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

Abstract

To provide a binder composition for a lithium ion battery which has high workability in manufacturing a cathode and a battery and makes it possible to manufacture a flexible cathode, and slurry for forming a cathode mixture layer of a lithium ion battery, a cathode for a lithium ion battery, and a lithium ion battery which contain the binder composition; in addition, by using the binder composition, to provide a cathode for a lithium ion battery and a lithium ion battery having excellent charge-discharge behavior, and reduced cycle-life depression.SOLUTION: For a cathode for a lithium ion battery, a composition comprising a polyacrylamide and a specific sugar alcohol is used, as a binder for binding a cathode active material, a conductive assistant, and a current collector.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a binder composition for a lithium ion battery positive electrode, a slurry for forming a lithium ion battery positive electrode composite layer, a lithium ion battery positive electrode, and a lithium ion battery. [Background technology]

[0002] Lithium-ion batteries have been widely used in recent years as power sources for electrical devices, etc. Furthermore, their use has recently expanded to include power sources for electric vehicles, and there is a demand for improved performance, such as higher capacity, higher output, and improved cycle life, as well as high safety.

[0003] The positive electrode of a lithium-ion battery has a structure in which a porous body consisting mainly of powdered positive electrode active material, a conductive additive, and a binder is layered and bound onto a current collector, and it is known that its performance is greatly affected not only by the properties of the positive electrode active material but also by the type of binder.

[0004] Traditionally, polyvinylidene fluoride (PVDF) has been the mainstream binder for positive electrodes, but because organic solvents are used during electrode manufacturing, it 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, and the need for increased binder addition due to the low adhesive strength of PVDF to active materials.

[0005] On the other hand, as aqueous binders that can be dispersed and dissolved in water without using organic solvents, the use of polyvinyl alcohol and methyl cellulose (see, for example, Patent Document 1), the use of xanthan gum (see, for example, Patent Document 2), the use of starch-type polysaccharides such as amylose and aminopectin (see, for example, Patent Documents 3 and 4), and the use of alginic acid or alginic acid derivatives (see, for example, Patent Documents 5, 6, 7, 8, and 9) have been disclosed. Aqueous binders are preferable because they do not require decontamination equipment during electrode sheet production and also improve the working environment, but 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 cause cracks in the electrode sheet or peeling of the electrode mixture layer during battery production or charge / discharge, posing problems. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 53-41732 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-68292 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-27904 [Patent Document 4] International Publication No. 2012 / 133120 [Patent Document 5] Japanese Patent Application Publication No. 10-92415 [Patent Document 6] Japanese Patent Application Laid-Open No. 2001-15114 [Patent Document 7] Japanese Patent Application Laid-Open No. 2014-96238 [Patent Document 8] Japanese Patent Application Laid-Open No. 2014-195018 [Patent Document 9] Japanese Patent Application Laid-Open No. 2015-191862 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a binder composition for lithium-ion battery positive electrodes that allows for the production of flexible positive electrodes that are easy to work with 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. [Means for solving the problem]

[0008] As a result of intensive research to solve the above problems, the inventors have found that the above object can be achieved by using a composition comprising a polysaccharide and a specific sugar alcohol as a positive electrode binder, and have thus completed the present invention.

[0009] That is, the embodiments of the present invention are [1] to [5] shown below.

[0010] [1] A binder composition for a lithium ion battery positive electrode, which binds a positive electrode active material, a conductive additive, and a current collector together in the positive electrode of a lithium ion battery, the binder composition comprising 80 to 99.5% of a polysaccharide and 0.5 to 20% of a sugar alcohol having 4 to 24 carbon atoms relative to the total amount of the polysaccharide and the sugar alcohol.

[0011] [2] The binder composition according to claim 1, wherein the polysaccharide is alginic acid or an alginic acid derivative.

[0012] [3] A slurry for forming a positive electrode mixture layer for a lithium ion battery, comprising a positive electrode active material, a conductive additive, the binder composition according to [1] or [2], and water.

[0013] [4] A positive electrode for a lithium ion battery, comprising the binder composition according to [1] or [2].

[0014] [5] A lithium ion battery comprising the positive electrode for a lithium ion battery according to [4].

[0015] The present inventors speculate as follows as to why the above object can be achieved by the binder composition for a lithium ion battery positive electrode of the present invention.

[0016] That is, 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 fabrication, cracking of 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, thereby preventing breakdown of the conductive channel. The absence of cracks in the positive electrode sheet and the absence of peeling of the positive electrode composite layer during battery operation contributes significantly to improved battery performance, such as excellent charge and discharge characteristics and extended cycle life. Furthermore, the specific sugar alcohol used in the present invention does not dissolve in the electrolyte and therefore does not adversely affect battery performance. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a binder composition for a lithium-ion battery positive electrode that allows for the production of a flexible positive electrode with excellent workability during the production of the positive electrode and battery, as well as a slurry for forming a lithium-ion battery positive electrode composite layer using the same, a positive electrode for a lithium-ion battery, and a lithium-ion battery. Furthermore, by using the same, it is possible to provide a positive electrode for a lithium-ion battery and a lithium-ion battery that have excellent charge-discharge characteristics and suppressed reduction in cycle life. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is an SEM image (magnification: 1500 times) of the surface of the positive electrode obtained in Example 1. [Figure 2] 1 is an SEM image (magnification: 1500x) of the surface of the positive electrode obtained in Example 2. [Figure 3] 1 is an SEM image (magnification: 3000 times) of the surface of the positive electrode obtained in Example 3. [Figure 4] 1 is an SEM image (magnification: 1500x) of the surface of the positive electrode obtained in Comparative Example 1. [Figure 5] 1 is an SEM image (magnification: 3000 times) of the surface of the positive electrode obtained in Comparative Example 2. [Figure 6] 1 is an SEM image (magnification: 1500 times) of the surface of the positive electrode obtained in Comparative Example 3. [Figure 7] 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 1, Example 2, and Comparative Example 1. [Figure 8] 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 3 and Comparative Example 2. [Figure 9] 10 is a graph showing the charge-discharge cycle characteristics of the coin cell obtained in Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described in detail below based on preferred embodiments thereof.

[0020] A binder composition for a lithium ion battery positive electrode, which is one embodiment of the present invention, is a composition comprising a polysaccharide and a specific sugar alcohol.

[0021] Polysaccharides refer to those having a structure in which multiple monosaccharides are linked by 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. Among these, neutral and acidic polysaccharides are preferred 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. The alginic acid derivative refers to a compound in which various substituents have been introduced into alginic acid, and examples thereof include alkali metal alginates, alkaline earth metal alginates, ammonium alginates, alginic acid esters, and sulfated alginic acid.

[0022] The molecular weight of the polysaccharide is preferably a weight-average molecular weight of 10,000 to 2,000,000. If the weight-average molecular weight is in the above range, the mechanical properties are good and the slurry viscosity during electrode production is also at an appropriate value, which is preferable.

[0023] Specific examples of alginic acid or alginic acid derivatives include those represented by the following general formula (1):

[0024] [ka]

[0025] (In the formula, m and n each independently represent an integer of 1 or greater; R1 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 -R3-OH; and R3 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.

[0026] 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.

[0027] Alginic acid is defined as the compound represented by formula (1) where all R1s are hydrogen atoms. Alginate salts are defined as compounds represented by formula (1) where at least some of the R1s are alkali metals (e.g., lithium, sodium, potassium), alkaline earth metals (e.g., magnesium, calcium), or quaternary ammonium (e.g., ammonium, tetramethylammonium). Alkynoic acid esters are defined as compounds represented by formula (1) where at least some of the R1s are -R3-OH (R3 is a divalent hydrocarbon group having 2 to 6 carbon atoms). Such alkynoic acid esters can be produced by reacting alginic acid with an epoxy compound.

[0028] Furthermore, 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.

[0029] The ratio of mannuronic acid to guluronic acid, which are the constituent components of alginic acid, can be any ratio, and 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.

[0030] 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 CaCl. The sulfate groups are introduced at hydroxyl group sites in the polysaccharide structure, where the hydrogen atoms of the hydroxyl groups are replaced by -SO3H.

[0031] The polysaccharide having a sulfate group and / or an alkali metal sulfate group is represented by the following general formula (2):

[0032] [ka]

[0033] (In the formula, m and n are each independently an integer of 1 or greater; R1 may be the same or different and each represents one selected from the group consisting of hydrogen, an alkali metal, an alkaline earth metal, a quaternary ammonium, and a group represented by -R3-O-R2; R3 represents a divalent hydrocarbon group having 2 to 6 carbon atoms; R2 may be the same or different and each represents hydrogen or -S03M (M is hydrogen or an alkali metal), and at least one R2 is -S03M.) Preferably, the compound has the structure shown below.

[0034] Examples of the divalent hydrocarbon group having 2 to 6 carbon atoms for R3 in formula (2) include those exemplified as the divalent hydrocarbon group having 2 to 6 carbon atoms for R3 in formula (1).

[0035] The amount of sulfate group and / or alkali metal sulfate base introduced into the polysaccharide is preferably 0.5 to 6.0 mmol / g. When the amount of sulfate group and / or alkali metal sulfate base introduced is within the above range, oxidation resistance is improved, and excellent affinity for the electrolyte is achieved while maintaining binding strength to the active material, which is preferable.

[0036] Since sulfate groups and alkali metal sulfate groups 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.

[0037] Furthermore, 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.

[0038] The method for producing a polysaccharide having sulfate groups and / or alkali metal sulfate salt groups is not particularly limited, and it can be obtained, for example, by reacting a polysaccharide with a sulfating reagent in an aprotic polar solvent. The reaction can be carried out in a slurry system or a homogeneous system.

[0039] The solvent used in the reaction is preferably an aprotic polar solvent, and specific examples thereof 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.

[0040] 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, alginate, alginate esters, agarose, carrageenan, and chondroitin sulfate. More preferred are alginic acid and alginate esters.

[0041] The sulfating reagent used is a reagent capable of reacting 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 dimethyl sulfoxide complex, and sulfur trioxide dioxane complex. Among these sulfating reagents, those that cause minimal reduction in the molecular weight of polysaccharides are preferably used 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. The amount of these sulfating reagents used is not particularly limited, but is preferably in the range of 1 to 5 times the molar amount of monosaccharides, which are the structural units of polysaccharides. Using a sulfating reagent in this range is preferred because it allows for a large amount of sulfate groups to be introduced, suppresses reduction in the molecular weight of the polysaccharide, and results in a high molecular weight product.

[0042] Other reaction conditions can be set arbitrarily, and it is preferable that the reaction temperature is selected from the range of 0°C to 100°C, the reaction time is selected from the range of 30 minutes to 12 hours, and the polysaccharide concentration during the reaction is selected from the range of 0.1 to 50% by mass.

[0043] 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. Examples of methods that can be used include a method in which the solvent is heated and removed to isolate the polysaccharide having sulfate groups and / or alkali metal sulfate bases, and a method in which the reaction solution is dropped into a poor solvent to precipitate the polysaccharide having sulfate groups and / or alkali metal sulfate bases, and then the precipitate is recovered by filtration.

[0044] Polysaccharides having sulfate groups and / or alkali metal sulfate salt groups are obtained in which the introduced sulfate groups are in the regenerated form (-SO3H) at the time of production. However, in the present invention, at least a portion of the regenerated groups may be ion-exchanged to the alkali metal salt form (-SO3M' (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 contacting the regenerated binder with a solution containing alkali metal ions, as in the case of ordinary ion exchangers.

[0045] Polysaccharides having functional groups other than sulfate groups introduced therein are also suitable for use in the present invention. Examples of functional groups other than sulfate groups include chelating groups such as iminodiacetic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, aminomethylphosphate, and polyethyleneimine groups, and epoxy groups such as glycidyl groups.

[0046] Although not essential, the binder composition for a lithium ion battery positive electrode of the present invention may contain, as other binder components, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polymaleic acid copolymer, polyfumaric acid, polyitaconic acid, or salts thereof.

[0047] Next, the sugar alcohol contained in the binder composition of the present invention will be described.

[0048] 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. 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 suppressing migration to the negative electrode and reductive decomposition at the negative electrode, thereby preventing 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.

[0049] 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 polysaccharide and sugar alcohol, and it is preferable that the polysaccharide content be 85 to 95% by mass and the sugar alcohol content be 5 to 15% by mass. When the polysaccharide and sugar alcohol contents are within these ranges, the polysaccharide is sufficiently plasticized, imparting flexibility, which is preferable.

[0050] The blending amount of the binder composition in the positive electrode mixture layer is preferably 1 to 20 mass %, more preferably 3 to 10 mass %, based on the total amount of the positive electrode active material, binder composition, and conductive additive.

[0051] 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, the resulting peeling and detachment of the active material, and destruction of the conductive channel. Furthermore, the binder composition of the present invention is insoluble in the electrolyte but 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. By coating the positive electrode active material, decomposition of the electrolyte or electrolyte by the positive electrode active material can be suppressed. These and other outstanding properties make the binder composition suitable for use in lithium-ion battery positive electrodes.

[0052] 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. For example, transition metal oxides such as CuO, Cu2O, MnO2, MoO3, V2O5, CrO3, Fe2O3, Ni2O3, and CoO3, and Li X CoO2, Li X NiO2, Li X MnO2, Li X Mn2O4, LiNi X Co (1-X) O2, LiNi X Mn (2-X) O4, LiMn a Ni b Co c Among these, a composite oxide of lithium and at least one transition metal selected from transition metals such as Co, Ni, and Mn is preferred, and specific examples include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and LiNi X Co (1-X) O2, LiNi X Mn (2-X) O4, LiMn a Ni b Co c O2 (a+b+c=1). These lithium composite oxides may be doped with a small amount of an element such as fluorine, boron, Al, Cr, Zr, Mo, or Fe, or the particle surfaces of the lithium composite oxide may be surface-treated with carbon, MgO, Al2O3, SiO2, or the like.

[0053] The conductive additive contained in the positive electrode is not particularly limited, and any electron-conductive material that does not adversely affect the battery characteristics can be used. Specific examples include conductive carbons 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 the conductive additive to be added is preferably 1 to 30% by mass relative to the positive electrode active material.

[0054] One embodiment of the present invention provides a slurry for forming a positive electrode composite layer for a lithium-ion battery, which comprises 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 modifier such as an acid or alkali. The solids concentration of the slurry is not particularly limited, but is preferably 20 to 80 mass % in consideration of the viscosity of the slurry, the dispersibility of the solids, and the load on the drying process. Furthermore, the mass ratio of the components in the slurry is preferably positive electrode active material:conductive additive:binder composition = 70 to 98:1 to 30:1 to 20. The method for producing the slurry is also not particularly limited. Examples include a method in which the binder composition, positive electrode active material, and conductive additive are mixed together 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 an 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 limitations on the mixer used to prepare the slurry, and a mortar, roll mill, ball mill, screw mill, vibration mill, homogenizer, planetary mixer, etc. can be used.

[0055] A positive electrode for a lithium ion battery according to one embodiment of the present invention contains the binder composition. Such a positive electrode for a lithium ion battery is composed of a positive electrode mixture 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 mixture layer, which is made of a positive electrode active material, a binder composition, and a conductive additive, is preferably 10 to 200 μm. To form a positive electrode mixture layer of this thickness on the positive electrode current collector, the basis weight of the positive electrode mixture layer should be 4 to 25 mg / cm. 2 It is recommended to set it to .

[0056] 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, and examples thereof 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 shapes such as foil, film, sheet, net, expanded metal, punched metal, and foam can be used. There are also no particular restrictions on the thickness of the positive electrode current collector, and it is preferably about 1 to 100 μm.

[0057] There are no particular limitations on the method for producing a positive electrode for a lithium ion battery, and the electrode can be produced by applying the above-mentioned slurry to a positive electrode current collector and drying it. There are also no particular limitations on the method for applying the slurry, and 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 limitations on the drying method and conditions, and a typical hot air circulation dryer, reduced pressure dryer, infrared dryer, or microwave heating dryer can be used. There are also no limitations on the heating temperature, and the electrode can be heated and dried at 50 to 150°C. Furthermore, the porous structure can be made uniform by pressing the positive electrode during or after drying.

[0058] One embodiment of the present invention relates to a lithium-ion battery characterized by having the above-described positive electrode for a lithium-ion battery. Using 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 other components. 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. 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, consisting of the following negative electrode active material and the above-described conductive additive bound to the negative electrode current collector using a binder. The separator is typically a porous film such as polyolefin, sandwiched between the positive and negative electrodes to provide a shutdown function in the event of thermal runaway. The nonaqueous electrolyte is formed by dissolving an electrolyte salt such as LiPF4 in an organic solvent such as a 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.

[0059] The negative electrode used in the lithium ion battery of the present invention is not particularly limited and can be made of known materials. The negative electrode is composed of a negative electrode mixture layer made of a negative electrode active material, a conductive additive, and a binder, and a current collector.

[0060] Any material that can insert and extract lithium ions can be used as the negative electrode active material, and examples thereof 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; metallic lithium and alloys; and the like.

[0061] As the conductive additive for the negative electrode, the same conductive additives as those exemplified for the positive electrode can be used.

[0062] As the binder for the negative electrode, a known binder may be used, and examples thereof include fluorine-based resins 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.

[0063] As the negative electrode current collector, the same current collector as the current collector exemplified for the positive electrode can be used.

[0064] There are no particular restrictions on the non-aqueous electrolyte, 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 can be selected from the range of 0.1 to 5 mol / L, preferably 0.5 to 3 mol / L.

[0065] There are no particular restrictions on the separator, and any known separator can be used. Examples of the separator include a polyethylene microporous membrane, a polypropylene microporous membrane, a laminated membrane of a polyethylene microporous membrane and a polypropylene microporous membrane, a nonwoven fabric made of polyester fiber, aramid fiber, glass fiber, or the like, and a separator in which the surface of the above-mentioned microporous membrane is coated with ceramics, aramid, or PVDF. [Example]

[0066] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0067] <Production of sulfated alginates> (Reference Example 1) Production of sulfated alginic acid To 150 ml of dimethyl sulfoxide (DMSO), 3.0 g (17 mmol as monosaccharide units) of H-alginic acid (Kimika Acid SA, manufactured by Kimika Corporation) was added in small portions with stirring and dispersed uniformly. Next, 6.75 g (42.5 mmol) of sulfur trioxide pyridine complex was added with stirring, and after uniform dispersion, the mixture was heated to 40°C and reacted for 5 hours. The alginic acid dissolved as the reaction proceeded, resulting in 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 on a glass filter, 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 groups introduced was calculated to be 3.2 mmol / g from the sulfur content.

[0068] Example 1 <Production of positive electrodes> 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 O4) 80 parts by mass Conductive additive: 10 parts by mass of acetylene black (AB: Li-400 manufactured by Denka Co., Ltd.) 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.

[0069] Sulfated alginic acid was neutralized with a 1 mol / L lithium hydroxide aqueous solution to prepare a lithium sulfated alginate aqueous solution. Next, xylitol was added to the lithium sulfated alginate aqueous solution and dissolved. LNMO and AB were mixed in a mortar, and the mixture was mixed with the lithium sulfated alginate / xylitol aqueous solution and then mixed again in a 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 and adhesion of the positive electrode composite layer of the obtained positive electrode were evaluated using the following method. 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.

[0070] <Positive electrode flexibility test> The positive electrode was punched into a disk 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 judged to have flexibility. If the positive electrode composite layer was broken and the underlying aluminum foil was exposed, it was judged to have no flexibility.

[0071] <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. If 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, if the aluminum foil was exposed and peeling or delamination of the positive electrode composite layer was observed, the adhesion was judged to be poor.

[0072] <Coin cell manufacturing> A lithium metal foil was used for the negative electrode, and lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (1 / 1) to a concentration of 1 mol / L to prepare an electrolyte. A polyolefin microporous film (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 cells to prepare a 2032-type coin cell.

[0073] 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 produced 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.

[0074] 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.

[0075] (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 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.

[0076] (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.

[0077] (Comparative Example 3) A positive electrode and 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.

[0078] <SEM observation of the positive electrode surface> The surfaces of the positive electrodes obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were observed by 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, and showed only slight cracks. 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, showed frequent cracks. This shows 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. The presence of only slight cracks indicates that the addition of an elastomer can also suppress cracking.

[0079] <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.

[0080] 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, indicating that the addition of sugar alcohols did not adversely affect the battery characteristics. 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, indicating that the addition of sugar alcohols did not adversely affect the battery characteristics. 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, led to oxidative decomposition of the SBR as the cycle count increased, resulting in a decrease in battery characteristics. [Industrial Applicability]

[0081] As described above, according to the present invention, it is possible to provide a binder composition for a lithium-ion battery positive electrode that allows for the production of a flexible positive electrode with excellent workability during the production of the positive electrode and battery, as well as a slurry for forming a lithium-ion battery positive electrode composite layer using the same, a positive electrode for a lithium-ion battery, and a lithium-ion battery. Furthermore, by using the same, it is possible to provide a positive electrode for a lithium-ion battery and a lithium-ion battery that have excellent charge-discharge characteristics and suppressed reduction in cycle life.

Claims

1. The binder composition for a lithium ion battery positive electrode is used to bind a positive electrode active material, a conductive additive, and a current collector together in a positive electrode of a lithium ion battery, and contains 80 to 99.5 mass % of a polysaccharide and 0.5 to 20 mass % of a sugar alcohol having 4 to 24 carbon atoms relative to the total amount of the polysaccharide and the sugar alcohol.

2. The binder composition of claim 1 , wherein the polysaccharide is alginic acid or an alginic acid derivative.

3. A slurry for forming a positive electrode mixture layer for a lithium ion battery, comprising a positive electrode active material, a conductive additive, the binder composition according to claim 1 or 2, and water.

4. A positive electrode for a lithium ion battery, comprising the binder composition according to claim 1 or 2.

5. A lithium ion battery comprising the positive electrode for a lithium ion battery according to claim 4.

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