Cathode for lithium-ion secondary batteries and lithium-ion secondary batteries

Abstract

The present invention provides a positive electrode for a lithium-ion secondary battery capable of suppressing the occurrence of cracks or wrinkles in the positive electrode active material layer. The positive electrode for a lithium-ion secondary battery according to the present invention comprises a current collector and a positive electrode active material layer installed on the current collector, wherein the positive electrode active material layer comprises lithium iron phosphate particles, a latex-derived aqueous polymer binder, and sodium carboxymethylcellulose or calcium carboxymethylcellulose, wherein the aqueous polymer binder and the sodium carboxymethylcellulose or calcium carboxymethylcellulose constitute a binder complex, and the elastic modulus of the binder complex is in the range of 30 to 100 MPa.

Description

Technology Field

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

[0002] The positive active material layer of a lithium-ion battery is typically produced by applying a slurry containing positive active material particles, a PVdF-based binder, N-methylpyrrolidone, and a conductive additive onto a current collector and drying the coating layer. However, N-methylpyrrolidone is on the list of restricted substances under the REACH rule due to concerns regarding its toxicity.

[0003] A method for preparing a positive active material layer that does not use N-methylpyrrolidone is known, which involves applying a slurry containing positive active material particles, an aqueous binder, a conductive aid, and water onto a current collector and drying the coating layer (see, for example, Patent Document 1). Prior art literature

[0004] Japanese Patent Publication No. 2017-091789 The problem to be solved

[0005] However, when a conventional positive active material slurry with water as a dispersion medium is thickly coated onto a current collector and the coating layer is dried, cracks or wrinkles occur in the positive active material layer (for example, a positive active material layer having a thickness of 150 μm or more). If cracks occur in the positive active material layer, the positive active material layer becomes prone to peeling off from the current collector, which is disadvantageous for manufacturing the battery.

[0006] In addition, if wrinkles occur in the positive active material layer, twisting occurs in the positive electrode, making it disadvantageous for battery manufacturing.

[0007] The present invention has been made in consideration of these circumstances and provides a positive electrode for a lithium-ion secondary battery that can suppress the occurrence of cracks or wrinkles in the positive electrode active material layer. means of solving the problem

[0008] The present invention provides a positive electrode for a lithium-ion secondary battery, comprising a current collector and a positive electrode active material layer installed on the current collector, wherein the positive electrode active material layer comprises lithium iron phosphate particles, a latex-derived aqueous polymer binder, and CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose), wherein the aqueous polymer binder and the CMC form a binder composite, and the elastic modulus of the binder composite is in the range of 30 to 100 MPa.

[0009] In addition, the present invention also provides a lithium-ion secondary battery comprising the anode, cathode, separator, and non-aqueous electrolyte of the present invention. Effects of the invention

[0010] According to the present invention, cracks or wrinkles can be suppressed in the positive active material layer, and peeling of the positive active material layer from the current collector can be suppressed.

[0011] According to the present invention, since the thickness of the positive active material layer can be increased, the amount of positive active material included in the lithium-ion secondary battery can be increased, and the battery capacity can be increased.

[0012] 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 inventors. Brief explanation of the drawing

[0013] FIG. 1(a) is a schematic plan view of a positive electrode for a lithium-ion secondary battery according to one embodiment of the present invention, and (b) is a schematic cross-sectional view of the positive electrode at the dashed line AA in (a). FIG. 2 is a schematic cross-sectional view of a lithium-ion secondary battery according to one embodiment of the present invention. FIG. 3 is a schematic structural diagram of an electrode stack included in a lithium-ion secondary battery according to one embodiment of the present invention. Specific details for implementing the invention

[0014] Hereinafter, an embodiment of the present invention will be described using the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0015] The positive electrode (5) for a lithium-ion secondary battery of the present embodiment comprises a current collector (3) and a positive electrode active material layer (2) installed on the current collector (3), wherein the positive electrode active material layer (2) comprises lithium iron phosphate particles, a latex-derived water-based polymer binder, and CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose), wherein the water-based polymer binder and the CMC form a binder composite, and the elastic modulus of the binder composite is in the range of 30 to 100 MPa.

[0016] The lithium-ion secondary battery (30) of the present embodiment comprises a positive electrode (5), a negative electrode (32), a separator (34), and a non-aqueous electrolyte (15).

[0017] 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).

[0018] The positive electrode (5) for a lithium-ion secondary battery comprises a positive electrode current collector (3) and a porous positive electrode active material layer (2) installed on the positive electrode current collector (3).

[0019] The positive current collector (3) is a sheet that serves as a substrate for installing the positive active material layer (2), and is a conductor that electrically connects the positive connection member (13) and the positive active material layer (2). The positive current collector (3) is, for example, an aluminum foil. The positive connection member (13) is electrically connected to an external connection terminal (18a). The positive active material layer (2) may be installed on one side of the positive current collector (3), or may be installed on both sides of the positive current collector (3).

[0020] The positive active material layer (2) is a layer containing a positive active material. The thickness of the positive active material layer (2) (after pressing) may be 150 μm or more. By doing so, the amount of positive active material contained in the positive (5) increases, and the battery capacity of the lithium-ion secondary battery (30) can be increased.

[0021] The positive active material layer (2) comprises lithium iron phosphate particles, which are the positive active material, a latex-derived aqueous polymer binder, and CMC. Additionally, the positive active material layer (2) may include a conductive aid. The aqueous polymer binder and CMC form a binder composite.

[0022] The lithium iron phosphate particles are particles of lithium iron phosphate (LiFePO4), which is the positive electrode active material. The lithium iron phosphate particles may have a conductive film on their surface. This can improve the conductivity of the surface of the fine particles where the intercalation reaction proceeds and lower the internal resistance of the positive electrode (5). The conductive film is, for example, a carbon film. The lithium iron phosphate particles may be included in the positive electrode active material layer (2) as secondary particles. The average particle size of the secondary lithium iron phosphate particles can be 10 μm or more and 60 μm or less. The average particle size can be calculated by measuring the particle sizes of 100 secondary particles randomly selected from a cross-sectional photograph of the positive electrode active material layer (2) and averaging these particle sizes.

[0023] The conductive aid is, for example, furnace black, acetylene black, hard carbon, soft carbon, etc. By including the conductive aid in the positive active material layer (2), the conductivity of the positive active material layer (2) can be improved and the internal resistance of the positive (5) can be reduced.

[0024] A latex-derived water-based polymer binder is a polymer binder that can be dispersed in a colloidal phase in an aqueous dispersion medium. The water-based polymer binder forms a binder complex together with CMC. Examples of water-based polymer binders include acrylic polymer binders and styrene-butadiene rubber (SBR). By using such a water-based polymer binder, it becomes possible to use water or an aqueous solution as a dispersion medium for the positive active material slurry used to form the positive active material layer (2). The glass transition point (Tg) of the polymer compound that is the water-based polymer binder is, for example, 20°C or lower, preferably 10°C or lower, and more preferably 0°C or lower. This allows the water-based polymer binder to have flexibility.

[0025] By including a water-based polymer binder in the positive active material layer (2), the delamination of the positive active material layer (2) from the positive current collector (3) and the occurrence of cracks in the positive active material layer (2) can be suppressed.

[0026] The specific gravity of the acrylic rubber included in the acrylic polymer binder is 1.1, the hardness range of the acrylic rubber measured according to JIS standards is 40–90, the tensile strength of the acrylic rubber is 70–120 kg / cm², the elongation of the acrylic rubber is 100–600%, and the volume resistivity of the acrylic rubber is 10 8 ~10 10 It is Ω / cm.

[0027] The specific gravity of styrene-butadiene rubber (SBR) is 0.94, the hardness range of SBR measured according to JIS standards is 30–100, the tensile strength of SBR is 50–200 kg / cm², the elongation of SBR is 100–800%, and the volume resistivity of SBR is 10 10 ~10 15 It is Ω / cm.

[0028] CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose) acts as a binder for the positive active material layer (2) and is a material that can control the viscosity of the positive active material slurry used to form the positive active material layer (2). The CMC forms a binder complex together with a water-based polymer binder. By controlling the type or amount of CMC, the flexibility or peel strength of the positive active material layer (2) can be changed. In addition, by including CMC in the positive active material slurry, the viscosity of the positive active material slurry can be increased, and the positive active material layer (2) can be made to a uniform thickness. As a result, it becomes possible to apply the positive active material slurry thickly, and it becomes possible to form a thick positive active material layer (2) (for example, a thickness of 150 μm or more (after pressing)).

[0029] It is preferable that the elastic modulus of the binder composite, which consists of a water-based polymer binder and CMC in the positive active material layer (2), be in the range of 30 to 100 MPa, with an upper limit of 85 MPa. By setting the elastic modulus of the binder composite to this range, it becomes possible to form a good positive active material layer (2). If the elastic modulus of the binder composite exceeds 85 MPa, it becomes difficult to form a flexible positive active material layer. Also, if the elastic modulus of the binder composite becomes less than 30 MPa, there is a concern that a problem may occur in maintaining the shape of the positive active material layer (2). When the elastic modulus of the binder composite is 25 MPa, the contact portion is transferred when the electrode is wound onto a roll after drying, and the thickness of the positive active material layer (2) becomes uneven.

[0030] The elastic modulus of the binder composite composed of a water-based polymer binder and CMC included in the positive electrode active material layer (2) can be measured, for example, by producing a binder composite having the same composition as the binder composite included in the positive electrode active material layer (2) and performing a tensile test using this binder composite.

[0031] The elastic region elongation rate of the binder composite in the positive active material layer (2) is 10% or more. By keeping the elastic region elongation rate within this range of 10%, a good positive active material layer (2) can be formed. If the elongation rate is less than 10%, it becomes easy to cause cracks or delamination. There is no special limit on the upper limit of the elongation rate, but it is preferable that it be 30% or less.

[0032] For example, a positive active material slurry can be prepared by mixing and kneading lithium iron phosphate particles, a conductive aid, a latex of a polymer binder (an emulsion in which a polymer compound serving as a binder is dispersed in water in a colloidal state), CMC, and water, and then applying this slurry onto a positive current collector (3) and drying the coating layer to form a positive active material layer (2). Additionally, a press treatment may be performed on the positive active material layer (2). In the press treatment, for example, pressure may be applied to the positive active material layer (2) so that the porosity of the positive active material layer (2) is within the range of 10% to 40%.

[0033] The negative electrode (32) is an electrode having a porous negative active material layer (36). The negative active material layer (36) is, for example, a porous layer containing a negative active material installed on a sheet-shaped negative current collector (38). The negative current collector (38) is electrically connected to a negative connection member (14). Also, the negative connection member (14) is electrically connected to an external connection terminal (18b). The negative current collector (38) is, for example, a copper foil.

[0034] The negative electrode active material is a material that is directly involved in the transfer of electrons accompanied by charge transfer at the negative electrode. Examples of negative electrode active materials include graphite, partially graphitized carbon, hard carbon, soft carbon, lithium titanate (LTO), Sn alloy, etc. The negative electrode active material layer (36) may include these negative electrode active materials either individually or as a mixture of multiple types.

[0035] The separator (34) is in the form of a sheet and is placed between the positive electrode (5) and the negative electrode (32). Additionally, the separator (34) can form an electrode stack (22) as shown in FIG. 3 together with the positive electrode (5) and the negative electrode (32). By installing the separator (34), short-circuit current can be prevented from flowing between the positive electrode (5) and the negative electrode (32).

[0036] The separator (34) is not particularly limited as long as it can prevent short-circuit current from flowing and allows ions to pass through that conduct between the positive and negative electrodes, but for example, it can be a microporous film of polyolefin, a cellulose sheet, or an aramid sheet. Also, the separator (34) may be a nonwoven fabric comprising at least one of cellulose fibers, polyester fibers, polypropylene fibers, polyacrylonitrile fibers, and polyethylene terephthalate fibers.

[0037] As shown in FIG. 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 negative electrodes (32) are arranged alternately. In addition, the electrode stack (22) may have a structure in which a separator (34) is arranged between adjacent positive electrodes (5) and negative electrodes (32).

[0038] The non-aqueous electrolyte (15) may use carbonates, lactones, ethers, esters, ionic liquids, etc. as solvents, and may also use a mixture of two or more types of these solvents. Among these, it is particularly preferable to use a mixture of cyclic carbonates and chain carbonates. The non-aqueous electrolyte (15) is, for example, a solution in which a lithium salt solute such as LiCF3SO3, LiAsF6, LiClO4, LiBF4, LiPF6, LiBOB, LiN(CF3SO2)2, LiN(C2F5SO2) is dissolved in an organic solvent. Additionally, if necessary, additives such as VC (vinylene carbonate), PS (propanesulfone), VEC (vinyl ethyl carbonate), PRS (propensulfone), and flame retardants may be added individually or in combination.

[0039] The battery case (11) is a battery outer body that accommodates an electrode laminate (22) (including a positive electrode (5), a negative electrode (32), and a separator (34)) and a non-aqueous electrolyte (15). The battery case (11) may be formed into a bag shape by welding a laminate film at the welding portion. In this case, the lithium-ion secondary battery (30) is a pouch battery. Also, the battery case (11) may be a metal case or a hard resin case. Additionally, the battery case (11) may have a lid member (12).

[0040] Measurement of elastic modulus and elongation and fabrication of anodes

[0041] (Test Method)

[0042] The elastic modulus and elongation of the binder composite were measured according to JIS K 7127 (tensile test).

[0043] A prepared specimen was set on an arm of a tensile testing machine with an arm spacing of 5–10 mm, and a tensile test was performed by pulling the specimen at a tensile speed of 55 mm / min. A graph was plotted with the measurement results plotted with load on the vertical axis and displacement on the horizontal axis; the vertical axis was converted to stress (load / cross-sectional area) and the horizontal axis to elongation (displacement / distance between arms), thereby obtaining an SS curve. Subsequently, the elastic modulus was calculated from the initial slope of the SS curve, and the elongation of the elastic region was obtained from the inflection point.

[0044] (Production of sample version)

[0045] A binder, sodium carboxymethylcellulose (CMC), and pure water (dispersion medium) were mixed to prepare binder complex solutions for samples 1 to 9, 14, and 15. Additionally, binder solutions for samples 10 to 13 were prepared by mixing CMC with the dispersion medium. A binder complex or binder body was prepared by applying the mixed binder complex solution or binder solution onto a PET sheet using a bar coater (Gap: 1 mm) and drying the coating layer at 95°C. It is preferable that the drying conditions of the coating layer be similar to those of the anode active material slurry coating layer, and the temperature may be 80 to 110°C. The binder complex or binder body used in this measurement was prepared by drying the coating film at 95°C and then drying it by vacuum drying for 8 hours. After drying, the binder complex or binder body was peeled off from the PET sheet and cut into an appropriate size to prepare a sample piece. Tables 1 and 2 show the types of binder and CMC and the weight ratio after drying. Also, Tables 1 and 2 show the values ​​for the width, length, and thickness of the specimens. The effective length of the specimen is the distance between the arms at the point when tensile load begins to be applied to the specimen. It was not possible to produce specimens that did not contain CMC containing only binder.

[0046] (anode)

[0047] Anodes of samples 1 to 15 corresponding to the binder composites or binder bodies of samples 1 to 15 were prepared. Specifically, anode active material slurries of samples 1 to 9, 14, and 15 were prepared by mixing lithium iron phosphate (LiFePO4) powder (90 parts by weight), a conductivity aid (10 parts by weight of acetylene black), CMC, a binder, and pure water (dispersion medium). In addition, anode active material slurries of samples 10 to 13 were prepared by mixing lithium iron phosphate (LiFePO4) powder (90 parts by weight), a conductivity aid (10 parts by weight of acetylene black), CMC, and pure water (dispersion medium). The binders and CMCs shown in Tables 1 and 2 were used. In addition, CMC and binder are added to the mixture such that the ratio (a:b) of the weight of the solid content of CMC (a) to the weight of the solid content of binder (b) is the weight ratio shown in Tables 1 and 2. The prepared anode active material slurry is coated onto an aluminum foil (current collector), and the coating film is dried to produce the anodes of samples 1 to 15.

[0048]

[0049]

[0050] As for CMC,

[0051] Sodium carboxymethylcellulose (CMC) BSH6 of Dai-ichi Kogyo Seiyaku Co., Ltd. (degree of etherification: 0.65–0.75, viscosity of 1% CMC aqueous solution at 25°C: 3000–4000 mPa·s, abbreviation: BSH6),

[0052] CMC BSH12 of Dai-ichi Kogyo Seiyaku Co., Ltd. (average degree of polymerization: 1600–1800, average molecular weight: 330,000–360,000, degree of etherification: 0.65–0.75, viscosity of 1% CMC aqueous solution at 25°C: 6000–8000 mPa·s, abbreviation: BSH12),

[0053] CMC EP of Dai-ichi Kogyo Seiyaku Co., Ltd. (Degree of etherification: 0.90–0.96, viscosity of 1% CMC aqueous solution at 25°C: 2500 mPa·s or higher, abbreviation: EP), or

[0054] CMC F350HC of Nippon Paper Industries Co., Ltd. (average degree of polymerization: 1400 (F300HC), average molecular weight: 330,000 (F300HC), degree of etherification: 0.8–1.0, viscosity of 1% CMC aqueous solution at 25°C: 2500–4000 mPa·s, abbreviation: F350HC) was used.

[0055] As for the binder,

[0056] Acrylic polymer binder AXA391 of ZEON CORPORATION. (latex, active ingredient concentration: 40 wt%, glass transition point (Tg): -35℃, pH: 7–9, Type B viscosity: 5–50 cP, average particle size: 180 µm, particle size distribution: peak at 170 µm (100–500 µm), abbreviation: AXA391),

[0057] Acrylic polymer binder BEVEL012 of TOYO INK CO., LTD. (latex, active ingredient concentration: 40.3 wt, glass transition point (Tg): -25℃, particle size distribution: peaks at 15㎛ and 0.16㎛, abbreviation: BEVEL012),

[0058] Styrene-butadiene rubber (SBR) XG4015 of NIPPON A and L INC. (latex, active ingredient concentration: 48.3 wt%, glass transition point: -10℃, pH: 7.3, type B viscosity: 68 cP, particle size distribution: peak at 150 µm (100–250 µm), abbreviation: XG4015),

[0059] SBR AL1002 of NIPPON A and L INC. (latex, active ingredient concentration: 48.3 wt%, glass transition point: 45℃, pH: 6.4, type B viscosity: 148 cP, particle size distribution: peak at 130 µm (80–200 µm), abbreviation: AL1002) was used.

[0060] (Sample 1: Example)

[0061] The elastic modulus of the binder composite formed using the slurry of Sample 1, which includes XG4015 (SBR) as a binder and BSH6 as a CMC, was in the range of 30 MPa to 85 MPa, and the elongation of the elastic region was 10% or more.

[0062] In addition, when a positive active material layer (thickness 200 μm) was fabricated using the positive active material slurry of Sample 1 corresponding to the binder composite of Sample 1, no cracks or wrinkles were observed in the positive active material layer.

[0063] (Samples 2, 3: Comparative Examples)

[0064] The elastic modulus of the binder composite formed using the slurry of Sample 2 or Sample 3, which contains XG4015 (SBR) as the binder and F350HC or EP as the CMC, was 100 MPa or higher, and the elongation of the elastic region was 10% or lower. In addition, when a positive active material layer (thickness 200 μm) was fabricated using the positive active material slurries of Sample 2 and Sample 3, which correspond to the binder composites of Sample 2 and Sample 3, respectively, cracks were observed in the positive active material layer. This is thought to be caused by the elastic modulus of the binder composite being too high and the elongation being too low.

[0065] In addition, aggregates were confirmed in the positive active material layer formed using the slurries of samples 1 to 3.

[0066] (Samples 4, 5, 6: Examples)

[0067] The elastic modulus of the binder composite formed using the slurry of samples 4, 5, or 6, which includes BEVEL012 (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 of the elastic region was 10% or more. In addition, when a positive active material layer (thickness 200 μm) was fabricated using the positive 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 active material layer.

[0068] In addition, there was almost no variation in the film thickness of the positive active material layer formed using the slurries of samples 4 to 6.

[0069] (Samples 7, 8, 9: Examples)

[0070] The elastic modulus of the binder composite formed using the slurry of samples 7, 8, or 9, which includes AXA391 (acrylic polymer binder) as the binder and BSH6, F350HC, or EP as the CMC, was within the range of 30 MPa to 85 MPa, and the elongation of the elastic region was 10% or more. In addition, when a positive active material layer (thickness 200 μm) was fabricated using the positive 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 active material layer.

[0071] In addition, there was almost no variation in the film thickness of the positive active material layer formed using the slurries of samples 7 to 9.

[0072] (Samples 10, 11, 12, 13: Comparative Examples)

[0073] The elastic modulus of the binder body formed using a slurry of samples 10, 11, 12, or 13, which does not contain a binder and contains BSH6, F350HC, EP, or BSH12 as the CMC, was 100 MPa or higher. In addition, when a positive active material layer (thickness 200 μm) was fabricated using the positive 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 active material layer. This is thought to be caused by the excessively high elastic modulus of the binder composite.

[0074] (Sample 14: Comparative Example)

[0075] The elastic modulus of the binder composite formed using the slurry of Sample 14, prepared with a ratio (a:b) of the solid weight of BSH6 (a) of the CMC and the solid weight of AXA391 (b) of the binder (a) being 1:1, was 100 MPa or higher, and the elongation was 10% or higher. In addition, when a positive active material layer (thickness 200 μm) was fabricated using the positive active material slurry of Sample 14 corresponding to the binder composite of Sample 14, cracks were observed in the positive active material layer. This is thought to be caused by the excessively high elastic modulus of the binder composite.

[0076] (Sample 15: Comparative Example)

[0077] The elastic modulus of the binder composite formed using the slurry of Sample 15, which contains AL1002 (SBR) as a binder and BSH6 as a CMC, was 100 MPa or higher, and the elongation of the elastic region was 10% or lower. In addition, when a positive active material layer (thickness 200 μm) was fabricated using the positive active material slurry of Sample 15 corresponding to the binder composite of Sample 15, cracks were observed in the positive active material layer. This is thought to be caused by the elastic modulus of the binder composite being too high and the elongation being too low.

[0078] In addition, in a sample where the elastic modulus of the binder composite is 25 MPa, when the anode corresponding to this sample is wound onto a roll after drying, the anode active material layer in contact is transferred, and the thickness of the anode active material layer is not uniform.

[0079] Production of lithium-ion secondary batteries

[0080] A first positive active material powder (lithium iron phosphate, particle size of secondary particles: 16–20 μm), a second positive active material powder (lithium iron phosphate, particle size of secondary particles: 2–3 μm), CMC (BSH6), and an acrylic polymer binder (aqueous binder, AXA391) were mixed to achieve the respective solid compositions shown in Table 3. By adding water to these mixed powders and kneading, the positive active material slurries of Examples 1–7 and Comparative Examples 1–4 were prepared. The positive active material slurries were coated onto an aluminum foil (positive current collector sheet), and by drying the coating film, a positive active material layer with a thickness of about 200 μm was formed on the positive current collector sheet, thereby producing the positives of Examples 1–7 and Comparative Examples 1–4.

[0081] In addition, Table 3 shows the total ratio of CMC and water-based binder in the positive active material layer or the solid composition, and the weight ratio (b / a) of binder (b) to CMC (a).

[0082]

[0083] Next, the fabricated anode, a polyolefin separator, a carbonaceous cathode, and a non-aqueous electrolyte (1M LiPF6 electrolyte (carbonate-based solvent)) were placed in a coin cell case, and lithium-ion secondary batteries (coin cells) of Examples 1 to 7 and Comparative Examples 1 to 4 were fabricated.

[0084] Charge / Discharge Cycle Test

[0085] 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 5 times with an upper voltage limit of 3.6V and a lower voltage limit of 2.0V, and the battery capacity (discharge capacity) was calculated based on the discharge of the 5th cycle. The calculated battery capacities are shown in Table 3.

[0086] From Table 3, it was found that by making the sum of the ratio of water-based binder and the ratio of CMC in the positive electrode active material layer 1.0 wt% or more and 1.8 wt% or less, the lithium-ion battery has a battery capacity of 130 mAh or more. In addition, it was found that by making the ratio (b / a) of the mass of water-based binder (b) to the mass (a) of CMC included in the positive electrode active material layer 2.0 or more and 3.5 or less, good battery characteristics are exhibited. Explanation of the symbols

[0087] 2: Positive active material layer 3: Positive current collector 5: Anode 11: Battery case 12: Lid member 13: Positive connection member 14: Cathode connecting member 15: Non-aqueous electrolytes 18a, 18b: External connection terminals 22: Electrode laminate 25: Shrink film 30: Lithium-ion secondary battery 32: Cathode 34: Separator 36: Cathode active material layer 38: Cathode current collector

Claims

Claim 1 A positive electrode for a lithium-ion secondary battery, comprising a current collector and a positive electrode active material layer installed on the current collector, wherein the positive electrode active material layer comprises lithium iron phosphate particles, a latex-derived aqueous polymer binder, and sodium carboxymethylcellulose or calcium carboxymethylcellulose, wherein the aqueous polymer binder and the sodium carboxymethylcellulose or calcium carboxymethylcellulose constitute a binder composite, and wherein the elastic modulus of the binder composite is in the range of 30 to 100 MPa. Claim 2 An anode according to claim 1, wherein the elongation rate in the elastic region of the binder composite is 10% or more. Claim 3 In claim 1, the polymer binder is an anode that is an acrylic polymer binder or styrene-butadiene rubber. Claim 4 An anode according to claim 1, wherein the glass transition point of the polymer compound included in the polymer binder is 20°C or lower. Claim 5 In claim 1, the anode having a thickness of 150 μm or more of the anode active material layer. Claim 6 A lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte as described in any one of claims 1 to 5.

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