JP2019091525A - Method for manufacturing lithium ion secondary battery - Google Patents

Abstract

To provide a method for manufacturing a lithium ion secondary battery superior in charge and discharge cycle characteristics.SOLUTION: A method for manufacturing a lithium ion secondary battery comprises the steps of: immersing at least one electrode of a positive electrode including a positive electrode active substance and a negative electrode including a negative electrode active substance in an electrolyte including 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide and a lithium salt; subjecting a thermal treatment on the electrode after the immersion; and putting, in a battery container, the positive electrode, the negative electrode and the electrolyte including 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide and the lithium salt after the thermal treatment of the electrode.SELECTED DRAWING: None

Description

  The present invention relates to a method of manufacturing a lithium ion secondary battery.

A lithium ion battery (lithium ion secondary battery) is a lightweight, high energy density secondary battery, and is used as a power source for portable devices such as notebook computers and mobile phones by utilizing its characteristics.
In recent years, lithium ion secondary batteries have been developed not only for portable applications such as portable devices, but also for large-scale storage systems for natural energy such as in-vehicle mounted applications and solar power generation and wind power generation.

As an electrolyte of a lithium ion secondary battery, an electrolytic solution in which a lithium salt is dissolved in an organic solvent is generally used. In addition, since an organic solvent is easily volatilized, a non-volatile ionic liquid may be used instead of the organic solvent, and an electrolyte in which a lithium salt is dissolved in an ionic liquid may be used for a lithium ion secondary battery.
Here, the ionic liquid means a salt which is composed of a cation and an anion and which is in a liquid state at a relatively low temperature, for example, about 25.degree.

  However, a lithium ion secondary battery using an ionic liquid as an electrolyte has a problem that sufficient charge and discharge cycle characteristics can not be obtained. Therefore, in a non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte containing a normal temperature molten salt, after injecting the non-aqueous electrolyte, before the first charge A non-aqueous electrolyte secondary battery has been proposed in which the potential of the negative electrode is swept to below the potential at which the cation of the normal temperature molten salt is reduced and reduced by potential scanning (see, for example, Patent Document 1). A stable film can be formed on the negative electrode surface by reducing and decomposing the cation of the room temperature molten salt (ionic liquid), whereby the decomposition of the cation of the room temperature molten salt by subsequent charging is suppressed, and charge and discharge cycles are performed. It becomes a non-aqueous electrolyte secondary battery (lithium ion secondary battery) excellent in characteristics.

JP, 2009-230899, A

  Here, in addition to the method described in Patent Document 1, a method of manufacturing a lithium ion secondary battery capable of enhancing charge / discharge cycle characteristics of the lithium ion secondary battery is required.

  An embodiment of the present disclosure is made in view of the above-described conventional circumstances, and an object of the present disclosure is to provide a method of manufacturing a lithium ion secondary battery having excellent charge and discharge cycle characteristics.

The specific means for achieving the said subject are as follows.
<1> A step of immersing at least one electrode of a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material in an electrolyte containing 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide and a lithium salt; Heat treating the electrode after the heat treatment, and heat treating the electrode, the positive electrode, the negative electrode, and an electrolyte containing 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide and a lithium salt in a battery container And a step of containing the lithium ion secondary battery.
<2> A positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a step of housing an electrolyte containing 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide and a lithium salt in a battery container; Heat-treating the battery container containing the negative electrode and the electrolyte, and a method of manufacturing a lithium ion secondary battery.
The manufacturing method of the lithium ion secondary battery as described in <1> whose lithium salt density | concentration of the said electrolyte used in <3> said immersing process is 0.5 mol / kg-2.5 mol / kg.
<4> The lithium ion secondary battery according to any one of <1> to <3>, wherein the lithium salt concentration of the electrolyte contained in the battery container is 0.5 mol / kg to 2.5 mol / kg. Manufacturing method.
The manufacturing method of the lithium ion secondary battery as described in any one of <1>-<4> in which the <5> said negative electrode active material contains graphite.
<6> The method for producing a lithium ion secondary battery according to any one of <1> to <5>, wherein the positive electrode active material contains NMC (layered lithium-nickel-manganese-cobalt composite oxide).

  According to one aspect of the present disclosure, it is possible to provide a method of manufacturing a lithium ion secondary battery having excellent charge and discharge cycle characteristics.

Hereinafter, modes for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, the constituent elements (including element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and ranges thereof, and does not limit the present invention. In addition, various changes and modifications can be made by those skilled in the art within the scope of the technical idea in the present disclosure.
In the present disclosure, the term “step” includes, in addition to steps independent of other steps, such steps as long as the purpose of the step is achieved even if it can not be clearly distinguished from other steps. .
In the numerical value range indicated by using “to” in the present disclosure, the numerical values described before and after “to” are included as the minimum value and the maximum value, respectively.
The upper limit value or the lower limit value described in one numerical value range may be replaced with the upper limit value or the lower limit value of the other stepwise description numerical value range in the numerical value range described stepwise in the present disclosure. . In addition, in the numerical range described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the example.
In the present disclosure, each component may contain a plurality of corresponding substances. When there are a plurality of substances corresponding to each component, the content of each component means the total content of the plurality of substances unless otherwise specified.
In the present disclosure, particles corresponding to each component may contain a plurality of types. When there are a plurality of particles corresponding to each component, the particle diameter of each component means the value for a mixture of the plurality of particles unless otherwise specified.
In the present disclosure, the words “layer” or “film” mean that when the region in which the layer or film is present is observed, in addition to the case where the region is entirely formed, only a part of the region The case where it is formed is also included.
In the present disclosure, “solid content” of the positive electrode mixture or the negative electrode mixture means the remaining component of the slurry of the positive electrode mixture or the slurry of the negative electrode mixture excluding volatile components such as an organic solvent.

<Method of manufacturing lithium ion secondary battery>
First Embodiment
In a method of manufacturing a lithium ion secondary battery according to a first embodiment of the present disclosure, at least one electrode of a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material is 1-ethyl-3-methylimidazolium bis (fluoro A step of immersing in an electrolyte containing a sulfonyl) imide and a lithium salt, a step of heat treating the electrode after immersing, and a heat treatment of the electrode, the positive electrode, the negative electrode, and 1-ethyl-3-methylimidazolium Housing an electrolyte containing bis (fluorosulfonyl) imide and a lithium salt in a battery container.
In the manufacturing method of the present disclosure, the electrode is immersed in an electrolyte containing 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide (hereinafter also referred to as “EMI-FSI”) and a lithium salt, and heat-treated. And a film can be formed on the surface of the electrode. As a result, decomposition of components in the electrolyte due to subsequent charge and discharge, deterioration of the active material, and the like are suppressed, and a lithium ion secondary battery having excellent charge and discharge cycle characteristics can be obtained.
In addition, according to the manufacturing method of the present disclosure, a lithium ion secondary battery excellent in charge and discharge cycle characteristics can be obtained by a simple method without performing a battery reaction.
Note that an electrolyte containing EMI-FSI and a lithium salt (hereinafter, also simply referred to as "electrolyte") may be prepared by dissolving a lithium salt in EMI-FSI.

(Immersion process)
The manufacturing method of the present disclosure includes a step (immersion step) of immersing at least one of a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material in an electrolyte including EMI-FSI and a lithium salt. In this step, for example, an electrolyte prepared by dissolving lithium salt lithium bis (fluorosulfonyl) imide, which is a lithium salt, at a concentration of 1.2 mol / kg is filled in a container, and at least one of the positive electrode and the negative electrode is filled. The electrode may be immersed in the electrolyte in the container.
The concentration of the lithium salt of the electrolyte used in the immersion step is preferably 0.5 mol / kg to 2.5 mol / kg, and more preferably 0.8 mol / kg to 2.0 mol / kg, More preferably, it is 1.0 mol / kg to 1.8 mol / kg.
Moreover, as a specific example of a lithium salt, it is the same as that of the specific example of the lithium salt contained in the electrolyte accommodated in the battery container mentioned later.

Here, it is preferable to immerse the electrode in the electrolyte under the condition that the electrode and the electrolyte are not in contact with air, moisture, etc. For example, it is preferable to immerse the electrode in the inert gas atmosphere.
When immersing the electrode in the electrolyte, it is preferable to immerse the electrode in the liquid electrolyte, for example, at 25 ° C., from the viewpoint of suitably forming a film on the electrode surface in the electrode heat treatment step described later. The electrode may be immersed.

  In the manufacturing method of the present disclosure, at least one of the positive electrode and the negative electrode may be immersed in the electrolyte, and heat treatment may be performed on the electrode immersed in the electrolyte in an electrode heat treatment step described later. That is, in the manufacturing method of the present disclosure, both electrodes of the positive electrode and the negative electrode may be immersed in the electrolyte, and heat treatment may be performed on both the electrodes immersed in the electrolyte. And the other electrode immersed in the electrolyte may be heat-treated.

(Electrode heat treatment process)
The manufacturing method of the present disclosure includes a step of heat-treating the electrode after the immersion (electrode heat treatment step). The conditions for heat treatment of the electrode may be such that the components in the electrolyte are thermally decomposed to form a film on the electrode surface.

For example, the temperature of the heat treatment may be 35 ° C. to 100 ° C., 40 ° C. to 80 ° C., or 45 ° C. to 60 ° C. When the temperature of the heat treatment is 35 ° C. or more, the film tends to be easily formed by thermal decomposition of the components in the electrolyte, and when the temperature of the heat treatment is 100 ° C. or less, the excessive thermal decomposition reaction can be suppressed. It is in.
The heat treatment temperature refers to the ambient temperature when the electrode is heat treated.

  The heat treatment time may be 2 hours to 24 hours, 3 hours to 18 hours, 5 hours to 12 hours, or 6 hours to 8 hours. Good. When the heat treatment time is 2 hours or more, the coating tends to be easily formed by thermal decomposition of the components in the electrolyte, and the heat treatment time is 24 hours or less, the production efficiency of the lithium ion secondary battery and It tends to be excellent in discharge load characteristics.

  In addition, as the atmosphere of the heat treatment, it is preferable that air, moisture, and the like do not enter during the heat treatment, and for example, a reduced pressure atmosphere or an inert gas atmosphere is preferable.

(Retention process after heat treatment)
The manufacturing method of the present disclosure includes the step of housing the positive electrode, the negative electrode, and the electrolyte containing the EMI-FSI and the lithium salt in the battery case (heat treatment after heat treatment) after heat treatment of the electrode. In this step, each component of the lithium ion secondary battery is accommodated in the battery container. In addition, the separator may be housed in the battery case such that a separator that insulates between the positive electrode and the negative electrode is disposed between the positive electrode and the negative electrode.

  For example, after heat treatment of the electrode, the electrolyte containing the EMI-FSI and the lithium salt is supplied into the battery case with the positive electrode, the negative electrode, and, if necessary, the separator disposed between the positive electrode and the negative electrode. You may

  Preferred configurations of the positive electrode, the negative electrode, and the electrolyte used in the production method of the present disclosure will be described below.

(Positive electrode)
It will not specifically limit, if it is a positive electrode applicable to a lithium ion secondary battery as a positive electrode containing the positive electrode active material used by the manufacturing method of this indication. For example, the positive electrode may be configured to have a positive electrode current collector and a positive electrode mixture layer disposed on the surface thereof and including a positive electrode active material.

The positive electrode active material preferably contains a layered lithium-nickel-manganese-cobalt composite oxide (hereinafter also referred to as “NMC”). NMC tends to have high capacity and also be excellent in safety.
The content of NMC is preferably 65% by mass or more, more preferably 70% by mass or more, and more preferably 80% by mass or more based on the total amount of the positive electrode mixture layer, from the viewpoint of increasing the capacity of the battery. Is more preferred.

As NMC, it is preferable to use one represented by the following composition formula (Formula 1).
Li _{(1 + δ)} Mn _x Ni _y Co _(1-x-y-z) M _z O ₂ (Formula 1)
In the composition formula (Formula 1), (1 + δ) is the composition ratio of Li (lithium), x is the composition ratio of Mn (manganese), y is the composition ratio of Ni (nickel), (1-x-y- z) each shows the composition ratio of Co (cobalt). z represents the compositional ratio of the element M. The composition ratio of O (oxygen) is 2.
The element M is Ti (titanium), Zr (zirconium), Nb (niobium), Mo (molybdenum), W (tungsten), Al (aluminum), Si (silicon), Ga (gallium), Ge (germanium) and Sn It is preferably at least one element selected from the group consisting of (tin).
Also, −0.15 <δ <0.15, 0.1 <x ≦ 0.5, 0.6 <x + y + z <1.0, 0 ≦ z ≦ 0.1.

Further, as the positive electrode active material, materials other than NMC may be used.
As a positive electrode active material other than NMC, any one commonly used in this field can be used, and spinel type lithium manganese composite oxide (sp-Mn), lithium containing composite metal oxide other than NMC and sp-Mn, olivine type lithium And salts, chalcogen compounds, manganese dioxide and the like.
The positive electrode active material may be used alone or in combination of two or more.

Next, the positive electrode mixture layer and the positive electrode current collector will be described in detail. The positive electrode mixture layer contains a positive electrode active material, a binder and the like, and is disposed on the positive electrode current collector. There is no restriction | limiting in the formation method of a positive mix layer, For example, it forms as follows. By mixing the other materials such as the positive electrode active material, the binder and the conductive agent used as needed, a thickener, etc. in a dry state to form a sheet, and pressing it on the positive electrode current collector (dry method) A positive electrode mixture layer can be formed. In addition, other materials such as a positive electrode active material, a binder and a conductive agent used as needed, a thickener and the like are dissolved or dispersed in a dispersion solvent to form a slurry of a positive electrode mixture, which is used as a positive electrode current collector. The positive electrode mixture layer can be formed by applying and drying (wet method).
As the positive electrode active material, as described above, layered lithium-nickel-manganese-cobalt composite oxide (NMC) is preferably used. The positive electrode active material is used in powder form (granular form) and mixed.
As particles of a positive electrode active material such as NMC, particles having a shape such as bulk, polyhedron, sphere, oval sphere, plate, needle, column and the like can be used.
The average particle size (d50) of the particles of the positive electrode active material such as NMC (when the primary particles are aggregated to form secondary particles, the average particle size (d50) of the secondary particles) is tap density (filling 1) from the viewpoint of mixing with other materials in forming the electrode, it is preferably 1 μm to 30 μm, more preferably 3 μm to 25 μm, and still more preferably 5 μm to 15 μm. The average particle diameter (d50) of the particles of the positive electrode active material is determined, for example, by measuring the particle size distribution based on volume using a particle size distribution measuring apparatus (SALD-3000, Shimadzu Corporation) using a laser light scattering method, It is a volume average particle diameter calculated as d50 (median diameter).

The positive electrode active specific surface area determined from nitrogen adsorption measurements at 77K particulate substances NMC like (hereinafter, simply referred to as "specific surface area".) Range ^is a 0.2m ² /g~4.0m 2 / g preferably ^there, more preferably 0.3m ² /g~2.5m 2 / ^g, still more preferably 0.4m ² /g~1.5m 2 / g.
If the specific surface area of the particles of the positive electrode active material is 0.2 m ² / g or more, excellent battery performance tends to be obtained. In addition, when the specific surface area of the particles of the positive electrode active material is 4.0 m ² / g or less, the tap density tends to increase, and the mixing property with other materials such as a binder and a conductive agent tends to be favorable. . The specific surface area can be measured from the nitrogen adsorption capacity according to JIS Z 8830: 2013. When measuring the specific surface area, it is preferable to perform pretreatment for removing water by heating first, since it is considered that the water adsorbed in the sample surface and structure affects the gas adsorption capacity.
In the pretreatment, the measurement cell into which 0.05 g of the measurement sample was charged was depressurized to 10 Pa or less with a vacuum pump and then heated at 110 ° C. and held for 3 hours or more, and then kept at room temperature while maintaining the depressurized state ( Allow to cool naturally to 25 ° C. After this pretreatment, the evaluation temperature is set to 77 K, and the evaluation pressure range is measured as a relative pressure (equilibrium pressure to saturated vapor pressure) less than 1. Nitrogen adsorption is measured by the multipoint method, and the specific surface area is calculated by the BET method.

Examples of the conductive agent for the positive electrode include metal materials such as copper and nickel; graphite (graphite) such as natural graphite and artificial graphite; carbon blacks such as acetylene black; carbon materials such as amorphous carbon such as needle coke . In addition, the electrically conductive agent for positive electrodes may be used individually by 1 type, and may be used in combination of 2 or more type.
The content of the conductive agent with respect to the mass of the positive electrode mixture layer is preferably 0.01 mass% to 50 mass%, more preferably 0.1 mass% to 30 mass%, and 1 mass% to 15 mass%. More preferably, it is mass%. If the content of the conductive agent is 0.01% by mass or more, sufficient conductivity tends to be easily obtained. If the content of the conductive agent is 50% by mass or less, a decrease in battery capacity tends to be suppressed.

The binder for the positive electrode is not particularly limited, and when forming the positive electrode mixture layer by a wet method, a material having good solubility or dispersibility in the dispersion solvent is selected. Specifically, resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polyimide and cellulose; rubber-like polymers such as SBR (styrene-butadiene rubber) and NBR (acrylonitrile-butadiene rubber), polyvinylidene fluoride (PVdF) , Fluorine-based polymers such as polytetrafluoroethylene, polytetrafluoroethylene-vinylidene fluoride copolymer, fluorinated polyvinylidene fluoride, etc .; polymer compositions having ion conductivity of alkali metal ions (particularly lithium ions), etc. It can be mentioned. In addition, the binder for positive electrodes may be used individually by 1 type, and may be used in combination of 2 or more type.
From the viewpoint of the stability of the positive electrode, it is preferable to use a fluorine-based polymer such as polyvinylidene fluoride (PVdF) or polytetrafluoroethylene-vinylidene fluoride copolymer as the binder.
The content of the binder relative to the mass of the positive electrode mixture layer is preferably 0.1% by mass to 60% by mass, more preferably 1% by mass to 40% by mass, and 3% by mass to 10% by mass. More preferably, it is%.
When the binder content is 0.1% by mass or more, the positive electrode active material can be sufficiently bound, sufficient mechanical strength of the positive electrode mixture layer can be obtained, and battery performance such as cycle characteristics is improved. There is a tendency. When the content of the binder is 60% by mass or less, sufficient battery capacity and conductivity tend to be obtained.

In order to improve the packing density of the positive electrode active material, the positive electrode mixture layer formed on the positive electrode current collector using a wet method or a dry method is preferably consolidated by a hand press or a roller press.
The density of compacted positive electrode mixture layer is preferably from the viewpoint of further improvement of the input-output characteristics and ^safety, in the range of ^{2.0g / cm 3 ~3.5g / cm 3} , 2.3g / cm more preferably ³ in a range of ~3.2g / cm ^3, more preferably in the range of ^{2.5g / cm 3 ~3.0g / cm 3} .
In addition, the amount of single-sided application of the positive electrode mixture slurry to the positive electrode current collector when forming the positive electrode mixture layer is 30 g / m ² as the solid content of the positive electrode mixture from the viewpoint of energy density and input / output characteristics. is preferably ~170g / ^{m 2,} more preferably from ^{40g / m 2 ~160g / m 2} , further preferably ^{40g / m 2 ~150g / m 2} .

There is no restriction | limiting in particular as a material of a positive electrode collector, Especially, a metal material is preferable and aluminum is more preferable. The shape of the positive electrode current collector is not particularly limited, and materials processed into various shapes can be used. The metal material may, for example, be a metal foil, a metal plate, a metal thin film, an expanded metal, etc. Among them, it is preferable to use a metal thin film. The thin film may be formed in a mesh shape as appropriate.
The average thickness of the positive electrode current collector is not particularly limited, and is preferably 1 μm to 1 mm, and 3 μm to 100 μm from the viewpoint of obtaining the strength and the flexibility required for the positive electrode current collector. Is more preferable, and 5 μm to 100 μm is more preferable.

(Negative electrode)
The negative electrode containing the negative electrode active material used in the manufacturing method of the present disclosure is not particularly limited as long as it is a negative electrode applicable to a lithium ion secondary battery. For example, the negative electrode may be configured to have a negative electrode current collector and a negative electrode mixture layer disposed on the surface thereof and including a negative electrode active material.

As the negative electrode active material, carbon materials such as graphite, low crystalline carbon, mesophase carbon and the like can be mentioned, and graphite is preferable because charge and discharge capacity can be easily increased. Examples of the graphite include artificial graphite, natural graphite, graphitized mesophase carbon, graphitized carbon fiber and the like, and examples of the shape of the graphite include scaly, spherical, massive and the like.
One type of negative electrode active material may be used alone, or two or more types may be used in combination.

The average particle size of the carbon material, preferably graphite, is preferably 2 μm to 30 μm, more preferably 2.5 μm to 25 μm, still more preferably 3 μm to 20 μm, and preferably 5 μm to 20 μm. Particularly preferred. When the average particle size is 30 μm or less, the discharge capacity and the discharge characteristics tend to be improved. When the average particle size is 2 μm or more, the initial charge and discharge efficiency tends to be improved.
The average particle size (d50) can be measured in the same manner as in the case of the positive electrode active material described above.

Carbon material, preferably in the range of the specific surface area of the graphite is ^preferably 0.5m ² / g~10m 2 / ^g, more preferably ^{0.8m 2 / g~8m 2 / g,} 1m 2 / G～7m further preferably from ² / ^g, particularly preferably 1.5m ² / g~6m 2 / g.
When the specific surface area is 0.5 m ² / g or more, excellent battery performance tends to be obtained. When the specific surface area is 10 m ² / g or less, the tap density tends to increase, and the mixing property with other materials such as a binder and a conductive agent tends to be favorable.
The specific surface area can be measured in the same manner as in the case of the positive electrode active material described above.

  The content of the carbon material, preferably graphite, is preferably 80% by mass or more, more preferably 85% by mass or more, based on the total amount of the negative electrode mixture layer, from the viewpoint of increasing the capacity of the battery. It is further preferable that the content is at least% by mass.

  Next, the negative electrode mixture layer and the negative electrode current collector will be described in detail. The negative electrode mixture layer contains a negative electrode active material, a binder and the like, and is disposed on the negative electrode current collector. There is no restriction | limiting in the formation method of a negative mix layer, For example, it forms as follows. A negative electrode active material, a binder, and other materials such as a conductive agent used as needed, and a thickener are dissolved or dispersed in a dispersion solvent to form a slurry of a negative electrode mixture, which is applied to a negative electrode current collector. The negative electrode mixture layer can be formed by drying (wet method).

  As the conductive agent for the negative electrode, carbon black such as acetylene black and amorphous carbon such as needle coke can be used. As the conductive agent for the negative electrode, one type may be used alone, or two or more types may be used in combination. Thus, by adding the conductive agent to the negative electrode mixture, an effect of reducing the resistance of the electrode or the like tends to be obtained.

  The content of the conductive agent with respect to the mass of the negative electrode mixture layer is preferably 1% by mass to 45% by mass, and is 2% by mass to 42% by mass from the viewpoint of improving the conductivity and reducing the initial irreversible capacity. It is more preferable that it is 3 mass%-40 mass%. If the content of the conductive agent is 1% by mass or more, sufficient conductivity tends to be easily obtained. If the content of the conductive agent is 45% by mass or less, a decrease in battery capacity tends to be suppressed.

  The binder for the negative electrode is not particularly limited as long as it is a material stable to the non-aqueous electrolytic solution or the dispersion solvent used in forming the electrode. Specifically, resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, cellulose and nitrocellulose; rubber-like polymers such as SBR (styrene-butadiene rubber) and NBR (acrylonitrile-butadiene rubber); polyvinylidene fluoride (PVdF) And fluorine-containing polymers such as polytetrafluoroethylene and fluorinated polyvinylidene fluoride; and polymer compositions having ion conductivity of alkali metal ions (particularly lithium ions). In addition, the binder for negative electrodes may be used individually by 1 type, and may be used in combination of 2 or more type. Among these, it is preferable to use SBR, a fluorine-based polymer represented by polyvinylidene fluoride, and the like.

The content of the binder relative to the mass of the negative electrode mixture layer is preferably 0.1% by mass to 20% by mass, more preferably 0.5% by mass to 15% by mass, and 0.6% by mass. More preferably, it is% to 10% by mass.
When the content of the binder is 0.1% by mass or more, the negative electrode active material can be sufficiently bonded, and sufficient mechanical strength of the negative electrode mixture layer tends to be obtained. When the content of the binder is 20% by mass or less, sufficient battery capacity and conductivity tend to be obtained.

  The content of the binder relative to the mass of the negative electrode mixture layer is 1% by mass to 15% by mass when a fluorine-based polymer represented by polyvinylidene fluoride is used as the main component as the binder. Is preferable, 2 to 10% by mass is more preferable, and 3 to 8% by mass is more preferable.

  Thickeners are used to adjust the viscosity of the slurry. The thickener is not particularly limited, and specific examples thereof include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein and salts thereof. The thickener may be used singly or in combination of two or more.

  The content of the thickener with respect to the mass of the negative electrode mixture layer is preferably 0.1% by mass to 5% by mass from the viewpoint of input / output characteristics and battery capacity, and is 0.5% by mass to 3% by mass The content is more preferably 0.6% by mass to 2% by mass.

  As a dispersion solvent for forming a slurry, any solvent which can dissolve or disperse a negative electrode active material, a binder, a conductive agent used as needed, a thickener and the like can be used. There is no limitation, and either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water, alcohol, a mixed solvent of water and alcohol, and the like. Examples of organic solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethyl sulfoxide , Benzene, xylene, hexane and the like. In particular, when using an aqueous solvent, it is preferable to use a thickener.

Density of the negative electrode mixture layer is preferably ^{0.7g / cm 3 ~2g / cm 3} , more preferably from ^{0.8g / cm 3 ~1.9g / cm 3} , 0.9g / cm More preferably, it is ^{3 to} 1.8 g / cm ³ .
When the density of the negative electrode mixture layer is 0.7 g / cm ³ or more, the conductivity between the negative electrode active materials can be improved, the increase in battery resistance can be suppressed, and the capacity per unit volume can be improved. . When the density of the negative electrode mixture layer is 2 g / cm ³ or less, the discharge characteristics deteriorate due to an increase in initial irreversible capacity and a decrease in the permeability of the non-aqueous electrolyte to the vicinity of the interface between the negative electrode current collector and the negative electrode active material Tend to reduce the risk of
In addition, the amount of single-sided application of the negative electrode mixture slurry to the negative electrode current collector when forming the negative electrode mixture layer is 30 g / m ² as the solid content of the negative electrode mixture from the viewpoint of energy density and input / output characteristics. it is preferably to 150 g / ^{m 2,} more preferably from ^{40g / m 2 ~140g / m 2} , further preferably ^{45g / m 2 ~130g / m 2} .

  There is no restriction | limiting in particular as a material of a negative electrode collector, As a specific example, metal materials, such as copper, nickel, stainless steel, nickel plating steel, are mentioned. Among them, copper is preferable in terms of ease of processing and cost.

There is no restriction | limiting in particular as a shape of a negative electrode collector, The material processed into various shapes can be used. As a specific example, a metal foil, a metal plate, a metal thin film, an expanded metal etc. are mentioned. Among them, metal foil is preferable, and copper foil is more preferable. The copper foil includes a rolled copper foil formed by a rolling method and an electrolytic copper foil formed by an electrolytic method, both of which are suitable as a negative electrode current collector.
The average thickness of the negative electrode current collector is not particularly limited. For example, the thickness is preferably 5 μm to 50 μm, more preferably 8 μm to 40 μm, and still more preferably 9 μm to 30 μm.
When the average thickness of the negative electrode current collector is less than 25 μm, the strength can be improved by using a stronger copper alloy (phosphor bronze, titanium copper, corson alloy, Cu-Cr-Zr alloy, etc.) than pure copper. it can.

(Electrolytes)
The electrolyte used in the manufacturing method of the present disclosure includes EMI-FSI and a lithium salt.

As lithium salts, LiPF ₆ , LiBF ₄ , LiFSI (lithium bis (fluorosulfonyl) imide), LiTFSI (lithium bis (trifluoromethanesulfonyl) imide), LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ And LiCF ₃ SO ₃ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) _{2 and the} like. A lithium salt may be used individually by 1 type, and may be used in combination of 2 or more type.

  The lithium salt concentration of the electrolyte contained in the battery container is, for example, preferably 0.5 mol / kg to 2.5 mol / kg, from 0.8 mol / kg to 2.0 mol / kg, from the viewpoint of electrical conductivity. Is more preferably 1.0 mol / kg to 1.8 mol / kg.

(Separator)
In the manufacturing method of the present disclosure, a separator that insulates between the positive electrode and the negative electrode may be used between the positive electrode and the negative electrode. The separator is not particularly limited as long as it has ion permeability and insulates between the positive electrode and the negative electrode, and has resistance to oxidation on the positive electrode side and reducibility on the negative electrode side. Resin, an inorganic substance, etc. are used as a material (material) of the separator which satisfy | fills such a characteristic.
As the resin, an olefin polymer, a fluorine polymer, a cellulose polymer, polyimide, nylon or the like is used. Among them, it is preferable to use a porous sheet or non-woven fabric made of polyolefin such as polyethylene and polypropylene as a raw material.
As the inorganic substance, oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, glass and the like are used. For example, the inorganic material in the form of fibers or particles may be used as a non-woven fabric, a woven fabric, or a thin film-shaped substrate such as a microporous film, which may be used as a separator. As the thin film-shaped substrate, one having a pore diameter of 0.01 μm to 1 μm and an average thickness of 5 μm to 50 μm is suitably used. In addition, it is also possible to use, as a separator, a composite porous layer in which the above-mentioned inorganic substance in the form of fibers or particles is made of a binder such as a resin. In addition, this composite porous layer may be formed on the surface of another separator to be a multilayer separator. Furthermore, this composite porous layer may be formed on the surface of the positive electrode or the negative electrode, and may be used as a separator.

Second Embodiment
A method of manufacturing a lithium ion secondary battery according to a second embodiment of the present disclosure includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide and lithium The method includes the steps of: storing an electrolyte containing a salt in a battery case; and heat treating the battery case containing the positive electrode, the negative electrode, and the electrolyte. The manufacturing method of the second embodiment differs from the manufacturing method of the first embodiment described above in that the battery case is heat-treated after the positive electrode, the negative electrode and the electrolyte are accommodated in the battery case.
In the manufacturing method of the present disclosure, a film can be formed on the surfaces of the positive electrode and the negative electrode by heat treating the battery case after housing the positive electrode, the negative electrode, and the electrolyte containing EMI-FSI and lithium salt in the battery case. As a result, decomposition of components in the electrolyte due to subsequent charge and discharge, deterioration of the active material, and the like are suppressed, and a lithium ion secondary battery having excellent charge and discharge cycle characteristics can be obtained.
In addition, according to the manufacturing method of the present disclosure, a lithium ion secondary battery excellent in charge and discharge cycle characteristics can be obtained by a simple method without performing a battery reaction.
In addition, about the preferable structure of a positive electrode, a negative electrode, and electrolyte, and the preferable structure of the manufactured lithium ion secondary battery, since it is the same as that of above-mentioned 1st embodiment, the description is abbreviate | omitted.

(Containment process before heat treatment)
The manufacturing method of the present disclosure includes a step of housing a positive electrode including a positive electrode active material, a negative electrode including the negative electrode active material, and an electrolyte including EMI-FSI and a lithium salt in a battery container (pre-heat treatment storage step). In this step, each component of the lithium ion secondary battery is accommodated in the battery container. In addition, the separator may be housed in the battery case such that a separator that insulates between the positive electrode and the negative electrode is disposed between the positive electrode and the negative electrode.

(Battery heat treatment process)
The manufacturing method of this indication has the process of heat-treating the said battery container in which the positive electrode, the negative electrode, and the electrolyte were accommodated. The conditions for heat-treating the battery case may be such that the components in the electrolyte are thermally decomposed to form a film on the surfaces of the positive electrode and the negative electrode.

For example, the temperature of the heat treatment may be 35 ° C. to 100 ° C., 40 ° C. to 80 ° C., or 45 ° C. to 60 ° C. When the temperature of the heat treatment is 35 ° C. or more, the film tends to be easily formed by thermal decomposition of the components in the electrolyte, and when the temperature of the heat treatment is 100 ° C. or less, the excessive thermal decomposition reaction can be suppressed. It is in.
In addition, the temperature of heat processing refers to the atmospheric temperature at the time of heat-processing a battery container.

  The heat treatment time may be 2 hours to 24 hours, 3 hours to 18 hours, 5 hours to 12 hours, or 6 hours to 8 hours. Good. When the heat treatment time is 2 hours or more, the coating tends to be easily formed by thermal decomposition of the components in the electrolyte, and when the heat treatment time is 24 hours or less, the discharge load characteristics tend to be excellent.

  In addition, as the atmosphere of the heat treatment, it is preferable that air, moisture, and the like do not enter during the heat treatment, and for example, a reduced pressure atmosphere or an inert gas atmosphere is preferable. Further, it is preferable to perform heat treatment after sealing the battery container.

  EXAMPLES Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples.

<Fabrication of positive electrode>
90 parts by mass of layered lithium-nickel-manganese-cobalt composite oxide (positive electrode active material), 5 parts by mass of acetylene black (conductive agent), 5 parts by mass of polyvinylidene fluoride (binding agent) The agent was prepared. The positive electrode mixture is dispersed in N-methyl-2-pyrrolidone as a dispersion medium, and the slurry is applied on an aluminum foil having a thickness of 20 μm so that the coating amount after drying of the dispersion medium is 100 g / m ² . The mixture was applied as such and dried at 100 ° C. for 1 hour to form a positive electrode mixture layer. After drying, the density of the positive electrode mixture layer was adjusted to 2.6 g / cm ³ by pressing. The density of the positive electrode mixture layer is expressed by the formula: density of positive electrode mixture layer = (mass of positive electrode−mass of positive electrode current collector [aluminum foil]) / (thickness of positive electrode mixture layer × area of positive electrode mixture layer) Calculated from). This was punched out to a diameter of 12 mm and used as a positive electrode.

<Fabrication of negative electrode>
Add 95 parts by mass of graphite (negative electrode active material, Hitachi Chemical Co., Ltd.), 3 parts by mass of acetylene black (conductive agent), 1 part by mass of styrene butadiene rubber (binder), and 1 part by mass of carboxymethylcellulose (thickener) The mixture was mixed to prepare a negative electrode mixture. The negative electrode mixture is dispersed in water as a dispersion medium, and the slurry is applied on a 20 μm thick copper foil so that the coating amount after drying of the dispersion medium is 54 g / m ^2. It dried at 80 degreeC for 1 hour, and formed the negative mix layer. After drying, the density of the negative electrode mixture layer was adjusted to 1.6 g / cm ³ by pressing. In addition, the density of the negative electrode mixture layer is represented by the formula: density of negative electrode mixture layer = (mass of negative electrode−mass of current collector [copper foil]) / (thickness of negative electrode mixture layer × area of negative electrode mixture layer) Calculated from This was punched to a diameter of 12 mm and used as a negative electrode.

Example 1
<Fabrication of evaluation cell>
A cell for evaluation was manufactured using a positive electrode, a polyethylene-based microporous membrane separator punched to a diameter of 14 mm, a negative electrode, and a 3-pole HS cell (Housen Co., Ltd.). After stacking the negative electrode and the separator, lithium bis (fluorosulfonyl) imide (LiFSI) is dissolved in 1-ethyl-3 methylimidazolium bis (fluorosulfonyl) imide (EMI-FSI) to a concentration of 1.2 mol / kg. The electrolyte prepared as above was poured into a 3-pole HS cell, and then the positive electrode was stacked to face the negative electrode and placed in the cell.
The evaluation cell was produced in a glove box under an argon atmosphere at a dew point of −80 ° C. or less.

<Heat treatment of evaluation cell>
As a heat treatment, the evaluation cell was allowed to stand in a thermostat set at 50 ° C. for 6 hours. Thereafter, the evaluation cell was taken out and allowed to stand at room temperature for 12 hours.

<Cycle test>
The evaluation cell after heat treatment was measured using the charge / discharge device under the following charge / discharge conditions.
(1) The first cycle is a constant current constant voltage (CCCV) charge that charges to a final voltage of 4.2 V with a current value of 0.1 C and then charges until the current value becomes 1/10 of the set current value at a constant voltage Did. Thereafter, constant current (CC) discharge was performed at a current value of 0.1 C to a termination voltage of 3.0 V. C means "current value (A) / battery capacity (Ah)".
(2) Next, charging was performed to a final voltage of 4.2 V at a current value of 1 C, and then CCCV charging was performed at a constant voltage until the current value became 1/10 of the set current value. Thereafter, CC discharge was performed at a current value of 1 C to a termination voltage of 3.0 V. This cycle was performed for 200 cycles.
From the results of the cycle test, Coulomb efficiency (%) and capacity retention rate (%) were calculated using the following formulas. The coulombic efficiency and the capacity retention rate can be said to be superior as the battery as the values are larger.
Coulomb efficiency (%) = [discharge capacity at 10th cycle obtained by (2) / charge capacity at 10th cycle of (2)] × 100
Capacity retention rate (%) = [Discharge capacity of 200th cycle obtained by (2) / Discharge capacity of 1st cycle of (2)] × 100

<Discharge load test>
After the cycle test, constant current constant voltage (CCCV) charging was performed by charging to a final voltage of 4.2 V at a current value of 1 C and then charging the current value to 1/10 of the set current value at a constant voltage . Thereafter, constant current (CC) discharge was performed at a current value of 1 C to a termination voltage of 3.0 V. After the charge and discharge cycle was performed five times, the current value at the time of discharge was changed in the order of 2C and 3C, and the charge and discharge cycle was performed five times at each current value at the time of the above discharge.
From the results of the discharge load test, the discharge load characteristics (%) were calculated using the following equation. The larger the value of the discharge load characteristic, the better the battery.
Discharge load characteristic = [3 C discharge capacity (5th cycle) / 1 C discharge capacity (5th cycle)] × 100

[Examples 2 to 6]
In the same manner as in Example 1 except that the lithium salt concentration, the heat treatment temperature and the heat treatment time were changed to the conditions shown in Table 1, the heat treatment of the evaluation cell was performed, and the evaluation cell after heat treatment was used. Cycle test and discharge load test were conducted.

[Example 7]
<Heat treatment of positive electrode>
In the glove box under an argon atmosphere with a dew point of −80 ° C. or less, a positive electrode was placed on a glass sample tube, and an electrolyte prepared by dissolving LiFSI in EMI-FSI to a concentration of 1.2 mol / kg was injected. The positive electrode was immersed in EMI-FSI. The sample tube was then capped and sealed from the atmosphere. The sample tube was allowed to stand in a thermostat set at 50 ° C. for 6 hours, and then allowed to stand at room temperature in a glove box.

  Thereafter, using this positive electrode, an evaluation cell was produced in the same manner as in Example 1. Then, in the same manner as in Example 1 except that the heat treatment of the produced evaluation cell was not performed, the cycle test and the discharge load test were performed using the produced evaluation cell.

[Example 8]
<Heat treatment of negative electrode>
In a glove box under an argon atmosphere with a dew point of -80 ° C or less, a negative electrode is placed on a glass sample tube, and an electrolyte prepared by dissolving LiFSI in EMI-FSI to a concentration of 1.2 mol / kg is injected. Liquid. The sample tube was then capped and sealed from the atmosphere. The sample tube was allowed to stand in a thermostat set at 50 ° C. for 6 hours, and then allowed to stand at room temperature in a glove box.

  Thereafter, using this negative electrode, an evaluation cell was produced in the same manner as in Example 1. Then, in the same manner as in Example 1 except that the heat treatment of the produced evaluation cell was not performed, the cycle test and the discharge load test were performed using the produced evaluation cell.

The results of Coulomb efficiency (%), capacity retention rate (%) and discharge load characteristics (%) in each Example and each Comparative Example are shown in Table 1.
In addition, "-" in Table 1 represents unmeasured.

  In Examples 1 to 8, the Coulomb efficiency (%) and the capacity retention ratio were superior to Comparative Examples 1 and 2, and it was confirmed that the charge / discharge cycle characteristics were excellent.

Further, comparing Example 3 in which the lithium salt concentration is 1.6 mol / kg and the heat treatment time 6 h with Comparative Example 2 in which the heat treatment is not performed at the lithium salt concentration 1.6 mol / kg, the discharge load characteristics in Example 3 are obtained. The result is an improvement.
The lithium salt concentration is the same as in Example 3, the heat treatment temperature is 40 ° C., and the heat treatment time is 6 hours. The heat treatment temperature is 40 ° C., and the heat treatment time is 12 hours. When the lithium salt concentration is the same as in Example 3 and Comparative Example 2 in which the heat treatment is not performed, the discharge load characteristics are improved in Examples 5 and 6.

Claims (6)

Immersing at least one electrode of a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material in an electrolyte containing 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide and a lithium salt;
Heat treating the electrode after immersion;
Storing the positive electrode, the negative electrode, and an electrolyte containing 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide and a lithium salt in a battery case after heat treatment of the electrode;
The manufacturing method of the lithium ion secondary battery which has. A step of housing a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolyte containing 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide and a lithium salt in a battery container;
Heat treating the battery container containing the positive electrode, the negative electrode, and the electrolyte;
The manufacturing method of the lithium ion secondary battery which has.   The method for producing a lithium ion secondary battery according to claim 1, wherein a concentration of lithium salt of the electrolyte used in the step of immersing is 0.5 mol / kg to 2.5 mol / kg.   The lithium salt concentration of the said electrolyte accommodated in the said battery container is 0.5 mol / kg-2.5 mol / kg, The manufacturing method of the lithium ion secondary battery of any one of Claims 1-3 .   The method for producing a lithium ion secondary battery according to any one of claims 1 to 4, wherein the negative electrode active material contains graphite.   The method for manufacturing a lithium ion secondary battery according to any one of claims 1 to 5, wherein the positive electrode active material contains NMC (layered lithium-nickel-manganese-cobalt composite oxide).