JP2013004229A - Binder for lithium secondary battery electrode, method for producing binder for lithium secondary battery electrode, lithium secondary battery electrode, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a binder for a lithium secondary battery electrode, capable of forming an active material layer excellent in adhesiveness with a collector and also excellent in flexibility.SOLUTION: A binder for a lithium secondary battery electrode includes a first polymer obtained by copolymerizing a first monomer mixture (1) containing 0.01-30 mass% of a carboxyl group-containing monomer (a-1) and 20-60 mass% of acrylonitrile (a-2), with the remainder being a copolymerizable monomer (a-3).

Description

  The present invention relates to a method for producing a binder for a lithium secondary battery electrode in which an active material layer excellent in adhesiveness with a current collector and having excellent flexibility is obtained, and a binder for a lithium secondary battery electrode obtained using the same The present invention relates to a slurry for a lithium secondary battery electrode, a lithium secondary battery electrode, and a lithium secondary battery.

  In recent years, portable information terminals such as notebook personal computers, mobile phones, and PDAs have become widespread. Furthermore, electric vehicles are being actively developed from the viewpoint of reducing environmental impact. Lithium secondary batteries having high energy density are widely used as power sources for these portable information terminals and electric vehicles.

  Lithium-containing metal composite oxides are mainly used as positive electrode active materials for lithium secondary batteries. As the negative electrode active material, a carbon-based material having a multilayer structure capable of inserting lithium ions between layers (forming a lithium intercalation compound) and releasing is used. The electrode of the lithium secondary battery is prepared by kneading these active materials, a binder and a solvent to prepare a slurry, which is applied to one or both sides of a metal foil as a current collector with a transfer roll or the like, and the solvent is dried. After removing and forming an active material layer, it is produced by compression molding with a roll press or the like, if necessary.

  Conventionally, styrene-butadiene rubber (hereinafter abbreviated as “SBR”) and polyvinylidene fluoride (hereinafter abbreviated as PVDF) have been used as binders for lithium secondary batteries. However, SBR and PVDF have poor adhesion to metal foils (aluminum foil, copper foil, etc.) that are current collectors. For this reason, in order to ensure sufficient adhesive force with the current collector of the active material layer, a large amount of binder must be blended with the active material. The binder contained in a large amount in the active material layer is a factor that hinders the increase in capacity of the lithium secondary battery by covering the active material and inhibiting the battery reaction of the active material or increasing the internal resistance of the lithium battery. Become.

  As binders capable of forming an active material layer having high adhesiveness with a current collector, there are those disclosed in Patent Document 1 and Patent Document 2. When these binders are used, the adhesiveness between the active material layer and the current collector is improved as compared with SBR and PVDF.

JP-A-8-287915 JP 2001-332265 A

However, even when the binders disclosed in Patent Document 1 and Patent Document 2 are used, in order to obtain sufficient adhesion between the active material layer and the current collector, the binder is still attached to the active material. Must be blended in large amounts, which hinders the increase in capacity of lithium secondary batteries.
In addition, an electrode formed by using a conventional binder to form an active material layer is insufficient in flexibility of the active material layer. Therefore, in the manufacturing process of a lithium secondary battery, the electrode is spirally wound through a separator. When rotating, there is a problem that part or all of the active material layer easily peels off from the current collector.

Therefore, in the conventional binder for lithium secondary batteries, it is required to improve the adhesiveness with the current collector in the active material layer using the binder and to improve the flexibility.
The present invention has been made in view of the above circumstances, and has a binder for a lithium secondary battery electrode that is capable of forming an active material layer that is excellent in adhesiveness with a current collector and that is excellent in flexibility, and a method for producing the same. It is an object to provide a slurry for a lithium secondary battery electrode used.
Also provided are a lithium secondary battery electrode and a lithium secondary battery having an active material layer excellent in adhesiveness and flexibility with a current collector obtained using such a binder for a lithium secondary battery electrode. Is an issue.

  As a result of intensive studies on the above problems, the present inventors have found that the above problems can be solved, and have completed the present invention.

1. A monomer (a-3) containing 0.01 to 30% by mass of a carboxyl group-containing monomer (а-1) and 20 to 60% by mass of acrylonitrile (а-2), the remainder being copolymerizable The binder for lithium secondary battery electrodes containing the 1st polymer obtained by copolymerizing the 1st monomer mixture (1) which becomes.
2. In the presence of the first polymer, one or both of the carboxyl group-containing monomer (b-1) and the phosphate group-containing monomer (b-2) can be copolymerized with 50% by mass or more. The second monomer mixture (2) composed of 0 to 50% by mass of the monomer (b-3) is changed to a mass ratio (first monomer) of the first monomer mixture (1) and the second monomer mixture (2). 2. The binder for a lithium secondary battery electrode according to 1 above, comprising a second polymer obtained by polymerizing the mixture (1) / second monomer mixture (2) as 99.99 / 0.01 to 80/20.

3. 3. A lithium secondary battery electrode slurry comprising the binder for a lithium secondary battery electrode according to the above 1 or 2, an active material, and a solvent.
4). 4. The slurry for a lithium secondary battery electrode according to 3 above, which contains a polyfunctional epoxy compound.
5. 5. A lithium secondary battery electrode, wherein the slurry for a lithium secondary battery electrode according to the above 3 or 4 is applied to a current collector and dried.
6). A lithium secondary battery comprising the electrode for a lithium secondary battery as described in 5 above.

7). A monomer (a-3) containing 0.01 to 30% by mass of a carboxyl group-containing monomer (а-1) and 20 to 60% by mass of acrylonitrile (а-2), the remainder being copolymerizable The manufacturing method of the binder for lithium secondary battery electrodes including the 1st superposition | polymerization process of copolymerizing the 1st monomer mixture (1) which becomes, and obtaining a 1st polymer.
8). In the presence of the first polymer, one or both of the carboxyl group-containing monomer (b-1) and the phosphate group-containing monomer (b-2) can be copolymerized with 50% by mass or more. The second monomer mixture (2) composed of 0 to 50% by mass of the monomer (b-3) is changed to a mass ratio (first monomer) of the first monomer mixture (1) and the second monomer mixture (2). 8. The lithium secondary battery comprising the above-mentioned 7, comprising a second polymerization step in which the mixture (1) / second monomer mixture (2)) is polymerized as 99.99 / 0.01 to 80/20 to obtain a second polymer. Manufacturing method of binder for electrodes.

  According to the binder for a lithium secondary battery electrode of the present invention, an active material layer excellent in adhesiveness with a current collector can be formed. Therefore, when forming the active material layer of the lithium secondary battery electrode using the binder for the lithium secondary battery electrode of the present invention, the amount of the binder to be used relative to the active material is small compared to the conventional, and the active material layer Since the content of the active material in can be increased, the capacity of the lithium secondary battery can be increased.

  Moreover, according to the binder for lithium secondary battery electrodes of this invention, the active material layer excellent in the softness | flexibility can be formed. In the present invention, the effect of improving the flexibility of the active material layer caused by the first polymer and / or the second polymer contained in the binder for the lithium secondary battery electrode and the amount of the binder used for the lithium secondary battery electrode are small. Due to the synergistic effect with the effect of improving the flexibility of the active material layer resulting from the above, an active material layer having very excellent flexibility can be obtained. Therefore, when a lithium secondary battery is manufactured using an electrode formed with an active material layer using the binder for a lithium secondary battery electrode of the present invention, the active material layer is removed from the current collector in the course of manufacturing. Lithium secondary batteries can be manufactured with good yield, which is difficult to peel off or fall off.

(Binder for lithium secondary battery electrode)
"First polymer"
The lithium secondary battery electrode binder of the present invention (hereinafter abbreviated as “binder”) is a carboxyl group-containing monomer (а-1) (hereinafter abbreviated as “a-1” component) of 0.01 to 30 mass. % And acrylonitrile (а-2) (hereinafter abbreviated as “component (a-2)”) 20 to 60% by mass, the remainder being copolymerizable with component (a-1) and component (a-2) Monomer (a-3) (hereinafter abbreviated as component (a-3)) 10 to 79.99 mass% of the first monomer mixture (1) (however, in the first monomer mixture (1) ( the total of a-1) to (a-3) is 100% by mass).

  Examples of the component (a-1) include acrylic carboxyl group-containing monomers such as acrylic acid and methacrylic acid, croton carboxyl group-containing monomers such as crotonic acid, maleic acid, and anhydrides thereof. Maleic carboxyl group-containing monomers, itaconic carboxyl group-containing monomers such as itaconic acid and its anhydride, citraconic carboxyl group-containing monomers such as citraconic acid and its anhydride, fumaric acid, mesacone Examples include acid, cinnamic acid, succinic acid mono (2- (meth) acryloyloxyethyl), and ω-carboxy-polycaprolactone mono (meth) acrylate. Among these, acrylic acid or methacrylic acid is preferable in terms of ease of polymerization. These (a-1) components may be used individually by 1 type, and may use 2 or more types together.

Specific examples of the component (a-3) include methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, alkyl (meth) acrylate such as 2-ethylhexyl (meth) acrylate, hydroxyethyl ( Monohydroxy-containing (meth) acrylates such as (meth) acrylate, hydroxybutyl (meth) acrylate, hydroxypropyl (meth) acrylate, and 1,2-dihydroxyethyl (meth) acrylate having two or more hydroxyl groups in one molecule; 1,2-dihydroxypropyl (meth) acrylate, 1,2-dihydroxybutyl (meth) acrylate, 1,2-dihydroxy5-ethylhexyl (meth) acrylate, 1,2,3-trihydroxypropyl (meth) acrylate, 1 , 2,3- Rehydroxybutyl (meth) acrylate, 1,1-dihydroxyethyl (meth) acrylate, 1,1-dihydroxypropyl (meth) acrylate, 1,1-dihydroxybutyl (meth) acrylate, 1,1,2-trihydroxypropyl (Meth) acrylate, 1,1,2-trihydroxybutyl (meth) acrylate, glycidyl (meth) acrylate, styrene, α-methylstyrene, acrylonitrile, acrylamide, methacrylamide, diethylaminoethyl (meth) acrylate, dimethylaminoethyl ( (Meth) acrylate, vinyl acetate, ethoxyethyl (meth) acrylate, methoxyethyl (meth) acrylate, ethoxydiethylene glycol (meth) acrylate, methoxytriethylene glycol (meth) Acrylate, phenoxydiethylene glycol (meth) acrylate, ethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, 1,3-butylene glycol di (meth) acrylate, 1,4-di (meth) acrylic acid 1,4- Butanediol, 1,6-hexanediol di (meth) acrylate, neopentyl glycol di (meth) acrylate, neopentyl glycol hydroxypivalate di (meth) acrylate, polypropylene glycol di (meth) acrylate, di (Meth) acrylic acid polytetramethylene glycol, trimethylolpropane tri (meth) acrylic acid ester, ethoxylated trimethylolpropane tri (meth) acrylic acid ester, propoxylated trimethylol propylene Tri (meth) acrylate, glycerin tri (meth) acrylate, ethoxylated glycerin tri (meth) can be mentioned acrylic acid esters and the like.
Among these, alkyl (meth) acrylate is preferable from the viewpoint of easy copolymerization of the components (a-1) and (a-2). These (a-3) components may be used individually by 1 type, and may use 2 or more types together.

The first monomer mixture (1) contains 0.01 to 30% by mass of the (а-1) component and 20 to 60% by mass of the (а-2) component, with the balance being the component (a-3) 10 to 79. .99% by mass.
The component (a-1) is a total of 100% by mass of the component (a-1) to the component (a-3) (in other words, 100% by mass of the first monomer mixture (1)), 0.01 to 30% by mass. It is contained in the range. By setting the content of the component (a-1) to 0.01% by mass or more, the active material layer obtained using the binder can be given adhesiveness with the current collector. (A-1) As for the lower limit of content of a component, 3 mass% or more is more preferable, More preferably, it is 7 mass% or more. When the content of the component (a-1) is 7% by mass or more, the adhesiveness of the active material layer obtained using the binder to the current collector tends to be further improved. (A-1) As for the upper limit of content of a component, 30 mass% or less is preferable. If content of (a-1) component is 30 mass% or less, a softness | flexibility can be given to the active material layer obtained using the binder of this invention. There exists a tendency for the softness | flexibility of an active material layer to improve, so that there is little content of (a-1) component.

  The (а-2) component is contained in the range of 20 to 60% by mass in a total of 100% by mass of the (a-1) component to the (a-3) component. When the component (a-2) is within the above range, both the adhesiveness and flexibility to the current collector can be imparted to the active material layer obtained using the binder. The component (a-2) is more preferably contained in an amount of 22% by mass or more, more preferably 28% by mass or more, in a total of 100% by mass of the components (a-1) to (a-3). . In addition, the component (a-2) is more preferably contained in an amount of 50% by mass or less in a total of 100% by mass of the components (a-1) to (a-3), and is contained in a range of 40% by mass or less. More preferably. When the component (a-2) is 28% by mass or more, the adhesiveness of the active material layer obtained using the binder to the current collector tends to be further improved. When the component (a-2) is 40% by mass or less, the flexibility of the active material layer obtained using the binder of the present invention tends to be further improved.

  The (а-3) component is contained in a range of 10 to 79.99 mass% in a total of 100 mass% of the components (a-1) to (a-3). The component (a-3) is preferably contained in an amount of 10% by mass or more, more preferably 40% by mass or more, in a total of 100% by mass of the components (a-1) to (a-3). . Further, the component (a-3) is more preferably 79% by mass or less in a total of 100% by mass of the components (a-1) to (a-3), and is contained in a range of 75% by mass or less. More preferably. When the component (a-3) is 75% by mass or less, the content of the component (a-1) and / or the component (a-2) is relatively increased, and the active material layer obtained using the binder There is a tendency that the adhesion to the current collector is further improved. Moreover, when the component (a-3) is 40% by mass or more, the flexibility of the active material layer obtained using the binder of the present invention tends to be further improved.

"Production method"
In order to produce the binder of the present invention containing the first polymer, a first polymerization step is performed in which the first monomer mixture (1) is copolymerized to obtain the first polymer.
The method for copolymerizing the first monomer mixture (1) is not particularly limited, and examples thereof include known methods such as bulk polymerization, suspension polymerization, emulsion polymerization, and solution polymerization. Of these, emulsion polymerization is preferred from the viewpoints of ease of synthesis of the first polymer, ease of post-treatment such as recovery and purification, and selectivity of the slurry solvent including water.

When copolymerizing the first monomer mixture (1) by emulsion polymerization, heating and polymerizing a material to be a first polymer obtained by mixing the first monomer mixture (1), an emulsifier and a polymerization initiator. It is preferable to obtain an emulsion containing the first polymer.
Examples of the emulsifier include an anionic emulsifier (sodium dodecylbenzenesulfonate, sodium lauryl sulfonate, sodium lauryl sulfate, etc.), an anionic emulsifier containing a polyoxyethylene group, a nonionic emulsifier (polyoxyethylene nonylphenyl ether, polyoxy Ethylene lauryl ether, etc.) and reactive emulsifiers having a vinyl polymerizable double bond in the molecule are used, but there is no particular limitation.

  Examples of the polymerization initiator include peroxides such as benzoyl peroxide, cumene hydroperoxide, and hydrogen peroxide; azo compounds such as azobisisobutyronitrile; persulfate compounds such as ammonium persulfate and potassium persulfate; A redox initiator composed of a chloric acid compound, a perboric acid compound or a combination of a peroxide and a reducing sulfoxy compound is used, and there is no particular limitation.

Moreover, the material used as the 1st polymer in the case of copolymerizing the 1st monomer mixture (1) by emulsion polymerization may contain a chain transfer agent. As the chain transfer agent, mercaptans such as n-dodecyl mercaptan, tert-dodecyl mercaptan, n-octyl mercaptan, n-tetradecyl mercaptan, n-hexyl mercaptan, and the like can be used.
Furthermore, the material to be the first polymer in the case of copolymerizing the first monomer mixture (1) by emulsion polymerization may contain a solvent such as water.

  Further, when copolymerizing the first monomer mixture (1) by emulsion polymerization, an emulsion containing seed particles is prepared in a container of a polymerization apparatus, and then a material that becomes the first polymer is added to the emulsion containing seed particles. It is preferable to produce an emulsion containing the first polymer by a method in which it is dropped little by little and polymerized by heating at a predetermined temperature. When the first polymer is produced by such a method, the particle diameter of the first polymer can be easily controlled.

  Seed particles are a method of producing an emulsion containing seed particles by charging a small amount of a material to be a first polymer into a container of a polymerization apparatus and heating and copolymerizing while stirring all at once. Is obtained. The material that becomes the first polymer used when producing the seed particles is a part of the material that is polymerized to become the first polymer, and is dropped into the container of the polymerization apparatus in which the seed particles are present. The composition may be the same as or different from the material that forms one polymer. The seed particles may be produced either in an air atmosphere or in an inert gas atmosphere such as nitrogen.

The amount of the material used as the first polymer when producing seed particles is preferably in the range of 0.1% by mass to 15% by mass.
Moreover, the heating temperature at the time of producing the seed particles is not particularly limited, but it is preferable to set the temperature range of the polymerization initiator to 10-hour half-life temperature ± 30 ° C. or less. The 10-hour half-life temperature is a temperature at which the time for reducing the concentration of the polymerization initiator to half of the initial value is 10 hours.
Moreover, it is preferable to perform the heating at the time of producing the seed particles while stirring the material to be the first polymer in order to reduce the variation in the particle diameter of the seed particles.

Further, the dropping speed when dropping the material that becomes the first polymer onto the seed particles is included in the material that becomes the first polymer when the total amount of monomers contained in the material that becomes the first polymer is 100% by mass. It is preferable that the monomer be dropped at a rate of 4% by mass or more per hour. Thereby, the first polymer capable of forming an active material layer excellent in adhesiveness and flexibility with the current collector can be obtained. Note that the entire amount of the material to be the first polymer may be charged all at once.
Moreover, although the heating temperature at the time of manufacturing a 1st polymer is not specifically limited, In order to obtain the 1st polymer which can form the active material layer excellent in adhesiveness and a softness | flexibility with a collector, Similar to the heating temperature at the time of producing the seed particles, it is not particularly limited, but it is preferable that the polymerization initiator has a 10-hour half-life temperature of ± 30 ° C. or less.

  In addition, the manufacturing method in the case of copolymerizing the first monomer mixture (1) by emulsion polymerization is not limited to a method including a step of manufacturing seed particles. For example, the first polymer is contained in a container of a polymerization apparatus. The first polymer may be produced by charging the total amount of the material to be obtained and copolymerizing the first monomer mixture (1) in a lump.

"Second polymer"
In the presence of the first polymer, the binder of the present invention comprises a carboxyl group-containing monomer (b-1) (hereinafter abbreviated as (b-1) component) and a phosphate group-containing monomer (b-2). ) (Hereinafter abbreviated as component (b-2)) and 50% by mass or more of copolymerizable monomer (b-3) (hereinafter abbreviated as component (b-3)). .) The second monomer mixture (2) consisting of 0 to 50% by mass is a mass ratio of the first monomer mixture (1) and the second monomer mixture (2) (first monomer mixture (1) / second It may contain a second polymer obtained by polymerizing the two-monomer mixture (2)) as 99.99 / 0.01 to 80/20.

As the component (b-1), the same component as the component (a-1) can be used.
Examples of the component (b-2) include 2-methacryloyloxyethyl phosphate, 2-methacryloyloxyethyl diphenyl phosphate (meth) acrylate, trimethacryloyloxyethyl phosphate (meth) acrylate, triacryloyloxyethyl phosphate (meth) acrylate, and the like. I can do it. These (b-2) components may be used individually by 1 type, and may use 2 or more types together.

  As a specific example of the component (b-3), the same component as the component (a-3) can be used. Moreover, acrylonitrile (a-2), methacrylonitrile, etc. can be used as (b-3) component other than the thing similar to (a-3) component. These (b-3) components may be used individually by 1 type, and may use 2 or more types together.

A 2nd monomer mixture (2) consists of 50 mass% or more of any one or both of (b-1) component and (b-2) component, and (b-3) component 0-50 mass%. The component (b-3) is a component that can be appropriately selected and used as necessary.
The component (b-1) and the component (b-2) are contained in order to obtain a binder capable of forming an active material layer excellent in adhesiveness with the current collector. The total content of the component (b-1) and the component (b-2) is 100% by mass in total of the components (b-1) to (b-3) (in other words, the second monomer mixture (2) 100). Preferably in the range of 50 to 100% by mass). By making the total content of the component (b-1) and the component (b-2) 50% by mass or more, the active material layer obtained using the binder can be provided with adhesiveness to the current collector. I can do it.

  The total content of the component (b-1) and the component (b-2) is contained in the range of 60 to 100% by mass in the total 100% by mass of the component (b-1) to the component (b-3). It is more preferable that it is contained in the range of 90 to 100% by mass. When the total content of the component (b-1) and the component (b-2) is 90% by mass or more, the adhesiveness of the active material layer obtained using the binder to the current collector tends to be further improved. is there.

  The component (b-3) is preferably contained in the range of 0 to 50% by mass in a total of 100% by mass of the components (b-1) to (b-3). The component (b-3) is more preferably contained in a range of 10% by mass to 40% by mass in a total of 100% by mass of the component (b-1) to the component (b-3). When the component (b-3) is 10% by mass or more, the adhesive of the active material layer obtained using the binder to the current collector can be sufficiently obtained, and the active material layer can be given flexibility. I can do it.

  The mass ratio (first monomer mixture (1) / second monomer mixture (2)) between the monomer mixture (1) and the monomer mixture (2) contained in the first polymer present when the second polymer is polymerized is: 99.99 / 0.01 to 80/20. When the mass ratio of the first monomer mixture (1) and the second monomer mixture (2) is within the above range, the active material layer obtained using the binder has adhesiveness and flexibility with the current collector. Can be given together. The mass ratio of the first monomer mixture (1) and the second monomer mixture (2) is more preferably in the range of 99.9 / 0.1 to 90/10. When it is within the above range, the adhesiveness and flexibility of the active material layer obtained using the binder with the current collector can be further improved.

"Production method"
In order to produce the binder of the present invention containing the second polymer, in the presence of the first polymer, the second monomer mixture (2) is added to the mass of the first monomer mixture (1) and the second monomer mixture (2). A second polymerization step is performed by polymerizing the ratio (first monomer mixture (1) / second monomer mixture (2)) to 99.99 / 0.01 to 80/20 to obtain a second polymer.

  In the present invention, when the first polymer is synthesized by performing the first polymerization step, and the second polymer is synthesized in the presence of the first polymer in the second polymerization step, the first polymer becomes a core. It is presumed that a binder having a core-shell structure in which the second polymer surrounds the first polymer is obtained. And since the binder has a core-shell structure, it is presumed that the active material layer formed using the binder will be more excellent in adhesiveness and flexibility with the current collector.

As a method for polymerizing the second monomer mixture (2), the second monomer mixture (2) is contained in an emulsion containing the first polymer obtained by copolymerizing the first monomer mixture (1) by emulsion polymerization. It is preferable to use a method in which the material to be the second polymer is dropped and polymerized in small portions.
The dropping speed when dropping the material to be the second polymer into the emulsion containing the first polymer is the second polymer when the total amount of monomers contained in the material to be the second polymer is 100% by mass. It is preferable that the monomer contained in the material is dropped at a rate of 4% by mass or more per hour. As a result, the first polymer becomes a core (core), and the first polymer has a core-shell structure surrounded by the second polymer, so that an active material layer excellent in adhesiveness and flexibility with the current collector can be formed. It is presumed that the synthesis of the second polymer is promoted. Note that the entire amount of the material to be the second polymer may be charged all at once.
Further, the heating temperature at the time of producing the second polymer is not particularly limited, but the second polymer having a core-shell structure capable of forming an active material layer excellent in adhesiveness and flexibility with the current collector is used. In order to obtain it, it is preferable to set it as the temperature range of 10-hour half-life temperature +/- 30 degreeC or less of a polymerization initiator.

It is preferable that the weight average molecular weight (Mw) of the binder for lithium secondary battery electrodes of the present invention containing the first polymer and / or the second polymer is 10,000 or more. When Mw is 10,000 or more, both the adhesiveness and flexibility to the current collector can be imparted to the active material layer obtained using the binder for the lithium secondary battery electrode.
Mw can be measured using gel permeation chromatography (GPC). For example, it can be determined as a molecular weight in terms of polystyrene using a solvent such as tetrahydrofuran as an eluent.

Moreover, it is preferable that the volume average particle diameter of the polymer particle (1st polymer and / or 2nd polymer) of the binder for lithium secondary battery electrodes of this invention is 1 nm-10 micrometers. When the volume average particle diameter of the polymer particles is within the above range, the polymer particles work well as a binder and the battery resistance is lowered.
Here, the volume average particle diameter is a value measured using a light scattering photometer. In addition, the volume average particle diameter in the present invention refers to the primary particle diameter at the time of production by radical polymerization, and is different from the secondary particle diameter which is the particle diameter when granulation is performed thereafter.

(Slurry for lithium secondary battery electrode)
The slurry for lithium secondary battery electrodes of the present invention is a composition containing the binder of the present invention, an active material, and a solvent. The slurry for a lithium secondary battery electrode of the present invention is obtained by kneading the binder of the present invention, an active material, and a solvent.

The active material used for the slurry for the lithium secondary battery electrode of the present invention is not particularly limited as long as it can reversibly insert and release lithium ions by charging and discharging the lithium secondary battery.
For example, examples of the positive electrode active material include lithium-containing metal composite oxides containing lithium and one or more metals selected from iron, cobalt, nickel, and manganese. Examples of the lithium-containing metal composite oxide include lithium cobaltate, lithium manganate, and lithium nickelate. These positive electrode active materials may be used alone or in combination of two or more.
On the other hand, examples of the negative electrode active material include carbon materials such as graphite, amorphous carbon, carbon fiber, coke, and activated carbon. A composite of such a carbon material and a metal such as silicon, tin, silver, or an oxide thereof can also be used. These negative electrode active materials may be used alone or in combination of two or more.

  There is no restriction | limiting in particular as a solvent used for the slurry for lithium secondary battery electrodes of this invention, What is necessary is just a solvent which can melt | dissolve or disperse | distribute a binder and an active material uniformly. As a solvent, the solvent used when preparing a varnish by melt | dissolving a binder can be used as it is. For example, water, hydrocarbons such as n-dodecane, decahydronaphthalene and tetralin, alcohols such as 2-ethyl-1-hexanol and 1-nonanol, ketones such as holon, acetophenone and isophorone, benzyl acetate, isopentyl butyrate , Esters such as γ-butyrolactone, methyl lactate, ethyl lactate and butyl lactate, amines such as o-toluidine, m-toluidine and p-toluidine, N-methyl-2-pyrrolidone, N, N-dimethylacetamide and dimethyl Examples include amides such as formamide, and sulfoxides and sulfones such as dimethyl sulfoxide and sulfolane. These solvents may be used alone or in combination of two or more. Of these, water is preferred because of its low environmental impact.

  When water is used as a solvent, 0.01 to 200 parts by mass of a water-soluble thickener may be used as an additive as needed with respect to 100 parts by mass of the active material for the purpose of adjusting the slurry viscosity. . Examples of the water-soluble thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, and casein. These water-soluble thickeners may be used alone or in combination of two or more.

When water is used as a solvent, an emulsion (emulsion) containing the first polymer and / or the second polymer obtained by emulsion polymerization in the first polymerization step or the second polymerization step is used as a binder containing the solvent as it is. Is preferred.
The slurry for lithium secondary battery electrodes of the present invention is preferably used at a viscosity of 100 to 50,000 mPa · s, more preferably 500 to 10,000 mPa · s. The slurry for lithium secondary battery electrodes in the above viscosity range can be obtained by appropriately adjusting the amount of the solvent.

  The amount of the binder contained in the slurry for the lithium secondary battery electrode of the present invention is preferably 0.1 to 4 parts by mass, more preferably 0.2 to 3 parts by mass, particularly preferably 100 parts by mass of the active material. Is 0.5-2 parts by mass. When there is too little binder amount with respect to an active material, there exists a possibility that an active material may fall out easily from the electrode which has the active material layer obtained using the slurry. Conversely, if the amount of the binder relative to the active material is too large, the active material contained in the active material layer may be covered with the binder and the battery reaction may be hindered or the internal resistance may increase.

  The slurry for the lithium secondary battery electrode of the present invention preferably contains a conductivity imparting material in addition to the binder, the active material, and the solvent. As the conductivity-imparting material, conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotube can be used. By using the conductivity imparting material, the electrical contact between the electrode active materials can be improved, and the discharge rate characteristic can be improved when used in a non-aqueous electrolyte secondary battery. It is preferable that the usage-amount of an electroconductivity imparting material is 0-20 mass parts normally with respect to 100 mass parts of electrode active materials.

  The slurry for lithium secondary battery electrodes of the present invention may contain a polyfunctional epoxy compound. By making a slurry for a lithium secondary battery electrode containing a polyfunctional epoxy compound, the adhesion between the active material layer and the current collector can be further improved, and the active material layer obtained using the binder of the present invention The flexibility can be increased.

  A polyfunctional epoxy compound is an epoxy compound having two or more epoxy groups in one molecule. Specifically, for example, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol diglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol , Diglycidyl ether, trimethylolpropane polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycidyl (meth) acrylate copolymer resin, bisphenol A, novolac phenol resin, orthocresol novolac phenol resin, etc. Can be mentioned. These polyfunctional epoxy compounds may be used alone or in combination of two or more.

  When the slurry for the lithium secondary battery electrode contains a polyfunctional epoxy compound, the molar ratio between the epoxy group of the polyfunctional epoxy compound and the carboxyl group in the binder of the present invention is expressed by (epoxy group) / (of the present invention). The carboxyl group in the binder) is preferably in the range of 10: 100 to 80: 100. If the molar ratio is within the above range, the adhesion between the active material layer and the current collector can be further increased, and the flexibility of the active material layer obtained using the binder of the present invention can be further increased. I can do it.

(Lithium secondary battery electrode)
The lithium secondary battery negative electrode of the present invention is characterized in that the lithium secondary battery electrode slurry is applied to a current collector and dried.
The current collector is not particularly limited as long as it is made of a conductive material, and examples thereof include those made of metal such as iron, copper, aluminum, nickel, and stainless steel. Although the shape is not particularly limited, it is preferable to use a sheet having a thickness of about 0.001 to 0.5 mm.

The slurry for the lithium secondary battery electrode may be applied on one side of the current collector or on both sides.
The method for applying the slurry for the lithium secondary battery electrode to the current collector is not particularly limited. For example, the doctor blade method, the dip method, the reverse roll (transfer roll) method, the direct roll method, the gravure method, and the extrusion method. , Dipping, brushing, and the like. The amount to be applied is not particularly limited. For example, the thickness of the active material layer formed after drying the slurry for the lithium secondary battery electrode is 0.005 to 1 mm, preferably 0.01 to 0.5 mm. It can be a certain amount.

An active material layer is formed on the surface of the current collector by drying and removing the solvent of the slurry for the lithium secondary battery electrode applied to the current collector.
The drying method of the lithium secondary battery electrode slurry is not particularly limited, and examples thereof include air drying, hot air drying, infrared heating, a far infrared heater, and vacuum drying. The drying temperature may be appropriately selected according to the boiling point of the solvent contained in the lithium secondary battery electrode slurry, but is usually around 150 ° C.

When the slurry for a lithium secondary battery electrode contains a polyfunctional epoxy compound, it is preferable to perform a heat treatment after drying for the purpose of reacting the epoxy group and the carboxyl group in the binder. The heat treatment is usually performed at a temperature of about 200 ° C. for about 30 minutes, but is not limited thereto.
Furthermore, the density of the active material of the electrode may be increased by compression-molding the dried lithium secondary battery negative electrode with a press machine or the like. Examples of the pressing method include a die press and a roll press, and are not particularly limited.

(Lithium secondary battery)
The lithium secondary battery of the present invention is characterized by comprising the above-described lithium secondary battery electrode. Specifically, it has the above lithium secondary battery electrode, an electrolytic solution, and a separator. In addition, the lithium secondary battery of this invention should just use said lithium secondary battery electrode in any one of a positive electrode or a negative electrode.

The electrolyte solution of the lithium ion secondary battery may be a commonly used one, and a battery that functions as a battery may be selected according to the type of the negative electrode active material and the positive electrode active material. As the electrolyte contained in the electrolytic solution, for example, any conventionally known lithium salt can be used, and examples thereof include LiClO ₄ , LiBF ₆ , and LiPF ₆ . The solvent of the electrolytic solution is not particularly limited as long as it is usually used. Carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; γ-butyl Lactones such as lactones; ethers such as trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2-ethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran. These solvents may be used alone or as a mixed solvent of two or more.

  The separator is made of a porous polymer film such as a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer. The produced porous polymer film can be used alone or in layers. In addition, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, or the like can be used, but is not limited thereto.

  There is no restriction | limiting in particular about the manufacturing method of the lithium secondary battery of the said invention. As a manufacturing method, for example, a positive electrode and a negative electrode are overlapped via a separator, wound in a spiral shape according to the battery shape, folded, put into a battery container, injected with an electrolyte, and sealed. A well-known method is mentioned. The shape of the battery may be any of a coin shape, a cylindrical shape, a square shape, a flat shape, and the like.

Hereinafter, the present invention will be described with reference to examples and comparative examples. In the following Examples and Comparative Examples, “part” means “part by mass”, and “%” means “% by mass”.
Evaluation methods in the following examples and comparative examples are as follows.

<Volume average particle diameter>
The volume average particle diameter of the polymer particles of the binder for a lithium secondary battery electrode was measured using a light scattering photometer FPAR-1000 type (manufactured by Otsuka Electronics Co., Ltd.).

<Adhesiveness>
Cellophane tape was affixed to the surface of the active material layer of the electrode produced by the method described later. After peeling the adhered cellophane tape at a constant speed, the state of the active material layer on the surface of the current collector was observed and evaluated according to the following criteria.
A: The active material layer bound on the current collector is hardly peeled off and the current collector surface is not visible.
○: The active material layer bound on the current collector is slightly peeled off, but the current collector surface is not visible.
Δ: A part of the active material layer bound on the current collector is peeled off, and a part of the current collector surface is seen.
X: The active material layer bound on the current collector is almost peeled off, and the current collector surface is entirely seen.

<Flexibility>
An electrode produced by the method described later is cut into a width of 10 mm and a length of 70 mm, and the active material layer is applied to a Teflon (registered trademark) rod having a diameter of 6 mmφ with the active material layer facing outward, and the state of the surface of the active material layer is observed. Evaluated by criteria.
A: The active material layer bound on the current collector is not cracked or peeled off at all.
○: Cracks are observed in the active material layer bound on the current collector, but no peeling is observed.
Δ: Cracks are seen in the active material layer bound on the current collector, and some peeling is also observed.
X: The active material layer bound on the current collector was cracked and peeled off.

<Production Examples 1, 5, 6, 8, 10>
In a container of a polymerization apparatus capable of heating and cooling, 100 parts of water (solvent), 0.35 part of sodium dialkylsulfosuccinate (manufactured by Kao Corporation, “Perex OTP”), and 0.05% potassium persulfate Part of the material (polymerization initiator) and 3 parts of normal butyl acrylate (component (a-3)) are mixed, and a part of the material that becomes the first polymer is added and stirred in a nitrogen atmosphere at a rotational speed of 200 rpm. The mixture was heated at 80 ° C. for 0.5 hours and polymerized to obtain an emulsion containing core particles (seed particles).

  In the emulsion containing the core particles, 50 parts of deionized water (solvent), 1 part of sodium dialkylsulfosuccinate (manufactured by Kao Corporation, “Perex OTP”) and 0.1 part of potassium persulfate (polymerization start) Agent) and 97 parts of a monomer mixture containing the components obtained by removing the seed particles from the monomer mixture (1) shown in Table 1 in the proportions shown in Table 1 to form a first polymer obtained by emulsification Was added dropwise over 4 hours, and then heated and polymerized at 80 ° C. for 1 hour to polymerize the binder for lithium secondary battery electrode (A-1, A-5, A-6, A-8) containing the first polymer. , A-10) was obtained.

In addition, the symbol in Table 1 shows the following materials.
n-BA: normal butyl acrylate AA: acrylic acid MAA: methacrylic acid AN: acrylonitrile P-1M: 2-methacryloyloxyethyl phosphate (Kyoeisha Chemical Co., Ltd. light ester P-1M)

<Production Example 2>
In an emulsion containing core particles (seed particles) obtained in the same manner as in Production Example 1, 45 parts of deionized water (solvent) and 1 part of sodium dialkylsulfosuccinate (manufactured by Kao Corporation, “Perex OTP”) (emulsifier) ), 0.1 part of potassium persulfate (polymerization initiator), and 97 parts of a monomer mixture containing the components obtained by removing seed particles from the monomer mixture (1) shown in Table 1 in the proportions shown in Table 1 Then, the material to be the first polymer obtained by the emulsification treatment was dropped in the same manner as in Production Example 1 and polymerized in the same manner as in Production Example 1 to obtain an emulsion containing the first polymer.
Next, 59.9 parts (solvent) of deionized water and 0.1 part of monomer mixture (2) containing each component in the proportions shown in Table 1 in 99.9% of the obtained emulsion containing the first polymer Was added dropwise over a period of 0.5 hour, and then held for 1 hour to obtain a lithium secondary battery electrode binder (A-2) containing the second polymer. .

<Production Example 3>
In an emulsion containing core particles (seed particles) obtained in the same manner as in Production Example 1, 50 parts of deionized water (solvent) and 1 part of sodium dialkylsulfosuccinate (manufactured by Kao Corporation, “Perex OTP”) (emulsifier) ), 0.1 part of potassium persulfate (polymerization initiator), and 97 parts of a monomer mixture containing the components obtained by removing seed particles from the monomer mixture (1) shown in Table 1 in the proportions shown in Table 1 Then, the material to be the first polymer obtained by the emulsification treatment was dropped in the same manner as in Production Example 1 and polymerized by heating and holding at 80 ° C. for 0.5 hours to obtain an emulsion containing the first polymer.
Next, 3 parts of the monomer mixture (2) containing each component at a ratio shown in Table 1 was added dropwise to 97% of the obtained emulsion containing the first polymer over 0.5 hours, and then for 1 hour. The lithium secondary battery electrode binder (A-3) containing the second polymer was obtained.

<Production Example 4>
In an emulsion containing core particles (seed particles) obtained in the same manner as in Production Example 1, 50 parts of deionized water (solvent) and 1 part of sodium dialkylsulfosuccinate (manufactured by Kao Corporation, “Perex OTP”) (emulsifier) ), 0.1 part of potassium persulfate (polymerization initiator), and 97 parts of a monomer mixture containing the components obtained by removing seed particles from the monomer mixture (1) shown in Table 1 in the proportions shown in Table 1 Then, the material to be the first polymer obtained by the emulsification treatment was dropped in the same manner as in Production Example 1, and an emulsion containing the first polymer was obtained in the same manner as in Production Example 3.
Subsequently, 7 parts of monomer mixture (2) containing each component in the ratio shown in Table 1 was dropped into 93% of the obtained emulsion containing the first polymer in the same manner as in Production Example 3, and Production Example 3 In the same manner as described above, a lithium secondary battery electrode binder (A-4) containing the second polymer was obtained.

<Production Example 7>
In an emulsion containing core particles (seed particles) obtained in the same manner as in Production Example 1, 45 parts of deionized water (solvent) and 1 part of sodium dialkylsulfosuccinate (manufactured by Kao Corporation, “Perex OTP”) (emulsifier) ), 0.1 part of potassium persulfate (polymerization initiator), and 97 parts of a monomer mixture containing the components obtained by removing seed particles from the monomer mixture (1) shown in Table 1 in the proportions shown in Table 1 Then, the material to be the first polymer obtained by the emulsification treatment was dropped in the same manner as in Production Example 1, and an emulsion containing the first polymer was obtained in the same manner as in Production Example 3.
Next, 99% of the obtained emulsion containing the first polymer was mixed with 5 parts (solvent) of deionized water and 1 part of the monomer mixture (2) containing each component at the ratio shown in Table 1. The material used as the second polymer was dropped in the same manner as in Production Example 2 and retained in the same manner as in Production Example 2 to obtain a binder for lithium secondary battery electrodes (A-7) containing the second polymer. .

<Production Example 9>
In a container of a polymerization apparatus capable of heating and cooling, 100 parts of water (solvent) is added and maintained at 80 ° C., 0.05 part of potassium persulfate (polymerization initiator), and 3 parts of normal butyl acrylate (( Part of the material to be the first polymer obtained by mixing the component a-3)) was charged and polymerized in the same manner as in Production Example 1 to obtain an emulsion containing core particles (seed particles).

  In the emulsion containing the core particles, 50 parts of deionized water (solvent), 1.35 parts (emulsifier) of sodium dialkylsulfosuccinate (manufactured by Kao Corporation, “Perex OTP”), 0.15 part of potassium persulfate ( Polymerization initiator) and a first polymer obtained by mixing and emulsifying 97 parts of a monomer mixture containing each component obtained by removing seed particles from the monomer mixture (1) shown in Table 1 in the ratio shown in Table 1. The resulting material was added dropwise in the same manner as in Production Example 1 and polymerized in the same manner as in Production Example 1 to obtain a lithium secondary battery electrode binder (A-9) containing the first polymer.

<Examples 1, 3-7, 9-11>
100 parts of CMC (manufactured by Aldric) 1% aqueous solution, 100 parts of active material (artificial graphite AGB-5 manufactured by Ito Graphite Industries Co., Ltd.), and binders for lithium secondary battery electrodes (A-1 to A) shown in Table 1 -9) 2.5 parts (1% in terms of binder resin solid content with respect to the active material) is added and kneaded, and slurry for lithium secondary battery electrodes (B-1, 3-7, 9-11) Got.

  The obtained slurry for lithium secondary battery electrode was applied onto a 18 μm thick copper foil so that the thickness of the negative electrode active material layer after drying was 50 μm, and then dried at 110 ° C. for 10 minutes to obtain a lithium secondary Battery electrodes (C-1, 3-7, 9-11) were obtained. Table 2 shows the evaluation results of the adhesion and flexibility of each electrode obtained.

<Example 2>
100 parts of CMC (manufactured by Aldric) 1% aqueous solution, 100 parts of active material (artificial graphite AGB-5 manufactured by Ito Graphite Industries Co., Ltd.), and lithium secondary battery electrode binder (A-1) 2 shown in Table 1 .5 parts (1% in terms of binder resin solid content with respect to the active material) and 0.033 parts (epoxy group / ethylene glycol diglycidyl ether (Denacol EX-810 manufactured by Nagase Chemtech), which is a polyfunctional epoxy compound. A carboxyl group in the binder = 50/100 (molar ratio)) was added and kneaded to obtain a lithium secondary battery electrode slurry (B-2).

  The obtained lithium secondary battery negative electrode slurry was applied onto a 18 μm thick copper foil and dried in the same manner as in Example 1, followed by heat treatment at 200 ° C. for 30 minutes under vacuum conditions to provide a lithium secondary battery. An electrode (C-2) was obtained. Table 2 shows the evaluation results of the adhesion and flexibility of the obtained electrode.

<Example 8>
100 parts of CMC (manufactured by Aldric) 1% aqueous solution, 100 parts of active material (artificial graphite AGB-5 manufactured by Ito Graphite Industries Co., Ltd.), and binder for lithium secondary battery electrodes (A-6) 2 shown in Table 1 .5 parts (1% in terms of binder resin solid content with respect to the active material) and 0.023 parts (epoxy group / ethylene glycol diglycidyl ether (Denacol EX-810, manufactured by Nagase Chemtech), which is a polyfunctional epoxy compound. The carboxyl group in the binder = 50/100 (molar ratio)) was added and kneaded to obtain a lithium secondary battery electrode slurry (B-8).

  The obtained slurry for the negative electrode of the lithium secondary battery was applied on a copper foil having a thickness of 18 μm, dried in the same manner as in Example 1, and heat treated in the same manner as in Example 2 to obtain a lithium secondary battery electrode ( C-8) was obtained. Table 2 shows the evaluation results of the adhesion and flexibility of the obtained electrode.

<Comparative Example 1>
100 parts of CMC (manufactured by Aldric) 1% aqueous solution, 100 parts of active material (artificial graphite AGB-5 manufactured by Ito Graphite Industries Co., Ltd.), and lithium secondary battery electrode binder (A-10) 2 shown in Table 1 Part (1% in terms of binder resin solid content with respect to the active material) was added and kneaded to obtain a slurry for lithium secondary battery electrode (B-12).

  The obtained slurry for the negative electrode of a lithium secondary battery was applied onto a copper foil having a thickness of 10 μm, dried in the same manner as in Example 1, and heat treated in the same manner as in Example 2 to obtain a lithium secondary battery electrode ( C-12) was obtained. Table 2 shows the evaluation results of the adhesion and flexibility of the obtained electrode.

As shown in Table 2, the lithium secondary battery electrodes of Examples 1 to 11 all had good evaluations of adhesion and flexibility.
In particular, the lithium secondary battery electrode obtained using the binder (A-2, 3, 4, 7) for the lithium secondary battery electrode obtained using the monomer mixture (2) was particularly excellent in adhesiveness. (Examples 3, 4, 5, 9).
In addition, the lithium secondary battery electrode obtained using the slurry for the secondary battery negative electrode containing the binder for the lithium secondary battery electrode and the polyfunctional epoxy compound of the present invention was particularly excellent in adhesion (Examples) 2, 8).

  On the other hand, the lithium secondary battery electrode (C-12) obtained using the binder (A-10) for lithium secondary battery electrode with a small amount of the component (a-2) is satisfactory in both adhesiveness and flexibility. (Comparative Example 1).

Claims (8)

  A monomer (a-3) containing 0.01 to 30% by mass of a carboxyl group-containing monomer (а-1) and 20 to 60% by mass of acrylonitrile (а-2), the remainder being copolymerizable The binder for lithium secondary battery electrodes containing the 1st polymer obtained by copolymerizing the 1st monomer mixture (1) which becomes.   In the presence of the first polymer, one or both of the carboxyl group-containing monomer (b-1) and the phosphate group-containing monomer (b-2) can be copolymerized with 50% by mass or more. The second monomer mixture (2) composed of 0 to 50% by mass of the monomer (b-3) is changed to a mass ratio (first monomer) of the first monomer mixture (1) and the second monomer mixture (2). The binder for lithium secondary battery electrodes according to claim 1, comprising a second polymer obtained by polymerizing the mixture (1) / second monomer mixture (2) as 99.99 / 0.01 to 80/20.   A slurry for a lithium secondary battery electrode, comprising the binder for a lithium secondary battery electrode according to claim 1 or 2, an active material, and a solvent.   The slurry for lithium secondary battery electrodes according to claim 3, comprising a polyfunctional epoxy compound.   A lithium secondary battery electrode, wherein the slurry for a lithium secondary battery electrode according to claim 3 or 4 is applied to a current collector and dried.   A lithium secondary battery comprising the electrode for a lithium secondary battery according to claim 5.   A monomer (a-3) containing 0.01 to 30% by mass of a carboxyl group-containing monomer (а-1) and 20 to 60% by mass of acrylonitrile (а-2), the remainder being copolymerizable The manufacturing method of the binder for lithium secondary battery electrodes including the 1st superposition | polymerization process which copolymerizes the 1st monomer mixture (1) which becomes, and obtains a 1st polymer.   In the presence of the first polymer, one or both of the carboxyl group-containing monomer (b-1) and the phosphate group-containing monomer (b-2) can be copolymerized with 50% by mass or more. The second monomer mixture (2) composed of 0 to 50% by mass of the monomer (b-3) is changed to a mass ratio (first monomer) of the first monomer mixture (1) and the second monomer mixture (2). The lithium secondary battery according to claim 7, further comprising a second polymerization step in which the mixture (1) / second monomer mixture (2)) is polymerized as 99.99 / 0.01 to 80/20 to obtain a second polymer. Manufacturing method of binder for electrodes.