Slurry for cathode films, cathode films, secondary batteries

Abstract

The present invention provides a slurry for cathode films, a cathode film, and a secondary battery that exhibit excellent viscosity stability. [Solution] A slurry for a cathode film containing a cathode active material comprising a manganese iron lithium phosphate-based cathode active material represented by the following chemical formula (I), a dispersant, a conductive material, a binder, and a non-aqueous solvent, wherein the conductive material comprises at least one selected from carbon nanotubes and carbon black, the dispersant comprises a dispersant (A) with an acid value of 35 mg KOH / g or more and 150 mg KOH / g or less, and the binder comprises a vinylidene fluoride-based polymer with an acid value of 1.0 mg KOH / g or more and 30.0 mg KOH / g or less. Chemical formula (I): Li 1+a Fe 1-x-y Mn x M y (PO 4-b )X b [In formula (I), M is selected from Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, and X is selected from F, S, and N.]

Description

Technical Field

[0001] Embodiments of the present invention relate to a slurry for a positive electrode film, a positive electrode film, and a secondary battery.

Background Art

[0002] With the spread of electric vehicles, the miniaturization, weight reduction, and high performance of portable devices, secondary batteries having a high energy density have attracted attention, and an increase in the capacity of secondary batteries is required. Under such circumstances, secondary batteries typified by lithium-ion secondary batteries have come to be used in many devices because of their characteristics of high energy density and high voltage.

[0003] The positive electrode of a secondary battery is manufactured by coating a current collector with a slurry for a positive electrode film containing a positive electrode active material. In the case of a secondary battery having a high energy density and a high capacity, generally, a lithium transition metal composite oxide is used as the positive electrode active material. Specifically, the lithium transition metal composite oxide is lithium cobaltate, lithium manganate, lithium nickelate, a compound in which some of these metal elements are substituted with different metal elements, etc., and has a layered or spinel crystal structure. The lithium transition metal composite oxide is characterized by having a high potential of about 4 V to about 5 V with respect to metallic lithium depending on the crystal structure, the type of metal element, and the mixing ratio of the metal elements. However, these lithium transition metal composite oxides have problems such as thermal stability, performance degradation due to compositional changes during charge and discharge, high cost due to the use of rare metals, and raw material supply instability.

[0004] As these improvement measures, lithium iron phosphate-based positive electrode active materials containing iron, which is abundant and inexpensive in resources and has excellent structural stability during charge and discharge, have attracted attention. The lithium iron phosphate-based positive electrode active material is expected as a highly practical material because it has a relatively high potential of about 3.4 V with respect to metallic lithium. [[ID=2...]] However, lithium iron phosphate-based cathode active materials with an olivine structure, represented as LiFePO4 (hereinafter also referred to as lithium iron phosphate), are crystals with very poor electronic conductivity compared to layered and spinel-type cathode active materials, and the conductivity of lithium ions within the crystal is also poor.

[0005] Therefore, methods are being investigated to improve the conductivity of lithium iron phosphate by coating its surface with a conductive carbon material, thereby increasing the discharge capacity of the battery (Patent Documents 1 and 2). Furthermore, Patent Document 3 discloses a method for producing a high-performance electrode using a composition containing lithium iron phosphate coated with a carbon material and carbon nanotubes (CNTs).

[0006] Furthermore, in order to improve the electrical conductivity and ionic conductivity of lithium iron phosphate-based cathode active materials, it has been investigated to improve electrical conductivity by coating the surface of the lithium iron phosphate-based cathode active material with carbon and to improve ionic conductivity by reducing the particle size of the lithium iron phosphate-based cathode active material (Patent Documents 4 and 5).

[0007] Patent Document 4 proposes that, in order to improve the dispersibility of lithium iron phosphate-based cathode active materials with reduced particle size, a predispersion of the cathode active material is prepared using hydrogenated nitrile butadiene rubber as a dispersant, and then a slurry composition for a cathode film is prepared by adding a conductive material and a binder. Patent Document 5 proposes a method for producing a cathode slurry by preparing a paste containing lithium iron phosphate-based cathode active material, a conductive material, a binder, and a solvent, in order to improve the dispersibility of lithium iron phosphate-based cathode active material with reduced particle size, and then mixing it while adding the solvent.

[0008] On the other hand, techniques are being investigated to improve the conductivity of a cathode by adding conductive materials such as carbon nanotubes to a slurry for cathode films to enhance their dispersibility. For example, the use of hydrogenated nitrile butadiene rubber is being considered to improve the dispersibility of conductive materials (Patent Documents 6 and 7). Patent Document 6 discloses a conductive material dispersion in which the dispersibility of the conductive material is enhanced by using copolymer A containing nitrile group-containing structural units and aliphatic hydrocarbon structural units as a dispersant, and further discloses providing an electrode film slurry by adding a binder resin and an electrode active material to this conductive material dispersion. Patent Document 7 discloses a carbon nanotube dispersion in which bundled carbon nanotubes are dispersed using a partially hydrogenated nitrile rubber in which the residual double bond value, which is the mass ratio of structural units derived from conjugated dienes to the total amount of structural units derived from hydrogenated conjugated dienes and structural units derived from hydrogenated conjugated dienes, is 0.5 to 40% by mass. Furthermore, it is disclosed that an electrode slurry is provided by adding a binder resin and an electrode active material to this carbon nanotube dispersion.

[0009] Patent Document 8 discloses a method for producing an electrode film slurry comprising a carbon nanotube dispersion containing carbon nanotubes and a polymer containing nitrile group-containing structural units, an active material, a low molecular weight acidic compound having a molecular weight of less than 10,000, and a fluorine-based polymer. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2016-149296 [Patent Document 2] Re-tabled publication 2017 / 154592 [Patent Document 3] Special Publication No. 2012-521065 [Patent Document 4] Special Publication No. 2020-515014 [Patent Document 5] Special Publication No. 2020-515012 [Patent Document 6] Patent No. 6933285 [Patent Document 7] Special Publication No. 2018-522803 [Patent Document 8] Japanese Patent Publication No. 2023-96879 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] As mentioned above, various studies have been conducted on slurries for electrode films, and it is known that partially hydrogenated nitrile butadiene rubber can contribute to improving the dispersibility of lithium iron phosphate-based cathode active materials. In the technologies described in Patent Documents 4 and 5, based on the finding that partially hydrogenated nitrile butadiene rubber has low solubility in solvents, the dispersibility of lithium iron phosphate is obtained by adjusting the order and method of using the partially hydrogenated nitrile butadiene rubber. Furthermore, in the technologies described in Patent Documents 4 and 5, a positive electrode containing conductive materials such as carbon materials in addition to lithium iron phosphate is prepared. However, it tends to be difficult to maintain good dispersibility with partially hydrogenated nitrile butadiene rubber until the lithium iron phosphate and carbon material are mixed.

[0012] According to the technologies of Patent Documents 6 and 7, hydrogenated nitrile butadiene rubber also contributes to the dispersibility of conductive materials such as carbon nanotubes. In particular, in the technology of Patent Document 7, the residual double bond value of hydrogenated nitrile butadiene rubber is adjusted to 40% by mass or less in order to disperse carbon nanotubes. The above-mentioned hydrogenated nitrile butadiene rubber is used to disperse conductive materials such as carbon nanotubes and has a different structure from the hydrogenated nitrile butadiene rubber disclosed in Patent Documents 4 and 5 for dispersing lithium iron phosphate.

[0013] As mentioned above, various studies have been conducted to improve the dispersibility of cathode film slurries using partially hydrogenated nitrile butadiene rubber. However, sufficient research has not been done to achieve dispersibility of cathode active materials such as lithium iron phosphate while simultaneously dispersing conductive materials such as carbon nanotubes in cathode film slurries. Furthermore, partially hydrogenated nitrile butadiene rubber has carbon-carbon double bonds that remain unhydrogenated as unsaturated bonds. Therefore, if the partially hydrogenated nitrile butadiene rubber oxidizes while contained in the electrode, it can cause a decrease in battery performance. Since the oxidation resistance of the dispersant affects the viscosity stability, electrode characteristics, and ultimately battery characteristics of the cathode film slurry, there is a demand for cathode film slurries composed using dispersants with excellent oxidation resistance.

[0014] The dispersibility of each component constituting the cathode film slurry, such as lithium iron phosphate (a positive electrode active material), conductive material, and binder, greatly affects viscosity stability, electrode characteristics, and even battery characteristics. Therefore, there is a demand for further improvement in dispersibility. In contrast, Patent Document 8 discloses a method for improving the viscosity stability of an electrode film slurry by suppressing the gelation of a fluorine-based polymer through the addition of a low molecular weight acidic compound. However, the disclosed method aims to improve the viscosity stability of the electrode film slurry by suppressing the gelation of the fluorine-based polymer, and has not adequately considered how to improve the dispersibility of positive electrode active materials such as lithium iron phosphate and conductive materials.

[0015] In cathode film slurries, if the dispersion of the cathode active material is poor and coarse particles exist due to aggregation, spontaneous discharge occurs in the battery, leading to a decrease in voltage and a deterioration of high-temperature storage characteristics. However, it is difficult to obtain sufficiently satisfactory dispersion in conventional cathode film slurries containing lithium iron phosphate-based cathode active material, conductive material, and binder. Therefore, there is a demand for cathode film slurries that can improve the dispersion of cathode active material such as lithium iron phosphate and conductive material, and form cathode films with excellent viscosity stability and superior electrode characteristics.

[0016] Among positive electrode active materials, lithium manganese iron phosphate has a larger specific surface area and smaller particle size than lithium iron phosphate. Therefore, when lithium manganese iron phosphate is used, the viscosity of the positive electrode slurry tends to increase and viscosity stability is poor, which may result in defects such as uneven coating during electrode film fabrication. Uneven coating of the electrode film can lead to differences in the amount of active material per electrode, which in turn can lead to differences in battery capacity and make it difficult to control the battery voltage. Furthermore, when lithium manganese iron phosphate is used, its higher specific surface area and smaller particle size compared to lithium iron phosphate makes it more prone to binder deficiency, which tends to reduce electrode adhesion. Thus, when using lithium manganese iron phosphate as a positive electrode active material, there is a demand for improved adhesion of the positive electrode film and improved battery characteristics.

[0017] In view of these circumstances, an embodiment of the present invention provides a slurry for a positive electrode film with excellent viscosity stability. Another embodiment of the present invention provides a positive electrode film with excellent conductivity and adhesion. Yet another embodiment of the present invention provides a secondary battery with excellent high-temperature storage characteristics and cycle characteristics. [Means for solving the problem]

[0018] The inventors focused on manganese iron lithium phosphate-based positive electrode active materials among positive electrode active materials and diligently investigated the dispersibility of a slurry for a positive electrode film using manganese iron lithium phosphate-based positive electrode active material and a conductive material. Since manganese iron lithium phosphate-based positive electrode active material tends to have lower conductivity than lithium iron phosphate-based positive electrode active material, it is preferable to use it in combination with a conductive material. Furthermore, when a secondary battery is stored at high temperatures, there is a risk of manganese and other substances leaching from the manganese iron lithium phosphate-based positive electrode active material into the electrolyte. However, the inventors found that by uniformly dispersing and coating the positive electrode active material with a conductive material, particularly carbon nanotubes, a secondary battery with excellent cycle characteristics can be provided. Moreover, they found that when a dispersant with a specific acid value and a binder with a specific acid value are used with such a combination of manganese iron lithium phosphate-based positive electrode active material and a conductive material, aggregation and sedimentation of the positive electrode active material and conductive material can be effectively suppressed, thus completing the present invention. Embodiments of the present invention relate to the following. However, the present invention is not limited to the following embodiments and includes various embodiments.

[0019] <1> A slurry for a cathode film containing a cathode active material, a dispersant, a conductive material, a binder, and a non-aqueous solvent, comprising a lithium iron manganese phosphate-based cathode active material represented by the following chemical formula (I), The conductive material comprises at least one selected from the group consisting of carbon nanotubes and carbon black. The aforementioned dispersant includes a dispersant (A) having an acid value of 35 mg KOH / g or more and 150 mg KOH / g or less. The amount of the dispersant is 0.01 parts by mass or more and 8 parts by mass or less per 100 parts by mass of the positive electrode active material. The binder is a slurry for cathode films containing a vinylidene fluoride polymer and having an acid value of 1.0 mg KOH / g or more and 30.0 mg KOH / g or less. Chemical formula (I): Li 1+a Fe 1-x-y Mn x M y (PO 4-b )X b [In the above chemical formula (I), M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X is at least one element selected from the group consisting of F, S, and N, a, b, x, and y are integers satisfying -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, 0.1 ≤ x ≤ 0.8, and 0 ≤ y ≤ 0.1, respectively.

[0020] <2> The average particle diameter D of the positive electrode active material 50 is 0.05 μm or more and 3.0 μm or less, and the slurry for a positive electrode film according to <1> above.

[0021] <3> The BET specific surface area of the positive electrode active material determined by nitrogen adsorption measurement is 1 m 2 / g or more and 50 m 2 / g or less, and the slurry for a positive electrode film according to <1> or <2> above.

[0022] <4> The lithium iron manganese phosphate-based positive electrode active material has a carbon coating on its surface, and the slurry for a positive electrode film according to any one of <1> to <3> above.

[0023] <5> The dispersant (A) contains an organic compound having at least one acidic group selected from the group consisting of -COOM, -SO3M, and -PO(OM)2 [where M represents a hydrogen atom, a quaternary amine, or an alkali metal], and the slurry for a positive electrode film according to any one of <1> to <4> above.

[0024] <6> The dispersant further contains a dispersant (B) different from the dispersant (A), and the slurry for a positive electrode film according to any one of <1> to <5> above.

[0025] <7> The dispersant (B) contains at least one selected from polyvinylpyrrolidone-based resins, polyvinylacetal-based resins, and optionally hydrogenated acrylonitrile-butadiene rubber, and the slurry for a positive electrode film according to <6> above.

[0026] <8> The dispersant (B) comprises a dispersant (B1), wherein the dispersant (B1) is a copolymer having structural units having nitrile groups and structural units derived from a conjugated diene in which the unsaturated bonds are partially hydrogenated, and the mass ratio (RDB) of residual double bonds calculated by the following formula (1) is 0.05 to 5% by mass. <6> or <7> A slurry for cathode films as described above. Formula (1): RDB(mass%)=(BD / (BD+HBD))×100 [In formula (1) above, BD is the mass ratio of structural units derived from conjugated dienes having unsaturated bonds to the total mass of structural units derived from conjugated dienes.] HBD is the mass ratio of structural units derived from conjugated dienes with hydrogenated unsaturated bonds to the total mass of structural units derived from conjugated dienes.

[0027] <9> The conductive material content is 0.1 parts by mass or more and 3 parts by mass or less per 100 parts by mass of solid content of the slurry for the positive electrode film. <1> ~ <8> A slurry for cathode films as described in any one of the following.

[0028] <10> The intrinsic viscosity of the vinylidene fluoride polymer, measured at 25°C after dissolving in N-methyl-2-pyrrolidone, is 0.5 dL / g or more and less than 3.5 dL / g. <1> ~ <9> A slurry for cathode films as described in any one of the following.

[0029] <11> the above <1> ~ <10> A positive electrode film formed using a positive electrode film slurry described in any one of the following.

[0030] <12> It contains a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is as described above. <1> ~ <10> A secondary battery comprising a positive electrode film formed using a positive electrode film slurry described in any one of the above. [Effects of the Invention]

[0031] According to embodiments of the present invention, a slurry for a positive electrode film with excellent viscosity stability can be provided. Furthermore, a positive electrode film with excellent conductivity and adhesion can be provided. In addition, a secondary battery with excellent high-temperature storage characteristics and cycle characteristics can be provided. [Modes for carrying out the invention]

[0032] The following describes in detail embodiments of the present invention, including a slurry for a cathode film, a cathode film, and a secondary battery. The present invention is not limited to the following embodiments, and includes embodiments that are implemented without changing the essence of the invention.

[0033] 1. Slurry for cathode film One embodiment of the present invention relates to a slurry for a cathode film containing a cathode active material including a lithium iron manganese phosphate-based cathode active material represented by chemical formula (I), a dispersant, a conductive material, a binder, and a non-aqueous solvent. The conductive material comprises at least one selected from the group consisting of carbon nanotubes and carbon black. The dispersant comprises a dispersant (A) having an acid value of 35 mg KOH / g or more and 150 mg KOH / g or less, and the content of the dispersant is 0.01 parts by mass or more and 8 parts by mass or less per 100 parts by mass of the positive electrode active material. The binder is characterized by containing a vinylidene fluoride polymer and having an acid value of 1.0 mg KOH / g or more and 30.0 mg KOH / g or less.

[0034] The slurry for the positive electrode film according to this embodiment contains a dispersant (A) with an acid value of 35 mg KOH / g to 150 mg KOH / g and a binder containing a vinylidene fluoride polymer with an acid value of 1.0 mg KOH / g to 30.0 mg KOH / g. This improves the dispersibility of the lithium manganese iron phosphate positive electrode active material and conductive material, suppresses aggregation and sedimentation, and improves viscosity stability. Furthermore, the positive electrode film obtained using this slurry has excellent conductivity and adhesion because the lithium manganese iron phosphate positive electrode active material is uniformly coated. Moreover, a secondary battery equipped with this positive electrode film can achieve excellent lifespan characteristics because the deterioration of the components constituting the secondary battery, such as the lithium manganese iron phosphate positive electrode active material, dispersant, and vinylidene fluoride polymer, is suppressed even during long-term use and high-temperature storage.

[0035] The following provides a detailed explanation of the components of the slurry for the positive electrode film. <Cathode active material> The positive electrode active material includes at least a lithium iron manganese phosphate (LMFP)-based positive electrode active material, and may optionally include other positive electrode active materials containing lithium. The content of the positive electrode active material is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, and even more preferably 90 parts by mass or more, based on 100 parts by mass of the total solid content in the slurry for the positive electrode film. The above content is preferably 99 parts by mass or less, more preferably 98 parts by mass or less, and even more preferably 97 parts by mass or less.

[0036] (Lithium iron manganese phosphate cathode active material) The cathode film slurry of this embodiment contains a manganese iron lithium (LMFP)-based cathode active material represented by the following chemical formula (I). Chemical formula (I): Li 1+a Fe 1-x-y Mn x M y (PO 4-b )X b

[0037] In the above chemical formula (I), M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y. X is at least one element selected from the group consisting of F, S, and N. a, b, x, and y are integers that satisfy -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, 0.1 ≤ x ≤ 0.8, and 0 ≤ y ≤ 0.1, respectively.

[0038] In some embodiments, a may preferably be -0.3 ≤ a ≤ 0.3, more preferably -0.2 ≤ a ≤ 0.2, and even more preferably -0.1 ≤ a ≤ 0.1. The value of b may preferably be 0 ≤ b ≤ 0.08, and more preferably 0 ≤ b ≤ 0.05. In one embodiment, the value of b may preferably be 0. The value of x may preferably be 0.3 ≤ x ≤ 0.8, more preferably 0.4 ≤ x ≤ 0.8, even more preferably 0.5 ≤ x ≤ 0.7, and even more preferably 0.6 ≤ x ≤ 0.7. The value of y may preferably be 0 ≤ y ≤ 0.08, and more preferably 0 ≤ y ≤ 0.05. In one embodiment, the value of y may preferably be 0.

[0039] In some embodiments, the lithium manganese iron phosphate-based cathode active material represented by the above chemical formula (I) (hereinafter referred to as "LMFP-based cathode active material") preferably contains the compound represented by the following chemical formula (I-1). Chemical formula (I-1): Li 1+a Fe 1-x Mn x (PO4) In the formula, a and x are as described above.

[0040] While not particularly limited, LiFe is a specific example of an LMFP-based cathode active material. 0.4 Mn 0.6 PO4, LiFe 0.3 Mn 0.7 PO4, LiFe 0.33 Mn0.62 Zn 0.05 PO4, LiFe 0.33 Mn 0.59 Zn 0.07 PO4, LiFe 0.2 Mn 0.75 Mg 0.05 PO4 is one example.

[0041] In some embodiments, from the viewpoint of conductivity, it is preferable that the surface of the primary particles of the LMFP-based positive electrode active material is coated with a carbon material. The coating with carbon material (hereinafter referred to as carbon coating) may cover the entire surface or a part of the surface of the LMFP-based positive electrode active material. Furthermore, "coating" means a state in which the LMFP-based positive electrode active material and the carbon material are in contact. The state of carbon coating on the surface of the primary particles of the LMFP-based positive electrode active material can be confirmed by TEM (transmission electron microscope) observation.

[0042] When using LMFP-based positive electrode active material with a carbon coating, a decrease in the amount of LMFP-based active material with an olivine structure in the positive electrode leads to a decrease in the amount of lithium involved in charging and discharging. Furthermore, the discharge capacity per unit mass or volume of the battery may decrease, making it difficult to construct a highly practical battery. Therefore, it is preferable to appropriately adjust the carbon coating content (carbon coating ratio).

[0043] In some embodiments, from the viewpoint of increasing the content of LMFP-based cathode active material having an olivine structure in the cathode, the carbon coating ratio is preferably 10% by mass or less per 100% by mass of the carbon-coated LMFP-based cathode active material. The carbon coating ratio is more preferably 8% by mass or less, even more preferably 6% by mass or less, and even more preferably 5% by mass or less. The above carbon coating ratio can be calculated by the method described in the examples below. By adjusting the carbon coating ratio to the above range, the lithium content per unit weight of the active material decreases, reducing the reactivity between lithium ions and the LMFP-based cathode active material, which can easily improve the problem of reduced battery capacity. Furthermore, it can easily improve the problem of inhibited lithium ion conduction between LMFP particles, which reduces battery performance.

[0044] In some embodiments, the carbon coating ratio, relative to 100% by mass of the carbon-coated LMFP positive electrode active material, is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1.0% by mass or more, and even more preferably 1.8% by mass or more, from the viewpoint of conductivity. When the carbon coating ratio is adjusted to the above range, the conductivity of the LMFP positive electrode active material becomes insufficient, making it difficult to use the active material uniformly during charging and discharging, and thus the problem of reduced battery life can be easily improved.

[0045] The preferred carbon coating content (carbon coating ratio) on the surface of LMFP-based positive electrode active material particles varies depending on the physical properties of the LMFP-based positive electrode active material, such as crystallite size, specific surface area, and particle size. From the viewpoint of battery performance, both too little and too much carbon coating ratio can lead to a decrease in performance; therefore, it is preferable to adjust it according to the physical properties of the LMFP-based positive electrode active material. Although not particularly limited, assuming the carbon coating ratio range described above, LMFP-based positive electrode active materials having the physical properties described later can be suitably used.

[0046] In some embodiments, the LMFP-based cathode active material has a BET specific surface area of ​​1 m². 2 / g or more and 50m 2 It is preferable that it is less than or equal to / g. The BET specific surface area is preferably 5m². 2 / g or more, more preferably 7m 2 / g or more, and more preferably 8m 2 It is 1 / g or more. Preferably, 45m 2 / g or less, more preferably 40m 2 It is 1 / g or more, and more preferably 30m 2 It is less than / g.

[0047] Furthermore, the LMFP-based cathode active material has a pore volume of 0.01 cm³. 3 / g or more and 0.40cm 3 It is preferable that it be less than or equal to / g. The pore volume is preferably 0.05 cm³. 3 / g or more and 0.30cm 3 It is less than or equal to / g, and more preferably 0.10cm 3 / g or more and 0.20cm 3 It is less than / g.

[0048] The BET specific surface area and pore volume were determined from nitrogen adsorption measurements using a gas adsorption measurement device. The pore volume was determined using the BJH method as the analytical method. The BET specific surface area of ​​the LMFP-based cathode active material is 1 m². 2 / g, and even 5m 2 If the value is less than / g, and the pore volume is 0.01 cm³, 3 / g, and even 0.05cm 3 When the values ​​fall below / g, the lithium ion insertion and removal reactions on the active material surface tend to be restricted. As a result, the rate of the battery reaction decreases, the time required for charging increases, and it tends to become difficult to obtain sufficient power for practical use. On the other hand, the BET specific surface area is 50m² 2 If the value exceeds / g, and the pore volume is 0.40 cm³, 3 When the values ​​exceed / g, the primary particle size becomes extremely small. This can make it difficult to control dispersion in the slurry preparation process for the cathode film and to achieve uniform coating control in the coating process of the cathode film slurry. Furthermore, it may become difficult to fill the cathode with cathode active material particles at a high density after coating. In this way, the cathode film slurry may become impractical, which can lead to a decrease in productivity.

[0049] In the slurry for the positive electrode film of this embodiment, in particular, when the BET specific surface area and pore volume of the LMFP-based positive electrode active material are within the above range, the dispersant (A) described later can disperse the particles, crystals, and carbon coating of the LMFP-based positive electrode active material well without damaging them, which tends to be advantageous for reducing the resistance and extending the lifespan of the secondary battery.

[0050] In some embodiments, the average particle size D of the LMFP-based positive electrode active material 50 The particle size is preferably between 0.05 μm and 3.0 μm. 50 The particle size may more preferably be 0.1 μm or more and 2.0 μm or less, even more preferably 0.5 μm or more and 1.5 μm or less, and even more preferably 0.6 μm or more and 1.2 μm or less. Average particle size D 50 This refers to the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve, and is a value obtained by measurement using the laser diffraction method. Average particle size D 50 If the above range is achieved, electrodes with excellent adhesion can be obtained, and the resistance reduction and extended lifespan of the secondary battery can be easily achieved. The LMFP-based cathode active material preferably has an average primary particle diameter of 30 nm or more and 500 nm or less. Preferably, it is 40 nm or more and 400 nm or less, and more preferably 50 nm or more and 300 nm or less. However, the average primary particle diameter is determined by directly observing the primary particles with a scanning electron microscope (SEM), measuring the size (maximum diameter) of the primary particles, and averaging the results.

[0051] If the average particle size falls below the above range, the slurry for the positive electrode film is prone to thickening, resulting in poor viscosity stability and potentially poor coating suitability. Furthermore, it may become difficult to increase the concentration of the slurry or the density of the positive electrode film, potentially leading to decreased productivity. On the other hand, if the average particle size exceeds the above range, the lithium ion diffusion distance within the solid of the LMFP-based positive electrode active material increases, potentially degrading the battery's charge and discharge performance.

[0052] (Method for manufacturing LMFP-based cathode active material) The method for producing the lithium manganese iron phosphate (LMFP) cathode active material used in this embodiment is not particularly limited, and methods well known in the art can be applied. For example, the LMFP cathode active material can be produced by applying the method described in Japanese Patent Application Publication No. 2018-037291, etc.

[0053] While not particularly limited, the manufacturing method may include, for example, a step of preparing a raw material slurry α by mixing a mixture containing a Li source, an Fe source, a Mn source, and a P source, and optionally other metal element M sources, with a solvent mainly composed of water, and synthesizing an LMFP-based cathode active material by heating and pressurizing the obtained raw material slurry α (synthesis step). The manufacturing method may further include a step of preparing a raw material slurry β by dispersing the LMFP-based cathode active material in an aqueous solvent containing a carbon source (carbon material), drying and granulating the obtained raw material slurry β, and then heating it to coat the surface of the LMFP-based cathode active material with the carbon material (coating step).

[0054] ·Synthesis process In the above manufacturing method, the method for synthesizing the LMFP-based cathode active material is not particularly limited, but for example, a mixture containing a Li source, an Fe source, a Mn source, and a P source, and optionally another metal element M source, is added to a solvent mainly composed of water, and these are stirred to prepare a raw material slurry α containing a precursor of the LMFP-based cathode active material. The above other metal element M (hereinafter sometimes simply referred to as M) may be at least one selected from Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements.

[0055] The mixture of Li source, Fe source, Mn source, P source, and other metal element M source is adjusted so that the molar ratio of the metal elements, i.e., the molar ratio of Li:Fe:Mn:P:M, is 2-3.5:0.05-1:0-1:0-0.95:0.95-1.07:0-0.14. The mixture with the adjusted molar ratio is added to a solvent mainly composed of water, stirred and mixed to prepare the raw material slurry α. From the viewpoint of ensuring uniform mixing, it is preferable that the Li source, Fe source, Mn source, P source, and other metallic element M source be mixed in aqueous solution form. From the viewpoint of obtaining a highly pure, highly crystalline, and very fine LMFP-based cathode active material, the molar concentrations of the Li source, Fe source, Mn source, P source, and M source in this raw material slurry α are preferably 1 mol / L or more and 4 mol / L or less.

[0056] Examples of lithium sources include hydroxides such as lithium hydroxide (LiOH), lithium inorganic salts such as lithium carbonate (Li2CO3), lithium chloride (LiCl), lithium nitrate (LiNO3), lithium phosphate (Li3PO4), dilithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4), lithium organic salts such as lithium acetate (LiCH3COO) and lithium oxalate ((COOLi)2), and their hydrates. One or more of these can be suitably used. Lithium phosphate (Li3PO4) can be used as both a lithium source and a phosphorus source.

[0057] Examples of iron sources include iron compounds and their hydrates such as iron(II) chloride (FeCl2), iron(II) sulfate (FeSO4), and iron(II) acetate (Fe(CH3COO)2), trivalent iron compounds such as iron(III) nitrate (Fe(NO3)3), iron(III) chloride (FeCl3), and iron(III) citrate (FeC6H5O7), and lithium iron phosphate. One or more of these can be suitably used.

[0058] As a source of Mn, Mn salts are preferred, such as manganese(II) chloride (MnCl2), manganese(II) sulfate (MnSO4), manganese(II) nitrate (Mn(NO3)2), manganese(II) acetate (Mn(CH3COO)2), and their hydrates. One or more of these can be suitably used.

[0059] Examples of phosphorus sources include phosphoric acid such as orthophosphoric acid (H3PO4) and metaphosphoric acid (HPO3), phosphates such as ammonium dihydrogen phosphate (NH4H2PO4), diammonium hydrogen phosphate ((NH4)2HPO4), ammonium phosphate ((NH4)3PO4), lithium phosphate (Li3PO4), dilithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4), and their hydrates. One or more of these can be suitably used.

[0060] As a source of Mg, Mg salts are preferred, such as magnesium(II) chloride (MgCl2), magnesium(II) sulfate (MgSO4), magnesium(II) nitrate (Mg(NO3)2), magnesium(II) acetate (Mg(CH3COO)2), and their hydrates. One or more of these can be suitably used.

[0061] As a source of Ca, calcium salts are preferred, such as calcium(II) chloride (CaCl2), calcium(II) sulfate (CaSO4), calcium(II) nitrate (Ca(NO3)2), calcium(II) acetate (Ca(CH3COO)2), and their hydrates. One or more of these can be suitably used.

[0062] As a source of Co, Co salts are preferred, such as cobalt(II) chloride (CoCl2), cobalt(II) sulfate (CoSO4), cobalt(II) nitrate (Co(NO3)2), cobalt(II) acetate (Co(CH3COO)2), and their hydrates. One or more of these can be suitably used.

[0063] As a source of Sr, Sr salts are preferred, such as strontium carbonate (SrCO3), strontium sulfate (SrSO4), and strontium hydroxide (Sr(OH)2). One or more of these can be suitably used.

[0064] As a source of barium, barium salts are preferred, such as barium(II) chloride (BaCl2), barium(II) sulfate (BaSO4), barium(II) nitrate (Ba(NO3)2), barium(II) acetate (Ba(CH3COO)2), and their hydrates. One or more of these can be suitably used.

[0065] As a source of Ti, Ti salts are preferred, such as titanium chloride (TiCl4, TiCl3, TiCl2), titanium oxide (TiO), and their hydrates. One or more of these can be suitably used.

[0066] Preferred Zn sources are Zn salts, such as zinc(II) chloride (ZnCl2), zinc(II) sulfate (ZnSO4), zinc(II) nitrate (Zn(NO3)2), zinc(II) acetate (Zn(CH3COO)2), and their hydrates. One or more of these can be suitably used.

[0067] Examples of boron sources include boron compounds such as chlorides, sulfur oxides, nitrates, acetic acid oxides, hydroxides, and oxides. One or more of these can be suitably used.

[0068] Examples of aluminum sources include aluminum compounds such as chlorides, sulfur oxides, nitrates, acetic acid oxides, and hydroxides. One or more of these can be suitably used.

[0069] Examples of gallium sources include gallium compounds such as chlorides, sulfur oxides, nitrates, acetic acid oxides, and hydroxides. One or more of these can be suitably used.

[0070] Examples of indium sources include indium compounds such as chlorides, sulfur oxides, nitrates, acetic acid oxides, and hydroxides. One or more of these can be suitably used.

[0071] Examples of silicon sources include sodium silicate, potassium silicate, silicon tetrachloride (SiCl4), silicates, and organosilicon compounds. One or more of these can be suitably used.

[0072] Examples of germanium compounds that can be used as a source of Ge include chlorides, sulfur oxides, nitrate oxides, acetic acid oxides, hydroxides, and other oxides. One or more of these can be suitably used.

[0073] Examples of rare earth element sources include chlorides, sulfides, nitrates, acetic acid oxides, hydroxides, and oxides of rare earth elements such as Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. One or more of these can be suitably used.

[0074] A solvent whose main component is water is either water alone, or an aqueous solvent whose main component is water and which may contain other aqueous solvents such as alcohol as needed. The aqueous solvent is not particularly limited as long as it is a solvent capable of dissolving the Li source, Fe source, Mn source, P source, and M source. Examples include alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol; esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone; ethers such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, and cyclohexanone; amides such as dimethylformamide, N,N-dimethylacetacetamide, and N-methylpyrrolidone; and glycols such as ethylene glycol, diethylene glycol, and propylene glycol. These aqueous solvents may be used individually or as a mixture of two or more.

[0075] In the above synthesis process, the raw material slurry α obtained as described above is placed in a pressure vessel and subjected to hydrothermal treatment to obtain an LMFP-based cathode active material. The hydrothermal treatment is carried out for 1 hour or more and 72 hours or less, by heating to a temperature in the range of 150°C or higher and 230°C or lower, preferably 170°C or higher and 200°C or lower, and adjusting the pressure inside the pressure vessel. When the temperature in the range of 150°C or higher and 230°C or lower is reached, the pressure inside the pressure vessel is, for example, 0.1 MPa or higher and 2 MPa or lower. By adjusting the temperature and time during hydrothermal treatment, it is possible to control the particle size of the LMFP-based cathode active material to a desired size.

[0076] • Coating process A raw material slurry β is prepared by dispersing an LMFP-based cathode active material in an aqueous solvent containing a carbon source. Next, this raw material slurry β is dried and granulated, and then heated at a temperature in the range of 550°C or higher and 830°C or lower for 1 hour or more and 36 hours or less to coat the surface of the iron manganese phosphate-based cathode active material (primary particles) with a carbon material, thereby obtaining an LMFP-based cathode active material having a carbon coating.

[0077] LMFPs are also available as commercial products. Examples include the LMFP64 and LMFP73 from SKYLAND, the LMFP-60M from EaspringTechnology (Changzhou) NewMaterial, and the 1200TY117 and 1261ZY117 from Hostec.

[0078] The slurry for the positive electrode film is characterized by containing the above-mentioned LMFP-based positive electrode active material, but may further contain positive electrode active materials other than the LMFP-based positive electrode active material. The positive electrode active material other than the LMFP-based positive electrode active material can be any material that can reversibly dope or intercalate lithium ions, and can include metal oxides, metal sulfides and other metal compounds, and conductive polymers. Examples include oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, inorganic compounds such as transition metal sulfides, and polyanionic compounds. Specifically, MnO, V2O5, V6O 13 Examples include transition metal oxide powders such as TiO2, lithium-transition metal composite oxide powders such as layered lithium nickelate, lithium cobaltate, lithium manganate, and lithium manganate with a spinel structure, transition metal sulfide powders such as TiS2 and FeS, and layered sodium ironate, sodium manganate, sodium chromate, and sodium nickelate. These may be used individually or in combination of two or more. Conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophene can also be used. These may be used individually or in combination of two or more. The above inorganic and organic compounds may also be mixed and used.

[0079] In particular, composite oxides of transition metals such as Al, Fe, Co, Ni, and Mn with lithium can be suitably used. More preferably, it may be a composite oxide of a transition metal such as Al, Fe, Co, Ni, and Mn with lithium, and even more preferably, it may be a composite oxide of lithium with one selected from the group consisting of Al, Co, Ni, and Mn. In some embodiments, a composite oxide of Ni and / or Mn with lithium is particularly preferred. Active materials containing Ni and / or Mn tend to become more basic due to the elution of raw material-derived components or metal ions, and this tendency becomes more pronounced when the total amount of Ni and / or Mn in the transition metal is 50 mol% or more. As a result, problems such as gelation of the binder and deterioration of the dispersion state are likely to occur, so in the case of a battery containing an active material containing Ni and / or Mn, the effect of the combination of a dispersant (A) having a specific acid value and a specific binder having a specific acid value in this embodiment is likely to be exhibited.

[0080] If the positive electrode film slurry of this embodiment further contains positive electrode active materials other than LMFP-based positive electrode active materials, the content of LMFP-based positive electrode active materials in the positive electrode active materials is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more, relative to the total amount of positive electrode active materials. That is, the content of positive electrode active materials other than LMFP-based positive electrode active materials is preferably less than 70% by mass, more preferably less than 50% by mass, and even more preferably less than 30% by mass, relative to the total amount of positive electrode active materials. The content of positive electrode active materials other than LMFP-based positive electrode active materials may be 0% by mass, and the positive electrode active materials may consist only of LMFP-based positive electrode active materials. By setting the content of LMFP-based positive electrode active materials within the above range, the electrode density can be increased, and the battery capacity per unit volume can be increased. In addition, by increasing the electrode density, the conductivity of the electrodes improves, and battery performance tends to improve as well.

[0081] <Dispersant> The dispersant only needs to have the function of dispersing the LMFP-based positive electrode active material, and may be either an inorganic acid or an organic acid, or a combination thereof. The content of the dispersant in the slurry for the positive electrode film is 0.01 parts by mass or more and 8 parts by mass or less, based on the mass of the positive electrode active material (assuming the mass of the positive electrode active material is 100 parts by mass), preferably 0.02 parts by mass or more and 5 parts by mass or less, more preferably 0.1 parts by mass or more and 2 parts by mass or less, and even more preferably 0.25 parts by mass or more and 1.5 parts by mass or less. In this embodiment, the dispersant includes dispersant (A) having an acid value of 35 mg KOH / g or more and 150 mg KOH / g or less. By including dispersant (A), the oxidation resistance of the dispersant can be easily improved, and the viscosity stability, electrode characteristics, and further battery characteristics of the slurry for the positive electrode film can be easily improved.

[0082] (Dispersant (A)) The dispersant (A) may be a compound having an acid value of 35 mg KOH / g or more and 150 mg KOH / g or less. Preferably, the acid value is 50 mg KOH / g or more and 120 mg KOH / g or less, and more preferably 80 mg KOH / g or more and 100 mg KOH / g or less. When the acid value of the dispersant (A) is within the above range, the viscosity stability of the slurry for the cathode film tends to be good.

[0083] Specific examples of the dispersant (A) include compounds having acidic groups such as carboxyl groups, sulfol groups, or phosphate groups. The compounds having acidic groups may also have counterions. In some embodiments, the dispersant (A) may be an organic compound having at least one acidic group selected from the group consisting of -COOM, -SO3M, and -PO(OM)2 [wherein M represents a hydrogen atom, a quaternary amine, or an alkali metal]. The above organic compound having acidic groups may be either an aliphatic compound or an aromatic compound, and may have two or more acidic groups in its molecule. If two carboxyl groups are present in adjacent positions in the above organic compound, an anhydride may be formed. The above organic compound having acidic groups may also be a polymer (hereinafter also referred to as a dispersion resin) formed using a polymerizable monomer having acidic groups.

[0084] In some embodiments, the dispersant (A) may be a dispersion resin having at least one acidic group selected from the group consisting of -COOM, -SO3M, and -PO(OM)2 [wherein M represents a hydrogen atom, a quaternary amine, or an alkali metal], and having an acid value of 35 mgKOH / g or more and 150 mgKOH / g or less. The dispersion resin may be polyester-based, polyvinyl-based, polyurethane-based, polyester-based, polyether-based, formalin condensate-based, silicone-based, and composites thereof, and may have at least one acidic group selected from the group consisting of -COOM, -SO3M, and -PO(OM)2 [wherein M represents a hydrogen atom, a quaternary amine, or an alkali metal] as the acidic group. Such a dispersion resin may be a homopolymer of a polymerizable monomer having the above-mentioned acidic group, or a copolymer of a polymerizable monomer having the above-mentioned acidic group and another polymerizable monomer. The dispersion resin may further have functional groups other than the acidic group. In some embodiments, the dispersion resin may be a resin composed of carboxyl group-containing monomers such as (meth)acrylic acid and (maleic anhydride), for example, a styrene-(maleic anhydride) copolymer, and may be further modified with functional groups other than acidic groups as needed. In other embodiments, the dispersion resin may be a resin composed of sulfo group-containing monomers such as acrylamide t-butylsulfonic acid and phosphoric acid-containing monomers such as 2-methchloroyloxyethyl acid phosphate.

[0085] Such dispersion resins containing acidic groups are available commercially. Examples include BYK-ET3004 (manufactured by BIC Chemie Japan, alkylammonium salt of copolymer containing acidic groups, acid value 65.1 mg KOH / g), BYK-ET3003 (manufactured by BIC Chemie Japan, phosphate group-containing copolymer, acid value 72.0 mg KOH / g), and DISPERBYK(registered trademark)-111 (manufactured by BIC Chemie Japan, phosphate group-containing copolymer, acid value 65.1 mg KOH / g).

[0086] When using a dispersion resin, the weight-average molecular weight is preferably 500 to 30,000, more preferably 1,000 to 20,000, and even more preferably 1,000 to 10,000. When the molecular weight of the dispersion resin is within the above range, the dispersibility of the LMFP-based cathode active material is good, and good viscosity stability of the cathode film slurry described later can be easily obtained. Furthermore, an electrode film with excellent conductivity and adhesion can be easily obtained in which the LMFP-based cathode active material, carbon nanotubes, and vinylidene fluoride polymer are uniformly dispersed. The weight-average molecular weight is the weight-average molecular weight on a polystyrene basis and can be measured by gel permeation chromatography (GPC).

[0087] The weight-average molecular weight (Mw) of the dispersant can be measured, for example, by gel permeation chromatography (GPC) equipped with an RI detector. Specifically, an HLC-8320GPC (manufactured by Tosoh Corporation) can be used as the instrument, with three separation columns connected in series, and Tosoh Corporation's "TSK-GEL SUPER AW-4000," "AW-3000," and "AW-2500" can be used as the packing materials, in that order. The measurement conditions may be an oven temperature of 40°C, using tetrahydrofuran (THF) as the eluent, and a flow rate of 0.6 mL / min. The sample to be measured is dried at 120°C for 60 minutes, then the concentration is adjusted using THF (to about 0.2%), and 20 microliters are injected. The value obtained from the measurement is converted to polystyrene equivalent to obtain Mw.

[0088] The content of the dispersant (A) in the slurry for the positive electrode film may be preferably 0.01 to 5 parts by mass, more preferably 0.1 to 5 parts by mass, even more preferably 0.1 to 1 part by mass, and even more preferably 0.1 to 0.5 parts by mass, based on the mass of the positive electrode active material (assuming the mass of the positive electrode active material is 100 parts by mass). As described above, the slurry for the positive electrode film of this embodiment is characterized by containing an LMFP-based positive electrode active material. Therefore, in some embodiments, the content of the dispersant (A) in the slurry for the positive electrode film may be preferably 0.01 to 5 parts by mass, more preferably 0.1 to 5 parts by mass, even more preferably 0.1 to 1 part by mass, and even more preferably 0.1 to 0.5 parts by mass, based on the mass of the LMFP-based positive electrode active material (assuming the mass of the LMFP-based positive electrode active material is 100 parts by mass).

[0089] (Dispersant (B)) The cathode film slurry of the present invention may further contain a dispersant (B) different from dispersant (A). The dispersant (B) is not particularly limited and can be any material that has the effect of dispersing the positive electrode active material and conductive material in a non-aqueous solvent. In some embodiments, from the viewpoint of adsorption to the conductive material and affinity to the non-aqueous solvent, the dispersant (B) may be at least one selected from polyvinylpyrrolidone resins, polyvinyl acetal resins, polyvinyl alcohol resins, and hydrogenated acrylonitrile butadiene rubber. The dispersant (B) may be at least one selected from polyvinylpyrrolidone resins, polyvinyl acetal resins, and hydrogenated acrylonitrile butadiene rubber.

[0090] The polyvinyl alcohol-based resin may be polyvinyl alcohol, modified polyvinyl alcohol having functional groups other than hydroxyl groups (e.g., acetyl groups, sulfo groups, carboxyl groups, carbonyl groups, amino groups), polyvinyl alcohol modified with various salts, or other anionically or cationically modified polyvinyl alcohol. The polyvinyl acetal resin may be a polyvinyl acetal obtained by acetal modification (acetacetal modification or butyral modification, etc.) of the above-mentioned polyvinyl alcohol with aldehydes, and examples include polyvinyl acetal, polyvinyl butyral, and polyvinyl formal.

[0091] As the dispersant (B), a commercially available dispersant may be used, or one synthesized by a known synthesis method may be used. For example, polyvinyl alcohol-based resins include Kuraray Poval (polyvinyl alcohol resin manufactured by Kuraray), Gosenol, Gosenex (polyvinyl alcohol resin manufactured by Nippon Synthetic Chemical Industry Co., Ltd.), Denka Poval (polyvinyl alcohol resin manufactured by Denka Co., Ltd.), and J-Poval (polyvinyl alcohol resin manufactured by Nippon Vinegar & Poval Co., Ltd.), and various grades are available. Modified polyvinyl alcohols with various functional groups are also available in the same manner.

[0092] Examples of polyvinylpyrrolidone resins include Luvitec K17 (K value: 15.0~19.0, low molecular weight), K30 (K value: 27.0~33.0), K90 (K value: 88.0~92.0), K90HM (K value: 92.0~96.0, high molecular weight) (manufactured by BASF Japan), K15, K30, K90, K120 (manufactured by ISP), polyvinylpyrrolidone K30 (K value: 27.0~33.0), K90 (K value: 88.0~96.0) (manufactured by Nippon Shokubai), and PVP. Examples include K12 (K value 10-14), K15 (K value 13-19), K30 (K 26-35), K60 (K value 50-62), and K90 (K value 88-100) (manufactured by DSP Gokei Food & Chemical). From the viewpoint of preventing viscosity increase, polyvinylpyrrolidone is preferably K value 150 or less, more preferably K value 100 or less, and even more preferably K value 85 or less.

[0093] Examples of acrylonitrile butadiene rubbers that may be hydrogenated include Therban (hydrogenated nitrile rubber manufactured by Alantheo), Baymod (nitrile rubber manufactured by Alantheo), Zetpole (hydrogenated nitrile rubber manufactured by Nippon Zeon), and Nipole NBR (nitrile rubber manufactured by Nippon Zeon), and various grades with different nitrile ratios, hydrogenation rates, and molecular weights are available.

[0094] The weight-average molecular weight of dispersant (B) is preferably 10,000 or more, more preferably 50,000 or more, and even more preferably 100,000 or more. The weight-average molecular weight of dispersant (B) is preferably 400,000 or less, more preferably 350,000 or less, and even more preferably 300,000 or less. When the weight-average molecular weight of dispersant (B) is 10,000 or more and 400,000 or less, the adsorption to the dispersed material and affinity to the solvent are good, and the stability of the cathode film slurry tends to improve. The weight-average molecular weight is the weight-average molecular weight on a polystyrene basis and can be measured by gel permeation chromatography (GPC). Specifically, it can be measured by the method described in the examples.

[0095] The content of dispersant (B) in the slurry for the positive electrode film is preferably adjusted so that the total amount of dispersant (B) and dispersant (A) is within the range of 0.02 to 8 parts by mass based on the mass of the positive electrode active material. In some embodiments, the content of dispersant (B) in the slurry for the positive electrode film is preferably 0.01 to 7.9 parts by mass, more preferably 0.01 to 5 parts by mass, even more preferably 0.05 to 3 parts by mass, and even more preferably 0.05 to 2 parts by mass, based on the mass of the positive electrode active material (assuming the mass of the positive electrode active material is 100 parts by mass).

[0096] When the dispersant (B) is hydrogenated acrylonitrile butadiene rubber, more preferably hydrogenated acrylonitrile butadiene rubber as described later, the Mooney viscosity (ML1 + 4, 100°C) of these acrylonitrile butadiene rubbers is preferably 20 or more and 100 or less. The Mooney viscosity is preferably 70 or less, more preferably 55 or less, and even more preferably 50 or less. When acrylonitrile butadiene rubber with a Mooney viscosity within the above range is used, the dispersibility of the dispersed material can be further improved. Furthermore, the viscosity increase of the resulting cathode film slurry can be suppressed, allowing for efficient removal of metallic foreign matter originating from the raw materials by methods such as magnetic iron removal, filtration, and centrifugation, thereby preventing a decrease in battery performance due to residual metallic foreign matter. Here, "Mooney viscosity (ML1 + 4, 100°C)" can be measured at a temperature of 100°C in accordance with JIS K6300-1.

[0097] The method for adjusting Mooney viscosity is not particularly limited. For example, Mooney viscosity can be adjusted by changing the composition of acrylonitrile butadiene rubber (e.g., structural unit types and content, hydrogenation rate), structure (e.g., linearity), molecular weight, and preparation conditions (e.g., polymerization temperature, amount of molecular weight adjuster).

[0098] In some embodiments, hydrogenated nitrile butadiene rubber (H-NBR) can be suitably used as the dispersant (B) because it is easy to increase its adsorption to the positive electrode active material and conductive material and its affinity to non-aqueous solvents. Hydrogenated nitrile butadiene rubber (H-NBR) has structural units having nitrile groups and structural units derived from conjugated dienes in which the unsaturated bonds are partially hydrogenated. In some embodiments, when hydrogenated nitrile butadiene rubber (H-NBR) (hereinafter also referred to as dispersant (B1)) is included as the dispersant (B), it is preferable that the mass ratio (RDB) of residual double bonds (RDB) calculated by the following formula (1) satisfies 0.05 to 5% by mass. Formula (1): RDB(mass%)=(BD / (BD+HBD))×100 In equation (1), BD is the mass ratio of structural units derived from conjugated dienes having unsaturated bonds to the total mass of structural units derived from conjugated dienes, and HBD is the mass ratio of structural units derived from conjugated dienes with hydrogenated unsaturated bonds to the total mass of structural units derived from conjugated dienes.

[0099] The mass ratio (RDB) of residual double bonds calculated by formula (1) is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. Within these ranges, the affinity of the dispersant (B1) to the solvent increases, improving the dispersibility and dispersion stability of the LMFP-based cathode active material and conductive material, and suppressing aggregation and sedimentation. When the mass ratio of residual double bonds (RDB) exceeds the above range, the probability of structural units containing residual double bonds being close to or adjacent to each other increases, and the more consecutive the structures forming conjugation, the more stable the π-conjugated orbital becomes, making it easier for oxidation reactions to proceed in a chain reaction. When oxidation reactions occur, battery performance may decrease due to swelling of the battery due to gas generation, deterioration of internal battery components due to the generation of corrosive substances such as hydrofluoric acid, and embrittlement of the electrode due to structural changes in the dispersant (B1). In particular, when secondary batteries are used for a long period of time and at high temperatures, the dispersant (B1) is less likely to oxidize, and a decrease in battery performance can be prevented. Furthermore, in these ranges, the number of π-conjugated electrons in the dispersant (B1) decreases, so the dispersant (B1) can act well on LMFP-type cathode active materials, which have less polarization compared to layered or spinel-type active materials. From the viewpoint of further enhancing the oxidation resistance of the dispersant (B1) and its effect on the LMFP-based cathode active material, the mass ratio of residual double bonds (RDB) calculated by formula (1) is preferably 2% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less.

[0100] The effect can be obtained when the mass ratio (RDB) of residual double bonds calculated by formula (1) is 0.05 mass% or higher, and may be, for example, 0.2 mass% or higher, or 0.3 mass% or higher. In addition, H-NBR with an RDB in the above range has the advantage of being readily available. Examples of commercially available products include Thermaln® AT 3404 (manufactured by ARLANXEO, hydrogenated acrylonitrile-butadiene rubber, RDB value 0.5), ZN35052 (manufactured by Zannan Scitech, hydrogenated acrylonitrile-butadiene rubber, RDB value 1.0), and ZN35053 (manufactured by Zannan Scitech, hydrogenated acrylonitrile-butadiene rubber, RDB value 1.0).

[0101] LMFP-based cathode active materials have a stable crystalline structure but low conductivity. Therefore, in some cases, a conductive material such as carbon material is coated on the surface of the LMFP compound to enhance its conductivity before using it as a cathode active material. Even in such cases, the dispersant (B1) can be adsorbed onto the carbon coating on the surface of the LMFP-based cathode active material, further improving its dispersibility. The nitrile groups of the dispersant (B1) readily adsorb to the carbon coating on the surface of the LMFP-based cathode active material, and the elasticity of the structural units derived from the conjugated diene acts as a repulsive site, further improving its dispersibility and dispersion stability.

[0102] Structural units derived from conjugated dienes containing unsaturated bonds are those structural units derived from conjugated dienes that have a carbon-carbon double bond. Structural units derived from conjugated dienes with hydrogenated unsaturated bonds are those derived from conjugated dienes that lack carbon-carbon double bonds but contain saturated carbon bonds. These structural units derived from conjugated dienes with hydrogenated unsaturated bonds can be formed by hydrogenating conjugated diene structural units. In equation (1), the mass ratio of structural units derived from conjugated dienes having unsaturated bonds, and the mass ratio of structural units derived from conjugated dienes with hydrogenated unsaturated bonds, to the total mass of structural units derived from conjugated dienes, can be measured using IR (infrared spectroscopy). For details, these can be measured by the method described in the examples.

[0103] The dispersant (B1) can be obtained by copolymerizing a monomer composition containing a monomer having a nitrile group and a conjugated diene, and then partially hydrogenating the carbon-carbon double bond of the structural unit derived from the conjugated diene by hydrogenation. Monomers having a nitrile group may include, for example, (meth)acrylonitrile. (Meth)acrylonitrile may be acrylonitrile, methacrylonitrile, or a combination thereof. Examples of conjugated dienes include 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, and 1,3-pentadiene, which may be used individually or in combination of two or more.

[0104] The dispersant (B1) may further contain structural units derived from other ethylenically unsaturated monomers other than structural units having a nitrile group and structural units derived from conjugated dienes. Other ethylenically unsaturated monomers copolymerizable with monomers having a nitrile group and conjugated dienes include, for example, ethylenically unsaturated carboxylic acids such as (meth)acrylic acid, (anhydride) maleic acid, fumaric acid, and itaconic acid; mono or dialkyl esters of the ethylenically unsaturated carboxylic acids such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, mono or dimethyl maleic acid, mono or diethyl fumaric acid, mono or di-n-butyl fumaric acid, mono or di-n-butyl itaconic acid; and ethylene acrylate such as methoxyacrylate, ethoxyacrylate, methoxyethoxyethyl acrylate, and methoxyethoxybutyl acrylate. Examples include alkoxyalkyl esters of ethylenically unsaturated carboxylic acids; (meth)acrylates having hydroxyalkyl groups such as 2-hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate; glycidyl (meth)acrylate; (meth)acrylamides and their derivatives such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; acrylates having amino groups such as dimethylaminomethyl acrylate and diethylaminomethyl acrylate; aromatic vinyl monomers such as styrene, α-methylstyrene, vinyltoluene, and chlorostyrene; α-olefins such as ethylene and propylene; and non-conjugated diene monomers such as dicyclopentadiene and vinylnorbornene. Other ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0105] Monomers containing nitrile groups and conjugated dienes can be synthesized by conventional emulsion polymerization methods. The polymerization agents used in emulsion polymerization, such as emulsifiers (surfactants), polymerization initiators, chelating agents, oxygen scavengers, and molecular weight modifiers, can be conventionally known agents and are not particularly limited.

[0106] Typically, anionic emulsifiers, nonionic emulsifiers, etc., are used as emulsifiers, and these may be used in combination. Examples of anionic emulsifiers include fatty acid salts such as potassium tallow fatty acid, partially hydrogenated potassium tallow fatty acid, potassium oleate, and sodium oleate; resin salts such as potassium rosinate, sodium rosinate, hydrogenated potassium rosinate, and hydrogenated sodium rosinate; and alkylbenzene sulfonates such as sodium dodecylbenzenesulfonate. Examples of nonionic emulsifiers include polyethylene glycol ester type, polyethylene glycol ester type, and Pluronic® type such as block copolymers of ethylene oxide and propylene oxide. The amount of emulsifier used is usually 0.1 to 10 parts by mass per 100 parts by mass of the monomer composition.

[0107] Examples of polymerization initiators include pyrolysis-type initiators such as persulfates like potassium persulfate and ammonium persulfate; organic peroxides such as t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, octanoyl peroxide, and 3,5,5-trimethylhexanoyl peroxide; azo compounds such as azobisisobutyronitrile; and redox initiators consisting of these compounds and reducing agents such as divalent iron ions. The amount of polymerization initiator used is usually 0.01 to 10 parts by mass per 100 parts by mass of the monomer composition.

[0108] Examples of molecular weight modifiers include alkyl mercaptans such as octyl mercaptan, nonyl mercaptan, decyl mercaptan, dodecyl mercaptan, and 3-mercapto-1,2-propanediol; thioglycolic acid esters such as octyl thioglycolate, nonyl thioglycolate, and 2-ethylhexyl thioglycolate; 2,4-diphenyl-4-methyl-1-pentene, 1-methyl-4-isopropylidene-1-cyclohexene, α-pinene, and β-pinene. The amount of molecular weight adjusting agent used is usually 0.01 to 10 parts by mass per 100 parts by mass of the monomer composition.

[0109] The emulsion polymerization reaction may be continuous or batch-based, and the polymerization temperature may be low to high temperature, but preferably 0 to 70°C, more preferably 0 to 50°C. Furthermore, the method of adding the monomer (e.g., all-at-once addition, divided addition), polymerization time, polymerization conversion rate, etc., are not particularly limited.

[0110] Next, by hydrogenating a copolymer of a monomer composition containing a monomer having a nitrile group and a conjugated diene, H-NBR can be obtained that contains structural units having a nitrile group and structural units derived from a conjugated diene having a partially hydrogenated unsaturated bond. The hydrogenation step can be carried out by conventional methods.

[0111] Hydrogenation is preferably carried out by a method that can selectively hydrogenate structural units derived from conjugated dienes, such as the oil layer hydrogenation method or the aqueous layer hydrogenation method. The aqueous layer hydrogenation method is preferred because it results in a low content of impurities (for example, coagulants and metals described later) in the resulting copolymer. Hydrogenation is preferably carried out using a hydrogenation catalyst. Any known selective hydrogenation catalyst can be used as the hydrogenation catalyst without limitation. For example, palladium catalysts include palladium salts of carboxylic acids such as formic acid, acetic acid, propionic acid, lauric acid, succinic acid, oleic acid, and phthalic acid; palladium chlorides such as palladium chloride, dichloro(cyclooctadiene)palladium, dichloro(norbornadiene)palladium, and ammonium hexachloropalladate(IV); iodates such as palladium iodide; and palladium sulfate dihydrate. Furthermore, the hydrogenation process may be carried out in one stage, but it is preferable to carry it out in two or more stages. By carrying it out in two or more stages, the hydrogenation rate can be further increased.

[0112] After the hydrogenation reaction is complete, it is preferable to remove the hydrogenation catalyst from the reaction product. For example, this can be done by adding an adsorbent such as activated carbon or an ion exchange resin and adsorbing the hydrogenation catalyst under stirring, and then filtering or centrifuging the reaction product. It is also possible to leave the hydrogenation catalyst in the reaction product without removing it, and the hydrogenation catalyst may remain in the slurry for the cathode film.

[0113] The content of structural units having nitrile groups is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the mass of the dispersant (B1) (i.e., when the mass of the dispersant (B1) is set to 100% by mass). The content of structural units having nitrile groups is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less, based on the mass of the dispersant (B1) (i.e., when the mass of the dispersant (B1) is set to 100% by mass). By setting the content of structural units having nitrile groups within these ranges, the adsorption to the dispersed material and affinity to the solvent can be controlled, allowing the dispersed material to exist more stably in the solvent. Furthermore, the affinity of the dispersant (B1) to the electrolyte can be better controlled, preventing problems such as the dispersant (B1) dissolving in the electrolyte within the battery and increasing the resistance of the electrolyte. Furthermore, by setting the content of structural units having strongly polarizing nitrile groups within the above range, the dielectric constant of the dispersant (B1) can be set to an appropriate range, and when used in combination with the dispersant (A), the affinity for LMFP-based cathode active materials, which have less polarization compared to layered or spinel-type active materials, can be increased.

[0114] The content of structural units derived from conjugated dienes is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, based on the mass of the dispersant (B1) (i.e., when the mass of H-NBR is set to 100% by mass). The content of structural units derived from conjugated dienes is preferably less than 85% by mass, more preferably 75% by mass or less, and even more preferably 70% by mass or less, based on the mass of the dispersant (B1) (i.e., when the mass of the copolymer is set to 100% by mass). By setting the content of structural units derived from conjugated dienes within these ranges, they act as sterically hindering sites that prevent aggregation of dispersed materials, allowing them to exist stably in the solvent.

[0115] The following are preferred embodiments of H-NBR used as a dispersant (B1). The total content of structural units derived from conjugated dienes and structural units having nitrile groups contained in the dispersant (B1) is 80% by mass or more and 100% by mass or less, based on the mass of the dispersant (B1). The above total content is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. The total content of structural units derived from conjugated dienes, structural units having nitrile groups, and structural units having amide groups contained in the dispersant (B1) is 80% by mass or more and 100% by mass or less, based on the mass of the dispersant (B1). The above total content is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. The total content of structural units derived from conjugated dienes, structural units having nitrile groups, structural units containing amide groups, and structural units having carboxyl groups in the dispersant (B1) is 80% by mass or more and 100% by mass or less, based on the mass of the dispersant (B1). The above total content is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. In this specification, the content of structural units in the dispersant (B1) can be determined by the amount of monomer used, NMR (nuclear magnetic resonance) measurement, and / or IR (infrared spectroscopy) measurement.

[0116] In the cathode film slurry of this embodiment, the dispersant preferably contains at least dispersant (A), and further preferably contains a dispersant (B) different from dispersant (A). In the cathode film slurry, the content of the dispersant may be preferably 0.02 to 20% by mass, more preferably 0.05 to 9% by mass, and even more preferably 0.1 to 3% by mass, based on the total amount of solids. The ratio of dispersant (A) to dispersant (B) may be preferably 5:1 to 1:100 by mass, more preferably 2:1 to 1:10, and even more preferably 1:1 to 1:2. In some embodiments, the dispersant preferably contains a phosphate group-containing compound as dispersant (A) and H-NBR (dispersant (B1)) as dispersant (B).

[0117] <Conductive material> The cathode film slurry of this embodiment includes a conductive material. The conductive material facilitates the formation of a conductive network between LMFP-based cathode active materials, thereby facilitating improvements in electrode and battery performance. The conductive material includes at least one selected from the group consisting of carbon nanotubes (CNTs) and carbon black (CB). Examples include carbon black such as acetylene black, furnace black, channel black, Ketjen black, and thermal black, and carbon nanotubes. One or more conductive materials other than carbon nanotubes and / or carbon black may be used in combination. From the viewpoint of conductivity, it is preferable that the conductive material includes carbon nanotubes. Carbon nanotubes can enhance conductivity with a smaller amount than carbon black, thus reducing the impact of using conductive materials. Specifically, it can reduce the problem of insufficient conductivity of the LMFP-based positive electrode active material, making it difficult to use the active material homogeneously during charging and discharging, and thus reducing battery life.

[0118] Carbon nanotubes may have a cylindrical shape formed by winding a planar graphite, and may include single-walled carbon nanotubes and multi-walled carbon nanotubes, or a mixture of these. Carbon nanotubes of different diameters may also be mixed. Single-walled carbon nanotubes have a structure in which a single layer of graphite is wound. Furthermore, the sidewalls of carbon nanotubes do not necessarily have a graphite structure.

[0119] The shape of the CNT is not limited. Examples of CNT shapes include needle-shaped, cylindrical tubular, fishbone-shaped (fishbone or cup-stacked), playing card-shaped (platelet), and coil-shaped. In this embodiment, the shape of the CNT is preferably needle-shaped or cylindrical tubular. The CNT may be a single shape or a combination of two or more shapes.

[0120] Examples of carbon nanotube (CNT) forms include graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. Carbon nanotubes may exist in these forms individually or in combination of two or more of these forms.

[0121] The outer diameter of the CNTs is preferably 1 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more. Furthermore, the outer diameter of the CNTs is preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 13 nm or less. By setting the average outer diameter within the above range, an efficient conductive network can be formed in the positive electrode. The average outer diameter of the CNTs can be calculated by observing and imaging the CNTs using a transmission electron microscope, then selecting 300 arbitrary CNTs from the obtained observation images and measuring the outer diameter of each.

[0122] When using two or more types of CNTs with different average outer diameters, the average outer diameter of the first CNT is preferably 1 nm or more and less than 5 nm. The average outer diameter of the second CNT is preferably 5 nm or more and 30 nm or less, and more preferably 20 nm or less. When using two or more types of CNTs with different average outer diameters, the mass ratio of the first CNT to the second CNT is preferably 100:1 to 1:100, and more preferably 10:1 to 1:10.

[0123] The average fiber length of the CNTs is preferably 0.5 μm or more, more preferably 0.8 μm or more, and even more preferably 1.0 μm or more. Furthermore, the fiber length of the CNTs is preferably 20 μm or less, and more preferably 10 μm or less. The average fiber length of the CNTs can be calculated by observing and imaging the CNTs with a scanning electron microscope, then selecting 300 arbitrary CNTs from the obtained observation images and measuring the fiber length of each. The average fiber length of the CNTs in the conductive dispersion is also preferably within the above range.

[0124] The specific surface area of ​​CNT is 100m² 2 It is preferable that it be 150m or more / g. 2 It is more preferable that it be 200m or more per gram. 2 It is even more preferable that the amount is 1 / g or more. Furthermore, the specific surface area of ​​the CNTs is 1200 m². 2 It is preferable that it be less than or equal to / g, and 1000m 2 It is more preferable that it be less than or equal to / g, and 800m 2 It is even more preferable that the value be less than or equal to / g. The specific surface area of ​​CNTs is calculated by the BET method using nitrogen adsorption measurement.

[0125] The carbon purity of CNTs is expressed as the carbon atom content (mass%) within the CNT. The carbon purity is preferably 80% by mass, more preferably 90% or higher, even more preferably 95% or higher, and particularly preferably 98% or higher, based on 100% by mass of CNT. By setting the carbon purity within the above range, problems such as dendrite formation and short circuits due to impurities can be prevented.

[0126] In some embodiments, the content of the conductive material is preferably 0.1 parts by mass or more and 3 parts by mass or less per 100 parts by mass of solid content of the cathode film slurry. The content of the conductive material may more preferably be 0.2 parts by mass or more and 2 parts by mass or less, and even more preferably 0.5 parts by mass or more and 2 parts by mass or less. When carbon nanotubes and carbon black are used in combination as the conductive material, it is preferable to adjust the total amount of these materials to within the above range.

[0127] The average primary particle diameter of carbon black (CB) is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 25 nm or more. It is also preferably 100 nm or less, more preferably 80 nm or less, and even more preferably 60 nm or less. The average primary particle diameter of CB can be calculated by observing and imaging the CB using a transmission electron microscope, then selecting 100 arbitrary spherical primary particles from the obtained observation images and measuring the outer diameter of each.

[0128] <Binder> The binder may be a resin capable of bonding substances such as positive electrode active material. In this specification, the resin used as a binder (hereinafter also referred to as the binder resin) is a different resin from the dispersants (A) and (B) described as dispersants. In other words, the binder may be selected from resins other than dispersants (A) and (B), and preferably contains at least a vinylidene fluoride polymer. In the slurry for the positive electrode film of this embodiment, the binder has an acid value of 1.0 mg KOH / g or more and 30.0 mg KOH / g or less, and contains a vinylidene fluoride polymer. By using such a specific binder, when the slurry for the positive electrode is coated onto a substrate such as a metal foil that will serve as a current collector, adhesion to the substrate is improved, and the durability of the electrode and the lifespan characteristics of the battery are improved.

[0129] The binder that can be used in this embodiment has an acid value of 1.0 mg KOH / g or more and 30.0 mg KOH / g or less. The acid value of the binder may preferably be 1.5 mg KOH / g or more, more preferably 2.0 mg KOH / g or more, and even more preferably 3.0 mg KOH / g or more. The acid value may preferably be 25 mg KOH / g or less, more preferably 20 mg KOH / g or less, and even more preferably 15 mg KOH / g or less. In some embodiments, the binder may consist only of a vinylidene fluoride polymer having an acid value within the above range. In other embodiments, the binder may be a mixed resin containing a vinylidene fluoride polymer and other resins, and the acid value of this mixed resin may be within the above range.

[0130] In this specification, the acid value of the binder is calculated as the acid value per unit of non-volatile content of the sample, obtained by titrating a sample obtained by adding the binder to NMP and methanol and stirring, using a 0.1 mol / L potassium hydroxide-ethanol aqueous solution as the titrant, and then considering the non-volatile content (binder) concentration of the sample. Specifically, it can be calculated according to the method described in the examples.

[0131] In some embodiments, the intrinsic viscosity of the vinylidene fluoride polymer is preferably 0.5 dL / g or more and less than 3.5 dL / g. The above intrinsic viscosity is the value measured at a temperature of 25°C after dissolving the vinylidene fluoride polymer in N-methyl-2-pyrrolidone. When a vinylidene fluoride polymer with an intrinsic viscosity within the above range is used, a positive electrode film with high adhesion can be easily obtained. The above intrinsic viscosity may more preferably be 0.8 dL / g or more, and even more preferably 1.0 dL / g or more. The above viscosity may more preferably be 3.0 dL / g or less, and even more preferably 2.5 dL / g or less. In this specification, the intrinsic viscosity of the binder is a value measured according to the method described in JIS K 6721-3.1-1977. More specifically, it can be measured according to the method described in the examples below.

[0132] The vinylidene fluoride polymer may be a polymer obtained using vinylidene fluoride, and the polymer may be either a homopolymer or a copolymer. In some embodiments, a homopolymer of vinylidene fluoride can be suitably used. Homopolymers of vinylidene fluoride (hereinafter also referred to as polyvinylidene fluoride or polyvinylidene fluoride resin) can be obtained as commercially available products. For example, solef5130 (manufactured by Solvay, polyvinylidene fluoride resin, intrinsic viscosity 1.2 dl / g, acid value 9.7 mg KOH / g), KF Polymer W#9700 (manufactured by Kureha, polyvinylidene fluoride resin, intrinsic viscosity 2.62 dl / g, acid value 3.2 mg KOH / g), KF Polymer W#7300 (manufactured by Kureha, polyvinylidene fluoride resin, intrinsic viscosity 3.20 dl / g, acid value 0.3 mg KOH / g), etc., can be suitably used.

[0133] In some embodiments, other resins may be used as the binder resin in addition to the vinylidene fluoride polymer described above. Examples include homopolymers or copolymers containing ethylene, propylene, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, acrylonitrile, styrene, vinyl butyral, vinyl acetal, vinylpyrrolidone, etc., as structural units; resins such as polyurethane resins, polyester resins, phenolic resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, and fluororesins; cellulose resins such as carboxymethylcellulose; rubbers such as styrene-butadiene rubber and fluororubber; and conductive resins such as polyaniline and polyacetylene. Modified forms, mixtures, or copolymers of these are also acceptable.

[0134] The weight-average molecular weight of the binder resin is preferably 10,000 to 5,000,000, more preferably 100,000 to 3,000,000, and even more preferably 200,000 to 1,500,000.

[0135] In some embodiments, the vinylidene fluoride polymer used as a binder resin may be a copolymer of polyvinylidene fluoride and another fluorine-containing monomer (hereinafter referred to as fluororesin). In some embodiments, the binder may further contain other fluororesins in addition to the vinylidene fluoride polymer. Fluororesins are resins containing fluorine atoms, exhibiting excellent heat resistance, chemical resistance, and tackiness, and functioning as binder resins. Fluororesins may have a structure in which hydrogen atoms in polyethylene are replaced with fluorine or trifluoromethyl atoms. Examples of fluororesins include homopolymers such as polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), and polychlorotrifluoroethylene (PCTFE); copolymers such as perfluoroalkoxyalkanes (PFA), perfluoroethylenepropene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and tetrafluoroethylene-perfluorodioxole copolymer (TFE / PDD), or homopolymers thereof. These may be used individually or in combination of two or more.

[0136] Among fluororesins, polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVdF), resins having these structural units, modified versions thereof, or combinations thereof are preferred in terms of resistance. Among these, polyvinylidene fluoride-based resins are particularly preferred, such as homopolymers of polyvinylidene fluoride; copolymers of polyvinylidene fluoride with hexafluoropropylene, tetrafluoroethylene, etc. Polyvinylidene fluoride-based resins may be modified, for example, by introducing acidic groups such as carboxyl groups. Fluororesins may be used individually or in combination of two or more types.

[0137] The weight-average molecular weight (Mw) of the fluororesin is preferably 100,000 to 5,000,000, more preferably 200,000 to 3,000,000, and even more preferably 500,000 to 1,500,000, in order to maintain a good balance between various resistances, adhesion, and resin viscosity. The glass transition temperature of the fluororesin is preferably 20°C or lower, more preferably 10°C or lower, and even more preferably 0°C or lower, from the viewpoint of film formation of the positive electrode film.

[0138] Examples of commercially available polyvinylidene fluoride and its modified forms include the KF Polymer series from Kureha Corporation, such as "W#7300, W#7200, W#1700, W#1300, W#1100, W#9700, W#9300, W#9100, L#7305, L#7208, L#1710, L#1320, L#1120," and the Solvay Solef series, such as "6008, 6010, 6012, 1015, 6020, 5130, 9007, 460, 41308, 11010, 21510, 31508, 60512" (all are product names).

[0139] The binder resin content in the slurry for the positive electrode film is preferably 0.1 to 30% by mass, more preferably 0.5 to 20% by mass, and even more preferably 1 to 10% by mass, based on the mass of the positive electrode active material (with the mass of the positive electrode active material being 100% by mass). Here, the positive electrode active material is preferably a manganese iron lithium (LMFP)-based positive electrode active material. However, when used in combination with other positive electrode active materials, it is preferable that the content of the dispersant and binder resin be within the above-mentioned ranges when the total amount of two or more positive electrode active materials is 100% by mass.

[0140] <Non-aqueous solvent> The slurry for the positive electrode film contains a non-aqueous solvent. The non-aqueous solvent is not particularly limited, but it is preferably a solvent that can dissolve the dispersant (A), and if it further contains a dispersant (B), it is preferable that the solvent also dissolves the dispersant (B).

[0141] As non-aqueous solvents, amide-based solvents (such as N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, and N-methylcaprolactam), heterocyclic solvents (such as cyclohexylpyrrolidone, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone, and γ-butyrolactone), sulfoxide-based solvents (such as dimethyl sulfoxide), sulfone-based solvents (such as hexamethylphosphorotriamide and sulfolane), lower ketone-based solvents (such as acetone and methyl ethyl ketone), and others such as tetrahydrofuran, urea, and acetonitrile can be used. The non-aqueous solvent preferably contains an amide-based organic solvent, and more preferably contains at least one selected from the group consisting of N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone.

[0142] The content of non-aqueous solvent in the cathode film slurry is not particularly limited. The amount of non-aqueous solvent used can be adjusted considering the amount of solids in the cathode film slurry. In some embodiments, the amount of solids in the cathode film slurry is preferably 30 to 90% by mass, more preferably 30 to 80% by mass, and even more preferably 40 to 75% by mass, based on the mass of the cathode film slurry (with the mass of the cathode film slurry being 100% by mass).

[0143] <Other ingredients> The cathode film slurry may further contain, if necessary, other optional components such as amine compounds, inorganic bases, wetting agents, defoaming agents, surfactants, pH adjusters, wetting and penetrating agents, antioxidants, preservatives, fungicides, and leveling agents.

[0144] <Method for manufacturing slurry for cathode film> The slurry for the positive electrode film can be manufactured by adding and mixing an LMFP-based positive electrode active material, a dispersant, a conductive material, a binder, and a non-aqueous solvent, either all at once or in separate steps. The dispersant preferably contains at least dispersant (A) and further dispersant (B). In some embodiments, the method for manufacturing a slurry for a positive electrode film is as follows: Step 1 involves preparing a conductive material dispersion containing a conductive material, a dispersant, and a non-aqueous solvent. It is preferable to include step 2 of mixing the conductive material dispersion, the positive electrode active material, and the binder to prepare a slurry for the positive electrode film.

[0145] In the above embodiment, in step 2 for preparing the slurry for the positive electrode film, it is preferable to adjust the solid content concentration to 60% or less. The solid content refers to the solid components used to constitute the slurry for the positive electrode film, such as conductive materials, dispersants, and binders. It is preferable to adjust the amount of each component so that the total amount of these components falls within a predetermined range. The solid components may be used in the form of a solution (dispersion) by mixing them with a solvent beforehand. In some embodiments, when a dispersant (B) is included, it is preferable that the dispersant (B) is pre-mixed with the solvent during the production of the cathode film slurry and used in the form of a dispersant-containing liquid. The content of the dispersant (B) is preferably 1 to 20% by mass, and more preferably 5 to 15% by mass, relative to the total mass of the dispersant-containing liquid. The binder is preferably used in the form of a binder-containing liquid, pre-mixed with a solvent during the production of the cathode film slurry. The binder content is preferably 1 to 40% by mass, and more preferably 3 to 20% by mass, relative to the total mass of the binder-containing liquid.

[0146] In a method for manufacturing a slurry for a cathode film, methods for mixing each component include, for example, using various dispersion devices such as a disperser, homogenizer, Silverson mixer, kneader, two-roll mill, three-roll mill, ball mill, horizontal sand mill, vertical sand mill, annular bead mill, attritor, planetary mixer, or high-pressure homogenizer.

[0147] 2. Cathode film One embodiment of the present invention is a positive electrode film obtained using a slurry for positive electrode films, the details of which are as described above. Specifically, the positive electrode film comprises a manganese iron lithium phosphate-based positive electrode active material, a conductive material, a dispersant, and a binder. The method for manufacturing the positive electrode film is not particularly limited; for example, the positive electrode film can be obtained by coating a current collector with the slurry for positive electrode films and drying it.

[0148] The material and shape of the current collector are not particularly limited, and those commonly used in various secondary batteries can be appropriately selected. For example, the material of the current collector can be a metal or alloy such as aluminum, copper, nickel, titanium, or stainless steel. In terms of shape, a flat foil is generally used, but current collectors with a roughened surface, perforated foil current collectors, or mesh current collectors can also be used.

[0149] There are no particular restrictions on the method for coating the current collector with a slurry for the positive electrode film, and known methods can be used. Specifically, examples include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic coating. As for drying methods, examples include drying by standing, or drying using a forced-air dryer, hot-air dryer, infrared heater, or far-infrared heater, but these are not particularly limited to these.

[0150] After coating, rolling may be performed using a flatbed press, calender roll, or the like. The thickness of the formed film is, for example, 1 μm to 500 μm, preferably 10 μm to 300 μm.

[0151] 3. Secondary battery A secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a positive electrode film obtained using a slurry for a positive electrode film, and the details of the slurry for the positive electrode film are as described above.

[0152] The positive electrode includes, for example, a current collector and a positive electrode film formed on the current collector. The positive electrode can be formed, for example, by coating a slurry for the positive electrode film on the current collector and drying it. That is, the positive electrode film contains at least a manganese iron phosphate lithium-based positive electrode active material, a conductive material, a dispersant (A), and a binder. Details are as described above.

[0153] The negative electrode includes, for example, a current collector and a negative electrode film formed on the current collector. The negative electrode film can be formed, for example, by coating a slurry for the negative electrode film on the current collector and drying it. As the current collector of the negative electrode, those described for the current collector of the positive electrode above can be used. The slurry for the negative electrode film is not particularly limited, but can contain, for example, a negative electrode active material, a binder, and a solvent. As the negative electrode active material, a material capable of reversibly doping or intercalating lithium ions can be used. For example, alloy systems such as metallic Li, its alloy tin alloy, silicon alloy, lead alloy, etc.; metal oxide systems such as LiXFe2O3, LiXFe3O4, LiXWO2 (x is a number where 0 < x < 1), lithium titanate, lithium vanadate, lithium silicate, etc.; conductive polymer systems such as polyacetylene, poly-p-phenylene, etc.; carbonaceous powders such as highly graphitized carbon materials, artificial graphite, natural graphite, etc.; carbon-based materials such as resin-fired carbon materials. The negative electrode active material can also be used alone or in combination of two or more.

[0154] As the binder for the negative electrode film, one or a combination of two or more of the resins described above as the binder for the positive electrode film can be used. As the binder for the negative electrode film, carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, etc. with good adhesion are preferred. The slurry for the negative electrode film may further contain optional additives such as a dispersant, a conductive material, etc. Examples of the dispersant include surfactant-type dispersants, resin-type dispersants, etc. Examples of the conductive material include carbon materials such as carbon black, carbon nanotubes, carbon nanofibers, fullerenes, etc.

[0155] The binder content in the slurry for the negative electrode film is preferably 0.1 to 30% by mass, more preferably 0.5 to 20% by mass, and even more preferably 1 to 10% by mass, based on the mass of the negative electrode active material (with the mass of the negative electrode active material being 100% by mass). If the slurry for the negative electrode film contains a dispersant, the amount of dispersant in the slurry for the negative electrode film is preferably 0.01 to 10% by mass, and more preferably 0.05 to 5% by mass, based on the mass of the negative electrode active material (with the mass of the negative electrode active material being 100% by mass). When a conductive material is included in the slurry for the negative electrode film, the content of the conductive material in the slurry for the negative electrode film is preferably 0.01 to 10% by mass, more preferably 0.02 to 5% by mass, and even more preferably 0.03 to 3% by mass, based on the mass of the negative electrode active material (with the mass of the negative electrode active material being 100% by mass).

[0156] The solid content in the slurry for the negative electrode film is preferably 30 to 90% by mass, more preferably 30 to 80% by mass, and even more preferably 40 to 75% by mass, based on the mass of the slurry for the negative electrode film (with the mass of the slurry for the negative electrode film being 100% by mass).

[0157] The electrolyte may be a liquid electrolyte, a gel electrolyte, or a solid electrolyte. For example, a liquid electrolyte may contain an electrolyte salt such as a lithium salt and a non-aqueous solvent. Various conventionally known electrolyte salts that allow ion movement can be used. Examples include, but are not limited to, lithium salts such as LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, Li(CF3SO2)3C, LiI, LiBr, LiCl, LiAlCl, LiHF2, LiSCN, or LiBPh4 (where Ph is a phenyl group). It is preferable to dissolve the electrolyte salt in a non-aqueous solvent and use it as an electrolyte solution.

[0158] Non-aqueous solvents are not particularly limited, but examples include carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; glycines such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile. These solvents may be used individually or in combination of two or more.

[0159] Examples of gel-like electrolytes include those obtained by dissolving a polymer in a liquid electrolyte to form a gel. Examples of solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes. The secondary battery may be any of the following: a non-aqueous electrolyte secondary battery using a liquid electrolyte or a gel-like electrolyte, an all-solid-state secondary battery using a solid electrolyte, or a semi-solid-state secondary battery.

[0160] The secondary battery may further include a separator. The separator is used by being placed between the positive electrode and the negative electrode. The separator is not particularly limited, but examples include polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, porous polyethylene, porous polypropylene, porous polyamide, etc., and those which have been treated to be hydrophilic.

[0161] The structure of the secondary battery in this embodiment is not particularly limited. Typically, the secondary battery comprises a positive electrode, a negative electrode, and a separator provided as needed, and can take on various shapes depending on the intended use, such as paper type, cylindrical type, button type, or laminated type.

[0162] The secondary battery of this embodiment can be used for various applications. For example, it can be used in vehicles, electronic devices, small communication devices, large-capacity batteries, stationary storage batteries, etc. Examples of vehicles include electric vehicles, hybrid vehicles, plug-in hybrid vehicles, trains, motorcycles, electric bicycles, motorized bicycles, construction vehicles, etc. Among them, electric vehicles, hybrid vehicles, and plug-in hybrid vehicles have increasing global demand, so the secondary battery of this embodiment can be preferably used in these vehicles. The slurry for the positive electrode film of this embodiment has excellent dispersibility of lithium manganese iron phosphate-based positive electrode active material and conductive material, and good storage stability (viscosity stability). Therefore, the positive electrode film formed using the slurry for the positive electrode film of this embodiment has excellent electrode characteristics such as adhesion and conductivity. Furthermore, the secondary battery obtained using the positive electrode film of this embodiment can achieve good high-temperature storage characteristics and high cycle characteristics, and can be preferably used for various applications.

Examples

[0163] The present invention will be further specifically described below with reference to examples. The present invention is not limited to the following examples as long as the gist thereof is not exceeded. Unless otherwise specified, "part" represents "part by mass" and "%" represents "mass%". In addition, in the examples, "hydrogenated nitrile butadiene rubber" may be referred to as "H-NBR" or "dispersant". Furthermore, in the examples, a "dispersant-containing liquid" containing a "dispersant" and a solvent may be referred to as a "dispersion liquid". In the following description, N-methyl-2-pyrrolidone is also denoted as NMP.

[0164] <1> Production Example of Raw Materials <1-1> Production of Positive Electrode Active Material (LMFP1) Add 200 g of dimethyl sulfoxide to 150 g of pure water, and add 360 mmol of lithium hydroxide monohydrate. To the resulting solution, add 120 mmol of phosphoric acid using an 85 wt% aqueous phosphoric acid solution, then add 90 mmol of manganese(II) sulfate monohydrate and 30 mmol of iron(II) sulfate heptahydrate. Transfer the resulting solution to an autoclave and heat and maintain the temperature inside the container at 150°C for 4 hours. After heating, discard the supernatant of the solution and obtain lithium iron manganese phosphate (LiMn) as a precipitate. 0.75 Fe 0.25 PO4 was obtained. The obtained lithium iron manganese phosphate was washed with pure water, and the supernatant was removed by centrifugation, a process that was repeated five times. Finally, pure water was added again to obtain a dispersion. Subsequently, 15% by weight of lithium iron manganese phosphate in the dispersion and the same weight of glucose were added to the dispersion and dissolved. Then, pure water was added to adjust the solid content concentration of the dispersion to 20% by weight to obtain an LMFP dispersion. The obtained LMFP dispersion was dried with hot air at 200°C using a spray dryer (Fujisaki Electric Co., Ltd., MDL-050B) to obtain secondary particles. The obtained secondary particles were heated in a rotary kiln under a nitrogen atmosphere at 700°C for 4 hours to obtain carbon-coated LMFP particles (hereinafter referred to as LMFP1).

[0165] <1-2> Example of preparation of dispersant (B) (Preparation of a dispersant-containing solution including H-NBR1) Capacity 1000cm 3 475 parts by mass of NMP4 and 25 parts by mass of NaOH (manufactured by Tosoh Corporation, Tosoh Pearl) were added to a plastic container. The mixture was then dispersed at a speed of 9000 rpm using a high-shear mixer (L5M-A, manufactured by Silverson) equipped with a fine emulsion screen until the mixture was uniform. Finally, the mixture was passed through a nylon filter with a mesh size of 150 μm using a filtration bell to prepare an NaOH dispersion (NaOH concentration 5% by mass). 864 parts by mass of NMP were charged into a reaction vessel equipped with a gas inlet tube, thermometer, condenser, and stirrer, and the vessel was purged with nitrogen gas. The reaction vessel was then heated to 80°C, and 80 parts by mass of Thermon(R)3406 (manufactured by ARLANXEO, hydrogenated acrylonitrile-butadiene rubber) were added and stirred until the hydrogenated nitrile-butadiene rubber was completely dissolved. Then, 56 parts by mass of NaOH dispersion were added, and the mixture was stirred while adding air. The reaction vessel was heated while maintaining the temperature at 80°C for 12 hours to obtain a dispersant-containing solution (solid content concentration 8% by mass) that included H-NBR1.

[0166] <2> Description of the raw materials used in the examples <2-1> Measurement methods for various physical properties <Carbon coating of positive electrode active material> The "carbon coating content (carbon coating rate)" on the surface of the positive electrode active material refers to the value obtained by the following method. Specifically, the surface of the positive electrode active material particles is observed in "Field of View 1" captured by electron microscopy using a TEM, and the surfaces of positive electrode active material particles that are not coated with carbon and the surfaces of positive electrode active material particles that are coated with carbon are identified. Next, the "circumference xn of the surface of the positive electrode active material particles that are not coated with carbon" and the "circumference xc of the surface of the positive electrode active material particles that are coated with carbon" are measured within this Field of View 1. The obtained xn and xc are added together to obtain the "total circumference xB of the surface of the positive electrode active material particles". Next, the values ​​of "total circumferential length xB of the surface of the positive electrode active material particles" and "circumferential length xc of the surface of the carbon-coated positive electrode active material particles" obtained as described above are introduced into the following equation (2) to calculate the carbon coverage rate (%) in field of view 1. Following this method, the values ​​obtained for 50 fields of view are averaged to obtain the carbon coverage rate (%) on the surface of the positive electrode active material particles. Formula (2): Carbon coverage (%) = [(Circumference of the surface of the carbon-coated positive electrode active material particle xc) / (Total circumference of the surface of particle b xB)] × 100

[0167] <BET specific surface area of ​​positive electrode active material> The specific surface area of ​​the positive electrode active materials in the examples and comparative examples was measured using a specific surface area meter (product name: BELSORP-mini, manufactured by Nippon Bell Co., Ltd.) by the BET method with nitrogen (N2) adsorption.

[0168] <Average particle size of positive electrode active material D 50 > Average particle size D of the positive electrode active material in the examples and comparative examples 50 This particle size corresponds to 50% of the cumulative volume in the particle size distribution curve measured using the laser diffraction method.

[0169] <Acid value of the binder> 3.0 g of 8% by mass NMP solution of binder was mixed with 50 g of NMP and 10 g of methanol, and stirred to dissolve uniformly. The solution was then titrated with a 0.1 mol / L potassium hydroxide-ethanol aqueous solution as the titrant using an automatic titrator ("COM-555," manufactured by Hiranuma Sangyo Co., Ltd.) to measure the acid value. The acid value per unit of non-volatile content of the sample was then calculated from the calculated acid value and the non-volatile content concentration of the sample.

[0170] The acid value (mgKOH / g) of the dry sample was determined using the following formula (3). Formula (3): Acid value (mgKOH / g) = {(5.611 × α × F) / S} / (non-volatile content concentration / 100) In the above formula, S represents the sample volume (g). α represents the amount of 0.1 mol / L potassium hydroxide-ethanol solution consumed (ml). F represents the titer of the 0.1 mol / L potassium hydroxide-ethanol solution.

[0171] <Binder's intrinsic viscosity> The intrinsic viscosity of the binder η i The measurements were performed in accordance with the method described in JIS K 6721-3.1-1977. Specifically, the measurements were performed using the following method. A binder solution was prepared by dissolving 80 mg of binder in 20 ml of N,N-dimethylformamide. The viscosity (intrinsic viscosity) η of the binder solution was measured using an Ubbelohde viscometer in a 30°C constant temperature bath. Similarly, the viscosity η0 of N,N-dimethylformamide was measured using an Ubbelohde viscometer in a 30°C constant temperature bath. The intrinsic viscosity η was then calculated based on the following formula (4). i They sought it. Formula (4): η i =(1 / C)·ln(η / η0) In the above formula, η0 is the viscosity of the solvent, N,N-dimethylformamide, and C is 0.4 g / dl.

[0172] <Measurement of residual double bonds in hydrogenated nitrile butadiene rubber> The mass ratio (RDB value) of residual double bonds in hydrogenated nitrile butadiene rubber was measured by infrared spectroscopy using total internal reflection in samples before and after hydrogenation. Solid copolymers were used directly for measurement. For dispersant-containing liquids (dispersant compositions), the mixture was treated with hot air at 100°C for 10 hours to allow it to dry completely, and the resulting solid copolymer was used as the measurement sample. IR measurements were performed on the measurement samples using an infrared spectrophotometer (Nicolet iS5 FT-IR spectrometer, Thermo Fisher Scientific, Inc.). The double bond originating from the conjugated diene monomer unit was 970 cm⁻¹. -1 A peak appears, and the hydrogenated single bond is 723 cm⁻¹. -1 Since a peak appeared, the hydrogenation rate was calculated from the ratio of the heights of these two peaks. The mass ratio of residual double bonds (RDB) is calculated using the following formula (1). Formula (1): RDB(mass%)=(BD / (BD+HBD))×100 In equation (1), BD is the mass ratio of structural units derived from conjugated dienes having unsaturated bonds to the total mass of structural units derived from conjugated dienes, and HBD is the mass ratio of structural units derived from conjugated dienes with hydrogenated unsaturated bonds to the total mass of structural units derived from conjugated dienes.

[0173] <2-2> Specific explanation of raw materials The raw materials used in the examples are as follows: (CNT) ·10B: JENOTUBE10B (manufactured by JEIO, multilayer CNT, average outer diameter 10 nm, specific surface area 230 m 2 / g) ·6A:JENOTUBE6A (manufactured by JEIO, multilayer CNT, average outer diameter 6nm, specific surface area 700m 2 / g) ·100T: K-Nanos 100T (manufactured by Kumho Petrochemical, multilayer CNT, average outer diameter 13 nm, specific surface area 210 m 2 / g)

[0174] (Cathode active material) • LMFP1: The LMFP-based cathode active material obtained in the manufacturing example described above was used. ·LMFP2:LMFP73(LiMn 0.7 Fe 0.3 PO4, made by SKYLAND) BET specific surface area 18.3m 2 / g, D 50 0.88μm, carbon coverage 1.5% by mass ·LMFP3:LMFP64(LiMn 0.6 Fe 0.4 PO4, made by SKYLAND) BET specific surface area 19.7m 2 / g, D 50 0.81μm, carbon coverage 1.8% by mass ·LMFP4:LMFP-60M(LiMn 0.6 Fe 0.4 PO4, made by Easpring Technology (Changzhou) New Material) BET specific surface area 15.8m 2 / g, D 50 0.7μm, carbon coverage 1.3% by mass • LFP: HED (trademark) LFP-400 (Lithium iron phosphate, manufactured by BASF)

[0175] (binder) • PVdF1:solef5130 (manufactured by Solvay, polyvinylidene fluoride resin), intrinsic viscosity 1.2 dl / g, acid value 9.7 mgKOH / g PVdF2:KF Polymer W#9700 (manufactured by Kureha Corporation, polyvinylidene fluoride resin), intrinsic viscosity 2.62 dl / g, acid value 3.2 mgKOH / g PVdF3:KF Polymer W#7300 (manufactured by Kureha Corporation, polyvinylidene fluoride resin), intrinsic viscosity 3.20 dl / g, acid value 0.3 mgKOH / g

[0176] (Dispersant (A)) • Dispersant A-1: ​​BYK-ET3004 (manufactured by Bic Chemie Japan, alkylammonium salt of copolymer with acidic groups, acid value 65.1 mg KOH / g) • Dispersant A-2: DISPERBYK(registered trademark)-111 (manufactured by Bic Chemie Japan, phosphate group-containing copolymer, acid value 65.1 mg KOH / g) • Dispersant A-3: BYK-ET3003 (manufactured by BIC Chemie Japan, phosphate group-containing copolymer, acid value 72.0 mg KOH / g) • Dispersant A-4: BYK-ET3000 (manufactured by Bic Chemie Japan, modified styrene-maleic acid copolymer solution, acid value 4.1 mg KOH / g)

[0177] (Dispersant (B)) • H-NBR1: (Dispersant obtained in the preparation example described above, 8% by mass NMP solution, RDB value of H-NBR1 0.3) • H-NBR2: Therban(R)AT 3404 (manufactured by ARLANXEO, hydrogenated acrylonitrile-butadiene rubber, RDB value 0.5) • PVP: Polyvinylpyrrolidone K-90 (manufactured by Nippon Shokubai) • PVB: Polyvinyl acetal (acetalization degree 15 mol%) A 10% aqueous solution of polyvinyl alcohol (degree of polymerization 300, degree of saponification 98-99 mol%) was prepared. To 100 parts by mass of the aqueous solution, 0.2 parts by mass of hydrochloric acid and 2 parts by mass of butyraldehyde were added dropwise while stirring. Subsequently, the temperature was raised to 80°C and held for 1 hour, then allowed to cool. This was dried and pulverized to obtain polyvinyl acetal.

[0178] <Evaluation of dispersants (oxidation resistance)> The oxidation resistance of the dispersants was evaluated by preparing half-cells according to the following procedure and measuring them using linear sweep voltammetry (LSV) with an HSV-110 (Hokuto Denko Co., Ltd.). The oxidation resistance was evaluated for the dispersants listed in Table 1 (dispersants (A) and (B)). When dispersants (A) and (B) were used together, they were mixed to a solid content ratio of 1:1 for evaluation. Denka Black Li-400 (manufactured by Denka Co., Ltd.), the dispersants listed in Table 1 (dispersants (A) and (B)), and NMP were added to a 150 mL plastic container. The mixture was then stirred at 2,000 rpm for 5 minutes using a rotation and revolution mixer (Awatori Rentaro, ARE-310, manufactured by Thinky Co., Ltd.) to obtain an evaluation slurry with a solid content of 10% (solid content ratio Li-400:dispersant = 1:1). The obtained evaluation slurry was applied to a 20 μm thick aluminum foil, which would serve as the current collector, using an applicator to prepare an evaluation electrode. The fabricated evaluation electrode was punched out to a diameter of 16 mm to serve as the working electrode, and a metallic lithium foil (thickness 0.20 mm) was used as the counter electrode. A separator made of porous polypropylene film was inserted and laminated between the working electrode and the counter electrode, and the cell was filled with an electrolyte (a non-aqueous electrolyte prepared by dissolving LiPF6 at a concentration of 1 M in a mixed solvent of ethylene carbonate and diethyl carbonate in a 1:1 ratio) to assemble a two-electrode sealed metal cell (Hosen Co., Ltd. HS flat cell). The half-cell was assembled in a glove box purged with argon gas. After the half-cell was assembled, LSV measurement was performed. The electrode weight was calculated by measuring the mass of the punched working electrode and subtracting the mass of the aluminum foil used for coating.

[0179] (LSV measurement conditions) Measurement potential: +3.0 to +3.8V Sweep speed: 10mV / min The current density (mA / mg) was calculated by dividing the current value obtained from LSV measurements by the electrode weight. Higher voltages and lower current densities indicate better oxidation resistance of the dispersant. (Judgment criteria) A: At a voltage of 3.8V, the current density does not reach 1mA / mg. B: Current density reaches 1 mA / mg at a voltage of 3.6V to 3.8V. C: Current density reaches 1 mA / mg at a voltage of less than 3.6V.

[0180] [Table 1]

[0181] As shown in Table 1, the form of the dispersant constituting the cathode slurry in this embodiment, namely dispersant (A) and a combination of dispersant (A) and dispersant (B), can be seen to achieve excellent oxidation resistance. In particular, among the compounds having acidic groups, a comparison between dispersants (A-1) to (A-3) with predetermined acid values ​​and dispersant (A-4) whose acid value is outside the predetermined range shows that the use of dispersant (A) can improve oxidation resistance. Furthermore, a comparison with the case where only dispersant (B) is used shows that the combined use of dispersant (A) and dispersant (B) can improve oxidation resistance.

[0182] <3> Manufacturing of CNT dispersion A conductive material dispersion was prepared according to the composition (type and amount of conductive material, dispersant (A), dispersant (B), and non-aqueous solvent) listed in Table 2, using the following manufacturing example. (Manufacturing Example 1) First, NMP and H-NBR1 (8% by mass NMP solution) were added to a stainless steel container and stirred while heating to 50°C. Next, CNT (10B) was added while stirring with a disperser, and a square-hole high-shear screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was performed at a speed of 8,000 rpm until the mixture was uniform and the dispersed particle size was 250 μm or less using a grind gauge with a maximum groove depth of 300 μm. At this time, the dispersed particle size confirmed with the grind gauge was 200 μm. Next, the dispersion liquid was supplied from the stainless steel container to a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine) via piping, and a circulating dispersion treatment was performed. The dispersion treatment was carried out using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After dispersion until the viscosity of the dispersion liquid at 60 rpm, measured with a B-type viscometer (TOKI SANGYO, VISCOMETER, MODEL: BL), was 3,000 mPa·s or less, a pass-type dispersion treatment (8 passes) was performed in the high-pressure homogenizer, and after dispersion, the mixture was passed through a nylon mesh with a mesh opening of 48 μm three times to obtain a conductive material dispersion liquid (conductive material dispersion liquid 1).

[0183] (Manufacturing example 2) First, NMP and H-NBR1 (8% by mass NMP solution) were added to a stainless steel container and stirred while heating to 50°C. Next, CNT (10B) was added while stirring with a disperser, and a square-hole high-shear screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was performed at a speed of 8,000 rpm until the mixture was uniform and the dispersed particle size was 250 μm or less using a grind gauge with a maximum groove depth of 300 μm. At this time, the dispersed particle size confirmed with the grind gauge was 200 μm. Next, the dispersion liquid was supplied from the stainless steel container to a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine) via piping, and a circulating dispersion treatment was performed. The dispersion treatment was carried out using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After dispersion until the viscosity of the dispersion liquid at 60 rpm, measured with a B-type viscometer (TOKI SANGYO, VISCOMETER, MODEL: BL), was 3,000 mPa·s or less, a pass-type dispersion treatment (8 passes) was performed in the high-pressure homogenizer. The obtained dispersion was transferred to another stainless steel container, and dispersant A-1 was added little by little while stirring with a disperser and mixed. The mixture was passed through a nylon mesh with a mesh size of 48 μm three times to obtain a conductive material dispersion (conductive material dispersion 2).

[0184] (Manufacturing Example 3) First, NMP and dispersant (A) were added to a stainless steel container and stirred while heating to 50°C. Next, CNT (10B) was added while stirring with a disperser, and a square-hole high-shear screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was performed at a speed of 8,000 rpm until the mixture was uniform and the dispersed particle size was 250 μm or less using a grind gauge with a maximum groove depth of 300 μm. At this time, the dispersed particle size confirmed with the grind gauge was 250 μm. Next, the dispersion liquid was supplied from the stainless steel container to a high-pressure homogenizer (Starburst Lab HJP-17007, manufactured by Sugino Machine) via piping, and a circulating dispersion treatment was performed. The dispersion treatment was carried out using a single nozzle chamber with a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After dispersion until the viscosity of the dispersion liquid at 60 rpm, measured with a B-type viscometer (TOKI SANGYO, VISCOMETER, MODEL: BL), was 4,000 mPa·s or less, a pass-type dispersion treatment (8 passes) was performed in the high-pressure homogenizer, and after dispersion, the mixture was passed through a nylon mesh with a mesh size of 48 μm three times to obtain a conductive material dispersion (conductive material dispersion liquid 3).

[0185] (Manufacturing examples 4-16) Conductive material dispersions 4 to 16 were obtained using the same method as in Production Example 2, according to the compositions (type and amount of conductive material, dispersant (A), dispersant (B), and non-aqueous solvent) listed in Table 2.

[0186] [Table 2]

[0187] <3> Slurry for cathode films <3-1> Preparation of slurry for cathode film (Example 1-1) In a 150 mL plastic container, 1.4 parts of conductive material dispersion 1 (equivalent to solid content), 0.2 parts of dispersant A-1 (equivalent to solid content), and an NMP solution containing 8% by mass of PVdF1 (equivalent to solid content of 2.5 parts) were added. Then, 93.9 parts of LMFP1 were added as the positive electrode active material, followed by the addition of NMP. The mixture was stirred at 2,000 rpm for 5 minutes using a rotation and revolution mixer (Awatori Rentaro, ARE-310, manufactured by Thinky Co., Ltd.) to obtain a slurry 1 for a positive electrode film with a solid content of 44%.

[0188] (Examples 1-2) In a 150 mL plastic container, 1.7 parts of conductive material dispersion 2 (equivalent to solid content) and 2.5 parts of NMP solution containing 8% by mass of PVdF1 were added. Then, 95.8 parts of LMFP1 were added as the positive electrode active material, followed by the addition of NMP. The mixture was stirred at 2,000 rpm for 5 minutes using a rotation and revolution mixer (Awatori Rentaro, ARE-310, manufactured by Thinky Co., Ltd.) to obtain a slurry 2 for a positive electrode film with a solid content of 44%.

[0189] (Examples 1-3) In a 150 mL plastic container, 1.4 parts of conductive material dispersion 3 (equivalent to solid content) and an NMP solution containing 8% by mass of PVdF1 (equivalent to solid content, 2.5 parts) were added. Then, 96.1 parts of LMFP1 were added as the positive electrode active material, followed by the addition of NMP. The mixture was stirred at 2,000 rpm for 5 minutes using a rotation and revolution mixer (Awatori Rentaro, ARE-310, manufactured by Thinky Co., Ltd.) to obtain a slurry 3 for a positive electrode film with a solid content of 44%.

[0190] (Examples 1-4 to 1-14) Slurries 4 to 14 for the positive electrode film were obtained using the same manufacturing method as in Example 2, except that conductive material dispersion 2 was replaced with conductive material dispersions 4 to 14.

[0191] (Examples 1-15) In a 150 mL plastic container, 4.8 parts of CB in terms of solid content, 0.2 parts of dispersant A-1 in terms of solid content, and an NMP solution (2.5 parts in terms of solid content) in which 8 mass% of PVdF1 was dissolved were added. Then, 92.5 parts of LMFP1 was added as a positive electrode active material, and further NMP was added. Using a rotation and revolution mixer (Sumitomo Riko Co., Ltd.'s Awatori Renjiro, ARE-310), it was stirred at 2,000 rpm for 5 minutes to obtain a slurry 15 for a positive electrode film with a solid content of 44%.

[0192] (Examples 1-16 to 1-17) Except for changing the type of dispersant (A) according to Table 3, in the same manner as in Example 15, slurries 16 to 17 for a positive electrode film were obtained.

[0193] (Example 1-18) In a 150 mL plastic container, 0.2 parts of conductive material dispersion liquid 1 in terms of solid content, 4.0 parts of CB in terms of solid content, 0.2 parts of dispersant A-1 in terms of solid content, and an NMP solution (2.5 parts in terms of solid content) in which 8 mass% of PVdF1 was dissolved were added. Then, 93.1 parts of LMFP1 was added as a positive electrode active material, and further NMP was added. Using a rotation and revolution mixer (Sumitomo Riko Co., Ltd.'s Awatori Renjiro, ARE-310), it was stirred at 2,000 rpm for 5 minutes to obtain a slurry 18 for a positive electrode film with a solid content of 44%.

[0194] (Examples 1-19 to 1-22, Comparative Examples 1-1 to 1-4) According to the compositions in Table 3, by the same production method as in Example 1, slurries 19 to 22 for a positive electrode film and comparative slurries 1 to 4 for a positive electrode film were obtained.

[0195] <3-2>Evaluation of Slurry for Positive Electrode Film For each slurry for a positive electrode film obtained in the examples and comparative examples, various evaluations were carried out according to the methods described below. The results are shown in Table 3.

[0196] <Viscosity of Slurry for Positive Electrode Film The viscosity of the slurry composition for the cathode film was measured using a viscoelasticity measuring device (Anton Paar "MCR 102e") with a cone plate of 0.5° / 25mm, a measurement temperature of 25°C, and a shear rate of 0.1s. -1 The viscosity was measured. A lower value indicates better dispersion of the active material. The evaluation criteria are as follows: (Evaluation Criteria) A: Less than 100,000 mPa·s B: 100,000 mPa·s or more and less than 300,000 mPa·s C: 300,000 mPa·s or more and less than 700,000 mPa·s D: 700,000mPa·s or more

[0197] <Storage stability of cathode film slurry> After preparing the slurry for the cathode film, a 50 mL vial was filled with the slurry and left to stand at 25°C for 5 days. The solid content concentrations of the supernatant and bottom were then measured. Storage stability was calculated using the following formula: ((Solid content concentration of the supernatant after 5 days of standing in the container) / (Solid content concentration of the bottom after 5 days of standing in the container)) × 100. The closer the value is to 100%, the better the sedimentation is suppressed and the superior the storage stability. The evaluation criteria are as follows. (Evaluation Criteria) A: Over 95% B: 90% or more but less than 95% C: 85% to less than 90% D: Less than 85%

[0198] [Table 3]

[0199] <4> Fabrication of positive electrode film <4-1> Fabrication of the positive electrode film (Example 2-1) A slurry 1 for the positive electrode film was applied to a 20 μm thick aluminum foil, which would serve as the current collector, using an applicator. The film was then dried in an electric oven at 120°C ± 5°C for 25 minutes, resulting in a positive electrode weight of 20 mg / cm² per unit area. 2It was adjusted so as to become. Further, rolling treatment was performed using a roll press (manufactured by Sanko Metal Co., Ltd., 3t hydraulic roll press), and the density of the positive electrode composite material layer was 3.1 g / cm 3 The positive electrode film 1 with such density was obtained.

[0200] (Examples 2-2 to 2-22, Comparative Examples 2-1 to 2-4) In accordance with the formulations shown in Table 4, positive electrode films 2 to 22 and comparative positive electrode films 1 to 4 were obtained in the same manner as in Example 2-1.

[0201] <4-2>Evaluation of the positive electrode film For each of the positive electrode films obtained in the examples and comparative examples, various evaluations were performed according to the methods described below. The results are shown in Table 4.

[0202] <Adhesion of the positive electrode film> The adhesion of the positive electrode film was evaluated by measuring the peel strength. The composite material slurry was coated on the aluminum foil using an applicator so that the coating amount per unit area of the positive electrode was 20 mg / cm 2 After that, the coating film was dried in an electric oven at 120 °C ± 5 °C for 25 minutes. Then, it was cut into two rectangles of 90 mm × 20 mm with the coating direction as the long axis. For the measurement of the peel strength, a tabletop tensile tester (manufactured by Toyo Seiki Seisakusho Co., Ltd., Strograph E3) was used and evaluated by the 180-degree peel test method. Specifically, a double-sided tape (No. 5000NS, manufactured by Nitto Denko Corporation) with a size of 100 mm × 30 mm was attached to a stainless steel plate, and the prepared battery electrode composite material layer was adhered to the other side of the double-sided tape, and peeled while pulling upward from below at a constant speed (50 mm / min), and the average value of the stress at this time was taken as the peel strength. The adhesion of the positive electrode film was evaluated from the value of the peel strength. The evaluation criteria are as follows. (Evaluation criteria) A: Peel strength is 1.0 N / cm or more B: Peel strength is 0.7 N / cm or more and less than 1.0 N / cm C: Peel strength is 0.5 N / cm or more and less than 0.7 N / cm D: Peel strength is less than 0.5 N / cm

[0203] <Electrode resistance of the positive electrode film> The asphalt slurry is applied using an applicator, and the basis weight per unit of the electrode is 20 mg / cm³. 2 After coating the aluminum foil in the manner described, the coating film was dried in an electric oven at 120°C ± 5°C for 25 minutes. Subsequently, the surface resistivity (Ω / □) of the dried coating film was measured using a resistivity meter (Mitsubishi Chemical Analytech Co., Ltd.: Rolester GP (MCP-T610), probe: AP2 probe (RMH333)). After measurement, the volume resistivity (Ω·cm) of the electrode film was obtained by multiplying the surface resistivity by the thickness of the electrode composite layer formed on the aluminum foil. The thickness of the electrode composite layer was determined by subtracting the thickness of the aluminum foil from the average value of three points measured in the electrode film using a film thickness gauge (NIKON, DIGIMICRO MH-15M) to obtain the volume resistivity (Ω·cm) of the electrode film. The evaluation criteria for volume resistivity are as follows. (Evaluation Criteria) A: Less than 8Ω·cm B: 8Ω·cm or higher, less than 12Ω·cm C: 12Ω·cm or more, less than 15Ω·cm D: 15Ω cm or more

[0204] <5> Fabrication and evaluation of secondary batteries Using the respective positive electrode films obtained in the examples and comparative examples, secondary batteries were fabricated according to the method described below. (Preparation of standard negative electrode slurry) A plastic container with a capacity of 150 mL was added with Denka Black Li-400 (manufactured by Denka Co., Ltd.), CMC #1190 (manufactured by Daicel Finechem Co., Ltd.), and water. Then, using a rotation and revolution mixer (Sumikiki Awatotoran, ARE-310 manufactured by Sumikiki Co., Ltd.), it was stirred at 2,000 rpm for 30 seconds. Further, artificial graphite CGB-20 (manufactured by Nippon Graphite Industry Co., Ltd.) was added as a negative electrode active material, and using the rotation and revolution mixer, it was stirred at 2,000 rpm for 150 seconds. Subsequently, SBR (styrene-butadiene rubber, TRD2001 (manufactured by JSR Corporation, solid content 48%)) was added, and using the rotation and revolution mixer, it was stirred at 2,000 rpm for 30 seconds to obtain a composite slurry for a standard negative electrode. The solid content of the composite slurry for the standard negative electrode was 48% by mass. The solid content ratio of the negative electrode active material: conductive material: CMC: SBR in the composite slurry for the standard negative electrode was 97:0.5:1:1.5.

[0205] (Fabrication of Standard Negative Electrode Film) The composite slurry for the negative electrode was coated on a copper foil with a thickness of 20 μm serving as a current collector using an applicator, and then dried in an electric oven at 80 °C ± 5 °C for 25 minutes so that the coating amount per unit area of the electrode was 10 mg / cm 2 It was adjusted. Further, rolling treatment was performed using a roll press (3t hydraulic roll press manufactured by Sanku Metal Co., Ltd.) to fabricate a negative electrode film with a density of the negative electrode composite layer of 1.5 g / cm 3

[0206] (Fabrication of Secondary Battery) ​The standard negative electrode and the respective positive electrode films prepared in the examples and comparative examples were punched out to 50 mm × 45 mm and 45 mm × 40 mm, respectively. A separator (porous polypropylene film) to be inserted between them was placed in an aluminum laminate bag and dried in an electric oven at 70°C for 1 hour. Subsequently, 2 mL of electrolyte was injected in a glove box filled with argon gas, and the aluminum laminate bag was sealed to prepare secondary battery example B-1. The electrolyte was a non-aqueous electrolyte prepared by mixing ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a 1:1:1 (volume ratio) mixture, and then adding 1 part VC (vinylene carbonate) per 100 parts of the electrolyte as an additive, followed by dissolving LiPF6 at a concentration of 1 M.

[0207] The characteristics of each secondary battery obtained as described above were evaluated according to the method described below. The results are shown in Table 4. <High-temperature storage test for secondary batteries> A secondary battery was placed in a constant temperature room at 25°C, and charge / discharge measurements were performed using a charge / discharge device (SM-8, manufactured by Hokuto Denko Co., Ltd.). Constant current and constant voltage charging (cutoff current 0.6mA) was performed with a charging current of 12mA (0.2C) and a charging termination voltage of 4.3V, followed by constant current discharge at a discharge current of 12mA (0.2C) and a discharge termination voltage of 3V. This operation was repeated three times, and the discharge capacity of the third discharge was taken as the 0.2C discharge capacity at 25°C. Subsequently, constant current and constant voltage charging (cutoff current 0.6mA) was performed with a charging current of 12mA (0.2C) and a charging termination voltage of 4.3V, and the battery was stored in a constant temperature room set to 55°C for 7 days. Finally, constant current discharge was performed with a discharge current of 12mA (0.2C) and a discharge termination voltage of 3V to determine the discharge capacity. The high-temperature storage characteristics were determined by the ratio of the 0.2C discharge capacity at 25°C to the 0.2C discharge capacity after 7 days of storage at 55°C (0.2C discharge capacity after 7 days of storage at 55°C / 0.2C discharge capacity at 25°C × 100) (%). The evaluation results are shown in Table 4. (Judgment criteria) A: High temperature storage characteristics are 80% or higher. B: High-temperature storage characteristics are between 70% and 80%. C: High-temperature storage characteristics are between 60% and 70%. D: High-temperature storage characteristics are less than 60%

[0208] <Secondary battery cycle testing> The secondary battery was placed in a constant temperature room at 45°C, and charge / discharge measurements were performed using a charge / discharge device (SM-8, manufactured by Hokuto Denko Co., Ltd.). The battery was fully charged using constant current and constant voltage charging at a charge rate of 0.5C (upper voltage 4.3V, cutoff current 0.025C), and discharged to the lower discharge voltage of 2.0V using the same constant current as during charging. This constituted one charge / discharge cycle (with a 30-minute pause between charge / discharge cycles). A total of 100 such cycles were performed, and the charge / discharge capacity retention rate was calculated as the ratio of the electrical capacity at the end of 100 cycles to the electrical capacity at the end of 5 cycles ((electrical capacity at the end of 100 cycles / electrical capacity at the end of 5 cycles) × 100) (%). A larger value indicates better cycle characteristics. The evaluation results are shown in Table 4. (Judgment criteria) A: Cycle characteristics are 90% or higher B: Cycle characteristics are between 85% and 90% C: Cycle characteristics are between 80% and 85% D: Cycle characteristics less than 80%

[0209] [Table 4] As shown in Table 4, the positive electrode films formed using the slurry for positive electrode films of this embodiment (Examples 2-1 to 2-22) are superior to the comparative positive electrode films in terms of adhesion, electrode resistance (conductivity), and battery characteristics such as high-temperature storage characteristics and cycle characteristics.

Claims

1. A slurry for a cathode film containing a cathode active material, a manganese iron lithium phosphate-based cathode active material represented by the following chemical formula (I), a dispersant, a conductive material, a binder, and a non-aqueous solvent, The conductive material comprises at least one selected from the group consisting of carbon nanotubes and carbon black. The dispersant comprises a dispersant (A) having an acid value of 35 mg KOH / g or more and 150 mg KOH / g or less. The amount of the dispersant is 0.01 parts by mass or more and 8 parts by mass or less with respect to 100 parts by mass of the positive electrode active material. The binder is a slurry for cathode films, comprising a vinylidene fluoride polymer and having an acid value of 1.0 mg KOH / g or more and 30.0 mg KOH / g or less. Chemical formula (I): Li １＋ａ Fe １－ｘ－ｙ Mn ｘ M ｙ (2O ４－ｂ )X ｂ [In the above chemical formula (I), M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y. X is at least one element selected from the group consisting of F, S, and N. a, b, x, and y are integers satisfying -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, 0.1 ≤ x ≤ 0.8, and 0 ≤ y ≤ 0.1, respectively.

2. The average particle size D of the positive electrode active material ５０ The slurry for a positive electrode film according to claim 1, wherein the particle size is 0.05 μm or more and 3.0 μm or less.

3. The BET specific surface area of the positive electrode active material determined by nitrogen adsorption measurement is 1 m ２ / g or more and 50 m ２ / g or less, and the slurry for a positive electrode film according to claim 1.

4. The slurry for a cathode film according to claim 1, wherein the lithium iron manganese phosphate-based cathode active material has a carbon coating on its surface.

5. The dispersant (A) is -COOM, -SO ３ M, and -PO(OM) ２ A slurry for a cathode film according to claim 1, comprising an organic compound having at least one acidic group selected from the group consisting of [wherein M represents a hydrogen atom, a quaternary amine, or an alkali metal].

6. The slurry for a cathode film according to claim 1, wherein the dispersant further comprises a dispersant (B) different from the dispersant (A).

7. The slurry for a cathode film according to claim 6, wherein the dispersant (B) comprises at least one selected from polyvinylpyrrolidone resin, polyvinyl acetal resin, and hydrogenated acrylonitrile butadiene rubber.

8. The slurry for a cathode film according to claim 6, wherein the dispersant (B) comprises a dispersant (B1), the dispersant (B1) is a copolymer having structural units having a nitrile group and structural units derived from a conjugated diene in which the unsaturated bond is partially hydrogenated, and the mass ratio of residual double bonds (RDB) calculated by the following formula (1) is 0.05 to 5% by mass. Formula (1): RDB (mass%) = (BD / (BD+HBD)) x 100 [In formula (1), BD is the mass ratio of structural units derived from conjugated dienes having unsaturated bonds to the total mass of structural units derived from conjugated dienes.] HBD is the mass ratio of structural units derived from conjugated dienes with hydrogenated unsaturated bonds to the total mass of structural units derived from conjugated dienes.

9. The cathode film slurry according to claim 1, wherein the content of the conductive material is 0.1 parts by mass or more and 3 parts by mass or less per 100 parts by mass of solid content of the cathode film slurry.

10. The cathode film slurry according to claim 1, wherein the intrinsic viscosity of the vinylidene fluoride polymer, measured at a temperature of 25°C after dissolving in N-methyl-2-pyrrolidone, is 0.5 dL / g or more and less than 3.5 dL / g.

11. A positive electrode film formed using the slurry for positive electrode films described in any one of claims 1 to 10.

12. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a positive electrode film formed using the slurry for positive electrode films described in any one of claims 1 to 10.

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