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

Abstract

The present invention provides a positive electrode which has a positive electrode active material layer that can be formed with use of a positive electrode active material slurry that does not contain N-methylpyrrolidone. A positive electrode for lithium ion secondary batteries according to the present invention comprises the positive electrode active material layer. The positive electrode active material layer is characterized by containing a positive electrode active material and an aqueous polymer binder that is derived from a latex, and having a thickness of 150 μm or more. A lithium ion secondary battery according to the present invention is provided with the positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte.

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 usually produced by applying a slurry containing positive electrode active material particles, a PVdF-based binder, N-methylpyrrolidone, and a conductive additive onto a current collector and drying the applied layer.

[0003] Due to concerns about its toxicity, N-methylpyrrolidone is included in the list of restricted substances under the REACH regulation. The present invention has been made in light of these circumstances, and provides a positive electrode having a positive electrode active material layer that can be formed using a positive electrode active material slurry that does not contain N-methylpyrrolidone.

[0004] The present invention provides a positive electrode for a lithium ion secondary battery, comprising a positive electrode active material layer, the positive electrode active material layer containing a positive electrode active material and a water-based polymer binder derived from latex, and having a thickness of 150 μm or more.

[0005] Since the positive electrode active material layer contains a water-based polymer binder derived from latex, a good positive electrode active material layer can be formed using a positive electrode active material slurry (dispersion medium: water) that does not contain N-methylpyrrolidone.

[0006] 1A is a schematic cross-sectional view of a lithium ion secondary battery according to one embodiment of the present invention; FIG. 1B is a schematic plan view of a positive electrode included in the lithium ion secondary battery according to one embodiment of the present invention; FIG. 1B is a schematic cross-sectional view of the positive electrode taken along dashed line A-A in FIG. 1A; FIG. 1B is a schematic plan view of a negative electrode included in the lithium ion secondary battery according to one embodiment of the present invention; FIG. 1B is a schematic cross-sectional view of the negative electrode taken along dashed line B-B in FIG. 1A; and FIG. 1C is a schematic structural diagram of an electrode stack included in the lithium ion secondary battery according to one embodiment of the present invention.

[0007] 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 examples, and the scope of the present invention is not limited to those shown in the drawings and the following description. A positive electrode 5 for a lithium ion secondary battery according to this embodiment includes a positive electrode active material layer 2, which contains a positive electrode active material and a latex-derived aqueous polymer binder and has a thickness of 150 μm or more. 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.

[0008] 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.

[0009] 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) is 150 μm or more, preferably 200 μm or more. This increases the amount of positive electrode active material contained in the positive electrode 5, thereby increasing the battery capacity of the lithium-ion secondary battery 30. Furthermore, the amount of current collector 3 contained in the lithium-ion secondary battery 30 can be reduced, thereby increasing the mass energy density of the lithium-ion secondary battery 30. Furthermore, if the thickness of the positive electrode active material layer 2 is too thick, the distance between the surface of the positive electrode active material layer 2 and the current collector 3 becomes too long, which may increase the electrode resistance. Therefore, the thickness of the positive electrode active material layer 2 is preferably 500 μm or less, and preferably 400 μm or less.

[0010] The positive electrode active material layer 2 contains a positive electrode active material and a latex-derived aqueous polymer binder. The positive electrode active material layer 2 may also contain CMC (sodium carboxymethyl cellulose or calcium carboxymethyl cellulose). The positive electrode active material layer 2 may also contain a thickener. The positive electrode active material layer 2 may also contain a conductive additive. The positive electrode active material layer 2 may also contain a surfactant.

[0011] Lithium iron phosphate (LiFePO4) is preferred as the positive electrode active material. Other positive electrode materials can also be used as long as they are compatible with the solvent of the positive electrode slurry. Lithium iron phosphate secondary particles, which are secondary particles formed by agglomeration of primary particles of lithium iron phosphate (LiFePO4), are preferred. The lithium iron phosphate primary particles may have a conductive coating on their surfaces. This can improve the conductivity of the particle surface 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 average particle size of the lithium iron phosphate primary particles is, for example, 30 nm or more and 400 nm or less. The average particle size can be calculated by measuring the particle sizes of 100 randomly selected primary particles from a cross-sectional photograph of the positive electrode active material layer 2 and arithmetically averaging these particle sizes.

[0012] The positive electrode active material layer 2 can include large-particle-size lithium iron phosphate secondary particles as a first positive electrode active material and small-particle-size lithium iron phosphate secondary particles as a second positive electrode active material having a particle size smaller than the first lithium iron phosphate secondary particles. The large-particle-size lithium iron phosphate secondary particles are lithium iron phosphate secondary particles having a particle size of 10 μm or more among the plurality of lithium iron phosphate secondary particles contained in the positive electrode active material layer 2, and have an average particle size of 15 μm or more and 22 μm or less. The small-particle-size lithium iron phosphate secondary particles are lithium iron phosphate secondary particles having a particle size of less than 10 μm among the plurality of lithium iron phosphate secondary particles contained in the positive electrode active material layer 2, and have an average particle size of 2 μm or more and 7 μm or less.

[0013] For example, the average particle diameter of large-particle lithium iron phosphate can be obtained by measuring the particle diameters of 100 randomly selected secondary particles of lithium iron phosphate from a cross-sectional photograph of the positive electrode active material layer 2, and arithmetically averaging the particle diameters of secondary particles having a particle diameter of 10 μm or more among the secondary particles whose particle diameters have been measured, and the average particle diameter of small-particle lithium iron phosphate can be obtained by arithmetically averaging the particle diameters of secondary particles having a particle diameter of less than 10 μm among the secondary particles whose particle diameters have been measured.

[0014] For example, when the particle diameters of 100 randomly selected lithium iron phosphate secondary particles are measured in a cross-sectional photograph of the positive electrode active material layer 2 and a particle size distribution of the lithium iron phosphate secondary particles is prepared, with the horizontal axis representing particle diameter and the vertical axis representing number, a peak for small particle size lithium iron phosphate can be found in the range of 2 μm to 7 μm, and a peak for large particle size lithium iron phosphate can be found in the range of 15 μm to 22 μm. This particle size distribution does not need to have any peaks other than the two peaks for the large particle size lithium iron phosphate and the small particle size lithium iron phosphate.

[0015] The ratio (a:b) of the mass (a) of the first positive electrode active material to the mass (b) of the second positive electrode active material is preferably 7:3 to 9:1 (the ratio (b / a) is (1 / 9) or more and (3 / 7) or less). This ratio stabilizes the positive electrode slurry and the positive electrode active material layer 2. It also improves the pressability of the positive electrode active material layer 2, making it easier to control the porosity. The masses of the first positive electrode active material and the second positive electrode active material can be calculated from the density and particle diameter of the positive electrode active material particles. The density of the positive electrode active material particles can be measured and calculated using a positive electrode active material (powder) equivalent to the positive electrode active material particles contained in the positive electrode active material layer 2. Alternatively, the density can be measured by dissolving the positive electrode active material layer 2 in a solvent or the like, recovering the positive electrode active material, and classifying it.

[0016] Examples of the conductive additive include furnace black, acetylene black, carbon black, fine particles of coke-based 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.

[0017] The latex-derived aqueous polymer binder is a polymer binder that can be colloidally dispersed in an aqueous dispersion medium. Examples of the aqueous polymer binder include an acrylic polymer binder and styrene-butadiene rubber (SBR). The use of such a 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 polymer binder is, for example, 20°C or lower, preferably 10°C or lower, and more preferably 0°C or lower. This allows the polymer binder to have flexibility and improve its binding properties. 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.

[0018] The ratio (d / c) of the mass (d) of the latex-derived aqueous polymer binder contained in the positive electrode active material layer 2 to the mass (c) of the positive electrode active material contained in the positive electrode active material layer 2 (the sum of the mass of the first positive electrode active material and the mass of the second positive electrode active material) is, for example, (1 / 98.7) or more and (1.6 / 97.8) or less. This allows for good binding. Furthermore, the small binder amount can improve the output characteristics of the lithium-ion secondary battery 30. The mass of the latex-derived aqueous polymer binder contained in the positive electrode active material layer 2 can be calculated, for example, from the mass of the aqueous polymer binder extracted from the positive electrode active material layer 2. Furthermore, the positive electrode active material layer 2 can be produced with a smaller amount of binder than a solvent-based binder. Furthermore, the binding of the positive electrode active material layer 2 to the positive electrode current collector 3 can be improved.

[0019] CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose) has the function of increasing the viscosity of the positive electrode active material slurry used to form the positive electrode active material layer 2. CMC may also function as a binder for the positive electrode active material layer 2. When the positive electrode active material slurry contains CMC, the viscosity of the positive electrode active material slurry can be increased, and the positive electrode active material layer 2 can have a uniform thickness. Furthermore, it becomes possible to apply the positive electrode active material slurry thickly, and it becomes possible to form a thick positive electrode active material layer 2 (for example, a thickness of 150 μm or more (after pressing)).

[0020] The ratio (e / c) of the mass (e) of the CMC contained in the positive electrode active material layer 2 to the mass (c) of the positive electrode active material contained in the positive electrode active material layer 2 (the sum of the mass of the first positive electrode active material and the mass of the second positive electrode active material) is, for example, (0.3 / 98.7) or more and (0.6 / 97.8) or less. This makes it possible to prevent cracks from occurring in the positive electrode active material layer 2 when the coated layer of the positive electrode active material slurry is dried.

[0021] The sum of the proportion of the latex-derived aqueous polymer binder and the proportion of CMC in the positive electrode active material layer 2 is, for example, 1.3 wt % or more and 2.0 wt % or less. This makes it possible to suppress poor bonding of the positive electrode active material layer 2 even when the positive electrode active material slurry is applied thickly in a single coating to form a positive electrode active material layer having a thickness of 150 μm or more (after pressing). Furthermore, it is possible to increase the battery capacity of a lithium-ion secondary battery having the positive electrode active material layer 2.

[0022] The ratio (d / e) of the mass (d) of the polymer binder to the mass (e) of the CMC contained in the positive electrode active material layer 2 may be 2.25 or more and 4.0 or less, which makes it possible to suppress the occurrence of cracks in the positive electrode active material layer 2 even when the positive electrode active material slurry is applied thickly in a single coating to form a positive electrode active material layer having a thickness (after pressing) of 150 μm or more.

[0023] The positive electrode may be prepared by mixing and kneading large-particle-size lithium iron phosphate secondary particles, which are the first positive electrode active material, having an average particle diameter D50 in the range of 15 μm to 22 μm; small-particle-size lithium iron phosphate secondary particles, which are the second positive electrode active material, having an average particle diameter D50 in the range of 2 μm to 7 μm; a latex of a water-based polymer binder (an emulsion in which a polymer compound serving as a binder is dispersed in a colloidal state in water); CMC; and water or an aqueous solution to prepare a positive electrode active material slurry. This slurry is then applied to a positive electrode current collector 3, and the applied layer is dried to form a positive electrode active material layer 2. If necessary, a thickener or a small amount of a surfactant may be added to the positive electrode active material slurry. The positive electrode active material layer 2 may also 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 20% to 40%. If the porosity is too low, impregnation of the electrolyte solution 15 will be poor, and if it is too high, the positive electrode active material layer 2 will become thick and the volumetric energy density will decrease. The porosity is preferably 30% to 40%.

[0024] The positive electrode active material layer 2 can be produced by repeatedly applying thin layers in a coating process. However, a positive electrode active material layer 2 produced by this method may have high resistance due to contact resistance at the interface between the layers. Furthermore, repeated application of multiple layers may dissolve the applied and dried layer in the solvent contained in the overcoated slurry, potentially causing the coating film to lose its shape. For these reasons, it is preferable to produce a uniformly thick positive electrode active material layer 2 by applying the layer once. As a reference example, the DCR resistance values ​​of electrodes with a thickness of approximately 200 μm, which were applied once and twice, are shown below. The resistance was 1.483 mΩ for the single-layer electrode and 1.523 mΩ for the two-layer electrode, demonstrating a higher resistance.

[0025] 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.

[0026] 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. 4 . 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.

[0027] 4, 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.

[0028] 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.

[0029] 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.

[0030] Preparation of positive electrode for lithium ion secondary battery 1 A mixture of carbon-coated lithium iron phosphate secondary particles A (average particle size (D50): 18 μm) and carbon-coated lithium iron phosphate secondary particles B (average particle size (D50): 4 μm) in a mass ratio of 9:1 was mixed with an acrylic polymer binder liquid (latex dispersion aqueous solution, AXA391, active ingredient concentration: 40 wt %, glass transition temperature Tg: −35°C, pH: 7 to 9, B-type viscosity: 5 to 50 cP, average particle size: 180 μm, particle size distribution: peak at 170 μm (100 to 500 μm)) and sodium carboxymethylcellulose (CMC, BSH6 manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., degree of etherification: 0.65 to 0.75, viscosity of 1% CMC aqueous solution at 25°C: 3000 to 4000 mPa s, average degree of polymerization: 1200 to 1350, average molecular weight: 255,000 to 270,000). Table 1 shows the mass ratios of the total positive electrode active material, CMC (solid content), and acrylic polymer binder (solid content), and the total CMC (solid content) and acrylic polymer binder (solid content) in the prepared positive electrode active material layer. A positive electrode active material slurry was prepared by mixing and kneading these materials with water as a dispersion medium. The positive electrode active material slurry was applied once to an aluminum foil (positive electrode current collector sheet), and the coating film was dried and further pressed to form a 221 μm-thick positive electrode active material layer on one side of the positive electrode current collector sheet, thereby preparing a positive electrode. No cracks were observed in the dried positive electrode active material layer.

[0031]

[0032] Preparation 2 of a Positive Electrode for a Lithium-Ion Secondary Battery A positive electrode was prepared by changing the mass ratio of lithium iron phosphate secondary particles A to lithium iron phosphate secondary particles B, which were mixed at a 7:3 mass ratio. Table 2 shows the mass ratios of the total positive electrode active material, CMC (solid content), and acrylic polymer binder (solid content), and the total CMC (solid content) and acrylic polymer binder (solid content), as well as the mass ratio (b / a) of the acrylic polymer binder (b) to CMC (a). A positive electrode was prepared using the same method as in Preparation 1, resulting in a positive electrode active material layer with a porosity of 34% and a thickness of 214 μm on one side of the positive electrode current collector sheet. No cracks were observed in the positive electrode active material layer after drying.

[0033]

[0034] Next, the prepared positive electrode, a polyolefin separator, a metallic lithium 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 8 and Comparative Examples 1 to 6.

[0035] Charge-Discharge Tests Charge-discharge tests were conducted using the lithium-ion secondary batteries of Examples 1 to 8 and Comparative Examples 1 to 6. Specifically, 1C charge-discharge tests were performed with an upper voltage limit of 3.6 V and a lower voltage limit of 2.0 V, and the battery capacity (discharge capacity) (mAh / g) per mass of the positive electrode active material layer was calculated based on the discharge current value. The calculated battery capacities are shown in Tables 1 and 2. Tables 1 and 2 indicate that by setting the total proportion of the acrylic polymer binder and the CMC in the positive electrode active material layer to 1.3 wt% or more and 2.0 wt% or less, the lithium-ion battery had a battery capacity of 130 mAh or more, thereby forming a good electrode. It was also found that by setting the ratio (b / a) of the mass (b) of the acrylic polymer binder to the mass (a) of the CMC contained in the positive electrode active material layer to 2.25 or more and 4.0 or less, a good electrode was formed.

[0036] Tape Peel Test A tape peel test was performed using the positive electrodes of Examples 1 and 4 and the positive electrodes of Comparative Examples 1, 3, and 4. The positive electrodes were cut into 5 cm x 5 cm pieces, and tape (Teraoka Seisakusho cloth tape No. 1535, adhesive strength: 11.53 N / 25 mm, tensile strength: 120.7 N / 25 mm, width: 50 mm) was applied to the entire positive electrode active material layer. After leaving the tape for 1 hour, the tape was pulled from the positive electrode in a 90-degree direction at a speed of 300 mm / min. The results are shown in Table 3.

[0037]

[0038] Peeling of the positive electrode active material layer was not observed in the positive electrodes of Examples 1 and 4 and the positive electrode of Comparative Example 3. Peeling was confirmed at the edges of the positive electrode active material layer of Comparative Example 1. The sum of the proportions of the acrylic polymer binder and the CMC was less than 1.3 wt%, which is thought to have weakened the adhesive strength. The positive electrode active material layer of Comparative Example 4 cracked, with a considerable number of small pieces adhering to the tape and exposing the positive electrode current collector foil. The mass ratio (b / a) of the acrylic polymer binder (b) to the CMC (a) was greater than 4.0, which is thought to have made the layer hard and prone to cracking.

[0039] For reference, the measurement results when the binder was changed to PvdF are also shown in Table 3. When the mass ratio of the binder in the positive electrode active material was 3.5 wt % (Reference Example 1), no peeling occurred, but when the mass ratio was 2.0 wt %, which is the same as that of the acrylic polymer binder (Reference Example 2), peeling occurred.

[0040] Furthermore, a plurality of positive electrodes having different positive electrode active materials in the positive electrode active material layer were fabricated and subjected to a tape peeling test. The results of the tape peeling test are shown in Table 4.

[0041]

[0042] In a positive electrode containing only secondary particles B (small particle size positive electrode active material) as the positive electrode active material layer, and in a positive electrode containing a mixture of secondary particles B (small particle size) and secondary particles A (large particle size), no peeling of the positive electrode active material layer was observed. However, in a positive electrode containing only secondary particles A (large particle size) in the positive electrode active material layer, the positive electrode active material layer cracked, with a considerable number of small pieces adhering to the tape, revealing the positive electrode current collector foil. Furthermore, when a positive electrode slurry was prepared using only secondary particles B (small particle size), the stability of the positive electrode slurry was poor, and the solids began to settle within a short time, resulting in gelation. This may result in poor productivity of the positive electrode.

[0043] Example of Lithium Ion Battery Fabrication A positive electrode was fabricated using the mass ratio of the positive electrode active material layer in Example 2, and a positive electrode active material layer (length: 142 mm, width: 84.5 mm) with a porosity of 38% and a thickness of 221 μm was formed to fabricate a positive electrode.

[0044] 94 wt% (solid content) graphite powder, 5 wt% (solid content) SBR binder, and 1 wt% (solid content) carboxymethyl cellulose sodium (CMC, EP manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were mixed. Water was added to the mixed powder and kneaded to prepare a negative electrode active material slurry. The negative electrode active material slurry was applied to a copper foil (negative electrode current collector sheet), and the applied film was dried and further pressed to form a 82.5 μm thick negative electrode active material layer (length: 145 mm, width: 86.8 mm) on one side of the negative electrode current collector sheet, thereby producing a negative electrode.

[0045] Next, the fabricated positive electrode, a separator (a three-layer separator manufactured by Toray Industries, Inc.), the fabricated negative electrode, and a non-aqueous electrolyte (1.2 M LiPF electrolyte (carbonate-based electrolyte)) were placed in a battery case to fabricate a lithium ion secondary battery. As a result, a lithium ion secondary battery with a battery capacity of 63 Ah was fabricated.

[0046] 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 positive electrode active material layer, the positive electrode active material layer including a positive electrode active material and a water-based polymer binder derived from latex, and having a thickness of 150 μm or more.

2. The positive electrode according to claim 1, wherein the positive electrode active material layer comprises first and second positive electrode active materials having different particle sizes, the first positive electrode active material having an average particle size of 15 μm or more and 22 μm or less, and the second positive electrode active material having an average particle size of 2 μm or more and 7 μm or less.

3. The positive electrode according to claim 2, wherein the ratio (b / a) of the mass (b) of the second positive electrode active material to the mass (a) of the first positive electrode active material is from (1 / 9) to (3 / 7).

4. The positive electrode according to claim 3, wherein the first and second positive electrode active materials contained in the positive electrode active material layer are each lithium iron phosphate secondary particles.

5. The positive electrode according to claim 1, wherein the positive electrode active material layer further contains sodium carboxymethylcellulose or calcium carboxymethylcellulose, and a sum (c+d) of a mass (c) of the aqueous polymer binder and a mass (d) of the sodium carboxymethylcellulose or calcium carboxymethylcellulose contained in the positive electrode active material layer is 1.3 wt % or more and 2.0 wt % or less with respect to a mass of the positive electrode active material layer.

6. The positive electrode according to claim 5, wherein a ratio (c / d) of a mass (c) of the aqueous polymer binder contained in the positive electrode active material layer to a mass (d) of the sodium carboxymethylcellulose or calcium carboxymethylcellulose contained in the positive electrode active material layer is 2.25 or more and 4.0 or less.

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

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