Positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

The present invention provides a positive electrode for a lithium-ion secondary battery which can suppress the generation of cracks and wrinkles in a positive electrode active material layer. This positive electrode for a lithium-ion secondary battery is characterized by comprising a current collector and a positive electrode active material layer provided on the current collector, wherein: the positive electrode active material layer contains lithium iron phosphate particles, a latex-derived aqueous polymer binder, and carboxymethyl cellulose sodium or carboxymethyl cellulose calcium; the aqueous polymer binder and the carboxymethyl cellulose sodium or carboxymethyl cellulose calcium form a binder composite; and the elastic modulus of the binder composite is in the range of 30-100 MPa.

Description

Positive electrode for lithium ion secondary battery and lithium ion secondary battery

[0001] The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery.

[0002] The positive electrode active material layer of a lithium-ion battery is typically produced by applying a slurry containing positive electrode active material particles, a PVdF-based binder, N-methylpyrrolidone, and a conductive additive to a current collector and drying the resulting coating. However, N-methylpyrrolidone is included in the list of restricted substances under the REACH Regulations due to concerns about its toxicity. A known method for producing a positive electrode active material layer without using N-methylpyrrolidone involves applying a slurry containing positive electrode active material particles, a water-based binder, a conductive additive, and water to a current collector and drying the resulting coating (see, for example, Patent Document 1).

[0003] JP 2017-091789 A

[0004] However, when a conventional cathode active material slurry using water as a dispersion medium is thickly applied to a current collector and the applied layer is dried, cracks and wrinkles occur in the cathode active material layer (e.g., a cathode active material layer having a thickness of 150 μm or more). When cracks occur in the cathode active material layer, the cathode active material layer is easily peeled off from the current collector, which is disadvantageous for battery fabrication. Furthermore, when wrinkles occur in the cathode active material layer, the cathode is distorted, which is disadvantageous for battery fabrication. The present invention has been made in view of these circumstances and provides a cathode for a lithium-ion secondary battery that can suppress the occurrence of cracks and wrinkles in the cathode active material layer.

[0005] The present invention provides a positive electrode for a lithium-ion secondary battery, comprising: a current collector; and a positive electrode active material layer provided on the current collector, the positive electrode active material layer containing lithium iron phosphate particles, a latex-derived aqueous polymer binder, and CMC (sodium carboxymethyl cellulose or calcium carboxymethyl cellulose), the aqueous polymer binder and the CMC forming a binder complex, the binder complex having a modulus of elasticity in the range of 30 to 100 MPa. The present invention also provides a lithium-ion secondary battery comprising the positive electrode of the present invention, a negative electrode, a separator, and a non-aqueous electrolyte.

[0006] According to the present invention, it is possible to suppress the occurrence of cracks and wrinkles in the positive electrode active material layer, and to suppress the positive electrode active material layer from peeling off from the current collector. According to the present invention, it is possible to increase the thickness of the positive electrode active material layer, and therefore the amount of positive electrode active material contained in the lithium ion secondary battery can be increased, and the battery capacity can be increased. A lithium ion secondary battery including the positive electrode of the present invention can have a large battery capacity. This has been demonstrated by experiments conducted by the present inventors.

[0007] 1A is a schematic plan view of a positive electrode for a lithium ion secondary battery according to one embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of the positive electrode taken along dashed line A-A in FIG. 1A. 2A is a schematic cross-sectional view of a lithium ion secondary battery according to one embodiment of the present invention. 3A is a schematic structural diagram of an electrode laminate included in a lithium ion secondary battery according to one embodiment of the present invention.

[0008] An embodiment of the present invention will be described below with reference to the drawings. The configurations shown in the drawings and the following description are merely illustrative, and the scope of the present invention is not limited to those shown in the drawings or the following description. A positive electrode 5 for a lithium-ion secondary battery according to this embodiment includes a current collector 3 and a positive electrode active material layer 2 provided on the current collector 3. The positive electrode active material layer 2 contains lithium iron phosphate particles, a latex-derived aqueous polymer binder, and CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose). The aqueous polymer binder and the CMC form a binder complex, and the binder complex has an elastic modulus of 30 to 100 MPa. A lithium-ion secondary battery 30 according to this embodiment includes the positive electrode 5 according to this embodiment, a negative electrode 32, a separator 34, and a non-aqueous electrolyte 15.

[0009] The positive electrode 5 for a lithium ion secondary battery is a positive electrode 5 included in a lithium ion secondary battery 30 or a positive electrode 5 used in the production of a lithium ion secondary battery 30. The positive electrode 5 for a lithium ion secondary battery includes a positive electrode current collector 3 and a porous positive electrode active material layer 2 provided on the positive electrode current collector 3. The positive electrode current collector 3 is a sheet that serves as a substrate for providing the positive electrode active material layer 2, and is a conductor that electrically connects a positive electrode connection member 13 and the positive electrode active material layer 2. The positive electrode current collector 3 is, for example, aluminum foil. The positive electrode connection member 13 is electrically connected to an external connection terminal 18a. The positive electrode active material layer 2 may be provided on one side of the positive electrode current collector 3, or may be provided on both sides of the positive electrode current collector 3.

[0010] The positive electrode active material layer 2 is a layer containing a positive electrode active material. The thickness of the positive electrode active material layer 2 (after pressing) may be 150 μm or more. This increases the amount of positive electrode active material contained in the positive electrode 5, and the battery capacity of the lithium-ion secondary battery 30 can be increased.

[0011] The positive electrode active material layer 2 includes lithium iron phosphate particles as a positive electrode active material, a latex-derived aqueous polymer binder, and CMC. The positive electrode active material layer 2 may also include a conductive additive. The aqueous polymer binder and CMC form a binder composite. The lithium iron phosphate particles are particles of lithium iron phosphate (LiFePO4), a positive electrode active material. The lithium iron phosphate particles may have a conductive coating on their surfaces. This can improve the conductivity of the particle surfaces where the intercalation reaction proceeds, thereby reducing the internal resistance of the positive electrode 5. The conductive coating is, for example, a carbon coating. The lithium iron phosphate particles may be contained in the positive electrode active material layer 2 as secondary particles. The average particle diameter of the lithium iron phosphate secondary particles can be 10 μm or more and 60 μm or less. The average particle diameter can be calculated by measuring the particle diameters of 100 randomly selected secondary particles from a cross-sectional photograph of the positive electrode active material layer 2 and averaging these particle diameters.

[0012] Examples of the conductive additive include furnace black, acetylene black, fine particles of hard carbon, soft carbon, etc. When the positive electrode active material layer 2 contains the conductive additive, the conductivity of the positive electrode active material layer 2 can be improved, and the internal resistance of the positive electrode 5 can be reduced.

[0013] The latex-derived aqueous polymer binder is a polymer binder that can be colloidally dispersed in an aqueous dispersion medium. The aqueous polymer binder forms a binder complex together with CMC. Examples of the aqueous polymer binder include an acrylic polymer binder and styrene-butadiene rubber (SBR). The use of such an aqueous polymer binder makes it possible to use water or an aqueous solution as the dispersion medium for the positive electrode active material slurry used to form the positive electrode active material layer 2. The glass transition temperature Tg of the polymer compound that is the aqueous polymer binder is, for example, 20°C or lower, preferably 10°C or lower, and more preferably 0°C or lower. This allows the aqueous polymer binder to have flexibility. The inclusion of the aqueous polymer binder in the positive electrode active material layer 2 can prevent the positive electrode active material layer 2 from peeling off from the positive electrode current collector 3 and the positive electrode active material layer 2 from cracking.

[0014] The specific gravity of the acrylic rubber contained in the acrylic polymer binder is 1.1, the hardness range of the acrylic rubber measured according to the JIS standard is 40 to 90, and the tensile strength of the acrylic rubber is 70 to 120 kg / cm 2 The elongation of the acrylic rubber is 100 to 600%, and the volume resistance of the acrylic rubber is 10 8 ~10 10 The specific gravity of styrene butadiene rubber (SBR) is 0.94, the hardness range of SBR measured according to the JIS standard is 30 to 100, and the tensile strength of SBR is 50 to 200 kg / cm 2 The elongation of SBR is 100 to 800%, and the volume resistivity of SBR is 10 10 ~10 15 Ω / cm.

[0015] CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose) functions as a binder for the positive electrode active material layer 2 and is also a substance that can control the viscosity of the positive electrode active material slurry used to form the positive electrode active material layer 2. CMC forms a binder complex together with an aqueous polymer binder. Controlling the type and amount of CMC can change the flexibility, peel strength, and other properties of the positive electrode active material layer 2. Furthermore, by including CMC in the positive electrode active material slurry, the viscosity of the positive electrode active material slurry can be increased, allowing the positive electrode active material layer 2 to have a uniform thickness. As a result, it becomes possible to apply the positive electrode active material slurry thickly, making it possible to form a thick positive electrode active material layer 2 (for example, a thickness of 150 μm or more (after pressing)).

[0016] The elastic modulus of the binder composite comprising the aqueous polymer binder and CMC in the positive electrode active material layer 2 is preferably in the range of 30 to 100 MPa, with the upper limit being 85 MPa. By setting the elastic modulus of the binder composite within this range, it becomes possible to form a good positive electrode active material layer 2. If the elastic modulus of the binder composite exceeds 85 MPa, it becomes difficult to form a flexible positive electrode active material layer. Furthermore, if the elastic modulus of the binder composite is less than 30 MPa, problems arise in maintaining the shape of the positive electrode active material layer 2. When the elastic modulus of the binder composite is 25 MPa, contact portions are transferred when the dried electrode is wound around a roll, resulting in an uneven thickness of the positive electrode active material layer 2. The elastic modulus of the binder composite comprising the aqueous polymer binder and CMC contained in the positive electrode active material layer 2 can be measured, for example, by preparing a binder composite having the same composition as the binder composite contained in the positive electrode active material layer 2 and performing a tensile test using this binder composite.

[0017] The elongation percentage of the binder composite in the elastic region of the positive electrode active material layer 2 is 10% or more. By setting the elongation percentage of the elastic region to this range of 10%, a good positive electrode active material layer 2 can be formed. If the elongation percentage is less than 10%, cracking and peeling are likely to occur. There is no particular upper limit to the elongation percentage, but it is preferably 30% or less.

[0018] For example, lithium iron phosphate particles, a conductive additive, a polymer binder latex (an emulsion in which a polymer compound serving as a binder is dispersed in a colloidal state in water), CMC, and water are mixed and kneaded to prepare a positive electrode active material slurry, which is then applied to the positive electrode current collector 3 and the applied layer is dried to form the positive electrode active material layer 2. Alternatively, the positive electrode active material layer 2 may be subjected to a press treatment. In the press treatment, pressure may be applied to the positive electrode active material layer 2 so that the porosity of the positive electrode active material layer 2 is within a range of 10% to 40%, for example.

[0019] The negative electrode 32 is an electrode having a porous negative electrode active material layer 36. The negative electrode active material layer 36 is, for example, a porous layer containing a negative electrode active material provided on a sheet-like negative electrode current collector 38. The negative electrode current collector 38 is electrically connected to the negative electrode connection member 14. The negative electrode connection member 14 is also electrically connected to the external connection terminal 18b. The negative electrode current collector 38 is, for example, copper foil. The negative electrode active material is a substance that directly participates in the transfer of electrons accompanying charge transfer in the negative electrode. Examples of negative electrode active materials include graphite, partially graphitized carbon, hard carbon, soft carbon, lithium titanate (LTO), and Sn alloys. The negative electrode active material layer 36 can contain these negative electrode active materials alone or in combination.

[0020] The separator 34 is sheet-shaped and is disposed between the positive electrode 5 and the negative electrode 32. The separator 34, together with the positive electrode 5 and the negative electrode 32, can constitute an electrode stack 22 as shown in FIG. 3 . The provision of the separator 34 can prevent a short-circuit current from flowing between the positive electrode 5 and the negative electrode 32. The separator 34 is not particularly limited as long as it can prevent a short-circuit current from flowing and is permeable to ions that conduct between the positive and negative electrodes. For example, the separator 34 can be a microporous polyolefin film, a cellulose sheet, or an aramid sheet. The separator 34 may also be a nonwoven fabric containing at least one of cellulose fibers, polyester fibers, polypropylene fibers, polyacrylonitrile fibers, and polyethylene terephthalate fibers.

[0021] 3, the electrode stack 22 may have a structure in which a plurality of positive electrodes 5 and a plurality of negative electrodes 32 are stacked so that the positive electrodes 5 and the negative electrodes 32 are alternately arranged. The electrode stack 22 may also have a structure in which a separator 34 is arranged between adjacent positive electrodes 5 and negative electrodes 32.

[0022] The nonaqueous electrolyte 15 may be formed using carbonates, lactones, ethers, esters, ionic liquids, or the like as a solvent, or a mixture of two or more of these solvents. Among these, a mixture of a cyclic carbonate and a chain carbonate is particularly preferred. The nonaqueous electrolyte 15 is a solution in which a lithium salt solute, such as LiCF3SO3, LiAsF6, LiClO4, LiBF4, LiPF6, LiBOB, LiN(CF3SO2)2, or LiN(CF2F5SO2), is dissolved in an organic solvent. If necessary, additives such as VC (vinylene carbonate), PS (propane sultone), VEC (vinyl ethyl carbonate), PRS (propene sultone), and flame retardants may be blended alone or in combination.

[0023] The battery case 11 is a battery exterior housing that houses the electrode stack 22 (including the positive electrode 5, the negative electrode 32, and the separator 34) and the nonaqueous electrolyte 15. The battery case 11 may be formed into a bag shape by welding a laminate film at a welding portion. In this case, the lithium-ion secondary battery 30 is a pouch battery. The battery case 11 may be a metal case or a hard resin case. The battery case 11 may also have a lid member 12.

[0024] Measurement of Elastic Modulus and Elongation Percentage and Preparation of Positive Electrode (Test Method) The elastic modulus and elongation percent of the binder composite were measured according to JIS K 7127 (tensile test). The prepared sample pieces were set on the arms of a tensile tester with an arm distance of 5 to 10 mm, and a tensile test was performed by pulling the sample pieces at a pulling speed of 55 mm / min. The measurement results were graphed with the vertical axis representing load and the horizontal axis representing displacement. The vertical axis was converted to stress (load / cross-sectional area) and the horizontal axis to elongation (displacement / arm distance), and an SS curve was obtained. The elastic modulus was then calculated from the initial slope of the SS curve, and the elongation percent of the elastic region was obtained from the inflection point.

[0025] (Preparation of Sample Pieces) A ​​binder, sodium carboxymethyl cellulose (CMC), and pure water (dispersion medium) were kneaded to prepare binder composite solutions for Samples 1 to 9, 14, and 15. Furthermore, CMC and a dispersion medium were kneaded to prepare binder solutions for Samples 10 to 13. The kneaded binder composite solution or binder solution was applied to a PET sheet using a bar coater (gap: 1 mm), and the coating layer was dried at 95°C to prepare a binder composite or binder body. The drying conditions for the coating layer are preferably similar to the drying conditions for the coating layer of the positive electrode active material slurry, and are preferably 80 to 110°C. The binder composite or binder body used in this measurement was prepared by drying the coating film at 95°C and then vacuum drying for 8 hours. The dried binder composite or binder body was peeled off from the PET sheet and cut to an appropriate size to prepare a sample piece. Tables 1 and 2 show the types of binder and CMC and their weight ratios after drying. Tables 1 and 2 also show the width, length, and thickness of the sample pieces. The effective sample length is the distance between the arms at the point when the tensile load begins to be applied to the sample piece. It was not possible to produce a sample piece containing only binder without CMC.

[0026] (Positive Electrode) Positive electrodes corresponding to the binder composites or binder bodies of Samples 1 to 15 were prepared. Specifically, lithium iron phosphate (LiFePO4) powder (90 parts by weight), a conductive additive (10 parts by weight of acetylene black), CMC, a binder, and pure water (dispersion medium) were mixed to prepare positive electrode active material slurries for Samples 1 to 9, 14, and 15. Furthermore, lithium iron phosphate (LiFePO4) powder (90 parts by weight), a conductive additive (10 parts by weight of acetylene black), CMC, and pure water (dispersion medium) were mixed to prepare positive electrode active material slurries for Samples 10 to 13. The binders and CMCs used were those shown in Tables 1 and 2. Furthermore, the CMC and binder were added to the mixture so that the ratio (a:b) of the weight of the solid content of the CMC (a) to the weight of the solid content of the binder (b) was the weight ratio shown in Tables 1 and 2. The prepared positive electrode active material slurry was applied onto an aluminum foil (current collector) and the applied film was dried to prepare positive electrodes of Samples 1 to 15.

[0027]

[0028]

[0029] CMC includes sodium carboxymethylcellulose (CMC) BSH6 (degree of etherification: 0.65 to 0.75, viscosity of 1% CMC aqueous solution at 25°C: 3000 to 4000 mPa·s, abbreviated as BSH6) manufactured by Daiichi Kogyo Seiyaku Co., Ltd., CMC BSH12 (average degree of polymerization: 1600 to 1800, average molecular weight: 330,000 to 360,000, degree of etherification: 0.65 to 0.75, viscosity of 1% CMC aqueous solution at 25°C: 6000 to 8000 mPa·s, abbreviated as BSH12) manufactured by Daiichi Kogyo Seiyaku Co., Ltd., CMC EP (degree of etherification: 0.90 to 0.96, viscosity of 1% CMC aqueous solution at 25°C: 2500 mPa·s or more, abbreviated as EP) manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and CMC manufactured by Nippon Paper Industries Co., Ltd. F350HC (average degree of polymerization: 1,400 (F300HC), average molecular weight: 330,000 (F300HC), degree of etherification: 0.8 to 1.0, viscosity of 1% CMC aqueous solution at 25°C: 2,500 to 4,000 mPa·s, abbreviation: F350HC) was used.

[0030] The binders used were AXA391, an acrylic polymer binder manufactured by Zeon Corporation (latex, active ingredient concentration: 40 wt%, glass transition temperature Tg: -35°C, pH: 7-9, B-type viscosity: 5-50 cP, average particle size: 180 μm, particle size distribution: peak at 170 μm (100-500 μm), abbreviated as AXA391), and BEVEL012, an acrylic polymer binder manufactured by Toyo Ink Co., Ltd. (latex, active ingredient concentration: 40.3 wt%, glass transition temperature Tg: -25°C, particle size distribution: peak at 15 μm and 0.16 μm, abbreviated as BEVEL012). Styrene butadiene rubber (SBR) XG4015 (latex, active ingredient concentration 48.3 wt%, glass transition point: -10°C, pH: 7.3, B-type viscosity: 68 cP, particle size distribution: peak at 150 μm (100 to 250 μm), abbreviated as XG4015) manufactured by Nippon A&L Inc., and SBR AL1002 (latex, active ingredient concentration: 48.3 wt%, glass transition point: 45°C, pH: 6.4, B-type viscosity: 148 cP, particle size distribution: peak at 130 μm (80 to 200 μm), abbreviated as AL1002) manufactured by Nippon A&L Inc. were used.

[0031] (Sample 1: Example) The binder composite formed using the slurry of Sample 1 containing XG4015 (SBR) as the binder and BSH6 as the CMC had an elastic modulus in the range of 30 MPa to 85 MPa and an elongation in the elastic region of 10% or more. Furthermore, when a positive electrode active material layer (thickness 200 μm) was produced using the positive electrode active material slurry of Sample 1 corresponding to the binder composite of Sample 1, no cracks or wrinkles were observed in the positive electrode active material layer.

[0032] (Samples 2 and 3: Comparative Examples) The elastic modulus of the binder composite formed using the slurry of Sample 2 or Sample 3, which contained XG4015 (SBR) as the binder and F350HC or EP as the CMC, was 100 MPa or more, and the elongation in the elastic region was 10% or less. Furthermore, when a positive electrode active material layer (thickness: 200 μm) was produced using the positive electrode active material slurries of Samples 2 and 3, which correspond to the binder composites of Samples 2 and 3, respectively, cracks were observed in the positive electrode active material layer. This is thought to be due to the binder composite having an excessively high elastic modulus and an excessively low elongation. Furthermore, aggregates were observed in the positive electrode active material layers formed using the slurries of Samples 1 to 3.

[0033] (Samples 4, 5, and 6: Examples) The elastic modulus of the binder composite formed using the slurry of Sample 4, 5, or 6, which contained BEVEL012 (an acrylic polymer binder) as the binder and BSH6, F350HC, or EP as the CMC, was in the range of 30 MPa to 85 MPa, and the elongation in the elastic region was 10% or more. Furthermore, when a positive electrode active material layer (thickness 200 μm) was produced using the positive electrode active material slurries of Samples 4, 5, and 6, which correspond to the binder composites of Samples 4, 5, and 6, respectively, no cracks or wrinkles were observed in the positive electrode active material layer. Furthermore, there was almost no variation in the film thickness of the positive electrode active material layer formed using the slurries of Samples 4 to 6.

[0034] (Samples 7, 8, and 9: Examples) The elastic modulus of the binder composite formed using the slurry of Sample 7, 8, or 9, which contained AXA391 (an acrylic polymer binder) as the binder and BSH6, F350HC, or EP as the CMC, was in the range of 30 MPa to 85 MPa, and the elongation in the elastic region was 10% or more. Furthermore, when a positive electrode active material layer (thickness: 200 μm) was produced using the positive electrode active material slurries of Samples 7, 8, and 9, which correspond to the binder composites of Samples 7, 8, and 9, respectively, no cracks or wrinkles were observed in the positive electrode active material layer. Furthermore, there was almost no variation in the film thickness of the positive electrode active material layer formed using the slurries of Samples 7 to 9.

[0035] (Samples 10, 11, 12, 13: Comparative Examples) The elastic modulus of the binder body formed using the slurry of Sample 10, 11, 12, or 13, which did not contain a binder and contained BSH6, F350HC, EP, or BSH12 as the CMC, was 100 MPa or more. Furthermore, when a positive electrode active material layer (thickness: 200 μm) was produced using the positive electrode active material slurries of Samples 10, 11, 12, and 13, which correspond to the binder composites of Samples 10, 11, 12, and 13, respectively, cracks were observed in the positive electrode active material layer. This is thought to be due to the binder composite's elastic modulus being too high.

[0036] (Sample 14: Comparative Example) The binder composite formed using the slurry of Sample 14, which was prepared with a ratio (a:b) of the weight a of the solid content of the CMC BSH6 to the weight b of the solid content of the binder AXA391, had an elastic modulus of 100 MPa or more and an elongation of 10% or more. Furthermore, when a positive electrode active material layer (thickness 200 μm) was produced using the positive electrode active material slurry of Sample 14, which corresponds to the binder composite of Sample 14, cracks were observed in the positive electrode active material layer. This is thought to be due to the binder composite's excessively high elastic modulus.

[0037] (Sample 15: Comparative Example) The binder composite formed using the slurry of Sample 15, which contained AL1002 (SBR) as the binder and BSH6 as the CMC, had an elastic modulus of 100 MPa or more and an elongation in the elastic region of 10% or less. Furthermore, when a positive electrode active material layer (thickness 200 μm) was produced using the positive electrode active material slurry of Sample 15, which corresponds to the binder composite of Sample 15, cracks were observed in the positive electrode active material layer. This is thought to be due to the binder composite's elastic modulus being too high and its elongation being too low.

[0038] Furthermore, in the sample in which the elastic modulus of the binder composite was 25 MPa, when the dried positive electrode corresponding to this sample was wound up on a roll, the positive electrode active material layer in contact with the sample was transferred, and the thickness of the positive electrode active material layer was not uniform.

[0039] Preparation of Lithium-Ion Secondary Batteries A first positive electrode active material powder (lithium iron phosphate, secondary particle diameter: 16-20 μm), a second positive electrode active material powder (lithium iron phosphate, secondary particle diameter: 2-3 μm), CMC (BSH6), and an acrylic polymer binder (aqueous binder, AXA391) were mixed to achieve the solid content compositions shown in Table 3. Water was added to these mixed powders and the mixture was kneaded to prepare the positive electrode active material slurries of Examples 1-7 and Comparative Examples 1-4. The positive electrode active material slurries were applied to aluminum foil (positive electrode current collector sheet), and the coating was dried to form a positive electrode active material layer with a thickness of approximately 200 μm on the positive electrode current collector sheet, thereby producing the positive electrodes of Examples 1-7 and Comparative Examples 1-4. Table 3 also shows the total ratio of CMC and aqueous binder in the positive electrode active material layer or the solid content composition, and the weight ratio (b / a) of binder (b) to CMC (a).

[0040]

[0041] Next, the prepared positive electrode, a polyolefin separator, a carbonaceous negative electrode, and a non-aqueous electrolyte (1 M LiPF electrolyte solution (carbonate-based solvent)) were placed in a coin cell case to prepare lithium ion secondary batteries (coin cells) of Examples 1 to 7 and Comparative Examples 1 to 4.

[0042] Charge-Discharge Cycle Test A charge-discharge cycle test was conducted using the lithium-ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 4. Specifically, a 1C charge-discharge cycle was repeated five times with an upper limit voltage of 3.6 V and a lower limit voltage of 2.0 V, and the battery capacity (discharge capacity) was calculated based on the discharge of the fifth cycle. The calculated battery capacities are shown in Table 3. Table 3 shows that by setting the total proportion of the aqueous binder and the CMC in the positive electrode active material layer to 1.0 wt % or more and 1.8 wt % or less, the lithium-ion battery had a battery capacity of 130 mAh or more. Furthermore, it was found that by setting the ratio (b / a) of the mass (b) of the aqueous binder to the mass (a) of the CMC contained in the positive electrode active material layer to 2.0 or more and 3.5 or less, good battery characteristics were exhibited.

[0043] 2: Positive electrode active material layer 3: Positive electrode current collector 5: Positive electrode 11: Battery case 12: Lid member 13: Positive electrode connecting member 14: Negative electrode connecting member 15: Non-aqueous electrolyte 18a, 18b: External connection terminals 22: Electrode laminate 25: Shrink film 30: Lithium ion secondary battery 32: Negative electrode 34: Separator 36: Negative electrode active material layer 38: Negative electrode current collector

Claims

1. A positive electrode for a lithium ion secondary battery comprising: a current collector; and a positive electrode active material layer provided on the current collector; the positive electrode active material layer contains lithium iron phosphate particles, a latex-derived aqueous polymer binder, and sodium carboxymethylcellulose or calcium carboxymethylcellulose; the aqueous polymer binder and the sodium carboxymethylcellulose or calcium carboxymethylcellulose form a binder complex; and the elastic modulus of the binder complex is in the range of 30 to 100 MPa.

2. The positive electrode according to claim 1, wherein the binder complex has an elongation of 10% or more in the elastic region.

3. The positive electrode according to claim 1, wherein the polymer binder is an acrylic polymer binder or a styrene-butadiene rubber.

4. The positive electrode according to claim 1, wherein the polymer compound contained in the polymer binder has a glass transition temperature of 20° C. or lower.

5. The positive electrode according to claim 1, wherein the positive electrode active material layer has a thickness of 150 μm or more.

6. A lithium ion secondary battery comprising the positive electrode according to any one of claims 1 to 5, a negative electrode, a separator, and a non-aqueous electrolyte.

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