Positive electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

This positive electrode (11) for a non-aqueous electrolyte secondary battery comprises a positive electrode mixture layer (31) containing a positive electrode active material and a conductive agent, and is characterized in that: the conductive agent contains fibrous carbon; and when the content ratio of the conductive agent to the mass of the positive electrode active material is defined as X [mass%], and the ratio of the area of the surface of the positive electrode active material coated with the conductive agent to the surface area of the positive electrode active material in the positive electrode 11 is defined as Y, the relationship of (Y / X)≥1.0 is satisfied.

Description

Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

[0001] This disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

[0002] In recent years, non-aqueous electrolyte secondary batteries have seen expanding applications, including power sources for electric vehicles and energy storage devices for utilizing renewable energy. The positive electrode significantly influences battery characteristics, including battery capacity, rate characteristics, and cycle characteristics, and therefore, much research has been conducted on positive electrodes. For example, Patent Document 1 discloses a positive electrode that uses both single-walled carbon nanotubes and multi-walled carbon nanotubes as conductive agents. According to the positive electrode of Patent Document 1, the conductivity of the positive electrode is improved, and the rate characteristics of the battery are improved.

[0003] International Publication No. 2022-079409

[0004] In non-aqueous electrolyte secondary batteries, improving cycle characteristics and low-temperature operating characteristics while simultaneously improving rate characteristics is a crucial challenge. Generally, increasing the amount of conductive agent improves rate characteristics, but it can also increase the likelihood of side reactions between the conductive agent and the non-aqueous electrolyte on the conductive agent surface, potentially degrading cycle characteristics. Furthermore, increasing the amount of conductive agent can reduce the voids in the positive electrode mixture layer, decreasing the diffusivity of the non-aqueous electrolyte and potentially degrading low-temperature operating characteristics. Therefore, improving cycle characteristics and low-temperature operating characteristics while simultaneously improving rate characteristics is not easy.

[0005] A positive electrode for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, is a positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode mixture layer containing a positive electrode active material and a conductive agent, wherein the conductive agent contains fibrous carbon, the content of the conductive agent relative to the mass of the positive electrode active material is X [mass%], and in the positive electrode, when Y is the ratio of the area covering the surface of the positive electrode active material with the conductive agent to the surface area of ​​the positive electrode active material, the relationship (Y / X) ≥ 1.0 is satisfied.

[0006] Furthermore, a non-aqueous electrolyte secondary battery according to one aspect of this disclosure is a non-aqueous electrolyte secondary battery comprising the above-mentioned positive electrode, negative electrode, and non-aqueous electrolyte, wherein the non-aqueous electrolyte is, as an electrolyte salt, lithium hexafluorophosphate (LiPF)６ ) and lithium bis(fluorosulfonyl)imide (LiFSI), LiPF ６ The ratio of the mass of LiFSI to the mass of is characterized by being between 10% and 50% by mass.

[0007] According to a positive electrode for a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, it is possible to improve cycle characteristics and low-temperature operating characteristics while improving rate characteristics.

[0008] This figure schematically shows an axial cross-section of a non-aqueous electrolyte secondary battery (cylindrical battery), which is an example of an embodiment.

[0009] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using said positive electrode will be described in detail. Note that the embodiments and modified forms described below, obtained by selectively combining the various components, are included within the scope of this disclosure.

[0010] In the following, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 is given as an example of a non-aqueous electrolyte secondary battery, but the battery outer casing is not limited to a cylindrical shape. The non-aqueous electrolyte secondary battery according to this disclosure may be, for example, a prismatic battery with a prismatic outer casing, a coin-type battery with a coin-type outer casing, or a pouch-type battery with an outer casing made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body is not limited to a wound type, and may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator in between. In addition, the design of the non-aqueous electrolyte secondary battery according to this disclosure is not limited to the example design of the non-aqueous electrolyte secondary battery, and known designs of non-aqueous electrolyte secondary batteries may be applied.

[0011] Figure 1 is an axial cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte, and an outer casing 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. Hereafter, for the sake of convenience of explanation, the side of the battery with the sealing body 17 will be referred to as "upper," and the bottom side of the outer casing 16 will be referred to as "lower."

[0012] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all rectangular elongated bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape in the longitudinal direction. The separator 13 separates the positive electrode 11 and the negative electrode 12 from each other. Two separators 13 are arranged, for example, so as to sandwich the positive electrode 11. The electrode body 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the short direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the end faces in the short direction of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode body 14.

[0013] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0014] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure the hermeticity inside the battery. The exterior body 16 is formed with a groove-in portion 22 that supports the sealing body 17, where a part of the side surface portion projects inward. The groove-in portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16 and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the upper part of the exterior body 16 by the groove-in portion 22 and the open end portion of the exterior body 16 caulked to the sealing body 17.

[0015] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and an insulating member 25 is interposed between the peripheral portions of each. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 is deformed and broken so as to push up the upper valve body 26 toward the cap 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further rises, the upper valve body 26 breaks and gas is discharged from the opening of the cap 27.

[0016] Hereinafter, the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte constituting the non-aqueous electrolyte secondary battery 10 will be described in detail, particularly the positive electrode 11.

[0017] [Positive Electrode] The positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 formed on the surface of the positive electrode core 30. The positive electrode mixture layer 31 is preferably formed on both surfaces of the positive electrode core 30. For the positive electrode core 30, a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, and a film or the like having the metal disposed on the surface layer can be used. The thickness of the positive electrode core 30 is, for example, 10 μm or more and 30 μm or less.

[0018] The positive electrode mixture layer 31 includes, for example, a positive electrode active material, a conductive agent containing fibrous carbon, and a binder. As will be described in detail later, the conductive agent covers a predetermined range on the surface of the particles of the positive electrode active material. The positive electrode mixture layer 31 is preferably provided on both surfaces of the positive electrode core 30 except for the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. to the surface of the positive electrode core 30, drying the coating film, and then compressing it to form the positive electrode mixture layer 31 on both surfaces of the positive electrode core 30.

[0019] The positive electrode active material includes particulate lithium transition metal composite oxide. The lithium transition metal composite oxide is a composite oxide containing metal elements such as Ni, Co, Mn, Al, etc. in addition to Li. The metal elements constituting the lithium transition metal composite oxide are, for example, at least one selected from Ni, Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Mo, Mg, Cu, V, Ga, Ge, Y, Sn, W, Bi, and Nb. Among them, it is preferable to contain at least one selected from Ni, Co, Al, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, Mn, and lithium transition metal composite oxides containing Ni, Co, Al. The content of the positive electrode active material is preferably 80% by mass or more and 99.5% by mass or less, more preferably 85% by mass or more and 99% by mass or less, based on the mass of the positive electrode mixture layer 31.

[0020] Lithium transition metal composite oxides include, for example, secondary particles formed by the aggregation of primary particles. The particle size of the primary particles constituting the secondary particles of lithium transition metal composite oxides is, for example, between 0.02 μm and 2 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM). The average particle diameter of the secondary particles of lithium transition metal composite oxides is, for example, between 2 μm and 30 μm. Here, the average particle diameter refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of secondary particles of lithium transition metal composite oxides can be measured using a laser diffraction particle size distribution analyzer (for example, Microtrac-Bell MT3000II) with water as the dispersion medium.

[0021] Lithium transition metal composite oxides have, for example, a layered structure. Examples of layered structures of lithium transition metal composite oxides include layered structures belonging to space group R-3m and layered structures belonging to space group C2 / m. From the viewpoint of increasing capacity and stabilizing the crystal structure, it is preferable for lithium transition metal composite oxides to have a layered structure belonging to space group R-3m.

[0022] Lithium transition metal composite oxides include, for example, those with the compositional formula Li ｙ Ni ｘ M １－ｘ O ２－δ (In the formula, 0.5 ≤ x ≤ 1.0, 0 < y ≤ 1.2, 0 ≤ δ ≤ 0.05, and M is at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Mo, Mg, Cu, V, Ga, Ge, Y, Sn, W, Bi, and Nb). In particular, it is preferable that M includes at least one element selected from the group consisting of Co, Mn, and Al. The content of the elements constituting the lithium transition metal composite oxide can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron beam microanalyzer (EPMA), or an energy dispersive X-ray spectrometer (EDX), etc.

[0023] In lithium transition metal composite oxides, the Ni content relative to the total number of moles of metal elements excluding Li is preferably 80 mol%, and may be 85 mol% or more. When the Ni content is 80% or more, the improvement in rate characteristics and cycle characteristics described later is more pronounced. The upper limit of the Ni content may be, for example, 98 mol%.

[0024] As described above, the positive electrode mixture layer 31 contains a conductive agent including fibrous carbon. The fibrous carbon may be carbon nanofibers (CNF) or the like, but is preferably carbon nanotubes (CNT). Carbon nanotubes are conductive carbon fibers with an outer diameter of several tens of nanometers or less, and have an extremely large aspect ratio (ratio of fiber length to fiber diameter). The average aspect ratio of carbon nanotubes is, for example, 20 times or more, preferably 50 times or more. With carbon nanotubes having a high aspect ratio, contact with the active material and core material becomes linear contact rather than point contact. Therefore, a good conductive path is formed with only a small amount of additive.

[0025] The average fiber diameter of carbon nanotubes is, for example, 30 nm or less, and may also be 20 nm or less. Note that fiber diameter refers to the length in the direction perpendicular to the fiber length. If the average fiber diameter is 30 nm or less, the plate resistance is further reduced, and the rate characteristics tend to improve more easily. There is no particular lower limit to the average fiber diameter of carbon nanotubes, but one example is 1 nm. The average fiber diameter of carbon nanotubes can be determined by image analysis using TEM. The average fiber diameter of carbon nanotubes can be determined by arbitrarily selecting 100 carbon nanotubes, measuring their fiber diameters, and taking the arithmetic mean of the measured values.

[0026] The average fiber length of the carbon nanotubes is, for example, 0.5 μm or more, and may be 1 μm or more. Note that the fiber length means the length when the carbon nanotubes are extended linearly. If the average fiber length is 0.5 μm or more, the polar plate resistance is further reduced, and the rate characteristics are likely to be improved. The upper limit value of the average fiber length of the carbon nanotubes is not particularly limited, but as an example, it is 100 μm. The average fiber length of the carbon nanotubes is determined by image analysis using a scanning electron microscope (SEM). The average fiber length of the carbon nanotubes is obtained by arbitrarily selecting 100 carbon nanotubes, measuring their lengths, and calculating the arithmetic mean of the measured values.

[0027] The BET specific surface area of the carbon nanotubes varies slightly depending on the type of carbon nanotubes. As an example, it is 200 m ２ / g or more, and more preferably 250 m ２ / g or more. The upper limit of the BET specific surface area is not particularly limited, but as an example, it is 2000 m ２ / g. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0028] The carbon nanotubes may be either single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT), but it is preferable to include single-walled carbon nanotubes and multi-walled carbon nanotubes respectively. When single-walled carbon nanotubes and multi-walled carbon nanotubes are included, the conductive agent is more likely to cover the surface of the particles of the positive electrode active material. Note that single-walled carbon nanotubes are carbon nanotubes having a structure in which a single layer of graphite sheet is formed in a tubular shape, and multi-walled carbon nanotubes are carbon nanotubes having a structure in which multiple layers of graphite sheets are formed in a tubular shape. An example of multi-walled carbon nanotubes is bilayer carbon nanotubes having a bilayer structure.

[0029] The mass ratio of single-walled carbon nanotubes to the total mass of fibrous carbon (carbon nanotubes) is preferably 1% by mass or more and 30% by mass or less, and more preferably 3% by mass or more and 25% by mass or less. When the mass ratio of single-walled carbon nanotubes is within the above range, a slurry containing a conductive agent can be easily prepared, and the conductive agent can more easily coat the surface of the positive electrode active material particles.

[0030] Carbon nanotubes are supplied to the cathode manufacturing process in the form of a conductive agent dispersion, in which they are dispersed in a liquid containing a dispersant and an aprotic polar solvent (dispersion medium), and are added to the cathode mixture slurry. In the dispersion, the dispersant is dissolved in the polar solvent, and the carbon nanotubes are dispersed in the polar solvent by the action of the dispersant. The solid content concentration (carbon nanotubes and dispersant) of the dispersion is, for example, 0.1% by mass or more and 30% by mass or less, and preferably 0.2% by mass or more and 20% by mass or less from the viewpoint of balancing the dispersibility of carbon nanotubes and productivity.

[0031] The dispersant may include, for example, nitrile group-containing rubber. The nitrile group-containing rubber is a copolymer of monomers containing unsaturated nitrile and conjugated diene as raw materials, and may be substantially a copolymer of only unsaturated nitrile and conjugated diene. The molar ratio of unsaturated nitrile to conjugated diene is, for example, 10:90 or more and 70:30 or less. The weight-average molecular weight of the nitrile group-containing rubber is not particularly limited, but as an example, it is 5000 or more and 500000 or less. In addition, at least a portion of the dispersant may function as a binder for the positive electrode mixture layer 31.

[0032] The nitrile group-containing rubber may be hydrogenated nitrile rubber. Hydrogenated nitrile rubber includes, for example, structural units derived from unsaturated nitriles, structural units derived from conjugated dienes, and hydrogenated structural units derived from conjugated dienes. A suitable example of hydrogenated nitrile rubber is a partially hydrogenated nitrile rubber in which 80 mol% or more of the structural units derived from conjugated dienes are hydrogenated. An example of an unsaturated nitrile is acrylonitrile or methacrylonitrile, preferably acrylonitrile. An example of a conjugated diene is a conjugated diene having 4 to 6 carbon atoms, preferably butadiene.

[0033] The dispersion of the conductive agent may contain only nitrile group-containing rubber as a dispersant, or other dispersants may be used in combination. Examples of other dispersants used in combination with nitrile group-containing rubber include polyvinyl alcohol, polyvinylpyrrolidone (PVP), polyalkylene oxide, polyvinyl acetal, polyvinyl ether, cellulose or its derivatives, chitins, chitosans, starch, and derivatives thereof. Among these, the use of PVP or its derivatives (PVPs) is preferred.

[0034] An aprotic polar solvent is used as the dispersion medium. Any aprotic polar solvent that can dissolve the dispersant and disperse the conductive agent is acceptable. Since the dispersion is added to the positive electrode slurry, the polar solvent is preferably miscible with the solvent in the positive electrode slurry, and may be the same type of solvent as the positive electrode slurry. Examples of aprotic polar solvents include N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, tetrahydrofuran, dimethylformamide, acetone, ethyl acetate, and dimethyl sulfoxide. Among these, NMP is preferred.

[0035] Here, the positive electrode 11 of this disclosure satisfies the relationship (Y / X) ≥ 1.0, where X [mass%] is the content of the conductive agent relative to the mass of the positive electrode active material, and Y is the ratio of the area covering the surface of the positive electrode active material with the conductive agent relative to the surface area of ​​the positive electrode active material. When (Y / X) ≥ 1.0 is satisfied, the proportion of the conductive agent present on the surface of the positive electrode active material increases, allowing electrons to be efficiently supplied to the positive electrode active material. As a result, the rate characteristics can be improved. Also, when (Y / X) ≥ 1.0 is satisfied, the proportion of the conductive agent present elsewhere than on the surface of the positive electrode active material decreases, ensuring voids in the positive electrode mixture layer. As a result, the diffusivity of the non-aqueous electrolyte is improved, and the low-temperature operating characteristics are improved. Furthermore, when (Y / X) ≥ 1.0 is satisfied, the area in contact between the conductive agent and the positive electrode active material increases, and the area in contact between the conductive agent and the non-aqueous electrolyte decreases. This suppresses side reactions between the conductive agent and the non-aqueous electrolyte on the conductive agent surface. As a result, when charging and discharging are repeated, the accumulation of reactants produced by the side reaction on the surface of the positive electrode active material and the leaching of transition metals from the positive electrode active material are suppressed, improving the cycle characteristics.

[0036] The relationship (Y / X) ≥ 1.0 is sufficient, but it is preferable that (Y / X) ≥ 1.05 be satisfied, and more preferably that (Y / X) ≥ 1.1 be satisfied. As (Y / X) increases, cycle characteristics and low-temperature operating characteristics can be further improved while rate characteristics are improved. There is no particular upper limit to (Y / X), but for example it is 2.0.

[0037] Here, the content of the conductive agent relative to the mass of the positive electrode active material, X [mass%], is preferably 0.05 mass% or more and 0.8 mass% or less, more preferably 0.07 mass% or more and 0.7 mass% or less, and even more preferably 0.1 mass% or more and 0.6 mass% or less. When the content of the conductive agent, X [mass%], is 0.05 mass% or more and 0.8 mass% or less, the conductive agent is more likely to coat the particle surface of the positive electrode active material, making it easier to satisfy the relationship (Y / X) ≥ 1.0. As a result, cycle characteristics and low-temperature operating characteristics can be improved while rate characteristics are improved.

[0038] Furthermore, the ratio Y of the area of ​​the surface of the positive electrode active material covered by the conductive agent to the surface area of ​​the positive electrode active material is preferably 0.05 or more and 0.8 or less, more preferably 0.1 or more and 0.7 or less, and even more preferably 0.2 or more and 0.6 or less. The ratio (Y) of the area of ​​the surface of the positive electrode active material covered by the conductive agent is calculated by the following method: (1) For the positive electrode 11 before charging and discharging, the outermost surface of the positive electrode 11 is removed with tape or the like, and a backscattered electron image of the exposed surface of the positive electrode 11 is taken using a scanning electron microscope. The area from which the backscattered electron image is taken is, for example, an area of ​​100 μm × 100 μm. (2) The surface image obtained above is imported into a computer, and binarization processing is performed using image analysis software (for example, ImageJ from the National Institutes of Health, USA), to obtain a binarized image in which the area where the conductive agent is present on the particle surface is made white, and the area where the conductive agent is not present on the particle surface is made black. The ratio of the area converted to white to the area of ​​the captured area becomes the ratio (Y) of the area covered by the conductive agent on the surface of the positive electrode active material.

[0039] The positive electrode mixture layer 31 may contain materials other than fibrous carbon as a conductive agent. Examples of materials other than fibrous carbon include carbon black (particulate carbon) such as acetylene black, furnace black, and channel black, as well as graphene. Preferably, the conductive agent contained in the positive electrode mixture layer 31 is mainly composed of fibrous carbon. Here, "main component" means the component that accounts for the largest mass percentage of the conductive agent. In this case, cycle characteristics and low-temperature operating characteristics can be further improved while improving rate characteristics. The positive electrode mixture layer 31 may contain only fibrous carbon as a conductive agent.

[0040] The positive electrode mixture layer 31 may contain a binder in addition to the positive electrode active material and conductive agent. Examples of binders include fluorine-containing polymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc. The binder content is, for example, 0.1% by mass or more and 2% by mass or less, preferably 0.2% by mass or more and 1.5% by mass or less, based on the total mass of the positive electrode active material.

[0041] As described above, the positive electrode 11 can be manufactured by applying a positive electrode mixture slurry to the surface of the positive electrode core 30, drying the coating, and then compressing it to form a positive electrode mixture layer 31 on both sides of the positive electrode core 30. Here, the positive electrode mixture slurry is prepared by kneading a positive electrode active material (lithium transition metal composite oxide), a conductive agent dispersion, and a dispersant such as N-methyl-2-pyrrolidone (NMP).

[0042] Mixing methods include, for example, mixing using various dispersers such as planetary mixers, homogenizers, ball mills, sand mills, and roll mills. Among these, mixing using a planetary mixer is preferable from the viewpoint of increasing the ratio (Y) of the surface area (Y) covered by the conductive agent on the surface of the positive electrode active material. An example of a planetary mixer is the Hibismix manufactured by PRIMIX, in which the stirring blades perform planetary motion.

[0043] Furthermore, the ratio (Y) of the surface area of ​​the positive electrode active material covered by the conductive agent can be adjusted by the mixing conditions. For example, when mixing using a planetary mixer, increasing the rotation speed can increase the ratio (Y) of the surface area covered by the conductive agent. The rotation speed can be set appropriately depending on the equipment used and is not particularly limited. Also, the ratio (Y) of the surface area of ​​the positive electrode active material covered by the conductive agent tends to increase as the conductive agent content X decreases.

[0044] [Negative Electrode] The negative electrode 12 may have, for example, a negative electrode core 40 and a negative electrode mixture layer 41 formed on the surface of the negative electrode core 40, or a metallic Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode core 40, and lithium metal may be deposited on the surface of the negative electrode core 40 by charging. When the negative electrode 12 has a negative electrode mixture layer 41, it is preferable that the negative electrode mixture layer 41 is formed on both sides of the negative electrode core 40. The negative electrode core 40 can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal arranged on its surface. The thickness of the negative electrode core 40 is, for example, 5 μm or more and 30 μm or less.

[0045] The negative electrode mixture layer 41 includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer 41 is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode core 40. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., to the surface of the negative electrode core 40, drying the coating film, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0046] The negative electrode active material contained in the negative electrode mixture layer 41 is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si and Sn, or lithium titanium composite oxides may be used as the negative electrode active material. Furthermore, materials with a carbon coating may be used. For example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0047] Examples of binders included in the negative electrode mixture layer 41 include styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0048] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0049] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0050] [Non-aqueous electrolytes] Non-aqueous electrolytes are liquid electrolytes (electrolyte solutions) that have ionic conductivity (e.g., lithium ion conductivity). Liquid electrolytes (electrolyte solutions) include, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Non-aqueous solvents can include, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. Non-aqueous solvents may contain halogen-substituted products in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted products include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0051] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0052] Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methylphenyl ether. Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0053] An example of an electrolyte salt is a lithium salt. An electrolyte salt is lithium hexafluorophosphate (LiPF). ６ Preferably, the solution contains ) and lithium bis(fluorosulfonyl)imide (LiFSI). The concentration of the electrolyte salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 1.8 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0054] Here, LiPF ６ The ratio of the mass of LiFSI to the mass of LiPF is preferably 5% by mass or more, and more preferably 10% by mass or more. ６ By setting the ratio of LiFSI mass to the mass of PF to 10% by mass or more, ionic conductivity is improved, and rate characteristics and cycle characteristics are more easily improved. ６The ratio of the mass of LiFSI to the mass of LiPF is preferably 50% by mass or less, and more preferably 45% by mass or less. ６ By setting the ratio of the mass of LiFSI to the mass of PF to 50% by mass or less, for example, the electrochemical stability is improved. Therefore, LiPF ６ The ratio of the mass of LiFSI to the mass of is preferably 5% by mass or more and 50% by mass or less, and more preferably 10% by mass or more and 45% by mass or less.

[0055] Note that the electrolyte salt contains LiPF ６ It may also contain lithium salts other than LiFSI. Examples of lithium salts include LiClO ４ LiBF ４ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO4). ２ F ２ Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２)), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) and others are used.

[0056] Non-aqueous electrolytes may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0057] Examples of unsaturated cyclic carbonate esters include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. Unsaturated cyclic carbonate esters may be used individually or in combination of two or more. Some hydrogen atoms in the unsaturated cyclic carbonate esters may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by the intermolecular condensation of multiple carboxylic acid molecules, but it is preferable that it be an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0058] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0059] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanate methylcyclohexane (BIMCH). Examples of sultone compounds include propanesultone and propensultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethylborate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethylphosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethylphosphite and tris(trimethylsilyl)phosphite.

[0060] The present disclosure will be further explained below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples. <Example 1> [Preparation of positive electrode] As the positive electrode active material, LiN ｉ０．８ Co ０．１０ Mn ０．１０ O ２ The following was used. In addition, single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) were used in combination as conductive agents. Then, a powder obtained by dry-blending the above carbon nanotubes and polyvinylpyrrolidone (PVP) as a dispersant in a mass ratio of 4:1 was added to N-methyl-2-pyrrolidone (NMP), and a conductive agent dispersion was obtained by dispersing using a ball mill.

[0061] Then, the above-mentioned positive electrode active material, the above-mentioned conductive agent dispersion, and polyvinylidene fluoride (PVDF) were mixed in a solid content mass ratio of 100:0.3:0.5, excluding the dispersant. Using N-methyl-2-pyrrolidone (NMP) as the dispersion medium, the mixture was kneaded using Hibismix manufactured by PRIMIX Corporation to prepare a positive electrode mixture slurry. During this process, the rotation speed was adjusted so that the ratio (Y) of the area covered by the conductive agent on the surface of the positive electrode active material was 0.3.

[0062] The positive electrode mixture slurry was applied to both sides of a positive electrode core made of aluminum foil, the coating was dried, and then the coating was rolled out using a roller and cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core. A portion of the positive electrode was provided with an exposed area where the surface of the positive electrode core was exposed.

[0063] [Fabrication of the negative electrode] Natural graphite and SiO were used as the negative electrode active material. Natural graphite, SiO, carboxymethylcellulose sodium (CMC-Na), and a dispersion of styrene-butadiene rubber (SBR) were mixed in a solid content mass ratio of 95:5:1:1, and a negative electrode mixture slurry was prepared using water as the dispersion medium. This negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and after the coating film was dried, the coating film was rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. An exposed portion was provided on a part of the negative electrode in which the surface of the negative electrode core was exposed.

[0064] [Preparation of Non-Aqueous Electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4 to prepare a non-aqueous solvent. Lithium hexafluorophosphate (LiPF) was added to this non-aqueous solvent. ６ A non-aqueous electrolyte (electrolyte solution) was prepared by dissolving an electrolyte salt containing ) and lithium bis(fluorosulfonyl)imide (LiPFSI) to a concentration of 1.2 mol / liter. ６ The ratio of the mass of LiFSI to the mass of the other substance was set to 10% by mass.

[0065] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] Aluminum leads were attached to the exposed part of the positive electrode and nickel leads to the exposed part of the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator to create a wound electrode body. This electrode body was placed inside a bottomed cylindrical outer container, the non-aqueous electrolyte was injected, and the opening of the outer container was sealed with a sealing body to obtain a test cell.

[0066] [Evaluation of Rate Characteristics (Evaluation of DC Resistance (DCR))] For the test cell, constant current charging was performed at a constant current of 0.2C at a temperature of 25°C until the cell voltage reached 4.2V. Then, constant voltage charging was performed until the current value was 0.02C at 4.2V. After that, the test cell was discharged at a constant current of 0.2C until the capacity equivalent to 50% of the capacity (SOC 50%) was reached. A constant current of 0.5C was then applied to the test cell for 10 seconds. The DC resistance (DCR) was calculated by dividing the resulting potential difference by the current value. A smaller DC resistance (DCR) indicates better rate characteristics.

[0067] [Evaluation of Cycle Characteristics] The test cell was charged to 4.2V with a constant current of 0.3C under a temperature environment of 25°C, and then charged at a constant voltage of 4.2V until the current value was equivalent to 0.01C. After a 1-hour rest period, it was discharged to 2.5V with a constant current of 0.5C, which was considered one cycle. The discharge capacity at 0.3C after 300 cycles was measured. The capacity retention rate was calculated using the following formula: Capacity retention rate = Discharge capacity after 300 cycles / Discharge capacity in the first cycle

[0068] [Evaluation of Low-Temperature Operating Characteristics] The test cell was charged to 4.2V with a constant current of 0.2C in a 25°C environment, and then charged again at a constant voltage of 4.2V until the current value was equivalent to 0.01C. After a 1-hour rest, it was discharged at a constant current of 1.0C until it reached 2.5V. The test cell was moved to a -10°C environment and left undisturbed for 24 hours. After that, it was charged to 4.2V with a constant current of 0.2C, and then charged again at a constant voltage of 4.2V until the current value was equivalent to 0.01C. After a 1-hour rest, it was discharged at a constant current of 1.0C until it reached 2.5V. The low-temperature operating characteristics were determined using the following formula: Low-temperature operating characteristics = Discharge capacity at a -10°C environment / Discharge capacity at a 25°C environment

[0069] <Example 2> In preparing the positive electrode, the content of the conductive agent (carbon nanotube) relative to the mass of the positive electrode active material (X) was set to 0.2% by mass, and the mixture was kneaded so that the ratio of the surface area covered by the conductive agent on the surface of the positive electrode active material (Y) was 0.22. Except for these other factors, a test cell was prepared and evaluated in the same manner as in Example 1.

[0070] <Example 3> In preparing the positive electrode, the rotation speed was increased compared to Example 1 so that the ratio (Y) of the area covered by the conductive agent on the surface of the positive electrode active material was 0.33. Otherwise, a test cell was prepared and evaluated in the same manner as in Example 1.

[0071] <Example 4> In preparing the positive electrode, the test cell was prepared and evaluated in the same manner as in Example 2, except that the rotation speed was increased compared to Example 2 so that the ratio (Y) of the area covered by the conductive agent on the surface of the positive electrode active material was 0.24.

[0072] <Comparative Example 1> In the preparation of the positive electrode, a test cell was prepared and evaluated in the same manner as in Example 1, except that the mixture was kneaded using a foam remover manufactured by Thinky Co., Ltd. to prepare the positive electrode slurry. At this time, the ratio (Y) of the area covered by the conductive agent on the surface of the positive electrode active material was 0.27.

[0073] <Comparative Example 2> In the preparation of the positive electrode, a test cell was prepared and evaluated in the same manner as in Example 3, except that only multi-walled carbon nanotubes were used as carbon nanotubes. At this time, the ratio (Y) of the area covered by the conductive agent on the surface of the positive electrode active material was 0.21.

[0074] Table 1 shows the evaluation results for DC resistance (DCR), capacitance retention rate, and low-temperature operating characteristics of the test cells of Examples 1-3 and Comparative Examples 1 and 2. Note that the DC resistance (DCR), capacitance retention rate, and low-temperature operating characteristics shown in Table 1 are expressed relatively, with the evaluation result of Comparative Example 1 set to 100. A smaller DC resistance (DCR) value indicates lower resistance and superior rate characteristics. A larger capacitance retention rate indicates superior cycle characteristics. Similarly, a larger low-temperature operating characteristic indicates superior low-temperature operating characteristics.

[0075]

[0076] As shown in Table 1, the test cell of the embodiment satisfying the relationship (Y / X) ≥ 1.0 exhibits reduced DC resistance (DCR), improved capacitance retention, and improved low-temperature operating characteristics compared to the test cell of the comparative example. Therefore, it can be said that satisfying the relationship (Y / X) ≥ 1.0 improves rate characteristics, cycle characteristics, and low-temperature operating characteristics.

[0077] This disclosure is further illustrated by the following embodiments. Configuration 1: A positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode mixture layer containing a positive electrode active material and a conductive agent, wherein the conductive agent contains fibrous carbon, the content of the conductive agent relative to the mass of the positive electrode active material is X [mass%], and in the positive electrode, when Y is the ratio of the area covering the surface of the positive electrode active material with the conductive agent to the surface area of ​​the positive electrode active material, the relationship (Y / X) ≥ 1.0 is satisfied. Configuration 2: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the fibrous carbon includes single-walled carbon nanotubes and multi-walled carbon nanotubes. Configuration 3: The positive electrode active material has the compositional formula Li ｙ Ni ｘ M １－ｘ O ２－δ A positive electrode for a non-aqueous electrolyte secondary battery according to configuration 1 or 2, comprising a lithium transition metal composite oxide represented by the formula (wherein 0.8 ≤ x ≤ 1.0, 0 < y ≤ 1.2, 0 ≤ δ ≤ 0.05, and M is at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Mo, Mg, Cu, V, Ga, Ge, Y, Sn, W, Bi, and Nb). Configuration 4: A positive electrode for a non-aqueous electrolyte secondary battery according to any one of configurations 1 to 3, wherein X is 0.05 or more and 0.8 or less. Configuration 5: A positive electrode for a non-aqueous electrolyte secondary battery according to any one of configurations 1 to 4, wherein Y is 0.05 or more and 0.8 or less. Configuration 6: A non-aqueous electrolyte secondary battery comprising a positive electrode according to any one of configurations 1 to 5, a negative electrode, and a non-aqueous electrolyte. Configuration 7: A non-aqueous electrolyte secondary battery as described in Configuration 6, wherein the non-aqueous electrolyte is lithium hexafluorophosphate (LiPF) as the electrolyte salt. ６) and lithium bis(fluorosulfonyl)imide (LiFSI), LiPF ６ A non-aqueous electrolyte secondary battery in which the ratio of the mass of LiFSI to the mass of the battery is 5% by mass or more and 50% by mass or less.

[0078] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core body, 31 Positive electrode mixture layer, 40 Negative electrode core body, 41 Negative electrode mixture layer

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode mixture layer containing a positive electrode active material and a conductive agent, wherein the conductive agent contains fibrous carbon, the content of the conductive agent relative to the mass of the positive electrode active material is X [mass%], and in the positive electrode, when Y is the ratio of the area covering the surface of the positive electrode active material with the conductive agent to the surface area of ​​the positive electrode active material, the relationship (Y / X) ≥ 1.0 is satisfied.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the fibrous carbon includes single-walled carbon nanotubes and multi-walled carbon nanotubes.

3. The positive electrode active material has the composition formula Li ｙ Ni ｘ M １－ｘ O ２－δ A positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, comprising a lithium transition metal composite oxide represented by the formula (wherein 0.5 ≤ x ≤ 1.0, 0 < y ≤ 1.2, 0 ≤ δ ≤ 0.05, and M is at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Mo, Mg, Cu, V, Ga, Ge, Y, Sn, W, Bi, and Nb).

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein X is 0.05 or more and 0.8 or less.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein Y is 0.05 or more and 0.8 or less.

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

7. A non-aqueous electrolyte secondary battery according to claim 6, wherein the non-aqueous electrolyte is an electrolyte salt, lithium hexafluorophosphate (LiPF) ６ ) and lithium bis(fluorosulfonyl)imide (LiPF) ６ A non-aqueous electrolyte secondary battery in which the ratio of the mass of LiFSI to the mass of the battery is 5% by mass or more and 50% by mass or less.

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