JP7488018B2 - How lithium-ion batteries are manufactured - Google Patents

Description

The present invention relates to a method for manufacturing a lithium-ion battery.

In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to protect the environment. In the automotive industry, there is hope that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will help reduce carbon dioxide emissions, and efforts are being made to develop secondary batteries for driving motors, which hold the key to putting these into practical use. As a secondary battery, attention is being focused on lithium-ion batteries (also known as lithium-ion secondary batteries), which can achieve high energy density and high power density.

Lithium ion batteries are manufactured by stacking or rolling a positive electrode, a negative electrode, and a separator to form a power generating element, and injecting an electrolyte into the electrode. However, in order to manufacture a high-capacity battery for use in an electric vehicle (EV), the electrodes become larger and the amount of active material contained therein increases, so it is necessary to uniformly impregnate the electrodes with the electrolyte to prevent or suppress uneven impregnation of the electrolyte.
Known examples of injection methods for preventing or suppressing uneven impregnation of the electrolyte include a method in which negative pressure is created inside a laminated container having a storage section that stores a power generating element and a space that stores electrolyte, thereby allowing the electrolyte in the space to gradually permeate the power generating element (see Patent Document 1).

JP 2009-76248 A

However, in the method described in Patent Document 1, the electrolyte is injected laterally into the laminate and gradually spreads laterally to impregnate the active material layer. This means that when the electrode surface area is large or when only a short time has passed since the electrolyte was injected, it is difficult to uniformly impregnate the electrolyte, and there is a problem in that uneven impregnation cannot be sufficiently prevented or suppressed.

The present invention was made in consideration of the above problems, and aims to provide a method for manufacturing a lithium-ion battery that can uniformly impregnate an electrolyte even when impregnating an electrode with a large surface area or when only a short time has passed since the electrolyte was injected.

The present inventors conducted extensive research to solve the above problems and arrived at the present invention.
That is, the present invention relates to a method for producing a lithium ion battery comprising a laminate unit in which an electrode active material layer (A) and an electrode active material layer (B) serving as a counter electrode of the electrode active material layer (A) are laminated via a separator, an electrolyte, and an exterior body for accommodating the laminate unit and the electrolyte, the method comprising a lamination step of obtaining a laminate unit in which the electrode active material layer (A), the separator, and the electrode active material layer (B) are laminated, and prior to forming the laminate unit, at least one of the separator and the electrode active material layer (B) contains the electrolyte, and the electrode active material layer (A), the separator, and the negative electrode active material layer (B) satisfy at least one of the following conditions (1) to (2):
(1) All of the pores in the separator are filled with the electrolytic solution, and the total volume of the pores in the electrode active material layer (A) in a state in which the electrode active material layer (A) does not contain an electrolytic solution is larger than the volume of the electrolytic solution in the electrode active material layer (A).
(2) At least the electrode active material layer (B) contains the electrolytic solution, and the ratio of the volume of the electrolytic solution contained in the electrode active material layer (B) to the total volume of pores in the electrode active material layer (B) in a state in which the electrode active material layer (B) does not contain the electrolytic solution is greater than the ratio of the volume of the electrolytic solution contained in the electrode active material layer (A) to the total volume of pores in the electrode active material layer (A) in a state in which the electrode active material layer (B) does not contain the electrolytic solution.

The method for producing a lithium ion battery of the present invention can uniformly impregnate an electrode with a large area with the electrolyte in the planar direction even when the electrode is impregnated with the electrolyte or when only a short time has passed since the electrolyte was injected.

FIG. 1 is a diagram showing a schematic example of a lamination process. FIG. 2 is an explanatory diagram showing a schematic example of the movement of the electrolyte immediately after the formation of the laminate unit. FIG. 3 is an explanatory diagram that illustrates another example of the movement of the electrolyte immediately after the formation of the laminate unit.

The present invention will be described in detail below.
In this specification, the term "lithium ion battery" is intended to include the concept of a lithium ion secondary battery.

The method for producing a lithium ion battery of the present invention is a method for producing a lithium ion battery comprising a laminate unit in which an electrode active material layer (A) and an electrode active material layer (B) serving as a counter electrode of the electrode active material layer (A) are laminated via a separator, an electrolyte, and an exterior body for accommodating the laminate unit and the electrolyte, the method comprising a lamination step of obtaining a laminate unit in which the electrode active material layer (A), the separator, and the electrode active material layer (B) are laminated, and before forming the laminate unit, at least one of the separator and the electrode active material layer (B) contains the electrolyte, and the electrode active material layer (A), the separator, and the negative electrode active material layer (B) satisfy at least one of the following conditions (1) to (2):
(1) All of the pores in the separator are filled with the electrolytic solution, and the total volume of the pores in the electrode active material layer (A) in a state in which the electrode active material layer (A) does not contain an electrolytic solution is larger than the volume of the electrolytic solution in the electrode active material layer (A).
(2) At least the electrode active material layer (B) contains the electrolytic solution, and the ratio of the volume of the electrolytic solution contained in the electrode active material layer (B) to the total volume of pores in the electrode active material layer (B) in a state in which the electrode active material layer (B) does not contain the electrolytic solution is greater than the ratio of the volume of the electrolytic solution contained in the electrode active material layer (A) to the total volume of pores in the electrode active material layer (A) in a state in which the electrode active material layer (B) does not contain the electrolytic solution.

In the method for producing a lithium ion battery of the present invention, before the stack unit is formed, at least one of the separator and the electrode active material layer (B) contains an electrolytic solution and satisfies at least one of the following conditions (1) and (2):
(1) All of the pores in the separator are filled with the electrolytic solution, and the total volume of the pores in the electrode active material layer (A) in a state in which the electrode active material layer (A) does not contain an electrolytic solution is larger than the volume of the electrolytic solution in the electrode active material layer (A).
(2) At least the electrode active material layer (B) contains the electrolytic solution, and the ratio of the volume of the electrolytic solution contained in the electrode active material layer (B) to the total volume of pores in the electrode active material layer (B) in a state in which the electrode active material layer (B) does not contain the electrolytic solution is greater than the ratio of the volume of the electrolytic solution contained in the electrode active material layer (A) to the total volume of pores in the electrode active material layer (A) in a state in which the electrode active material layer (B) does not contain the electrolytic solution.

In the case of (1), all of the pores in the separator are filled with the electrolyte, while the total volume of the pores in the electrode active material layer (A) in a state in which the electrode active material layer (A) does not contain the electrolyte is larger than the volume of the electrolyte in the electrode active material layer (A), so that it can be said that the electrode active material layer (A) has pores that do not contain the electrolyte. Therefore, a part of the electrolyte contained in the separator is supplied to the electrode active material layer (A).
At this time, since the electrode active material layer (A) and the separator are in surface-to-surface contact, the speed at which the electrolyte is supplied from the separator to the electrode active material layer (A) is faster than in the conventional method (a method in which the electrolyte is injected into the exterior body).

In the case of (2), the ratio of the volume of the electrolyte contained in the electrode active material layer (B) to the total volume of the pores in the electrode active material layer (B) in a state where it does not contain an electrolyte is greater than the ratio of the volume of the electrolyte contained in the electrode active material layer (A) to the total volume of the pores in the electrode active material layer (A) in a state where it does not contain an electrolyte.
Therefore, a part of the electrolyte contained in the electrode active material layer (B), which occupies a relatively large amount of electrolyte in the total volume of the pores when the electrolyte is not contained, is supplied to the electrode active material layer (A), which occupies a relatively small amount of electrolyte in the total volume of the pores when the electrolyte is not contained. At this time, a part of the electrolyte contained in the electrode active material layer (B) is supplied to the electrode active material layer (A) through the separator that is in surface contact with the electrode active material layer (B), so that the speed at which the electrolyte is supplied from the electrode active material layer (B) to the electrode active material layer (A) through the separator is faster than that of a normal method (a method in which the electrolyte is injected into the exterior body).
In the case of (2), the electrode active material layer (A) may or may not contain an electrolyte solution. When the electrode active material layer (A) does not contain an electrolyte solution, the ratio of the volume of the electrolyte solution contained in the electrode active material layer (A) to the total volume of the pores in the electrode active material layer (A) in a state in which the electrode active material layer (A) does not contain an electrolyte solution is 0%.

In the above cases of (1) and (2), the movement of the electrolyte between the electrode active material layer (A) and the separator and the movement of the electrolyte between the separator and the electrode active material layer (B) occur at the contact surface between the separator and the electrode active material layer (A) or the electrode active material layer (B) and the electrolyte diffuses in the thickness direction. Therefore, the diffusion distance of the electrolyte is short and the concentration of the electrolyte is less likely to vary.
Therefore, the electrolyte can be uniformly impregnated into the lithium ion battery electrode, and this effect is more remarkable as the area of the electrode becomes larger.

In the manufacturing method for a lithium ion battery of the present invention, in the above case of (2), the difference between the ratio of the volume of the electrolyte contained in the electrode active material layer (B) to the total volume of the pores that the electrode active material layer (B) has in a state where it does not contain an electrolyte (hereinafter also referred to as the electrolyte ratio in the electrode active material layer (B)) and the ratio of the volume of the electrolyte contained in the electrode active material layer (A) to the total volume of the pores that the electrode active material layer (A) has in a state where it does not contain an electrolyte (hereinafter also referred to as the electrolyte ratio in the electrode active material layer (A)) is not particularly limited, but is preferably 45% or more.
When the difference in the proportion of the electrolyte is 45% or more, the electrolyte can be rapidly supplied from the electrode active material layer (B) to the electrode active material layer (A).

In the method for producing a lithium ion battery of the present invention, at least one of the above conditions (1) and (2) is satisfied before the stack unit is formed, and it is preferable that both conditions (1) and (2) are satisfied.
When both of the above conditions (1) and (2) are satisfied, the electrode active material layer (A) can be rapidly and uniformly impregnated with a sufficient amount of the electrolyte.

An example of a method for satisfying the above condition (1) before forming the laminate unit is a method in which the separator is impregnated with the electrolyte without impregnating the electrode active material layer (A) with the electrolyte before forming the laminate unit.
Methods for impregnating the separator with the electrolytic solution include a method in which the separator is placed (immersed) in a container containing a predetermined amount of electrolytic solution, and a method in which the electrolytic solution is dripped onto the surface of the separator using a dropper or the like.
Whether or not all of the pores in the separator are filled with the electrolyte can be confirmed by checking whether the appearance of the separator surface changes from white to translucent.
The separator may contain an electrolyte solution in a volume larger than the total volume of the pores, in which case the electrolyte solution that does not enter the pores is retained on the surface of the separator.

Examples of a method for satisfying the above condition (2) before forming the stacking unit include a method of putting (immersing) the electrode active material layer (B) into a container containing a predetermined amount of electrolyte solution, and a method of dripping the electrolyte solution onto the surface of the electrode active material layer (B) using a dropper or the like.
Furthermore, when preparing the electrode active material layer (B), an electrolytic solution may be added in addition to the components constituting the electrode active material layer, and the resulting mixture may be mixed and molded to prepare an electrode active material layer (B) containing the electrolytic solution.

The lamination step constituting the method for producing a lithium ion battery of the present invention will be specifically described with reference to FIG.
FIG. 1 is a diagram showing a schematic example of a lamination process.
As shown in FIG. 1, in the lamination step, an electrode active material layer (A) 10 and an electrode active material layer (B) 30 serving as a counter electrode are laminated via a separator 20 to obtain a laminate unit 100 .
Both the electrode active material layer (A) 10 and the electrode active material layer (B) 30 are porous bodies having pores capable of containing an electrolytic solution. The separator 20 is also a porous body having pores capable of containing an electrolytic solution therein.
At least one of the electrode active material layer (B) 30 and the separator 20 contains an electrolyte.

The movement of the electrolyte after the formation of the laminate unit will be described.
Fig. 2 is an explanatory diagram showing a schematic example of the movement of the electrolyte immediately after the formation of the laminate unit. Fig. 2 shows an example in which the electrolyte is contained only in the separator before the formation of the laminate unit.
In the case shown in Fig. 2, initially, the electrolyte is contained only in the separator 20. When the separator 20 comes into contact with the electrode active material layer (A) 10 and the electrode active material layer (B) 30 due to the formation of the laminate unit 100, the electrolyte moves from the contact surface toward the electrode active material layer (A) 10 and the electrode active material layer (B) 30 (the arrows shown in Fig. 2 indicate the direction in which the electrolyte diffuses).

Fig. 3 is an explanatory diagram showing a schematic diagram of another example of the movement of the electrolyte immediately after the formation of the laminate unit. Fig. 3 shows an example in which the electrolyte is contained only in the electrode active material layer (B) before the formation of the laminate unit.
In the case shown in Fig. 3, initially, the electrolyte is contained only in the electrode active material layer (B) 30. When the separator 20, the electrode active material layer (A) 10, and the electrode active material layer (B) 30 come into contact with each other as a result of the formation of the laminate unit 100, the electrolyte first moves from the contact surface between the electrode active material layer (B) 30 and the separator 20 to the separator 20. Then, the electrolyte moves from the contact surface between the separator 20 and the electrode active material layer (A) 10 to the electrode active material layer (A) 10 (the arrows shown in Fig. 3 indicate the direction in which the electrolyte diffuses).
Even when the electrolyte is contained in both the separator and the electrode active material layer (B) before the lamination unit is formed, the electrolyte moves from the separator to the electrode active material layer (A).

The electrode active material layer is a porous body that contains an electrode active material and has pores that can contain an electrolyte solution. In addition to the electrode active material, the electrode active material layer may also contain a conductive assistant, an adhesive resin, and an electrode binder.

In the method for producing a lithium ion battery of the present invention, different types of electrode active materials are used for the electrode active material layer (A) and the electrode active material layer (B).
That is, the electrode active material is either a positive electrode active material or a negative electrode active material, and when the electrode active material layer (A) is a positive electrode active material layer using a positive electrode active material as the electrode active material, the electrode active material layer (B) is a negative electrode active material layer using a negative electrode active material as the electrode active material.

The positive electrode active material layer is a porous body that contains a positive electrode active material and has pores that can contain an electrolyte.

Positive electrode active materials include composite oxides of lithium and transition metals {composite oxides containing one type of transition metal (LiCoO ₂ , LiNiO ₂ , LiAlMnO ₄ , LiMnO ₂ , LiMn ₂ O ₄ , etc.), composite oxides containing two types of transition metal elements (for example, LiFeMnO ₄ , LiNi _1-x Co _x O ₂ , LiMn _1-y Co _y O ₂ , LiNi _1/3 Co _1/3 Al _1/3 O ₂ , and LiNi _0.8 Co _0.15 Al _0.05 O ₂ ), and composite oxides containing three or more types of transition metal elements [for example, LiM _a M' _b M'' _c O ₂ (M, M', and M'' are different transition metal elements and satisfy a + b + c = 1, for example, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), etc.}, lithium-containing transition metal phosphates (for example, LiFePO ₄ , LiCoPO ₄ , LiMnPO ₄ and LiNiPO ₄ ), transition metal oxides (for example, MnO ₂ and V ₂ O ₅ ), transition metal sulfides (for example, MoS ₂ and TiS ₂ ), and conductive polymers (for example, polyaniline, polypyrrole, polythiophene, polyacetylene, poly-p-phenylene and polyvinylcarbazole), and two or more of them may be used in combination.
The lithium-containing transition metal phosphate may have some of the transition metal sites substituted with other transition metals.

From the viewpoint of the electrical characteristics of the lithium ion battery, the volume average particle diameter of the positive electrode active material is preferably 0.01 to 100 μm, more preferably 0.1 to 35 μm, and even more preferably 1 to 30 μm.

The negative electrode active material layer contains a negative electrode active material and is a porous body having pores that can contain an electrolyte.

Examples of the negative electrode active material include carbon-based materials [e.g., hard carbon, non-graphitizable carbon, amorphous carbon, baked resins (e.g., baked and carbonized phenolic resins and furan resins), cokes (e.g., pitch coke, needle coke, and petroleum coke), silicon carbide, and carbon fibers], conductive polymers (e.g., polyacetylene and polypyrrole), metals (tin, silicon, aluminum, zirconium, and titanium), metal oxides (titanium oxide, lithium-titanium oxide, and silicon oxide), and metal alloys (e.g., lithium-tin alloys, lithium-silicon alloys, lithium-aluminum alloys, and lithium-aluminum-manganese alloys), and mixtures of these with carbon-based materials.
Among the above-mentioned negative electrode active materials, those that do not contain lithium or lithium ions inside may be subjected to a pre-doping treatment in which lithium or lithium ions are contained in a part or the whole of the negative electrode active material in advance.

From the viewpoint of the electrical characteristics of the lithium ion battery, the volume average particle diameter of the negative electrode active material is preferably 0.01 to 100 μm, more preferably 0.1 to 40 μm, and even more preferably 2 to 35 μm.

In this specification, the volume average particle diameter of the electrode active material means the particle diameter at 50% of the cumulative value in the particle size distribution determined by the Microtrack method (laser diffraction/scattering method) (Dv50). The Microtrack method is a method for determining the particle size distribution by irradiating particles with laser light and using scattered light obtained. Note that the volume average particle diameter can be measured using a Microtrack manufactured by Nikkiso Co., Ltd., or the like.

The conductive assistant is selected from materials having electrical conductivity.
Specific examples include metals [nickel, aluminum, stainless steel (SUS), silver, copper, titanium, etc.], carbon [graphite and carbon black (acetylene black, ketjen black, furnace black, channel black, thermal lamp black, etc.)], and mixtures thereof, but are not limited to these.
These conductive assistants may be used alone or in combination of two or more. In addition, alloys or metal oxides of these may be used. From the viewpoint of electrical stability, aluminum, stainless steel, carbon, silver, copper, titanium and mixtures thereof are preferable, silver, aluminum, stainless steel and carbon are more preferable, and carbon is even more preferable. In addition, these conductive assistants may be those in which a conductive material (a metal among the conductive assistants described above) is coated around the particulate ceramic material or resin material by plating or the like.

The average particle size of the conductive assistant is not particularly limited, but from the viewpoint of the electrical characteristics of the lithium ion battery, it is preferably 0.01 to 10 μm, more preferably 0.02 to 5 μm, and even more preferably 0.03 to 1 μm.
The term "particle diameter" refers to the maximum distance L between any two points on the contour of a particle. The value of the "average particle diameter" is calculated as the average value of particle diameters of particles observed in several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

The shape (form) of the conductive additive is not limited to a particulate form, but may be a form other than a particulate form, and may be a form that is in practical use as a so-called filler-based conductive material, such as carbon nanotubes.

The conductive assistant may be in the form of conductive fibers.
Examples of the conductive fiber include carbon fibers such as PAN-based carbon fibers and pitch-based carbon fibers, conductive fibers obtained by uniformly dispersing conductive metals or graphite in synthetic fibers, metal fibers obtained by fiberizing metals such as stainless steel, conductive fibers obtained by coating the surface of organic fibers with metals, and conductive fibers obtained by coating the surface of organic fibers with resins containing conductive substances. Among these conductive fibers, carbon fibers are preferred. Polypropylene resins kneaded with graphene are also preferred.
When the conductive assistant is a conductive fiber, the average fiber diameter is preferably 0.1 to 20 μm.

The electrode active material may be a coated electrode active material having at least a part of its surface coated with a coating agent containing a coating resin. The coated electrode active material can be molded under simpler conditions because the coating agent containing the electrolyte swells and exhibits adhesion.
Therefore, it becomes easier to prepare an electrode active material layer that contains an electrolytic solution from the beginning.
When the electrode active material is a positive electrode active material, the coated electrode active material is also called a coated positive electrode active material, and when the electrode active material is a negative electrode active material, the coated electrode active material is also called a coated negative electrode active material.

The coating agent contains a coating resin, and may further contain a conductive material as required.
Examples of the coating resin include thermoplastic resins and thermosetting resins, such as fluororesins, acrylic resins, urethane resins, polyester resins, polyether resins, polyamide resins, epoxy resins, polyimide resins, silicone resins, phenolic resins, melamine resins, urea resins, aniline resins, ionomer resins, polycarbonates, polysaccharides (such as sodium alginate), and mixtures thereof. Among these, acrylic resins, urethane resins, polyester resins, and polyamide resins are preferred, and acrylic resins are more preferred.
Among these, coating resins that have a liquid absorption rate of 10% or more when immersed in an electrolyte and a tensile elongation at break of 10% or more when saturated with electrolyte are more preferred.
Among the above-mentioned coating resins, those described as coating resins in WO 2015/005117 are particularly suitable for use as coating resins constituting the coating agent in the method for producing an electrode composition molded body for lithium ion batteries of the present invention.

The coated active material can be obtained by known methods such as those described in International Publication No. WO 2015/041184.

The electrode active material layer may be a porous body formed by forming a mixture containing an electrode active material and an electrode binder and/or an adhesive resin. The electrode active material layer is preferably a porous body formed by molding a mixture containing an electrode active material and an adhesive resin without containing an electrode binder.
Examples of the electrode binder include known solvent-drying type binders for lithium ion batteries used to bind and fix electrode active materials to each other and to a current collector (materials such as starch, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinylpyrrolidone, tetrafluoroethylene, styrene-butadiene rubber, polyethylene, polypropylene, and styrene-butadiene copolymers, which are used by being dissolved or dispersed in a solvent and which bind by volatilizing and distilling off the solvent to precipitate as a solid).
The electrode binder is a material that, when dried by volatilizing the solvent component, the surface of the electrode binder solidifies without showing any adhesiveness, and firmly fixes the electrode active material to itself and the electrode active material to the current collector. On the other hand, the adhesive resin means a resin that has adhesiveness without solidifying even when the solvent component is volatilized and dried. Here, the adhesive means a phenomenon seen in a high viscosity liquid, which is defined in JIS Z0109:2015 Adhesive Tape and Adhesive Sheet Terms, in which the adhesive resin adheres to the adherend by applying only a slight pressure, and the adhesive resin is a resin that can adhere to the adherend by applying only a slight pressure. The electrode binder, which cannot be adhered to the adherend by applying only a slight pressure, is a different material from the solid electrode binder and the adhesive resin, and is distinguished from them. The electrode binder is also simply called a binding agent, binder, etc.

The adhesive resin is a resin capable of reversibly adhering to an adherend with slight pressure for a short time at room temperature, and preferably contains at least one low Tg monomer selected from the group consisting of vinyl acetate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, butyl acrylate, and butyl methacrylate as an essential constituent monomer. The total weight ratio of the low Tg monomer is preferably 45% by weight or more based on the total weight of the constituent monomers.
In addition, the coating resin does not exhibit adhesiveness unless a solvent that dissolves part of it is used, whereas the adhesive resin exhibits adhesiveness in the electrode active material composition without the use of a solvent that dissolves part of it, and thus the coating resin is distinguished from the adhesive resin.

The adhesive resin may be a polymer containing 2-hydroxyethyl methacrylate, acrylic acid, etc., in addition to the essential constituent monomers.

When using an adhesive resin, it is preferable to use 0.01 to 10% by weight of the adhesive resin based on the total weight of the electrode active material and the conductive assistant.

As the conductive material, the same conductive assistant as described above can be suitably used.
However, they can be clearly distinguished in that the conductive material is contained in a coating agent that constitutes a part of the coated electrode active material, whereas the conductive assistant is not contained in the coating agent.

The electrode active material layer can be obtained by mixing the electrode active material with an electrode binder and an adhesive resin, etc., which are used as required, using a known powder stirring and mixing device such as a planetary mixer, a universal mixer, or a tumbler mixer to obtain a mixture, and molding this mixture by a known method, such as compression molding, extrusion molding, and calendar molding.
In addition, when the electrode active material layer is a porous body obtained by molding a mixture containing an electrode active material and an adhesive resin, the adhesive resin may be attached to the surface of the electrode active material, and then mixed with a conductive assistant or the like used as needed and molded, or the electrode active material, the adhesive resin, and the conductive assistant used as needed may be mixed together and molded.

The mixture may be in a dry state or a wet state, but is preferably in a wet state. A wet state is obtained by adding a small amount of liquid (electrolyte or a solvent that constitutes the electrolyte) to the above-mentioned dried mixture. When the mixture is in a wet state, it is preferably in a pendular or funicular state.

The proportion of electrolyte in the wet body is not particularly limited, but in order to achieve a pendular or funicular state, it is preferable that the proportion of electrolyte be 0.5 to 15% by weight of the entire wet body in the case of a positive electrode, and 0.5 to 25% by weight of the entire wet body in the case of a negative electrode.

As a method for obtaining an electrode active material layer by compression molding, there can be mentioned a method in which the above mixture is placed in a mold of any shape and compressed using a known compression device.
The shape of the mold is not particularly limited as long as it has a bottom surface and a side surface. In addition, the mold constituting the side surface and the mold constituting the bottom surface may be configured to be separable.

Examples of materials constituting the mold include general materials such as metals used in metal molds.
In order to reduce friction occurring between the mold and the electrode composition molded article, the surface of the mold may be coated with fluorine or the like.
The shape of the space formed in the mold (hereinafter also simply referred to as the space) may be adjusted according to the shape of the electrode active material layer to be obtained, and it is preferable that the shape does not change in the compression direction, for example, a cylindrical shape, a rectangular prism shape, etc.
When a mold in which the shape of the space is cylindrical is used, an electrode composition molded article that is approximately circular in plan view is obtained, and when a mold in which the shape of the space is prismatic is used, an electrode composition molded article that is rectangular in plan view is obtained.

As a method for obtaining an electrode active material layer by extrusion molding, a method using a known extrusion molding machine can be mentioned.
An example of an extrusion molding machine is one that has a raw material tube into which raw material (the above-mentioned mixture) is supplied, a die (also called a mold) attached to the raw material discharge side of the raw material tube, and a rotating shaft-shaped screw for extruding the raw material placed in the raw material tube toward the die.
The mixture is fed into the raw material cylinder, and the mixture that moves through the raw material cylinder by rotating the screw is extruded through the die to obtain a cylindrical compact. The shape of the compact can be appropriately adjusted by adjusting the shape of the die and the rotation speed of the screw.

The shape of the cylindrical compact discharged from the die is not particularly limited, but a cylindrical or rectangular prism shape is preferable. The electrode active material layer is obtained by cutting the cylindrical compact discharged from the die to a predetermined length.

As a method for obtaining an electrode active material layer by calendar molding, a method using a known roll press device can be mentioned.
The mixture is fed from a continuous mixer such as a kneader, and the mixture is spread to a certain thickness on a smooth surface such as a film using a doctor blade or the like, and then roll-pressed to obtain a sheet-like molded product. The sheet-like molded product is cut to a predetermined length to obtain an electrode active material layer.

The ratio of the pore volume of the electrode active material layer to the apparent volume of the electrode active material layer (hereinafter referred to as the porosity of the electrode active material layer) is not particularly limited, but is preferably 35 to 50% in the case of a positive electrode active material layer, and is preferably 30 to 45% in the case of a negative electrode active material layer.
By adjusting the pressure in the above-mentioned molding method, the porosity of the electrode active material layer can be adjusted.
The porosity of the electrode active material layer is the ratio of the total volume of the pores contained in the electrode active material layer to the apparent volume of the electrode active material layer in a state in which the electrolyte is not contained, and in the present invention, the total volume of the pores contained in the electrode active material layer is a value calculated by subtracting the total volume of the electrode active material used to form the electrode active material layer from the apparent volume of the electrode active material layer in a state in which the electrolyte is not contained. The volume of the electrode active material used to form the electrode active material layer is obtained by multiplying the mass of the electrode active material by the true density.
The true density of the electrode active material can be obtained by the method described in OECD Test No. 109, OECD GUIDELINE FOR THE TESTING OF CHEN|MICALS, Density of Liquids and Solids.

Of the electrode active material layers, at least the electrode active material layer (A) is preferably a non-binding material in which the electrode active materials are not bound to each other by an electrode binder.
The electrode active material layer (A), which is a non-binding material, can be obtained by preparing a mixture without using an electrode binder and molding it.
When the electrode active material constituting the electrode active material layer (A) is a coated electrode active material, the coating resin is present in the electrode active material layer. However, the coating resin coats the surface of the electrode active material particles, and does not irreversibly bond the electrode active material particles together, but the adhesion is temporary (reversible), and can be easily loosened by hand. Therefore, even when a mixture containing a coated electrode active material is molded, a non-bonded body in which the electrode active materials are not bonded to each other by the electrode binder can be obtained by not adding an electrode binder.

In addition, when the mixture for obtaining the electrode active material layer contains an adhesive resin, the surface of the electrode active material is reversibly fixed by the adhesive resin. In this case, the electrode active materials can be easily separated from each other, and even if the electrode active material is a coated electrode active material, the coated electrode active materials can be separated from each other without destroying the coating resin.
Therefore, even when molding is performed using a mixture prepared using an electrode active material (or a coated electrode active material) and an adhesive resin without using an electrode binder, it is possible to obtain a non-bonded body in which the electrode active materials are not bonded to each other by the electrode binder.

The separator used in the method for producing a lithium ion battery of the present invention may be a known lithium ion battery separator, such as a separator made of porous polyolefin (such as Hipore, Celgard, and Ube Industries, Ltd.), or a separator having a porous membrane made of insulating particles such as alumina on its surface.
The ratio of the total volume of pores to the apparent volume of the separator (hereinafter referred to as the porosity of the separator) is preferably 40 to 60%, and commercially available separators with porosity in this range can be used as they are.
The porosity of a separator can be confirmed from the value described in the specifications of a commercially available separator, or can be confirmed by using a porosimeter that quantitatively measures the pore distribution by injecting a liquid such as mercury into the pores, calculating the total volume of the pores from the obtained pore distribution, and calculating the ratio of the total volume of the pores to the apparent volume calculated from the outer dimensions of the separator.

The thickness of the separator is not particularly limited, but is preferably 5 to 200 μm.
When the thickness of the separator is within the above range, the thickness of the separator is sufficiently thin, and the speed at which the electrolyte is supplied from the electrode active material layer (B) to the electrode active material layer (A) through the separator is good.
Furthermore, when the thickness of the separator is within the above range, the amount of electrolyte that the separator can contain is smaller than the amount of electrolyte that the electrode active material layer (A) and the electrode active material layer (B) can contain. Therefore, even when the electrolyte ratio in the electrode active material layer (B) is greater than the electrolyte ratio in the electrode active material layer (A) (i.e., condition (2)) and the separator does not contain electrolyte, the electrolyte contained in the electrode active material layer (B) is supplied not only to the separator but also to the electrode active material layer (A).

The electrolyte solution may be an electrolyte solution containing an electrolyte and a non-aqueous solvent, which is used in the manufacture of lithium-ion batteries.

As the electrolyte, those used in ordinary electrolytic solutions can be used, and preferred ones include, for example, lithium salts of inorganic anions such as _LiPF6 , _LiBF4 , _LiSbF6 , _LiAsF6 , _LiClO4 , and LiN( _SO2F ) ₂ , and lithium salts of organic anions such as LiN( _SO2CF3 ) ₂ and LiN( _SO2C2F5 ) ₂ . Of these, _LiPF6 and LiN( _SO2F ) ₂ are preferred from the viewpoint of battery output and _charge / _discharge cycle characteristics _.

As the non-aqueous solvent, an aprotic solvent can be preferably used.
The aprotic solvent is a solvent that does not have a hydrogen ion donating group (a group having a dissociable hydrogen atom, for example, an amino group, a hydroxyl group, and a thio group). Solvents that can be preferably used include lactone compounds, cyclic or chain carbonates, chain carboxylates, cyclic or chain ethers, phosphates, nitrile compounds, amide compounds, sulfones, and mixtures thereof, and more preferably cyclic carbonates, chain carbonates, and mixed solvents of cyclic carbonates and chain carbonates.

Examples of lactone compounds include 5-membered ring (such as γ-butyrolactone and γ-valerolactone) and 6-membered ring lactone compounds (such as δ-valerolactone).

Examples of the cyclic carbonate ester include propylene carbonate, ethylene carbonate, butylene carbonate, and vinylene carbonate.
Examples of the chain carbonate ester include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate, and di-n-propyl carbonate.

Examples of the chain carboxylate include methyl acetate, ethyl acetate, propyl acetate, and methyl propionate.
Examples of cyclic ethers include tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, and 1,4-dioxane.
Examples of the chain ethers include dimethoxymethane and 1,2-dimethoxyethane.

Examples of the phosphate ester include trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, tributyl phosphate, tri(trifluoromethyl phosphate), tri(trichloromethyl phosphate), tri(trifluoroethyl phosphate), tri(triperfluoroethyl phosphate), 2-ethoxy-1,3,2-dioxaphospholan-2-one, 2-trifluoroethoxy-1,3,2-dioxaphospholan-2-one, and 2-methoxyethoxy-1,3,2-dioxaphospholan-2-one.
Examples of the nitrile compound include acetonitrile, etc. Examples of the amide compound include N,N-dimethylformamide (hereinafter, also referred to as DMF), etc. Examples of the sulfone include linear sulfones such as dimethyl sulfone and diethyl sulfone, and cyclic sulfones such as sulfolane, etc.
The aprotic solvent may be used alone or in combination of two or more kinds.

The concentration of the above electrolyte contained in the electrolytic solution is preferably 0.3 to 3 mol/L from the viewpoint of battery characteristics at low temperatures, etc.

In the manufacturing method for a lithium ion battery of the present invention, in the case where the above condition (2) is met, the amounts of the electrolyte contained in the electrode active material layer (A), the separator, and the electrode active material layer (B) used in the lamination step are not particularly limited, so long as the ratio of the volume of the electrolyte contained in the electrode active material layer (B) to the total volume of the pores that the electrode active material layer (B) has in a state where it does not contain the electrolyte is greater than the ratio of the volume of the electrolyte contained in the electrode active material layer (A) to the total volume of the pores that the electrode active material layer (A) has in a state where it does not contain the electrolyte.
In particular, from the viewpoint of uniformly impregnating the electrolyte solution, the ratio of the volume of the electrolyte solution contained in the electrode active material layer (A) to the total volume of the pores that the electrode active material layer (A) has in a state in which the electrolyte solution is not contained is preferably 0 to 90%, and more preferably 0 to 50%.

The ratio of the volume of the electrolyte contained in the electrode active material layer (B) to the total volume of the pores in the electrode active material layer (B) in a state in which the electrolyte is not contained may exceed 100%. When the above ratio exceeds 100%, the electrode active material layer (B) becomes an active material layer having fluidity.

The method for producing a lithium ion battery of the present invention preferably further includes a liquid absorption step of impregnating at least one of the separator and the electrode active material layer (B) with the electrolyte solution prior to the stacking step.

The above-mentioned liquid absorption process can be carried out by a method of putting (immersing) the electrode active material layer (B) into a container containing a predetermined amount of electrolyte solution, a method of dripping the electrolyte solution onto the surface of the electrode active material layer (B) using a dropper or the like, or a method of adding the electrolyte solution in addition to the components constituting the electrode active material layer when preparing the electrode active material layer (B), and then mixing and molding the mixture to prepare an electrode active material layer (B) containing the electrolyte solution.

Methods for obtaining an electrode active material layer (B) having an electrolyte ratio of more than 100% include, for example, a method in which an amount of electrolyte greater than the amount that the pores of the electrode active material layer (B) obtained by molding a mixture containing the electrode active material is added to the electrode active material layer (B) obtained by molding, and a method in which an amount of electrolyte greater than the amount that the pores of the electrode active material (B) after molding is added to a mixture containing the electrode active material in advance, and then the mixture is molded.

In the liquid absorption process, methods for impregnating the separator with the electrolyte include placing (immersing) the separator in a container containing a predetermined amount of electrolyte, or dripping the electrolyte onto the surface of the separator using a dropper, etc.

The method for producing a lithium ion battery of the present invention preferably further includes a housing step of obtaining a housing in which the laminate unit is housed in an exterior body.
The accommodation step may be performed by accommodating the laminate unit in an outer casing after the lamination step, or by directly laminating the electrode active material layer (A), the separator, and the electrode active material layer (B) inside the outer casing to form a laminate unit (i.e., performing the lamination step and the accommodation step simultaneously).

Before sealing the container, a positive electrode current collector is connected to the electrode active material layer on the positive electrode side, and a negative electrode current collector is connected to the electrode active material layer on the negative electrode side.
The positive electrode current collector is preferably provided on the main surface of the electrode active material layer that becomes the positive electrode, opposite the separator, and the negative electrode current collector is preferably provided on the main surface of the electrode active material layer that becomes the negative electrode, opposite the separator.

The exterior body may be a known metal can case or a laminate film containing aluminum. The laminate film may be a three-layer laminate film made by laminating polypropylene, aluminum, and nylon in this order. Among these, a laminate film is preferred because it is easy to apply pressure to the laminate unit after storage to promote impregnation of the electrolyte.

A metal collector or a resin collector can be used as the positive electrode collector and the negative electrode collector. A known metal collector can be used as the metal collector. For example, the metal collector is preferably made of one or more selected from the group consisting of copper, aluminum, titanium, nickel, tantalum, niobium, hafnium, zirconium, zinc, tungsten, bismuth, antimony, alloys containing one or more of these elements, and stainless steel alloys. The metal collector may be formed from a thin plate or metal foil, or a metal layer may be formed on the surface of the substrate by a method such as sputtering, electrodeposition, or coating.

The resin current collector is a current collector made of a polymer composition having electrical conductivity, and is preferably made of a mixture of a non-conductive polymer material and a conductive filler. Those described in Japanese Patent Publication No. 2012-150905 and International Publication No. WO2015/005116, etc., can be used.
Examples of polymeric materials include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polycycloolefin (PCO), polyethylene terephthalate (PET), polyethernitrile (PEN), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVdF), epoxy resin, silicone resin, and mixtures thereof.
From the viewpoint of electrical stability, polyethylene (PE), polypropylene (PP), polymethylpentene (PMP) and polycycloolefin (PCO) are preferred, and polyethylene (PE), polypropylene (PP) and polymethylpentene (PMP) are more preferred.

The conductive filler is selected from materials having electrical conductivity, and from the viewpoint of suppressing ion permeation in the current collector, it is preferable to use a material that does not have conductivity with respect to the ions used as the charge transfer medium. Specific examples include, but are not limited to, carbon materials, aluminum, gold, silver, copper, iron, platinum, chromium, tin, indium, antimony, titanium, and nickel. These conductive fillers may be used alone or in combination of two or more. In addition, alloy materials of these, such as stainless steel (SUS), may be used. From the viewpoint of corrosion resistance, aluminum, stainless steel, carbon materials, and nickel are preferable, and carbon materials are more preferable. In addition, these conductive fillers may be formed by coating the particulate ceramic material or resin material with the above-mentioned metal by plating or the like.

The resin collector can be obtained by known methods described in Japanese Patent Publication No. 2012-150905 and International Publication No. WO2015/005116, and examples thereof include a method in which 5 to 20 parts of acetylene black is dispersed in polypropylene as a conductive filler, and then the polypropylene is rolled using a hot press machine. There are also no particular limitations on the thickness, and the thickness can be the same as known methods or can be modified as appropriate.

After the laminate unit is housed in the exterior body, a step of injecting electrolyte into the exterior body may be further carried out. By injecting further electrolyte into the exterior body, the amount of electrolyte in the lithium ion battery can be adjusted.

In the method for producing a lithium ion battery of the present invention, an exhaust step of exhausting gas from within the container may be performed.
The exhaust step promotes the discharge of gas from the pores of the electrode active material layer (A), the separator, and the electrode active material layer (B), and the electrolyte moves rapidly into the pores, facilitating more uniform impregnation with the electrolyte.
The gas inside the container can be exhausted by a method in which a known pressure reducing device (rotary vacuum pump, aspirator, etc.) is connected to the opening of the exterior body to reduce the pressure inside the container and exhaust the gas; a method in which the container is placed in a dry chamber or the like that is connected to the pressure reducing device and has a reduced pressure inside, thereby placing the container in a reduced pressure environment and exhausting the gas inside the container; and a method in which pressure is applied from the outside of the container to compress the stacking unit and exhaust the gas inside the container, among which the method in which the gas inside the container is exhausted by placing the container in a reduced pressure environment is preferred.
In the exhaust step of discharging the gas from within the container, the excess electrolyte may be discharged to the outside of the container at the same time as the gas.

After the exhaust step, the opening of the housing is sealed by heat sealing or the like to obtain a lithium ion battery. The housing is preferably sealed in a state where the gas inside the housing has been exhausted.
When the exhaust process is performed by placing the container in a sealed container such as a depressurized dry chamber, it is preferable to seal the opening of the container while the pressure inside the sealed container is still reduced (i.e., while the container is placed in a reduced pressure environment).
If the container is sealed while being kept in a reduced pressure environment, when the container is returned to a normal pressure environment, atmospheric pressure is applied uniformly from the outside of the container, improving the adhesion between the electrode active material layer (A), the electrode active material layer (B), and the separator constituting the laminate unit and between the laminate unit and the current collector, thereby enabling the electrical resistance value of the battery to be reduced.

The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples as long as they do not deviate from the spirit of the present invention. Note that, unless otherwise specified, parts are parts by weight and % is % by weight.

<Production Example 1: Production of adhesive resin>
A four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel and a nitrogen gas inlet tube was charged with 5.0 parts of vinyl acetate, 23.7 parts of 2-ethylhexyl acrylate and 185.5 parts of ethyl acetate, and the temperature was raised to 75°C. 11.1 parts of vinyl acetate, 21.0 parts of 2-ethylhexyl acrylate, 28.1 parts of 2-hydroxyethyl methacrylate, 11.1 parts of acrylic acid, 0.200 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) and 0.200 parts of 2,2'-azobis(2-methylbutyronitrile) were mixed. The obtained monomer mixture was continuously dropped into the flask over 4 hours using the dropping funnel while blowing nitrogen into the flask, to carry out radical polymerization. After the dropwise addition, a solution of 0.800 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) dissolved in 12.4 parts of ethyl acetate was continuously added using a dropping funnel over the 8th hour from the start of polymerization. Furthermore, polymerization was continued for 2 hours at the boiling point, and 702.4 parts of ethyl acetate were added to obtain an adhesive resin solution with a resin concentration of 10% by weight. Thereafter, the solution was placed in a vacuum dryer at 100°C for 3 hours to remove ethyl acetate, thereby obtaining an adhesive resin. The weight average molecular weight (hereinafter abbreviated as Mw) of the obtained adhesive resin was 420,000.
The Mw was determined by gel permeation chromatography under the following conditions.
Apparatus: "HLC-8120GPC" [manufactured by Tosoh Corporation]
Column: "TSKgel GMHXL" (2 columns) and "TSKgel Multipore HXL-M, one column each connected together" [both manufactured by Tosoh Corporation]
Sample solution: 0.25% by weight tetrahydrofuran solution Injection amount: 10 μL
Flow rate: 0.6 mL/min Measurement temperature: 40° C.
Detector: Refractive index detector Reference material: Standard polystyrene [manufactured by Tosoh Corporation]

<Production Example 2: Preparation of Electrolyte Solution>
LiN( _FSO2 ) ₂ was dissolved at a ratio of 2 mol/L in a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (volume ratio of EC:PC = 1:1) to prepare an electrolyte for lithium ion batteries. The specific gravity (20°C) of the electrolyte measured with a hydrometer according to JIS B 7525-3:2013 was 1.4.

[Preparation of mixture of adhesive resin and negative electrode active material (granulated mixture for negative electrode)]
<Production Example 3>
235 parts of hard carbon powder [manufactured by Kureha Corporation, true density 1.5 g/ ^cm3 ] was placed in a universal mixer and stirred at room temperature at 150 rpm. 50 parts of the adhesive resin solution (resin solids concentration 10% by weight) obtained in Production Example 1 was added dropwise over a period of 60 minutes and mixed, followed by stirring for an additional 30 minutes.
Next, 10 parts of carbon nanofibers [Teijin Limited, true density 2.2 g/cm ³ ] and 10 parts of acetylene black [Denka Black, Denki Kagaku Kogyo Co., Ltd., true density 1.8 g/cm ³ ] were mixed in a stirred state, and the mixture was heated to 70°C while stirring for 30 minutes, and the pressure was reduced to 0.01 MPa and maintained for 30 minutes to distill off ethyl acetate contained in the adhesive resin solution. By this operation, 260 parts of a granulated mixture for negative electrode (NM-1) was obtained.

<Production Example 4>
595 parts of hard carbon powder was placed in a universal mixer and stirred at room temperature at 150 rpm. In this state, 130 parts of the adhesive resin solution (resin solids concentration: 10% by weight) obtained in Production Example 1 was added dropwise over 60 minutes and mixed, followed by stirring for an additional 30 minutes.
Next, 26 parts of carbon nanofibers and 26 parts of acetylene black were mixed in a stirred state, and the mixture was heated to 70°C while stirring for 30 minutes, and the pressure was reduced to 0.01 MPa and held for 30 minutes to distill off ethyl acetate contained in the adhesive resin solution. By this operation, 660 parts of a granulated mixture for negative electrode (NM-2) was obtained.

<Production Example 5>
926 parts of hard carbon powder was placed in a universal mixer and stirred at room temperature at 150 rpm. In this state, 200 parts of the adhesive resin solution (resin solids concentration: 10% by weight) obtained in Production Example 1 was added dropwise and mixed over a period of 60 minutes, and the mixture was further stirred for 30 minutes.
Next, 37 parts of carbon nanofibers and 37 parts of acetylene black were mixed in a stirred state, and the mixture was heated to 70°C while stirring for 30 minutes, and the pressure was reduced to 0.01 MPa and held for 30 minutes to distill off ethyl acetate contained in the adhesive resin solution. This operation yielded 1000 parts of a granulated mixture for negative electrode (NM-3).

<Production Example 6>
926 parts of hard carbon powder was placed in a universal mixer and stirred at room temperature at 150 rpm. In this state, 200 parts of the adhesive resin solution (resin solids concentration: 10% by weight) obtained in Production Example 1 was added dropwise and mixed over a period of 60 minutes, and the mixture was further stirred for 30 minutes.
Next, 74 parts of acetylene black was mixed in while stirring, and the mixture was heated to 70° C. while stirring for 30 minutes, and the pressure was reduced to 0.01 MPa and maintained for 30 minutes to distill off ethyl acetate contained in the adhesive resin solution. This operation yielded 1,000 parts of a granulated mixture for negative electrode (NM-4).

[Preparation of mixture of adhesive resin and positive electrode active material (granulated mixture for positive electrode)]
<Production Example 7>
950 parts of nickel-based positive electrode material powder [manufactured by Toda Kogyo Co., Ltd., true density 4.8 g/cm ³ ] was placed in a universal mixer, and while stirring at room temperature and 150 rpm, 200 parts of the adhesive resin solution (resin solid concentration 10 wt %) obtained in Production Example 1 and 30 parts of carbon nanofibers were added and mixed, stirred for 30 minutes, and further heated to 70°C while stirring, reduced the pressure to 0.01 MPa, and maintained for 30 minutes to distill off ethyl acetate contained in the adhesive resin solution. By this operation, 1000 parts of a granulated mixture for positive electrode (PM-1) was obtained.

<Production Example 8>
855 parts of nickel-based positive electrode material powder was placed in a universal mixer, and while stirring at room temperature and 150 rpm, 180 parts of the adhesive resin solution (resin solid content concentration 10% by weight) obtained in Production Example 1 and 27 parts of carbon nanofiber were added and mixed, stirred for 30 minutes, and further heated to 70°C while stirring, reduced pressure to 0.01 MPa, and held for 30 minutes to distill off ethyl acetate contained in the adhesive resin solution. By this operation, 900 parts of a granulated mixture for positive electrode (PM-2) was obtained.

[Preparation of negative electrode active material layer]
<Production Example 9>
260 parts of the negative electrode granulated mixture (NM-1) prepared in Production Example 3 and 740 parts of the electrolyte obtained in Production Example 2 were placed in a planetary stirring type mixing and kneading device {Awatori Rentaro [manufactured by Thinky Corporation]} and mixed at 2000 rpm for 5 minutes to prepare a negative electrode active material composition. The negative electrode active material composition was applied to the entire surface of a 10 cm x 10 cm copper foil equipped with a current extraction terminal using a film applicator with a film thickness adjustment function so that the coating amount per 1 cm ² of copper foil was 163.4 mg, and a negative electrode active material layer (N-1) with a thickness of 1143 μm was formed on the copper foil. The weight of the hard carbon powder placed on the copper foil per 1 cm ² area was 38.4 mg. The ratio of the volume of the electrolyte contained in the negative electrode active material layer to the value obtained by subtracting the total volume of the hard carbon, which is the negative electrode active material particles contained in the negative electrode active material layer, from the volume of the negative electrode active material layer (the total volume of the pores in the negative electrode active material layer in a state in which the negative electrode active material layer does not contain the electrolyte) (i.e., the electrolyte ratio in the negative electrode active material layer) is 97%.

<Production Example 10>
660 parts of the negative electrode granulated mixture (NM-2) prepared in Production Example 4 and 340 parts of the electrolyte solution obtained in Production Example 2 were placed in a planetary stirring type mixing and kneading device {Awatori Rentaro [manufactured by Thinky Corporation]} and mixed at 2000 rpm for 5 minutes to prepare a negative electrode active material composition, which was then applied to the entire surface of a copper foil similar to that in Production Example 9 using a film applicator with a film thickness adjustment function so that the coating amount per ¹ cm2 of copper foil was 104.6 mg, forming a negative electrode active material layer (N-2) with a thickness of 715 μm on the copper foil. The weight of the hard carbon powder placed on the copper foil per ¹ cm2 area was 62.2 mg. The ratio of the volume of the electrolyte contained in the negative electrode active material layer (i.e., the electrolyte ratio in the negative electrode active material layer) to the value obtained by subtracting the total volume of the hard carbon, which is the negative electrode active material particles contained in the negative electrode active material layer, from the volume of the negative electrode active material layer (the total volume of the pores in the negative electrode active material layer in a state in which the negative electrode active material layer does not contain the electrolyte) was 86%.

<Production Example 11>
The negative electrode granulated mixture (NM-3) prepared in Production Example 5 was placed on the same copper foil as in Production Example 9 at a weight of 53.8 mg per 1 cm ² of copper foil, and compression molded at a pressure of 3400 kgf/cm ² using a tablet press to form a negative electrode active material layer (N-3) having a thickness of 511 μm on the entire surface of the copper foil. The weight of the hard carbon powder placed on the copper foil per 1 cm ² area is 38.4 mg. The ratio of the volume of the electrolyte contained in the negative electrode active material layer to the value obtained by subtracting the total volume of the hard carbon, which is the negative electrode active material particle contained in the negative electrode active material layer, from the volume of the negative electrode active material layer (the total volume of the pores of the negative electrode active material layer in a state in which the electrolyte is not contained) (i.e., the electrolyte ratio of the negative electrode active material layer) is 0%.

<Production Example 12>
The negative electrode granulated mixture (NM-4) prepared in Production Example 6 was placed on the same copper foil as in Production Example 9 at a weight of 53.8 mg per 1 cm ² of copper foil, and compression molded at a pressure of 3400 kgf/cm ² using a tablet press to form a negative electrode active material layer (N-4) having a thickness of 521 μm on the entire surface of the copper foil. The weight of the hard carbon powder placed on the copper foil per 1 cm ² area is 38.4 mg. The ratio of the volume of the electrolyte contained in the negative electrode active material layer to the value obtained by subtracting the total volume of the hard carbon, which is the negative electrode active material particle contained in the negative electrode active material layer, from the volume of the negative electrode active material layer (the total volume of the pores of the negative electrode active material layer in a state in which the electrolyte is not contained) (i.e., the electrolyte ratio of the negative electrode active material layer) is 0%.

[Preparation of Positive Electrode Active Material Layer]
<Production Example 13>
The positive electrode granulated mixture (PM-1) prepared in Production Example 7 was placed on a 10 cm x 10 cm carbon-coated aluminum foil with a current extraction terminal attached at a weight of 84.2 mg per 1 cm ² of carbon-coated aluminum foil, and compression molded at a pressure of 3400 kgf / cm ² using a tablet press to form a positive electrode active material layer (P-1) having a thickness of 352 μm on the entire surface of the carbon-coated aluminum foil. The weight of the nickel-based positive electrode material placed on the carbon-coated aluminum foil per 1 cm ² area is 80.0 mg. The ratio of the volume of the electrolyte contained in the positive electrode active material layer (P-1) to the value obtained by subtracting the total volume of the positive electrode active material particles contained in the positive electrode active material layer from the volume of the positive electrode active material layer (the total volume of the pores in the positive electrode active material layer in a state in which the electrolyte is not contained) (i.e., the electrolyte ratio of the positive electrode active material layer) is 0%.

<Production Example 14>
900 parts of the positive electrode granulated mixture (PM-2) prepared in Production Example 8 and 100 parts of the electrolyte obtained in Production Example 2 were placed in a planetary stirring type mixing and kneading device {Awatori Rentaro [manufactured by Thinky Corporation]} and mixed for 5 minutes at 2000 rpm to produce a positive electrode active material composition, which was placed on the same carbon-coated aluminum foil as in Production Example 13 at a weight of 92.2 mg per 1 cm ² of carbon-coated aluminum foil, and compression-molded at a pressure of 3400 kgf / cm ² using a tablet press to form a positive electrode active material layer (P-2) having a thickness of 347 μm on the entire surface of the carbon-coated aluminum foil. The weight of the nickel-based positive electrode material placed on the carbon-coated aluminum foil per area of 1 cm ² is 80.0 mg. The ratio of the volume of the electrolyte contained in the positive electrode active material layer (P-2) to the value obtained by subtracting the total volume of the positive electrode active material particles contained in the positive electrode active material layer from the volume of the positive electrode active material layer (the total volume of the pores in the positive electrode active material layer in a state in which the positive electrode active material layer does not contain the electrolyte) (i.e., the electrolyte ratio in the positive electrode active material layer) is 37%.

<Production Example 15>
[Preparation of separator impregnated with electrolyte]
5,500 parts of the electrolyte obtained in Production Example 2 were dropped onto 100 parts of a separator (Celgard (registered trademark) 3501, manufactured by Celgard Corporation, thickness 25 μm, porosity 55%) (S-0) to prepare a separator (S-1) impregnated with the electrolyte. When about 85 parts of the electrolyte were dropped, all of the pores of the separator were filled with the electrolyte and the electrolyte no longer penetrated into the separator. Thereafter, the remaining electrolyte was slowly dropped so as not to spill from the separator surface, and the entire amount was placed on the surface of the separator. Note that the surface tension of the electrolyte allowed it to be placed on the surface of the separator without spilling. In the separator (S-1), the excess electrolyte that did not enter the pores was placed on the separator surface.

<Production Example 16>
A separator (S-2) having the pores impregnated with the electrolytic solution was produced in the same manner as in Production Example 15, except that the amount of the electrolytic solution was changed to 450 parts. The excess electrolytic solution that did not enter the pores was on the surface of the separator.

Example 1
In a dry chamber, the three sides of two rectangular laminate films were heat-sealed to prepare an exterior body, and the carbon-coated aluminum foil, the positive electrode active material layer (P-1), the separator (S-0) not impregnated with the electrolyte, the negative electrode active material layer (N-1) and the copper foil were laminated in this order. Next, the pressure in the dry chamber was reduced to -1 atm, and a Teflon plate (Teflon is a registered trademark) was placed on the exterior body in the dry chamber, and a weight was applied from above to compress the exterior body, and compression was continued until 6.4 g of excess electrolyte was discharged from the exterior body. Next, the opening was sealed with the current extraction terminal protruding, and then the dry chamber was returned to normal pressure with dry air to prepare an evaluation battery. Then, the following cycle characteristic evaluation was performed.

[Evaluation of cycle characteristics]
The current output terminal of the evaluation battery was connected to a charge/discharge measuring device "Battery Analyzer 1470" (manufactured by Toyo Corporation).
Next, using the charge/discharge measuring device, the stacked unit was discharged at a current of 0.05 C until the positive electrode potential reached 2.5 V (vs. Li ⁺ /Li), and then rested for 10 minutes, after which it was charged until the positive electrode potential reached 4.2 V (vs. Li ⁺ /Li) to perform a preliminary charge.
Next, the evaluation battery after the preliminary charging process was further discharged at a current of 0.05 C until the positive electrode potential reached 2.5 V (vs. Li ⁺ /Li), rested for 10 minutes, charged at a current of 0.05 C until the positive electrode potential reached 4.2 V (vs. Li ⁺ /Li), and rested for 10 minutes, and the following cycle was repeated three times.
The first charge capacity was taken as 100%, and the ratio of the third charge capacity to the first charge capacity (third cycle capacity retention rate) was calculated. The results are shown in Table 1.
If the electrolyte is unevenly distributed in the electrode active material layer, the electrode active material in the portion not impregnated with the electrolyte cannot participate in the movement of lithium ions, so that the first charge capacity is smaller than the design value, and as the charge-discharge cycle is repeated, the electrolyte spreads throughout the entire electrode over time, so that the charge capacity becomes larger than the first charge capacity. In other words, the capacity retention rate at the third cycle exceeds 100%.
On the other hand, when the electrolyte is uniformly distributed from the first cycle, the charge capacity at the third cycle is close to the design value. Since the design value is the first cycle capacity minus the irreversible capacity, the capacity retention rate at the third cycle when the electrolyte is uniformly distributed is less than 100%.

Example 2
In a dry chamber, a carbon-coated aluminum foil, a positive electrode active material layer (P-2), a separator (S-0) not impregnated with an electrolyte, a negative electrode active material layer (N-2), and a copper foil were laminated in this order in an exterior body similar to that of Example 1. Next, the pressure in the dry chamber was reduced to -1 atm. Thereafter, the opening was sealed with a current extraction terminal protruding from it, and then the pressure in the dry chamber was returned to normal pressure with dry air to prepare a battery for evaluation. A battery for evaluation was prepared.
The cycle characteristics were evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Example 3
In a dry chamber, a carbon-coated aluminum foil, a positive electrode active material layer (P-1), a separator (S-1) impregnated with an electrolytic solution, a negative electrode active material layer (N-3), and a copper foil were laminated in this order in an exterior body similar to that of Example 1. The pressure in the dry chamber was then reduced to -1 atm. Thereafter, the dry chamber was sealed with a current extraction terminal protruding from the opening, and the pressure in the dry chamber was returned to normal pressure with dry air to prepare a battery for evaluation.
The cycle characteristics were evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Example 4
In a dry chamber, a carbon-coated aluminum foil, a positive electrode active material layer (P-1), a separator (S-2) impregnated with an electrolytic solution, a negative electrode active material layer (N-4), and a copper foil were laminated in this order in an outer casing similar to that in Example 1. Then, 5.6 g of an electrolytic solution was poured in through the opening. Next, the inside of the outer casing was evacuated using a vacuum pump through the opening of the outer casing while keeping the inside of the draft chamber at normal pressure, and then the outer casing was sealed with the current extraction terminal exposed to prepare an evaluation battery.
The cycle characteristics were evaluated in the same manner as in Example 1, and the results are shown in Table 1.

<Comparative Example 1>
In a dry chamber, a carbon-coated aluminum foil, a positive electrode active material layer (P-1), a separator (S-0) not impregnated with an electrolyte, a negative electrode active material layer (N-4), and a copper foil were laminated in this order in an outer casing similar to that of Example 1. Then, 6.2 g of electrolyte was poured into the opening. Then, the pressure in the dry chamber was reduced to -1 atm. Thereafter, the dry chamber was sealed with a current extraction terminal protruding from the opening, and the pressure in the dry chamber was returned to normal pressure with dry air to prepare a battery for evaluation.
The cycle characteristics were evaluated in the same manner as in Example 1, and the results are shown in Table 1.

All of the electrodes of the evaluation batteries shown in Table 1 were squares measuring 10 cm x 10 cm.
In addition, in Table 1, a separator that is not impregnated with an electrolyte solution is indicated as "S-0", the ratio of the volume of the electrolyte solution contained in the positive electrode active material layer to the value obtained by subtracting the total volume of the positive electrode active material contained in the positive electrode active material layer from the volume of the positive electrode active material layer is indicated as "electrolyte solution proportion [%] in positive electrode active material layer", and the ratio of the volume of the electrolyte solution contained in the negative electrode active material layer to the value obtained by subtracting the total volume of the negative electrode active material contained in the negative electrode active material layer from the volume of the negative electrode active material layer is indicated as "electrolyte solution proportion [%] in negative electrode active material layer".
In both separators (S-1) and (S-2), all of the pores are filled with the electrolyte, and the excess electrolyte that did not enter the pores is on the separator surface, with separator (S-1) having more electrolyte.

If the electrolyte is unevenly contained in the electrode active material layer, the electrode active material in the areas without electrolyte cannot participate in the movement of lithium ions, resulting in a small charge/discharge capacity. Over time, however, the electrolyte becomes incorporated throughout the electrode active material, increasing the charge/discharge capacity.
From the results in Table 1, it can be seen that the evaluation batteries according to the examples produced by the lithium ion battery manufacturing method of the present invention had third cycle capacity retention rates less than 100% compared to the evaluation batteries according to the comparative examples, and thus the electrolyte was able to be uniformly impregnated.

10 Electrode active material layer (A)
20 Separator 30 Electrode active material layer (B)
100 stacking units

Claims (3)

A method for producing a lithium ion battery comprising: a lamination unit in which an electrode active material layer A and an electrode active material layer B serving as a counter electrode of the electrode active material layer A are laminated with a separator interposed therebetween; an electrolyte; and an exterior body for accommodating the lamination unit and the electrolyte,
a lamination step of obtaining a lamination unit in which the electrode active material layer A , the separator, and the electrode active material layer B are laminated ;
a housing step of obtaining a housing in which the laminate unit is housed in the exterior body;
and an exhaust step of exhausting gas from the container by placing the container in a reduced pressure environment ,
a method for producing a lithium ion battery, characterized in that, before the stacking unit is formed, the electrode active material layer B contains the electrolyte solution, and the electrode active material layer A and the electrode active material layer B satisfy the following condition (2) , so that after the stacking unit is formed, the electrolyte solution is moved along a stacking direction of the stacking unit and the electrolyte solution is supplied from the electrode active material layer B to the electrode active material layer A :
( 2 ) When the electrode active material layer B contains the electrolyte solution, and the ratio of the volume of the electrolyte solution contained in the electrode active material layer B to the total volume of the pores in the electrode active material layer B in a state in which the electrode active material layer B does not contain the electrolyte solution is defined as the electrolyte solution ratio of the electrode active material layer B , and the ratio of the volume of the electrolyte solution contained in the electrode active material layer A to the total volume of the pores in the electrode active material layer A in a state in which the electrode active material layer A does not contain the electrolyte solution is defined as the electrolyte solution ratio of the electrode active material layer A,
The electrolyte ratio in the electrode active material layer B is greater than the electrolyte ratio in the electrode active material layer A. The method for producing a lithium ion battery according to claim 1 , further comprising a liquid absorption step of impregnating the electrode active material layer B with the electrolytic solution before the lamination step. The method for producing a lithium ion battery according to claim 1 or 2 , wherein the electrode active material layer A and/or the electrode active material layer B contains an adhesive resin.

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