Positive electrode for secondary batteries

Abstract

A positive electrode for a secondary battery comprises: a positive electrode mixture layer comprising a positive electrode active material and a conductive material, wherein the positive electrode active material comprises a lithium transition metal composite oxide containing at least Ni, and the conductive material comprises carbon fibers having an average fiber diameter d of greater than or equal to 5 μm and less than or equal to 30 μm and an average fiber length L of greater than or equal to 50 μm and less than or equal to 2000 μm.

Description

Technical Field

[0001] The present invention relates to a positive electrode for a secondary battery. Background Art

[0002] Secondary batteries, particularly lithium-ion secondary batteries, are expected to be used as power sources for small consumer applications, power tools, electric vehicles, hybrid vehicles, and more due to their high output and high energy density. Composite oxides of lithium and transition metals (such as cobalt) are used as the positive electrode active material for lithium-ion secondary batteries. Replacing at least part of the cobalt with nickel can achieve higher capacity.

[0003] Secondary batteries used in power tools and hybrid vehicles require particularly high input / output characteristics. To improve this, the conductivity of the positive electrode needs to be increased. Therefore, it is necessary to ensure a sufficient conductive path within the positive electrode mixture layer. Previously, improvements in positive electrode conductivity have been achieved by including carbon nanotubes in the positive electrode mixture layer.

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

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-238944 Summary of the Invention

[0008] In order to improve the conductivity of the positive electrode using carbon nanotubes (CNTs), it is necessary to fully disperse the CNTs within the positive electrode mixture layer so that the CNTs are bonded to the positive electrode active material. However, the fiber diameter of CNTs is nanometer-scale, and the degree of entanglement between CNTs is large, which easily forms aggregates. Therefore, it is not easy to improve the dispersion of CNTs within the positive electrode mixture layer, and there is a limit to improving the conductivity of the positive electrode.

[0009] One aspect of the present invention relates to a positive electrode for a secondary battery, which has a positive electrode mixture layer comprising a positive electrode active material and a conductive material, wherein the positive electrode active material comprises a lithium transition metal composite oxide containing at least Ni, and the conductive material comprises carbon fibers having an average fiber diameter d of greater than 5 μm and less than 30 μm and an average fiber length L of greater than 50 μm and less than 2000 μm.

[0010] According to the present invention, the conductivity of the positive electrode can be improved, and thus high input / output characteristics of the secondary battery can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a partially cutaway plan view schematically showing the structure of a secondary battery according to one embodiment of the present invention.

[0012] Figure 2 yes Figure 1 A cross-sectional view of the non-aqueous secondary battery taken along line XX' is shown. DETAILED DESCRIPTION

[0013] A secondary battery positive electrode according to one embodiment of the present invention includes a positive electrode mixture layer containing a positive electrode active material and a conductive material. The positive electrode mixture layer is formed by forming a positive electrode mixture into a layer. The positive electrode mixture is a mixture containing a positive electrode active material and a conductive material as essential components. The positive electrode mixture layer is formed, for example, on the surface of a sheet-shaped positive electrode current collector.

[0014] The positive electrode active material includes a lithium transition metal composite oxide (hereinafter also referred to as composite oxide A) containing at least Ni. Composite oxide A has, for example, a layered rock salt type crystal structure. Lithium is released from composite oxide A during charging of the secondary battery and absorbed by composite oxide A during discharge. Composite oxide A containing Ni has a high energy density. The positive electrode active material may include a lithium transition metal composite oxide (such as LiCoO2) other than composite oxide A, but preferably, more than 50% by mass, and further more than 80% by mass, of the positive electrode active material is composite oxide A.

[0015] For example, the Ni content in the composite oxide A may be 20 mol% (atomic %) or more, or 30 mol% or more, relative to the total of all metal elements excluding lithium contained in the composite oxide A. On the other hand, from the perspective 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, further 80 mol% or less, or even 50 mol% or less.

[0016] The composite oxide A preferably further contains Co. Co has the effect of improving the thermal stability of the composite oxide A without impairing the high energy density of the composite oxide A containing Ni. For example, the Co content in the composite oxide A may be 50 mol% (atomic %) or less, and further may be 40 mol% or less, relative to the total of all metal elements excluding lithium contained in the composite oxide A. In order to fully obtain the effect of improving the conductivity brought about by Co, the Co content in the composite oxide A may be 20 mol% (atomic %) or more, and further may be 30 mol% or more, relative to the total of all metal elements excluding lithium contained in the composite oxide A.

[0017] The composite oxide A may further contain Mn. Mn, like Co, has the effect of improving the thermal stability of the composite oxide A without impairing the high energy density of the composite oxide A containing Ni. For example, the Mn content in the composite oxide A may be 50 mol% (atomic %) or less, and further 40 mol% or less, relative to the total of all metal elements contained in the composite oxide A except lithium. For example, the Mn content in the composite oxide A may be 15 mol% (atomic %) or more, and further 20 mol% or more, relative to the total of all metal elements contained in the composite oxide A except lithium.

[0018] The composite oxide A may further contain an element different from any one of 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 or more elements selected from these elements.

[0019] The composite oxide A can be represented by the general formula: Li a Ni x Co y M z O b F c Represented. Wherein, the above general formula satisfies 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. As a specific example, LiNi 0.35 Co 0.35 Mn 0.30 O2、LiNi 0.9 Co 0.05 Al 0.05 O2, etc.

[0020] The content of each element contained in the positive electrode active material can be measured, for example, by the following method. First, the positive electrode obtained by disassembling a fully discharged secondary battery is washed with dimethyl carbonate (DMC). Next, the positive mixture layer is separated from the positive electrode and weighed, immersed in a hydrochloric acid aqueous solution (1+1), and maintained at 90°C for 2 hours. Then, the dissolved residual binder and conductive material are filtered. The filtrate is subjected to high-frequency inductively coupled plasma atomic emission spectrometry (ICP-AES) to perform quantitative analysis of each element.

[0021] The conductive material comprises a carbon fiber (hereinafter referred to as carbon fiber 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. The average fiber diameter d and the average fiber length L of carbon fiber A are large, so the conductivity and strength are excellent. By including carbon fiber A in the positive electrode mixture layer, a high current collection 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. Through the high current collection network, the high input and output characteristics of the composite oxide A are further amplified.

[0022] The average fiber diameter d can be from 7 μm to 25 μm, or from 9 μm to 20 μm. Furthermore, the average fiber length L can be from 70 μm to 1500 μm, or from 100 μm to 1000 μm. Furthermore, the aspect ratio (L / d) of the carbon fibers A can be, for example, 5 or greater.

[0023] The average fiber diameter d and the average fiber length L can be measured, for example, by the following method. First, the positive electrode obtained by disassembling a fully discharged secondary battery is washed with dimethyl carbonate (DMC). Next, the positive compound layer is separated from the positive electrode, immersed in a hydrochloric acid aqueous solution (1+1), and maintained at 90°C for 2 hours. Then, the dissolved residual binder and conductive material are filtered to separate the carbon fibers A. Randomly select 100 of the obtained carbon fibers A, measure the fiber diameter and fiber length, and average them.

[0024] The conductive material may contain carbon fibers (eg, CNTs) other than the carbon fibers A, but preferably the carbon fibers A account for 90% by mass or more of all the carbon fibers.

[0025] By using the positive electrode of the present invention, a secondary battery with high input / output characteristics can be obtained. Such a secondary battery is useful as a power source for hybrid vehicles and power tools, for example. Among them, the power source for hybrid vehicles has low resistance and is valued for being able to exert high input / output characteristics when necessary. On the other hand, the power source for hybrid vehicles only needs to play a role in assisting a portion of the energy required to drive the vehicle. In such applications, even when the positive electrode mixture layer contains a sufficient amount of conductive material, the required capacity can be ensured.

[0026] 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 this case, the DC-IR of the positive electrode can be more effectively reduced while ensuring sufficient capacity.

[0027] The content of carbon fiber A contained in the positive electrode mixture layer can be measured, for example, by the following method. First, the positive electrode obtained by disassembling a fully discharged secondary battery is washed with dimethyl carbonate (DMC). Next, the positive mixture layer is separated from the positive electrode, and the weighed positive electrode mixture of known mass is immersed in a hydrochloric acid aqueous solution (1+1) and kept in a state heated to 90°C for 2 hours. Then, the dissolved residual binder and conductive material are filtered to separate the carbon fiber A. 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.

[0028] The ratio L / T of the average fiber length L of the carbon fiber A to the thickness T of the positive electrode mixture layer can be greater than 2 and less than 50, and can be greater than 5 and less than 45. By using carbon fibers A that are long enough relative to the thickness T of the positive electrode mixture layer, the current collection network connecting the positive electrode 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 refers to the distance from the surface of the positive electrode collector side of the positive electrode mixture layer (the bonding surface with the positive electrode collector) to the surface on the side opposite to the negative electrode. The thickness T is calculated as follows: a cross-section of the positive electrode mixture layer along the thickness direction is photographed with a scanning electron microscope (SEM), and the distance from the surface of the positive electrode collector side of the positive electrode mixture layer to the surface on the side opposite to the negative electrode is measured at any 10 points of the cross-section, and the distance is averaged to obtain the result.

[0029] The conductive material may further include carbon particles. By surrounding the positive electrode active material with carbon particles, the conductive path between the positive electrode active material and the carbon fibers A is increased. The carbon fibers A form a microscopic conductive path that electrically connects the positive electrode mixture layer to the positive electrode current collector. The carbon particles form even finer conductive paths that branch from the microscopic conductive path. The fine conductive path is between the positive electrode active material and the carbon fibers A and electrically connects the positive electrode active materials to each other.

[0030] Examples of carbon particles that serve as conductive materials include carbon black, graphite, easily graphitized carbon (soft carbon), and difficultly graphitized carbon (hard carbon). Among them, carbon black is suitable for forming a conductive path. Examples of carbon black include acetylene black, Ketjen black, furnace black, and lamp black. These can be used alone or in combination of two or more. The average particle size of the primary particles of carbon black can be, for example, 5 nm or more and 500 nm or less. These average particle sizes can be determined as the average value of the maximum diameters of any 100 particles observed using an SEM.

[0031] The amount of carbon particles contained in the positive electrode mixture layer can be from 0.5% to 15% by mass, or from 5% to 11% by mass. In this case, the DC-IR of the positive electrode can be more effectively reduced while ensuring sufficient capacity. The content of carbon particles contained in the positive electrode mixture layer can be determined based on the content of carbon fibers A described above.

[0032] The total amount of carbon fibers 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, the DC-IR of the positive electrode can be more effectively reduced while ensuring sufficient capacity.

[0033] In the positive electrode mixture layer, the proportion of carbon fibers A in the total of carbon fibers A and carbon particles can be, for example, 5% to 50% by mass, 20% to 50% by mass, or 20% to 40% by mass. Within these ranges, a dense electrical network with an excellent balance between micro-pitch conductive paths and even finer conductive paths branching therefrom can be easily formed.

[0034] The smaller the thickness of the positive electrode mixture layer, the easier it is to improve the input-output characteristics. The thickness T of the positive electrode mixture layer can be, for example, 40 μm or less, or 30 μm or less. By controlling the thickness T of the positive electrode mixture layer to the above range, the movement path of the lithium ions is sufficiently shortened, and the movement of the electrolyte inside the positive electrode mixture layer becomes easy. Therefore, it is easy to achieve high input-output characteristics. From the perspective of ensuring sufficient capacity, it is preferred that the thickness T of the positive electrode mixture layer be set to, for example, 5 μm or more, and more preferably to 10 μm or more.

[0035] The ratio d / D of the average fiber diameter d of the carbon fiber A relative to the average particle size D of the positive electrode active material can 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 of sufficient size relative to the average particle size 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. On the other hand, the average particle size D of the positive electrode active material is limited to a relatively small size, which is conducive to the improvement of the input and output characteristics.

[0036] The positive electrode active material generally has the form of secondary particles formed by aggregation of primary particles. The average particle size D of the positive electrode active material can be, for example, less than 15 μm, less than 10 μm, or less than 6 μm. By controlling the average particle size D of the positive electrode active material to be within the above range, the surface area of ​​the positive electrode active material becomes larger, which is more beneficial for improving the input-output characteristics. From the perspective of suppressing side reactions, it is preferred that the average particle size D of the positive electrode active material be, for example, greater than 1 μm, and more preferably greater than 2 μm.

[0037] The average particle size D of the positive electrode active material can be measured, for example, by the following method. First, the positive electrode obtained by disassembling a fully discharged secondary battery is washed with dimethyl carbonate (DMC). Next, a cross-section of the positive electrode mixture layer along the thickness direction is photographed using a scanning electron microscope (SEM), and the maximum diameter of 10 particles of the positive electrode active material observed in the cross-section is measured and averaged to obtain the average particle size.

[0038] The density of the positive electrode mixture layer can be, for example, 2 g / cm 3 Above and 3.8g / cm 3 By controlling the density of the positive electrode mixture layer to the above range, it becomes easy to achieve high input / output characteristics. From the perspective of obtaining higher input / output characteristics, it is preferred that the above density be sufficiently less than 3.8 g / cm 3 , preferably 3.0 g / cm 3 Below, more preferably 2.6g / cm 3 the following.

[0039] The density (d) of the positive electrode mixture layer can be calculated, for example, as follows: a positive electrode sheet of a specified size is cut out from the positive electrode, the thickness (t) and area (S) of the positive electrode mixture layer of the positive electrode sheet are measured, the mass (M) of the positive electrode mixture layer of the positive electrode sheet is measured, and the density (d) is calculated using the formula: d = M / (t×S).

[0040] Hereinafter, a secondary battery in which the positive electrode according to the present invention can be used will be described. The secondary battery includes a positive electrode, a negative electrode, a nonaqueous electrolyte, and a separator.

[0041] [positive electrode]

[0042] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer having the above-described structure 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 containing a positive electrode mixture 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.

[0043] The positive electrode mixture layer contains a positive electrode active material and a conductive material as essential components, and a binder and other optional components. The binder provides binding 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.

[0044] As the binder used in the positive electrode mixture layer, known materials can be utilized, for example, fluororesins (polytetrafluoroethylene, polyvinylidene fluoride, etc.), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, etc. These can be used alone or in combination of two or more.

[0045] For example, a metal sheet or metal foil can be used as the positive electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0046] [negative electrode]

[0047] The negative electrode, for example, includes 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 containing a negative electrode mixture including a negative electrode active material, a binder, etc. 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 can be a lithium metal foil or a lithium alloy foil.

[0048] The negative electrode mixture layer contains a negative electrode active material as an essential component and contains a binder, a conductive material, a thickener, etc. as optional components. As the binder, the conductive material, and the thickener, known materials can be used.

[0049] As the binder used in the negative electrode mixture layer, known materials can be used, for example, fluororesins (polytetrafluoroethylene, polyvinylidene fluoride, etc.), polyacrylonitrile (PAN), rubber materials (copolymers of styrene and butadiene, etc.), polyimide resins, acrylic resins, polyolefin resins, etc. These can be used alone or in combination of two or more.

[0050] Negative electrode active materials include materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, and the like. Carbon materials and alloy-based materials can be used as materials that electrochemically absorb and release lithium ions. Examples of carbon materials include graphite, easily graphitizable carbon, and non-graphitizable carbon. Of these, graphite is preferred due to its excellent charge and discharge stability and low irreversible capacity.

[0051] The alloy-based material refers to a material containing an element capable of forming an alloy with lithium. Examples of the element capable of forming an alloy with lithium include silicon and tin, with silicon (Si) being particularly preferred.

[0052] For example, a metal sheet or metal foil can be used as the negative electrode current collector. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0053] [Non-aqueous electrolyte]

[0054] The non-aqueous electrolyte may be, for example, an electrolyte solution containing a non-aqueous solvent and a solute dissolved therein. The solute is an electrolyte salt that undergoes ion dissociation in the non-aqueous solvent, including lithium salts. The non-aqueous electrolyte may contain additives other than the non-aqueous solvent and solute.

[0055] As non-aqueous solvent, for example, cyclic carbonate, chain carbonate, cyclic carboxylate, chain carboxylate etc. can be used. As cyclic carbonate, propylene carbonate (PC), ethylene carbonate (EC), vinylene carbonate (VC) etc. can be listed. As chain carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) etc. can be listed. In addition, as cyclic carboxylate, gamma-butyrolactone (GBL), gamma-valerolactone (GVL) etc. can be listed. As chain carboxylate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP) etc. can be listed. Non-aqueous solvent can be used alone or in combination of two or more. These solvents can be fluorinated solvents in which a part of the hydrogen atom is replaced by a fluorine atom. As fluorinated solvent, fluoroethylene carbonate (FEC) can be used.

[0056] As the lithium salt, for example, lithium salts containing chloric acid (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorinated acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorinated imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.). The lithium salts may be used alone or in combination of two or more.

[0057] The concentration of the lithium salt in the electrolyte solution may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less.

[0058] [Separator]

[0059] The separator is placed between the positive and negative electrodes. It is ion-permeable and has insulating properties. Microporous membranes, woven fabrics, nonwoven fabrics, etc. can be used as separators. Polyolefins are preferred as the material for the separator.

[0060] A secondary battery has an electrode assembly and a non-aqueous electrolyte housed in an outer casing. The electrode assembly is not particularly limited; it can be constructed by winding the positive and negative electrodes with a separator interposed between them, or by laminating the positive and negative electrodes with a separator interposed between them. Secondary batteries can be of any type, including cylindrical, rectangular, coin-shaped, button-shaped, and laminated.

[0061] Below, refer to Figure 1 and Figure 2 , a non-aqueous electrolyte secondary battery according to one embodiment of the present invention is described. Figure 1 It is a partially cutaway plan view schematically showing an example of the structure of a non-aqueous electrolyte secondary battery. Figure 2 yes Figure 1 A cross-sectional view along line XX'.

[0062] like Figure 1 and Figure 2 As shown, the secondary battery 100 is a sheet-type battery including an electrode plate assembly 4 and an outer case 5 for housing the electrode plate assembly 4 .

[0063] The electrode plate assembly 4 is a structure in which the negative electrode 10, the separator 30, and the positive electrode 20 are laminated in this order, and the negative electrode 10 and the positive electrode 20 face each other with the separator 30 interposed therebetween. This forms the electrode plate assembly 4. The electrode plate assembly 4 is impregnated with an electrolyte solution.

[0064] 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.

[0065] 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.

[0066] The negative electrode tab lead 1 c is connected to the negative electrode current collector 1 b , and the positive electrode tab lead 2 c is connected to the positive electrode current collector 2 b . The negative electrode tab lead 1 c and the positive electrode tab lead 2 c each extend outside the outer case 5 .

[0067] The negative electrode tab lead 1 c and the outer case 5 , and the positive electrode tab lead 2 c and the outer case 5 , are insulated by insulating sheet films 6 .

[0068] Hereinafter, the present invention will be specifically described based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[0069] Comparative Example 1

[0070] (1) Preparation of positive electrode

[0071] The positive electrode active material (LiNi with an average particle size D of 4 μm) was contained in an amount of 90.3% by mass. 0.35Co 0.35 Mn 0.30 A positive electrode mixture of 2.7% by mass of polyvinylidene fluoride (PVDF) as a binder was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was then applied to one side of a positive electrode current collector (aluminum foil), and after the coating was dried, it was rolled with a calendering roller to obtain a film with a thickness of 25 μm and a density of 2.4 g / cm 3 The positive electrode of the positive electrode mixture layer.

[0072] The positive electrode was cut into a specified shape to obtain a positive electrode for evaluation. A 40 mm x 30 mm area functioning as the positive electrode and a 5 mm x 5 mm area for connection to the tab lead were provided on the positive electrode. The positive electrode mixture layer formed on the connection area was removed to expose the positive electrode current collector. The exposed portion of the positive electrode current collector was then connected to the positive tab lead, and a specified area around the positive tab lead was covered with an insulating film.

[0073] (2) Preparation of negative electrode

[0074] 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 styrene butadiene copolymer (SBR) as a binder, and 0.6% by mass of carboxymethyl cellulose (CMC) as a thickener was dispersed in water to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to one side of a negative electrode current collector (electrolytic copper foil). After the coating was dried, it was rolled with a calendering roller to obtain a film with a thickness of 60 μm and a density of 1.2 g / cm 3 The negative electrode of the negative electrode mixture layer.

[0075] 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, similarly formed as for the positive electrode, was removed to expose the negative electrode current collector. The exposed portion of the negative electrode current collector was then connected to the negative electrode tab lead, and a designated area around the negative electrode tab lead was covered with an insulating film.

[0076] (3) Preparation of electrolyte

[0077] 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.

[0078] (4) Preparation of evaluation batteries

[0079] The battery was made using the positive and negative electrodes for evaluation as described above. First, the positive electrode and the negative electrode were placed opposite each other with a polypropylene separator (30 μm thick) interposed so that the positive electrode mixture layer and the negative electrode mixture layer overlapped exactly, to obtain a plate group. Next, the Al laminate film (100 μm thick) cut into a rectangle of 60×90 mm 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 tube. Then, the prepared plate group was placed in the tube, and the end face of the Al laminate film was aligned with the position of the hot-melt resin of each tab lead, and heat-sealed at 230°C. Next, 0.7 cm was injected from the short side of the Al laminate film that was not heat-sealed. 3 After injection, the non-aqueous electrolyte was allowed to stand for 5 minutes under a reduced pressure of 0.06 MPa to allow the electrolyte to penetrate the positive electrode mixture layer. Finally, the end face of the Al laminate film on the injection 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 below -50°C.

[0080] (5) Evaluation of battery DC-IR

[0081] The evaluation battery was clamped by a pair of 80×80 cm stainless steel (thickness 2 mm) clamps and fixed under pressure at 0.2 MPa.

[0082] First, in a thermostatic chamber at 25°C, charge and discharge were repeated five times at a constant current of 0.05C (1C is the current value required to discharge 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. Between charging and discharging, the battery was left to stand in an open circuit for 20 minutes. Based on the discharge curve of the fifth cycle, the voltage V at which the battery reaches 50% discharge was calculated. 50 .

[0083] Then, the battery was charged to V in a 25°C constant temperature chamber at a constant current of 0.05C. 50 The voltage is then maintained at V 50 The battery was then discharged at a constant current of 1C for 30 seconds in a 25°C constant temperature chamber until the current value was less than 0.02C. After that, the battery was left to stand in an open circuit for 20 minutes. The battery was then discharged at a constant current of 1C for 30 seconds in a 25°C constant temperature chamber. The voltage at the 10-second moment was determined. After the 30-second discharge, the battery was charged with the same amount of electricity as that used for the 0.05C constant current discharge. The discharge current value was set to 2C, 3C, 4C, 5C, 10C, 15C, 20C, 25C, 30C, and 36C, and the voltage at the 10-second moment was determined in the same manner. The DC-IR was determined from the slope of the approximate straight line representing the relationship between each current value I and the voltage V at the 10-second moment.

[0084] Example 1

[0085] In the preparation of the positive electrode, evaluation battery A1 was prepared in the same manner as in Comparative Example 1, except that a positive electrode mixture containing 87.4% by mass of the positive electrode active material, 7% by mass of acetylene black (AB), 2.6% by mass of polyvinylidene fluoride, and 3% by mass of carbon fibers (CF) A1 having an average fiber diameter d and an average fiber length L shown in Table 1 was used. In this example, L / T = 8 and d / D = 3.63 were satisfied. Furthermore, the proportion of carbon fibers A1 in the total of carbon fibers A1 and carbon particles was 30% by mass.

[0086] Example 2

[0087] In the production of the positive electrode, evaluation battery A2 was prepared in the same manner as in Comparative Example 1, except that a positive electrode mixture containing 85.45% by mass of the positive electrode active material, 7% by mass of acetylene black, 2.55% by mass of polyvinylidene fluoride, and 5% by mass of carbon fibers A2 having the average fiber diameter d and average fiber length L shown in Table 1 was used. In this example, L / T = 40 and d / D = 3.63 were satisfied. Furthermore, the proportion of carbon fibers A2 in the total of carbon fibers A2 and carbon particles was 41.7% by mass.

[0088] Comparative Example 2

[0089] Evaluation battery C2 was prepared 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 to prepare the positive electrode.

[0090] Comparative Example 3

[0091] In the preparation of the positive electrode, the evaluation battery C3 was prepared in the same manner as in Comparative Example 1, except that a positive electrode mixture containing 90.06 mass% of positive electrode active material, 7 mass% of acetylene black, 2.69 mass% of polyvinylidene fluoride, and 0.25 mass% of carbon fiber a3 having an average fiber diameter d and an average fiber length L shown in Table 1 was used.

[0092] The results are shown in Table 1. DC-IR is expressed as a relative value with the value of Comparative Example 1 being 100. A smaller value means a smaller DC-IR and better electrical conductivity and input-output characteristics of the positive electrode.

[0093] [Table 1]

[0094]

[0095] As shown in Table 1, in batteries A1 and A2 using a positive electrode mixture containing carbon fiber A, the reduction in DC-IR was significant. On the other hand, even if the content of the conductive material was the same, the reduction in DC-IR was insufficient in battery C2 that did not use carbon fiber A. In addition, although a certain degree of reduction in DC-IR was observed in battery C3 using CNT instead of carbon fiber A, it was not as good as that of carbon fiber A. It should be noted that CNTs are large in volume and easily aggregated, so the time required to disperse in the positive electrode slurry is long, and it is difficult to include more than 0.25% by mass in the positive electrode slurry.

[0096] Industrial applicability

[0097] The secondary battery positive electrode according to the present invention is suitable for use as a power source for, for example, hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHVs).

[0098] Description of Reference Numerals

[0099] 1a Negative electrode active material layer

[0100] 1b negative electrode collector

[0101] 1c negative electrode tab lead

[0102] 2a Positive electrode mixture layer

[0103] 2b positive electrode collector

[0104] 2c positive electrode tab lead

[0105] 4-electrode plate group

[0106] 5External shell

[0107] 6Insulation film

[0108] 10 negative electrode

[0109] 20 positive electrode

[0110] 30 dividers

[0111] 100 lithium-ion secondary batteries

Claims

1. A positive electrode for a secondary battery, comprising: a positive electrode mixture layer comprising a positive electrode active material and a conductive material; The positive electrode active material comprises 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 to 30 μm and an average fiber length L of 200 μm to 2000 μm. The ratio d / D of the average fiber diameter d of the carbon fibers to the average particle diameter D of the positive electrode active material is 0.5 or more and 5 or less. A ratio L / T of the average fiber length L of the carbon fibers to the thickness T of the positive electrode mixture layer is 5 or more and 50 or less.

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

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

4. The secondary battery positive electrode according to any one of claims 1 to 3, wherein The amount of the carbon fibers contained in the positive electrode mixture layer is 7% by mass or less.

5. The secondary battery positive electrode according to any one of claims 1 to 3, wherein The conductive material further comprises carbon particles.

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

7. The secondary battery positive electrode according to any one of claims 1 to 3, wherein The average particle size D of the positive electrode active material is less than 15 μm.

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