element distribution in the thickness direction of a thin film by irradiating the sample surface with rare gas ions to release atoms from the sample and continuously analyzing the atomic emission lines.

First, the fluorine element is analyzed by glow discharge spectroscopy. The fluorine element is contained only in the binder in the positive electrode active material layer 24. Therefore, the element ratio of the fluorine element can be converted into the proportion of the binder present in the positive electrode active material layer 24. As described above, glow discharge spectroscopy can measure the element distribution in the thickness direction, so the proportion of the binder present in each of the first region A1 to the sixth region A6 can be determined.

Then, the carbon element is analyzed by glow discharge spectroscopy. The carbon element is contained in the binder and the conductive additive in the positive electrode active material layer 24. Therefore, the ratio of the carbon element in the positive electrode active material layer 24 cannot be directly converted into the proportion of the conductive additive in the positive electrode active material layer 24. Therefore, a process is performed on the positive electrode active material layer 24 to remove the carbon element portion caused by the binder from the carbon element ratio.

First, a film of only the binder is formed on the underlayer. The binder is the same as the binder contained in the positive electrode active material layer 24. Glow discharge spectroscopy is performed on the binder. From the results of the glow discharge spectroscopy, the elemental ratio (C/F) of carbon element to fluorine element contained in the binder is obtained. The elemental ratio (C/F) is, for example, 0.5 or more and 2.1 or less.

Next, the elemental ratio of the carbon element caused by the binder in the positive electrode active material layer 24 is calculated by multiplying the elemental ratio of the fluorine element in the positive electrode active material layer 24 previously determined by the elemental ratio of the carbon element to the fluorine element contained in the binder (C/F). Next, the elemental ratio of the carbon element caused by the conductive assistant is determined by subtracting the elemental ratio of the carbon element caused by the binder from the elemental ratio of the carbon element in the positive electrode active material layer 24. The elemental ratio of the carbon element caused by the conductive assistant can be converted into the abundance ratio of the conductive assistant in the positive electrode active material layer 24. Since glow discharge spectroscopy can measure the elemental distribution in the thickness direction as described above, the abundance ratio of the conductive assistant in each of the first region A1 to the sixth region A6 can be determined.

Then, the first ratio is calculated by dividing the calculated proportion of conductive additive by the proportion of binder. The overall average of the first ratio is calculated by averaging the values calculated for each region.

Furthermore, it is preferable that the first ratio of the sixth region A6 is, for example, 119% or less of the overall average. Furthermore, it is preferable that the difference between the first ratio of the first region A1 and the first ratio of the sixth region A6 is 25% or less.

<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 formed on at least one surface of the negative electrode current collector 32.

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material and may further contain a conductive assistant, a binder, and a solid electrolyte, as necessary.

The negative electrode active material may be any compound capable of absorbing and releasing ions, and may be any negative electrode active material used in known lithium ion secondary batteries. Examples of the negative electrode active material include metallic lithium, lithium alloys, graphite (natural graphite, artificial graphite) capable of absorbing and releasing ions, carbon materials such as carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature fired carbon, metals capable of combining with metals such as lithium, such as aluminum, silicon, tin, and germanium, amorphous compounds mainly composed of oxides such as SiO _x (0<x<2) and tin dioxide, and particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

The negative electrode active material layer 34 may contain silicon, tin, and germanium. Silicon, tin, and germanium may exist as single elements or as compounds. The compounds are, for example, alloys, oxides, and the like. As an example, when the negative electrode active material is silicon, the negative electrode 30 may be called a Si negative electrode. The negative electrode active material may be, for example, a mixture of a simple substance or compound of silicon, tin, or germanium and a carbon material. The carbon material is, for example, natural graphite. The negative electrode active material may also be, for example, a simple substance or compound of silicon, tin, or germanium, the surface of which is coated with carbon. The carbon material and the coated carbon increase the conductivity between the negative electrode active material and the conductive additive. When the negative electrode active material layer contains silicon, tin, or germanium, the capacity of the lithium ion secondary battery 100 increases.

The negative electrode active material layer 34 may contain, for example, lithium as described above. The lithium may be metallic lithium or a lithium alloy. The negative electrode active material layer 34 may be metallic lithium or a lithium alloy. The lithium alloy is, for example, an alloy of lithium and one or more elements selected from the group consisting of Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al. As an example, when the negative electrode active material is metallic lithium, the negative electrode 30 may be called a Li negative electrode. The negative electrode active material layer 34 may be a sheet of lithium.

The negative electrode 30 may be made only of the negative electrode current collector 32 without having the negative electrode active material layer 34. When the lithium-ion secondary battery 100 is charged, metallic lithium is precipitated on the surface of the negative electrode current collector 32. Metallic lithium is elemental lithium with lithium ions precipitated therein, and metallic lithium functions as the negative electrode active material layer 34.

The conductive additive and binder can be the same as those in the positive electrode 20. In addition to those listed in the positive electrode 20, the binder in the negative electrode 30 can be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc. The cellulose can be, for example, carboxymethyl cellulose (CMC).

<Separator>
The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 may be, for example, a monolayer or laminate of a film made of a polyolefin such as polyethylene or polypropylene, or a stretched film of a mixture of the above resins, or 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 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.

(Terminal)
The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. The terminals 60 and 62 are preferably protected with insulating tape to prevent short circuits.

(Exterior body)
The power generating element 40 and the non-aqueous electrolyte are sealed inside the exterior body 50. The exterior body 50 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and the like from entering the lithium ion secondary battery 100 from the outside.

As shown in FIG. 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

The metal foil 52 may be, for example, aluminum foil. The resin layer 54 may be a polymer film such as polypropylene. The materials constituting the resin layer 54 may be different on the inside and outside. For example, the material on the outside may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the material for the polymer film on the inside may be polyethylene (PE), polypropylene (PP), etc.

(Non-aqueous electrolyte)
The non-aqueous electrolyte is sealed in the exterior body 50 and impregnates the power generating element 40. The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent.

The non-aqueous solvent contains, for example, a cyclic carbonate and a chain carbonate. The cyclic carbonate solvates the electrolyte. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, and butylene carbonate. It is preferable that the cyclic carbonate contains at least propylene carbonate. The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent may also contain methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like.

It is preferable that the ratio of cyclic carbonate to chain carbonate in the non-aqueous solvent is 1:9 to 1:1 by volume.

The electrolyte is, for example, a lithium salt. The electrolyte is, for example, _LiPF6 , LiClO4, _LiBF4 , _LiCF3SO3 , _LiCF3CF2SO3 , LiC _{( CF3SO2} ) _{3 , LiN( CF3SO2 ) 2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2 ) , LiN ( CF3CF2CO)2 , LiBOB , etc. The lithium salt} may be used _alone or in combination of two or more. From the viewpoint of the degree of ionization, _it is preferable that the electrolyte contains _LiPF6 .

"Method of manufacturing lithium-ion secondary batteries"
First, the positive electrode 20 is produced. When producing the positive electrode 20, a conditioning step is first performed to define the manufacturing conditions of the positive electrode 20. The actual drying step is performed based on the conditions (first conditions) obtained in the conditioning step.

The conditioning process consists of a first step and a second step. The first step is to dry the coating film under various conditions. The second step is to analyze the dried coating film and extract the appropriate conditions.

The first step is carried out as follows. First, a paste-like positive electrode slurry (coating) is applied to at least one surface of the positive electrode collector 22. A commercially available positive electrode collector 22 can be used. There are no particular limitations on the application method. Examples include a slit die coating method and a doctor blade method. The positive electrode slurry is a paste made by mixing a positive electrode active material, a conductive assistant, a binder, and a solvent.

Then, the positive electrode slurry is dried to remove the solvent. The positive electrode slurry is dried, for example, by passing it through a drying furnace. Examples of drying conditions include the conveying speed of the positive electrode current collector 22 coated with the positive electrode slurry, the drying temperature, the drying time, and the gas speed relative to the positive electrode slurry. The drying temperature and drying time each have conditions for the coating film preheating period, the coating film temperature holding period, and the coating film temperature increase period. That is, there are parameters for the drying temperature and drying time during the coating film preheating period, the drying temperature and drying time during the coating film temperature holding period, and the drying temperature and drying time during the coating film temperature increase period. In the first step, the coating film is dried under multiple conditions in which at least one of the conveying speed, drying temperature, and drying time is changed.

In the second step, glow discharge spectroscopy is performed on the positive electrode active material layers 24 produced under each of the multiple conditions. By performing the glow discharge spectroscopy, the first ratio of each positive electrode active material layer 24 is determined. Then, the condition (first condition) under which the first ratio of the first region A1 is 94% or more of the overall average is extracted.

The first condition tends to be met when the temperature during the coating preheating period and coating temperature holding period is 90°C or less. This is thought to be because a high drying temperature increases the rate of solvent removal on the surface of the positive electrode slurry, making it easier for the binder to segregate on the surface of the positive electrode slurry. Segregation of the binder inhibits convection within the positive electrode slurry. Furthermore, when the coating temperature holding period at a temperature of 90°C or less is 40 seconds or more, the first condition tends to be met. Furthermore, when the conveying speed is 9.5 m/min or less, the first condition tends to be met.

The positive electrode 20 is produced by carrying out an actual drying process based on the results of the above-mentioned condition setting process. The actual drying process is carried out under the first condition determined in the condition setting process. The dried coating film may then be pressed to densify the positive electrode active material layer 24. For example, a roll press, a hydrostatic press, or the like can be used as the pressing means.

Next, the negative electrode 30 is prepared. The negative electrode 30 can be prepared in the same manner as the positive electrode 20. A paste-like negative electrode slurry is applied to at least one surface of the negative electrode current collector 32. The negative electrode slurry is a mixture of a negative electrode active material, a binder, a conductive additive, and a solvent, which is then made into a paste. The negative electrode slurry is applied to the negative electrode current collector 32 and dried to obtain the negative electrode 30.

Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. If the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

Finally, the power generating element 40 is enclosed in the exterior body 50. The non-aqueous electrolyte is injected into the exterior body 50. After the non-aqueous electrolyte is injected, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte is impregnated into the power generating element 40. The exterior body 50 is sealed by applying heat, etc., to obtain the lithium ion secondary battery 100.

As shown in the examples described below, the lithium ion secondary battery according to the first embodiment has improved rate characteristics because the first ratio in the first region A1 satisfies the above-mentioned conditions.

The reason for this is unclear, but it is believed that the first region A1 is closest to the positive electrode current collector 22 and thus has a large effect on the rate characteristics. The rate characteristics are the capacity retention rate when rapidly discharged, based on the 0.2C discharge capacity after 0.2C CCCV charging. The higher the rate characteristics, the more the battery can perform during rapid discharge. The lithium ion conductivity and electronic conductivity in the active material layer have a large effect on the rate characteristics.

The fact that the first ratio in the first region A1 is 94% or more of the overall average means that there is a sufficient conductive assistant in the first region A1. If the amount of conductive assistant in the first region A1 is sufficient, the electronic conductivity in the positive electrode active material layer 24 can be sufficiently ensured. In addition, the conductive assistant has high liquid retention. Therefore, if the amount of conductive assistant in the first region A1 is sufficient, there is sufficient electrolyte in the first region A1. The electrolyte is responsible for the conduction of lithium ions between the positive electrode active materials. Therefore, the lithium ion conductivity in the positive electrode active material layer 24 can also be ensured. In other words, if the first ratio in the first region A1 is 94% or more of the overall average, it is considered that the lithium ion conductivity and electronic conductivity in the first region A1 are sufficiently high, and the rate characteristics are improved.

In addition, if the difference between the first ratio in the first region A1 and the first ratio in the sixth region A6 is 25% or less, the variation in lithium ion conductivity and electronic conductivity in the positive electrode active material layer 24 will be reduced, and the rate characteristics will be further improved.

The above describes the embodiments of the present invention in detail with reference to the drawings, but each configuration and their combinations in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible without departing from the spirit of the present invention.

For example, although an example has been described in which the first ratio in the positive electrode active material layer satisfies a predetermined condition, the first ratio in the negative electrode active material layer may also satisfy a predetermined condition.

"Example 1"
First, a positive electrode was prepared by coating one surface of an aluminum foil having a thickness of 15 μm with a positive electrode slurry, which was prepared by mixing a positive electrode active material, a conductive assistant, a binder, and a solvent.

The positive electrode active material used was NCM811 manufactured by ECOPRO. NCM811 is a nickel-cobalt-manganese compound with nickel:cobalt:manganese=8:1:1. Carbon black (Super-P) and Ketjen Black (ECP600JD) were used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. The mass ratio of NCM811 (positive electrode active material), carbon black (conductive additive), Ketjen Black (conductive additive), and PVDF (binder) was 96:1:1:2. The amount of the positive electrode active material carried in the positive electrode active material layer after drying was 17.4 mg/ ^cm2 .

The positive electrode slurry was then dried. The conveying speed during drying was 5.0 m/min. The coating preheating period was 104 seconds, and the temperature was raised to 90°C. The coating temperature was then maintained at 90°C for 146 seconds. Thereafter, the coating temperature was raised to 120°C for 182 seconds. The positive electrode slurry was dried under the above conditions to produce a positive electrode.

A glow discharge spectroscopy analysis was then performed on a positive electrode prepared under the same conditions as the positive electrode. In the glow discharge spectroscopy analysis, a first ratio was calculated by dividing the proportion of conductive additive by the proportion of binder. The first ratio was calculated for each thickness width of 16.7% (approximately 10 μm in this example) of the total thickness of the positive electrode active material layer from the surface of the positive electrode current collector, and the overall average of these was also calculated.

Next, a negative electrode was prepared. First, a negative electrode slurry was applied to one side of a copper foil having a thickness of 10 μm. The negative electrode slurry was prepared by mixing a negative electrode active material, a conductive assistant, a binder, and a solvent. Silicon was used as the negative electrode active material. Carbon black (Super-P) and Ketjen Black (ECP600JD) were used as the conductive assistant. Polyvinylidene fluoride (PVDF) was used as the binder. The mass ratio of silicon (negative electrode active material), carbon black (conductive assistant), Ketjen Black (conductive assistant), and PVDF (binder) was 96:1:1:2. The amount of the negative electrode active material carried in the negative electrode active material layer after drying was 1.3 mg/ ^cm2 .

(Preparation of evaluation lithium-ion secondary battery (full cell))
The negative and positive electrodes thus prepared were alternately laminated with 10 μm thick polypropylene separators in between, and six negative electrodes and five positive electrodes were laminated to produce a laminate. Furthermore, in the negative electrode of the laminate, a nickel negative electrode lead was attached to the protruding end of the copper foil on which the negative electrode active material layer was not provided. In the positive electrode of the laminate, an aluminum positive electrode lead was attached to the protruding end of the aluminum foil on which the positive electrode active material layer was not provided, by an ultrasonic welding machine.

The laminate was inserted into the exterior of the laminate film and heat-sealed except for one peripheral portion to form a closed portion. A non-aqueous electrolyte was injected into the exterior. The non-aqueous electrolyte was a solvent of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 1:9, to which 1.0 M (mol/L) LiPF ₆ was added as a lithium salt. The remaining portion was then sealed by heat sealing while reducing the pressure with a vacuum sealer, to produce a lithium ion secondary battery (full cell).

The rate characteristics were measured using a secondary battery charge/discharge test device. The rate characteristics were evaluated in terms of the 5C discharge capacity retention rate (%), with a voltage range of 4.2V to 3.0V, 1C = 1000mAh per full cell design capacity. The 5C discharge capacity retention rate is the ratio of the discharge capacity when CCCV charging (constant current constant voltage charging, end 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, end current value 0.05C) at a current value of 0.2C and discharging at a current value of 0.2C, and is expressed by the following formula (1).

(5C capacity retention rate (%)) = (0.2CCCV charge/5C discharge capacity)/(0.2CCCV charge/0.2C discharge capacity) x 100...(1)

The cycle characteristics were evaluated in an environment of 45°C using a secondary battery charge/discharge tester (manufactured by Hokuto Denko Corporation). The cycle characteristics were evaluated by repeating a charge/discharge cycle of constant current/constant voltage charging at 0.5C to 4.2V and constant current discharging at 1C to 3.0V for 200 cycles. The capacity retention rate is the discharge capacity at the 200th cycle when the discharge capacity at the initial (first) cycle is taken as 100%.

"Examples 2 to 5, Comparative Examples 1 and 2"
This example differs from Example 1 in that the drying conditions for the positive electrode slurry were changed. The drying conditions were changed in any one of the following: conveying speed, drying temperature, and drying time. Details of the manufacturing conditions for Examples 1 to 5 and Comparative Examples 1 and 2 are summarized in Table 1. Drying conditions 1z to 6z in Table 1 are obtained by dividing the entire distance of the drying oven in which the coating film preheating period, coating film temperature holding period, and coating film temperature increase period are performed into six zones (1z to 6z) by six. The drying temperature in Table 1 is the temperature of the coating film at the center of each zone.

In Examples 2 to 5 and Comparative Examples 1 and 2, the rate characteristics and the first ratio were determined in the same manner as in Example 1.

The conditions and measurement results for Examples 1 to 5 and Comparative Examples 1 and 2 are summarized in Table 1.

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 generating element 50 Exterior body 52 Metal foil 54 Resin layers 60, 62 Terminal 100 Lithium ion secondary battery A1 First region A2 Second region A3 Third region A4 Fourth region A5 Fifth region A6 Sixth region

Claims (5)

A battery comprising a current collector and an active material layer in contact with at least one surface of the current collector,
The active material layer includes an active material, a conductive assistant, and a binder,
a first ratio, which is obtained by dividing the abundance ratio of the conductive assistant, which is determined by performing glow discharge spectroscopy on the active material layer, by the abundance ratio of the binder in a first region that is within 16.7% of the total thickness of the active material layer from the surface on the current collector side in a thickness direction of the active material layer, is 94% or more of the overall average,
the first ratio in a sixth region within 16.7% of the total thickness of the active material layer in a thickness direction from a surface of the active material layer opposite to the current collector is greater than the first ratio in the first region;
An electrode , wherein a difference between a ratio of the first ratio in the sixth region to the overall average and a ratio of the first ratio in the first region to the overall average is greater than or equal to 20% and less than or equal to 25% . The electrode of claim 1, wherein the first ratio in the first region is 130% or less of the overall average. 3. The electrode according to claim 1 , wherein the atomic ratio of carbon to fluorine in the binder is 0.5 or more and 2.1 or less. A lithium ion secondary battery comprising the electrode according to any one of claims 1 to 3 . The method includes a conditioning step and an actual drying step,
The conditioning step includes:
a first step of drying a coating film containing an active material, a conductive assistant, and a binder, which is applied onto a current collector, under a plurality of conditions in which at least one of a conveying speed, a drying temperature, and a drying time is changed;
a second step of analyzing each of the active material layers obtained by drying the coating film in the first step, and determining a first condition in which a first ratio, obtained by dividing the abundance ratio of the conductive assistant, obtained by performing glow discharge spectroscopy on the active material layer in a first region within 16.7% of the total thickness of the active material layer in a thickness direction from the surface of the current collector of the active material layer, by the abundance ratio of the binder, is 94% or more of an overall average,
The method for producing an electrode, wherein the actual drying step is performed under the first condition.

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