JP2015088487A - Composition for lithium secondary battery electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composition for a lithium secondary battery electrode which is superior in the active material dispersibility, adhesiveness and ion conductivity, and which allows a high-capacity lithium secondary battery to be fabricated even when an addition amount of a binder is small.SOLUTION: A composition for a lithium secondary battery electrode comprises an active material, a binder and an organic solvent. The binder contains a polyvinyl acetal resin. The polyvinyl acetal resin has: a constitutional unit having a hydroxyl group in a side chain; a constitutional unit having an acetyl group in a side chain; a constitutional unit having an acetal group in a side chain; and a constitutional unit having an ethylene oxide group in a side chain.

Description

The present invention relates to a composition for a lithium secondary battery electrode that is excellent in dispersibility, adhesiveness, and ion conductivity of an active material, and that can produce a high-capacity lithium secondary battery even when the amount of binder added is small. .

In recent years, with the widespread use of portable electronic devices such as portable video cameras and portable personal computers, the demand for secondary batteries as mobile power sources has increased rapidly. In addition, there are very high demands for such secondary batteries to be reduced in size, weight, and energy density.
Thus, as secondary batteries that can be repeatedly charged and discharged, conventionally, water-based batteries such as lead batteries and nickel-cadmium batteries have been mainstream, but these water-based batteries have excellent charge / discharge characteristics. However, in terms of battery weight and energy density, it cannot be said that the battery has sufficient characteristics as a power source for moving portable electronic devices.

Therefore, research and development of lithium secondary batteries using lithium or a lithium alloy as a negative electrode as a secondary battery has been actively conducted. This lithium secondary battery has excellent characteristics such as high energy density, low self-discharge, and light weight.
An electrode of a lithium secondary battery is usually a thin film obtained by kneading an active material and a binder together with a solvent, dispersing the active material into a slurry, applying the slurry on a current collector by a doctor blade method or the like, and drying the slurry. It is formed by forming.

Currently, in particular, fluorine-based resins represented by polyvinylidene fluoride (PVDF) are most widely used as binders for electrodes (negative electrodes) of lithium secondary batteries.
However, when a fluororesin is used as a binder, a flexible negative electrode thin film can be produced, but the binding property between the current collector and the negative electrode active material is inferior. There was a risk that some or all of the part would peel off or fall off from the current collector. In addition, when the battery is charged / discharged, lithium ions are repeatedly inserted and released in the negative electrode active material, and there is a problem that the negative electrode active material may be peeled off or dropped off from the current collector. .
In order to solve such a problem, an attempt has been made to add an excessive amount of a binder. However, along with this, the amount of the negative electrode active material added is relatively reduced, and the problem that the battery capacity is reduced is newly introduced. Had occurred.

On the other hand, Patent Document 1 discloses a technique using a mixture of a polymer insoluble in an electrolytic solution and a polymer soluble in the electrolytic solution as a binder used for a positive electrode and a negative electrode.
However, in such a technique, by using a polymer that is soluble in the electrolytic solution, the binder flows out to the electrolytic solution, and eventually, peeling and dropping of the negative electrode active material from the current collector cannot be suppressed. It was.

JP-A-10-214629

The present invention provides a composition for a lithium secondary battery electrode that is excellent in dispersibility, adhesiveness, and ion conductivity of an active material and that can produce a high-capacity lithium secondary battery even when the amount of binder added is small. The purpose is to provide.

The present invention is a composition for a lithium secondary battery electrode containing an active material, a binder, and an organic solvent, wherein the binder contains a polyvinyl acetal resin, and the polyvinyl acetal resin is represented by the following formula (1). The structural unit having a hydroxyl group, the structural unit having an acetyl group represented by the following formula (2), the structural unit having an acetal group represented by the following formula (3), and the following formula (4) It is a composition for lithium secondary battery electrodes which has a structural unit.
The present invention is described in detail below.

式（３）中、Ｒ^１は、水素原子又は炭素数１〜２０のアルキル基を表し、式（４）中、Ｒ^２は、エチレンオキサイド基を有する基を表す。

In formula (3), R ¹ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and in formula (4), R ² represents a group having an ethylene oxide group.

As a result of intensive studies, the present inventors use a polyvinyl acetal resin having a predetermined structural unit as a binder for forming a lithium secondary battery electrode, thereby being excellent in dispersibility, adhesiveness, and ion conductivity of the active material. The inventors have found that a high-capacity lithium secondary battery can be produced even when the amount of binder added is small, and have completed the present invention.

The composition for a lithium secondary battery electrode of the present invention contains an active material.
The composition for a lithium secondary battery electrode of the present invention may be used for either a positive electrode or a negative electrode, or may be used for both a positive electrode and a negative electrode. Accordingly, the active material includes a positive electrode active material and a negative electrode active material.

As the positive electrode active material, a lithium-containing transition metal oxide containing a transition metal element capable of occluding and releasing lithium is preferable.
As a typical example of the lithium-containing transition metal compound, for example, a general formula Li _x M1 _(1-yz) M2 _y M3 _z O ₂ (0.9 ≦ x ≦ 1.1, 0 ≦ y ≦ 1) 0.0, 0 ≦ z ≦ 1.0, M1, M2, and M3 include Co, Ni, Mn, Fe, Ti, and the like. Specifically, LiCoO ₂ , LiNi _1/2 Mn _1/2 O ₂ , LiNi _1/2 Co _1/2 O ₂ , LiNiO ₂ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , LiNi _{1 / 2 Fe 1/2 O 2, LiMnO} 2 , and the like.
Further, a lithium-excess active material having a layered structure is represented by the general formula Li [(Ni _0.5 Mn _0.5 ) _a Co _b (Li _1/3 Mn _1/3 ) _c ] O ₂ (where a + b + c = 1, c > 0) or Li _α Ni _β Mn _γ O ₂ (α is 1.1 or more and β: γ = 1: 1). Specifically, Li _1.2 Ni _0.4 Mn _0.4 O ₂ , Li [(Ni _0.5 Mn _0.5 ) _5/12 Co _1/4 (Li _1/3 Mn _2/3 ) _{1 / 3} ] O _{2 and the} like. Moreover, a spinel structure and an olivine structure may be sufficient. Specifically, LiMn ₂ O ₄ , LiFePO ₄ , LiCoPO ₄ , LiMnPO _4, etc. may be mentioned, and Al or the like may be included. Only one type of positive electrode active material may be used alone, or two or more types may be used in combination.

As the negative electrode active material, for example, a material conventionally used as a negative electrode active material of a lithium secondary battery can be used. For example, natural graphite (such as flake graphite), graphite such as artificial graphite, acetylene, etc. Carbon blacks such as black, ketjen black, channel black, furnace black, lamp black, and thermal black, carbon fibers, metal fibers, alloys, lithium metals, tin compounds, silicides, and nitrides can be used. These materials may be used alone or in combination of two or more.

The composition for a lithium secondary battery electrode of the present invention may contain a conductivity-imparting agent.
Examples of the conductivity-imparting agent include carbon materials such as graphite, acetylene black, carbon black, ketjen black, and vapor grown carbon fiber. In particular, acetylene black and carbon black are preferable as the conductivity imparting agent for the positive electrode, and acetylene black and scaly graphite are preferable as the conductivity imparting agent for the negative electrode.

The composition for a lithium secondary battery electrode of the present invention contains a polyvinyl acetal resin. In the present invention, when a polyvinyl acetal resin is used as a binder (binder) for a positive electrode, an attractive interaction acts between a hydroxyl group of the polyvinyl acetal resin and an oxygen atom of the positive electrode active material, and the positive electrode active material is converted into a polyvinyl acetal resin. Takes a structure that surrounds. Further, another hydroxyl group in the same molecule exerts an attractive interaction with the conductivity-imparting agent, so that the distance between the active material and the conductivity-imparting agent can be kept within a certain range. Thus, the dispersibility of an active material is improved significantly by taking a characteristic structure in a moderate distance between an active material and a conductivity-imparting agent. Moreover, compared with the case where resin, such as PVDF, is used, adhesiveness with a collector can be improved. Furthermore, there is an advantage that the solvent solubility is excellent and the range of solvent selection is widened.

The polyvinyl acetal resin has a structural unit having a hydroxyl group represented by the above formula (1), a structural unit having an acetyl group represented by the above formula (2), and an acetal group represented by the above formula (3). It has a structural unit and a structural unit represented by the above formula (4).
By having these structural units, the polyvinyl acetal resin can have excellent resistance to an electrolytic solution, adhesion with a current collector, and ion conductivity, even when the amount of binder added is reduced. There is an advantage that a high capacity lithium ion battery can be manufactured.

The polyvinyl acetal resin has a structural unit having a hydroxyl group represented by the formula (1).
The minimum with preferable content (hydroxyl group amount) of the structural unit which has a hydroxyl group represented by the said Formula (1) in the said polyvinyl acetal resin is 40 mol%, and a preferable upper limit is 65 mol%. When the amount of the hydroxyl group is less than 40 mol%, the resistance to the electrolytic solution becomes insufficient, and when the electrode is immersed in the electrolytic solution, the resin component may be eluted in the electrolytic solution. If it exceeds, not only industrial synthesis is difficult, but also the solution viscosity becomes high and it may be difficult to sufficiently disperse the active material.
A more preferred lower limit of the hydroxyl group content is 45 mol%, and a more preferred upper limit is 60 mol%.

The polyvinyl acetal resin has a structural unit having an acetyl group represented by the above formula (2).
The minimum with preferable content (acetyl group amount) of the structural unit which has an acetyl group represented by the said Formula (2) in the said polyvinyl acetal resin is 1 mol%, and a preferable upper limit is 20 mol%. If the amount of acetyl groups is less than 1 mol%, the flexibility of the resin may be insufficient, and the adhesive strength to the current collector may be insufficient. If the amount of acetyl groups exceeds 20 mol%, electrolysis will occur. Resistance to the liquid may be significantly reduced, and it may cause elution into the electrolyte and cause a short circuit. The more preferable lower limit of the acetyl group amount is 3 mol%, and the more preferable upper limit is 10 mol%.

The polyvinyl acetal resin has a structural unit having an acetal group represented by the above formula (3).
It is preferable that content (acetalization degree) of the structural unit which has an acetal group represented by the said Formula (3) in the said polyvinyl acetal resin is 40-65 mol%. When the degree of acetalization is less than 40 mol%, the solubility in a solvent is lowered, so that it is difficult to use the composition. When the degree of acetalization exceeds 65 mol%, the resistance to the electrolytic solution becomes insufficient, and the resin component may be eluted in the electrolytic solution when the electrode is immersed in the electrolytic solution. More preferably, it is 45-60 mol%.
In this specification, the degree of acetalization is the ratio of the number of hydroxyl groups acetalized with butyraldehyde in the number of hydroxyl groups of polyvinyl alcohol, and the calculation method of the degree of acetalization is as follows. Since the acetal group is formed by acetalization from two hydroxyl groups, a method of counting the two acetal hydroxyl groups is employed to calculate the mol% of the degree of acetalization.

In the above formula (3), R ¹ is a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and particularly preferably an alkyl group having 1 carbon atom or an alkyl group having 3 carbon atoms.
The polyvinyl acetal resin in which R ¹ is an alkyl group having 1 carbon atom or an alkyl group having 3 carbon atoms can be obtained by acetalization with acetaldehyde and butyraldehyde.
In the polyvinyl acetal resin, the ratio of the portion acetalized with acetaldehyde and the portion acetalized with butyraldehyde is preferably 0/100 to 50/50. Thereby, a polyvinyl acetal resin becomes flexible and the adhesive force to a collector becomes favorable. More preferably, the ratio of the portion acetalized with acetaldehyde and the portion acetalized with butyraldehyde is 0/100 to 20/80.

The said polyvinyl acetal resin has a structural unit represented by following formula (4).
By having the structural unit represented by the following formula (4), the oxygen atom of the ethylene oxide group can efficiently coordinate with the lithium ion, and the flow of the lithium ion can be efficiently formed. Therefore, the polyvinyl acetal resin can have ion conductivity and has an advantage that the capacity of the lithium ion battery can be increased.
In order to increase the ion conductivity, lithium and the structural unit represented by the following formula (4) have a bulky structure, and the glass transition point is lowered and the flexibility is increased by increasing the intermolecular distance. Therefore, there is also an advantage that the adhesive force to the current collector is remarkably improved.

式（４）中、Ｒ^２は、エチレンオキサイド基を有する基を表す。

In formula (4), R ² represents a group having an ethylene oxide group.

Examples of the structural unit represented by the above formula (4) include those having a plurality of ethylene oxide groups such as polyethylene glycol in the structural unit, those having an ethylene glycol unit alone in the structural unit, and discontinuous. In particular, those having a plurality of ethylene glycol units or polyethylene glycol units may be mentioned.
The structural unit represented by the above formula (4) is preferably a structural unit having an ethylene oxide group represented by the following formula (5). By having the ethylene oxide group, there is an advantage that the oxygen atom of the ethylene oxide group and lithium ions can be coordinated efficiently and have high ionic conductivity.
In order to further enhance the ion conductivity, the weight ratio of the polyvinyl acetal resin to the active material (the weight of the polyvinyl acetal resin / the weight of the active material) is preferably 0.002 to 0.2. By being in the above range, the polyvinyl acetal resin layer adsorbed on the active material surface does not become too thick, and the lithium ion atoms in the active material and the oxygen atoms in ethylene oxide in the polyvinyl acetal resin are efficiently provided. Therefore, the ionic conductivity can be increased.

式（５）中、Ｒ^３及びＲ^４は、Ｃ、Ｈ及びＯからなる群より選択される少なくとも１種を有する連結基又は単結合を表し、ｎは整数を表す。

In formula (5), R ³ and R ⁴ represent a linking group or a single bond having at least one selected from the group consisting of C, H and O, and n represents an integer.

R ³ is a linking group or a single bond having at least one selected from the group consisting of C, H and O. R ³ is preferably an alkylene group having 1 to 10 carbon atoms or a carbonyl group. When the number of carbon atoms of R ³ is within the above range, the ethylene oxide structural unit is not greatly separated from the main chain, and the hydroxyl group and the oxygen atom of the ethylene oxide unit interact with each other. It can be expanded. If it is out of the above range, the interaction between ethylene oxide works strongly, the ethylene oxide unit cannot be expanded, and the ionic conductivity cannot be increased efficiently. Examples of R ³ include a methylene group, an ethylene group, a carbonyl group, and an ether group. R ³ may be a single bond. R ⁴ is a linking group or a single bond having at least one selected from the group consisting of C, H and O. R ⁴ is preferably an alkylene group having 1 to 10 carbon atoms or a carbonyl group. Examples of R ⁴ include a methylene group, an ethylene group, a propylene group, a carbonyl group, and an ether group. R ⁴ may be a single bond.
Furthermore, although the integer n which is the repeating number of ethylene oxide is not specifically limited, 2-50 are preferable and 5-20 are more preferable. When the number of ethylene oxide repeats is within the above range, lithium ions cannot be held in a single molecule, and thus lithium ions can be efficiently conducted.

The minimum with preferable content of the structural unit represented by the said Formula (4) in the said polyvinyl acetal resin is 1 mol%, and a preferable upper limit is 20 mol%. If the content is less than 1 mol%, sufficient ion conductivity may not be obtained, and if the content exceeds 20 mol%, it may be difficult to maintain resistance to the electrolytic solution. There is. The more preferable lower limit of the content is 3 mol%, and the more preferable upper limit is 10 mol%.

The mol% ratio of the content of the structural unit having a hydroxyl group represented by the formula (1) in the polyvinyl acetal resin and the content of the structural unit represented by the formula (4) [formula (4) / formula ( 1) Ratio] is preferably 0.01 to 0.5. By setting it within the above range, ethylene oxide can freely move and lithium ions can be efficiently conducted.
When the ratio of the above formula (4) / formula (1) is less than 0.01, the interaction between the oxygen atom and the hydroxyl group of ethylene oxide is too strong and the structural unit represented by the above formula (4) is free. Cannot move efficiently with lithium ions. Moreover, when the ratio of the above formula (4) / formula (1) exceeds 0.5, the interaction between the oxygen atoms of the structural unit represented by the above formula (4) works too strongly, and the lithium ion and the efficiency In some cases, sufficient ionic conductivity may not be obtained. The ratio of the above formula (4) / formula (1) is more preferably 0.02 to 0.4.

The minimum with a preferable polymerization degree of the said polyvinyl acetal resin is 250, and a preferable upper limit is 4000. If the degree of polymerization is less than 250, it may be difficult to obtain industrially. When the polymerization degree exceeds 4000, the solution viscosity becomes high and it may be difficult to sufficiently disperse the active material. The more preferable lower limit of the degree of polymerization is 280, and the more preferable upper limit is 800.

The polyvinyl acetal resin preferably has an anionic group.
By having the anionic group, the polyvinyl acetal resin easily adheres to the surface of the active material, and the dispersibility of the active material can be improved.

Although content of the said polyvinyl acetal resin in the composition for lithium secondary battery electrodes of this invention is not specifically limited, A preferable minimum is 0.2 weight% and a preferable upper limit is 5 weight%. When the content of the polyvinyl acetal resin is less than 0.2% by weight, the adhesive force to the current collector may be insufficient. When the content exceeds 5% by weight, the discharge capacity of the lithium secondary battery decreases. May end up. More preferably, it is 0.5 to 3% by weight.

The polyvinyl acetal resin is obtained by acetalizing polyvinyl alcohol with an aldehyde.
In particular, as a method for producing the polyvinyl acetal resin, a method of preparing polyvinyl alcohol having a structural unit represented by the above formula (4) in advance and then acetalizing it, a structural unit represented by the above formula (4) And a method of adding a portion corresponding to R ² of the structural unit represented by the above formula (4) after acetalizing polyvinyl alcohol having no benzene.

As a method for producing polyvinyl alcohol having the structural unit represented by the above formula (4), for example, after copolymerizing hydroxyalkyl vinyl ether containing oxyalkylene group and vinyl acetate, Examples thereof include a method for saponifying an alcohol solution by adding an acid or an alkali. In addition to the above hydroxyalkyl vinyl ether, poly (ethylene glycol) vinyl ethers such as diethylene glycol vinyl ether and triethylene glycol monovinyl ether may be used.
Moreover, as a method of adding the part corresponding to R ² of the structural unit represented by the above formula (4), for example, a method of changing the kind of the alkyl vinyl ether can be mentioned.

Polyvinyl alcohol not having the structural unit represented by the above formula (4) (hereinafter also simply referred to as polyvinyl alcohol) can be obtained, for example, by saponifying a copolymer of vinyl ester and ethylene. Examples of the vinyl ester include vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate and the like. Of these, vinyl acetate is preferred from the viewpoint of economy.

The polyvinyl alcohol may be a copolymer of 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 acrylic acid, methacrylic acid, (anhydrous) phthalic acid, (anhydrous) maleic acid, (anhydrous) itaconic acid, acrylonitrile methacrylonitrile, acrylamide, and methacrylamide. , Trimethyl- (3-acrylamido-3-dimethylpropyl) -ammonium chloride, acrylamido-2-methylpropanesulfonic acid and its sodium salt, ethyl vinyl ether, butyl vinyl ether, N-vinyl pyrrolidone, vinyl chloride, vinyl bromide, fluorine Vinyl chloride, vinylidene chloride, vinylidene fluoride, tetrafluoroethylene, sodium vinyl sulfonate, sodium allyl sulfonate and the like. 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.

The polyvinyl alcohol may be a saponified copolymer obtained by copolymerizing the vinyl ester and an α-olefin. Furthermore, it is good also as polyvinyl alcohol which copolymerizes the said ethylenically unsaturated monomer and contains the component originating in an ethylenically unsaturated monomer. Also used is a terminal polyvinyl alcohol obtained by copolymerizing a vinyl ester monomer such as vinyl acetate with an α-olefin in the presence of a thiol compound such as thiol acetic acid or mercaptopropionic acid, and saponifying it. be able to. The α-olefin is not particularly limited, and examples thereof include methylene, ethylene, propylene, isopropylene, butylene, isobutylene, pentylene, hexylene, cyclohexylene, cyclohexylethylene, and cyclohexylpropylene.

In addition to the said polyvinyl acetal resin, the composition for lithium secondary battery electrodes of this invention may contain the polyvinylidene fluoride resin.
By using the polyvinylidene fluoride resin in combination, the resistance to the electrolytic solution can be further improved, and the discharge capacity can be improved.

When the polyvinylidene fluoride resin is contained, the weight ratio of the polyvinyl acetal resin to the polyvinylidene fluoride resin is preferably 0.5: 9.5 to 7: 3.
By setting it within such a range, it is possible to impart resistance to the electrolytic solution while having adhesive force to the current collector that is significantly insufficient for polyvinylidene fluoride.
A more preferred weight ratio of the polyvinyl acetal resin to the polyvinylidene fluoride resin is 1: 9 to 4: 6.

Although content of the whole binder in the composition for lithium secondary battery electrodes of this invention is not specifically limited, A preferable minimum is 1 weight% and a preferable upper limit is 20 weight%. If the content of the binder is less than 1% by weight, the adhesive force to the current collector may be insufficient, and if it exceeds 20% by weight, the discharge capacity of the lithium secondary battery may be reduced. There is.

The composition for a lithium secondary battery electrode of the present invention contains an organic solvent.
The organic solvent is not particularly limited as long as it can dissolve the polyvinyl acetal resin. For example, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, tetrahydrofuran, toluene, isopropyl alcohol, N-methylpyrrolidone, ethanol, distillation Water etc. are mentioned. Of these, N-methylpyrrolidone is preferable.
The said organic solvent may be used independently and may use 2 or more types together.

Although content of the organic solvent in the composition for lithium secondary battery electrodes of this invention is not specifically limited, A preferable minimum is 20 weight% and a preferable upper limit is 50 weight%. If the content of the organic solvent is less than 20% by weight, the viscosity may be high, so that paste coating may be difficult. If the content exceeds 50% by weight, unevenness may occur when the solvent is dried. . A more preferred lower limit is 25% by weight, and a more preferred upper limit is 40% by weight.

In addition to the above-described active material, polyvinyl acetal resin, and solvent, the composition for a lithium secondary battery electrode of the present invention includes a flame retardant aid, a thickener, an antifoaming agent, a leveling agent, and an adhesion as necessary. An additive such as a property-imparting agent may be added.

The method for producing the composition for a lithium secondary battery electrode of the present invention is not particularly limited. For example, the active material, the polyvinyl acetal resin, the solvent, and various additives to be added as necessary may be a ball mill, a blender mill, The method of mixing using various mixers, such as a 3 roll, is mentioned.

The electrode for lithium secondary battery electrode composition of the present invention is formed, for example, through a process of coating on a conductive substrate and drying.
A lithium secondary battery using the composition for a lithium secondary battery electrode of the present invention is also one aspect of the present invention.
As a coating method when the composition for a lithium secondary battery electrode of the present invention is applied to a conductive substrate, various coating methods such as an extrusion coater, a reverse roller, a doctor blade, an applicator and the like can be adopted. it can.

ADVANTAGE OF THE INVENTION According to this invention, the composition for lithium secondary battery electrodes which is excellent in the dispersibility of an active material, adhesiveness, and ionic conductivity, and can produce a high capacity | capacitance lithium secondary battery even when there is little addition amount of a binder. Can provide things.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

(Synthesis of polyvinyl alcohol (a) to (j))
In a flask equipped with a stirrer, a thermometer, a dropping funnel and a reflux condenser, 1000 parts by weight of vinyl acetate, R ^{3 of the} chemical formula represented by the formula (5) is an ethylene group, and n = 10 After adding 320 parts by weight of ethylene) vinyl ether and 300 parts by weight of methanol and performing nitrogen substitution in the system, the temperature was raised to 60 ° C. To this system, 1.1 parts by weight of 2,2-azobisisobutyronitrile was added to initiate polymerization. The polymerization was stopped in 5 hours from the start of polymerization. When the polymerization was stopped, the solid content concentration in the system was 53% by weight, and the polymerization yield based on all monomers was 65% by weight. After removing unreacted monomers under reduced pressure, a 45 wt% methanol solution of the copolymer was obtained. It was confirmed by quantitative determination of unreacted monomers that the obtained copolymer contained 94 mol% of vinyl acetate units and 6 mol% of alkyl vinyl ether units containing ethylene oxide.

While stirring 100 parts by weight of the methanol solution of the copolymer at 40 ° C., 25 parts by weight of 3% NaOH methanol solution was added and mixed well, and then allowed to stand. After 30 minutes, the solidified polymer was pulverized with a pulverizer, washed with methanol, and dried to obtain a polymer powder (hereinafter referred to as polyvinyl alcohol (a)).
Polyvinyl alcohol (a) had a saponification degree of 99 mol% and a polymerization degree of 1200.
Moreover, polyvinyl alcohol (b)-(j) was obtained like polyvinyl alcohol (a) except having described in Table 1.

(Synthesis of polyvinyl acetal resins (A) to (J))
350 parts by weight of polyvinyl alcohol (a) was added to 3000 parts by weight of pure water, and stirred at a temperature of 90 ° C. for about 2 hours for dissolution. The solution was cooled to 40 ° C., and 230 parts by weight of hydrochloric acid having a concentration of 35% by weight was added thereto, the liquid temperature was lowered to 20 ° C., and 155 parts by weight of n-butyraldehyde was added to maintain the temperature, and the acetal was maintained. The reaction product was precipitated by the reaction. Thereafter, the liquid temperature was maintained at 50 ° C. for 3 hours to complete the reaction, and neutralized, washed with water and dried by a conventional method to obtain a white powder of polyvinyl acetal resin. The obtained polyvinyl acetal resin is dissolved in DMSO-d ₆ (dimethylsulfoxaside), and the content of the structural unit represented by the above formula (1) using ¹³ C-NMR (nuclear magnetic resonance spectrum) [hydroxyl group] Amount], the content of the structural unit represented by the above formula (2) [acetyl group amount], the content of the structural unit represented by the above formula (3) [degree of acetalization], and the above formula (4). When the content of the structural unit represented by the formula (4), the amount of hydroxyl groups is 50 mol%, the amount of acetyl groups is 1 mol%, the degree of acetalization (degree of butyralization) is 44 mol%, The unit amount of the formula (4) was 5 mol%.
The structural unit represented by the above formula (4) was a structure represented by the above formula (5) (R ³ = ethylene group, n = 10).
Moreover, polyvinyl acetal resin (B)-(J) was synthesize | combined like polyvinyl acetal resin (A) except having set it as the conditions shown in Table 2 (use polyvinyl alcohol, n-butyraldehyde usage-amount).

Example 1
(Preparation of composition for positive electrode of lithium secondary battery)
90 parts by weight of N-methylpyrrolidone was added to 10 parts by weight of the polyvinyl acetal resin (A) obtained as a binder to prepare a 10% by weight solution of the polyvinyl acetal resin. With respect to 32.5 parts by weight of this solution, 55 parts by weight of lithium manganate (manufactured by Nippon Chemical Industry Co., Ltd., Cell Seed C-5H) as the positive electrode active material, and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., Denka Black) Was added and mixed with 6 parts by weight of N-methylpyrrolidone as a solvent to obtain a composition for a positive electrode of a lithium secondary battery.

(Examples 2-9)
A composition for a positive electrode of a lithium secondary battery was obtained in the same manner as in Example 1 except that the polyvinyl acetal resin shown in Table 3 was used and the addition amount of N-methylpyrrolidone was changed.

(Example 10)
(Preparation of composition for negative electrode of lithium secondary battery)
96 parts by weight of N-methylpyrrolidone was added to 4 parts by weight of the polyvinyl acetal resin (A) obtained as a binder to prepare a 4% by weight solution of the polyvinyl acetal resin. With respect to 100 parts by weight of this solution, 43 parts by weight of spherical natural graphite (manufactured by Nippon Graphite Industry Co., Ltd., CGB-10) as a negative electrode active material was added and mixed to obtain a composition for a negative electrode of a lithium secondary battery.

(Examples 11 to 17)
A composition for a lithium secondary battery negative electrode was obtained in the same manner as in Example 10 except that the polyvinyl acetal resin shown in Table 3 was used and the addition amount of N-methylpyrrolidone was changed.

(Example 18)
Polyvinyl chloride shown in Table 3 was used instead of 43 parts by weight of spherical natural graphite (manufactured by Nippon Graphite Industry Co., Ltd., CGB-10) as a negative electrode active material, and 43 parts by weight of artificial graphite (manufactured by Nippon Graphite Industry Co., Ltd., PAG-5). A composition for a negative electrode of a lithium secondary battery was obtained in the same manner as in Example 10 except that the amount of N-methylpyrrolidone was changed using an acetal resin.

(Comparative Examples 1-3)
A composition for a positive electrode of a lithium secondary battery was obtained in the same manner as in Example 1 except that the polyvinyl acetal resin shown in Table 3 was used and the addition amount of N-methylpyrrolidone was changed.

(Comparative Example 4)
A lithium secondary battery positive electrode composition was obtained in the same manner as in Example 1 except that polyvinylidene fluoride (PVDF, manufactured by Kureha, # 1100) was used instead of the polyvinyl acetal resin (A).

(Comparative Examples 5-7)
A composition for a lithium secondary battery negative electrode was obtained in the same manner as in Example 10 except that the polyvinyl acetal resin shown in Table 3 was used and the addition amount of N-methylpyrrolidone was changed.

(Comparative Example 8)
A lithium secondary battery negative electrode composition was obtained in the same manner as in Example 10 except that polyvinylidene fluoride (PVDF, manufactured by Kureha Corporation, # 1100) was used instead of the polyvinyl acetal resin (A).

<Evaluation>
The following evaluation was performed about the composition for lithium secondary battery electrodes (for positive electrodes and negative electrodes) obtained by the Example and the comparative example. The results are shown in Table 3.

(1) Adhesiveness About the compositions for lithium secondary battery positive electrodes obtained in Examples 1 to 9 and Comparative Examples 1 to 4, the adhesiveness to aluminum foil was evaluated, and Examples 10 to 18 and Comparative Examples 5 to 5 were evaluated. About the composition for lithium secondary battery negative electrodes obtained by 8, adhesiveness with respect to copper foil was evaluated.

(1-1) Examples 1 to 9 and Comparative Examples 1 to 4
On the aluminum foil (thickness 20 μm), the electrode composition was applied and dried so that the film thickness after drying was 20 μm, to obtain a test piece in which the electrode was formed in a sheet shape on the aluminum foil.
This sample was cut into a length of 1 cm and a width of 2 cm, and the electrode sheet was pulled up while fixing the test piece using AUTOGRAPH (manufactured by Shimadzu Corporation, “AGS-J”) until the electrode sheet was completely peeled from the aluminum foil. The required peel force (N) was measured.

(1-2) Examples 10 to 18 and Comparative Examples 5 to 8
In the above (1-1), the peel force was measured in exactly the same manner except that the aluminum foil was changed to a copper foil (thickness 20 μm).

(2) Dispersibility After mixing and diluting 10 parts by weight of the obtained composition for a lithium secondary battery electrode and 90 parts by weight of N-methylpyrrolidone, an ultrasonic disperser (manufactured by SNDI, “US-303”) was used. And stirred for 10 minutes. Thereafter, the particle size distribution was measured using a laser diffraction particle size distribution meter (LA-910, manufactured by Horiba, Ltd.), and the average particle size was measured.

(3) Solvent solubility (production of electrode sheet)
On the polyethylene terephthalate (PET) film subjected to the release treatment, the lithium secondary battery electrode compositions obtained in Examples and Comparative Examples were applied and dried so that the film thickness after drying was 20 μm. A sheet was produced.
The electrode sheet was cut into a 2 cm square to prepare an electrode sheet test piece.

(Elution evaluation)
The weight of the obtained test piece was accurately measured, and the weight of the resin contained in the test piece was calculated from the component weight ratio contained in the sheet. Thereafter, the test piece was put in a bag-like mesh, and the total weight of the mesh bag and the test piece was accurately measured.
Next, the mesh bag containing the test piece was immersed in diethyl carbonate, which is an electrolytic solution, and left overnight at room temperature. After standing, the mesh bag was taken out and dried under conditions of 150 ° C. and 8 hours to completely dry the solvent.
After taking out from the dryer, it was left at room temperature for 1 hour and weighed. The elution amount of the resin was calculated from the weight change before and after the test, and the elution rate of the resin was calculated from the ratio between the elution amount and the weight of the resin calculated in advance, and evaluated according to the following criteria. In addition, it means that resin is easy to elute in electrolyte solution, so that the value of elution rate is high.

(4) A composition for a lithium secondary battery electrode is coated on an ion conductive aluminum foil (thickness 20 μm) so that the film thickness after drying is 20 μm, and the electrode is formed into a sheet on the aluminum foil. A test piece formed in the above was obtained.
While the obtained test piece was dried under reduced pressure in a glove box, a lead wire was attached, and ion conductivity was measured using an impedance analyzer (manufactured by Toyo Technica).

(5) Battery performance evaluation (a) Preparation of coin battery The lithium secondary battery positive electrode composition obtained in Example 1 was applied to an aluminum foil, dried to a thickness of 0.2 mm, and punched out to φ12 mm. A positive electrode layer was obtained.
Moreover, the composition for lithium secondary battery negative electrodes obtained in Example 10 was applied to a copper foil and dried to a thickness of 0.2 mm, which was punched into φ12 mm to obtain a negative electrode layer.
A mixed solvent with ethylene carbonate containing LiPF ₆ (1M) was used as the electrolytic solution, and after impregnating the electrolytic solution into the positive electrode layer, the positive electrode layer was placed on the positive electrode current collector, and further on the electrolytic solution A porous PP membrane (separator) having a thickness of 25 mm impregnated with was placed.
Further, a lithium metal plate serving as a negative electrode layer was placed thereon, and a negative electrode current collector covered with insulating packing was placed thereon. Pressure was applied to the laminate with a caulking machine to obtain a sealed coin battery.

(B) Evaluation of discharge capacity and evaluation of charge / discharge cycle The obtained coin battery was subjected to a discharge capacity evaluation and a charge / discharge cycle evaluation using a charge / discharge test apparatus manufactured by Hosen Co., Ltd.
This discharge capacity evaluation and charge / discharge cycle evaluation were performed at a voltage range of 3.0 to 4.4 V and an evaluation temperature of 20 ° C.

ADVANTAGE OF THE INVENTION According to this invention, the composition for lithium secondary battery electrodes which is excellent in the dispersibility of an active material, adhesiveness, and ionic conductivity, and can produce a high capacity | capacitance lithium secondary battery even when there is little addition amount of a binder. Can provide things.

Claims (7)

A composition for an electrode of a lithium secondary battery containing an active material, a binder and an organic solvent,
The binder contains a polyvinyl acetal resin,
The polyvinyl acetal resin has a structural unit having a hydroxyl group represented by the following formula (1), a structural unit having an acetyl group represented by the following formula (2), and an acetal group represented by the following formula (3). It has a structural unit and a structural unit represented by following formula (4), The composition for lithium secondary battery electrodes characterized by the above-mentioned.

In formula (3), R ¹ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and in formula (4), R ² represents a group having an ethylene oxide group. 2. The composition for a lithium secondary battery electrode according to claim 1, wherein the structural unit represented by the formula (4) is a structural unit having an ethylene oxide group represented by the following formula (5).

In Formula (5), R ³ and R ⁴ represent a linking group or a single bond having at least one selected from the group consisting of C, H and O, and n represents an integer. The composition for lithium secondary battery electrodes according to claim 1 or 2, wherein the polyvinyl acetal resin contains 1 to 20 mol% of a structural unit represented by the formula (4). The composition for a lithium secondary battery electrode according to claim 1, wherein the polyvinyl acetal resin has a polymerization degree of 250 to 4000. 5. The composition for a lithium secondary battery electrode according to claim 1, comprising 0.2 to 5% by weight of a polyvinyl acetal resin. 6. The composition for a lithium secondary battery electrode according to claim 1, wherein the organic solvent is contained in an amount of 20 to 50% by weight. A lithium secondary battery comprising the composition for a lithium secondary battery electrode according to claim 1, 2, 3, 4, 5 or 6.