JP2015115289A - Composition for lithium ion secondary battery electrodes, lithium ion secondary battery electrode, method for manufacturing lithium ion secondary battery electrode, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composition for lithium ion secondary battery electrodes which is low in both of the degree of elution and the degree of swelling, and exhibits good battery characteristics.SOLUTION: A composition for lithium ion secondary battery electrodes comprises: an active material; a binder; and a solvent (B). The binder includes a polyvinyl acetal-based resin (A) including structural units expressed by the formulas (1) and (2) below. The content of the structural unit (1) is 80-99.9 mol%. The average polymerization degree of the polyvinyl acetal-based resin (A) is 500 or larger. The content of the structural unit (2) is 0.1-10 mol%. In the solvent (B), the water content is less than 20 wt.% of the solvent (B). (R1 represents hydrogen or an alkyl group having 1-10 carbon atoms.)

Description

  The present invention relates to a composition for a lithium ion secondary battery electrode containing a polyvinyl acetal resin, a lithium ion secondary battery electrode, and a lithium ion secondary battery. Specifically, the acetal structural unit content is a specific small amount. Lithium ion secondary battery electrode composition comprising a polyvinyl acetal resin, a lithium ion secondary battery electrode using such a composition, a lithium ion secondary battery electrode containing a crosslinked product of such a polyvinyl acetal resin, lithium ion secondary The present invention relates to a method for manufacturing a secondary battery electrode and a lithium ion secondary battery.

  In recent years, lithium-ion secondary batteries have been required to have higher capacities and higher performance, such as mounting on electric / hybrid vehicles that require large capacities. As a solution for this, for example, silicon or tin having a large amount of lithium occlusion per unit volume as an anode active material, or an alloy containing these, or mixed with carbon such as conventional graphite is used to charge and discharge capacity. There has been proposed a method for increasing the value.

  However, when an active material having a large charge / discharge capacity such as silicon, tin, or an alloy containing these is used, the active material undergoes a very large volume change with charge / discharge. Therefore, when using as a binder resin a rubber-based resin such as polyvinylidene fluoride or SBR, which has been widely used in electrodes using carbon such as graphite as an active material, a very large volume change Cannot follow, and peeling occurs at the interface between the current collector and the active material layer. Accordingly, there is a problem that the current collecting structure in the electrode is destroyed, the electron conductivity of the electrode is lowered, and the cycle characteristics of the battery are easily lowered.

  For this reason, the composition for lithium ion secondary battery electrodes using the binder resin component which can follow a very big volume change of an active material was calculated | required.

  On the other hand, a technique using a polyvinyl acetal resin as a binder resin component of a composition for a lithium ion secondary battery electrode is known (for example, Patent Document 1). This technique uses a polyvinyl acetal resin having a hydroxyl group content of 33 to 55 mol%, thereby having excellent adhesion between the electrode active material and the current collector, and a composition capable of producing a high-capacity lithium ion secondary battery. It provides things. This document describes that the electrode sheet test piece obtained by using such a technique has a low elution rate when left in an electrolytic solution at room temperature for 1 hour.

  However, even with the above technique, the binder binder resin component in the electrode elutes in a very small amount into the electrolyte (carbonate solvent), so there is a problem when trying to use the battery stably for a long period of time. It was. In addition, since the degree of swelling of the binder resin component with respect to the electrolytic solution is high, there is a problem that the discharge capacity retention rate associated with the charge / discharge cycle is reduced (see Non-Patent Document 1).

  Therefore, a resin having a low elution degree and swelling degree with respect to the carbonate electrolyte solution is required as a binder resin component in the electrode.

  Moreover, the technique which coat | covers a lithium ion secondary battery electrode active material with the crosslinked material of polyvinyl acetal resin is known (for example, patent document 2). This technology requires the combined use of a binder for a lithium ion secondary battery electrode active material in addition to the polyvinyl acetal resin, improves the adhesive force between the active material and the binder, and suppresses the collapse of the active material. For the purpose, the active material is coated with a crosslinked product of polyvinyl acetal resin. However, in such a technique, the crosslinked product of the polyvinyl acetal resin is only used as a coating material for the active material, and the binder resin component is polytetrafluoroethylene, ethylene-propylene-diene rubber, styrene-butadiene copolymer. Since it is a conventionally used resin such as a rubber-based resin made of a coalescence or polyester amide, it has the same problem as described above.

  Further, a composition for a lithium secondary battery electrode using a polyvinyl acetal resin having a specific hydroxyl amount and a solubility of 1 g or more with respect to 100 g of water and water has been proposed (for example, Patent Document 3). However, after applying to the electrode current collector, a great deal of energy must be spent to remove water, and there is a problem in manufacturing a battery with resource saving.

Hitachi Chemical Technical Report No. 45 (July 2005)

JP 2012-195289 A JP 2000-48806 A JP 2013-178862 A

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a battery having good electrical characteristics with low elution degree and swelling degree with respect to a carbonate ester electrolyte. It is providing the composition for lithium ion secondary battery electrodes.

  As a result of intensive studies, the present inventors have used a polyvinyl acetal resin having a specific amount of acetal structural unit content as a binder binder resin component as an electrode composition, a specific amount of water as a solvent, or substantially It has been found that the object of the present invention can be achieved by using water that does not contain water.

（Ｒ１は水素または炭素数１〜10のアルキル基である。） That is, the gist of the present invention is a composition for a lithium ion secondary battery electrode containing an active material, a binder and a solvent (B), wherein the binder has a structure represented by the following chemical formulas (1) and (2). A polyvinyl acetal resin containing a unit, wherein the content of the structural unit (2) in the polyvinyl acetal resin (A) is 0.1 mol% or more and less than 10 mol%, and water in the previous solvent (B) Content exists in the composition for lithium ion secondary battery electrodes characterized by being less than 20 weight% of a solvent (B).

(R1 is hydrogen or an alkyl group having 1 to 10 carbon atoms.)

By using the polyvinyl acetal resin (A) having a specific amount of acetal structural unit content as the binder binder resin component as described above, the binder binder resin component in the obtained electrode is , Has the advantage of low elution and swelling.
Furthermore, since the elastic modulus is high even in a state swollen by the electrolytic solution, the deformation of the entire electrode layer can be reduced when the active material particles expand and contract due to charge and discharge, and contact between the active materials and the active material Since the contact state with the current collector can be maintained satisfactorily, the initial charge / discharge efficiency and the charge / discharge cycle characteristics can be improved.
Moreover, it becomes possible to manufacture a battery with high efficiency because the composition for a lithium ion secondary battery electrode includes a solvent and the content of water as the solvent is a specific small amount and substantially does not include water.

  The description of the constituent requirements described below is an example (representative example) of an embodiment of the present invention, and is not limited to these contents.

（Ｒ１は水素または炭素数１〜10のアルキル基である。） The present invention is a composition for a lithium ion secondary battery electrode containing an active material, a binder and a solvent (B), wherein the binder comprises a structural unit represented by the following chemical formulas (1) and (2) The content of the structural unit (2) in the polyvinyl acetal resin (A) is 0.1 mol% or more and less than 10 mol%, and the water content in the solvent (B) is It is a composition for lithium ion secondary battery electrodes, characterized by being less than 20% by weight of the solvent (B).

(R1 is hydrogen or an alkyl group having 1 to 10 carbon atoms.)

（Ｒ２はエチレン性不飽和結合を含む官能基である。） In addition, in a polyvinyl acetal type resin (A), you may contain 0.1-10 mol% of crosslinkable group containing acetal structural units as shown to following Chemical formula (3).

(R2 is a functional group containing an ethylenically unsaturated bond.)

[binder]
The binder used for the composition for lithium ion secondary battery electrodes of the present invention is an adhesive that bonds the active materials of the electrodes and the active material and the current collector. In an electrode obtained using such a binder, the binder is exposed to an environment in contact with the carbonate ester electrolyte for a long period of time.
The binder used in the lithium ion secondary battery electrode composition of the present invention has a low elution degree and swellability with respect to the ester electrolyte solution, and maintains a high elastic modulus even when swollen with the electrolyte solution. When expanded and contracted by charging and discharging, the deformation of the entire electrode layer can be reduced, and the contact between the active materials and the contact state between the active material and the current collector can be kept good, so that the initial charging and discharging The efficiency and charge / discharge cycle characteristics can be improved.

<Description of (A) Polyvinyl Acetal Resin>
As the binder of the lithium ion secondary battery electrode composition of the present invention, a polyvinyl acetal resin (A) having a specific amount of acetal structural unit content is used. Hereinafter, the polyvinyl acetal resin (A) will be described.

Chemical formula (1) in the polyvinyl acetal resin (A) represents a vinyl alcohol structural unit. Such a structural unit is usually derived from a vinyl alcohol resin that is a precursor of a polyvinyl acetal resin.

  The content of the structural unit represented by the chemical formula (1) in the polyvinyl acetal resin (A) is usually 80 to 99 mol%. Preferably it is 85-97 mol%, Most preferably, it is 85-95 mol%. When the content of the structural unit is within the above range, the effect of the present invention tends to be obtained efficiently.

（Ｒ１は水素または炭素数１〜10のアルキル基である。） The chemical formula (2) in the polyvinyl acetal resin (A) represents an acetal structural unit. Such a structural unit is usually obtained by acetalizing a vinyl alcohol resin with an aldehyde compound.

(R1 is hydrogen or an alkyl group having 1 to 10 carbon atoms.)

In the chemical formula (2), R1 is hydrogen or an alkyl group having 1 to 10 carbon atoms, preferably hydrogen or an alkyl group having 1 to 5 carbon atoms, and particularly preferably an alkyl group having 1 to 5 carbon atoms.
Since R1 is a functional group derived from an aldehyde compound, it is possible to adjust R1 by selecting an aldehyde compound to be used. As such aldehyde compounds, formaldehyde (R1 is hydrogen), acetaldehyde (R1 is an alkyl group having 1 carbon atom), butyraldehyde (R1 is an alkyl group having 3 carbon atoms), pentylaldehyde (R1 is an alkyl group having 4 carbon atoms) And decanaldehyde (R1 is an alkyl group having 9 carbon atoms). One R1 may be used, or two or more may be used in combination. That is, one type of aldehyde compound may be used, or two or more types may be used in combination.

The content of the chemical formula (2) in the polyvinyl acetal resin (A) is based on the number of hydroxyl groups of the vinyl alcohol resin which is a precursor of the polyvinyl acetal resin, and the ratio of the number of hydroxyl groups acetalized with an aldehyde compound among these. (This ratio is referred to as “degree of acetalization”). That is, as a method for calculating the degree of acetalization, a method of counting the number of hydroxyl groups subjected to acetalization is employed to calculate the mol% of the degree of acetalization.
The degree of acetalization according to the structural unit (2) in the polyvinyl acetal resin (A) is 0.1 mol% or more and less than 10 mol%. Preferably it is 1 mol% or more and less than 10 mol%, Most preferably, it is 3-5 mol%. When the degree of acetalization is too high, the elution degree to the electrolyte and the degree of swelling tend to increase, and when it is too low, the effect of the present invention tends to be difficult to obtain.

  In the binder of the lithium ion secondary battery electrode composition of the present invention, the acetal structural unit content of the polyvinyl acetal resin (A) is only set to a specific small amount, and both the elution degree and the swelling degree with respect to the electrolytic solution are low. An effect is obtained. Therefore, it is not necessary to prepare another new copolymer compound or modifying compound, and a battery having good electrical characteristics can be obtained economically.

（Ｒ２はエチレン性不飽和結合を含む官能基である。） In addition, in a polyvinyl acetal type resin (A), you may contain a crosslinkable group containing acetal structural unit as shown in following Chemical formula (3). Such a structural unit is usually obtained by acetalizing vinyl alcohol with an aldehyde compound having a functional group containing an ethylenically unsaturated bond.

(R2 is a functional group containing an ethylenically unsaturated bond.)

In chemical formula (3), R2 is a functional group containing an ethylenically unsaturated bond. When such a functional group contains an ethylenically unsaturated bond, it is crosslinked by carrying out a crosslinking reaction with heat or light, and becomes a resin having a network structure. Such an ethylenically unsaturated bond is at least one selected from a terminal vinyl structure and an internal alkene structure.
The number of ethylenically unsaturated bonds in the functional group containing an ethylenically unsaturated bond is usually 1 to 5, preferably 1 to 3. At this time, since the ethylenically unsaturated bond constituting the aromaticity of the aromatic compound does not have a crosslinking ability, it is not included in the “ethylenically unsaturated bond”.
Note that the functional group containing an ethylenically unsaturated bond may have an unsaturated bond other than the ethylenically unsaturated bond, such as a carbonyl group or an amide bond.

R2 is a functional group containing an ethylenically unsaturated bond whose molecular weight per ethylenically unsaturated group (hereinafter referred to as an unsaturated group equivalent) is usually 200 or less, particularly preferably ethylene having an ethylenically unsaturated group equivalent of 150 or less. It is a functional group containing a polymerizable unsaturated bond.
The carbon number of R2 is usually 3-20, preferably 3-15, particularly preferably 3-10.

This means, for example, an alkyl group containing a terminal vinyl structure such as CH ₂ ═CH— (unsaturated group equivalent 27), CH ₂ ═C (CH ₃ ) — (unsaturated group equivalent 41), CH ₃ —CH═CH - (unsaturated group equivalent _{41), CH 3 CH 2 CH} = CH- ( unsaturated group equivalent _{55), CH 3 -CH = CH} -CH = CH- ( unsaturated group equivalent 33.5), _(CH 3) Alkyl groups containing an internal alkene structure such as ₂ C = CH—CH ₂ CH ₂ CH (CH ₃ ) CH ₂ — (unsaturated group equivalent 125), C ₆ H ₅ CH═CH— (unsaturated group equivalent 103), etc. alkyl groups containing aromatic functional _group, CH ₂ = CHCONHCH 2 - (unsaturated group equivalent _{84), CH 2 = C (} CH 3) CONHCH 2 - ( unsaturated group equivalent 98) alkyl group containing an amide bond such as Is mentioned.

Since R2 is a functional group derived from an aldehyde compound, it is possible to adjust R2 by selecting an aldehyde compound to be used. Such aldehyde compounds, such as acrolein (R2 is _CH 2 = CH-), crotonaldehyde (R2 is _CH 3 -CH = CH-), methacrolein (R2 is _{CH 2 = C (CH 3)} -), trans - 2- pentenal (R2 is _{CH 3 CH 2 CH = CH-)} , trans, trans-2,4-hexa diene knurl (CH- R2 is _{CH 3 -CH = CH-CH =} ), the citronellal (R2 (CH ₃ ) ₂ C = CH—CH ₂ CH ₂ CH (CH ₃ ) CH ₂ —), cinnamic aldehyde (R 2 is C ₆ H ₅ CH═CH—), acrylamide acetaldehyde (R 2 is CH ₂ ═CHCONHCH ₂ —), methacrylamide acetaldehyde (R2 is _{CH 2 = C (CH 3)} CONHCH 2 -), all-trans-retinal (R2 is _{C 19} H _27- ) and the like.

  The content of the chemical formula (3) in the polyvinyl acetal resin (A) is based on the number of hydroxyl groups of polyvinyl alcohol which is a precursor of the polyvinyl acetal resin, and among these, an aldehyde compound having a functional group containing an ethylenically unsaturated bond Means the ratio of the number of hydroxyl groups acetalized with (this ratio is referred to as “degree of crosslinking group modification”). That is, as a method for calculating the degree of crosslinking group modification, a method of counting the number of hydroxyl groups subjected to acetalization with an aldehyde compound having a functional group containing an ethylenically unsaturated bond is used to calculate mol% of the degree of crosslinking group modification. .

  The degree of crosslinking group modification of the structural unit (3) in the polyvinyl acetal resin (A) is usually 0.1 to 10 mol%. Preferably it is 0.1-8 mol%, Most preferably, it is 0.1-2 mol%. If the degree of modification of the crosslinking group is too high, the durability of the battery may be lowered, for example, because the elastic modulus becomes too high, the electrode becomes brittle, or the adhesiveness with the current collector decreases. Moreover, when too low, the effect of bridge | crosslinking will be thin and there exists a tendency for the effect of this invention not to be fully acquired.

  In the present invention, by using the polyvinyl acetal resin (A) having a specific amount of acetal structural unit content as a binder binder resin component, it is possible to provide a binder having a low elution degree and swelling degree. .

  In general, it is known that even when an uncrosslinked polymer is dissolved in a solvent, the crosslinked polymer does not dissolve in the solvent but swells. That is, when solvent molecules enter the network structure of the polymer, the uncrosslinked polymer is dissolved by unraveling the network structure. However, in the case of the crosslinked polymer, the force of the polymer chain between the networks to shrink due to rubber-like elasticity is exerted. This is because it becomes a deterrent and cannot be dissolved. (Shinji Furukawa, “Polymer physical properties” (Chemical Doujinshi) p.17) Accordingly, the degree of dissolution of the binder resin component and the degree of swelling are not correlated.

The polyvinyl acetal resin (A) may contain a structural unit derived from a vinyl ester monomer (hereinafter sometimes referred to as a vinyl ester structural unit). Such vinyl ester structural units are usually derived from a vinyl alcohol resin that is a precursor of a polyvinyl acetal resin.
The structural unit derived from the vinyl ester in the polyvinyl acetal resin (A) is usually 0 to 5 mol%. Preferably it is 0-3 mol%, Most preferably, it is 0-1 mol%. When the amount is too large, it tends to be eluted into the electrolyte solution and the intended effect tends to be difficult to obtain.

  The average degree of polymerization of the polyvinyl acetal resin (A) is the same as that of the polyvinyl alcohol resin as a precursor. The average degree of polymerization (measured in accordance with JIS K6726) of the polyvinyl acetal resin (A) is usually 500 or more. Preferably it is 1000-3000, Most preferably, it is 2500-3000. If it is too high, the viscosity of the binder tends to be too high and the electrode manufacturing workability tends to be lowered, and if it is too low, the effect of the present invention tends to be difficult to obtain efficiently.

  The polyvinyl alcohol resin which is a precursor of the polyvinyl acetal resin (A) can be obtained by saponifying the polyvinyl ester. Examples of the vinyl ester include vinyl formate, vinyl acetate, vinyl propionate, vinyl valelate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, versatic. Examples thereof include vinyl acid. Of these, vinyl acetate is preferred from the viewpoint of economy.

  The polyvinyl alcohol resin may be one obtained by copolymerizing an ethylenically unsaturated monomer as long as the effects of the present invention are not impaired. The ethylenically unsaturated monomer is not particularly limited, and examples thereof include α-olefins such as propylene, isopropylene, butylene, isobutylene, pentylene, hexylene, cyclohexylene, cyclohexylethylene, and cyclohexylpropylene; 3-butene-1 Hydroxy group-containing α-olefins such as ol, 4-penten-1-ol, 3-butene and 1,2 diols; vinylene carbonates, acrylic acid, methacrylic acid, (anhydrous) phthalic acid, (anhydrous) maleic Acids, unsaturated acids such as itaconic acid, etc. or salts thereof, mono- or dialkyl esters, nitriles such as acrylonitrile, amides such as acrylamide and methacrylamide, ethylene sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, etc. Olefin sulfo Acid or a salt thereof, 3,4-diasiloxy-1-butene such as 3,4-dihydroxy-1-butene, 3,4-diacetoxy-1-butene, 3-acyloxy-4-hydroxy-1-butene, 4 -Acyloxy-3-hydroxy-1-butene, 3,4-diacyloxy-2-methyl-1-butene, 4,5-dihydroxy-1-pentene, 4,5-diacyloxy-1-pentene, 4,5-dihydroxy Glycerol monoallyl which is -3-methyl-1-pentene, 4,5-diacyloxy-3-methyl-1-pentene, 5,6-dihydroxy-1-hexene, 5,6-diacyloxy-1-hexene Ether, 2,3-diacetoxy-1-allyloxypropane, 2-acetoxy-1-allyloxy-3-hydroxypropane, 3-acetoxy-1-ali Examples include luoxy-2-hydroxypropane, glycerin monovinyl ether, and glycerin monoisopropenyl ether. Vinylethylene carbonate, vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, allyltrimethoxysilane, allylmethyldimethoxysilane, allyldimethylmethoxysilane, Allyltriethoxysilane, allylmethyldiethoxysilane, allyldimethylethoxysilane, vinyltris (β-methoxyethoxy) silane, vinylisobutyldimethoxysilane, vinylethyldimethoxysilane, vinylmethoxydibutoxysilane, vinyldimethoxybutoxysilane, vinyltributoxysilane , Vinylmethoxydihexyloxysilane, vinyldimethoxyhexyloxysilane, vinyltrihexyloxysilane Vinyl methoxydioctyloxysilane, vinyldimethoxyoctyloxysilane, vinyltrioctyloxysilane, vinylmethoxydilauryloxysilane, vinyldimethoxylauryloxysilane, vinylmethoxydioleoyloxysilane, vinyldimethoxyoleyloxysilane, etc. . Also use terminal-modified polyvinyl alcohol obtained by copolymerizing vinyl ester monomer such as vinyl acetate with ethylene in the presence of thiol compounds such as thiol acetic acid and mercaptopropionic acid, and saponifying it. Can do.

  Polymerization of the vinyl ester monomer can be carried out by any known polymerization method such as solution polymerization, suspension polymerization, emulsion polymerization and the like. For the saponification of the obtained polymer, a conventionally known saponification method can be employed. That is, it can be carried out using an alkali catalyst or an acid catalyst in a state where the copolymer is dissolved in alcohol or water / alcohol solvent. Examples of the alkali catalyst include alkali metal hydroxides and alcoholates such as potassium hydroxide, sodium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and lithium methylate.

  A polyvinyl acetal resin is manufactured by acetalizing the obtained polyvinyl alcohol-type resin using an aldehyde compound. Such acetalization reaction may be performed under known general conditions. Usually, an aldehyde compound is blended with an aqueous polyvinyl alcohol resin solution in the presence of an acid catalyst to cause an acetalization reaction. With the progress of the acetalization reaction, a method in which polyvinyl acetal-based resin particles are precipitated and the reaction proceeds in a heterogeneous system is generally performed thereafter.

Such acetalization reaction is preferably initiated at a low temperature. For example, I) a method in which an aqueous polyvinyl alcohol resin solution is cooled and an acid and a predetermined amount of an aldehyde compound are mixed to start the reaction. II) an aldehyde compound is mixed in an aqueous polyvinyl alcohol resin solution and cooled, and then the acid is added. The method of mix | blending and starting reaction, III) The method of mix | blending an acid with a polyvinyl alcohol-type resin aqueous solution, cooling, and mix | blending an aldehyde compound and starting reaction is mentioned.
In preparing an aqueous solution containing a polyvinyl alcohol-based resin, it is usually preferable to dissolve the solution by heating. In the above I) to III), unless otherwise specified, it means an aqueous solution cooled to room temperature after dissolution by heating. .

As such an aldehyde compound, a compound giving the structure shown in the structural unit (2) {and optionally (3)} is used. That is, as the structural unit (2), formaldehyde (R1 is hydrogen), acetaldehyde (R1 is a C1 alkyl group), butyraldehyde (R1 is a C3 alkyl group), pentylaldehyde (R1 is a C4 carbon group). Alkyl group) and decane aldehyde (R1 is an alkyl group having 9 carbon atoms), and the above-mentioned aldehyde compound is used as the structural unit (3).
These aldehyde compounds to be the structural unit (2) {and optionally (3)} are usually blended together at the time of blending the aldehyde compounds in the above a) and b). If necessary, one can be blended before cooling and one can be blended after cooling. Preferably, a polyvinyl alcohol-based resin is dissolved in water to prepare an aqueous solution, which is cooled to 5 to 50 ° C., and an aldehyde compound to be the structural unit (2) {and (3)} is blended therein if desired. Then, an acetalization method in which the reaction is started by blending an acid.

  Examples of the acid used for the acetalization reaction include inorganic acids such as hydrochloric acid, sulfuric acid and nitric acid, and organic acids such as p-toluenesulfonic acid. These may be used alone or in combination of two or more. it can. Alkali compounds used to neutralize the powdered reaction product obtained after the acetalization reaction include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, as well as ammonia, triethylamine, and pyridine. And amine compounds such as

The acetalization reaction is preferably performed with stirring. Moreover, in order to perform acetalization reaction completely, it is preferable to continue reaction normally at 50-80 degreeC. The acetalization reaction is usually performed for 1 to 10 hours.
The powdery reaction product obtained after the acetalization reaction is filtered, neutralized with an aqueous alkali solution, washed with water, and dried to obtain the intended polyvinyl acetal resin (A).

<Solvent (B)>
The composition for lithium ion secondary battery electrodes of the present invention contains a solvent (B). Such solvent (B) dissolves the polyvinyl acetal resin (A). Furthermore, the content of water in the solvent (B) is a specific small amount or substantially free of water. The water content in the solvent (B) in the lithium ion secondary battery electrode composition of the present invention is less than 20% by weight, preferably less than 10% by weight, particularly preferably 5 to 0% by weight. %. When the solvent (B) contains a large amount of water, it tends to be difficult to completely dissolve the polyvinyl acetal resin (A).

Examples of the solvent (B) include amide solvents such as N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, and methylformamide, sulfoxides such as dimethylsulfoxide, 1,1,1,3,3, Alcohol solvents such as 3-hexafluoro-2-propanol can be used. From the viewpoint of easy handling and relatively low cost, an amide solvent is preferable, and N-methylpyrrolidone is particularly preferable.
When the solvent (B) contains a specific amount of water as described above or substantially does not contain water, the composition of the present invention is applied to the electrode current collector and dried as described later. Since a larger amount of energy is required than when the above exemplified solvents are used, there is a tendency to be economically disadvantageous. Preferably, the solvent (B) is substantially free of water.

<Active material>
The composition for lithium ion secondary battery electrodes of the present invention contains an active material.
As such an active material, a known general active material used for a lithium ion secondary battery electrode can be used.
As the positive electrode active material, for example, olivine-type lithium iron phosphate, lithium cobaltate, lithium manganate, lithium nickelate, ternary nickel cobalt lithium manganate, lithium nickel cobalt aluminum composite oxide, or the like can be used. .
A carbon material is preferable as the negative electrode active material. Examples of the carbon material include graphite-based carbon materials (graphite) such as natural graphite, artificial graphite, and expanded graphite, carbon black, activated carbon, carbon fiber, coke, soft carbon, and hard carbon. In particular, it is preferable to use a lithium-containing metal composite oxide, carbon powder, silicon powder, tin powder, or a mixture thereof because the effects of the present invention are effectively exhibited. The content of the active material in the slurry is 10 to 95% by weight, 20 to 80% by weight, preferably 35 to 65% by weight.

  The average particle diameter of the active material is usually 1 to 100 μm, more preferably 1 to 50 μm, and still more preferably 1 to 25 μm. In addition, the value measured by the laser diffraction type particle size distribution measurement (laser diffraction scattering method) shall be employ | adopted for the average particle diameter of an active material.

<Other ingredients>
The electrode composition of the present invention may contain other known ingredients such as those generally contained in the electrode composition in addition to the active material and the binder. For example, conductive aids, flame retardant aids, thickeners, antifoaming agents, leveling agents, adhesion promoters, and the like can be mentioned. In particular, the conductive auxiliary agent refers to a compound that is blended to improve conductivity. Examples of the conductive assistant include carbon powders such as graphite and acetylene black, and various carbon fibers such as vapor grown carbon fiber (VGCF).
The blending amounts of these components are within a known general range, and the blending ratio thereof can also be adjusted by appropriately referring to the known knowledge about lithium ion secondary batteries.

The composition for a lithium ion secondary battery electrode of the present invention may contain other binder performance or active material dispersibility resin, crosslinking agent, silane coupling agent, etc., as long as the effects of the present invention are not impaired. good. Examples of resins having other binding performance and active material dispersibility include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyvinyl pyrrolidone, polyethylene oxide, polyvinyl acetate, cellulose acetate, carboxymethyl cellulose, and the like. It is done.
In particular, when the polyvinyl acetal resin (A) having the structural unit (3) is used as a binder, it is preferable to contain a polymerization initiator (C).
Further, in addition to the above additives, a small amount of impurities contained in the raw materials for producing the constituent components of the adhesive may be included.

Examples of the polymerization initiator (C) include a photopolymerization initiator (c1) and a thermal polymerization initiator (c2).
Examples of the photopolymerization initiator (c1) include diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyldimethyl ketal, 4- (2-hydroxyethoxy) phenyl- (2 -Hydroxy-2-propyl) ketone, 1-hydroxycyclohexyl phenylketone, 2-methyl-2-morpholino (4-thiomethylphenyl) propan-1-one, 2-benzyl-2-dimethylamino-1- (4- Acetophenones such as morpholinophenyl) butanone and 2-hydroxy-2-methyl-1- [4- (1-methylvinyl) phenyl] propanone oligomer; benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl Ben such as ether Ins; benzophenone, methyl o-benzoylbenzoate, 4-phenylbenzophenone, 4-benzoyl-4'-methyl-diphenyl sulfide, 3,3 ', 4,4'-tetra (t-butylperoxycarbonyl) benzophenone, 2,4,6-trimethylbenzophenone, 4-benzoyl-N, N-dimethyl-N- [2- (1-oxo-2-propenyloxy) ethyl] benzenemethananium bromide, (4-benzoylbenzyl) trimethylammonium Benzophenones such as chloride; 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2,4-diethylthioxanthone, 2,4-dichlorothioxanthone, 1-chloro-4-propoxythioxanthone, 2- (3-dimethylamino-2-hydroxy ) -3 Thioxanthones such as 4-dimethyl-9H-thioxanthone-9-one mesochloride; 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, bis (2,6-dimethoxybenzoyl) -2,4,4-trimethyl- And acyl phosphine oxides such as pentylphosphine oxide and bis (2,4,6-trimethylbenzoyl) -phenylphosphine oxide. In addition, as for these photoinitiators (c1), only 1 type may be used independently and 2 or more types may be used together.

  These auxiliary agents include triethanolamine, triisopropanolamine, 4,4′-dimethylaminobenzophenone (Michler ketone), 4,4′-diethylaminobenzophenone, 2-dimethylaminoethylbenzoic acid, 4-dimethylaminobenzoic acid. Ethyl, 4-dimethylaminobenzoic acid (n-butoxy) ethyl, isoamyl 4-dimethylaminobenzoate, 2-ethylhexyl 4-dimethylaminobenzoate, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone Etc. can be used in combination.

  Among these, benzyl dimethyl ketal, 1-hydroxycyclohexyl phenyl ketone, benzoyl isopropyl ether, 4- (2-hydroxyethoxy) -phenyl (2-hydroxy-2-propyl) ketone, 2-hydroxy-2-methyl-1- It is preferable to use phenylpropan-1-one.

  Examples of the thermal polymerization initiator (c2) include methyl ethyl ketone peroxide, cyclohexanone peroxide, methyl cyclohexanone peroxide, methyl acetoacetate peroxide, acetyl acetate peroxide, 1,1-bis (t-hexyl peroxide). ) -3,3,5-trimethylcyclohexane, 1,1-bis (t-hexylperoxy) -cyclohexane, 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, 1, 1-bis (t-butylperoxy) -2-methylcyclohexane, 1,1-bis (t-butylperoxy) -cyclohexane, 1,1-bis (t-butylperoxy) cyclododecane, 1,1- Bis (t-butylperoxy) butane, 2,2-bis (4 -Di-t-butylperoxycyclohexyl) propane, p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, t-hexyl hydroper Oxide, t-butyl hydroperoxide, α, α′-bis (t-butylperoxy) diisopropylbenzene, dicumyl peroxide, 2,5-dimethyl-2,5-bis (t-butylperoxy) hexane, t-Butyl cumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-bis (t-butylperoxy) hexyne-3, isobutyryl peroxide, 3,5,5-trimethylhexa Noyl peroxide, octanoyl per Oxide, lauroyl peroxide, stearoyl peroxide, succinic acid peroxide, m-toluoyl benzoyl peroxide, benzoyl peroxide, di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, bis (4-t-butyl) (Cyclohexyl) peroxydicarbonate, di-2-ethoxyethyl peroxydicarbonate, di-2-ethoxyhexyl peroxydicarbonate, di-3-methoxybutyl peroxydicarbonate, di-s-butyl peroxydicarbonate, Di (3-methyl-3-methoxybutyl) peroxydicarbonate, α, α'-bis (neodecanoylperoxy) diisopropylbenzene, cumylperoxyneodecanoate, 1,1,3,3-te Lamethylbutyl peroxyneodecanoate, 1-cyclohexyl-1-methylethylperoxyneodecanoate, t-hexylperoxyneodecanoate, t-butylperoxyneodecanoate, t-hexylperoxy Pivalate, t-butylperoxypivalate, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, 2,5-dimethyl-2,5-bis (2-ethylhexanoylper) Oxy) hexanoate, 1-cyclohexyl-1-methylethylperoxy-2-ethylhexanoate, t-hexylperoxy-2-ethylhexanoate, t-butylperoxy-2-ethylhexanoate, t- Hexyl peroxyisopropyl monocarbonate, t-butyl peroxyisobutyrate t-butyl peroxymalate, t-butyl peroxy-3,5,5-trimethylhexanoate, t-butyl peroxylaurate, t-butyl peroxyisopropyl monocarbonate, t-butyl peroxy-2 -Ethylhexyl monocarbonate, t-butyl peroxyacetate, t-butyl peroxy-m-toluylbenzoate, t-butyl peroxybenzoate, bis (t-butylperoxy) isophthalate, 2,5-dimethyl-2,5 -Bis (m-toluylperoxy) hexane, t-hexylperoxybenzoate, 2,5-dimethyl-2,5-bis (benzoylperoxy) hexane, t-butylperoxyallyl monocarbonate, t-butyltrimethylsilylper Oxide, 3,3 ', 4,4'-tetra Organic peroxide initiators such as (t-butylperoxycarbonyl) benzophenone and 2,3-dimethyl-2,3-diphenylbutane; 2-phenylazo-4-methoxy-2,4-dimethylvaleronitrile, 1- [(1-cyano-1-methylethyl) azo] formamide, 1,1′-azobis (cyclohexane-1-carbonitrile), 2,2′-azobis (2-methylbutyronitrile), 2,2′- Azobisisobutyronitrile, 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (2-methylpropionamidine) dihydrochloride, 2,2'-azobis (2-methyl- N-phenylpropionamidine) dihydrochloride, 2,2'-azobis [N- (4-chlorophenyl) -2-methylpropionamidine] Hydride chloride, 2,2'-azobis [N- (4-hydrophenyl) -2-methylpropionamidine] dihydrochloride, 2,2'-azobis [2-methyl-N- (phenylmethyl) propionamidine] dihydrochloride 2,2'-azobis [2-methyl-N- (2-propenyl) propionamidine] dihydrochloride, 2,2'-azobis [N- (2-hydroxyethyl) -2-methylpropionamidine] dihydrochloride, 2,2'-azobis [2- (5-methyl-2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl) Lopan] dihydrochloride, 2,2'-azobis [2- (3,4,5,6-tetrahydropyrimidin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (5-hydroxy-3 , 4,5,6-tetrahydropyrimidin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane] dihydrochloride 2,2'-azobis [2- (2-imidazolin-2-yl) propane], 2,2'-azobis [2-methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] ] Propionamide], 2,2'-azobis [2-methyl-N- [1,1-bis (hydroxymethyl) ethyl] propionamide], 2,2'-azobis [2-me Til-N- (2-hydroxyethyl) propionamide], 2,2'-azobis (2-methylpropionamide), 2,2'-azobis (2,4,4-trimethylpentane), 2,2'- Azobis (2-methylpropane), dimethyl-2,2-azobis (2-methylpropionate), 4,4'-azobis (4-cyanopentanoic acid), 2,2'-azobis [2- (hydroxymethyl ) Propionitrile] and the like; and the like. In addition, only 1 type may be used independently for these thermal polymerization initiators, and 2 or more types may be used together.

  The content of the polymerization initiator (C) is usually 0.5 to 20 parts by weight, preferably 0.8 to 15 parts by weight, more preferably 100 parts by weight of the polyvinyl acetal resin (A). 1 to 10 parts by weight. When there is too little content of the said polymerization initiator (C), there exists a tendency for the efficiency of a crosslinking reaction to raise easily, and when too large, there exists a tendency for an unreacted product to have a bad influence on resin after bridge | crosslinking.

(Preparation of electrode composition)
The lithium ion secondary battery electrode composition of the present invention can be prepared by mixing the binder, active material, solvent, and other components as required. For such mixing, a stirrer, a defoamer, a bead mill, a high-pressure homogenizer, or the like can be used. Moreover, it is preferable to perform preparation of the composition for electrodes under reduced pressure. Thereby, it can prevent that a bubble arises in the active material layer obtained.

  The content ratio of the active material and the polyvinyl acetal resin (A) in the composition for a lithium ion secondary battery electrode of the present invention is generally 0. 0% by weight for the polyvinyl acetal resin (A) with respect to 100 parts by weight of the active material. 1 to 10 parts by weight, preferably 0.1 to 5 parts by weight, particularly preferably 0.1 to 4 parts by weight. If the content of the polyvinyl acetal resin (A) is too high, there are many inactive portions in the active material layer, and the internal resistance tends to increase, and the weight energy density tends to decrease. On the other hand, if the amount is too small, a desired binding force cannot be obtained, and the active material tends to fall off or easily peel off from the current collector, which tends to lower the battery performance.

  The content of the polyvinyl butyral resin (A) in the resin solid content contained in the composition for lithium ion secondary battery electrodes of the present invention is usually 50 to 100% by weight, preferably 70 to 100% by weight, More preferably, it is 80 to 100% by weight. If this value is too low, the effects of the present invention tend to be difficult to obtain efficiently. The resin solid content concentration in the electrode composition is usually 0.5 to 18% by weight, preferably 1 to 15% by weight, more preferably 1 to 12% by weight, as measured with a ket. is there. If this value is too low, it is necessary to remove a large amount of the solvent (B) at the time of electrode preparation, which tends to reduce the economic efficiency. If it is too high, work efficiency will be reduced at the time of electrode preparation, and the accuracy of electrode density control There is a tendency to decrease.

  The composition for a lithium ion secondary battery electrode of the present invention is usually in a slurry form. An electrode (layer) is formed by applying the electrode composition prepared as described above onto a current collector. An electrode can be manufactured by drying this. Moreover, when making a polymerization initiator (C) contain the composition for electrodes, it is preferable to bridge | crosslink a polyvinyl acetal type resin (A) with a heat | fever or light so that it may mention later.

  As the current collector used for the electrode, those used as the current collector of the electrode of the lithium ion secondary battery can be used. Specifically, a metal foil such as copper or nickel, an etching metal foil, an expanded metal, or the like is used as the negative electrode current collector. Examples of the current collector for the positive electrode include metal materials such as aluminum, copper, nickel, tantalum, stainless steel, and titanium, which can be appropriately selected and used according to the type of the target power storage device.

Examples of the method of applying the electrode composition to the current collector include an extrusion coater method, a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, and an applicator method.
As drying conditions after electrode formation, the processing temperature is usually 20 to 250 ° C, and more preferably 50 to 150 ° C. Moreover, processing time is 1 to 120 minutes normally, and it is more preferable that it is 5 to 60 minutes.
The thickness of the obtained electrode layer is usually 20 to 500 μm, more preferably 20 to 300 μm, and further preferably 20 to 150 μm.

When the polymerization initiator (C) is contained, the electrode (layer) is exposed to heat or light, and the polyvinyl acetal resin (A) as a binder is crosslinked to obtain an electrode that exhibits further effects. If necessary, it is preferable to press after forming an electrode on the current collector to increase the resin density before the crosslinking reaction.
When the crosslinking reaction is performed with light, methods such as ultraviolet irradiation and electron beam irradiation can usually be employed. From the viewpoints of economy and productivity, it is preferable to adopt a method called ultraviolet irradiation. For the ultraviolet irradiation, a xenon lamp, a high-pressure mercury lamp, an ultra-high pressure mercury lamp, a metal halide lamp, or the like can be used. When using a high-pressure mercury lamp, it is preferable to cure at a conveying speed of 5 to 60 m / min for one lamp having an energy of 80 to 240 W / cm. It is preferable to perform a crosslinking reaction with heat after the crosslinking reaction with light, because the initiator can be used efficiently.
When performing a crosslinking reaction with heat, it is usually treated at a treatment temperature of 20 ° C. to 250 ° C. for 1 to 120 minutes. More preferably, a method of reacting at 50 to 150 ° C. for 5 to 60 minutes is preferable.
In the case where the crosslinking reaction is carried out by heat, it can be cured at the same time during the drying treatment after the electrode is formed, which is preferable.

The composition for a lithium ion secondary battery electrode of the present invention joins an active material and a current collector (that is, forms a target electrode) in a state where the polyvinyl acetal resin (A) as a binder is uncrosslinked. It is preferable that the polyvinyl acetal resin (A) is crosslinked by drying and optionally applying heat or light.
When a cross-linking reaction is performed before joining the active material and the current collector, there is a tendency that the active material and the current collector cannot be favorably bonded.

[Description of electrode]
The electrode obtained using the composition for lithium ion secondary battery electrodes of the present invention has the following characteristics.

The loss elastic modulus maximum point temperature of the binder in the electrode prepared using the lithium ion secondary battery electrode composition of the present invention is usually 70 to 100 ° C, more preferably 80 to 100 ° C, and particularly 85 to 100 ° C. It is.
Moreover, when a polyvinyl acetal type resin (A) contains a crosslinking group and this is bridge | crosslinked, the crosslinking density of a binder is 5 * 10 < ^-5 > ^-5 * 10 < ^-4 > mol / cm < ³ > normally, Furthermore, 1 * 10 ⁻⁴ to 4 × 10 ⁻⁴ mol / cm.
The loss elastic modulus maximum point temperature and the crosslinking density are measured as follows.
Binder film (25 x 5 x 0.02 to 0.05 mm) was lost at a measurement frequency of 10 Hz according to JIS K-7244-4 using a dynamic viscoelasticity measuring device DVA-225 manufactured by IT Measurement Control Co., Ltd. Measure the relationship between elastic modulus and temperature.
The temperature at which the loss elastic modulus rapidly decreases after the loss elastic modulus reaches a maximum at room temperature or higher is defined as the loss elastic modulus maximum point.
When obtaining the crosslinking density of the binder component, the loss elastic modulus maximum point temperature in the film of the binder having no crosslinking group is defined as Tg0. Moreover, the loss elastic modulus maximum point temperature in the film after the crosslinking of the binder in which the polyvinyl acetal resin (A) has a crosslinking group is defined as Tg.
Based on the loss modulus maximum point temperatures Tg and Tg0, the crosslinking density is calculated from the following Nielsen empirical formula.
(Nielsen's empirical formula)
Tg−Tg0 = 3.9 × 10 ⁻⁴ × ν
Tg: Loss modulus maximum point temperature of binder film subjected to crosslinking reaction Tg0: Loss modulus maximum point temperature v of binder film having no crosslinking group ν: Crosslink density (mol / cm ³ )

The degree of swelling of the electrolyte in the binder in the electrode prepared using the composition for a lithium ion secondary battery electrode of the present invention is usually 0 to 20%, preferably 0 to 10%, particularly preferably 0 to 5%. %.
The degree of swelling is 100 mm × 15 mm × 20 to 50 μm of a binder (when the polyvinyl acetal resin (A) as a binder has a crosslinking group and is crosslinked, after crosslinking), the pressure is reduced at 120 ° C. for 4 hours. Dry and measure the weight (W0 (g)). This film is immersed in about 10 g of a predetermined electrolyte at 23 ° C. and kept until a constant weight is reached. Thereafter, the film is taken out, the surface of the film is gently wiped with filter paper, and the weight is measured (W1 (g)). From them, the degree of swelling of the electrolyte is calculated according to the following formula.
Electrolyte swelling degree (%) = ((W1−W0) / W0) × 100

The degree of swelling of the electrolyte in the binder in the electrode prepared using the lithium ion secondary battery electrode composition of the present invention is usually 0 to 20% and 0 to 10%, although it depends on the electrolyte used. Is preferable, and 0 to 5% is particularly preferable.
The degree of swelling is 100 mm × 15 mm × 20 to 50 μm of a binder (when the polyvinyl acetal resin (A) as a binder has a crosslinking group and is crosslinked, after crosslinking), the pressure is reduced at 120 ° C. for 4 hours. Dry and measure the weight (W ₀ (g)). This film is immersed in about 10 g of a predetermined electrolyte at 23 ° C. and kept until a constant weight is reached. Thereafter, the film is taken out, the surface of the film is gently wiped with a filter paper, and the weight is measured (W ₁ (g)). From them, the degree of swelling of the electrolyte is calculated according to the following formula.
Electrolytic solution swelling degree (%) = ((W ₁ −W ₀ ) / W ₀ ) × 100

Moreover, although the elasticity modulus of the binder in the electrode created using the composition for lithium ion secondary battery electrodes of this invention is dependent also on the electrolyte solution to immerse, it is 0.5-5 GPa normally, Furthermore, it is 1-4 GPa. Yes, especially 2-3 GPa.
Such elastic modulus is measured as follows. First, a 100 mm × 15 mm × 20-50 μm binder film (polyvinyl acetal resin (A) has a crosslinkable group and, after crosslinking) is immersed in about 10 g of an electrolytic solution at 23 ° C. to obtain a constant weight. Keep up. Then, take it out, wipe the surface of the film lightly with filter paper, and at 23 ° C. and 50 RH%.
Obtain the elastic modulus according to JIS K-7161 10.3.

[Lithium ion secondary battery]
The lithium ion secondary battery provided with the electrode produced as described above will be described.

As an electrolytic solution of the lithium ion secondary battery, an aprotic polar solvent that dissolves a lithium salt is used. For example, chain carbonate solvents such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, isopropyl methyl carbonate, ethyl propyl carbonate, isopropyl ethyl carbonate, butyl methyl carbonate, butyl ethyl carbonate, dipropyl carbonate, ethylene carbonate , Cyclic carbonate solvents such as propylene carbonate and butylene carbonate, carbonate solvents such as halogen-containing cyclic carbonate solvents such as chloroethylene carbonate and trifluoropropylene carbonate, ether solvents such as ethylene glycol dimethyl ether and propylene glycol dimethyl ether , Γ-butyrolactone, tetrahydrofuran, 2-methyltetra Examples include heterocyclic compound solvents such as drofuran, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 1,3-dioxolane, and carboxylic acid ester solvents such as methyl acetate, ethyl acetate, methyl formate, and ethyl formate. . These solvents usually have 2 to 15 carbon atoms, preferably 3 to 10 carbon atoms, and particularly preferably 3 to 8 carbon atoms. These may be used alone, but are preferably mixed and used.
In particular, when the electrolytic solution contains a carbonate ester solvent, the effects of the present invention tend to be obtained effectively. The electrolyte is a high-permittivity / high-boiling point ethylene carbonate, propylene carbonate, or other cyclic carbonate ester solvent having 1 to 10 carbon atoms, low viscosity solvent such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, carbon It is often used by containing a chain carbonate ester solvent of several 3 to 10.

Examples of the lithium salt of the electrolyte include inorganic salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCl, and LiBr, LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , organic salts such as LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) _2, etc., which are commonly used as electrolytes for non-aqueous electrolytes may be used. Of these, LiPF ₆ , LiBF ₄ or LiClO ₄ is preferably used.

  Although it does not specifically limit as a separator, The nonwoven fabric of polyolefin, a porous film, etc. can be used.

  The structure of the secondary battery is not particularly limited, and can be applied to any conventionally known form / structure such as a stacked (flat) battery or a wound (cylindrical) battery. In addition, the electrical connection form (electrode structure) in the lithium ion secondary battery can be applied to both (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

  The lithium ion secondary battery obtained as described above is based on the use of the electrode composition of the present invention, and adheres well even if the active materials and the active material and the current collector are repeatedly charged and discharged. In addition, since the binder component hardly swells in the electrolytic solution, it has good output characteristics.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to description of a following example.
In the examples, “parts” and “%” mean weight basis unless otherwise specified. “V” means volume reference.

[Measurement evaluation method]
First, the measurement evaluation method employed in the following examples will be described.

(1) Measurement of electrolyte swelling degree Polyvinyl acetal resin (A) is formed into a film of 100 × 15 × 0.02-0.05 mm, dried at 120 ° C. for 4 hours at 0.005 MPa or less, and its weight is measured. (W0 (g)). Next, this film was immersed in about 10 g of a predetermined electrolytic solution at 23 ° C. and kept until a constant weight was reached. Thereafter, the film was taken out, the surface of the film was lightly wiped with filter paper, and the weight was measured (W1 (g)). From these, the degree of swelling of the electrolyte was calculated according to the following formula.
Electrolyte swelling degree (%) = ((W1−W0) / W0) × 100
Diethyl carbonate and propylene carbonate were used as the electrolytic solution.

(2) Measurement of electrolyte solution elution degree The film after the electrolyte solution swelling degree measurement was dried at 120 ° C. and 0.005 MPa or less for 4 hours and weighed (W2 (g)).
Electrolytic solution elution degree (%) = (W0-W2) / W0 × 100

(3) Measurement of elastic modulus of swollen film Polyvinyl acetal resin (A) is formed into a film of 100 × 15 × 0.02 to 0.05 mm, immersed in about 10 g of a predetermined electrolytic solution at 23 ° C., until a constant weight is obtained. Kept. Thereafter, the film surface was taken out and lightly wiped with a filter paper, and the film surface was determined according to JIS K-7161 using Autograph AG-IS manufactured by Shimadzu Corporation at 23 ° C. and 50 RH%.
Diethyl carbonate and propylene carbonate were used as the electrolytic solution.

[Example 1] <Synthesis of polyvinyl acetal resin (A)>
A 6% by weight aqueous solution of a polyvinyl alcohol resin (degree of polymerization 2600 measured by JIS K6726, degree of saponification 99.5 mol%) was prepared. 670 parts of this aqueous solution was cooled to 10 ° C., and 4.2 parts of hydrochloric acid (concentration 35% by weight) was added.
While stirring this, 3.3 parts of butyraldehyde was added dropwise over 10 minutes, and stirring was continued for 1 hour. To this, 24.5 parts of hydrochloric acid (concentration 35% by weight) was added dropwise over 10 minutes. Thereafter, the temperature was raised to 25 ° C. and stirring was continued for 30 minutes, and then the temperature was raised to 60 ° C. and stirred for 6 hours to perform an acetalization reaction.
The reaction solution was neutralized with sodium carbonate, and the resulting precipitate was thoroughly washed with water and dried to obtain a light yellow solid polyvinyl acetal resin having a crosslinking group.
The obtained polyvinyl acetal resin was dissolved in DMSO-D6, and the degree of acetalization and the degree of crosslinking group modification were determined from the integral ratio of 1H-NMR. The structural unit content of the obtained polyvinyl acetal resin represented by the chemical formula (1) is 91 mol%, and the degree of acetalization of the structural unit represented by the chemical formula (2) (R1 is an alkyl group having 3 carbon atoms) is 9 Mol%.

<Creation of polyvinyl acetal resin (A) film>
Using the polyvinyl acetal resin (A) obtained as described above as a binder, 8 parts of the polyvinyl acetal resin (A) was dissolved in 92 parts of N-methylpyrrolidone (NMP) to prepare a solution. Such a solution does not contain water.
This solution was cast on a polyethylene terephthalate film with a 350 μm applicator and dried under reduced pressure at 60 ° C. and not more than 0.005 MPa to prepare a film with a thickness of 25 μm. Furthermore, this film was dried under reduced pressure at 120 ° C. and 0.005 MPa or less, and NMP was completely removed to obtain a polyvinyl acetal resin (A) film.
About this film, said (1)-(5) evaluation was performed.

[Comparative Example 1]
<Synthesis of polyvinyl acetal resin having many structural units represented by chemical formula (2)>
In Example 1, a polyvinyl acetal resin (light yellow solid) was obtained in the same manner except that 21.0 parts of butyraldehyde was used during the acetalization reaction. The structural unit content of the obtained polyvinyl acetal resin represented by the chemical formula (1) is 35 mol%, and the degree of acetalization of the structural unit represented by the chemical formula (2) (R1 is an alkyl group having 3 carbon atoms) is 65 Mol%.
Using this polyvinyl acetal resin, a film was produced in the same manner as in Example 1. About this film, said (1)-(5) evaluation was performed.
The evaluation results are shown in Table 1.

[Comparative Example 2]
<Synthesis of polyvinyl acetal resin having many structural units represented by chemical formula (2)>
In Example 1, a polyvinyl acetal resin (light yellow solid) was obtained in the same manner except that 13.0 parts of butyraldehyde was used during the acetalization reaction. The structural unit content of the obtained polyvinyl acetal resin represented by the chemical formula (1) is 58 mol%, and the degree of acetalization of the structural unit represented by the chemical formula (2) (R1 is an alkyl group having 3 carbon atoms) is 42 Mol%.
Using this polyvinyl acetal resin, a film was produced in the same manner as in Example 1. About this film, said (1)-(5) evaluation was performed.
The evaluation results are shown in Table 1.

[Table 1]

Example 1 uses the electrode composition of the present invention. When diethyl carbonate was used as the electrolytic solution, the swelling degree and elution degree of Example 1 were about 10% of Comparative Example 1 using a polyvinyl acetal resin having a very high degree of acetalization of 65 mol%, and the degree of acetalization. Was suppressed to about 25% of Comparative Example 2 using a slightly large polyvinyl acetal resin of 42 mol%, which was a very excellent result.
Moreover, the elasticity modulus of the film swollen with diethyl carbonate in Example 1 was as high as 35 times that of Comparative Example 1 and 14 times that of Comparative Example 2.

When propylene carbonate was used as the electrolyte, the swelling degree of Example 1 was about 50% of Comparative Example 1 using a polyvinyl acetal resin having a large degree of acetalization of 65 mol%, and the degree of acetalization was 42 mols. %, Which was suppressed to about 75% of Comparative Example 2 using a slightly large polyvinyl acetal resin, and the elution degree was suppressed to about 5% and about 33%, respectively.
Moreover, the elastic modulus of the film swollen with diethyl carbonate in Example 1 was as high as about twice that of Comparative Example 1 and 1.7 times that of Comparative Example 2.

  When evaluating the performance of the electrode obtained in the present invention, it is possible to evaluate a higher level of performance difference by evaluating the binder itself rather than evaluating the state where the electrode, that is, the active material and the binder coexist. It is. The composition for a lithium ion secondary battery electrode of the present invention is an electrode composition from which an electrode having very excellent performance can be obtained by evaluating the elution degree, swelling degree, and elastic modulus of the binder itself in the obtained electrode. It is clearly understood that Therefore, the electrode obtained by using the composition for a lithium ion secondary battery electrode of the present invention and the lithium ion secondary battery using the electrode can be used stably for a long time, and the discharge capacity is maintained with the charge / discharge cycle. It is clear that the rate is good.

  The binder contained in the composition for a lithium ion secondary battery electrode of the present invention has a low elution degree and swelling degree with respect to a carbonate ester electrolyte, and exhibits good adhesion to an active material and a current collector. The composition for a lithium ion secondary battery electrode of the present invention is useful as a composition for a lithium ion secondary battery electrode for obtaining a lithium ion secondary battery having good battery characteristics.

Claims (10)

A composition for a lithium secondary battery electrode containing an active material, a binder, and a solvent (B),
The binder includes a polyvinyl acetal resin (A) including structural units represented by the following chemical formulas (1) and (2),
The content of the structural unit (2) in the polyvinyl acetal resin (A) is 0.1 mol% or more and less than 10 mol%,
Content of water in the said solvent (B) is less than 20 weight% of a solvent (B), The composition for lithium ion secondary battery electrodes characterized by the above-mentioned.

(R1 is hydrogen or an alkyl group having 1 to 10 carbon atoms.)   The composition for a lithium ion secondary battery electrode according to claim 1, wherein the content of the structural unit (1) is 80 to 99.9 mol% in the polyvinyl acetal resin (A).   The composition for a lithium ion secondary battery electrode according to any one of claims 1 to 2, wherein the polyvinyl acetal resin (A) has an average degree of polymerization of 500 or more.   The lithium ion secondary battery electrode obtained using the composition for lithium ion secondary battery electrodes in any one of Claims 1-3. The composition for lithium ion secondary battery electrodes according to claim 1, wherein the polyvinyl acetal resin (A) further has a structural unit represented by the following chemical formula (3).

(R2 is a functional group containing an ethylenically unsaturated bond.)   The composition for a lithium ion secondary battery electrode according to claim 5, wherein the content of the structural unit (3) is 0.1 to 10 mol% in the polyvinyl acetal resin (A). The composition for a lithium ion secondary battery electrode according to any one of claims 5 and 6, comprising a polymerization initiator (C). The lithium ion secondary battery electrode containing the crosslinked material of the polyvinyl acetal type resin (A) of Claim 5. 9. The method for producing a lithium ion secondary battery electrode according to claim 8, wherein the polyvinyl acetal resin (A) is crosslinked by heat or light.   A lithium ion secondary battery comprising the lithium ion secondary battery electrode according to claim 4.