JP6434772B2 - Lithium secondary battery electrode composition - Google Patents

Description

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

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.

At present, in particular, fluorine-based resins represented by polyvinylidene fluoride (PVDF) are most widely used as binders for electrodes of lithium secondary batteries.
However, when a fluororesin is used as a binder, a flexible thin film can be produced, but the binding property between the current collector and the active material is inferior. May peel off from the current collector. In addition, when the battery is charged / discharged, lithium ions are repeatedly inserted and released in the active material, and there is a problem that the 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 use a binder other than PVDF. However, when the conventional resin is used, there is a new problem that the viscosity of the paste increases when the electrode paste is left for a long time. When such thickening occurs over time, it has become difficult to adjust the viscosity to be suitable for application to the current collector, resulting in a new problem of greatly reducing productivity.

On the other hand, Patent Document 1 discloses a technique using a special urethane polymer as a binder used for the positive electrode and the negative electrode for the purpose of improving the storage stability of the slurry.
However, with such a technique, although the adhesive force is improved to some extent, it has not been possible to prevent thickening of the paste over time.

Japanese Patent Laid-Open No. 2007-200897

The present invention is a lithium secondary battery that is excellent in dispersibility and adhesiveness of an active material, has little viscosity increase with time, and can produce a high-capacity lithium secondary battery even when the amount of binder added is small. It aims at providing the composition for electrodes.

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). structural unit having a hydroxyl group is a structural unit having an acetal group represented by the following formula (2), and, have a structural unit having a nitrile group represented by the following formula (3), the organic solvent 20 It is a composition for lithium secondary battery electrodes containing -50 wt% .
The present invention is described in detail below.

式（２）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（３）中、Ｒ^２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種を有する官能基を表し、Ｒ^３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 (2), R ¹ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and in Formula (3), R ² and R ⁴ are at least selected from the group consisting of C, H and O. A functional group having one type is represented, R ³ represents a linking group or a single bond having at least one type selected from the group consisting of C, H and O, and n represents an integer.

As a result of intensive studies, the present inventors have achieved excellent dispersibility and adhesiveness of the active material by using a polyvinyl acetal resin having a predetermined structural unit as a binder for forming a lithium secondary battery electrode. The present inventors have found that a high capacity lithium secondary battery can be produced even when the amount of thickening is small and the amount of binder added is small, and the present invention has been completed.

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.

Examples of the positive electrode active material include lithium-containing composite metal oxides such as lithium nickel oxide, lithium cobalt oxide, and lithium manganese oxide. Specifically, for _{example, LiNiO 2, LiCoO 2, LiMn} 2 O 4 and the like.
In addition, these may be used independently and may use 2 or more types together.

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, spherical natural graphite, natural graphite, artificial graphite, amorphous carbon, carbon black, or And those obtained by adding different elements to these components.

The composition for a lithium secondary battery electrode of the present invention preferably contains 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, by using a polyvinyl acetal resin as a binder (binder), an attractive interaction acts between the hydroxyl group of the polyvinyl acetal resin and the oxygen atom of the positive electrode active material, and the positive electrode active material is surrounded by the polyvinyl acetal resin. Take. 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 includes a structural unit having a hydroxyl group represented by the above formula (1), a structural unit having an acetal group represented by the above formula (2), and a nitrile group represented by the above formula (3). A structural unit having
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.
Further, even when the electrode paste is left for a long time, there is an advantage that the viscosity does not increase and it is easy to adjust the viscosity to be suitable for application to the current collector.

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 acetal group represented by the above formula (2).
It is preferable that content (acetalization degree) of the structural unit which has an acetal group represented by the said Formula (2) 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 (2), 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 polyvinyl acetal resin has a structural unit having a nitrile group represented by the above formula (3).
By having the structural unit represented by the above formula (3), the nitrogen atom of the nitrile group is adsorbed on the surface of the carbon or the active material, so that when the paste is made from steric hindrance, the viscosity increase with time is prevented. There is an advantage that can be.

The structural unit having a nitrile group represented by the above formula (3) has a structure in which a polymer unit having a nitrile group is bonded to the main chain via an acetyl group. As a result, the distance between the nitrile group and the main chain is increased, so that the nitrile group can move freely without being hindered by the bulkiness of the main chain, and it is easy to adsorb on the surface of the active material. Can effectively prevent thickening.
Further, the structural unit having a nitrile group represented by the above formula (3) has a structure like a graft polymer in which a polymer unit having a nitrile group is grafted. Thereby, it becomes possible to prevent thickening with time without impairing the excellent adhesiveness and dispersibility of the polyvinyl acetal resin.

R ² is a functional group having at least one selected from the group consisting of C, H and O. Examples of R ² include a hydrogen atom, an alkyl group, an allyl group, an alkoxy group, a hydroxyl group, a carboxyl group, and an amino group. Of these, a functional group consisting only of C, H, and O is preferable, and specifically, a hydrogen atom or an alkyl group is preferable. Moreover, a relatively small functional group is preferable so as not to inhibit polymerization reactivity, and the alkyl group is preferably a linear alkyl group or an alkyl group having 1 to 6 carbon atoms.

R ³ is a linking group or a single bond having at least one selected from the group consisting of C, H and O. Examples of the linking group include an alkylene group, an alkyleneoxy group, an arylene group, a carbonyl group, an ether group, an imino group, and an amide bond.
Especially, the functional group which consists only of C, H, and O is preferable, and as said R < ³ >, a single bond, an alkylene group, a carbonyl group, and an ether group are preferable. Moreover, a relatively small functional group is preferable so as not to inhibit polymerization reactivity, and the alkylene group is preferably a linear alkylene group or an alkylene group having 1 to 6 carbon atoms.

R ⁴ is a functional group having at least one selected from the group consisting of C, H and O. Examples of R ⁴ include a hydrogen atom, an alkyl group, an allyl group, an alkoxy group, a hydroxyl group, a carboxyl group, and an amino group. Especially, the functional group which consists only of C, H, and O is preferable, and a hydrogen atom, an alkoxy group, and a hydroxyl group are preferable.
As said alkoxy group, a C1-C6 alkoxy group is preferable. Specific examples include a methoxy group, an ethoxy group, an n-propoxy group, an isopropoxy group, an n-butoxy group, an isobutoxy group, a sec-butoxy group, a tert-butoxy group, and a pentyloxy group.

In said formula (3), the preferable minimum of n is 30 and a preferable upper limit is 1000.
If n is less than 30, the number of nitrile groups relative to the molecular chain is too small, and the effect of preventing thickening over time may not be sufficiently obtained. When n exceeds 1000, the characteristics of the polyvinyl acetal resin cannot be sufficiently obtained, and the adhesiveness and dispersibility may be insufficient.
The more preferable lower limit of n is 100, and the more preferable upper limit is 500.

The minimum with preferable content of the structural unit which has a nitrile group represented by the said Formula (3) 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%, the number of nitrile groups relative to the molecular chain is too small, so the effect of preventing thickening over time may not be sufficiently obtained. If the content exceeds 20 mol%, It may be difficult to maintain resistance to the electrolyte. The more preferable lower limit of the content is 3 mol%, and the more preferable upper limit is 10 mol%.

The polyvinyl acetal resin preferably has a structural unit having an acetyl group represented by the following formula (4).
The minimum with preferable content (acetyl group amount) of the structural unit which has an acetyl group 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 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 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 in which polyvinyl alcohol having a structural unit having a nitrile group represented by the above formula (3) is prepared in advance and then acetalized, represented by the above formula (3) is used. Examples include a method of acetalizing polyvinyl alcohol having no nitrile group-containing structural unit and then adding a portion to be a structural unit having a nitrile group represented by the above formula (3).

As a method for producing polyvinyl alcohol having a structural unit having a nitrile group represented by the above formula (3), for example, a nitrile monomer represented by the following formula (5) and a vinyl ester such as vinyl acetate are used. Examples thereof include a method of saponification by adding an acid or an alkali to the alcohol solution of the obtained copolymer after the copolymerization.
Moreover, you may use the method of making a nitrile monomer react with the polyvinyl alcohol which does not have the structural unit which has a nitrile group represented by the said Formula (3).

式（５）中、Ｒ^２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種を有する官能基を表し、Ｒ^３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 ² and R ⁴ represent a functional group having at least one selected from the group consisting of C, H and O, and R ³ is selected from the group consisting of C, H and O A linking group having at least one kind or a single bond, and n represents an integer.

Moreover, as a method of adding a part corresponding to the structural unit having a nitrile group represented by the above formula (3), for example, a polyvinyl acetal having no structural unit having a nitrile group represented by the above formula (3) Examples thereof include a method of reacting a nitrile monomer with a resin.
Examples of the nitrile monomer include acrylonitrile, methacrylonitrile, 2-cyanomethyl-1-butene, 2-cyanocarbonyl-1-hexene, 2-chloroacrylonitrile, 2-methyleneglutaronitrile, allyl cyanoacetate, and the like.
In the method of adding a portion corresponding to the structural unit having a nitrile group represented by the above formula (3), the nitrile polymer composed of the nitrile monomer is bonded to the acetyl group of the polyvinyl acetal resin to form a graft polymer. It is preferable.

When using the method of adding a portion corresponding to the structural unit having a nitrile group represented by the above formula (3), the polyvinyl acetal resin not having the structural unit having a nitrile group represented by the above formula (3) is It is preferable to have a structural unit having an acetyl group represented by the above formula (4).
The minimum with preferable content of the structural unit which has an acetyl group represented by the said Formula (4) in the polyvinyl acetal resin which does not have the structural unit which has a nitrile group represented by the said Formula (3) is 0.5 mol%, A preferable upper limit is 20 mol%. If the amount of acetyl groups is less than 0.5 mol%, the amount may be insufficient. If the amount of acetyl groups exceeds 20 mol%, the resistance to the electrolyte solution is significantly reduced and the electrolyte solution is eluted. May 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%.

Polyvinyl alcohol not having a structural unit having a nitrile group represented by the above formula (3) (hereinafter also simply referred to as polyvinyl alcohol) can be obtained, for example, by saponifying a copolymer of vinyl ester and ethylene. it can. 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.

According to the present invention, a lithium secondary battery that is excellent in dispersibility and adhesiveness of an active material, has little viscosity increase with time, and can produce a high-capacity lithium secondary battery even when the amount of binder added is small. A composition for a secondary battery electrode can be provided.

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 acetal resins (a) to (e))
350 parts by weight of polyvinyl alcohol 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., 230 parts by weight of hydrochloric acid having a concentration of 35% by weight was added thereto, the temperature of the solution was lowered to 5 ° C., and 155 parts by weight of n-butyraldehyde was added to maintain the temperature and acetal was maintained. The reaction product was precipitated by the reaction. Thereafter, the liquid temperature was maintained at 30 ° 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], and the content of the structural unit represented by the above formula (3) [degree of acetalization] were measured. 50 mol%, the amount of acetyl groups was 1 mol%, and the degree of acetalization (degree of butyralization) was 49 mol%.
Further, the polyvinyl acetal resins (b) to (b) were used in the same manner as the polyvinyl acetal resin (a) except that the conditions shown in Table 1 were used (polyvinyl alcohol, n-butyraldehyde use amount, hydrochloric acid use amount, charging temperature). e) was synthesized.

(Synthesis of modified polyvinyl acetal resins (A) to (K))
In a reaction vessel equipped with a thermometer, a stirrer, a nitrogen introduction tube and a cooling tube, 8 parts by weight of the obtained polyvinyl butyral (a), 8 parts by weight of acrylonitrile as a graft monomer, and 84 parts by weight of N-methylpyrrolidone are added and stirred. While the polyvinyl butyral was dissolved. Next, nitrogen gas was blown for 30 minutes to replace the inside of the reaction vessel with nitrogen, and then the inside of the reaction vessel was heated to 70 ° C. while stirring. After 30 minutes, 0.5 part by weight of t-butyl peroxypivalate as a polymerization initiator was added dropwise to the reactor over 5 hours. Thereafter, the reaction was further carried out at 70 ° C. for 3 hours.
Subsequently, the reaction solution is cooled to obtain a resin solution (A) having a solid content of 15% by weight, which contains the modified polyvinyl acetal resin (A) having a structural unit having a nitrile group represented by the above formula (3). It was. The obtained polyvinyl acetal resin (A) is dissolved in DMSO-d ₆ (dimethylsulfoxaside), and contains the structural unit represented by the above formula (1) using ¹³ C-NMR (nuclear magnetic resonance spectrum). Amount [hydroxyl group amount], content of structural unit represented by the above formula (2) [degree of acetalization], structural unit having a nitrile group represented by formula (3) [nitrile-containing group content], the above formula ( When the content of the structural unit represented by 4) [acetyl group content] was measured, the hydroxyl group content was 50 mol%, the degree of acetalization (degree of butyralization) was 49 mol%, the amount of nitrile-containing groups was 1 mol%, The amount of acetyl groups was 0 mol%.
Further, the modified polyvinyl acetal resin (B) to the modified polyvinyl acetal resin (A) are the same as the modified polyvinyl acetal resin (A) except that the conditions shown in Table 2 (used polyvinyl acetal, polyvinyl acetal usage amount, graft monomer, graft monomer usage amount) are used. (K) [Resin solutions (B) to (K)] were synthesized.

(Synthesis of modified polyvinyl acetal resin (L))
A reaction flask equipped with a stirrer was charged with 400 parts by weight of a raw material polyvinyl alcohol having a saponification degree of 98 mol% and a polymerization degree of 1200 parts, and 3000 parts by weight of water, and after dissolving the polyvinyl alcohol, a 40% by mass benzyltrimethylammonium hydroxide aqueous solution 30 Part by weight was added, 30 parts by weight of acrylonitrile and 1800 parts by weight of acetone were added thereto, and reacted at 32 ° C. for 6 hours.
Then, a 20% by mass aqueous acetic acid solution containing acetic acid equivalent to benzyltrimethylammonium hydroxide was added. The reaction solution was divided into two phases, an aqueous phase and an organic phase. The organic phase was extracted and stirred while pouring it into water to precipitate polyvinyl alcohol.
The obtained polyvinyl alcohol is repeatedly washed with water, then redissolved in acetone, precipitated again in water, dehydrated and dried under reduced pressure to have a 2-cyanoethyl group containing a structural unit represented by the following formula (6). Modified polyvinyl alcohol was obtained.

350 parts by weight of the obtained modified polyvinyl alcohol 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 kept 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 the modified polyvinyl acetal resin (L). The obtained modified polyvinyl acetal resin (L) was added to N-methylpyrrolidone to obtain a resin solution (L) having a solid content of 15% by weight.
The obtained polyvinyl acetal resin is dissolved in DMSO-d ₆ (dimethylsulfoxaside), and the content of the structural unit represented by the formula (1) using ¹³ C-NMR (nuclear magnetic resonance spectrum) [hydroxyl group content] ], The content of the structural unit represented by the formula (4) [acetyl group amount], the content of the structural unit represented by the formula (3) [degree of acetalization], and the formula (6). When the content of the structural unit [formula (6) unit amount] was measured, the hydroxyl group amount was 50 mol%, the acetyl group amount was 1 mol%, the acetalization degree (butyralization degree) was 44 mol%, and the formula (6 ) The unit amount was 5 mol%.

Example 1
(Preparation of composition for positive electrode of lithium secondary battery)
In 20 parts by weight of the resin solution (A) containing the modified polyvinyl acetal resin (A) thus obtained (modified polyvinyl acetal resin (A): 3 parts by weight), lithium cobalt oxide (manufactured by Nippon Chemical Industry Co., Ltd., 50 parts by weight of cell seed C-5H), 5 parts by weight of acetylene black (Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductivity-imparting agent, and 25 parts by weight of N-methylpyrrolidone were added and mixed at Shintaro's Awatori Kentaro And the composition for lithium secondary battery positive electrodes was obtained.

(Examples 2 to 11)
A lithium secondary battery positive electrode composition was obtained in the same manner as in Example 1 except that the binder shown in Table 3 was used and the addition amount (total amount) of N-methylpyrrolidone was changed.

(Example 12)
(Preparation of composition for negative electrode of lithium secondary battery)
In 13.3 parts by weight (modified polyvinyl acetal resin (A): 2 parts by weight) of the resulting resin solution (A) containing the modified polyvinyl acetal resin (A), spherical natural graphite (Nippon Graphite Industries Co., Ltd.) Manufactured, CGB-10) 50 parts by weight and N-methylpyrrolidone 36.7 parts by weight were added and mixed by Shintaro Co., Ltd., Narotaro Awatori to obtain a composition for a positive electrode of a lithium secondary battery.

(Examples 13 to 17)
A composition for a lithium secondary battery negative electrode was obtained in the same manner as in Example 12 except that the binder shown in Table 3 was used and the addition amount (total amount) of N-methylpyrrolidone was changed.

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

(Comparative Example 2)
Instead of the resin solution (A) containing the modified polyvinyl acetal resin (A), an N-methylpyrrolidone solution (solid content 15% by weight) of the polyvinyl acetal resin (a) was used in the same manner as in Example 1. A composition for a positive electrode of a lithium secondary battery was obtained.

(Comparative Example 3)
Instead of the resin solution (A) containing the modified polyvinyl acetal resin (A), an N-methylpyrrolidone solution (solid content 15% by weight) of polyvinylidene fluoride (PVDF, manufactured by Kureha, # 1100) was used. In the same manner as in Example 1, a lithium secondary battery positive electrode composition was obtained.

(Comparative Example 4)
Instead of the resin solution (A) containing the modified polyvinyl acetal resin (A), a resin solution (L) which is an N-methylpyrrolidone solution (solid content 15% by weight) of the modified polyvinyl acetal resin (L) was used. In the same manner as in Example 1, a lithium secondary battery negative electrode composition was obtained.

(Comparative Example 5)
Instead of the resin solution (A) containing the modified polyvinyl acetal resin (A), an N-methylpyrrolidone solution (solid content 15% by weight) of the polyvinyl acetal resin (a) was used in the same manner as in Example 12. A composition for a negative electrode of a lithium secondary battery was obtained.

(Comparative Example 6)
Instead of the resin solution (A) containing the modified polyvinyl acetal resin (A), an N-methylpyrrolidone solution (solid content 15% by weight) of polyvinylidene fluoride (PVDF, manufactured by Kureha, # 1100) was used. In the same manner as in Example 12, a lithium secondary battery negative electrode composition was obtained.

(Comparative Example 7)
Instead of the resin solution (A) containing the modified polyvinyl acetal resin (A), a resin solution (L) which is an N-methylpyrrolidone solution (solid content 15% by weight) of the modified polyvinyl acetal resin (L) was used. In the same manner as in Example 12, a lithium secondary battery negative electrode composition was obtained.

<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 electrodes obtained in Examples 1 to 11 and Comparative Examples 1 to 4, the adhesiveness to aluminum foil was evaluated, and Examples 12 to 17 and Comparative Examples 5 to 5 were evaluated. About the composition for lithium secondary battery electrodes obtained by 7, the adhesiveness with respect to copper foil was evaluated.

(1-1) Examples 1 to 11 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 / m) was measured.

(1-2) Examples 12 to 17 and Comparative Examples 5 to 7
In the above (1-1), the peel strength was evaluated 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 dispersion diameter was measured.

(3) Paste viscosity About 10 parts by weight of the obtained composition for a lithium secondary battery electrode, using a rheometer (manufactured by Rheological Instrument Co., Ltd .: using a parallel plate having a diameter of 10 mm), the viscosity at a shear rate of 0.1 to 1000 s ⁻¹ Was measured. Among them, the viscosity at a shear rate of 1 s ⁻¹ was defined as the paste viscosity of the sample. The measurement temperature was 20 ° C.
Moreover, the viscosity after being allowed to stand at 20 ° C. for 1 week was also measured. The viscosity increase rate was calculated from the viscosity immediately after production and after 1 week according to the following formula.

Viscosity increase rate (%) = (viscosity after 1 week / viscosity immediately after production) × 100

(4) Solvent solubility (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.

(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 lithium secondary battery negative electrode composition obtained in Example 12 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.
In addition, when performing battery evaluation of Examples 2-11 and Comparative Examples 1-4, it replaces with the composition for lithium secondary battery positive electrodes obtained in Example 1, and Examples 2-11 and Comparative Examples 1- 1 The composition for positive electrode of the lithium secondary battery obtained in 4 was used.
Moreover, when performing battery evaluation of Examples 13-17 and Comparative Examples 5-7, it replaced with the composition for lithium secondary battery negative electrodes obtained in Example 12, and Examples 13-17 and Comparative Examples 5-5. The lithium secondary battery negative electrode composition obtained in 7 was used.

According to the present invention, a lithium secondary battery that is excellent in dispersibility and adhesiveness of an active material, has little viscosity increase with time, and can produce a high-capacity lithium secondary battery even when the amount of binder added is small. A composition for a secondary battery electrode can be provided.

Claims (5)

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 includes a structural unit having a hydroxyl group represented by the following formula (1), a structural unit having an acetal group represented by the following formula (2), and a nitrile group represented by the following formula (3). have a structural unit having a,
A composition for a lithium secondary battery electrode, comprising 20 to 50% by weight of the organic solvent .

In Formula (2), R ¹ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and in Formula (3), R ² and R ⁴ are at least selected from the group consisting of C, H and O. A functional group having one type is represented, R ³ represents a linking group or a single bond having at least one type 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, wherein the polyvinyl acetal resin contains 1 to 20 mol% of a structural unit having a nitrile group represented by the formula (3). The composition for a lithium secondary battery electrode according to claim 1 or 2, wherein the polyvinyl acetal resin has a polymerization degree of 250 to 4000. The composition for lithium secondary battery electrodes according to claim 1, wherein the composition contains 0.2 to 5% by weight of a polyvinyl acetal resin. Claim 1, 2, 3 or a lithium secondary battery, characterized by comprising using a lithium secondary battery electrode composition 4 according.