JP2016189256A - Binder composition for lithium ion secondary battery electrode, and slurry composition for lithium ion secondary battery electrode, lithium ion secondary battery negative electrode and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve battery characteristics of a lithium ion secondary battery without impairing the binding properties of an electrode binder.SOLUTION: There are provided a binder composition for a lithium ion secondary battery electrode and the like. The binder composition for a lithium ion secondary battery electrode contains a neutralized salt of an α-olefin-maleic acid copolymer obtained by copolymerization of an α-olefin and a maleic acid, and is characterized in that an amount of change between reduction current values at a second potential scan and a third potential scan when potential scans from 0 V to 3 V are repeated three times, the reduction current value being obtained by cyclic voltammetry in a predetermined condition, is 5.0 μA/cmor less.SELECTED DRAWING: None

Description

  The present invention relates to a binder composition for a lithium ion secondary battery electrode, and a slurry composition for a lithium ion secondary battery electrode, a lithium ion secondary battery negative electrode and a lithium ion secondary battery using the binder composition.

  In recent years, mobile terminals such as mobile phones, notebook computers, and pad-type information terminal devices have been widely used. Lithium ion secondary batteries are frequently used as secondary batteries used for the power sources of these portable terminals. Since portable terminals are required to have more comfortable portability, miniaturization, thinning, weight reduction, and high performance have rapidly progressed, and have come to be used in various places. This trend continues today, and batteries used in mobile terminals are further required to be smaller, thinner, lighter, and higher in performance.

Lithium ion secondary batteries have a positive electrode and a negative electrode installed via a separator, and LiPF ₆ , LiBF ₄ LiTFSI (lithium (bistrifluoromethylsulfonylimide)), LiFSI (lithium (bisfluorosulfonylimide)) and other lithium It has a structure in which a salt is stored in a container together with an electrolytic solution in which a salt is dissolved in an organic liquid such as ethylene carbonate.

The negative electrode and the positive electrode are usually obtained by dissolving or dispersing a binder and a thickener in water and mixing an active material, a conductive aid (conductivity imparting agent) and the like with this, Hereinafter, it may be simply formed as a mixed layer by coating the current collector on the current collector and drying the water. More specifically, for example, for the negative electrode, a carbonaceous material capable of occluding and releasing lithium ions, which is an active material, and, if necessary, acetylene black, a conductive auxiliary agent, are secondary to a current collector such as copper. They are bound together by a binder for battery electrodes. On the other hand, for the positive electrode, LiCoO ₂ that is an active material and, if necessary, a conductive aid similar to that of the negative electrode are bound to a current collector such as aluminum using a secondary battery electrode binder. Is.

  Up to now, diene rubbers such as styrene-butadiene rubber and acrylics such as polyacrylic acid have been used as binders for aqueous media (for example, Patent Documents 1 and 2). Examples of the thickener include methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropoxycellulose, carboxymethylcellulose sodium salt (CMC-Na), sodium polyacrylate, etc. Among them, CMC-Na is often used. (For example, patent document 3).

  However, diene rubbers such as styrene-butadiene rubber have low adhesion to metal collectors such as copper, and there is a problem that the amount used cannot be reduced in order to increase the adhesion between the collector and the electrode material. . In addition, there is a problem that the capacity maintenance rate is low due to weakness against heat generated during charging and discharging. Recently, demands for extending the usage time of mobile devices and shortening charging time have increased, and there is an urgent need to improve battery capacity (low resistance), life (cycle characteristics), and charging speed (rate characteristics). In particular, it is an obstacle.

  In a lithium ion secondary battery, since the battery capacity is affected by the amount of active material, it is effective to reduce the amount of binder and thickener in order to increase the active material in a limited space of the battery. . Moreover, since the rate characteristics are also affected by the ease of electron movement, it is effective to suppress the amount of binder and thickener that are non-conductive and prevent electron movement. However, if the amount of the binder and the thickener is reduced, the binding property between the collector electrode and the electrode material and the active material in the electrode is lowered, and the durability (battery life) for long-term use is significantly reduced. However, the electrode becomes brittle. Thus, it has been difficult to improve battery characteristics such as battery capacity while maintaining the binding property between the collector electrode and the electrode material.

JP 2000-67917 A JP 2008-288214 A JP 2014-13693 A

  The present invention has been made in view of the above-described problems, and an object thereof is to improve battery characteristics without impairing the function as a binder, that is, binding properties between active materials and collector electrodes.

  As a result of diligent research to solve the above problems, the present inventors have found that the above object can be achieved by using a binder composition for a lithium ion secondary battery electrode having the following configuration, and further based on this finding. The present invention was completed through repeated studies.

That is, the binder composition for a lithium ion secondary battery electrode according to one aspect of the present invention (hereinafter, also simply referred to as a binder composition) includes an α-olefin-maleic acid copolymer obtained by copolymerizing an α-olefin and a maleic acid. The amount of change in the reduction current value from the second time to the third time when the potential scan from 0 V to 3 V obtained by cyclic voltammetry under the following condition I is repeated three times, including the neutral salt of the polymer It is 5.0 μA / cm ² or less.
<Cyclic voltammetry conditions I>
Using a two-electrode electrolytic cell having the following specifications, the current value observed during each scan is measured at a sweep rate of 1 mV / s and a sweep range of 0 to 3 V. The reduction current value is a value obtained by dividing the current value at a voltage value of 0.45 V by the area of the working electrode.
≪Specifications of 2-electrode electrolytic cell≫
Working electrode: A layer made of the above binder composition on a copper foil (area φ14 mm, film thickness 10 μm)
Reference electrode, counter electrode: lithium foil Electrolytic solution: 1 mol / L lithium hexafluorophosphate solution (solvent: ethylene carbonate / diethyl carbonate = 1/1 (volume ratio))

  With such a configuration, it is considered that the battery characteristics can be improved without impairing the binding property of the electrode binder.

In the binder composition, the amount of change in the reduction current value from the second time to the third time when the potential scan from 0 V to 3 V obtained by cyclic voltammetry under the condition I is repeated three times is 5.0 μA / cm ² or less. Thus, it is considered that an electrochemically stable binder composition can be obtained.

  Moreover, the binder aqueous solution for lithium ion secondary battery electrodes which concerns on the other situation of this invention consists of the said binder composition and water, It is characterized by the above-mentioned.

  Moreover, the slurry composition for lithium ion secondary battery electrodes which concerns on the other situation of this invention contains the said binder composition, an active material, and a solvent, It is characterized by the above-mentioned.

  A negative electrode for a lithium ion secondary battery according to still another aspect of the present invention is obtained by binding a current collector to a mixed layer containing at least the binder composition for a lithium ion secondary battery electrode and an active material. It is characterized by that.

  Moreover, the lithium ion secondary battery which concerns on the other situation of this invention is equipped with the said lithium ion secondary battery negative electrode, a positive electrode, and electrolyte solution, It is characterized by the above-mentioned.

  According to the present invention, a binder composition for a lithium ion secondary battery electrode having excellent binding properties can be obtained, and further, the battery characteristics of the lithium ion secondary electrode can be improved using the binder composition. .

  Hereinafter, although embodiment of this invention is described in detail, this invention is not limited to these.

The binder composition for a lithium ion secondary battery electrode of the present embodiment contains a neutralized salt of an α-olefin-maleic acid copolymer obtained by copolymerizing an α-olefin and a maleic acid, and the following conditions: The amount of change in the reduction current value from the second time to the third time when the potential scan from 0 V to 3 V obtained by cyclic voltammetry is repeated three times is 5.0 μA / cm ² or less.
<Cyclic voltammetry conditions>
Using a two-electrode electrolytic cell having the following specifications, the current value observed during each scan is measured at a sweep rate of 1 mV / s and a sweep range of 0 to 3 V. The reduction current value is a value obtained by dividing the current value at a voltage value of 0.45 V by the area of the working electrode.
≪Specifications of 2-electrode electrolytic cell≫
Working electrode: A layer made of the above binder composition on a copper foil (area φ14 mm, film thickness 10 μm)
Reference electrode, counter electrode: lithium foil Electrolytic solution: 1 mol / L lithium hexafluorophosphate solution (solvent: ethylene carbonate / diethyl carbonate = 1/1 (volume ratio))

  By setting it as such a structure, the electrochemical stability of the binder composition of this embodiment can be acquired. According to the cyclic voltammetry (hereinafter also referred to as CV) method, a potential is scanned in a specimen at a constant rate, and when an oxidation reaction or a reduction reaction occurs, a peak current value is obtained. As the peak current value is higher, a substance that causes an oxidation reaction or a reduction reaction is included in the specimen. The binder used for the secondary battery is exposed to repeated charge and discharge. When cyclic voltammetry is repeated, an electrode manufactured using a binder that gradually shows a high peak current value is liable to cause an oxidation reaction or a reduction reaction by charging / discharging. Only batteries with inferior characteristics can be given.

Therefore, when the redox current of the binder composition of the present embodiment is measured by cyclic voltammetry under the above conditions, the change in the reduction current value from the second time to the third time when the potential scan from 0 V to 3 V is repeated three times. It is important that the amount is 5.0 μA / cm ² or less. In particular, the amount of change is more preferably 4.0 μA / cm ² or less.

  In the present embodiment, the amount of change in the reduction current value can be adjusted according to the degree of neutralization of the binder composition, but is not limited thereto. In particular, in the copolymer in the binder composition, it is considered that the amount of carboxylic acid described below can be more reliably suppressed than the amount of change in the reduction current value.

  In the present embodiment, an α-olefin-maleic acid copolymer obtained by copolymerizing an α-olefin and maleic acid is composed of a unit based on the α-olefin (A) and a unit based on the maleic acid (B). The components (A) and (B) preferably satisfy (A) / (B) = 1/1 to 1/3 (molar ratio). Moreover, it is preferable that it is a linear random copolymer whose weight average molecular weight is 10,000-500,000.

In this embodiment, the unit (A) based on α-olefins is represented by the general formula —CH ₂ CR ¹ R ² — (wherein R ¹ and R ² may be the same or different from each other, hydrogen Represents an alkyl or alkenyl group having 1 to 10 carbon atoms). The α-olefin used in the present embodiment is a linear or branched olefin having a carbon-carbon unsaturated double bond at the α-position. In particular, olefins having 2 to 12 carbon atoms, particularly 2 to 8 carbon atoms are preferred. Representative examples that may be used include ethylene, propylene, n-butylene, isobutylene, n-pentene, isoprene, 2-methyl-1-butene, 3-methyl-1-butene, n-hexene, 2-methyl- 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 2-ethyl-1-butene, 1,3-pentadiene, 1,3-hexadiene, 2,3-dimethylbutadiene, 2,5 -Pentadiene, 1,4-hexadiene, 2,2,4-trimethyl-1-pentene and the like. Among these, isobutylene is particularly preferable from the viewpoints of availability, polysynthesis, and product stability. Here, the isobutylene includes a mixture containing isobutylene as a main component, for example, a BB fraction (C4 fraction). These olefins may be used alone or in combination of two or more.

  In the present embodiment, maleic anhydride, maleic acid, maleic acid monoester (for example, methyl maleate, ethyl maleate, propyl maleate, phenyl maleate, etc.), maleic acid, as the unit (B) based on maleic acids Maleic anhydride derivatives such as diesters (eg, dimethyl maleate, diethyl maleate, dipropyl maleate, diphenyl maleate, etc.), maleic imides or N-substituted derivatives thereof (eg maleic imide, N-methylmaleimide, N N-substituted alkylmaleimide such as ethylmaleimide, N-propylmaleimide, Nn-butylmaleimide, Nt-butylmaleimide, N-cyclohexylmaleimide, N-methylphenylmaleimide, N-ethyl Phenyl male N-substituted alkylphenylmaleimide such as N-substituted alkoxyphenylmaleimide such as N-methoxyphenylmaleimide and N-ethoxyphenylmaleimide), and further halides thereof (for example, N-chlorophenyl) Maleimide), citraconic anhydride, citraconic acid, citraconic acid monoester (eg, methyl citraconic acid, ethyl citraconic acid, propyl citraconic acid, phenyl citraconic acid, etc.), citraconic acid diester (eg, dimethyl citraconic acid, diethyl citraconic acid, citraconic acid) Citraconic anhydride derivatives such as dipropyl acid, diphenyl citraconic acid, etc., citraconic acid imide or N-substituted derivatives thereof (for example, citraconic acid imide, 2-methyl-N-methylmaleimide, 2-methyl-N-ethylmaleic acid) N-substituted alkylmaleimides such as 2-methyl-N-propylmaleimide, 2-methyl-Nn-butylmaleimide, 2-methyl-Nt-butylmaleimide, 2-methyl-N-cyclohexylmaleimide 2-methyl-N-substituted alkylphenylmaleimide such as methyl-N-phenylmaleimide, 2-methyl-N-methylphenylmaleimide, 2-methyl-N-ethylphenylmaleimide, or 2-methyl-N- Methoxyphenylmaleimide, 2-methyl-N-substituted alkoxyphenylmaleimide such as 2-methyl-N-ethoxyphenylmaleimide), and further halides thereof (for example, 2-methyl-N-chlorophenylmaleimide) Is preferred. Among these, use of maleic anhydride is preferable from the viewpoint of availability, polymerization rate, and ease of molecular weight adjustment. These maleic acids may be used alone or in combination.

As for the content rate of each said structural unit in the copolymer of this embodiment, it is desirable for (A) / (B) to exist in the range of 1 / 1-1 / 3 by molar ratio. This is because the advantages of hydrophilicity, water solubility, and affinity for metals and ions as a high molecular weight substance that dissolves in water can be obtained. In particular, the molar ratio of (A) / (B) is desirably 1/1 or a value close thereto, in which case the unit based on α-olefin, that is, —CH ₂ CR ¹ R ² — A copolymer having a structure in which the units shown and units based on maleic acids are alternately repeated is obtained.

  The mixing ratio of α-olefins and maleic acids for obtaining the copolymer of the present embodiment varies depending on the composition of the target copolymer, but α- 1 to 3 times the number of moles of maleic acids. Use of olefin is effective for increasing the reaction rate of maleic acids.

  The method for producing the copolymer of the present embodiment is not particularly limited, and for example, the copolymer can be obtained by radical polymerization. In this case, the polymerization catalyst used is an azo catalyst such as azobisisobutyronitrile, 1,1-azobiscyclohexane-1-carbonitrile, or an organic peroxide catalyst such as benzoyl peroxide or dicumyl peroxide. preferable. The amount of the polymerization catalyst used is required to be in the range of 0.1 to 5 mol% with respect to maleic acids, but is preferably 0.5 to 3 mol%. As a method for adding the polymerization catalyst and the monomer, they may be added all at the beginning of the polymerization, but it is desirable to add them sequentially as the polymerization proceeds.

  In the method for producing a copolymer of this embodiment, the molecular weight can be appropriately adjusted mainly depending on the monomer concentration, the amount of catalyst used, and the polymerization temperature. For example, as a substance for reducing the molecular weight, a metal salt of Group I, II or III of the periodic table, a hydroxide, a metal halide of Group IV, a general formula N≡, HN =, H2N- or H4N- It is also possible to adjust the molecular weight of the copolymer by adding the amines shown, nitrogen compounds such as ammonium acetate and urea, or mercaptans, at the initial stage of polymerization or during the progress of polymerization. The polymerization temperature is preferably 40 ° C to 150 ° C, more preferably 60 ° C to 120 ° C. If the polymerization temperature is too high, the resulting copolymer tends to be in a block form, and the polymerization pressure may be significantly increased. The polymerization time is usually preferably about 1 to 24 hours, more preferably 2 to 10 hours. The amount of the polymerization solvent used is preferably adjusted so that the concentration of the obtained copolymer is 5 to 40% by weight, more preferably 10 to 30% by weight.

  As described above, it is preferable that the copolymer of the present embodiment usually has a weight average molecular weight of 10,000 to 500,000. A more preferred weight average molecular weight is 15,000 to 450,000. When the weight average molecular weight of the copolymer of the present embodiment is less than 10,000, the crystallinity is high and the adhesive strength between particles may be reduced. On the other hand, when it exceeds 500,000, the solubility in water or a solvent becomes small, and it may precipitate easily.

  The weight average molecular weight of the copolymer of this embodiment can be measured by, for example, a light scattering method or a viscosity method. When the intrinsic viscosity ([η]) in dimethylformamide is measured using a viscosity method, the copolymer of this embodiment preferably has an intrinsic viscosity in the range of 0.05 to 1.5. The copolymer of the present embodiment is usually obtained in the form of a powder having a grain size of about 16 to 60 mesh.

  In this embodiment, the neutralized salt of a copolymer is a neutralized product in which active hydrogen of carbonyl acid generated from maleic acids reacts with a basic substance to form a salt. Is preferred. In the neutralized product of α-olefin-maleic acid copolymer used in the present embodiment, a basic substance containing a monovalent metal and / or ammonia as the basic substance from the viewpoint of binding properties as a binder. Is preferably used.

  The degree of neutralization is not particularly limited. However, when used as a binder, the reactivity with the electrolytic solution is taken into consideration, and the amount of neutralization is preferably about 0.1 with respect to 1 mol of carboxylic acid produced from maleic acid. It is desirable to use a neutralized one in the range of 5 to 1 mol, more preferably in the range of 0.6 to 1 mol. With such a neutralization degree, it is considered that the amount of change in the reduction current value of the binder composition of the present embodiment and the mixed layer described later can be reliably suppressed within a desired range, and further, the acidity is low and the electrolytic solution is decomposed. There is an advantage of suppression.

  In this embodiment, the degree of neutralization can be determined by a method such as titration with a base, an infrared spectrum, or an NMR spectrum. To measure the neutralization point simply and accurately, titration with a base can be performed. preferable. The specific titration method is not particularly limited, but it can be dissolved in water with little impurities such as ion-exchanged water, and a basic substance such as lithium hydroxide, sodium hydroxide, potassium hydroxide, It can be carried out by neutralization. The indicator for the neutralization point is not particularly limited, but an indicator such as phenolphthalein whose pH is indicated by a base can be used.

  In the present embodiment, the amount of the basic substance containing monovalent metal and / or ammonia is not particularly limited and is appropriately selected depending on the purpose of use and the like, but usually in the maleic acid copolymer. The amount is preferably 0.1 to 2 mol per mol of maleic acid unit. The amount of the basic substance containing a monovalent metal is preferably 0.6 to 2.0 mol, more preferably 0.7 to 2.0, per mol of maleic acid unit in the maleic acid copolymer. When the molar amount is obtained, a water-soluble copolymer salt with little alkali residue can be obtained.

  The reaction of the α-olefin-maleic acid copolymer with the basic substance containing monovalent metal and / or ammonia can be carried out according to a conventional method, but is carried out in the presence of water, and the α-olefin-maleic acid is obtained. A method for obtaining a neutralized copolymer as an aqueous solution is simple and preferable.

  Examples of basic substances containing monovalent metals that can be used in the present embodiment include hydroxides of alkali metals such as sodium hydroxide, potassium hydroxide, and lithium hydroxide; alkali metals such as sodium carbonate and potassium carbonate. Carbonates of alkali metals such as sodium acetate and potassium acetate; phosphates of alkali metals such as trisodium phosphate, and the like. Among these, ammonia, lithium hydroxide, sodium hydroxide, and potassium hydroxide are preferable. In particular, it is preferable to use ammonia or lithium hydroxide as a binder for a lithium ion secondary battery. The basic substance containing monovalent metal and / or ammonia may be used alone or in combination of two or more. Moreover, if it is in the range which does not have a bad influence on battery performance, the basic substance containing alkali metal hydroxides, such as sodium hydroxide, is used together, and the neutralized product of alpha-olefin-maleic acid copolymer May be prepared.

  Next, in this embodiment, the pH of the aqueous solution containing 10% by weight of the binder composition is preferably 6-11. When the pH is less than 6, the solubility in water or a solvent becomes small, and it easily precipitates, making slurry coating difficult. On the other hand, when the pH is higher than 11, a basic substance to be neutralized is excessively present in the mixed layer, which may be a resistance component.

  The method for measuring the pH of the present embodiment is not particularly limited, and can be measured by an indicator method, a hydrogen electrode method, an antimony electrode method, a glass electrode method, a semiconductor sensor method, or the like. Among them, it is preferable to use the glass electrode method because the potential equilibration time is fast and the reproducibility is good, and it is not affected by the oxidizing agent or the reducing agent and can be measured for various solutions. .

  Next, in this embodiment, the ring-opening rate of the copolymer represents the hydrolysis rate of the site of maleic anhydrides that are polymerized with α-olefins when maleic anhydride is used as the maleic acid. In the copolymer of the present embodiment, a preferable ring opening rate is 60 to 100%, more preferably 70% to 100%, and still more preferably 80 to 100%. If the ring-opening rate is too low, the structural freedom of the copolymer becomes small and the stretchability becomes poor, so that the force for adhering the electrode material particles to be bonded may be small, which is not preferable. Furthermore, there is a possibility that problems such as low affinity for water and poor solubility may occur. The ring-opening rate can be determined, for example, by measuring the hydrogen at the α-position of maleic acid opened by 1H-NMR with reference to the hydrogen at the α-position of maleic anhydride. The ratio of the carbonyl group derived from the carbonyl group and the ring-opened maleic anhydride can also be determined by IR measurement.

  In this embodiment, when the maleic acid is maleic anhydride, the neutralized salt of the copolymer means that the active hydrogen of the carbonyl acid generated by the ring opening of maleic anhydride is a basic substance as described above. It forms a salt by forming a salt. The degree of neutralization in this case is not particularly limited. However, when used as a binder, in consideration of reactivity with the electrolytic solution, usually with respect to 1 mol of carbonyl acid generated by ring opening, A range of 0.5 to 1 mol is preferable, and a neutralized range of 0.6 to 1 mol is more preferable. Such a neutralization degree has the advantage of low acidity and suppression of electrolyte decomposition. The degree of neutralization of the copolymer when maleic anhydride is used can be measured by the same method as described above.

  The binder composition for a lithium ion secondary battery electrode of the present embodiment is preferably used as a slurry for a negative electrode. Usually, in addition to the α-olefin-maleic acid copolymer described above, a negative electrode active material and a solvent are further used. It is preferably used as a slurry composition for a negative electrode of a lithium ion secondary battery (hereinafter also simply referred to as a negative electrode slurry composition).

  Further, in the present embodiment, the lithium ion secondary battery negative electrode is formed by binding a current collector to a mixed layer containing at least the lithium ion secondary battery negative electrode binder composition of the present embodiment and the negative electrode active material. Features. This negative electrode can be formed by applying the above slurry composition for a lithium ion secondary battery negative electrode to a current collector and then removing the solvent by a method such as drying. If necessary, a thickener, a conductive aid and the like can be added to the mixed layer.

  In the slurry composition for a negative electrode of a lithium ion secondary battery, the amount of the α-olefin-maleic acid copolymer neutralized salt used is usually 0.1 to 4 parts by weight with respect to 100 parts by weight of the negative electrode active material. It is preferably 0.3 to 3 parts by weight, more preferably 0.5 to 2 parts by weight. If the amount of the copolymer is too small, the viscosity of the slurry for secondary battery negative electrode may be too low and the thickness of the mixed layer may be reduced. Conversely, if the amount of the copolymer is excessive, the discharge capacity may be reduced. There is sex.

  On the other hand, the amount of the solvent in the negative electrode slurry composition is usually preferably 40 to 150 parts by weight, more preferably 70 to 130 parts by weight with respect to 100 parts by weight of the negative electrode active material.

  Examples of the solvent in the negative electrode slurry composition of the present embodiment include water, alcohols such as methanol, ethanol, propanol, and 2-propanol, cyclic ethers such as tetrahydrofuran and 1,4-dioxane, N, N-dimethyl, and the like. Examples include amides such as formamide and N, N-dimethylacetamide, cyclic amides such as N-methylpyrrolidone and N-ethylpyrrolidone, and sulfoxides such as dimethylsulfoxide. In these, use of water is preferable from a viewpoint of safety.

  Moreover, when using water as a solvent of the slurry composition for negative electrodes of this embodiment, you may use together the organic solvent described next in the range used preferably in 20 weight% or less of the whole solvent. As such an organic solvent, those having a boiling point of 100 ° C. or more and 300 ° C. or less at normal pressure are preferable, for example, hydrocarbons such as n-dodecane; alcohols such as 2-ethyl-1-hexanol and 1-nonanol. Esters such as γ-butyrolactone and methyl lactate; amides such as N-methylpyrrolidone, N, N-dimethylacetamide and dimethylformamide; organic dispersion media such as sulfoxides and sulfones such as dimethyl sulfoxide and sulfolane.

Examples of the negative electrode active material (sometimes abbreviated as active material) added to the negative electrode slurry composition of the present embodiment include amorphous carbon, graphite, natural graphite, mesocarbon microbeads (MCMB), and pitch-based carbon. Carbonaceous materials such as fibers; conductive polymers such as polyacene; complex metal oxides represented by SiOx, SnOx, LiTiOx and other metal oxides, lithium metals such as lithium metals and lithium alloys; TiS ₂ , LiTiS and metal compounds such as ₂ are exemplified.

  In this embodiment, a thickener can be further added to the negative electrode slurry composition as necessary. The thickener that can be added is not particularly limited, and various alcohols, in particular, polyvinyl alcohol and modified products thereof, celluloses, starches, and other polysaccharides can be used.

  It is preferable that the usage-amount of the thickener mix | blended with the slurry composition for negative electrodes as needed is 0.1-4 weight part with respect to 100 parts of negative electrode active materials, More preferably, it is 0.3-3 weight Parts, more preferably 0.5 to 2 parts by weight. If the thickener is too small, the viscosity of the secondary battery negative electrode slurry may be too low and the thickness of the mixed layer may be reduced. Conversely, if the thickener is excessively large, the discharge capacity may be reduced. .

  Moreover, as a conductive support agent mix | blended with the slurry composition for negative electrodes as needed, metal powder, a conductive polymer, acetylene black etc. are mentioned, for example. The amount of the conductive aid used is usually preferably 0.5 to 10 parts by weight, more preferably 1 to 7 parts by weight with respect to 100 parts by weight of the negative electrode active material.

  The current collector used for the lithium ion secondary battery negative electrode of this embodiment is not particularly limited as long as it is made of a conductive material. For example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold Metal materials such as platinum can be used. One of these may be used alone, or two or more of these may be used in combination at any ratio.

  In particular, when copper is used as the negative electrode, the effect of the slurry for the negative electrode of the lithium ion secondary battery of the present invention is most apparent. The shape of the current collector is not particularly limited, but usually a sheet shape having a thickness of about 0.001 to 0.5 mm is preferable.

  The method for applying the negative electrode slurry to the current collector is not particularly limited. Examples thereof include a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a dipping method, and a brush coating method. The amount to be applied is not particularly limited, but the thickness of the mixed layer containing the active material, conductive additive, binder and thickener formed after removing the solvent or dispersion medium by a method such as drying is 0.005 to 5 mm. It is preferable that the amount is preferably 0.01 to 2 mm.

  The drying method of the solvent such as water contained in the negative electrode slurry composition is not particularly limited, and examples thereof include aeration drying with hot air, hot air, and low-humidity air; vacuum drying; irradiation radiation drying such as infrared rays, far infrared rays, and electron beams. Can be mentioned. The drying conditions are preferably adjusted so that the solvent can be removed as soon as possible while the active material layer is cracked by stress concentration or the active material layer does not peel from the current collector. Furthermore, in order to increase the density of the active material of the electrode, it is effective to press the current collector after drying. Examples of the pressing method include a die press and a roll press.

  In the present embodiment, a mixture obtained by drying a solvent such as water from the slurry composition to remove the solvent is referred to as a mixed layer. That is, the mixed layer of the present embodiment includes at least a binder composition for a negative electrode of a lithium ion secondary battery and a negative electrode active material.

In the mixed layer of this embodiment, when the potential scan from 0 V to 3 V obtained by cyclic voltammetry under the following condition II is repeated five times, the amount of change in the reduction current value from the third time to the fourth time is 100 μA or less. It is desirable to be.
<Cyclic voltammetry conditions II>
Using a two-electrode electrolytic cell having the following specifications, the current value observed during each scan is measured at a sweep rate of 1 mV / s and a sweep range of 0 to 3 V. The reduction current value is a value obtained by dividing the current value at a voltage value of 0.2 V by the area of the working electrode.
≪Specifications of 2-electrode electrolytic cell≫
Working electrode: The above mixed layer provided on copper foil (area φ14 mm, film thickness 45 μm)
Reference electrode: lithium foil Electrolytic solution: 1 mol / L lithium hexafluorophosphate solution (solvent: ethylene carbonate / ethyl methyl carbonate = 3/7 (volume ratio), vinylene carbonate 2 wt% added)

Therefore, when the oxidation-reduction current of the mixed layer of this embodiment is measured by cyclic voltammetry under the above conditions, the amount of change in the reduction current value from the third to the fourth when the potential scan from 0 V to 3 V is repeated five times. Is 100 μA / cm ² or less. In particular, the amount of change is more preferably 85 μA / cm ² or less.

When the amount of change in the reduction current value in the mixed layer is 100 μA / cm ² or less, there is an advantage that the electrochemical stability of the mixed layer of the present embodiment can be obtained. More preferably, the amount of change in the reduction current value is 85 μA / cm ² or less.

  In the present embodiment, the amount of change in the reduction current value of the mixed layer can be adjusted by the neutralization degree and molecular weight of the binder composition contained in the mixed layer, but is not limited thereto. In particular, in the copolymer in the binder composition, it is considered that the smaller amount of carboxylic acid can be more reliably suppressed than the amount of change in the reduction current value.

  Furthermore, the present invention also includes a lithium secondary battery including the above-described lithium ion secondary battery negative electrode, a positive electrode, and an electrolytic solution.

In the present embodiment, as the positive electrode, a positive electrode normally used for a lithium ion secondary battery is used without any particular limitation. For example, as the positive electrode active material, TiS ₂ , TiS ₃ , amorphous MoS ₃ , Cu ₂ V ₂ O ₃ , amorphous V ₂ O—P ₂ O ₅ , MoO ₃ , V ₂ O ₅ , V ₆ O Transition metal oxides such as ₁₃ and lithium-containing composite metal oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiMn ₂ O ₄ are used. Further, the positive electrode active material is made of a conductive additive similar to that of the negative electrode, and a binder such as SBR, NBR, acrylic rubber, hydroxyethyl cellulose, carboxymethyl cellulose, polyvinylidene fluoride, and the boiling point at 100 ° C. in water or the above normal pressure. The positive electrode slurry prepared by mixing in a solvent of 300 ° C. or lower can be applied to a positive electrode current collector such as aluminum and the solvent can be dried to obtain a positive electrode.

Moreover, the lithium ion secondary battery of this embodiment can use the electrolyte solution which melt | dissolved electrolyte in the solvent. The electrolyte solution may be liquid or gel as long as it is used for a normal lithium ion secondary battery, and an electrolyte solution that functions as a battery can be appropriately selected according to the type of the negative electrode active material and the positive electrode active material. That's fine. Specific electrolytes, for example, also known lithium salt is any conventionally _{available, LiClO 4, LiBF 6, LiPF} 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiB 10 Cl 10 _{, LiAlC l4, LiCl, LiBr,} LiB (C 2 H 5) 4, CF 3 SO 3 Li, CH 3 SO 3 Li, LiCF 3 SO 3, LiC 4 F 9 SO 3, Li (CF 3 SO 2) 2 N And lower aliphatic lithium carboxylates.

  A solvent (electrolyte solvent) for dissolving such an electrolyte is not particularly limited. Specific examples include carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, and diethyl carbonate; lactones such as γ-butyllactone; trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, and 2-ethoxyethane. Ethers such as tetrahydrofuran, 2-methyltetrahydrofuran; sulfoxides such as dimethyl sulfoxide; oxolanes such as 1,3-dioxolane, 4-methyl-1,3-dioxolane; nitrogen-containing compounds such as acetonitrile and nitromethane; formic acid Organic acid esters such as methyl, methyl acetate, ethyl acetate, butyl acetate, methyl propionate and ethyl propionate; inorganic acid esters such as triethyl phosphate, dimethyl carbonate and diethyl carbonate Terigres; diglymes; triglymes; sulfolanes; oxazolidinones such as 3-methyl-2-oxazolidinone; sultones such as 1,3-propane sultone, 1,4-butane sultone, naphtha sultone, etc. Alternatively, two or more kinds can be mixed and used. When a gel electrolyte is used, a nitrile polymer, an acrylic polymer, a fluorine polymer, an alkylene oxide polymer, or the like can be added as a gelling agent.

  Although there is no limitation in particular as a method of manufacturing the lithium ion secondary battery of this embodiment, For example, the following manufacturing method is illustrated. That is, the negative electrode and the positive electrode are overlapped with each other via a separator such as a polypropylene porous membrane, wound or folded according to the shape of the battery, put into a battery container, injected with an electrolyte, and sealed. The shape of the battery may be any known coin type, button type, sheet type, cylindrical type, square type, flat type, and the like.

  The lithium ion secondary battery of this embodiment is a battery that achieves both improved adhesion and improved battery characteristics, and is useful for various applications. For example, the battery is very useful as a battery used in a portable terminal that is required to be small, thin, light, and have high performance.

  Examples of the present invention will be described below, but the present invention is not limited thereto.

<Test Example 1>
Example 1
<Negative electrode binder composition and CV electrode>
A water-soluble lithium-modified isobutene-maleic anhydride copolymer resin (average molecular weight 325,000, neutralization degree 1.0, ring opening rate 100%) was used as a negative electrode binder composition, and a 10 w% aqueous solution was prepared. Using a bar coater (T101, made by Matsuo Sangyo Co., Ltd.), it was coated on a copper foil (CST8G, made by Fukuda Metal Foil Powder Industry Co., Ltd.) as a current collector, and then heated at 80 ° C for 30 minutes with a hot air dryer (manufactured by Yamato Kagaku). The primary drying was performed. Then, after punching out as a battery electrode (φ14 mm), a CV electrode was produced by secondary drying under reduced pressure conditions at 120 ° C. for 3 hours.

<Preparation of CV Bipolar Electrolytic Cell>
The CV electrode obtained above was transferred to a glove box (manufactured by Miwa Seisakusho) under an argon gas atmosphere. A metal lithium foil (thickness 0.2 mm, φ16 mm) was used as the reference and counter electrode. In addition, a polypropylene system (Celgard # 2400, manufactured by Polypore) is used as a separator, and an electrolytic solution is a mixed solvent system (1M-LiPF ₆ , EC) of ethylene carbonate and diethyl carbonate of lithium hexafluorophosphate (LiPF ₆ ). / EMC = 1/1 vol%) to inject a coin type bipolar cell.

<Measurement of CV>
The coin-type bipolar electrolysis cell produced above was subjected to redox current measurement using a potentio / galvanostat 1287 type (manufactured by Solartron). The measurement conditions were a sweep rate of 1 mV / s and a sweep range of 0 to 3 V, and the current value observed during each scan was measured. The reduction current value at the voltage value of 0.45 V for the second time and the third time was read, and the value obtained by dividing the difference in current by the area of the working electrode was taken as the amount of change. The results are shown in Table 1 below.

<Preparation of slurry for negative electrode>
The slurry for an electrode was prepared by adding 3 parts by weight of a 10 w% aqueous solution of the above-mentioned negative electrode binder composition as a solid content to 96 parts by weight of natural graphite (DMGS, manufactured by BYD) as an active material for a negative electrode, and a conductive auxiliary agent (conducting conductivity). 1 part by weight of Super-P (manufactured by Timcal Co., Ltd.) as a solid component was put into a special container and kneaded using a planetary stirrer (ARE-250, manufactured by Shinky Corporation). In order to adjust the viscosity of the slurry, an electrode coating slurry was prepared by adding water at the time of kneading and kneading again. The composition ratio of the active material and the binder in the slurry is, as a solid content, graphite powder: conductive aid: binder composition = 96: 1: 3.

<Preparation of negative electrode for battery>
The obtained slurry was coated on a current collector copper foil (CST8G, manufactured by Fukuda Metal Foil Co., Ltd.) using a bar coater (T101, manufactured by Matsuo Sangyo Co., Ltd.), and heated at 80 ° C. for 30 minutes with a hot air dryer (Yamato Scientific). After the primary drying, a rolling process was performed using a roll press (made by Hosen). Then, after punching out as a battery electrode (φ14 mm), a coin battery electrode was produced by secondary drying under reduced pressure conditions at 120 ° C. for 3 hours.

<Production of battery>
The battery coating electrode obtained above was transferred to a glove box (Miwa Seisakusho) under an argon gas atmosphere. A metal lithium foil (thickness 0.2 mm, φ15 mm) was used for the positive electrode. Also, a polypropylene system (Celgard # 2400, manufactured by Polypore) was used as a separator, and the electrolyte was a mixed solvent system (1M) in which ethyl methyl carbonate was added to lithium hexafluorophosphate (LiPF6) ethylene carbonate and diethyl carbonate. _{-LiPF 6, EC / EMC = 3} / 7vol%, were injected with VC2wt%), to prepare a coin battery (2032 type).

<Evaluation method: charge / discharge characteristic test>
The produced coin battery was subjected to a charge / discharge test using a commercially available charge / discharge tester (TOSCAT3100, manufactured by Toyo System). The coin battery is placed in a constant temperature bath at 25 ° C., and charging is performed with a constant current of 0.1 C (about 0.5 mA / cm ² ) with respect to the amount of active material until the voltage reaches 0 V with respect to the lithium potential. On the other hand, the constant voltage charge of 0V was implemented to the electric current of 0.02 mA. The capacity at this time was defined as a charging capacity (mAh / g). Next, a constant current discharge of 0.1 C (about 0.5 mA / cm ² ) was performed up to 1.5 V with respect to the lithium potential, and the capacity at this time was defined as a discharge capacity (mAh / g). The difference between the initial discharge capacity and the charge capacity was taken as the irreversible capacity, and the percentage of the discharge capacity / charge capacity was taken as the charge / discharge efficiency. As the direct current resistance of the coin battery, a resistance value after being charged once (fully charged state) was adopted. The results are shown in Table 1 below.

  The discharge capacity maintenance rate (%) of the coin battery was defined as the ratio of the tenth discharge capacity to the first discharge capacity using the charge / discharge conditions. The results are shown in Table 1 below.

(Example 2)
A 10 w% aqueous solution of a water-soluble lithium-modified isobutene-maleic anhydride copolymer resin (average molecular weight 165,000, neutralization degree 1.0, ring opening rate 100%) was prepared as a binder composition for a negative electrode.
And as a result of producing and measuring a bipolar electrolysis cell by the method similar to the said Example 1, the variation | change_quantity of the reduction current value was 1.37 microampere / cm < ² >.
Next, a negative electrode slurry was prepared in the same manner as in Example 1. Further, a coated negative electrode was prepared by the same method as in Example 1 to obtain a coin battery, and a charge / discharge characteristic test was performed. The results are shown in Table 1 below.

Example 3
A 10 w% aqueous solution of a water-soluble lithium-modified isobutene-maleic anhydride copolymer resin (average molecular weight 325,000, neutralization degree 0.5, ring opening rate 96%) was prepared as a binder composition for a negative electrode.
Then, a bipolar electrolysis cell was prepared by the same method as in Example 1 and measured. As a result, the amount of change in the reduction current value was 3.12 μA / cm ² .
Next, a negative electrode slurry was prepared in the same manner as in Example 1. Further, a coated negative electrode was prepared by the same method as in Example 1 to obtain a coin battery, and a charge / discharge characteristic test was performed. The results are shown in Table 1 below.

(Comparative Example 1)
A 10 w% aqueous solution of a water-soluble lithium-modified isobutene-maleic anhydride copolymer resin (average molecular weight 325,000, neutralization degree 0.3, ring opening rate 82%) was prepared as a binder composition for a negative electrode.
And as a result of producing and measuring a bipolar electrolysis cell by the method similar to the said Example 1, the variation | change_quantity of the reduction current value was 6.54 microampere / cm < ² >.
Next, a negative electrode slurry was prepared in the same manner as in Example 1. Further, a coated negative electrode was prepared by the same method as in Example 1 to obtain a coin battery, and a charge / discharge characteristic test was performed. The results are shown in Table 1 below.

  The binder compositions for negative electrodes of Examples 1 to 3 exhibit excellent coating properties without adding a thickener, and as is clear from Table 1, battery characteristics with low resistance and high capacity retention ratio are obtained. It was shown to be realized.

  On the other hand, in Comparative Example 1 having a large change amount, not only the resistance was high, but also electrically unstable, so that it deteriorated as it was repeatedly charged and discharged, resulting in a low capacity retention rate.

<Test Example 2>
<Making slurry of mixed layer>
3 parts by weight of a 10 w% aqueous solution of the binder composition obtained in Examples 1 to 3 and Comparative Example 1 as solids, 96 parts by weight of natural graphite (DMGS, manufactured by BYD) as a negative electrode active material, and a conductive additive ( 1 part by weight of Super-P (manufactured by Timcal Co., Ltd.) as a solid content was put into a special container and kneaded using a planetary stirrer (ARE-250, manufactured by Shinky Corporation). In order to adjust the viscosity of the slurry, an electrode coating slurry was prepared by adding water at the time of kneading and kneading again. The composition ratio of the active material and the binder in the slurry is, as a solid content, graphite powder: conductive aid: binder composition = 96: 1: 3.

<Preparation of mixed layer>
The obtained slurry was coated on a current collector copper foil (CST8G, manufactured by Fukuda Metal Foil Co., Ltd.) using a bar coater (T101, manufactured by Matsuo Sangyo Co., Ltd.), and heated at 80 ° C. for 30 minutes with a hot air dryer (Yamato Scientific). After the primary drying, a mixed layer was obtained by rolling using a roll press (made by Hosen).

For each of the obtained mixed layers, the amount of change in the reduction current value from the third time to the fourth time when the potential scan from 0 V to 3 V obtained by cyclic voltammetry under the condition II below was repeated five times was determined.
<Cyclic voltammetry conditions II>
Using a three-electrode electrolytic cell having the following specifications, the current value observed during each scan is measured at a sweep rate of 1 mV / s and a sweep range of 0 to 3 V. The reduction current value is a value obtained by dividing the current value at a voltage value of 0.2 V by the area of the working electrode.
≪Specifications of 2-electrode electrolytic cell≫
Working electrode: A layer made of the above binder composition on a copper foil (area φ14 mm, film thickness 45 μm)
Reference electrode, counter electrode: lithium foil Electrolytic solution: 1 mol / L lithium hexafluorophosphate solution (solvent: ethylene carbonate / ethyl methyl carbonate = 3/7 (volume ratio), vinylene carbonate added at 2 wt%)

  The results are shown in Table 2. In addition, about the charging / discharging characteristic test, the coin battery was produced by the same method as the said test example 1 using the said mixed layer, and the same evaluation method as the said test example 1 was performed.

In the mixed layer using the binder composition for negative electrodes of Examples 1 to 3, as shown in Table 2, since the amount of change in the reduction current value was low, it was shown to be electrically stable. On the other hand, in Comparative Example 1 in which the amount of change in the reduction current value of the mixed layer is large, not only the resistance is high as in Test Example 1, but it is electrically unstable and deteriorates as it is repeatedly charged and discharged, resulting in a capacity retention rate. The result was low.

Claims (5)

A binder composition for a lithium ion secondary battery electrode, comprising a neutralized salt of an α-olefin-maleic acid copolymer obtained by copolymerizing an α-olefin and maleic acid,
The amount of change in the reduction current value from the second time to the third time when the potential scan from 0 V to 3 V obtained by cyclic voltammetry under the following condition I is repeated three times is 5.0 μA / cm ² or less. A binder composition for a lithium ion secondary battery electrode.
<Cyclic voltammetry conditions I>
Using a two-electrode electrolytic cell having the following specifications, the current value observed during each scan is measured at a sweep rate of 1 mV / s and a sweep range of 0 to 3 V. The reduction current value is a value obtained by dividing the current value at a voltage value of 0.45 V by the area of the working electrode.
≪Specifications of 2-electrode electrolytic cell≫
Working electrode: A layer made of the above binder composition on a copper foil (area φ14 mm, film thickness 10 μm)
Reference electrode, counter electrode: lithium foil Electrolytic solution: 1 mol / L lithium hexafluorophosphate solution (solvent: ethylene carbonate / diethyl carbonate = 1/1 (volume ratio))   A binder aqueous solution for a lithium ion secondary battery electrode, comprising the binder composition according to claim 1 and water.   The slurry composition for lithium ion secondary battery electrodes containing the binder composition of Claim 1, an active material, and water.   A lithium ion secondary battery negative electrode obtained by binding a mixed layer containing the binder composition according to claim 1 and an active material to a current collector. A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode according to claim 4.