Positive electrodes for secondary batteries

Abstract

A secondary-battery positive electrode has a positive-electrode mixture layer that includes a positive-electrode active material and an electroconductive material, the positive-electrode active material including a lithium transition metal composite oxide containing at least Ni, and the electroconductive material including carbon fibers having an average fiber diameter d of 5-30 µm (inclusive) and an average fiber length L of 50-2000 µm (inclusive).

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode for a secondary battery. [Background technology]

[0002] Secondary batteries, especially lithium-ion secondary batteries, have high output and high energy density, and are therefore expected to be used as power sources for small consumer applications, power tools, electric vehicles, hybrid vehicles, etc. A composite oxide of lithium and a transition metal (e.g., cobalt) is used as the positive electrode active material for lithium-ion secondary batteries. Replacing at least a portion of the cobalt with nickel can increase the capacity.

[0003] Secondary batteries used as power sources for power tools and hybrid vehicles require particularly high input / output characteristics. To improve input / output characteristics, it is necessary to improve the conductivity of the positive electrode. To do this, it is necessary to ensure sufficient conductive paths inside the positive electrode mixture layer. Conventionally, the conductivity of the positive electrode has been improved by incorporating carbon nanotubes into the positive electrode mixture layer.

[0004] For example, Patent Document 1 proposes a positive electrode for a lithium ion secondary battery in which a positive electrode mixture layer having a positive electrode active material, a conductive agent, and a binder is provided on a positive electrode current collector, in which the conductive agent contains at least carbon nanotubes and the binder is an acrylic binder. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-238944 Summary of the Invention

[0006] To improve the conductivity of the positive electrode using carbon nanotubes (CNTs), it is necessary to sufficiently disperse the CNTs within the positive electrode mixture layer and bond the CNTs to the positive electrode active material. However, CNTs have a fiber diameter on the order of nanometers, and the degree of entanglement between CNTs is high, making them prone to forming aggregates. Therefore, it is not easy to improve the dispersibility of CNTs within the positive electrode mixture layer, and there has been a limit to the improvement of the conductivity of the positive electrode.

[0007] One aspect of the present disclosure relates to a positive electrode for a secondary battery, comprising a positive electrode mixture layer including a positive electrode active material and a conductive material, wherein the positive electrode active material includes a lithium transition metal composite oxide containing at least Ni, and the conductive material includes carbon fibers having an average fiber diameter d of 5 μm or more and 30 μm or less and an average fiber length L of 50 μm or more and 2000 μm or less.

[0008] According to the present disclosure, the conductivity of the positive electrode can be increased, thereby making it possible to achieve high input / output characteristics of the secondary battery. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a partially cutaway plan view schematically illustrating the structure of a secondary battery according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a cross-sectional view taken along line XX' of the nonaqueous secondary battery shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] A positive electrode for a secondary battery according to an embodiment of the present disclosure has a positive electrode mixture layer containing a positive electrode active material and a conductive material. The positive electrode mixture layer is formed in a layer shape from a positive electrode mixture, which is a mixture containing the positive electrode active material and the conductive material as essential components. The positive electrode mixture layer is formed on the surface of, for example, a sheet-shaped positive electrode current collector.

[0011] The positive electrode active material contains a lithium transition metal composite oxide containing at least Ni (hereinafter also referred to as composite oxide A). Composite oxide A has, for example, a layered rock salt type crystal structure. Lithium is released from composite oxide A when the secondary battery is charged and is absorbed into composite oxide A when the secondary battery is discharged. Composite oxide A containing Ni has a high energy density. The positive electrode active material may contain a lithium transition metal composite oxide other than composite oxide A (e.g., LiCoO2), but it is desirable that composite oxide A account for 50 mass % or more, and even 80 mass % or more of the positive electrode active material.

[0012] The Ni content in composite oxide A may be, for example, 20 mol % (atomic %) or more, or 30 mol % or more, based on the total of all metal elements other than lithium contained in composite oxide A. On the other hand, from the viewpoint of stabilizing the crystal structure and maintaining low resistance and high input / output characteristics for a long period of time, the Ni content may be 95 mol % or less, or even 80 mol % or less, or even 50 mol % or less.

[0013] It is desirable that composite oxide A further contains Co. Co has the role of increasing the thermal stability of composite oxide A without impairing the high energy density of composite oxide A containing Ni. The Co content in composite oxide A may be, for example, 50 mol % (atomic %) or less, or even 40 mol % or less, based on the total of all metal elements other than lithium contained in composite oxide A. From the viewpoint of fully obtaining the effect of Co in improving conductivity, the Co content in composite oxide A may be, for example, 20 mol % (atomic %) or more, or even 30 mol % or more, based on the total of all metal elements other than lithium contained in composite oxide A.

[0014] The composite oxide A may further contain Mn. Like Co, Mn plays a role in increasing the thermal stability of the composite oxide A without impairing the high energy density of the Ni-containing composite oxide A. The Mn content in the composite oxide A may be, for example, 50 mol % (atomic %) or less, or even 40 mol % or less, based on the total of all metal elements other than lithium contained in the composite oxide A. The Mn content in the composite oxide A may be, for example, 15 mol % (atomic %) or more, or even 20 mol % or more, based on the total of all metal elements other than lithium contained in the composite oxide A.

[0015] The composite oxide A may further contain an element other than Li, Ni, Co, and Mn. Examples of such elements include Al, Fe, Ti, Si, Nb, Zr, Mo, and Zn. The composite oxide A may contain one element selected from these elements, or may contain two or more elements selected from these elements.

[0016] The composite oxide A is, for example, a compound represented by the general formula: Li a Ni x Co y M z O b F c However, the above general formula satisfies the following conditions: 0.95≦a≦1.2, 0.2≦x≦0.95, 0≦y≦0.5, 0≦z≦0.5, x+y+z=1, b+c=2. M is at least one selected from the group consisting of Mn, Al, Fe, Ti, Si, Nb, Zr, Mo, Sr, W, P, Ca, Mg, Sb, Na, B, V, Cr, Cu, Ge, Ru, K, Bi, and Zn. Specific examples include LiNi 0.35 Co 0.35 Mn 0.30 O2, LiNi 0.9 Co 0.05 Al 0.05 Examples include O2.

[0017] The content of each element contained in the positive electrode active material can be measured, for example, by the following method. First, a fully discharged secondary battery is disassembled, and the resulting positive electrode is washed with dimethyl carbonate (DMC). Next, the positive electrode mixture layer is separated from the positive electrode, weighed, and immersed in a hydrochloric acid solution (1+1) and heated to 90°C for 2 hours. After that, the remaining binder and conductive material are filtered. The filtrate is subjected to inductively coupled plasma atomic emission spectroscopy (ICP-AES) to quantitatively analyze each element.

[0018] The conductive material includes carbon fibers (hereinafter also referred to as carbon fibers A) having an average fiber diameter d of 5 μm or more and 30 μm or less and an average fiber length L of 50 μm or more and 2000 μm or less. Carbon fibers A have a large average fiber diameter d and average fiber length L, and therefore have excellent conductivity and strength. By incorporating carbon fibers A into the positive electrode mixture layer, an advanced current collecting network is formed inside the positive electrode mixture layer. As a result, the DC-IR of the positive electrode is reduced and the conductivity of the positive electrode is significantly improved. The advanced current collecting network further amplifies the high input / output characteristics of complex oxide A.

[0019] The average fiber diameter d may be 7 μm or more and 25 μm or less, or 9 μm or more and 20 μm or less. The average fiber length L may be 70 μm or more and 1500 μm or less, or 100 μm or more and 1000 μm or less. The aspect ratio (L / d) of the carbon fiber A may be, for example, 5 or more.

[0020] The average fiber diameter d and the average fiber length L can be measured, for example, by the following method. First, a fully discharged secondary battery is disassembled, and the obtained positive electrode is washed with dimethyl carbonate (DMC). Next, the positive electrode mixture layer is separated from the positive electrode, immersed in a hydrochloric acid aqueous solution (1+1), and heated to 90°C for 2 hours. After that, the remaining binder and conductive material are filtered, and carbon fiber A is separated. 100 fibers are randomly selected from the obtained carbon fiber A, and the fiber diameter and fiber length are measured and averaged.

[0021] The conductive material may contain carbon fibers other than carbon fiber A (for example, CNT), but it is preferable that carbon fiber A accounts for 90 mass % or more of all carbon fibers.

[0022] By using a positive electrode according to the present disclosure, a secondary battery with high input / output characteristics can be obtained. Such a secondary battery is useful, for example, as a power source for hybrid vehicles, power tools, and the like. In particular, low resistance and the ability to demonstrate high input / output characteristics when needed are important for power sources for hybrid vehicles. On the other hand, the power source for a hybrid vehicle only needs to fulfill the role of supplementing a portion of the energy required to drive the vehicle. In such applications, the required capacity can be ensured even when a sufficient amount of conductive material is included in the positive electrode mixture layer.

[0023] The amount of carbon fiber A contained in the positive electrode mixture layer may be 7% by mass or less, 0.3% by mass or more and 7% by mass or less, 1.5% by mass or more and 5% by mass or less, or 1.0% by mass or more and 5% by mass or less, in which case sufficient capacity can be ensured while more effectively reducing DC-IR of the positive electrode.

[0024] The content of carbon fiber A contained in the positive electrode mixture layer can be measured, for example, by the following method. First, a fully discharged secondary battery is disassembled, and the obtained positive electrode is washed with dimethyl carbonate (DMC). Next, the positive electrode mixture layer is separated from the positive electrode, and the weighed positive electrode mixture with a known mass is immersed in a hydrochloric acid aqueous solution (1+1) and heated to 90°C for 2 hours. Thereafter, the remaining binder and conductive material are filtered, and the carbon fiber A is separated. The obtained carbon fiber A is dried at 100°C for 12 hours, and the content of carbon fiber A is calculated from its mass and the mass of the positive electrode mixture used in the analysis.

[0025] The ratio of the average fiber length L of the carbon fibers A to the thickness T of the positive electrode mixture layer (L / T) may be 2 or more and 50 or less, or 5 or more and 45 or less. By using carbon fibers A that are sufficiently long relative to the thickness T of the positive electrode mixture layer, the current collecting network connecting the positive electrode current collector and the positive electrode mixture layer becomes stronger, and the DC-IR of the positive electrode is significantly reduced. The thickness T of the positive electrode mixture layer is the distance from the surface of the positive electrode mixture layer facing the positive electrode current collector (the surface that joins with the positive electrode current collector) to the surface facing the negative electrode. The thickness T can be determined by photographing a cross section of the positive electrode mixture layer along the thickness direction with a scanning electron microscope (SEM), measuring the distances from the surface of the positive electrode mixture layer facing the positive electrode current collector to the surface facing the negative electrode at any 10 points on the cross section, and averaging the measured values.

[0026] The conductive material may further contain carbon particles. The carbon particles surround the positive electrode active material, thereby increasing the conductive path between the positive electrode active material and the carbon fibers A. The carbon fibers A form macro conductive paths that electrically connect the positive electrode mixture layer and the positive electrode current collector. The carbon particles form finer conductive paths that branch off from the macro conductive paths. The fine conductive paths are interposed between the positive electrode active material and the carbon fibers A and electrically connect the positive electrode active materials to each other.

[0027] Examples of carbon particles that can serve as conductive materials include carbon black, graphite, easily graphitized carbon (soft carbon), and non-graphitized carbon (hard carbon). Of these, carbon black is particularly suitable for forming conductive paths. Examples of carbon black include acetylene black, ketjen black, furnace black, and lamp black. These may be used alone or in combination of two or more. The average particle size of the primary particles of carbon black may be, for example, 5 nm or more and 500 nm or less. These average particle sizes can be determined by averaging the maximum diameters of any 100 particles observed with an SEM.

[0028] The amount of carbon particles contained in the positive electrode mixture layer may be 0.5% by mass or more and 15% by mass or less, or may be 5% by mass or more and 11% by mass or less. In this case, sufficient capacity can be ensured while more effectively reducing the DC-IR of the positive electrode. The content of carbon particles contained in the positive electrode mixture layer can be measured in accordance with the content of carbon fiber A already described.

[0029] The total amount of carbon fiber A and carbon particles contained in the positive electrode mixture layer may be 1% by mass or more and 20% by mass or less, 3.3% by mass or more and 20% by mass or less, 4% by mass or more and 15% by mass or less, 5% by mass or more and 15% by mass or less, or 7% by mass or more and 12% by mass or less. In this case, sufficient capacity can be ensured while more effectively reducing DC-IR of the positive electrode.

[0030] In the positive electrode mixture layer, the proportion of carbon fiber A in the total of carbon fiber A and carbon particles may be, for example, 5% by mass or more and 50% by mass or less, 20% by mass or more and 50% by mass or less, or 20% by mass or more and 40% by mass or less. Within such a range, a dense current collecting network with an excellent balance between macroscopic conductive paths and finer conductive paths branching from the macroscopic conductive paths is likely to be formed.

[0031] The smaller the thickness of the positive electrode mixture layer, the more easily the input / output characteristics are improved. The thickness T of the positive electrode mixture layer may be, for example, 40 μm or less, or may be 30 μm or less. By controlling the thickness T of the positive electrode mixture layer within the above range, the migration path of lithium ions becomes sufficiently short, and the migration of the electrolyte inside the positive electrode mixture layer becomes easy. Therefore, high input / output characteristics are easily achieved. From the viewpoint of ensuring sufficient capacity, the thickness T of the positive electrode mixture layer is preferably, for example, 5 μm or more, and more preferably 10 μm or more.

[0032] The ratio of the average fiber diameter d of the carbon fiber A to the average particle diameter D of the positive electrode active material (d / D) may be 0.5 or more and 5 or less, or 1 or more (or 2 or more) and 4 or less. In this case, the average fiber diameter d of the carbon fiber A is sufficiently large relative to the average particle diameter D of the positive electrode active material, so the current collection network becomes stronger. As a result, the DC-IR of the positive electrode is further significantly reduced. Meanwhile, the average particle diameter D of the positive electrode active material is limited to a relatively small size, which is advantageous for improving the output / input characteristics.

[0033] The positive electrode active material usually has the form of secondary particles formed by aggregation of primary particles. The average particle size D of the positive electrode active material may be, for example, 15 μm or less, 10 μm or less, or 6 μm or less. By controlling the average particle size D of the positive electrode active material within the above range, the surface area of ​​the positive electrode active material increases, which is further advantageous in improving the input / output characteristics. From the viewpoint of suppressing side reactions, the average particle size D of the positive electrode active material is preferably, for example, 1 μm or more, and more preferably 2 μm or more.

[0034] The average particle size D of the positive electrode active material can be measured, for example, by the following method. First, a fully discharged secondary battery is disassembled, and the obtained positive electrode is washed with dimethyl carbonate (DMC). Next, a cross section of the positive electrode mixture layer along the thickness direction is photographed with a scanning electron microscope (SEM), and the maximum diameters of any 10 particles of the positive electrode active material observed in the cross section are measured and averaged to determine the average particle size D.

[0035] The density of the positive electrode mixture layer is, for example, 2 g / cm 3 More than 3.8g / cm 3 By controlling the density of the positive electrode mixture layer within the above range, high input / output characteristics can be easily achieved. From the viewpoint of obtaining higher input / output characteristics, the density is preferably set to 3.8 g / cm or less. 3 It is desirable to make it sufficiently smaller than 3.0 g / cm 3 Less than 2.6g / cm is desirable. 3 The following is more preferable:

[0036] The density (d) of the positive electrode mixture layer can be calculated, for example, by cutting out a positive electrode piece of a predetermined size from a positive electrode, measuring the thickness (t) and area (S) of the positive electrode mixture layer on the positive electrode piece, and measuring the mass (M) of the positive electrode mixture layer on the positive electrode piece, using the formula: d = M / (t × S).

[0037] A secondary battery that can use a positive electrode according to the present disclosure will now be described. The secondary battery includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator.

[0038] [Positive electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer having the above-described configuration formed on the surface of the positive electrode current collector. The positive electrode mixture layer is formed, for example, by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector, drying the slurry, and rolling the dried coating. The positive electrode mixture layer is formed on one or both surfaces of the positive electrode current collector.

[0039] The positive electrode mixture layer contains a positive electrode active material and a conductive material as essential components, and optionally contains a binder, etc. The binder provides bonding strength between the positive electrode active materials, between the positive electrode active material and the conductive material, and between the positive electrode mixture and the positive electrode current collector. The positive electrode active material contains composite oxide A as an essential component.

[0040] Known materials can be used as the binder for the positive electrode mixture layer, and examples thereof include fluororesins (polytetrafluoroethylene, polyvinylidene fluoride, etc.), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, etc. These may be used alone or in combination of two or more.

[0041] The positive electrode current collector may be, for example, a metal sheet or a metal foil, and may be made of, for example, stainless steel, aluminum, an aluminum alloy, or titanium.

[0042] [Negative electrode] The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer (negative electrode mixture layer) formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry, in which a negative electrode mixture containing a negative electrode active material, a binder, etc. is dispersed in a dispersion medium, to the surface of the negative electrode current collector, drying the slurry, and rolling the dried coating. The negative electrode active material layer is formed on one or both surfaces of the negative electrode current collector. The negative electrode active material layer may be a lithium metal foil or a lithium alloy foil.

[0043] The negative electrode mixture layer contains a negative electrode active material as an essential component, and optionally contains a binder, a conductive material, a thickener, etc. Known materials can be used as the binder, conductive material, and thickener.

[0044] Known materials can be used as the binder for the negative electrode mixture layer, and examples thereof include fluororesins (polytetrafluoroethylene, polyvinylidene fluoride, etc.), polyacrylonitrile (PAN), rubber materials (styrene-butadiene copolymers, etc.), polyimide resins, acrylic resins, polyolefin resins, etc. These may be used alone or in combination of two or more.

[0045] The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions, lithium metal, lithium alloy, etc. As the material that electrochemically absorbs and releases lithium ions, a carbon material, an alloy material, etc. are used. Examples of the carbon material include graphite, graphitizable carbon, and non-graphitizable carbon. Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity.

[0046] Alloy materials are materials that contain elements that can form alloys with lithium. Elements that can form alloys with lithium include silicon and tin, with silicon (Si) being particularly promising.

[0047] The negative electrode current collector may be, for example, a metal sheet or a metal foil, and examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0048] [Non-aqueous electrolyte] The nonaqueous electrolyte may be, for example, an electrolytic solution containing a nonaqueous solvent and a solute dissolved therein. The solute refers to an electrolyte salt that ionizes in the nonaqueous solvent, including a lithium salt. The nonaqueous electrolyte may also contain additives other than the nonaqueous solvent and the solute.

[0049] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination. These solvents may be fluorinated solvents in which some of the hydrogen atoms are substituted with fluorine atoms. Fluoroethylene carbonate (FEC) may be used as the fluorinated solvent.

[0050] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0051] The concentration of the lithium salt in the electrolyte may be 1 mol / liter or more and 2 mol / liter or less, or may be 1 mol / liter or more and 1.5 mol / liter or less.

[0052] [Separator] A separator is interposed between the positive electrode and the negative electrode. The separator has ion permeability and insulating properties. The separator may be made of a microporous membrane, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin.

[0053] The secondary battery has a structure in which an electrode plate group and a non-aqueous electrolyte are housed in an exterior body. The electrode plate group is not particularly limited, but may be configured by winding a positive electrode and a negative electrode with a separator interposed therebetween, or may be configured by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0054] A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure will be described below with reference to Figures 1 and 2. Figure 1 is a partially cutaway plan view schematically illustrating an example of the structure of a nonaqueous electrolyte secondary battery. Figure 2 is a cross-sectional view taken along line XX' in Figure 1.

[0055] As shown in FIGS. 1 and 2, the secondary battery 100 is a sheet-type battery, and includes an electrode plate group 4 and an exterior case 5 that houses the electrode plate group 4.

[0056] The electrode plate group 4 has a structure in which a negative electrode 10, a separator 30, and a positive electrode 20 are stacked in this order, with the negative electrode 10 and the positive electrode 20 facing each other with the separator 30 interposed therebetween, thereby forming the electrode plate group 4. The electrode plate group 4 is impregnated with an electrolyte.

[0057] The negative electrode 10 includes a negative electrode active material layer 1a and a negative electrode current collector 1b. The negative electrode active material layer 1a is formed on the surface of the negative electrode current collector 1b.

[0058] The positive electrode 20 includes a positive electrode mixture layer 2a and a positive electrode current collector 2b. The positive electrode mixture layer 2a is formed on the surface of the positive electrode current collector 2b.

[0059] A negative electrode tab lead 1c is connected to the negative electrode current collector 1b, and a positive electrode tab lead 2c is connected to the positive electrode current collector 2b. The negative electrode tab lead 1c and the positive electrode tab lead 2c each extend to the outside of the outer case 5.

[0060] The negative electrode tab lead 1c and the exterior case 5, and the positive electrode tab lead 2c and the exterior case 5 are insulated by insulating tab films 6, respectively.

[0061] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0062] Comparative Example 1 (1) Preparation of the positive electrode Positive electrode active material (LiNi with an average particle size D of 4 μm) 0.35 Co 0.35 Mn 0.30 A positive electrode mixture containing 90.3 mass% of acetylene black (AB, primary particle average particle size 50 nm) as carbon particles, 7 mass% of acetylene black (AB, primary particle average particle size 50 nm), and 2.7 mass% of polyvinylidene fluoride as a binder was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to one side of a positive electrode current collector (aluminum foil), the coating was dried, and then the coating was rolled with a rolling roller to a thickness of 25 μm and a density of 2.4 g / cm. 3 A positive electrode having a positive electrode mixture layer of this formula was obtained.

[0063] The positive electrode was cut into a predetermined shape to obtain a positive electrode for evaluation. The positive electrode had a 40 mm × 30 mm area to function as a positive electrode and a 5 mm × 5 mm area for connection with the tab lead. The positive electrode mixture layer formed on the connection area was scraped off to expose the positive electrode current collector. The exposed portion of the positive electrode current collector was then connected to the positive electrode tab lead, and a predetermined area around the periphery of the positive electrode tab lead was covered with an insulating tab film.

[0064] (2) Preparation of the negative electrode A negative electrode mixture containing 99% by mass of a negative electrode active material (graphite with an average particle size of 10 μm), 0.4% by mass of a binder styrene butadiene copolymer (SBR), and 0.6% by mass of a thickener carboxymethyl cellulose (CMC) was dispersed in water to prepare a negative electrode slurry. The negative electrode slurry was then applied to one side of a negative electrode current collector (electrolytic copper foil). The coating film was dried and then rolled with a rolling roller to a thickness of 60 μm and a density of 1.2 g / cm. 3 Thus, a negative electrode having a negative electrode mixture layer of this formula was obtained.

[0065] The negative electrode was cut into the same shape as the positive electrode to obtain a negative electrode for evaluation. The negative electrode mixture layer formed on the connection area formed in the same manner as the positive electrode was scraped off to expose the negative electrode current collector. The exposed portion of the negative electrode current collector was then connected to a negative electrode tab lead, and a predetermined area around the periphery of the negative electrode tab lead was covered with an insulating tab film.

[0066] (3) Preparation of electrolyte An electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent containing EC, EMC, DMC, and MP in a volume ratio of 25:37:35:3.

[0067] (4) Preparation of evaluation cells A cell was fabricated using the above-described evaluation positive electrode and negative electrode. First, the positive electrode and negative electrode were placed opposite each other with a polypropylene separator (30 μm thick) interposed between them so that the positive electrode mixture layer and the negative electrode mixture layer overlapped, to obtain an electrode plate assembly. Next, an Al laminate film (100 μm thick) cut into a 60 × 90 mm rectangle was folded in half, and the end of the 60 mm long side was heat-sealed at 230 ° C to form a 60 × 45 mm cylindrical shape. The electrode plate assembly was then placed into the cylinder, and the end face of the Al laminate film was aligned with the heat-sealed resin of each tab lead, and heat-sealed at 230 ° C. Next, 0.7 cm of nonaqueous electrolyte was poured from the short side of the Al laminate film that was not heat-sealed. 3After the injection, the positive electrode mixture layer was left standing under a reduced pressure of 0.06 MPa for 5 minutes to allow the electrolyte to penetrate the positive electrode mixture layer. Finally, the end face of the Al laminate film on the injected side was heat-sealed at 230 °C to obtain evaluation cell C1. The evaluation cell was fabricated in a dry environment with a dew point of -50 °C or less.

[0068] (5) Battery DC-IR evaluation The evaluation cell was clamped between a pair of 80 x 80 cm stainless steel clamps (thickness: 2 mm) and pressurized and fixed at 0.2 MPa.

[0069] First, in a thermostatic chamber at 25°C, the battery was charged and discharged five times at a constant current of 0.05C (1C is the current value that discharges the designed capacity in one hour). Charging was terminated at a battery voltage of 4.15V, and discharging was terminated at a battery voltage of 2.5V. The battery was left in an open circuit for 20 minutes between charging and discharging. From the discharge curve of the fifth cycle, the voltage V 50 was calculated.

[0070] Next, in a constant temperature bath at 25°C, 50 Charge at a constant current of 0.05C until the voltage is below 0.02C. 50 The battery was then held at a constant voltage of 1 C. After leaving it in an open circuit for 20 minutes, it was discharged for 30 seconds at a constant current of 1 C in a thermostatic chamber at 25°C, and the voltage at 10 seconds was measured. After the 30-second discharge, the battery was charged with the same amount of power as when it was discharged at a constant current of 0.05 C. The voltage at 10 seconds was similarly measured at discharge current values ​​of 2 C, 3 C, 4 C, 5 C, 10 C, 15 C, 20 C, 25 C, 30 C, and 36 C. The DC-IR was calculated from the slope of the approximation line showing the relationship between each current value I and the voltage V at 10 seconds.

[0071] Example 1 In preparing the positive electrode, a positive electrode mixture containing 87.4 mass% of the positive electrode active material, 7 mass% of acetylene black (AB), 2.6 mass% of polyvinylidene fluoride, and 3 mass% of carbon fiber (CF) A1 having the average fiber diameter d and average fiber length L shown in Table 1 was used. An evaluation cell A1 was prepared in the same manner as in Comparative Example 1. In this example, L / T = 8 and d / D = 3.63 were satisfied. The proportion of carbon fiber A1 in the total of carbon fiber A1 and carbon particles was 30 mass%.

[0072] Example 2 In preparing the positive electrode, a positive electrode mixture containing 85.45 mass% of the positive electrode active material, 7 mass% of acetylene black, 2.55 mass% of polyvinylidene fluoride, and 5 mass% of carbon fiber A2 having the average fiber diameter d and average fiber length L shown in Table 1 was used. Evaluation cell A2 was prepared in the same manner as in Comparative Example 1. In this example, L / T = 40 and d / D = 3.63 were satisfied. The proportion of carbon fiber A2 in the total of carbon fiber A2 and carbon particles was 41.7 mass%.

[0073] Comparative Example 2 Evaluation cell C2 was produced in the same manner as in Comparative Example 1, except that a positive electrode mixture containing 85.45 mass% of positive electrode active material, 12 mass% of acetylene black, and 2.55 mass% of polyvinylidene fluoride was used in the production of the positive electrode.

[0074] Comparative Example 3 Evaluation cell C3 was prepared in the same manner as in Comparative Example 1, except that a positive electrode mixture containing 90.06 mass% of a positive electrode active material, 7 mass% of acetylene black, 2.69 mass% of polyvinylidene fluoride, and 0.25 mass% of carbon fiber a3 having the average fiber diameter d and average fiber length L shown in Table 1 was used in the preparation of the positive electrode.

[0075] The results are shown in Table 1. DC-IR is shown as a relative value with the value of Comparative Example 1 being 100. The smaller the value, the smaller the DC-IR, which means that the conductivity and input / output characteristics of the positive electrode are excellent.

[0076] [Table 1]

[0077] As shown in Table 1, cells A1 and A2, which use positive electrode mixtures containing carbon fiber A, show a significant decrease in DC-IR. On the other hand, cell C2, which does not use carbon fiber A despite having the same conductive material content, shows an insufficient decrease in DC-IR. Cell C3, which uses CNTs instead of carbon fiber A, shows a certain degree of decrease in DC-IR, but not as much as carbon fiber A. Because CNTs are bulky and prone to agglomeration, it takes a long time to disperse them in the positive electrode slurry, and it is difficult to include them in the positive electrode slurry in amounts of 0.25% by mass or more. [Industrial Applicability]

[0078] The positive electrode for a secondary battery according to the present disclosure is suitable for use as a power source for, for example, a hybrid electric vehicle (HEV) or a plug-in hybrid electric vehicle (PHV). [Explanation of symbols]

[0079] 1a Negative electrode active material layer 1b Negative electrode current collector 1c Negative electrode tab lead 2a Positive electrode mixture layer 2b Positive electrode current collector 2c Positive electrode tab lead 4 Plate group 5. Outer case 6 Insulation tab film 10 negative electrode 20 positive electrode 30 Separator 100 Lithium-ion secondary battery

Claims

1. a positive electrode mixture layer containing a positive electrode active material and a conductive material, the positive electrode active material includes a lithium transition metal composite oxide containing at least Ni, the conductive material includes carbon fibers having an average fiber diameter d of 5 μm or more and 30 μm or less and an average fiber length L of 200 μm or more and 2000 μm or less, and a ratio L / T of the average fiber length L of the carbon fibers to a thickness T of the positive electrode mixture layer is 5 or more and 50 or less.

2. 2. The positive electrode for a secondary battery according to claim 1, wherein the lithium transition metal composite oxide further contains Co.

3. 2. The positive electrode for a secondary battery according to claim 1, wherein the lithium transition metal composite oxide further contains Mn.

4. 4. The positive electrode for a secondary battery according to claim 1, wherein the amount of the carbon fiber contained in the positive electrode mixture layer is 7% by mass or less.

5. 4. The positive electrode for a secondary battery according to claim 1, wherein the conductive material further contains carbon particles.

6. The positive electrode for secondary batteries according to any one of claims 1 to 3, wherein the ratio of the average fiber diameter d of the carbon fibers to the average particle diameter D of the positive electrode active material: d / D is 0.5 or more and 5 or less.

7. The positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the thickness T of the positive electrode mixture layer is 40 µm or less.

8. 4. The positive electrode for a secondary battery according to claim 1, wherein the average particle diameter D of the positive electrode active material is 15 μm or less.

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