JP7759054B2 - Binder composition for lithium ion battery positive electrode, slurry for forming lithium ion battery positive electrode composite layer, positive electrode for lithium ion battery, and lithium ion battery - Google Patents

Description

The present invention relates to a binder composition for lithium-ion battery positive electrodes, a slurry for forming a lithium-ion battery positive electrode composite layer, a lithium-ion battery positive electrode, and a lithium-ion battery.

Lithium-ion batteries have been widely used in recent years as power sources for electrical devices and other devices. Furthermore, their use has recently expanded to include electric vehicles as power sources, and there is a demand for improved performance, such as higher capacity, higher output, and improved cycle life, as well as high safety.

The positive electrode of a lithium-ion battery has a structure in which a porous body composed primarily of powdered positive electrode active material, a conductive additive, and a binder is layered and bound onto a current collector. It is known that its performance is greatly affected not only by the characteristics of the positive electrode active material, but also by the type of binder.

Traditionally, polyvinylidene fluoride (PVDF) has been the mainstream binder for positive electrodes, but because organic solvents are used during electrode manufacturing, this has drawbacks, such as the need for decontamination equipment in the slurry preparation process and electrode sheet application process, and solvent recovery equipment in the electrode sheet drying process. Also, PVDF's low adhesive strength to the active material means that a larger amount of binder needs to be added.

On the other hand, methods have been disclosed for using aqueous binders that are dispersible and soluble in water without using organic solvents, such as those using polyvinyl alcohol and methyl cellulose (see, for example, Patent Document 1), xanthan gum (see, for example, Patent Document 2), starch-type polysaccharides such as amylose and aminopectin (see, for example, Patent Documents 3 and 4), and alginic acid or alginic acid derivatives (see, for example, Patent Documents 5, 6, 7, 8, and 9). Aqueous binders are preferable because they do not require decontamination equipment during electrode sheet production and improve the working environment. However, when polysaccharides such as alginic acid or its derivatives are used as binders, the electrode sheet can become hard and lack flexibility in some cases, which can lead to cracks in the electrode sheet or peeling of the electrode composite layer during battery production or charge/discharge, posing problems.

Japanese Unexamined Patent Publication No. 53-41732 Japanese Patent Application Laid-Open No. 2003-68292 JP 2008-27904 A International Publication No. 2012/133120 Japanese Patent Application Publication No. 10-92415 Japanese Patent Application Laid-Open No. 2001-15114 JP 2014-96238 A Japanese Patent Application Laid-Open No. 2014-195018 JP 2015-191862 A

The present invention aims to provide a binder composition for lithium-ion battery positive electrodes that allows for the production of flexible positive electrodes with excellent workability during the production of positive electrodes and batteries, as well as a slurry for forming a lithium-ion battery positive electrode composite layer using the same, a lithium-ion battery positive electrode, and a lithium-ion battery.

As a result of extensive research into resolving the above-mentioned issues, the inventors discovered that the above objective could be achieved by using a composition consisting of polysaccharides and specific sugar alcohols as the positive electrode binder, leading to the completion of the present invention.

That is, the various aspects of the present invention are [1] to [5] shown below.

[1] A binder composition for a lithium-ion battery positive electrode that binds together a positive electrode active material, a conductive additive, and a current collector in the positive electrode of the lithium-ion battery, characterized in that the binder composition contains 80 to 99.5% polysaccharide and 0.5 to 20% sugar alcohol having 4 to 24 carbon atoms relative to the total amount of polysaccharide and sugar alcohol.

[2] The binder composition according to claim 1, wherein the polysaccharide is alginic acid or an alginic acid derivative.

[3] A slurry for forming a positive electrode composite layer for a lithium ion battery, comprising a positive electrode active material, a conductive additive, the binder composition described in [1] or [2], and water.

[4] A positive electrode for a lithium ion battery, comprising the binder composition described in [1] or [2].

[5] A lithium ion battery characterized by having the lithium ion battery positive electrode described in [4].

The inventors speculate as follows as to why the above-mentioned objectives are achieved by the binder composition for lithium-ion battery positive electrodes of the present invention.

In other words, the specific sugar alcohol used in the present invention acts as a plasticizer for polysaccharides, imparting flexibility to the binder composition composed of polysaccharides and sugar alcohols. As a result, cracks in the positive electrode sheet that occur during electrode drying or battery assembly, cracks in the positive electrode composite layer due to volume changes during charge and discharge, and peeling and detachment of the positive electrode composite layer and active material are suppressed, preventing destruction of the conductive channel. The absence of cracks in the positive electrode sheet and peeling of the positive electrode composite layer during battery operation contributes greatly to improved battery performance, such as excellent charge and discharge characteristics and extended cycle life. Furthermore, because the specific sugar alcohol used in the present invention does not dissolve in the electrolyte, it does not adversely affect battery characteristics.

The present invention makes it possible to provide a binder composition for lithium-ion battery positive electrodes that allows for the production of flexible positive electrodes with excellent workability during the production of positive electrodes and batteries, as well as a slurry for forming a lithium-ion battery positive electrode composite layer using the same, a positive electrode for lithium-ion batteries, and a lithium-ion battery.Furthermore, by using the same, it is possible to provide a positive electrode for lithium-ion batteries and a lithium-ion battery that have excellent charge/discharge characteristics and suppressed reduction in cycle life.

1 is an SEM image (magnification: 1500 times) of the surface of the positive electrode obtained in Example 1. 1 is an SEM image (magnification: 1500 times) of the surface of the positive electrode obtained in Example 2. 1 is an SEM image (magnification: 3000 times) of the surface of the positive electrode obtained in Example 3. 1 is an SEM image (magnification: 1500 times) of the surface of the positive electrode obtained in Comparative Example 1. 1 is an SEM image (magnification: 3000 times) of the surface of the positive electrode obtained in Comparative Example 2. 1 is an SEM image (magnification: 1500x) of the surface of the positive electrode obtained in Comparative Example 3. 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 1, Example 2, and Comparative Example 1. 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 3 and Comparative Example 2. 10 is a graph showing the charge-discharge cycle characteristics of the coin cell obtained in Comparative Example 3.

The present invention will be described in detail below based on its preferred embodiments.

One aspect of the present invention is a binder composition for lithium-ion battery positive electrodes, which is a composition consisting of a polysaccharide and a specific sugar alcohol.

Polysaccharides refer to those having a structure in which multiple monosaccharides are linked together via glycosidic bonds. Examples of polysaccharides contained in the binder composition of the present invention include amylose, amylopectin, dextrin, glycogen, cellulose, carboxymethylcellulose, hydroxyalkylcellulose, acetylcellulose, pectin, pullulan, curdlan, chitin, chitosan, agarose, xanthan gum, guar gum, gellan gum, locust bean gum, gum arabic, tamarind seed gum, psyllium seed gum, carrageenan, alginic acid, alginic acid derivatives, heparin, hyaluronic acid, chondroitin sulfate, xyloglucan, and glucomannan. Of these, neutral and acidic polysaccharides are preferred for use in the present invention. Specific examples include amylose, amylopectin, carboxymethylcellulose, guar gum, gellan gum, xanthan gum, gum arabic, alginic acid, alginic acid derivatives, agarose, carrageenan, and chondroitin sulfate. More preferably, alginic acid or an alginic acid derivative is used. Alginic acid derivatives refer to compounds in which various substituents have been introduced into alginic acid, and examples include alkali metal alginates, alkaline earth metal alginates, ammonium alginates, alginic acid esters, and sulfated alginic acid.

The molecular weight of the polysaccharide is preferably a weight-average molecular weight of 10,000 to 2,000,000. A weight-average molecular weight within this range is preferable because it provides good mechanical properties and an appropriate slurry viscosity during electrode production.

Specific examples of alginic acid or alginic acid derivatives include those represented by the following general formula (1):

(In the formula, m and n each independently represent an integer of 1 or greater; R ₁ may be the same or different and each represents one selected from the group consisting of hydrogen, alkali metals, alkaline earth metals, quaternary ammonium, and groups represented by —R ₃ —OH; and R ₃ represents a divalent hydrocarbon group having 2 to 6 carbon atoms.)
Alginic acid, alginate salts, and alginate esters represented by the following formula are preferably used.

Examples of the divalent hydrocarbon group having 2 to 6 carbon atoms for _R3 include an ethylene group, an ethylidene group, a vinylene group, a trimethylene group, a methylethylene group, a 1-methylethylidene group, a propenylene group, a tetramethylene group, a methyltrimethylene group, a dimethylethylene group, a 1-ethylethylidene group, an ethylethylene group, a pentamethylene group, a methyltetramethylene group, a dimethyltrimethylene group, a methylethylethylene group, a hexamethylene group, a cyclohexylene group, and a cyclohexylidene group.

Alginic acid refers to the case where all R ₁ in formula (1) are hydrogen; alginate refers to the case where at least some of R ₁ in formula (1) are alkali metals (e.g., lithium, sodium, potassium), alkaline earth metals (e.g., magnesium, calcium), or quaternary ammonium (e.g., ammonium, tetramethylammonium); and alkynoic acid ester refers to the case where at least some of R ₁ in formula (1) are groups represented by -R ₃ -OH (R ₃ is a divalent hydrocarbon group having 2 to 6 carbon atoms), and such alkynoic acid esters can be produced by reacting alginic acid with an epoxy compound.

In addition, m and n in the general formula (1) may each independently be an integer of 1 or greater, but are preferably selected so that the weight-average molecular weight of the alginic acid or alginic acid derivative represented by the general formula (1) is 10,000 to 2,000,000, and m and n are preferably each independently 30 to 6,000.

The ratio of mannuronic acid and guluronic acid, which are components of alginic acid, can be any ratio; either alginic acid with a high mannuronic acid ratio, which produces a flexible gel, or alginic acid with a high guluronic acid ratio, which produces a rigid gel, can be used.

Sulfated polysaccharides in which sulfate groups and/or alkali metal sulfate groups have been introduced into the above polysaccharides are also suitable. The sulfate groups are ion-exchange groups that can exchange not only basic salts but also neutral salts such as NaCl and CaCl2. The sulfate groups are introduced at hydroxyl group sites in the polysaccharide structure, and the hydrogen atoms of the _hydroxyl groups are replaced with _-SO3H .

Polysaccharides having sulfate groups and/or alkali metal sulfate groups are represented by the following general formula (2):

(In the formula, m and n are each independently an integer of 1 or greater; R1 _'s may be the same or different and each represent one selected from the group consisting of hydrogen, alkali metals, alkaline earth metals, quaternary ammonium, and groups represented by _-R3 -O- _R2 ; _R3 represents a divalent hydrocarbon group having 2 to 6 carbon atoms; _R2 's may be the same or different and each represent hydrogen or _-SO3M (M is hydrogen or an alkali metal), and at least one _R2 is _-SO3M .)
Preferably, the compound has the structure shown below.

Examples of the divalent hydrocarbon group having 2 to 6 carbon atoms for R ₃ in formula (2) include those exemplified as the divalent hydrocarbon group having 2 to 6 carbon atoms for R ₃ in formula (1).

The amount of sulfate groups and/or alkali metal sulfate bases introduced into the polysaccharide is preferably 0.5 to 6.0 mmol/g. When the amount of sulfate groups and/or alkali metal sulfate bases introduced is within this range, oxidation resistance is improved, and excellent electrolyte affinity is achieved while maintaining binding strength to the active material, which is preferable.

Since sulfate groups and alkali metal sulfate bases are strongly acidic cation exchange groups, they can efficiently exchange and capture transition metal ions such as manganese ions without being affected by the pH of the system.

In addition, m and n in the general formula (2) may each independently be an integer of 1 or greater, but are preferably selected so that the weight-average molecular weight of the polysaccharide having sulfate groups and/or alkali metal sulfate groups represented by the general formula (2) is 10,000 to 2,000,000, and m and n are preferably each independently 30 to 6,000.

There are no particular limitations on the method for producing polysaccharides containing sulfate groups and/or alkali metal sulfate groups. For example, they can be obtained by reacting a polysaccharide with a sulfating reagent in an aprotic polar solvent. The reaction can be carried out in either a slurry system or a homogeneous system.

Aprotic polar solvents are preferred as the solvent used in the reaction, and specific examples include dimethyl sulfoxide, formamide, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone (NMP), 1,3-dimethyl-2-imidazolidinone, acetonitrile, hexamethylphosphoric triamide, tetramethylurea, N,N'-dimethylpropyleneurea, tetrahydrofuran, dioxane, pyridine, bipyridine, and phenanthroline.

The polysaccharides used are preferably those with excellent solubility in aprotic polar solvents, such as amylose, amylopectin, dextrin, glycogen, cellulose, carboxymethylcellulose, hydroxyalkylcellulose, acetylcellulose, pectin, pullulan, curdlan, chitin, chitosan, agarose, xanthan gum, guar gum, gellan gum, locust bean gum, gum arabic, tamarind seed gum, psyllium seed gum, carrageenan, alginic acid, alginates, alginate esters, heparin, hyaluronic acid, chondroitin sulfate, xyloglucan, and glucomannan. Preferred examples include amylose, amylopectin, carboxymethylcellulose, guar gum, gellan gum, xanthan gum, gum arabic, alginic acid, alginates, alginate esters, agarose, carrageenan, and chondroitin sulfate. More preferred are alginic acid and alginate esters.

The sulfating reagent used is one that can react with hydroxyl groups to introduce sulfate groups. Specific examples include concentrated sulfuric acid, chlorosulfonic acid, sulfur trioxide pyridine complex, sulfur trioxide trimethylamine complex, sulfur trioxide dimethylformamide complex, sulfur trioxide dimethylsulfoxide complex, and sulfur trioxide dioxane complex. Among these sulfating reagents, those that result in minimal reduction in the molecular weight of polysaccharides are preferred for use in the present invention. A preferred sulfating reagent is sulfur trioxide complex, and particularly preferred sulfating reagents are sulfur trioxide pyridine complex and sulfur trioxide dimethylformamide complex. There are no particular restrictions on the amount of sulfating reagent used, but a range of 1 to 5 times the molar amount of monosaccharides that constitute polysaccharides is preferred. Using a sulfating reagent within this range is preferred because it introduces a large amount of sulfate groups, suppresses reduction in the molecular weight of the polysaccharide, and results in a high molecular weight product.

Other reaction conditions can be set as desired, but it is preferable that the reaction temperature be selected from the range of 0°C to 100°C, the reaction time be 30 minutes to 12 hours, and the polysaccharide concentration during the reaction be selected from the range of 0.1 to 50% by mass.

There are no particular limitations on the method for isolating the polysaccharide having sulfate groups and/or alkali metal sulfate bases from the reaction solution after completion of the reaction. Possible methods include heating and removing the solvent to isolate the polysaccharide having sulfate groups and/or alkali metal sulfate bases, or adding the reaction solution dropwise to a poor solvent to precipitate the polysaccharide having sulfate groups and/or alkali metal sulfate bases, and then filtering and recovering it.

Polysaccharides having sulfate groups and/or alkali metal sulfate salt groups are obtained in which the introduced sulfate groups are in the regenerated form (-SO ₃ H) at the time of production. However, in the present invention, at least a portion of the regenerated groups may be ion-exchanged into alkali metal salt forms (-SO ₃ M'(M' represents an alkali metal (e.g., lithium, sodium, potassium)). Ion exchange from the regenerated form to the salt form can be carried out by bringing the regenerated binder into contact with a solution containing alkali metal ions, as in the case of ordinary ion exchangers.

Polysaccharides incorporating functional groups other than sulfate groups are also suitable for use in the present invention. Examples of functional groups other than sulfate groups include chelating groups such as iminodiacetic acid groups, nitrilotriacetic acid groups, ethylenediaminetetraacetic acid groups, aminomethylphosphate groups, and polyethyleneimine groups, as well as epoxy groups such as glycidyl groups.

Although not required, the binder composition for lithium-ion battery positive electrodes of the present invention may contain other binder components such as polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polymaleic acid copolymer, polyfumaric acid, polyitaconic acid, or salts thereof.

Next, we will explain the sugar alcohols contained in the binder composition of the present invention.

Sugar alcohols are a type of sugar produced by the reduction of the carbonyl group of aldoses or ketoses, and broadly include cyclitols, which are polyhydroxycycloalkanes. The sugar alcohols contained in the binder composition of the present invention are sugar alcohols with 4 to 24 carbon atoms, and specific examples include erythritol, D-threol, L-threitol, D-arabinitol, L-arabinitol, xylitol, ribitol, D-iditol, galactitol, sorbitol, mannitol, volemitol, inositol, quercitol, maltitol, and reduced starch syrup. Having a carbon number within the above range makes the sugar alcohol insoluble in the electrolyte, thereby inhibiting migration to the negative electrode and reductive decomposition at the negative electrode, thereby suppressing deterioration of battery characteristics. Furthermore, having a carbon number within the above range increases the affinity between the sugar alcohol and polysaccharides, enhancing the plasticizing effect and imparting flexibility to the electrode sheet.

In the binder composition of the present invention, the polysaccharide content must be 80 to 99.5% by mass and the sugar alcohol content must be 0.5 to 20% by mass, based on the total amount of polysaccharides and sugar alcohols. It is preferable that the polysaccharide content be 85 to 95% by mass and the sugar alcohol content be 5 to 15% by mass. Having the polysaccharide and sugar alcohol contents within these ranges is preferable because the polysaccharides are sufficiently plasticized, imparting flexibility.

The amount of the binder composition in the positive electrode mixture layer is preferably 1 to 20 mass %, and more preferably 3 to 10 mass %, of the total amount of the positive electrode active material, binder composition, and conductive additive.

The binder composition of the present invention is highly flexible, allowing it to adapt to volume changes in the positive electrode during charge and discharge, preventing cracking of the positive electrode due to volume changes, as well as the resulting peeling and detachment of the active material and destruction of the conductive channel. Furthermore, although the binder composition of the present invention is insoluble in the electrolyte, it has excellent affinity, resulting in excellent lithium ion conductivity. It also has excellent oxidation resistance, ensuring stability even under harsh oxidative conditions near the positive electrode. Therefore, by coating the positive electrode active material, it is possible to suppress decomposition of the electrolyte or electrolyte by the positive electrode active material, making it an excellent binder composition for lithium-ion battery positive electrodes.

In the present invention, the positive electrode active material is not particularly limited as long as it is capable of inserting and extracting lithium ions, and examples thereof include transition metal oxides such as CuO, Cu ₂ O, MnO ₂ , MoO ₃ , V ₂ O ₅ , CrO ₃ , Fe ₂ O ₃ , Ni ₂ O ₃ , and CoO _3, and lithium composite oxides such as Li _x CoO ₂ , Li _x NiO ₂ , Li _x MnO ₂ , Li _x Mn ₂ O ₄ , LiNi _x Co _(1-X) O ₂ , LiNi _x Mn _(2-X) O ₄ , LiMna _a Ni _b Co _c O ₂ (a + b + c = 1), and LiFePO ₄ . Among these, composite oxides of lithium and at least one transition metal selected from transition metals such as Co, Ni, and Mn are preferred, and specific examples include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _X Co _(1-X) O ₂ , LiNi _X Mn _(2-X) O ₄ , and LiMna _{Ni b} Co _c O ₂ (a + b + c = 1). These lithium composite oxides may be doped with small amounts of elements such as fluorine, boron, Al, Cr, Zr, Mo, and Fe, or the particle surfaces of the lithium composite oxide may be surface-treated with carbon, MgO, Al ₂ O ₃ , SiO _2, or the like.

There are no particular restrictions on the conductive additive contained in the positive electrode, and any electronically conductive material that does not adversely affect battery characteristics can be used. Specific examples include conductive carbon such as ketjen black and acetylene black; carbon materials such as natural graphite, artificial graphite, carbon whiskers, carbon nanotubes, and carbon fiber powder; metal powders or fibers such as Cu, Fe, Ag, Ni, Pd, Au, Pt, In, and W; and conductive metal oxides such as indium oxide and tin oxide. The amount of these conductive additives to be added is preferably 1 to 30% by mass of the positive electrode active material.

One embodiment of the present invention is a slurry for forming a positive electrode composite layer for a lithium-ion battery, which contains the binder composition, a positive electrode active material, a conductive additive, and water. The slurry may optionally contain a viscosity modifier such as carboxymethyl cellulose, or a pH adjuster such as an acid or alkali. The solids concentration of the slurry is not particularly limited, but is preferably 20 to 80% by mass, taking into consideration the viscosity of the slurry, the dispersibility of the solids, the load on the drying process, and other factors. Furthermore, the mass ratio of components in the slurry is preferably positive electrode active material:conductive additive:binder composition = 70 to 98:1 to 30:1 to 20. There are no particular restrictions on the method for producing the slurry, and examples include a method in which the binder composition, positive electrode active material, and conductive additive are mixed, dispersed, and dissolved all at once in water to produce a slurry; a method in which the binder composition is first dissolved in water, and then the positive electrode active material and conductive additive are added to the aqueous binder composition solution and mixed to produce a slurry; and a method in which the positive electrode active material and conductive additive are first mixed, and then mixed with the aqueous binder composition solution. There are also no particular restrictions on the mixer used to produce the slurry, and a mortar, roll mill, ball mill, screw mill, vibration mill, homogenizer, planetary mixer, etc. can be used.

A lithium-ion battery positive electrode according to one embodiment of the present invention includes the binder composition. This lithium-ion battery positive electrode comprises a positive electrode composite layer obtained by applying the slurry to a positive electrode current collector and drying the slurry, and the positive electrode current collector. The thickness of the positive electrode composite layer, which is composed of the positive electrode active material, the binder composition, and the conductive additive, is preferably 10 to 200 μm. To form a positive electrode composite layer of this thickness on the positive electrode current collector, the basis weight of the positive electrode composite layer is preferably set to 4 to 25 mg/ ^cm² .

The positive electrode current collector may be any conductor as long as the surface in contact with the positive electrode composite layer exhibits electrical conductivity. Examples include conductors formed from metals such as copper, gold, aluminum, titanium, nickel, stainless steel, or alloys thereof; conductive metal oxides such as indium oxide or tin oxide; and conductive materials such as conductive carbon. There are no particular restrictions on the shape of the positive electrode current collector, and it can be in the form of foil, film, sheet, net, expanded metal, punched metal, foam, or the like. There are also no particular restrictions on the thickness of the positive electrode current collector, with a thickness of approximately 1 to 100 μm being preferred.

There are no particular restrictions on the manufacturing method of positive electrodes for lithium-ion batteries; they can be produced by applying the above-mentioned slurry to a positive electrode current collector and drying it. There are also no particular restrictions on the method for applying the slurry; methods such as slit coating, die coating, roll coating, dip coating, blade coating, knife coating, and wire bar coating can be used. There are also no particular restrictions on the drying method or conditions; a standard hot air circulation dryer, reduced pressure dryer, infrared dryer, or microwave heating dryer can be used. There are also no restrictions on the heating temperature; drying can be carried out at a temperature between 50 and 150°C. Furthermore, the porous structure can be made uniform by applying pressure to the positive electrode during or after drying.

One aspect of the present invention is a lithium-ion battery characterized by having the above-described positive electrode for a lithium-ion battery. Use of the above-described positive electrode for a lithium-ion battery can provide a high-performance lithium-ion battery with excellent charge/discharge characteristics and a long cycle life. A lithium-ion battery generally comprises a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte, and the like. The positive electrode is formed by binding the above-described positive electrode active material together with the above-described conductive additive to a positive electrode current collector using the above-described binder composition, and has a structure in which a positive electrode composite layer composed of the positive electrode active material, binder composition, and conductive additive is formed on the current collector. The negative electrode has a similar structure to the positive electrode, and is formed by binding the below-described negative electrode active material and the above-described conductive additive to the negative electrode current collector using a binder. The separator is typically a porous film such as polyolefin, and is sandwiched between the positive and negative electrodes to provide a shutdown function in the event of thermal runaway of the battery. The nonaqueous electrolyte is formed by dissolving an electrolyte salt such as _LiPF4 in an organic solvent such as cyclic carbonate. The inside of the battery is filled with a non-aqueous electrolyte, and lithium ions move from the positive electrode to the negative electrode during charging, and from the negative electrode to the positive electrode during discharging.

There are no particular restrictions on the negative electrode used in the lithium-ion battery of the present invention, and it can be made using known materials. The negative electrode is composed of a negative electrode composite layer made of a negative electrode active material, a conductive additive, and a binder, and a current collector.

Any material capable of inserting and detaching lithium ions can be used as the negative electrode active material. Examples include carbon-based materials such as natural graphite, artificial graphite, non-graphitizable carbon, and easily graphitizable carbon; silicon-based materials such as silicon, silicon alloys, and silicon oxide; tin-based materials such as tin and tin alloys; and metallic lithium and alloys.

The same conductive additives as those exemplified for the positive electrode can be used as conductive additives for the negative electrode.

Any known binder can be used for the negative electrode binder, including fluororesins such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, polytetrafluoroethylene, and fluororubber; hydrocarbon elastomers such as styrene-butadiene copolymer and ethylene-propylene copolymer; polysaccharides such as carboxymethyl cellulose, alginic acid, and sodium alginate; and polyimides.

The negative electrode current collector can be the same as the current collector exemplified for the positive electrode.

The non-aqueous electrolyte is not particularly limited, and known materials can be used. The non-aqueous electrolyte is obtained by dissolving an electrolyte salt in an organic solvent _. Examples of the electrolyte salt include _CF3SO3Li , ( _CF3SO2 ) _{2NLi , (CF3SO2)2CLi, LiBF4, LiB(C6H8)4 , LiPF4 , LiClO4 , LiAsF6 , LiCl ,} and LiBr. Examples of organic solvents that dissolve the electrolyte salt include ethylene carbonate, vinylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,4-dioxane, anisole, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, dimethylformamide, dimethyl sulfoxide, trimethyl phosphate, triethyl phosphate, etc. The concentration of the electrolyte salt in the nonaqueous electrolyte solution can be selected from the range of 0.1 to 5 mol/L, preferably 0.5 to 3 mol/L.

There are no particular restrictions on the separator, and any known separator can be used. Examples of separators include polyethylene microporous membranes, polypropylene microporous membranes, laminated membranes of polyethylene microporous membranes and polypropylene microporous membranes, nonwoven fabrics made of polyester fibers, aramid fibers, glass fibers, etc., and separators in which the surface of the above microporous membranes is coated with ceramics, aramid, or PVDF.

The present invention will be explained in more detail below based on examples, but the present invention is not limited to these examples.

<Production of sulfated alginates>
(Reference Example 1) Production of sulfated alginic acid To 150 ml of dimethyl sulfoxide (hereinafter abbreviated as DMSO), 3.0 g (17 mmol as monosaccharide units) of H-type alginic acid (manufactured by Kimika Co., Ltd., trade name Kimika Acid SA) was added little by little with stirring and uniformly dispersed. Next, 6.75 g (42.5 mmol) of sulfur trioxide pyridine complex was added with stirring, and after uniform dispersion, the temperature was raised and the reaction was carried out at 40 ° C. for 5 hours. The alginic acid dissolved as the reaction progressed and became a homogeneous solution at the end of the reaction. After the reaction was completed, the reaction solution was added dropwise to ethanol, and the precipitated product was collected with a glass filter and further washed with ethanol, and then dried under reduced pressure at room temperature to isolate the product. The isolated yield was 5.0 g, the sulfur content determined by elemental analysis was 10.1% by mass, and the weight average molecular weight measured by gel permeation chromatography (GPC) was 48,000. The amount of sulfate group introduction calculated from the sulfur content was 3.2 mmol / g.

Example 1
<Production of positive electrode>
First, a positive electrode mixture was prepared from the following materials to fabricate a positive electrode.
Positive electrode active material: Lithium nickel manganese composite oxide
(LNMO: LiNi _0.5 Mn _1.5 O ₄ ) 80 parts by mass Conductive additive: Acetylene black (AB: Li-400 manufactured by Denka Co., Ltd.) 10 parts by mass Binder composition: Sulfated alginic acid (Reference Example 1) 9 parts by mass
Xylitol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) 1 part by mass.

Sulfated alginic acid was neutralized with a 1 mol/L aqueous solution of lithium hydroxide to prepare a lithium sulfated alginate aqueous solution. Xylitol was then added to the lithium sulfated alginate aqueous solution and dissolved. LNMO and AB were mixed in a mortar, and this was mixed with the lithium sulfated alginate/xylitol aqueous solution and mixed again in the mortar to prepare a positive electrode composite slurry. The slurry was then applied to aluminum foil using a doctor blade, dried at 80°C for 24 hours, and further dried under reduced pressure at 80°C for 24 hours to prepare a positive electrode. The flexibility of the resulting positive electrode and the adhesion of the positive electrode composite layer were evaluated using the following methods. The resulting positive electrode was flexible, no peeling of the positive electrode composite layer was observed even when bent, and adhesion was also good.

<Positive electrode flexibility test>
The positive electrode was punched into a disk shape with a diameter of 10 mm, bent 180°, and then opened again to observe the state of the bent portion. Hereinafter, this test will be referred to as the 180° bending test. If the positive electrode composite layer at the bent portion was not broken and maintained its shape, it was determined to have flexibility. If the positive electrode composite layer was broken and the underlying aluminum foil was exposed, it was determined to have no flexibility.

<Adhesion of the positive electrode mixture layer>
In the 180° bending test, the adhesion between the positive electrode composite layer and the aluminum foil at the bent portion was observed. When the positive electrode composite layer and the aluminum foil were in close contact and no peeling or delamination of the positive electrode composite layer was observed, the adhesion was judged to be good. On the other hand, when the aluminum foil was exposed and peeling or delamination of the positive electrode composite layer was observed, the adhesion was judged to be poor.

<Coin cell manufacturing>
A lithium metal foil was used for the negative electrode, and lithium hexafluorophosphate was dissolved in a 1/1 ethylene carbonate/dimethyl carbonate mixed solvent to a concentration of 1 mol/L to prepare an electrolyte solution. A polyolefin microporous membrane (manufactured by Toray Industries, Inc., thickness: 20 μm) was used as the separator. The positive and negative electrodes were then placed on either side of the separator, stacked, and the electrolyte was poured into the resulting 2032-type coin cell.

Example 2
Except for using sorbitol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) instead of xylitol, a positive electrode and a coin cell were fabricated in the same manner as in Example 1. The flexibility of the obtained positive electrode and the adhesion of the positive electrode composite layer were evaluated in the same manner as in Example 1. The obtained positive electrode was flexible, no peeling of the positive electrode composite layer was observed even when bent, and the adhesion was also good.

Example 3
A positive electrode and a coin cell were produced in the same manner as in Example 1, except that lithium alginate, which was obtained by neutralizing H-type alginic acid (manufactured by Kimica Corporation, product name Kimica Acid SA) with lithium hydroxide, was used instead of sulfated alginic acid. The flexibility of the obtained positive electrode and the adhesion of the positive electrode composite layer were evaluated in the same manner as in Example 1. The obtained positive electrode was flexible, no peeling of the positive electrode composite layer was observed even when bent, and the adhesion was also good.

(Comparative Example 1)
Except for not using xylitol, a positive electrode and a coin cell were produced in the same manner as in Example 1. The flexibility of the obtained positive electrode and the adhesion of the positive electrode mixture layer were evaluated in the same manner as in Example 1. The obtained positive electrode had poor flexibility, and in a 180° bending test, the aluminum foil was exposed at the bent portion, and peeling of the positive electrode mixture layer was observed.

(Comparative Example 2)
Except for not using sorbitol, a positive electrode and a coin cell were produced in the same manner as in Example 3. The flexibility of the obtained positive electrode and the adhesion of the positive electrode composite layer were evaluated in the same manner as in Example 1. The obtained positive electrode had poor flexibility, and in a 180° bending test, the aluminum foil was exposed at the bent portion, and peeling of the positive electrode composite layer was observed.

(Comparative Example 3)
A positive electrode and a coin cell were produced in the same manner as in Example 1, except that the amount of sulfated alginic acid was 8.5 parts by mass and 1.5 parts by mass of SBR (styrene-butadiene rubber) (manufactured by JSR Corporation, (grade name) TRD2001) was used instead of xylitol. The flexibility of the obtained positive electrode and the adhesion of the positive electrode composite layer were evaluated in the same manner as in Example 1. The obtained positive electrode was flexible, and no peeling of the positive electrode composite layer was observed even when bent, indicating good adhesion.

<SEM Observation of Positive Electrode Surface>
The surfaces of the positive electrodes obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were observed using an SEM. The results are shown in Figures 1 to 6. Figures 1, 2, and 3 are SEM images of the surfaces of the positive electrodes obtained in Examples 1, 2, and 3, respectively; cracks were minimal. In contrast, SEM images (Figures 4 and 5) of the surfaces of the positive electrodes obtained in Comparative Examples 1 and 2, to which no sugar alcohol was added, confirmed the occurrence of numerous cracks. This demonstrates that cracking cannot be suppressed without the addition of a sugar alcohol. Figure 6 is an SEM image of the surface of the positive electrode obtained in Comparative Example 3, to which SBR was added; cracks were minimal, demonstrating that the addition of an elastomer can also suppress cracking.

<Charge/discharge characteristic evaluation>
The coin cells obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were used to evaluate the charge/discharge characteristics under the following conditions.
Charging: Constant current (CC mode)
Discharge: Constant current (CC mode)
Potential range: 3.5 to 5.0 V
Current density: 20mA/g.

Under the above conditions, 50 charge/discharge cycles were performed. The results are shown in Figures 7 to 9. Figure 7 shows the charge/discharge curves of the coin cells obtained in Examples 1 and 2 and Comparative Example 1. Even after 50 cycles, there was only a slight decrease in discharge capacity, demonstrating that the addition of sugar alcohols does not adversely affect battery performance. Figure 8 shows the charge/discharge curves of the coin cells obtained in Example 3 and Comparative Example 2. Even after 50 cycles, there was only a slight decrease in discharge capacity, demonstrating that the addition of sugar alcohols does not adversely affect battery performance. In contrast, Figure 9 shows the charge/discharge curve of the coin cell obtained in Comparative Example 3. It can be seen that the discharge capacity decreased with increasing cycle count. This indicates that the addition of SBR, which has poor oxidation resistance, causes oxidative decomposition of the SBR as the cycle count increases, resulting in a decrease in battery performance.

As explained above, the present invention makes it possible to provide a binder composition for lithium-ion battery positive electrodes that allows for the production of flexible positive electrodes with excellent workability during the production of positive electrodes and batteries, as well as a slurry for forming a lithium-ion battery positive electrode composite layer using the same, a positive electrode for lithium-ion batteries, and a lithium-ion battery.Furthermore, by using the same, it is possible to provide a positive electrode for lithium-ion batteries and a lithium-ion battery that have excellent charge/discharge characteristics and suppressed reduction in cycle life.

Claims (5)

A binder composition for lithium-ion battery positive electrodes that binds together a positive electrode active material, a conductive additive, and a current collector. The binder composition contains 80 to 99.5% by mass of polysaccharides and 0.5 to 20% by mass of sugar alcohols having 4 to 24 carbon atoms, based on the total amount of polysaccharides and sugar alcohols. The binder composition according to claim 1, wherein the polysaccharide is alginic acid or an alginic acid derivative. A slurry for forming a positive electrode composite layer for a lithium ion battery, comprising a positive electrode active material, a conductive additive, the binder composition of claim 1 or 2, and water. A positive electrode for a lithium ion battery comprising the binder composition according to claim 1 or 2. A lithium ion battery having the lithium ion battery positive electrode described in claim 4.