JP6978259B2 - Positive electrode for lithium-ion batteries and lithium-ion batteries - Google Patents

Description

The present invention relates to a positive electrode for a lithium ion battery and a lithium ion battery.

In recent years, there has been an urgent need to reduce carbon dioxide emissions in order to protect the environment. In the automobile industry, expectations are high for the reduction of carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and the development of secondary batteries for driving motors, which hold the key to their practical application, is earnest. It is done. As a secondary battery, a lithium ion secondary battery that can achieve high energy density and high output density is attracting attention.

Lithium-ion secondary batteries are required to have further improved battery characteristics, and in particular, when used as a power source for electric vehicles, it is necessary to secure a large output.
As a lithium ion battery that can secure a large output, for example, by stacking a plurality of resin collectors coated with a positive electrode active material on one surface and a negative electrode active material on the other surface, a connection between battery cells can be established. A bipolar battery in which the part is omitted has been developed (see Patent Document 1).

Since the resin current collector used in the bipolar battery described in Patent Document 1 has lower electrical conductivity than the conventional metal current collector, conductivity such as _{LiPF 6 is used to reduce the internal resistance of the entire battery.} Highly high-quality electrolyte salt is used.

Japanese Unexamined Patent Publication No. 2006-190649

However, the bipolar battery described in Patent Document 1 has a problem that the cycle characteristics are deteriorated when the battery is operated in a high temperature environment.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a positive electrode for a lithium ion battery which is excellent in cycle characteristics when the battery is operated in a high temperature environment, that is, excellent in high temperature durability. ..

The present inventors have arrived at the present invention as a result of diligent studies to solve the above problems.
That is, the present invention is for a lithium ion battery including a positive electrode current collector, a positive electrode active material layer formed on the surface of the positive electrode current collector, and an electrolytic solution composed of an electrolyte containing lithium ions and a non-aqueous solvent. A positive electrode, the positive electrode current collector is a resin current collector composed of a conductive material and a resin, and the electrolyte contains at least an imide-based electrolyte, and the electrolyte concentration of the electrolytic solution is 1 to 5 mol / L. It is characterized by being.

The positive electrode for a lithium-ion battery of the present invention has a high thermal decomposition temperature and contains an imide-based electrolyte that does not generate hydrofluoric acid, which causes deterioration of the battery. Excellent cycle characteristics, that is, high temperature durability when operated in.

The positive electrode active material layer constituting the positive electrode for a lithium ion battery of the present invention comprises a positive electrode active material, and the positive electrode active material is a coated positive electrode in which a part or all of the surface of the positive electrode active material particles is covered with a coating layer. It is preferably an active material (described later).
As the positive electrode active material, a conventionally known material can be preferably used, which is a compound capable of inserting and removing lithium ions by applying a certain potential, and has a higher potential than the negative electrode active material used for the counter electrode. A compound capable of inserting and removing lithium ions can be used.

As the positive electrode active material, composite oxide of lithium and transition metal {composite oxide is a transition metal is one _{(LiCoO 2, LiNiO 2, LiAlMnO} 4, LiMnO 2 and LiMn ₂ O _4, etc.), transition metal elements Two types of composite oxides (eg 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 a composite oxide metal element is three or more [e.g. _{LiM a M 'b M''} c O 2 (M, M' and M '' is different from the transition metal elements, respectively And a + b + c = 1. For example, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) etc.}, lithium-containing transition metal phosphates (eg LiFePO ₄ , LiCoPO ₄ , LiMnPO ₄ and LiNiPO _4). ), Transition metal oxides (eg MnO ₂ and V ₂ O ₅ ), transition metal sulfides (eg MoS ₂ and TiS ₂ ) and conductive polymers (eg polyaniline, polypyrrole, polythiophene, polyacetylene and poly-p-phenylene and Polyvinylcarbazole) and the like may be mentioned, and two or more kinds may be used in combination.
The lithium-containing transition metal phosphate may be obtained by substituting a part of the transition metal site with another transition metal.

The volume average particle size of the positive electrode active material is preferably 0.01 to 100 μm, more preferably 0.1 to 35 μm, and even more preferably 2 to 30 μm from the viewpoint of the electrical characteristics of the battery. ..

In the present specification, the volume average particle size of the positive electrode active material means the particle size (Dv50) at an integrated value of 50% in the particle size distribution obtained by the microtrack method (laser diffraction / scattering method). The microtrack method is a method for obtaining a particle size distribution by using scattered light obtained by irradiating particles with laser light. A microtrack manufactured by Nikkiso Co., Ltd. can be used for measuring the volume average particle size.

The thickness of the positive electrode active material layer is not particularly limited, but is preferably 100 μm or more and 1500 μm or less, more preferably more than 150 μm and 1200 μm or less, and further preferably 200 or more and 800 μm or less.

The positive electrode current collector is a resin current collector composed of a conductive material and a resin.
When the positive electrode current collector is made of aluminum, there is a problem that it is corroded by an imide-based electrolyte. However, in the positive electrode for a lithium ion battery of the present invention, the positive electrode current collector is a resin current collector made of a conductive material and a resin. , The problem of corrosion due to the imide-based electrolyte does not occur.

The conductive material constituting the resin current collector is selected from the materials having conductivity.
Specifically, 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.), etc.), etc. ], And a mixture thereof, etc., but is not limited thereto.
These conductive materials may be used alone or in combination of two or more. Moreover, you may use these alloys or metal oxides. 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 more preferable. Further, as these conductive materials, a conductive material (a metal material among the above-mentioned conductive material materials) may be coated around a particle-based ceramic material or a resin material by plating or the like.

The average particle size of the conductive material is not particularly limited, but is preferably 0.01 to 10 μm, more preferably 0.02 to 5 μm, and 0. It is more preferably 03 to 1 μm. The "particle size of the conductive material" means the maximum distance among the distances between any two points on the contour line of the conductive material. As the value of the "average particle size", an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM) is used, and the average value of the particle size of the particles observed in several to several tens of fields is used. The calculated value shall be adopted.

The shape (form) of the conductive material is not limited to the particle form, and may be a form other than the particle form, and is in a form practically used as a so-called filler-based conductive resin composition such as carbon nanofiller and carbon nanotube. There may be.

The conductive material may be a conductive fiber whose shape is fibrous.
The conductive fibers include carbon fibers such as PAN-based carbon fibers and pitch-based carbon fibers, conductive fibers in which a metal having good conductivity and graphite are uniformly dispersed in synthetic fibers, and a metal such as stainless steel. Examples thereof include fibrous metal fibers, conductive fibers in which the surface of organic fibers is coated with metal, and conductive fibers in which the surface of organic fibers is coated with a resin containing a conductive substance. Among these conductive fibers, carbon fibers are preferable. Further, a polypropylene resin kneaded with graphene is also preferable.
When the conductive material is conductive fiber, the average fiber diameter thereof is preferably 0.1 to 20 μm.

The resins constituting the resin collector include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polycycloolefin (PCO), polyethylene terephthalate (PET), polyether nitrile (PEN), and polytetra. Fluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polymethylacrylate (PMA), polymethylmethacrylate (PMMA), polyvinylidene fluoride (PVdF), epoxy resin, silicone resin, or a mixture thereof, etc. Can be mentioned.
From the viewpoint of electrical stability, polyethylene (PE), polypropylene (PP), polymethylpentene (PMP) and polycycloolefin (PCO) are preferable, and polyethylene (PE), polypropylene (PP) and polymethylpentene are more preferable. (PMP).

The resin collector can be obtained by using the above conductive material and resin in the same manner as the resin collector described in JP-A-2012-150905 and International Publication No. 2015/005116. As a specific manufacturing method, for example, a known heat-melt kneader such as a twin-screw extruder is used to melt-knead the resin, the conductive material, and other components used if necessary, and then melt-knead the obtained product. Examples thereof include a method of rolling and forming a finished material with a hot press machine. As the molding method, injection molding, compression molding, calender molding, slush molding, rotary molding, extrusion molding, blow molding, film molding (cast method, tenter method, inflation method, etc.) and the like can also be performed.

As the electrolytic solution, an electrolytic solution composed of an electrolyte and a non-aqueous solvent used for manufacturing a lithium ion battery can be used.

The electrolyte contains at least an imide-based electrolyte.
Examples of the imide-based electrolyte include LiN (FSO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ and LiN (C ₂ F ₅ SO ₂ ) ₂ . _{Of these, LiN (FSO 2} ) ₂ is preferable from the viewpoint of ionic conductivity and thermal decomposition temperature at high concentrations. LiN (FSO ₂ ) ₂ may be used in combination with other electrolytes (including electrolytes other than the above-mentioned imide-based electrolytes), but it is preferably used alone.
That is, the electrolyte constituting the electrolytic solution preferably contains at least lithium bis (fluorosulfonyl) imide [LiN (FSO ₂ ) ₂ ], and is composed only of lithium bis (fluorosulfonyl) imide [LiN (FSO ₂ ) _2]. Is more preferable.
When the electrolyte constituting the electrolytic solution _{contains an imide-based electrolyte such as LiN (FSO 2} ) ₂ , LiN (CF ₃ SO ₂ ) ₂ and LiN (C ₂ F ₅ SO ₂ ) ₂ , the internal resistance of the battery is suppressed to a low level. , The output density can be improved.
_{Furthermore, unlike LiPF 6} , the imide-based electrolyte does not generate hydrofluoric acid, which causes deterioration of the battery, even when it comes into contact with water, so that it is less likely to deteriorate over time due to the intrusion of water, and further, the thermal decomposition temperature. Is _{higher than LiPF 6} and has excellent thermal stability.
In addition, when the positive electrode current collector is aluminum, there is a problem that it is corroded by an imide-based electrolyte. However, in the positive electrode for a lithium ion battery of the present invention, the positive electrode current collector is a resin current collector composed of a conductive material and a resin. Therefore, the problem of corrosion due to the imide-based electrolyte does not occur.

The electrolyte concentration of the electrolytic solution is 1 to 5 mol / L, preferably 1.5 to 4 mol / L, and more preferably 2 to 3 mol / L.
If the electrolyte concentration of the electrolytic solution is less than 1 mol / L, sufficient input / output characteristics of the battery cannot be obtained, and if it exceeds 5 mol / L, the electrolyte will precipitate.
The electrolyte concentration of the electrolytic solution can be confirmed by extracting the positive electrode for a lithium ion battery or the electrolytic solution constituting the lithium ion battery without using a solvent or the like and measuring the concentration.
Specifically, for example, a part of the member (separator or positive electrode active material layer) containing the electrolytic solution is cut off, or a hole is made in the outer body (battery outer body) of the lithium ion battery to reach the inside. The electrolyte concentration can be measured by centrifugally extracting the electrolytic solution using a centrifuge or the like and measuring the volume of the extracted electrolytic solution and the contained electrolytic mass.

As the non-aqueous solvent, those used in known electrolytic solutions can be used, for example, lactone compounds, cyclic or chain carbonate esters, chain carboxylic acid esters, cyclic or chain ethers, phosphoric acid esters, nitriles. Compounds, amide compounds, sulfones and the like and mixtures thereof can be used.

Examples of the lactone compound include a 5-membered ring (γ-butyrolactone, γ-valerolactone, etc.) and a 6-membered ring lactone compound (δ-valerolactone, etc.).

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

Examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, propyl acetate, methyl propionate and the like.
Examples of the cyclic ether include tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,4-dioxane and the like.
Examples of the chain ether include dimethoxymethane and 1,2-dimethoxyethane.

Examples of the phosphoric acid ester include trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, tributyl phosphate, tri (trifluoromethyl) phosphate, and tri (trichloromethyl) phosphate. Tri (trifluoroethyl) phosphate, Tri (triperfluoroethyl) phosphate, 2-ethoxy-1,3,2-dioxaphosphoran-2-one, 2-trifluoroethoxy-1,3,2- Examples thereof include dioxaphosphoran-2-one and 2-methoxyethoxy-1,3,2-dioxaphosphoran-2-one.
Examples of the nitrile compound include acetonitrile and the like. Examples of the amide compound include DMF and the like. Examples of the sulfone include chain sulfones such as dimethyl sulfone and diethyl sulfone, and cyclic sulfones such as sulfolane.
One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

Among the non-aqueous solvents, lactone compounds, cyclic carbonate esters, chain carbonate esters and phosphate esters are preferable from the viewpoint of battery output and charge / discharge cycle characteristics, and nitrile compounds are preferably not contained. Further preferable are a lactone compound, a cyclic carbonate ester and a chain carbonate ester, and particularly preferable is a mixed solution of a cyclic carbonate ester and a chain carbonate ester. The most preferable is a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC), or a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC).

The positive electrode active material layer preferably further contains a conductive auxiliary agent such as conductive particles and conductive fibers.
When the positive electrode active material layer contains a conductive auxiliary agent such as conductive particles or conductive fibers, the electrical conductivity of the positive electrode active material layer can be improved. At this time, the positive electrode active material is more preferably a coated positive electrode active material described later. As the conductive particles and the conductive fibers, any material may be used as long as it can impart conductivity to the positive electrode active material layer, and those described as the conductive materials can be preferably used, but the conductive material layer of the positive electrode is conductive. Conductive fibers are more preferable from the viewpoint of improving the above.
Examples of conductive fibers include carbon fibers such as PAN-based carbon fibers and pitch-based carbon fibers, and metal fibers obtained by fiberizing metals such as stainless steel. Among these conductive fibers, carbon fibers are preferable. The content of these conductive fibers is preferably 0.5 to 10.0% by weight based on the weight of the positive electrode active material layer.
When the positive electrode active material layer further contains conductive fibers, the positive electrode active material is preferably a coated positive electrode active material described later.

The positive electrode active material is preferably a coated positive electrode active material in which a part or all of the surface of the positive electrode active material particles is coated with a coating layer containing a polymer compound and a conductive agent.
When the surface of the positive electrode active material particles is covered with a coating layer, the volume change of the positive electrode is alleviated, and the expansion of the positive electrode can be suppressed. Further, the wettability of the coated positive electrode active material with respect to a non-aqueous solvent can be improved, and the process time for absorbing the electrolytic solution into the active material layer of the electrode can be shortened.
Further, by coating a part or all of the surface of the positive electrode active material particles with a coating layer containing a polymer compound and a conductive agent, the conductivity of the positive electrode can be increased, so that the battery capacity can be increased. Become.
As the positive electrode active material particles, particles made of the above-mentioned positive electrode active material constituting the positive electrode active material layer can be used. As the conductive agent, the same conductive material as that constituting the resin current collector can be preferably used.

As the polymer compound constituting the coating layer, a polymer compound having a liquid absorption rate of 10% or more when immersed in an electrolytic solution and a tensile elongation at break in a saturated liquid absorption state of 10% or more is preferable.

The liquid absorption rate when immersed in the electrolytic solution is obtained by the following formula by measuring the weight of the polymer compound before and after immersion in the electrolytic solution.
Liquid absorption rate (%) = [(Weight of polymer compound after immersion in electrolyte solution-Weight of polymer compound before immersion in electrolyte solution) / Weight of polymer compound before immersion in electrolyte solution] × 100
The electrolytic solution for determining the liquid absorption rate is preferably a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed in a volume ratio of EC: DEC = 3: 7 and LiPF ₆ as an electrolyte is 1 mol / mol /. An electrolytic solution dissolved so as to have a concentration of L is used.
Immersion in the electrolytic solution for determining the liquid absorption rate is performed at 50 ° C. for 3 days. By immersing at 50 ° C. for 3 days, the polymer compound is in a saturated liquid absorbing state. The saturated liquid absorbing state means a state in which the weight of the polymer compound does not increase even if it is further immersed in the electrolytic solution.
The electrolytic solution used in manufacturing the lithium ion battery is not limited to the above electrolytic solution, and other electrolytic solutions may be used.

When the liquid absorption rate is 10% or more, lithium ions can easily permeate the polymer compound, so that the ion resistance in the positive electrode active material layer can be kept low. If the liquid absorption rate is less than 10%, the conductivity of lithium ions becomes low, and the performance as a lithium ion battery may not be fully exhibited.
The liquid absorption rate is preferably 20% or more, and more preferably 30% or more.
The preferable upper limit value of the liquid absorption rate is 400%, and the more preferable upper limit value is 300%.

For the tensile elongation at break in the saturated liquid absorption state, the polymer compound is punched out in a dumbbell shape and immersed in the electrolytic solution at 50 ° C. for 3 days in the same manner as the above measurement of the liquid absorption rate to saturate the polymer compound. As a state, it can be measured according to ASTM D683 (test piece shape TypeII). The tensile elongation at break is a value calculated by the following formula as the elongation at break until the test piece breaks in the tensile test.
Tensile fracture elongation rate (%) = [(Test piece length at break-Test piece length before test) / Test piece length before test] x 100

When the tensile elongation at break of the polymer compound in the saturated liquid absorption state is 10% or more, the polymer compound has appropriate flexibility, and therefore the coating layer is peeled off due to the volume change of the positive electrode active material during charging and discharging. It becomes easier to suppress that.
The tensile elongation at break is preferably 20% or more, and more preferably 30% or more.
Further, the preferable upper limit value of the tensile elongation at break is 400%, and the more preferable upper limit value is 300%.

Subsequently, the polymer compound constituting the coating layer will be described.
Examples of the polymer compound constituting the coating layer include a thermoplastic resin and a thermosetting resin. For example, vinyl resin, urethane resin, polyester resin, polyamide resin, epoxy resin, polyimide resin, silicone resin, phenol resin, etc. Examples thereof include melamine resin, urea resin, aniline resin, ionomer resin, polycarbonate, polysaccharoid (sodium alginate, etc.) and mixtures thereof. Among these, vinyl resin, urethane resin, polyester resin or polyamide resin are preferable.
Among these, polymer compounds having a liquid absorption rate of 10% or more when immersed in an electrolytic solution and a tensile elongation at break in a saturated liquid absorption state of 10% or more are more preferable.

The ratio of the total weight of the polymer compound and the conductive agent to the weight of the positive electrode active material is not particularly limited, but is preferably 2.11 to 25% by weight.

The ratio of the weight of the polymer compound to the weight of the positive electrode active material is not particularly limited, but is preferably 0.11 to 11% by weight. The ratio of the weight of the conductive agent to the weight of the positive electrode active material is not particularly limited, but is preferably 2 to 14% by weight.

The ratio of the weight of the polymer compound contained in the coating layer to the weight of the conductive agent contained in the coating layer is not particularly limited, but is preferably 1 to 10% by weight, preferably 1 to 4% by weight. It is more preferable to have.

The conductivity of the coating layer is preferably 0.001 to 10 mS / cm, more preferably 0.01 to 5 mS / cm.
The conductivity of the coating layer can be determined by the four-terminal method.
When the conductivity of the coating layer is 0.001 mS / cm or more, the electric resistance to the positive electrode active material is not high, and charging / discharging is possible.

The resin current collector constituting the positive electrode for a lithium ion battery of the present invention may have a metal layer in contact with its surface (excluding the surface in contact with the positive electrode active material layer).
The metal layer is, for example, a nickel vapor-deposited film for assisting the exchange of current between the surface of the positive electrode current collector and the outside of the battery, or an aluminum foil attached to the positive electrode current collector, and these are positive electrode current collectors. It shall not be included in the composition of.

Hereinafter, the method for manufacturing the positive electrode for the lithium ion battery of the present invention described above will be described.
As a method for producing a positive electrode for a lithium ion battery of the present invention, for example, a dispersion liquid in which a positive electrode active material and conductive fibers are dispersed at a concentration of 30 to 60% by weight based on the weight of water or a solvent to form a slurry. Is applied to the positive electrode current collector with a coating device such as a bar coater, dried to remove water or a solvent, and pressed with a press machine if necessary. The positive electrode active material layer obtained by drying the dispersion liquid does not need to be directly formed on the positive electrode current collector. For example, the positive electrode obtained by applying the dispersion liquid to the surface of an aramid separator or the like and drying it. The positive electrode for a lithium ion battery of the present invention can also be manufactured by arranging the active material layer so as to be in contact with the positive electrode current collector.

If necessary, the dispersion liquid contains a known binder resin (also referred to as a binder, for example, starch, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinylpyrrolidone, tetrafluoro) contained in a positive electrode for a conventional lithium ion battery. Polymer compounds such as ethylene, styrene-butadiene rubber, polyethylene and polypropylene) may be added.
The binder resin is a binder used for the purpose of binding the active material particles and the current collector and adhering the active materials to each other in the electrode of the lithium ion secondary battery.

The positive electrode active material layer of the positive electrode for a lithium ion battery of the present invention preferably does not contain a binder resin that adheres the positive electrode active materials to each other. In particular, when the active material contained in the positive electrode for a lithium ion battery of the present invention is a coated positive electrode active material, the conductive path is maintained by the action of the coating layer without binding the positive electrode active material particles into the electrode. Not only is it not essential to include a binder, but also because the positive electrode active material particles are not bound to the inside of the electrode by not using a binder, the volume change of the active material due to charging and discharging and relaxation against external stress are alleviated. It is preferable because the capacity is good and the durability of the electrode is improved.
Whether or not the active material layer contains a binder resin that adheres the positive electrode active materials to each other should be observed by observing whether or not the active material layer collapses when the active material layer is completely immersed in the electrolytic solution. You can check with. When the active material layer contains the binder resin, the shape can be maintained for 1 minute or more, but when the active material layer does not contain the binder resin, disintegration occurs in less than 1 minute.

As the positive electrode active material for a lithium ion battery, the conductive fiber and the positive electrode current collector used in the method for manufacturing a positive electrode for a lithium ion battery, those described in the positive electrode for a lithium ion battery of the present invention can be preferably used. ..

Examples of the solvent include 1-methyl-2-pyrrolidone, methylethylketone, DMF, dimethylacetamide, N, N-dimethylaminopropylamine, tetrahydrofuran and the like.
Examples of the binder include starch, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinylpyrrolidone, tetrafluoroethylene, styrene-butadiene rubber, polyethylene and polypropylene.

The positive electrode active material is preferably a coated positive electrode active material in which a part or all of the surface of the positive electrode active material particles is coated with a coating layer containing a polymer compound and a conductive agent.
For the coated positive electrode active material, for example, in a state where the positive electrode active material particles are placed in a universal mixer and stirred at 30 to 50 rpm, a polymer solution containing a polymer compound is added dropwise over 1 to 90 minutes, and a conductive agent is further prepared. Can be obtained by mixing, raising the temperature to 50 to 200 ° C. with stirring, reducing the pressure to 0.007 to 0.04 MPa, and then holding the mixture for 10 to 150 minutes.

The blending ratio of the positive electrode active material and the polymer compound is not particularly limited, but the weight ratio is preferably positive electrode active material: polymer compound = 1: 0.001 to 1: 0.1.

When the positive electrode active material is a coated positive electrode active material in which a part or all of the surface of the positive electrode active material particles is coated with a coating layer containing a polymer compound and a conductive agent, the above-mentioned positive electrode for a lithium ion battery is produced. It is preferable not to add the above binder to the dispersion used in the step.
The conductive auxiliary agent such as conductive particles and conductive fibers used for manufacturing the electrode is different from the conductive agent contained in the coating layer, exists outside the coating layer of the coated positive electrode active material, and is a positive electrode active material. It has a function of improving electron conductivity from the surface of the coated positive electrode active material in the layer.

When manufacturing a lithium ion battery using the positive electrode for a lithium ion battery of the present invention, a method of combining electrodes to be opposite electrodes, housing the cells together with a separator in a cell container, injecting an electrolytic solution, and sealing the cell container, etc. Can be manufactured by
Further, a positive electrode is formed on one surface of the current collector, and a negative electrode is formed on the other surface to prepare a bipolar electrode. The bipolar electrode is laminated with a separator and stored in a cell container to store an electrolytic solution. It can also be obtained by injecting and sealing the cell container.

As the separator, polyethylene, a microporous film made of polypropylene, a multilayer film of a porous polyethylene film and polypropylene, a non-woven fabric made of polyester fiber, aramid fiber, glass fiber, etc., and silica, alumina, titania, etc. on their surfaces. Examples thereof include those to which the ceramic fine particles of the above are adhered.

As the electrolytic solution, an electrolytic solution containing an electrolyte and a non-aqueous solvent used in the production of a lithium ion battery can be used, and the electrolyte and the electrolytic solution constituting the positive electrode for the lithium ion battery of the present invention are preferably used. be able to.

The lithium ion battery of the present invention is characterized by comprising the positive electrode for the lithium ion battery of the present invention. Since the lithium ion battery of the present invention includes the positive electrode for the lithium ion battery of the present invention, it is excellent in high temperature durability.

Next, the present invention will be specifically described with reference to Examples, but the present invention is not limited to the Examples as long as it does not deviate from the gist of the present invention. Unless otherwise specified, parts mean parts by weight and% means parts by weight.

<Production Example 1: Preparation of Polymer Compound for Coating and Its Solution>
407.9 parts of DMF was placed in a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel and a nitrogen gas introduction tube, and the temperature was raised to 75 ° C. Next, a monomer compounding solution containing 242.8 parts of methacrylic acid, 97.1 parts of methyl methacrylate, 242.8 parts of 2-ethylhexyl methacrylate, and 116.5 parts of DMF, and 2,2'-azobis (2,4-dimethyl). While blowing nitrogen into a four-necked flask, an initiator solution prepared by dissolving 1.7 parts of valeronitrile) and 4.7 parts of 2,2'-azobis (2-methylbutyronitrile) in 58.3 parts of DMF was added to the flask. Under stirring, radical polymerization was carried out by continuously dropping with a dropping funnel over 2 hours. After completion of the dropping, the reaction was continued at 75 ° C. for 3 hours. Then, the temperature was raised to 80 ° C. and the reaction was continued for 3 hours to obtain a copolymer solution having a resin concentration of 50%. 789.8 parts of DMF was added thereto to obtain a polymer compound solution for coating having a resin solid content concentration of 30% by weight.

<Example 1>
[Preparation of coated positive electrode active material particles]
Place 100 parts of the positive electrode active material powder (LiNi _0.8 Co _0.15 Al _0.05 O ₂ powder, volume average particle diameter 4 μm) in the universal mixer High Speed Mixer FS25 [manufactured by EarthTechnica Co., Ltd.] at room temperature. With stirring at 720 rpm, 6.1 parts of the polymer compound solution for coating obtained in Production Example 1 was added dropwise over 2 minutes, and the mixture was further stirred for 5 minutes.
Then, while stirring, 6.1 parts of acetylene black [Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.], which is a conductive agent, was added in 2 minutes while being divided, and stirring was continued for 30 minutes. Then, the pressure was reduced to 0.01 MPa while maintaining the stirring, then the temperature was raised to 140 ° C. while maintaining the stirring and the degree of decompression, and the stirring, the degree of decompression and the temperature were maintained for 8 hours to distill off the volatile components. .. The obtained powder was classified by a sieve having an opening of 212 μm to obtain coated positive electrode active material particles (P-1).

[Preparation of resin current collector]
With a twin-screw extruder, 70 parts of polypropylene [trade name "SunAllomer PL500A", manufactured by SunAllomer Ltd.], 25 parts of carbon nanotubes [trade name: "FloTube9000", manufactured by CNano] and dispersant [trade name "Umex 1001" , Sanyo Kasei Kogyo Co., Ltd.] 5 parts were melt-kneaded under the conditions of 200 ° C. and 200 rpm to obtain a resin mixture.
The obtained resin mixture was passed through a T-die extrusion film forming machine and stretched and rolled to obtain a resin current collector having a film thickness of 100 μm.

[Manufacturing of lithium-ion batteries for positive electrode evaluation]
Lithium produced by dissolving _{lithium bis (fluorosulfonyl) imide [LiN (FSO 2} ) ₂ ] in a mixed solvent (volume ratio 1: 1) of ethylene carbonate (EC) and diethyl carbonate (DEC) at a ratio of 1 mol / L. 42 parts of electrolyte for ion batteries and 4.2 parts of carbon fiber [Donacabo Milled S-243 manufactured by Osaka Gas Chemical Co., Ltd .: average fiber length 500 μm, average fiber diameter 13 μm: electrical conductivity 200 mS / cm] are stirred by planetary. Using a mold mixing and kneading device {Awatori Kentarou [manufactured by Shinky Co., Ltd.]}, the mixture was mixed at 2000 rpm for 7 minutes, and then 30 parts of the above electrolytic solution and 206 parts of the coated positive electrode active material particles (P-1) were added. After that, the mixture was further mixed with Awatori Rentaro at 2000 rpm for 1.5 minutes, and after adding 20 parts of the above electrolytic solution, stirring by Awatori Rentaro was performed at 2000 rpm for 1 minute, and then 2.3 parts of the above electrolytic solution was added. Was further added and then stirred by Awatori Rentaro at 2000 rpm for 1.5 minutes to prepare a positive electrode active material slurry. The obtained positive electrode active material slurry was applied to one side of a resin current collector having a thickness of 100 μm, pressed at a pressure of 10 MPa for about 10 seconds, and the thickness was about 450 μm (thickness of the positive electrode active material layer was 350 μm). A positive electrode material (3 cm × 3 cm) according to Example 1 was produced.
The same carbon-coated aluminum foil (3 cm x 3 cm) with terminals (5 mm x 3 cm), one separator (made of Cellguard 2500 PP) (5 cm x 5 cm), and copper foil (3 cm x 3 cm) with terminals (5 mm x 3 cm). Laminate in order so that the two terminals come out in the direction, sandwich it between two commercially available heat-sealing aluminum laminate films (8 cm x 8 cm), and heat-fuse one side where the terminals come out to evaluate the positive electrode. Laminated cells were prepared.
Next, the positive electrode material (3 cm × 3 cm) according to Example 1 is inserted between the carbon-coated aluminum foil and the separator in the direction in which the carbon-coated aluminum foil of the laminated cell and the resin current collector of the positive electrode material are in contact with each other, and further, the electrodes are further inserted. 70 μL of the electrolytic solution was injected onto the electrode to absorb the electrolytic solution into the electrode. Next, 70 μL of the electrolytic solution was injected onto the separator. Then, a lithium foil was inserted between the separator and the copper foil, and the two sides orthogonal to the first heat-sealed side were heat-sealed.
After that, 70 μL of an electrolytic solution is further injected from the opening, and the laminated cell is sealed by heat-sealing the opening while evacuating the inside of the cell using a vacuum sealer to prepare a positive electrode according to Example 1 and to prepare a positive electrode. A lithium ion battery 1 for positive electrode evaluation was obtained.
In the positive electrode according to the first embodiment, a positive electrode active material layer is formed on one surface of a positive electrode current collector, and the positive electrode active material layer faces a lithium foil as a counter electrode via a separator. The other surface of the positive electrode current collector is in contact with the carbon coated aluminum foil with terminals.
The high temperature durability and initial output characteristics were measured by the following methods, and the results are shown in Table 1.

<Example 2>
A positive electrode for a lithium ion battery and a lithium ion battery for evaluation of a positive electrode according to Example 2 in the same procedure as in Example 1 except that the electrolyte concentration of the electrolytic solution produced in Example 1 was changed from 1 mol / L to 5 mol / L. 2 was produced.

<Example 3>
A positive electrode for a lithium ion battery and a lithium ion battery for evaluation of a positive electrode according to Example 3 in the same procedure as in Example 1 except that the electrolyte concentration of the electrolytic solution produced in Example 1 was changed from 1 mol / L to 2 mol / L. 3 was prepared.

<Example 4>
The electrolyte of the electrolyte prepared in Example 1 was changed from [LiN (FSO ₂ ) ₂ ] to lithium bis (trifluoromethanesulfonyl) imide [LiN (CF ₃ SO ₂ ) ₂ ], and the electrolyte concentration was changed from 1 mol / L. A positive electrode for a lithium ion battery and a lithium ion battery 4 for evaluation of a positive electrode according to Example 4 were produced in the same procedure as in Example 1 except that the value was changed to 2 mol / L.

<Example 5>
The electrolyte of the electrolyte prepared in Example 1 was changed from [LiN (FSO ₂ ) ₂ ] to lithium bis (pentafluoroethanesulfonyl) imide [LiN (C ₂ F ₅ SO ₂ ) ₂ ], and the electrolyte concentration was 1 mol. A positive electrode for a lithium ion battery and a lithium ion battery 5 for evaluating a positive electrode according to Example 5 were produced in the same procedure as in Example 1 except that the value was changed from / L to 2 mol / L.

<Example 6>
The electrolyte of the electrolyte prepared in Example 1 is a mixture of [LiN (FSO ₂ ) ₂ ] to [LiN (FSO ₂ ) ₂ ]: [LiN (CF ₃ SO ₂ ) ₂ ] = 3: 1 (molar ratio). The positive electrode for the lithium ion battery and the lithium ion battery 6 for evaluation of the positive electrode according to Example 6 were produced in the same procedure as in Example 1 except that the electrolyte concentration was changed from 1 mol / L to 2 mol / L.

<Example 7>
Examples except that the carbon fiber used in Example 1 was changed to acetylene black [Denka Black (registered trademark) NH-100, manufactured by Denka Co., Ltd.] and the electrolyte concentration was changed from 1 mol / L to 2 mol / L. The positive electrode for the lithium ion battery and the lithium ion battery 7 for evaluation of the positive electrode according to Example 7 were produced by the same procedure as in 1.

<Example 8>
The coated positive electrode active material particles (P-1) used in Example 1 are not coated with the uncoated positive electrode active material powder (LiNi _0.8 Co _0.15 Al _0.05 O ₂ powder, volume average particle diameter 4 μm) ( The positive electrode for the lithium ion battery and the lithium ion battery for evaluation of the positive electrode 8 according to Example 8 were carried out in the same procedure as in Example 1 except that the change was made to P-2) and the electrolyte concentration was changed from 1 mol / L to 2 mol / L. Was produced.

<Example 9>
The coated positive electrode active material particles (P-1) used in Example 1 were added to the uncoated positive electrode active material powder (P-2), and carbon fibers were added to acetylene black [Denka Black (registered trademark) NH-100, Denka. Made by Co., Ltd.], and the same procedure as in Example 1 except that the electrolyte concentration was changed from 1 mol / L to 2 mol / L. A battery 9 was manufactured.

<Comparative Examples 1 to 4>
Comparative evaluation of positive electrodes according to Comparative Examples 1 to 4 in the same procedure as in Example 1 except that the type and concentration of the electrolyte in the electrolytic solution for the lithium ion battery and the type of the current collector were changed to those shown in Table 1. Lithium-ion batteries 1 to 4 for use were prepared, high-temperature durability and initial output characteristics were measured by the following methods, and the results are shown in Table 1. The type of current collector described as resin indicates that it is the same resin current collector as in Example 1, and the type described as metal (Al) is a carbon coat having a thickness of 50 μm as a current collector. Indicates that aluminum foil was used.

<Measurement of high temperature durability of lithium-ion batteries>
Evaluation of lithium ion batteries 1 to 9 for positive electrode evaluation and lithium ion batteries 1 to 4 for positive electrode comparison evaluation by the following method using the charge / discharge measuring device "HJ-SD8" [manufactured by Hokuto Denko Co., Ltd.] at 70 ° C. Was done. The evaluation was carried out by allowing the lithium ion batteries 1 to 9 for positive electrode evaluation and the lithium ion batteries 1 to 4 for positive electrode comparison evaluation to stand in a constant temperature bath at 70 ° C., respectively.
It was charged to 4.2 V with a current of 0.05 C by a constant current constant voltage charging method (also referred to as CCCV mode), and after a 10-minute rest, it was discharged to 2.5 V with a current of 0.05 C.
The above charge and discharge were repeated for 10 cycles, and the discharge capacity (%) of the 10th cycle was defined as high temperature durability (10 cycles) with respect to the discharge capacity of the 1st cycle.

<Measurement of initial output characteristics of lithium-ion batteries>
Evaluation of lithium ion batteries 1 to 9 for positive electrode evaluation and lithium ion batteries 1 to 4 for positive electrode comparison evaluation by the following method using the charge / discharge measuring device "HJ-SD8" [manufactured by Hokuto Denko Co., Ltd.] at 45 ° C. Was done. The evaluation was carried out by allowing the lithium ion batteries 1 to 9 for positive electrode evaluation and the lithium ion batteries 1 to 4 for positive electrode comparison evaluation to stand in a constant temperature bath at 45 ° C., respectively.
It was charged to 4.2 V with a current of 0.05 C by a constant current constant voltage charging method (also referred to as CCCV mode), and after a 10-minute rest, it was discharged to 2.5 V with a current of 0.05 C.
Then, it was charged to 4.2 V with a current of 0.05 C, paused for 10 minutes, and then discharged to 2.5 V with a current of 1 C.
The discharge capacity (%) at a 1C current value with respect to the charge capacity in 0.05C charging was defined as the initial output characteristic (1C / 0.05C).

From the results in Table 1, it can be seen that the lithium ion battery using the positive electrode for the lithium ion battery of the present invention is excellent in initial output characteristics, particularly high temperature durability.

The positive electrode for a lithium ion battery of the present invention is particularly useful as a positive electrode for a bipolar secondary battery and a lithium ion battery used for a mobile phone, a personal computer, a hybrid vehicle and an electric vehicle.

Claims (4)

A positive electrode for a lithium ion battery including a positive electrode current collector, a positive electrode active material layer formed on the surface of the positive electrode current collector, and an electrolytic solution composed of an electrolyte containing lithium ions and a non-aqueous solvent.
The positive electrode active material layer further contains carbon fibers and contains
The positive electrode active material constituting the positive electrode active material layer is a coated positive electrode active material in which a part or all of the surface of the positive electrode active material particles is coated with a coating layer containing a polymer compound and a conductive agent.
The positive electrode current collector is a resin current collector made of a conductive material and a resin.
The electrolyte consists only of lithium bis (fluorosulfonyl) imide, and the electrolyte concentration of the electrolyte solution is 1 . A positive electrode for a lithium ion battery, characterized by having a concentration of 5 to 4 mol / L. The positive electrode for a lithium ion battery according to claim 1, wherein the positive electrode active material layer does not contain a binder resin that adheres positive electrode active material particles to each other. The positive electrode for a lithium ion battery according to claim 1 or 2, wherein the electrolyte concentration of the electrolytic solution is 2 to 3 mol / L. A lithium ion battery comprising the positive electrode for a lithium ion battery according to any one of claims 1 to 3.