WO2025115780A1 - Positive electrode for lithium ion secondary batteries, and lithium ion secondary battery - Google Patents

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

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

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 then drying the applied layer.

N-methylpyrrolidone is on the REACH restricted substances list due to concerns about its toxicity.
The present invention has been made in view of the above 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.

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

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.

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

Hereinafter, an embodiment of the present invention will be described 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.
The positive electrode 5 for a lithium ion secondary battery of this embodiment includes a positive electrode active material layer 2. The positive electrode active material layer 2 contains a positive electrode active material and a water-based polymer binder derived from latex, and is characterized in that it has a thickness of 150 μm or more.
The lithium ion secondary battery 30 of this embodiment includes the positive electrode 5 of this embodiment, a negative electrode 32, a separator 34, and a non-aqueous electrolyte 15.

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 serving as a base material for providing the positive electrode active material layer 2, and is a conductor that electrically connects the positive electrode connection member 13 and the positive electrode active material layer 2. The positive electrode current collector 3 is, for example, an aluminum foil. The positive electrode connection member 13 is electrically connected to the 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.

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, and preferably 200 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. Furthermore, the amount of the current collector 3 contained in the lithium ion secondary battery 30 can be reduced, and the mass energy density of the lithium ion secondary battery 30 can be increased.
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. For this reason, the thickness of the positive electrode active material layer 2 is preferably 500 μm or less, and more preferably 400 μm or less.

The positive electrode active material layer 2 includes a positive electrode active material and a water-based polymer binder derived from latex. The positive electrode active material layer 2 may also include CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose). The positive electrode active material layer 2 may also include a thickener. The positive electrode active material layer 2 may also include a conductive assistant. The positive electrode active material layer 2 may also include a surfactant.

The positive electrode active material is preferably lithium iron phosphate ( _LiFePO4 ), although other positive electrode materials may be used as long as they are compatible with the solvent of the positive electrode slurry.
As the lithium iron phosphate, lithium iron phosphate secondary particles, which are secondary particles formed by agglomeration of primary particles of lithium iron phosphate (LiFePO ₄ ), are preferred. The lithium iron phosphate primary particles may have a conductive film on the surface. This can improve the conductivity of the fine particle surface where the intercalation reaction proceeds, and can reduce the internal resistance of the positive electrode 5. The conductive film is, for example, a carbon film. 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 in a cross-sectional photograph of the positive electrode active material layer 2 and arithmetically averaging these particle sizes.

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 smaller particle size than the first lithium iron phosphate secondary particles.
The large particle size secondary lithium iron phosphate particles are lithium iron phosphate secondary particles having a particle size of 10 μm or more among the multiple 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 secondary lithium iron phosphate particles are lithium iron phosphate secondary particles having a particle size of less than 10 μm among the multiple 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.

For example, the particle diameters of 100 randomly selected lithium iron phosphate secondary particles from a cross-sectional photograph of the positive electrode active material layer 2 can be measured, and the average particle diameter of large-diameter lithium iron phosphate can be obtained by 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-diameter 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.

For example, if the particle diameters of 100 randomly selected lithium iron phosphate secondary particles from a cross-sectional photograph of the positive electrode active material layer 2 are measured and a particle size distribution of the lithium iron phosphate secondary particles is created with the horizontal axis representing particle diameter and the vertical axis representing number, the particle size distribution can have a peak for small particle size lithium iron phosphate in the range of 2 μm to 7 μm and a peak for large particle size lithium iron phosphate 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 of the large particle size lithium iron phosphate and the small particle size lithium iron phosphate.

The ratio a:b of the mass (b) of the second positive electrode active material to the mass (a) of the first 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). By setting this ratio, the positive electrode slurry and the positive electrode active material layer 2 are stabilized. In addition, the pressing property of the positive electrode active material layer 2 is improved, and the porosity can be easily controlled.
The masses of the first and second positive electrode active materials 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. In addition, the positive electrode active material layer 2 can be dissolved in a solvent or the like, and the positive electrode active material can be recovered and classified to measure the density.

The conductive additive is, for example, furnace black, acetylene black, carbon black, or fine particles of coke-based soft carbon. By including a conductive additive in the positive electrode active material layer 2, 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.

The latex-derived aqueous polymer binder is a polymer binder that can be dispersed in a colloidal state in an aqueous dispersion medium. The aqueous polymer binder is, for example, an acrylic polymer binder, styrene butadiene rubber (SBR), etc. By using such a polymer binder, it becomes possible to use water or an aqueous solution as a dispersion medium of the positive electrode active material slurry used to form the positive electrode active material layer 2. The glass transition point Tg of the polymer compound that is the polymer binder is, for example, 20° C. or less, preferably 10° C. or less, and more preferably 0° C. or less. This allows the polymer binder to have flexibility and further improves binding properties.
By including an aqueous polymer binder in the positive electrode active material layer 2, it is possible to prevent the positive electrode active material layer 2 from peeling off from the positive electrode current collector 3 and to prevent the positive electrode active material layer 2 from cracking.

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 good binding properties to be obtained. Furthermore, since the amount of binder is small, the output characteristics of the lithium ion secondary battery 30 can be improved. 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.
Moreover, the positive electrode active material layer 2 can be produced with a smaller amount of binder than in the case of a solvent-based system.
Furthermore, the adhesion of the positive electrode active material layer 2 to the positive electrode current collector 3 can be improved.

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. By including CMC in the positive electrode active material slurry, 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. In addition, 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)).

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 suppress the occurrence of cracks in the positive electrode active material layer 2 when the coating layer of the positive electrode active material slurry is dried.

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 adhesion of the positive electrode active material layer 2 even when the positive electrode active material slurry is applied thickly in one application to form a positive electrode active material layer having a thickness of 150 μm or more (after pressing). In addition, the battery capacity of a lithium-ion secondary battery having the positive electrode active material layer 2 can be increased.

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. This 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 application to form a positive electrode active material layer having a thickness (after pressing) of 150 μm or more.

The positive electrode can be prepared by mixing and kneading a first positive electrode active material, large-diameter lithium iron phosphate secondary particles having an average particle diameter D50 in the range of 15 μm to 22 μm, a second positive electrode active material, small-diameter lithium iron phosphate secondary particles 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, which 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 may be added to the positive electrode active material slurry, 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, for example, 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 the range of 20% to 40%. If the porosity is too low, impregnation of the electrolyte 15 will be poor, and if it is too high, the positive electrode active material layer 2 will become too thick and the volumetric energy density will decrease. The porosity is preferably 30% to 40%.

The positive electrode active material layer 2 may be produced by coating thin layers in a coating process, but the positive electrode active material layer 2 produced by coating may have high resistance due to contact resistance at the interface between the layers. In addition, when coating is performed, the coated and dried layer may be dissolved by the solvent contained in the overcoated slurry, and the shape of the coating film may be distorted. For these reasons, it is preferable to produce a uniform thick layer of the positive electrode active material layer 2 by coating once.
As a reference example, the DCR resistance values of an electrode having a thickness of about 200 μm when applied once and twice are shown below.
It can be seen that the resistance is larger, ie, 1.483 mΩ for the single-layer electrode and 1.523 mΩ for the two-layer electrode.

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-shaped negative electrode collector 38. The negative electrode 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 collector 38 is, for example, a copper foil.
The negative electrode active material is a material that is directly involved in the transfer of electrons accompanying charge transfer in the negative electrode. Examples of the negative electrode active material include graphite, partially graphitized carbon, hard carbon, soft carbon, lithium titanate (LTO), Sn alloy, etc. The negative electrode active material layer 36 can contain one or more of these negative electrode active materials in combination.

The separator 34 is in a sheet form 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 form an electrode laminate 22 as shown in Fig. 4. By providing the separator 34, it is possible to 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, but can be, for example, a microporous film of polyolefin, a cellulose sheet, an aramid sheet, etc. 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.

As shown in FIG. 4, the electrode laminate 22 can have a structure in which a plurality of positive electrodes 5 and a plurality of negative electrodes 32 are laminated so that the positive electrodes 5 and the negative electrodes 32 are arranged alternately. The electrode laminate 22 can also have a structure in which a separator 34 is arranged between adjacent positive electrodes 5 and negative electrodes 32.

The non-aqueous electrolyte 15 may use carbonates, lactones, ethers, esters, ionic liquids, etc. as a solvent, and may also use a mixture of two or more of these solvents. Among these, it is particularly preferable to use a mixture of a cyclic carbonate and a chain carbonate. The non-aqueous electrolyte 15 is a solution in which a lithium salt solute such as LiCF ₃ SO ₃ , LiAsF ₆ , LiClO ₄ , LiBF ₄ , LiPF ₆ , LiBOB, LiN(CF ₃ SO ₂ ) ₂ , or LiN(C ₂ F ₅ SO ₂ ) is dissolved in an organic solvent. In addition, additives such as VC (vinylene carbonate), PS (propane sultone), VEC (vinyl ethyl carbonate), PRS (propene sultone), and flame retardants may be mixed alone or in combination as necessary.

The battery case 11 is a battery exterior housing that houses the electrode laminate 22 (including the positive electrode 5, the negative electrode 32, and the separator 34) and the non-aqueous 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.

Preparation of positive electrode for lithium ion secondary battery 1
A mixture of carbon-coated lithium iron phosphate secondary particles A (average particle diameter (D50): 18 μm) and carbon-coated lithium iron phosphate secondary particles B (average particle diameter (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 point Tg: -35°C, pH: 7 to 9, B-type viscosity: 5 to 50 cP, average particle diameter: 180 μm, particle diameter distribution: peak at 170 μm (100 to 500 μm)) and sodium carboxymethylcellulose (CMC, Daiichi Kogyo Seiyaku Co., Ltd. BSH6, 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: 255000 to 270000). Table 1 shows the total mass ratio of the positive electrode active material in the positive electrode active material layer, CMC (solid content), acrylic polymer binder (solid content), and the total mass ratio of CMC (solid content) and acrylic polymer binder (solid content). The 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 on an aluminum foil (positive electrode current collector sheet), and the coating film was dried and further pressed to form a positive electrode active material layer with a thickness of 221 μm on one side of the positive electrode current collector sheet to prepare a positive electrode. No cracks were generated in the positive electrode active material layer after drying.

Preparation of positive electrode for lithium ion secondary battery 2
A positive electrode was prepared by changing the ratio of lithium iron phosphate secondary particles A and lithium iron phosphate secondary particles B to a mixture of 7:3 by mass. Table 2 shows the total mass ratio of the positive electrode active material in the prepared positive electrode active material layer, CMC (solid content), acrylic polymer binder (solid content), and the total mass ratio of CMC (solid content) and acrylic polymer binder (solid content), and the mass ratio (b/a) of the acrylic polymer binder (b) to CMC (a). When a positive electrode was prepared in the same manner as in Preparation 1, a positive electrode active material layer with a porosity of 34% and a thickness of 214 μm was formed on one side of the positive electrode current collector sheet. No cracks were generated in the positive electrode active material layer after drying.

Next, the prepared positive electrode, a polyolefin separator, a metallic lithium negative electrode, and a non-aqueous electrolyte ( ₁ M LiPF6 electrolyte (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.

Charge/Discharge Test Charge/Discharge tests were performed using the lithium ion secondary batteries of Examples 1 to 8 and Comparative Examples 1 to 6. Specifically, 1C charge/discharge was performed with the upper limit voltage set to 3.6 V and the lower limit voltage set to 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.
From Tables 1 and 2, it was found that by setting the total ratio 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, a lithium ion battery has a battery capacity of 130 mAh or more, and therefore a good electrode is formed. 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 is formed.

Tape Peel Test A tape peel test was carried out using the positive electrodes of Examples 1 and 4 and the positive electrodes of Comparative Examples 1, 3 and 4.
The positive electrode was cut to 5 cm x 5 cm, 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 and left for 1 hour, after which 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.

In the positive electrodes of Examples 1 and 4 and the positive electrode of Comparative Example 3, no peeling of the positive electrode active material layer was observed.
It was confirmed that the positive electrode active material layer in Comparative Example 1 peeled off at the edge portions. This is considered to be because the total ratio of the acrylic polymer binder and the CMC was less than 1.3 wt %, and the binding strength was weak.
The positive electrode active material layer of Comparative Example 4 was cracked, with a considerable amount of small pieces sticking to the tape, and the positive electrode current collector foil was exposed. It is considered that the mass ratio (b/a) of the acrylic polymer binder (b) to the CMC (a) was greater than 4.0, making the layer hard and prone to cracking.

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 it was 2.0 wt%, which is the same mass ratio as the acrylic polymer binder (Reference Example 2), peeling occurred.

In addition, a plurality of positive electrodes having different positive electrode active materials contained in the positive electrode active material layer were fabricated and subjected to a tape peel test.
The results of the tape peel test are shown in Table 4.

Peeling of the positive electrode active material layer was not observed in the positive electrode in which the positive electrode active material layer contained only secondary particles B (positive electrode active material with small particle size) and in which the positive electrode active material layer contained a mixture of secondary particles B (small particle size) and secondary particles A (large particle size). However, in the positive electrode in which the positive electrode active material layer contained only secondary particles A (large particle size), the positive electrode active material layer cracked and a considerable amount of small pieces stuck to the tape, exposing the positive electrode current collector foil.
In addition, when a positive electrode slurry was prepared using only the secondary particles B (small particle size), it was observed that the stability of the positive electrode slurry was poor, and the solid content began to settle in a short time, causing gelation. This may result in poor productivity of the positive electrode.

A positive electrode was prepared using the mass ratio of the positive electrode active material layer in Example 2 of the lithium ion battery preparation example , 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 prepare a positive electrode.

94 wt% (solids) graphite powder, 5 wt% (solids) SBR binder, and 1 wt% (solids) sodium carboxymethylcellulose (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 copper foil (negative electrode current collector sheet), and the applied film was dried and further pressed to form a negative electrode active material layer (length: 145 mm, width: 86.8 mm) with a thickness of 82.5 μm on one side of the negative electrode current collector sheet, thereby producing a negative electrode.

Next, the prepared positive electrode, a separator (a three-layer separator manufactured by Toray Industries, Inc.), the prepared negative electrode, and a non-aqueous electrolyte (1.2 M _LiPF6 electrolyte (carbonate-based electrolyte)) were placed in a battery case to prepare a lithium-ion secondary battery. As a result, a lithium-ion secondary battery with a battery capacity of 63 Ah was prepared.

2: Positive electrode active material layer 3: Positive electrode current collector 5: Positive electrode 11: Battery case 12: Lid member 13: Positive electrode connection member 14: Negative electrode connection member 15: Non-aqueous electrolyte 18a, 18b: External connection terminal 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 (7)

A positive electrode active material layer is provided,
The positive electrode active material layer comprises a positive electrode active material and a water-based polymer binder derived from latex, and has a thickness of 150 μm or more. the positive electrode active material layer has first and second positive electrode active materials having different particle sizes,
The first positive electrode active material has an average particle size of 15 μm or more and 22 μm or less,
The positive electrode according to claim 1 , wherein the second positive electrode active material has an average particle size of 2 μm or more and 7 μm or less. 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 (1/9) or more and (3/7) or less. 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. The positive electrode active material layer further contains sodium carboxymethylcellulose or calcium carboxymethylcellulose,
2. The positive electrode according to claim 1, wherein 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. The positive electrode according to claim 5, wherein the ratio (c/d) of the mass (c) of the aqueous polymer binder contained in the positive electrode active material layer to the 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. 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.