Binder 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

Provided are: a binder for the positive electrodes of lithium-ion batteries which brings about excellent operation efficiency in positive-electrode production and makes it possible to produce positive electrodes that are excellent in terms of charge / discharge characteristics including cycling characteristics and rate characteristics and have a prolonged cycle life; and a slurry for forming the positive-electrode mix layer of a lithium-ion battery, the slurry including the binder, a positive electrode for lithium-ion batteries, and a lithium-ion battery. In the positive electrode of a lithium-ion battery, a binder including a polysaccharide containing, introduced thereinto, at least one ion-exchange group selected from the group consisting of a sulfuric acid group and alkali-metal sulfate groups is used for bonding a positive-electrode active material, an electroconduction aid, and a current collector. Thus, excellent charge / discharge characteristics and a prolonged cycle life can be imparted to the lithium-ion battery.

Description

[Technical Field]

[0001] The present invention relates to a binder 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 or 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 improve the working environment, but there has been a problem in that the adhesive strength with the active material can be reduced depending on the combination of the active material and binder.

[0006] The positive electrode active material is LiMn2O4, LiNi 0.5 Mn 1.5 O4, LiMn 0.2 Ni 0.6 Co 0.2 Lithium composite oxides such as O2 are often used, but it is known that manganese and other elements in the active material of these lithium composite oxides dissolve and ionize during charging and discharging, depositing and accumulating on the negative electrode, resulting in a decrease in cycle characteristics and rate performance. [Prior art documents] [Patent documents]

[0007] [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]

[0008] The present invention aims to provide a binder for lithium-ion battery positive electrodes that allows for the production of positive electrodes that are easy to work with when producing the positive electrodes and have excellent charge / discharge characteristics such as cycle characteristics and rate characteristics, and that have an extended cycle life; and to provide a slurry for forming a lithium-ion battery positive electrode composite layer that uses the binder, a lithium-ion battery positive electrode, and a lithium-ion battery that uses the binder. [Means for solving the problem]

[0009] As a result of intensive investigations to solve the above-mentioned problems, the present inventors have found that the above-mentioned object can be achieved by using, as a positive electrode binder, a polysaccharide into which at least one ion exchange group selected from the group consisting of a sulfate group and an alkali metal sulfate group has been introduced, and have thus completed the present invention.

[0010] The embodiments of the present invention are [1] to [8] shown below.

[0011] [1] A binder that binds a positive electrode active material, a conductive additive, and a current collector in a positive electrode of a lithium ion battery, a polysaccharide having at least one carboxy group selected from the group consisting of carboxymethylcellulose, gellan gum, xanthan gum, gum arabic, alginic acid, alginate salts, alginate esters, and chondroitin sulfate; At least one ion exchange group selected from the group consisting of sulfate groups and alkali metal sulfate groups was introduced Contains polysaccharides, The polysaccharide having a carboxy group The ion exchange group was introduced A binder for a lithium-ion battery positive electrode, wherein the weight-average molecular weight of the polysaccharide is 10,000 to 1,000,000, and the amount of the ion-exchange group introduced into the polysaccharide is 1.0 to 6.0 mmol / g.

[0014] [ 2 ] The polysaccharide having a carboxy group The ion exchange group was introduced The polysaccharide is at least one selected from the group consisting of sulfated alginic acid, sulfated alkali metal alginate, sulfated alginic acid ester, sulfated alkali metal alginate, sulfated carboxymethylcellulose, and sulfated alkali metal carboxymethylcellulose. [1] The binder according to claim 1.

[0015] [ 3 ] The polysaccharide having a carboxy group The ion exchange group was introduced The polysaccharide has the following general formula (1):

[0016] [ka]

[0017] (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 a group selected from the group consisting of hydrogen, an alkali metal, an alkaline earth metal, 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 a group selected from the group consisting of hydrogen and a group represented by -S03M (wherein M represents a group selected from the group consisting of hydrogen and an alkali metal), and at least one R2 is a group represented by -S03M.) The above having a structure represented by [1] or [2] The binder according to claim 1.

[0018] [ 4 ] Positive electrode active material, conductive additive, the above [1] to [ 3 10. A slurry for forming a positive electrode mixture layer for a lithium ion battery, comprising the binder according to any one of claims 1 to 9 and water.

[0019] [ 4 ]Above [1]~[ 3 10. A positive electrode for a lithium ion battery, comprising the binder according to any one of claims 1 to 9.

[0020] [ 7 ]the above[ 6 A lithium ion battery having the positive electrode for a lithium ion battery according to any one of the preceding claims.

[0021] The present inventors speculate as follows as to why the above object can be achieved by the binder for a lithium ion battery positive electrode of the present invention.

[0022] That is, in the present invention, by using a polysaccharide into which at least one ion exchange group selected from the group consisting of a sulfate group and an alkali metal sulfate group has been introduced as a positive electrode binder, the following effects are exhibited: improved adhesion between the positive electrode active material or conductive additive and the current collector; improved affinity between the binder and the electrolyte; and the ion exchange group captures manganese ions and the like eluted from the positive electrode active material near the positive electrode, thereby suppressing their deposition and accumulation on the negative electrode, thereby achieving excellent charge-discharge characteristics and a long cycle life.

[0023] Furthermore, the polysaccharides having ion-exchange groups used in the positive electrode binder of the present invention not only have excellent adhesion to the positive electrode active material and current collector, but also have high strength, toughness, and flexibility, allowing them to adapt to volume changes in the positive electrode during charge and discharge, preventing cracking of the positive electrode, peeling and detachment of the active material, and destruction of the conductive channel due to volume changes. Therefore, using them as binders for lithium-ion battery positive electrodes provides lithium-ion batteries with excellent charge-discharge characteristics and extended cycle life. Furthermore, introducing the ion-exchange groups into the polysaccharides improves their affinity with the electrolyte, thereby achieving high performance in lithium-ion batteries.

[0024] Furthermore, because sulfate groups and alkali metal sulfate bases are strongly acidic cation exchange groups, they are highly effective at capturing transition metal ions such as manganese, making it possible to capture manganese ions eluted from the positive electrode active material near the positive electrode, thereby suppressing their deposition and accumulation on the negative electrode. As a result, excellent charge-discharge characteristics and a long cycle life can be achieved. [Effects of the Invention]

[0025] According to the present invention, it is possible to provide a binder for a lithium ion battery positive electrode and a slurry for forming a lithium ion battery positive electrode composite layer, which enable the production of a positive electrode that is excellent in workability during the production of the positive electrode and has excellent charge / discharge characteristics such as cycle characteristics and rate characteristics, and has an extended cycle life. Furthermore, by using the binder and a slurry, it is possible to provide a lithium ion battery positive electrode and a lithium ion battery that have excellent charge / discharge characteristics and an extended cycle life. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a graph showing the charge / discharge curves of the coin cell obtained in Example 1. [Figure 2] 1 is a graph showing the charge / discharge curves of the coin cell obtained in Example 2. [Figure 3] 1 is a graph showing the charge / discharge curves of the coin cell obtained in Comparative Example 1. [Figure 4] 10 is a graph showing the charge / discharge curves of the coin cell obtained in Comparative Example 2. [Figure 5] 10 is a graph showing the charge / discharge curves of the coin cell obtained in Comparative Example 3. [Figure 6] 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 1, Comparative Example 1, and Comparative Example 2. [Figure 7] 1 is a graph comparing the rate characteristics of the coin cells obtained in Example 1, Comparative Example 1, and Comparative Example 2. [Figure 8] 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 2, Comparative Example 1, and Comparative Example 3. [Figure 9] 1 is a graph comparing the rate characteristics of the coin cells obtained in Example 2, Comparative Example 1, and Comparative Example 3. [Figure 10] 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 3 and Comparative Example 4. [Figure 11] 1 is a graph comparing the charge-discharge cycle characteristics of the coin cells obtained in Example 4 and Comparative Example 5. [Figure 12] These are SEM images of the positive electrode surfaces of Comparative Example 1 (a-1 and a-2), Comparative Example 2 (b-1 and b-2), and Example 1 (c-1 and c-2) after 50 cycles in a cycle characteristic test, where a-1, b-1, and c-1 are at a magnification of 500x, and a-2, b-2, and c-2 are at a magnification of 1500x. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in detail below based on preferred embodiments thereof.

[0028] A binder for a lithium ion battery positive electrode according to one embodiment of the present invention contains a polysaccharide having at least one ion exchange group selected from the group consisting of a sulfate group and an alkali metal sulfate group.

[0029] The term "polysaccharide" refers to a polysaccharide having a structure in which a large number of monosaccharides are linked together through glycosidic bonds. The polysaccharides having the ion exchange groups (sulfate groups and / or alkali metal sulfate salt groups) contained in the binder of the present invention include polysaccharides in which at least a portion of the hydrogen atoms of the hydroxyl groups contained in the polysaccharide structure have been substituted with -SO3H (sulfate groups), and further include polysaccharides in which the protons of the sulfate groups have been ion-exchanged with alkali metal ions (e.g., lithium ions, sodium ions, potassium ions).

[0030] Examples of polysaccharides 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, alginates, alginate esters, heparin, hyaluronic acid, chondroitin sulfate, xyloglucan, glucomannan, etc. Among these, neutral polysaccharides and acidic polysaccharides are preferably used in the present invention. Specific examples include amylose, amylopectin, carboxymethylcellulose, guar gum, gellan gum, xanthan gum, gum arabic, alginic acid, alginates, alginic acid esters, agarose, carrageenan, and chondroitin sulfate, and among these, alginic acid, alginates, alginic acid esters, carboxymethylcellulose, and guar gum are preferred. More preferred are those represented by the following general formula (2):

[0031] [ka]

[0032] (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, an alkali metal, an alkaline earth metal, and a group 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.

[0033] 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.

[0034] Alginic acid is when all R1 in formula (2) are hydrogen; alginate is when at least some of R1 in formula (2) are alkali metals (e.g., lithium, sodium, potassium) or alkaline earth metals (e.g., magnesium, calcium); and alkynoic acid ester is when at least some of R1 in formula (2) are groups represented by -R3-OH (R3 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.

[0035] 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.

[0036] The sulfate groups introduced into the polysaccharides 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 the hydroxyl group sites in the polysaccharide structure, and the hydrogen atoms of the hydroxyl groups are replaced by -SOH.

[0037] The polysaccharide having an ion exchange group introduced therein and contained in the binder of the present invention is preferably at least one polysaccharide selected from the group consisting of amylose, amylopectin, carboxymethylcellulose, guar gum, gellan gum, xanthan gum, gum arabic, alginic acid, alginates, alginate esters, agarose, carrageenan, and chondroitin sulfate, into which the ion exchange group (sulfate group and / or alkali metal sulfate group) has been introduced. Among these, at least one polysaccharide selected from the group consisting of sulfated alginic acid, sulfated alkali metal alginate, sulfated alginate, sulfated alkali metal alginate, sulfated carboxymethylcellulose, sulfated alkali metal carboxymethylcellulose, sulfated guar gum, and sulfated alkali metal guar gum is more preferred.

[0038] As the polysaccharide having an ion exchange group, one having a structure represented by the following general formula (1) is particularly preferred.

[0039] [ka]

[0040] (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 a group selected from the group consisting of hydrogen, an alkali metal, an alkaline earth metal, 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 a group selected from the group consisting of hydrogen and a group represented by -S03M (wherein M represents a group selected from the group consisting of hydrogen and an alkali metal), and at least one R2 is a group represented by -S03M.) The amount of the ion-exchange groups (sulfate groups and / or alkali metal sulfate groups) introduced into the polysaccharide is preferably 1.0 to 6.0 mmol / g. When the amount of the ion-exchange groups introduced is within this range, manganese ions eluted from the positive electrode active material can be efficiently captured near the positive electrode, and further, the binding strength to the active material is high and the affinity for the electrolyte is excellent, which is preferable.

[0041] 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.

[0042] The molecular weight of the polysaccharide having an ion exchange group is preferably a weight-average molecular weight of 10,000 to 1,000,000. 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.

[0043] 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 polysaccharide represented by the general formula (1) is 10,000 to 1,000,000, and m and n are preferably each independently 30 to 3,000.

[0044] The method for producing the polysaccharide having ion exchange groups is not particularly limited, and it can be obtained, for example, by reacting a polysaccharide with a sulfation reagent in an aprotic polar solvent. The reaction can be carried out in a slurry system or a homogeneous system.

[0045] 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.

[0046] The polysaccharides used are preferably polysaccharides that have 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 examples of the polysaccharides to be used include alginic acid, alginates, alginic acid esters, carboxymethylcellulose, and guar gum, and particularly preferred examples include alginic acid, alginates, and alginic acid esters.

[0047] 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 dimethylsulfoxide 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.

[0048] 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.

[0049] There are no particular limitations on the method for isolating the polysaccharide having ion exchange groups from the reaction solution after completion of the reaction. Examples of methods that can be used include a method in which the solvent is removed by heating to isolate the polysaccharide having ion exchange groups, and a method in which the reaction solution is dropped into a poor solvent to precipitate the polysaccharide having ion exchange groups, followed by filtration and recovery.

[0050] The polysaccharides having ion exchange 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 an 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.

[0051] Although not essential, the binder for a lithium ion battery positive electrode of the present invention may contain, as other binder components, in addition to the polysaccharide having an ion exchange group, polysaccharides not having a sulfate group, such as alginic acid and carboxymethyl cellulose, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polymaleic acid copolymer, polyfumaric acid, polyitaconic acid and salts thereof, styrene-butadiene copolymer, and the like.

[0052] The binder for a lithium-ion battery positive electrode of the present invention may consist solely of the polysaccharide having an ion-exchange group. However, when other binder components are contained, the content of the polysaccharide having an ion-exchange group is preferably 10 mass % or more.

[0053] The blending amount of the binder is preferably 1 to 20 mass %, more preferably 3 to 10 mass %, based on the total amount of the positive electrode active material, binder, and conductive additive.

[0054] The binder of the present invention is highly flexible and can 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 of the present invention has excellent ionic conductivity due to its excellent affinity with the electrolyte, and excellent oxidation resistance, ensuring the stability of the positive electrode, making it an outstanding binder for lithium-ion battery positive electrodes.

[0055] Furthermore, in the binder of the present invention, at least one functional group selected from the group consisting of a sulfate group and an alkali metal sulfate group is introduced. Since these functional groups are strongly acidic cation exchange groups, they are highly effective in capturing transition metal ions such as manganese, and can capture manganese ions eluted from the positive electrode active material in the vicinity of the positive electrode, thereby suppressing their deposition and accumulation on the negative electrode, thereby achieving excellent charge-discharge characteristics and a long cycle life.

[0056] 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 XCo (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.

[0057] 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.

[0058] The slurry for forming a positive electrode composite layer of a lithium-ion battery according to one embodiment of the present invention contains the binder, a positive electrode active material, a conductive additive, and water. The slurry may contain a viscosity modifier such as carboxymethyl cellulose or a pH modifier such as an acid or alkali, as needed. 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 solids ratio in the slurry is preferably 70 to 98:1 to 30:1 to 20 by mass (positive electrode active material:conductive additive:binder). The method for producing the slurry is also not particularly limited. Examples include a method in which the binder, positive electrode active material, and conductive additive are mixed, dispersed, and dissolved in water together to produce a slurry; a method in which the binder is first dissolved in water, and then the positive electrode active material and conductive additive are added to the aqueous binder 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 solution. There are also no particular restrictions on the mixer used to prepare the slurry, and a mortar, roll mill, ball mill, screw mill, vibration mill, homogenizer, planetary mixer, etc. may be used.

[0059] A positive electrode for a lithium ion battery according to one embodiment of the present invention contains the binder. Such a positive electrode for a lithium ion battery is preferably 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 the positive electrode active material, binder, and conductive additive, is preferably 10 to 200 μm. In order 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 .

[0060] 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.

[0061] 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.

[0062] A lithium-ion battery according to one embodiment of the present invention includes the above-described lithium-ion battery positive electrode. Using the above-described lithium-ion battery positive electrode allows for the provision of 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 comprises the above-described positive electrode active material and the above-described conductive additive bound to a positive electrode current collector with a binder, and a positive electrode composite layer comprising the positive electrode active material, binder, and conductive additive is formed on the current collector. The negative electrode has a similar structure to the positive electrode, comprising the following negative electrode active material and the above-described conductive additive bound to a negative electrode current collector with 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 comprises an electrolyte salt such as LiPF4 dissolved 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.

[0063] 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.

[0064] 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.

[0065] As the conductive additive for the negative electrode, the same conductive additives as those exemplified for the positive electrode can be used.

[0066] 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.

[0067] As the negative electrode current collector, the same current collector as the current collector exemplified for the positive electrode can be used.

[0068] 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.

[0069] 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, and a nonwoven fabric made of polyester fiber, aramid fiber, glass fiber, or the like. [Example]

[0070] Example 1 The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0071] <Production of sulfated polysaccharides> (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.

[0072] (Reference Example 2) Production of sulfated alginate A sulfated alginate was produced in the same manner as in Reference Example 1, except that 3.0 g (13 mmol as monosaccharide units) of an alginate ester (manufactured by Kimica Co., Ltd., trade name Kimiloid HV) was used instead of H-form alginic acid, and 5.25 g (33 mmol) of sulfur trioxide-pyridine complex was added. The yield of the obtained sulfated alginate was 4.3 g, the sulfur content determined by elemental analysis was 9.2 mass%, and the weight-average molecular weight measured by GPC was 310,000. The amount of sulfate groups introduced calculated from the sulfur content was 2.9 mmol / g.

[0073] (Reference Example 3) Production of sulfated carboxymethyl cellulose To 150 ml of dimethyl sulfoxide (DMSO), 3.0 g (13 mmol as monosaccharide units) of carboxymethylcellulose (Daicel Miraize Co., Ltd., product number 1220, degree of etherification 0.8-1.0) was added in small portions with stirring and dispersed uniformly. Next, 6.0 g (37.8 mmol) of sulfur trioxide pyridine complex was added with stirring and dispersed uniformly. The temperature was then raised to 40°C for 3 hours. The carboxymethylcellulose 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 4.7 g. The sulfur content determined by elemental analysis was 12.1% by mass, and the weight-average molecular weight measured by gel permeation chromatography (GPC) was 50,000. The amount of sulfate groups introduced was calculated to be 3.8 mmol / g from the sulfur content.

[0074] (Reference Example 4) Production of sulfated guar gum Sulfated guar gum was produced in the same manner as in Reference Example 3, except that 3.0 g (6 mmol as monosaccharide units) of guar gum (manufactured by Somar Co., Ltd., trade name PROCOL S-1) was used instead of carboxymethyl cellulose and the reaction time was 5 hours. The yield of the obtained sulfated guar gum was 3.9 g, the sulfur content determined by elemental analysis was 6.6 mass%, and the weight-average molecular weight measured by GPC was 450,000. The amount of sulfate groups introduced calculated from the sulfur content was 2.1 mmol / g.

[0075] <Coin cell manufacturing> Example 1 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: sulfated alginic acid (Reference Example 1) 10 parts by mass.

[0076] Sulfated alginic acid was neutralized with a 1 mol / L lithium hydroxide solution to prepare a lithium sulfated alginate aqueous solution. LNMO and AB were mixed in a mortar, and the mixture was mixed with the lithium sulfated alginate aqueous solution and then 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.

[0077] Next, a coin cell was fabricated using the obtained positive electrode as follows. That is, a lithium metal foil was used for the negative electrode, and lithium hexafluorophosphate was dissolved in an ethylene carbonate / dimethyl carbonate (1 / 1) mixed solvent to a concentration of 1 mol / L to prepare an electrolyte solution. A polyolefin microporous film (manufactured by Toray Industries, Inc., thickness 20 μm) was used as the separator. The positive electrode and negative electrode were then placed on either side of the separator, stacked, and the electrolyte was poured into the cells to fabricate a 2032-type coin cell.

[0078] Example 2 A coin cell was prepared in the same manner as in Example 1, except that the sulfated alginate obtained in Reference Example 2 was used as the binder.

[0079] Example 3 A coin cell was produced in the same manner as in Example 1, except that the sulfated carboxymethyl cellulose obtained in Reference Example 3 was used as the binder and the composition of the positive electrode mixture was changed to 87 parts by mass of lithium nickel manganese composite oxide (LNMO), 10 parts by mass of conductive additive (AB), and 3 parts by mass of binder (sulfated carboxymethyl cellulose (Reference Example 3)).

[0080] Example 4 A coin cell was produced in the same manner as in Example 1, except that the sulfated guar gum obtained in Reference Example 4 was used as the binder.

[0081] (Comparative Example 1) A coin cell was produced in the same manner as in Example 1, except that PVDF (manufactured by Polysciences, catalog No. 18734-100) was used as the binder and N-methyl-2-pyrrolidone was used as the solvent to prepare the positive electrode mixture slurry.

[0082] (Comparative Example 2) A coin cell was produced in the same manner as in Example 1, except that H-type alginic acid (manufactured by Kimica Corporation, trade name Kimica Acid SA) was used as the binder.

[0083] (Comparative Example 3) A coin cell was produced in the same manner as in Example 1, except that an alginate ester (manufactured by Kimica Corporation, trade name: Kimiloid HV) was used as the binder.

[0084] Comparative Example 4 A coin cell was produced in the same manner as in Example 1, except that carboxymethyl cellulose (manufactured by Daicel Miraize Co., Ltd., product number 1220, degree of etherification 0.8 to 1.0) was used as the binder and the composition of the positive electrode mixture was changed to 87 parts by mass of lithium nickel manganese composite oxide (LNMO), 10 parts by mass of conductive additive (AB), and 3 parts by mass of binder (carboxymethyl cellulose).

[0085] (Comparative Example 5) A coin cell was produced in the same manner as in Example 1, except that guar gum (manufactured by Somar Co., Ltd., trade name PROCOL S-1) was used as the binder.

[0086] <Charge / discharge characteristic evaluation> The coin cells obtained in Examples 1 and 2 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.

[0087] Under the above conditions, 50 charge-discharge cycles were performed. The results are shown in Figures 1 to 5. The numbers in the figures indicate the number of cycles. Figures 1 and 2 show the charge-discharge curves of the coin cells obtained in Examples 1 and 2, respectively. It can be seen that the decrease in discharge capacity was slight even after 50 cycles, and the relationship between discharge capacity and electrode voltage at each cycle was stable. In contrast, Figure 3 shows the charge-discharge curve of the coin cell obtained in Comparative Example 1. It can be seen that the discharge capacity decreased with increasing cycle count. Figure 4 shows the charge-discharge curve of the coin cell obtained in Comparative Example 2. It can be seen that the discharge capacity decreased after 30 cycles or more. Furthermore, Figure 5 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 compared to Figure 1 (Example 1). From these results, it can be seen that the system using lithium sulfated alginate according to the present invention as the binder (Figure 1, Example 1) was able to charge and discharge more stably than the systems using PVDF or lithium alginate as the binder. Furthermore, a system using the sulfated alginate ester (lithium sulfated alginate ester) of the present invention as a binder (Figure 2, Example 2) also enabled more stable charging and discharging than systems using PVDF or alginate ester (lithium alginate ester) as a binder.

[0088] 6 is a graph showing the charge-discharge cycle characteristics of the coin cells obtained in Example 1, Comparative Example 1, and Comparative Example 2, and shows the change in discharge capacity over time as the number of cycles increases. The results show that the system of Example 1, which used lithium sulfated alginate according to the present invention as the binder, showed no decrease in discharge capacity even with an increase in the number of cycles, allowing stable charge and discharge, whereas the systems of Comparative Examples 1 and 2, which used PVDF or lithium alginate as the binder, showed a decrease in discharge capacity as the number of cycles increased.

[0089] Next, to evaluate rate characteristics, the charge and discharge rates were set to C / 10, C / 5, C / 2, 1C, and 2C, and the discharge capacities of the coin cells obtained in Example 1, Comparative Example 1, and Comparative Example 2 were measured. The results are shown in Figure 7. In Example 1, which used lithium alginate sulfate according to the present invention as the binder, the discharge capacity decreased little even when the rate was increased. In Comparative Example 1, which used PVDF as the binder, the discharge capacity decreased significantly as the rate increased. Furthermore, in Comparative Example 2, which used lithium alginate as the binder, a decrease in discharge capacity was observed, although not as severe as in Comparative Example 1.

[0090] 8 is a graph showing the charge-discharge cycle characteristics of the coin cells obtained in Example 2, Comparative Example 1, and Comparative Example 3, showing the change in discharge capacity over the number of cycles. The results show that the system of Example 2, which used the sulfated alginate ester (lithium sulfated alginate ester) according to the present invention as the binder, showed only a slight decrease in discharge capacity even with an increase in the number of cycles, allowing for stable charge and discharge, whereas the systems of Comparative Examples 1 and 2, which used PVDF or alginate ester (lithium alginate ester) as the binder, showed a decrease in discharge capacity as the number of cycles increased.

[0091] Next, to evaluate rate characteristics, the charge and discharge rates were set to C / 10, C / 5, C / 2, 1C, and 2C, and the discharge capacities of the coin cells obtained in Example 2, Comparative Example 1, and Comparative Example 3 were measured. The results are shown in FIG. 9. In the system of Example 2, which used the sulfated alginate ester (lithium sulfated alginate ester) according to the present invention as the binder, there was little decrease in discharge capacity even when the rate was increased. In contrast, in the system of Comparative Example 1, which used PVDF as the binder, there was a significant decrease in discharge capacity as the rate increased. Furthermore, in the system of Comparative Example 3, which used alginate ester (lithium alginate ester) as the binder, there was also a decrease in discharge capacity, although not as severe as in Comparative Example 1.

[0092] Next, the coin cells obtained in Examples 3 and 4 and Comparative Examples 4 and 5 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.

[0093] 10 shows the charge-discharge cycle characteristics of the coin cells obtained in Example 3 and Comparative Example 4. The results show that the system of Example 3, which used the sulfated lithium carboxymethyl cellulose according to the present invention as the binder, did not experience a decrease in discharge capacity even with an increase in the number of cycles, allowing for stable charge-discharge, whereas the system of Comparative Example 4, which used lithium carboxymethyl cellulose without sulfate groups in the binder, showed a significant decrease in discharge capacity as the number of cycles increased.

[0094] 11 shows the charge-discharge cycle characteristics of the coin cells obtained in Example 4 and Comparative Example 5. The results show that the system of Example 4, which used the sulfated lithium guar gum according to the present invention as the binder, did not experience a decrease in discharge capacity even with an increase in the number of cycles, allowing for stable charge-discharge, whereas the system of Comparative Example 5, which used lithium guar gum without sulfate groups introduced into the binder, had a lower discharge capacity than the system of Example 4, and the discharge capacity slightly decreased as the number of cycles increased.

[0095] <Adhesion evaluation> In the charge-discharge characteristic evaluation, the appearance of the positive electrodes in Example 1, Comparative Example 1, and Comparative Example 2 after a 50-cycle charge-discharge test was observed using a scanning electron microscope (SEM). The results are shown in FIG. 12. Large cracks occurred on the surface of the positive electrode composite layer of the positive electrode in Comparative Example 1 (a-1 and a-2), while many small cracks occurred on the surface of the positive electrode composite layer of the positive electrode in Comparative Example 2 (b-1 and b-2). In contrast, few cracks occurred on the surface of the positive electrode composite layer of the positive electrode in Example 1 (c-1 and c-2). This confirmed that the sulfated lithium alginate according to the present invention used as the binder in Example 1 has higher adhesion to the positive electrode active material, conductive additive, and current collector than the PVDF and unmodified lithium alginate used in the comparative examples. [Industrial Applicability]

[0096] As described above, according to the present invention, it is possible to provide a binder for a lithium-ion battery positive electrode and a slurry for forming a lithium-ion battery positive electrode composite layer, which enable the production of a positive electrode that is excellent in workability during the production of the positive electrode and has excellent charge-discharge characteristics such as cycle characteristics and rate characteristics, and has an extended cycle life. Furthermore, by using the binder and a slurry for forming a lithium-ion battery positive electrode, which has excellent charge-discharge characteristics and an extended cycle life, it is possible to provide a lithium-ion battery positive electrode and a lithium-ion battery.

Claims

1. A binder for binding a positive electrode active material, a conductive additive, and a current collector in a positive electrode of a lithium ion battery, the binder comprising a polysaccharide having at least one carboxy group selected from the group consisting of carboxymethyl cellulose, gellan gum, xanthan gum, gum arabic, alginic acid, alginate salts, alginate esters, and chondroitin sulfate, to which at least one ion exchange group selected from the group consisting of a sulfate group and an alkali metal sulfate salt has been introduced; a weight-average molecular weight of the polysaccharide obtained by introducing the ion exchange group into the polysaccharide having a carboxy group of 10,000 to 1,000,000, and an amount of the ion exchange group introduced into the polysaccharide of 1.0 to 6.0 mmol / g.

2. A binder as described in claim 1, wherein the polysaccharide having the carboxy group into which the ion exchange group has been introduced is at least one selected from the group consisting of sulfated alginic acid, sulfated alginic acid alkali metal salt, sulfated alginic acid ester, sulfated alginic acid ester alkali metal salt, sulfated carboxymethylcellulose, and sulfated carboxymethylcellulose alkali metal salt.

3. A binder according to claim 1 or 2, wherein the polysaccharide having the carboxyl group to which the ion exchange group has been introduced has a structure represented by the following general formula (1): 【Chemical 1】 (In the formula, m and n each independently represent an integer of 1 or more, and R １ may be the same or different and each represents hydrogen, an alkali metal, an alkaline earth metal, and -R ３ -O-R ２ R represents one selected from the group consisting of groups represented by ３ represents a divalent hydrocarbon group having 2 to 6 carbon atoms, and R ２ may be the same or different and each represents hydrogen and —SO ３ M (M represents one selected from the group consisting of hydrogen and alkali metals), R ２ At least one of is -SO ３ M is a group represented by

4. 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 according to any one of claims 1 to 3, and water.

5. A positive electrode for a lithium ion battery, comprising the binder according to any one of claims 1 to 3.

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

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