Secondary battery, positive electrode, and composition for positive electrode mixture layer

Abstract

This secondary battery comprises: a positive electrode containing a positive electrode collector and a positive electrode mixture layer that is disposed on the positive electrode collector and that includes a positive electrode active substance; a negative electrode containing a negative electrode collector and a negative electrode mixture layer that is disposed on the negative electrode collector and that includes a negative electrode active substance; and a housing material housing the positive electrode and the negative electrode. The secondary battery has a structure in which multiple positive electrodes and negative electrodes are layered in the thickness direction. The positive electrode mixture layer includes filamentous carbon. When the secondary battery is discharged at a 5 C current from the fully charged state to the discharge end voltage, σ / A is 0.35 (°C / Ah) or less, where σ (°C) is the standard deviation of temperature rise at the temperature measurement point on the housing material and A (Ah) is the rated capacity of the battery.

Description

Secondary battery, positive electrode, and composition for positive electrode mixture layer

[0001] The present disclosure relates to a secondary battery, a positive electrode, and a composition for a positive electrode mixture layer.

[0002] Taking advantage of their small size, light weight, and high voltage characteristics, secondary batteries are widely used in electronic devices such as notebook PCs, mobile phones, smartphones, tablet PCs, etc. In recent years, against the backdrop of environmental concerns, secondary batteries such as lithium-based secondary batteries have become widespread in electric vehicles (EVs) that run solely on batteries and hybrid electric vehicles (HEVs) that combine a gasoline engine with a battery.

[0003] The active materials expected to be used in the future are the so-called ternary compounds with a high nickel ratio or olivine-type phosphates for the positive electrode active material, and silicon for the negative electrode active material. ａ (Ni ｘ Mn ｙ Co ｚ ) O ２ (0≦a≦1, x+y+z=1) or Li ａ MPO ４ (0≦a≦1, M is one or more elements selected from Fe, Co, Mn, Ni, and Cu).

[0004] There are some drawbacks to using these positive electrode active materials. ａ CoO ２ , Li ａ (Ni １／３ Mn １／３ Co １／３ ) O ２ (0≦a≦1), it is difficult to achieve both cycle characteristics and rate characteristics.

[0005] As an example of an attempt to solve such problems, a positive electrode for a lithium ion secondary battery has been disclosed, which is characterized by containing olivine-type lithium iron phosphate and a carbon black composite formed by linking fibrous carbon and carbon black and having an ash content of 1.0 mass % or less as specified in JIS K 1469 (see, for example, Patent Document 1).

[0006] JP 2010-108889 A

[0007] Patent Document 1 does not consider a method for improving the rate characteristics and cycle characteristics of a secondary battery having a structure in which a plurality of positive electrodes and negative electrodes are stacked in the thickness direction, such as a laminate-type secondary battery.

[0008] The present disclosure has been made in view of the above-described conventional circumstances, and aims to provide a secondary battery having a structure in which a plurality of positive electrodes and negative electrodes are stacked in the thickness direction and which has excellent rate characteristics and cycle characteristics, and a composition for a positive electrode and a positive electrode mixture layer that can be used to fabricate a secondary battery having a structure in which a plurality of positive electrodes and negative electrodes are stacked in the thickness direction and which has excellent rate characteristics and cycle characteristics.

[0009] Specific means for achieving the above object are as follows: <1> A secondary battery including a positive electrode including a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector, a negative electrode including a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector, and an exterior material housing the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode have a structure in which a plurality of the positive electrodes and the negative electrodes are laminated in a thickness direction, and the positive electrode mixture layer contains fibrous carbon, and when the secondary battery is discharged from a fully charged state to a discharge end voltage at a current of 5 C, σ / A is 0.35 (°C / Ah) or less, where σ (°C) is a standard deviation of temperature rise at temperature measurement points on the exterior material, and A (Ah) is a rated capacity of the battery. <2> The secondary battery of <1>, wherein when the secondary battery is discharged from a fully charged state to a discharge end voltage at a current of 5 C, the σ / A is 0.30 (°C / Ah) or less. <3> The secondary battery according to <1> or <2>, wherein when the secondary battery is charged at a current of 5 C from a fully discharged state to a charge cut-off voltage, the σ / A is 0.25 (°C / Ah) or less. <4> The average electrode area per positive electrode is 20 cm ２ ~10,000 cm ２<5> The secondary battery according to any one of <1> to <3>, wherein the positive electrode and the negative electrode are laminated. <6> The secondary battery according to <5>, wherein the number of layers of each of the positive electrode and the negative electrode is 4 or more. <7> The secondary battery according to any one of <1> to <4>, wherein the secondary battery is wound. <8> The secondary battery according to <7>, wherein the number of windings is 4 or more. <9> The secondary battery according to any one of <1> to <8>, wherein the positive electrode mixture layer further contains carbon black. <10> The secondary battery according to any one of <1> to <9>, wherein the thickness of the positive electrode mixture layer and the thickness of the negative electrode mixture layer are each independently 30 μm or more. <11> The density of the positive electrode mixture layer is 3.0 g / cm ３ The density of the negative electrode mixture layer is 1.3 g / cm ３ <12> The secondary battery according to any one of <1> to <10>, wherein the positive electrode mixture layer has a basis weight of 10.0 mg / cm. ２ The negative electrode mixture layer has a basis weight of 5.0 mg / cm ２ <13> The secondary battery according to any one of <1> to <11>, wherein the positive electrode active material is LiNi ｘ Mn ｙ Co ｚ Al ｗ O ２ (x, y, z, w≧0, x+y+z+w=1), LiMPO ４ (M is one or more selected from Fe, Co, Mn and Ni) and LiMn ａ Ni ｂ O ４ (a, b≧0, a+b=2). ｘ Mn ｙ Co ｚ Al ｗ O ２ <15> The secondary battery according to any one of <1> to <13>, wherein the positive electrode active material contains LiMPO ４<16> The secondary battery according to any one of <1> to <14>, wherein M is at least one selected from the group consisting of Fe, Co, Mn, and Ni. <16> The positive electrode mixture layer has a density of 2.0 g / cm ３ The density of the negative electrode mixture layer is 1.3 g / cm ３ <17> The secondary battery according to <15>, wherein the negative electrode active material is Si, SiO ｘ (0<x≦2), soft carbon, hard carbon, graphite, silicon-carbon composite, Li ４ Ti ５ O １２ and metallic Li. <18> The secondary battery according to any one of <1> to <17>, wherein the negative electrode active material includes graphite. <19> A positive electrode comprising: a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector, wherein the positive electrode mixture layer contains fibrous carbon, and a secondary battery comprising: the positive electrode, a negative electrode, and a housing material that houses the positive electrode and the negative electrode, wherein a plurality of the positive electrodes and the negative electrodes are stacked in a thickness direction, when the secondary battery is discharged from a fully charged state to a discharge end voltage at a current of 5 C, σ / A is 0.35 (°C / Ah) or less, where σ (°C) is the standard deviation of temperature rise at temperature measurement points on the housing material, and A (Ah) is the rated capacity of the battery. <20> A positive electrode mixture layer composition used to form a positive electrode mixture layer containing a positive electrode active material, the positive electrode mixture layer composition comprising fibrous carbon, and a secondary battery comprising: a positive electrode including a positive electrode current collector and a positive electrode mixture layer formed from the positive electrode mixture layer composition and disposed on the positive electrode current collector; a negative electrode; and an exterior material accommodating the positive electrode and the negative electrode, the secondary battery having a structure in which a plurality of the positive electrodes and the negative electrodes are stacked in a thickness direction; when the secondary battery is discharged from a fully charged state to a discharge end voltage at a current of 5 C, σ (°C) is a standard deviation of a temperature rise at temperature measurement points on the exterior material, and A (Ah) is a rated capacity of the battery, the positive electrode mixture layer composition having a σ / A of 0.35 (°C / Ah) or less.

[0010] According to the present disclosure, it is possible to provide a secondary battery having a structure in which a plurality of positive electrodes and negative electrodes are stacked in the thickness direction and having excellent rate characteristics and cycle characteristics, and a composition for a positive electrode and a positive electrode mixture layer that can be used to fabricate a secondary battery having a structure in which a plurality of positive electrodes and negative electrodes are stacked in the thickness direction and having excellent rate characteristics and cycle characteristics.

[0011] 1 is a cross-sectional view in the thickness direction of a laminated secondary battery; 2 is a perspective view of a laminated secondary battery; 3 is a perspective view of a positive electrode, a negative electrode, and a separator housed in an exterior material of a laminated secondary battery; 4 is a graph showing measurement results of initial DCR in Example 1 and Comparative Example 1. In Example 1 and Comparative Example 1, the positive electrode basis weight was 17.0 mg / cm ２ 1 is a graph showing the DCR change rate when the positive electrode weight is 17.0 mg / cm in Example 1 and Comparative Example 1. ２ 1 is a graph showing the relationship between the current value and the charge capacity ratio when the positive electrode weight is 24.5 mg / cm in Example 1 and Comparative Example 1. ２ 1 is a graph showing the relationship between the current value and the charge capacity ratio when a high-rate charge-discharge test is performed in Example 2 and Comparative Example 2. FIG. 2 is a graph showing the discharge capacity retention rate when a high-rate charge-discharge test is performed in Example 2 and Comparative Example 2. FIG. 3 is a graph showing the ratio of the discharge capacity at 5C charge-discharge when the discharge capacity at 0.2C charge-discharge is set to 100% in Example 2 and Comparative Example 2. FIG. 4 is a graph showing the change in the voltage curve over the course of cycles in Comparative Example 2. FIG. 5 is a graph showing the voltage curve over the course of cycles in Example 2 and Comparative Example 2. FIG. 6 is a graph showing the voltage curve during charge-discharge after 400 cycles of a high-rate charge-discharge test are performed in Example 2 and Comparative Example 2.

[0012] Hereinafter, embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure.

[0013] In the present disclosure, the term "process" includes not only processes that are independent of other processes, but also processes that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved. In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another staged numerical range. Furthermore, in numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In the present disclosure, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, each component may contain multiple types of particles. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component means the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified. In this disclosure, the terms "layer" or "film" include cases where the layer or film is formed over the entire area when the area where the layer or film is present is observed, as well as cases where the layer or film is formed over only a portion of the area. In this disclosure, the term "laminated" refers to stacking layers, and two or more layers may be bonded together, or two or more layers may be detachable. When specific examples of secondary batteries are described with reference to drawings in this disclosure, the present disclosure is not limited to the forms shown in the drawings. Furthermore, the sizes of components in each drawing are conceptual, and the relative size relationships between components are not limited thereto.

[0014] [Secondary Battery] The secondary battery of the present disclosure is a secondary battery including: a positive electrode including a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector; a negative electrode including a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector; and an exterior material that houses the positive electrode and the negative electrode, wherein the secondary battery has a structure in which a plurality of the positive electrodes and the negative electrodes are stacked in a thickness direction, and the positive electrode mixture layer contains fibrous carbon, and when the secondary battery is discharged from a fully charged state to a discharge end voltage at a current of 5 C, σ / A is 0.35 (°C / Ah) or less, where σ (°C) is a standard deviation of a temperature rise at temperature measurement points on the exterior material, and A (Ah) is a rated capacity of the battery.

[0015] In the secondary battery disclosed herein, the positive electrode and negative electrode are stacked in multiple layers in the thickness direction, allowing the temperature rise at temperature measurement points on the exterior material to be measured, and the standard deviation of the temperature rise, which is the variation in temperature rise at multiple temperature measurement points, can be evaluated. Furthermore, by employing a positive electrode mixture layer containing fibrous carbon, it is easy to adjust σ / A to 0.35 (°C / Ah) or less during high-rate discharge. By setting σ / A to 0.35 (°C / Ah) or less, it is believed that current flows more uniformly, the increase in DC resistance after cycling is reduced, and the cycle characteristics of the secondary battery are improved. Furthermore, by setting σ / A to 0.35 (°C / Ah) or less during high-rate discharge, localized temperature rise is suppressed, which is related to the suppression of gas generation, decomposition of the electrolyte, decomposition of the positive electrode active material, etc. As a result, it is believed that the capacity retention rate during high-rate charge / discharge compared to low-rate charge / discharge (excellent rate characteristics) is improved. In the present disclosure, the "fully charged state" refers to the state of a secondary battery after the battery has been charged at 0.2 C with an end voltage of 4.2 V or higher. In the present disclosure, the "fully discharged state" refers to the state of a secondary battery after the battery has been discharged at 0.2 C with an end voltage of 2.8 V or lower. In a general example, the end voltage of charge is set to 4.2 V or higher, and the end voltage of discharge is set to 2.8 V or lower.

[0016] The secondary battery may be in the form of a laminated secondary battery or a wound secondary battery, as long as it has a structure in which a plurality of positive electrodes and negative electrodes housed in an outer casing are stacked in the thickness direction. The wound secondary battery may be, for example, a cylindrical secondary battery in which an electrode pair obtained by winding a laminate formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are enclosed in a cylindrical outer casing, or a cylindrical secondary battery in which a cell obtained by winding a laminate formed by stacking a positive electrode and a negative electrode with a solid electrolyte interposed therebetween is enclosed in a cylindrical outer casing.

[0017] The secondary battery may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween and an electrolytic solution are housed in an exterior packaging material, or may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a solid electrolyte interposed therebetween is housed in an exterior packaging material.

[0018] The number of stacked positive electrodes and negative electrodes may each be 2 or more, 3 or more, or 4 or more. In the present disclosure, stacking the positive electrodes and negative electrodes makes it possible to measure the temperature rise at the temperature measurement point on the exterior material, and by setting the standard deviation of the temperature rise within an appropriate range, a secondary battery with excellent rate characteristics and cycle characteristics can be obtained. There is no particular upper limit on the number of stacked positive electrodes and negative electrodes, and it may be, for example, 100 or less, or 50 or less. In the case of a wound-type secondary battery, the number of stacked layers refers to the number of windings.

[0019] Examples of laminated secondary batteries are shown in Figures 1 to 3. Figure 1 is a cross-sectional view of the laminated secondary battery in the thickness direction, Figure 2 is a perspective view of the laminated secondary battery, and Figure 3 is a perspective view of the positive electrode, negative electrode, and separator housed in the exterior packaging of the laminated secondary battery.

[0020] In Fig. 1 , a secondary battery 10 includes a stack of a positive electrode 1 and a negative electrode 3 stacked with a separator 5 interposed therebetween, and the stacked body is housed in an exterior material 6. In addition, a positive electrode tab 2A and a negative electrode tab 2B extend to the outside of the exterior material 6. Fig. 1 illustrates a secondary battery having six stacked negative electrodes and five stacked positive electrodes, but the present disclosure is not limited to these numbers of stacked negative electrodes and positive electrodes.

[0021] 3 shows a configuration in which positive electrodes 1 and negative electrodes 3 are alternately arranged in the thickness direction. Furthermore, a separator 5 is arranged between the positive electrode 1 and the negative electrode 3. The plurality of positive electrodes 1 and negative electrodes 3 are arranged such that, when the respective components are stacked, the plurality of positive electrode tabs 2A overlap one another and the plurality of negative electrode tabs 2B overlap one another. The positive electrodes 1, negative electrodes 3, and separators 5 arranged in this positional relationship are housed in an exterior packaging material 6, and an electrolyte is further housed in the exterior packaging material 6, and the opening is then sealed by heat fusion or the like, thereby obtaining the laminate-type secondary battery 10 shown in FIG. 2.

[0022] The standard deviation of the temperature rise at temperature measurement points on the exterior packaging material can be determined, for example, as follows. First, a secondary battery is subjected to constant-current charging at a current of 0.2 C with an end-of-charge voltage of 4.2 V, followed by constant-current discharging at a current of 5 C with an end-of-discharge voltage of 2.5 V. During this constant-current discharging, the temperature is measured at seven points on the exterior packaging material. For example, when measuring the temperature using the laminated secondary battery 10 shown in FIG. 2, the temperature may be measured in seven regions indicated by dotted circles. In this case, in order to grasp the temperature variation at each measurement point, it is desirable to set the distance between adjacent measurement points to 10 mm or more. The temperature during charging is measured during constant-current charging, in which a constant-current discharge is performed at a current of 0.2 C with an end-of-discharge voltage of 2.5 V, followed by constant-current charging at a current of 5 C with an end-of-charge voltage of 4.2 V.

[0023] For example, during constant current charging and constant current discharging, the temperature is measured in seven regions, and the temperature rise during charging (maximum temperature - minimum temperature) and the temperature rise during discharging (maximum temperature - minimum temperature) at each measurement point are determined. Then, the standard deviation σ of the temperature rise during high-rate discharging can be calculated from each measurement result of the temperature rise during discharging. Similarly, the standard deviation σ of the temperature rise during high-rate charging can be calculated from each measurement result of the temperature rise during charging.

[0024] When a secondary battery is discharged at a current of 5 C, the standard deviation σ (°C) of the temperature rise at temperature measurement points on the exterior material is taken as A (Ah), and the rated capacity of the battery is taken as A (Ah). σ / A is 0.35 (°C / Ah) or less, and from the viewpoint of rate characteristics and cycle characteristics, σ / A is preferably 0.33 (°C / Ah) or less, more preferably 0.32 (°C / Ah) or less, and even more preferably 0.30 (°C / Ah) or less. The lower limit of the standard deviation of the temperature rise during discharge is not particularly limited, and may be, for example, 0.10 (°C / Ah) or more.

[0025] When the secondary battery is charged with a current of 5 C, from the viewpoint of rate characteristics and cycle characteristics, σ / A at the temperature measurement point on the exterior material is preferably less than 0.28 (°C / Ah), more preferably 0.25 (°C / Ah) or less, and even more preferably 0.20 (°C / Ah) or less. The lower limit of the standard deviation of the temperature rise during charging is not particularly limited and may be, for example, 0.05 (°C / Ah) or more.

[0026] The type of secondary battery is not particularly limited, and examples thereof include lithium-based secondary batteries, sodium-based secondary batteries, potassium-based secondary batteries, magnesium-based secondary batteries, and aluminum-based secondary batteries. Among these, lithium-based secondary batteries that can achieve high energy density at high voltage and sodium-based secondary batteries that can reduce costs are preferred. Below, an example of a lithium-based secondary battery will be described, but the present invention is not limited thereto.

[0027] [Positive Electrode] The secondary battery includes a positive electrode including a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector.

[0028] The material of the positive electrode current collector is not particularly limited as long as it is an electron-conductive material that does not oxidize and dissolve at high potential, and can be selected from aluminum, copper, nickel, titanium, stainless steel, etc. The state of the positive electrode current collector is not particularly limited, and can be selected from foil, perforated foil, mesh, etc. As an example, aluminum foil is used as the positive electrode current collector.

[0029] As will be described later, a composition for a positive electrode mixture layer containing a positive electrode active material and fibrous carbon, and further containing carbon black, a binder, a solvent, and the like as necessary, is applied onto a positive electrode current collector, the applied slurry is dried, and then pressed, thereby forming a positive electrode mixture layer on the positive electrode current collector.

[0030] From the viewpoints of energy density and safety, the thickness of the positive electrode mixture layer may be 30 μm or more, may be 50 μm to 70 μm, or may be 70 μm to 100 μm.

[0031] The density of the positive electrode mixture layer is set to 2.0 g / cm from the viewpoints of energy density and safety. ３ or more, and 3.0 g / cm ３ or more, and 3.0 g / cm ３ ~4.0 g / cm ３ may be.

[0032] The basis weight of the positive electrode mixture layer is 10.0 mg / cm from the viewpoint of energy density and safety. ２ or more, and may be 10.0 mg / cm ２ ~30.0mg / cm ２ may be.

[0033] The average electrode area per sheet (average positive electrode area and average negative electrode area) is 20 cm ２ ~10,000 cm ２ 300 cm ２ ~10,000 cm ２ may be.

[0034] (Positive electrode active material) The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material can be appropriately selected depending on the type of secondary battery, and examples thereof include compounds containing at least one of lithium, sodium, potassium, magnesium, and aluminum. Examples of the positive electrode active material include oxides containing nickel, phosphates having an olivine structure, etc. When the secondary battery is a lithium-based secondary battery, the positive electrode active material may be LiNi ｘ Mn ｙ Co ｚ Al ｗ O ２ (x, y, z, w≧0, x+y+z+w=1), LiMPO４ (M is at least one selected from Fe, Co, Mn, and Ni), LiMn ａ Ni ｂ O ４ (a, b≧0, a+b=2), etc.

[0035] The positive electrode active material is LiNi ｘ Mn ｙ Co ｚ Al ｗ O ２ (x, y, z, w≧0, x+y+z+w=1), or LiMPO ４ (M is at least one selected from Fe, Co, Mn and Ni).

[0036] LiNi ｘ Mn ｙ Co ｚ Al ｗ O ２ As for (x, y, z, w≧0, x+y+z+w=1), it is preferable that the proportion of nickel is relatively high, for example, x≧0.5 or more, and Li(Ni ｘ Mn ｙ Co ｚ ) O ２ It is more preferable that (x≧0.5, y≦0.3, z≦0.3, x+y+z=1). ｘ Mn ｙ Co ｚ ) O ２ As the positive electrode active material represented by (x≧0.5, y≦0.3, z≦0.3, x+y+z=1), for example, Li(Ni ０．８ Mn ０．１ Co ０．１ ) O ２ , Li(Ni ０．７ Mn ０．２ Co ０．１ ) O ２ , Li(Ni ０．７ Mn ０．１ Co ０．２ ) O ２ , Li(Ni ０．６ Mn ０．２ Co ０．２ ) O ２ , Li(Ni ０．５ Mn ０．３ Co ０．２ ) O ２and Li(Ni ０．５ Mn ０．２ Co ０．３ ) O ２ Examples include:

[0037] LiMPO ４ (M is at least one selected from Fe, Co, Mn, and Ni) ４ , LiFe ０．５ Mn ０．５ P.O. ４ , LiFe ０．３ Mn ０．７ P.O. ４ , LiCoPO ４ and LiCo ０．５ Mn ０．５ P.O. ４ Examples include:

[0038] At least a portion of the surface of the positive electrode active material may be coated with amorphous carbon. Examples of a method for coating with amorphous carbon include a known method of heating the positive electrode active material and flowing a hydrocarbon gas therethrough to perform CVD, and a method of mixing a carbon precursor and the positive electrode active material and carbonizing the carbon precursor by performing a heat treatment in a state in which the carbon precursor is attached to at least a portion of the surface of the positive electrode active material.

[0039] The 50% particle diameter (D) of the primary particle diameter of the positive electrode active material in the number-based cumulative particle size distribution ｎ５０ From the viewpoint of obtaining a sufficient capacitance ratio, the thickness is preferably 0.01 μm or more, more preferably 0.02 μm or more, and even more preferably 0.03 μm or more.

[0040] The 50% particle diameter (D) of the primary particle diameter of the positive electrode active material in the number-based cumulative particle size distribution ｎ５０ ) is preferably 2.0 μm or less, more preferably 1.7 μm or less, and even more preferably 1.5 μm or less, from the viewpoint of obtaining sufficient rate characteristics.

[0041] The 50% particle diameter (D) of the primary particle diameter of the positive electrode active material in the number-based cumulative particle size distribution ｎ５０) can be obtained by arbitrarily selecting 100 primary particles of the positive electrode active material from the SEM photograph and cross-sectional SEM photograph of the electrode, and arithmetically averaging the results of measuring the maximum length of the particles using image recognition software.

[0042] The 50% particle diameter (D ｎ５０ ) is preferably 0.5 μm or more from the viewpoint of excellent powder handling properties and easy increase in electrode density, and D ｎ５０ is more preferably 1.0 μm or more, and even more preferably 2.0 μm or more.

[0043] The 50% particle diameter (D ｎ５０ ) is preferably 30.0 μm or less, more preferably 25.0 μm or less, and even more preferably 20.0 μm or less, from the viewpoint of obtaining sufficient rate characteristics and preventing abnormal unevenness from occurring during electrode coating.

[0044] The 50% particle diameter (D ｎ５０ ) can be obtained by arbitrarily selecting 100 secondary particles of the positive electrode active material from the SEM photograph and cross-sectional SEM photograph of the electrode, and arithmetically averaging the results of measuring the maximum length of the particles using image recognition software.

[0045] In this disclosure, the primary particles of the positive electrode active material refer to particles that can be recognized as a single grain in an SEM photograph at 3,000 to 30,000 magnifications. In this specification, the secondary particles of the positive electrode active material refer to clumps formed by an aggregation of primary particles. The secondary particles of the positive electrode active material can be confirmed in an SEM photograph. In this specification, when the positive electrode active material is simply referred to as a "particle," it refers to either a primary particle or a secondary particle.

[0046] In the positive electrode mixture layer, the content of the positive electrode active material is preferably 90.0 mass % or more, more preferably 93.0 mass % or more, and even more preferably 95.0 mass % or more, from the viewpoint of positive electrode capacity.

[0047] In the positive electrode mixture layer, the content of the positive electrode active material is preferably 98.0 mass % or less, more preferably 97.0 mass % or less, and even more preferably 96.5 mass % or less, from the viewpoint of ensuring the amounts of other components.

[0048] (Fiberous Carbon) The positive electrode mixture layer contains fibrous carbon. Examples of fibrous carbon include carbon fiber, vapor-grown carbon fiber, carbon nanotubes such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT), and carbon nanofibers. Vapor-grown carbon fiber is preferred because it has excellent dispersibility, can reduce the resistance of the electrode, and can suitably suppress variations in temperature rise during high-rate discharge. In addition, vapor-grown carbon fiber may be combined with other fibrous carbon, such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). The positive electrode mixture layer may contain one type of fibrous carbon, or may contain two or more types of fibrous carbon.

[0049] From the viewpoint of obtaining excellent cycle characteristics and rate characteristics, the average fiber diameter of the fibrous carbon is preferably 1 nm to 200 nm, more preferably 100 nm to 200 nm, and even more preferably 120 nm to 180 nm.

[0050] The average fiber diameter of fibrous carbon can be determined from the arithmetic mean of the diameters of 300 randomly selected fibers observed under a SEM of the electrode. The diameter of a single fiber can be determined by measuring the width of a fiber at 10 randomly selected points in the SEM photograph and calculating the arithmetic mean. Here, the fiber width refers to the dimension of the fiber in the direction perpendicular to the longitudinal direction.

[0051] From the viewpoint of obtaining excellent cycle characteristics and rate characteristics, the average fiber length of the fibrous carbon is preferably 1 μm to 20 μm, more preferably 3 μm to 15 μm, and even more preferably 4 μm to 10 μm.

[0052] The average fiber length of the fibrous carbon can be measured as follows: First, the electrode is washed with a solvent, and the binder and the like are removed. The resulting powder is dispersed in a dispersion medium, spread on an aluminum foil or the like, dried, and observed under an SEM. The lengths of 300 randomly selected fibers along the fiber axes are measured, and the average fiber length can be measured by taking the arithmetic mean.

[0053] The fibrous carbon preferably contains fibrous carbon having an average fiber diameter of 100 nm to 200 nm and an average fiber length of 3 μm to 15 μm, and more preferably contains vapor-grown carbon fibers or carbon nanotubes having an average fiber diameter of 100 nm to 200 nm and an average fiber length of 3 μm to 15 μm.

[0054] Specific examples of the fibrous carbon include VGCF (registered trademark)-X (manufactured by Showa Denko KK) and VGCF (registered trademark)-H (manufactured by Showa Denko KK).

[0055] In the positive electrode mixture layer, the content of fibrous carbon is preferably 0.1 mass % or more, more preferably 0.3 mass % or more, and even more preferably 0.5 mass % or more, from the viewpoint of obtaining excellent cycle characteristics and rate characteristics.

[0056] In the positive electrode mixture layer, the content of fibrous carbon is preferably 3.0% by mass or less, more preferably 2.0% by mass or less, and even more preferably 1.0% by mass or less, from the viewpoint of ensuring the capacity of the positive electrode.

[0057] In the positive electrode mixture layer, when the fibrous carbon contains vapor-grown carbon fiber and carbon nanotubes such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT), the ratio of the vapor-grown carbon fiber to the carbon nanotubes is not particularly limited. From the viewpoint of cycle characteristics during high-rate charge and discharge, for example, the ratio of the vapor-grown carbon fiber to the carbon nanotubes may be 20:1 to 1:1, 10:1 to 1:1, or 5:1 to 2:1.

[0058] (Carbon Black) The positive electrode mixture layer may further contain carbon black. Carbon black is used as a conductive additive for secondary batteries. Examples of carbon black include Denka Black (registered trademark, manufactured by Denka Company Ltd.), C-NERGY (registered trademark) Super C45, C65 (manufactured by Imerys Graphite & Carbon), and Ketjen Black (manufactured by Ketjen Black International).

[0059] The primary particle diameter of the carbon black may be 10 nm to 100 nm. The term "primary particle" as used herein refers to the portion of a bead-like structure called an aggregate, which corresponds to a single bead. If the primary particle diameter of the carbon black falls within this range, it tends to be uniformly dispersed on the surface of the active material. From the viewpoint of improving dispersibility, the primary particle diameter of the carbon black is preferably 20 nm to 80 nm, and more preferably 30 nm to 70 nm.

[0060] The primary particle diameter of the carbon black can be determined by randomly selecting 100 primary particles of carbon black from the SEM photograph and cross-sectional SEM photograph of the electrode, and calculating the arithmetic average of the results of measuring the maximum length of the particles using image recognition software.

[0061] When the positive electrode mixture layer contains carbon black, the carbon black content is preferably 0.2 mass % or more, more preferably 0.6 mass % or more, and even more preferably 1.0 mass % or more, from the viewpoint of obtaining excellent cycle characteristics and rate characteristics.

[0062] In the positive electrode mixture layer, the carbon black content is preferably 6.0 mass % or less, more preferably 4.0 mass % or less, and even more preferably 2.0 mass % or less, from the viewpoint of ensuring the capacity of the positive electrode.

[0063] When the positive electrode mixture layer contains carbon black, the mass ratio of the carbon black to the fibrous carbon (carbon black:fibrous carbon) is preferably 1:5 to 5:1, more preferably 1:4 to 4:1, and even more preferably 1:1 to 3:1, from the viewpoint of further reducing the resistance of the positive electrode mixture layer by combining the two.

[0064] (Binder) The positive electrode mixture layer may contain a binder. As the binder, a binder used in a positive electrode mixture layer for a typical lithium ion secondary battery can be suitably used. Examples of the binder include polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE).

[0065] When the positive electrode mixture layer contains a binder, the content of the binder is preferably 1.0 mass % or more, more preferably 1.5 mass % or more, and even more preferably 2.0 mass % or more, from the viewpoint of obtaining the function as a binder.

[0066] In the positive electrode mixture layer, the binder content is preferably 5.0 mass % or less, more preferably 4.5 mass % or less, and even more preferably 4.0 mass % or less, from the viewpoint of suppressing an increase in the resistance of the positive electrode.

[0067] (Other Components) In addition to the above, the positive electrode mixture layer may contain other components such as a dispersant, an additive, etc. For example, it may contain various dispersants for dispersing the positive electrode active material, agents for surface modification of the positive electrode active material, etc.

[0068] [Negative Electrode] The secondary battery includes a negative electrode including a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector.

[0069] The material of the negative electrode current collector is not particularly limited as long as it is an electron-conductive material, and can be selected from aluminum, copper, nickel, titanium, stainless steel, etc. The state of the negative electrode current collector is not particularly limited, and can be selected from foil, perforated foil, mesh, etc. As an example, copper foil is used as the negative electrode current collector.

[0070] As will be described later, a composition for a negative electrode mixture layer containing a negative electrode active material and further containing a conductive additive, a binder, a solvent, and the like as necessary is applied onto a negative electrode current collector, the applied slurry is dried, and then pressed, thereby forming a negative electrode mixture layer on the negative electrode current collector.

[0071] From the viewpoints of energy density and safety, the thickness of the negative electrode mixture layer may be 30 μm or more, may be 50 μm to 100 μm, or may be 100 μm to 150 μm.

[0072] The density of the negative electrode mixture layer is set to 1.3 g / cm from the viewpoints of energy density and safety. ３ or more, and 1.5 g / cm ３ ~2.0 g / cm ３ may be.

[0073] The weight of the negative electrode mixture layer is 5.0 mg / cm from the viewpoint of energy density and safety. ２ or more, and may be 10.0 mg / cm ２ ~20.0mg / cm ２ may be.

[0074] (Negative Electrode Active Material) The negative electrode mixture layer contains a negative electrode active material. Examples of the negative electrode active material include Si, SiO ｘ (0<x≦2), soft carbon, hard carbon, graphite, silicon-carbon composite, Li ４ Ti ５ O １２ , metallic Li, etc. Among these, the negative electrode active material preferably contains graphite. Furthermore, at least a portion of the surface of the negative electrode active material may be coated with amorphous carbon.

[0075] (Conductive Aid) The negative electrode mixture layer may contain a conductive aid, such as the above-mentioned carbon materials including fibrous carbon, carbon black, and graphene.

[0076] (Binder) The negative electrode mixture layer may contain a binder. Examples of the binder include PVdF, PTFE, etc., as in the positive electrode mixture layer, as well as styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), etc.

[0077] (Packaging Material) The packaging material for housing the positive electrode and the negative electrode is not limited as long as it can house the positive electrode and the negative electrode, and if necessary, a separator and an electrolytic solution or a solid electrolyte, etc. Examples of packaging materials include commonly available battery packs, 18650-type cylindrical cells, and those packed in aluminum packaging material, and packaging materials can be freely designed and used.

[0078] (Separator) The secondary battery may include a separator between the positive electrode and the negative electrode. The separator can be freely selected from those that can be used in general secondary batteries, and examples thereof include microporous films made of polyethylene or polypropylene. ２ , Al ２ O ３ It is also possible to use a separator in which particles such as those mentioned above are mixed as a filler, or a separator in which these particles are attached to the surface.

[0079] (Electrolyte) In the secondary battery, an electrolyte may be contained in the exterior material. There are no particular limitations on the electrolyte, and an electrolyte that can be used in a typical secondary battery can be suitably used. For example, an organic solvent in which 0.5 mol / L to 2.0 mol / L of lithium salt is dissolved can be used.

[0080] The lithium salt is LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiFSI, etc.

[0081] Examples of organic solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DEC), diethyl carbonate (DEC), and propylene carbonate (PC). The organic solvents listed here and others may be appropriately selected and mixed for use. Examples of additives for the electrolyte solution include vinylene carbonate (VC), propane sultone (PS), and fluoroethylene carbonate (FEC). When an additive is used, the content of the additive is preferably 0.01% to 20% by mass, more preferably 0.1% to 10% by mass, and even more preferably 0.5% to 5% by mass, relative to 100% by mass of the organic solvent.

[0082] (Ionic Liquid) An ionic liquid may be used as the electrolyte, or an ionic liquid may be used in combination with the organic solvent described above. The ionic liquid is not particularly limited, and examples thereof include a combination of a cation such as an imidazolium cation, a pyrrolidinium cation, a piperidinium cation, or an ammonium cation with an anion such as a bis(trifluoromethane)sulfonamide anion.

[0083] (Solid Electrolyte) A solid electrolyte may be used as the electrolyte. When a solid electrolyte is used, a separator is not required, and a battery (for example, an all-solid-state lithium ion secondary battery) can be formed in which a positive electrode and a negative electrode are sandwiched between the solid electrolyte.

[0084] Examples of the solid electrolyte include polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte is not particularly limited, and examples thereof include those obtained by impregnating a polymer such as polyethylene oxide with the lithium salt. The inorganic solid electrolyte is not particularly limited, and examples thereof include Li １３ Ti １．７ Al ０．３ (P.O. ４ ) ３ , Li ２ S-P ２ S ５ etc.

[0085] The positive electrode included in the secondary battery of the present disclosure is obtained by coating a positive electrode mixture layer composition containing a positive electrode active material and fibrous carbon on a positive electrode current collector, and further containing carbon black, a binder, a solvent, and the like as needed, drying the coated slurry, and then pressing to form a positive electrode mixture layer on the positive electrode current collector.

[0086] [Positive Electrode Mixture Layer Composition] The positive electrode mix layer composition of the present disclosure is a positive electrode mix layer composition used to form a positive electrode mix layer containing a positive electrode active material, and the positive electrode mix layer composition contains fibrous carbon. Furthermore, when a secondary battery is formed, the secondary battery includes a positive electrode having a positive electrode mix layer formed using the positive electrode mix layer composition of the present disclosure, a negative electrode, and an exterior material housing the positive electrode and the negative electrode, and the positive electrode and the negative electrode are stacked in a thickness direction. When the secondary battery is discharged at a current of 5 C from a fully charged state to a discharge end voltage, the standard deviation of the temperature rise at temperature measurement points on the exterior material is σ (°C), and the rated capacity of the battery is A (Ah), σ / A is 0.35 (°C / Ah) or less. As a result, by using the positive electrode mix layer composition of the present disclosure, it is possible to fabricate a secondary battery with excellent rate characteristics and cycle characteristics.

[0087] The preferred configurations of the positive electrode active material and fibrous carbon contained in the composition for positive electrode mixture layer of the present disclosure, as well as the carbon black, binder, and the like that are contained as needed, are the same as the preferred configurations described above in the section on the secondary battery of the present disclosure.

[0088] The composition for a positive electrode mixture layer of the present disclosure may contain a solvent. The solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP).

[0089] The composition for a positive electrode mixture layer of the present disclosure can be prepared, for example, as follows: (1) A positive electrode active material and fibrous carbon are weighed, and if necessary, carbon black, a binder, etc. are also weighed, and these are mixed while adding a solvent. (2) The mixture obtained in (1) is kneaded while adding a solvent to obtain a composition for a positive electrode mixture layer with an appropriate viscosity.

[0090] Furthermore, the composition for the positive electrode mixture layer may be applied to a positive electrode current collector using a doctor blade or a coater, and the positive electrode current collector coated with the composition for the positive electrode mixture layer may be placed on a heating device such as a hot plate to dry the solvent. Vacuum drying is then performed to remove the remaining solvent. The product obtained after removing the solvent is pressed and molded using a roll press, uniaxial press, or the like to obtain a positive electrode.

[0091] [Positive Electrode] The positive electrode of the present disclosure comprises a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector, the positive electrode mixture layer containing fibrous carbon. Furthermore, when a secondary battery is formed comprising the positive electrode of the present disclosure, a negative electrode, and an exterior material housing the positive electrode and the negative electrode, and the positive electrode and the negative electrode are stacked in a thickness direction, when the secondary battery is discharged at a current of 5 C from a fully charged state to a discharge end voltage, the standard deviation of the temperature rise at temperature measurement points on the exterior material is σ (°C), and the rated capacity of the battery is A (Ah), where σ / A is 0.35 (°C / Ah) or less. Thus, by using the positive electrode of the present disclosure, a secondary battery with excellent rate characteristics and cycle characteristics can be produced. The positive electrode of the present disclosure can be prepared, for example, using the positive electrode mixture layer composition of the present disclosure.

[0092] The electrodes can be stacked and packed for use. Furthermore, single cells can be connected in series to form a battery, module, or the like. The secondary battery of the present disclosure can be used as a power source for electronic devices such as smartphones, tablet PCs, and personal digital assistants; as a power source for electric motors for power tools, vacuum cleaners, electric bicycles, drones, and electric vehicles; and for storing power obtained from fuel cells, solar power generation, wind power generation, and the like.

[0093] The present disclosure will be specifically described below using examples, but the scope of the present disclosure is not limited to these examples.

[0094] [Example 1] (Preparation of Positive Electrode) NMC811 (Li(Ni) ０．８ Mn ０．１ Co ０．１ ) O ２ 96.5 parts by mass of carbon black (primary particle D ｎ５０An NMP solution containing 1.0 part by mass of PVdF (average fiber diameter: 100 nm), 0.5 part by mass of fibrous carbon having an average fiber diameter of 150 nm and an average fiber length of 6 μm, and 2.0 parts by mass of PVdF was weighed and mixed in a kneader. Then, the mixture was mixed in the kneader while appropriately adding NMP to prepare a positive electrode slurry with adjusted viscosity. The 50% particle diameter (D ｎ５０ ) is 0.5 μm, and the 50% particle diameter (D ｎ５０ ) was 10.1 μm.

[0095] The positive electrode slurry was applied to an aluminum foil having a thickness of 20 μm using a roll coater, dried, and then vacuum-dried to obtain a positive electrode sheet. The thickness of the positive electrode mixture layer was adjusted to 53 μm by roll pressing, and the density of the positive electrode mixture layer was adjusted to 3.2 g / cm. ３ It was decided.

[0096] (Preparation of Negative Electrode) 95.7 parts by mass of graphite particles were used as the negative electrode active material, and carbon black (primary particle D ｎ５０ 1.3 parts by mass of carboxymethyl cellulose (CMC) (50 nm), 1.5 parts by mass of a 2% by mass aqueous solution of CMC containing 1.5 parts by mass of CMC as a binder, and 1.5 parts by mass of POLYSOL (registered trademark, manufactured by Showa Denko K.K.) were weighed and mixed in a kneader to obtain a negative electrode slurry. ｖ５０ ) was 14 μm.

[0097] The negative electrode slurry was applied to a copper foil having a thickness of 20 μm using a roll coater, dried, and then further vacuum-dried to obtain a negative electrode sheet. The thickness of the negative electrode mixture layer was adjusted to 88 μm by roll pressing, and the density of the negative electrode mixture layer was adjusted to 1.4 g / cm. ３ It was decided.

[0098] (Fabrication of Laminated Full Cell) A laminated full cell for evaluation in Example 1 was fabricated as follows in a glove box maintained in a dry argon gas atmosphere with a dew point of −80° C. or less. The positive electrode sheet and the negative electrode sheet obtained in the fabrication of the positive electrode and the negative electrode were punched out to form a sheet having an area of ​​20 cm. ２A positive electrode and a negative electrode were obtained. An Al tab was attached to the Al foil of the positive electrode, and a Ni tab was attached to the Cu foil of the negative electrode. A polypropylene microporous membrane as a separator was sandwiched between the negative electrode and the positive electrode, resulting in a stacked cell having multiple negative electrodes and multiple positive electrodes, with a separator disposed between the negative electrode and the positive electrode. The stacked cell was entirely wrapped in aluminum laminate packaging except for the tabs, and 2890 μL of electrolyte was injected into it. The opening was then sealed by heat sealing to prepare a laminated full cell for evaluation, which is a secondary battery. The electrolyte was prepared by mixing a solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2, with vinylene carbonate (VC) at 1 mass%, and this mixture was further mixed with the electrolyte LiPF ６ The liquid was prepared by dissolving the above-mentioned compounds to a concentration of 1.2 mol / L. The design capacity of the laminated full cell for evaluation was 1000 mAh. As shown in Table 1, laminated cells with different numbers of negative and positive electrodes were used to prepare laminated full cells for evaluation with different capacity ratios (negative electrode / positive electrode) and different basis weights of the positive electrode mixture layer.

[0099] [Comparative Example 1] (Preparation of Positive Electrode) The same NMC811(Li(Ni) ０．８ Mn ０．１ Co ０．１ ) O ２ An NMP solution containing 96.5 parts by mass of the above-mentioned cellulose acetate copolymer, 1.5 parts by mass of the same carbon black as used in Example 1, and 2.0 parts by mass of PVdF was weighed and mixed in a kneader. Thereafter, the mixture was mixed in a kneader while appropriately adding NMP to prepare a positive electrode slurry with adjusted viscosity. A positive electrode sheet was produced in the same manner as in Example 1 using the positive electrode slurry prepared in Comparative Example 1.

[0100] (Fabrication of Laminated Full Cells) Laminated full cells for evaluation were fabricated in the same manner as in Example 1 using the positive electrode sheet fabricated in Comparative Example 1 and the negative electrode sheet fabricated in Example 1. Furthermore, similar to Example 1, laminated full cells for evaluation with different capacity ratios (negative electrode / positive electrode) and different basis weights of the positive electrode mixture layer were fabricated using stacked cells with different numbers of negative electrodes and positive electrodes stacked, as shown in Table 1. The fabricated laminated full cells were subjected to an aging treatment.

[0101] (Charge-Discharge Evaluation of Laminated Full Cell) After the aging, the laminated full cell (the basis weight of the positive electrode mixture layer was 17.0 mg / cm ２ For a laminated full cell with a capacity ratio (negative electrode / positive electrode) of 1.1, 8 negative electrode layers, and 7 positive electrode layers, constant current charging was performed at a current of 0.2 C with an end voltage of 4.2 V. Next, constant current discharging was performed at a current of 5 C with an end voltage of 2.5 V. During this constant current discharging, the temperature was measured at 7 points on the exterior material. Next, constant current discharging was performed at a current of 0.2 C with an end voltage of 2.5 V. Next, constant current charging was performed at a current of 5 C with an end voltage of 4.2 V. During this constant current charging, the temperature was measured at 7 points on the exterior material. The temperature was measured at the following locations in a plan view of the laminated full cell: (1) center, (2) upper left center (lower left tab), (3) upper right center (lower right tab), (4) left center, (5) right center, (6) lower left center, and (7) lower right center of the exterior material. The temperature was measured at seven points on the exterior material during constant current charging and constant current discharging, and the temperature rise during charging (maximum temperature - minimum temperature) and the temperature rise during discharging (maximum temperature - minimum temperature) at each measurement point were calculated, and the standard deviation σ of the temperature rise at the temperature measurement points was calculated. Furthermore, σ / A (°C / Ah) was calculated, where A (Ah) is the rated capacity of the battery. The results are shown in Tables 2 and 3.

[0102] (Measurement of Initial DCR) After the aging, the laminated full cell (with a positive electrode mixture layer of 11.0 mg / cm ２ , 17.0mg / cm ２ or 24.5 mg / cm ２The battery (capacity ratio (negative electrode / positive electrode) was 1.1, with eight negative electrode layers and seven positive electrode layers) was charged and discharged once at a constant current of 1 C in a thermostatic chamber at 25°C, then fully charged and discharged at 0.1 C to half the discharge capacity of the first cycle. After a 30-minute rest, the battery was discharged at 1000 mA for 5 seconds. The internal resistance of the battery (DCR at soC = 50%) was calculated from the voltage drop at this time using Ohm's law (R = ΔV / 1). The results are shown in Table 3 and Figure 4.

[0103] (Evaluation of DCR Change) For the laminated full cell after the initial DCR measurement, the DCR was measured at the 100th, 200th, 300th, 400th, 500th, and 600th cycles as follows, and the DCR change (%) at each cycle was calculated relative to the reference value (initial DCR value of Comparative Example 1 in Figure 5) of 100%. Specifically, the cell was charged and discharged at a constant current of 1 C a predetermined number of times in a thermostatic chamber at 25°C, then fully charged, and discharged at 0.1 C to half the discharge capacity of the first cycle. After a 30-minute rest, the cell was discharged at 1000 mA for 5 seconds. The internal resistance of the battery (DCR at soC = 50%) was determined as the DCR at each cycle based on the voltage drop at this time according to Ohm's law (R = ΔV / 1). The cycles were performed on the laminated full cell in a thermostatic chamber at 25°C by constant-current charging at 1C with an end voltage of 4.2V, followed by constant-current discharging at 1C with an end voltage of 2.8V. The charging operation to fully charge the cell and the discharging operation to reduce the SOC to 50% immediately before the DCR measurement for each cycle were not counted in the cycle count. After the DCR measurement for each cycle, the charging and discharging operation was resumed from charging at 1C, and the number of cycles after resuming charging and discharging was added to the previous cycle count. The results are shown in Table 3 and Figure 5. The DCR change (%) shown in Table 3 is the percentage increase relative to the initial DCR value for Example 1 for Example 1, and the percentage increase relative to the initial DCR value for Comparative Example 1 for Comparative Example 1.

[0104] (Evaluation of rate characteristics) The basis weight of the positive electrode mixture layer was 17.0 mg / cm ２ or 24.5 mg / cm ２The change in charge capacity ratio of the aged laminated full cell was evaluated when the current value was varied between 0.1 C and 1.0 C in a constant temperature bath at 25°C. Specifically, constant current charging and discharging was performed at 0.1 C, followed by constant current charging at each rate to determine the charge capacity. The charge capacity ratio (%) at 0.2 C, 0.5 C, or 1.0 C was then calculated, with the charge capacity at 0.1 C being taken as 100%. The results are shown in Table 3 and Figures 6 and 7. The higher the charge capacity ratio, the better the rate characteristics. Table 3 shows the charge capacity ratio (%) at 1.0 C relative to the charge capacity at 0.1 C.

[0105]

[0106]

[0107]

[0108] As shown in Tables 2 and 3, it was confirmed that the initial DCR value was reduced by reducing the standard deviation of the temperature rise during high-rate discharge. Furthermore, it was confirmed that the positive electrode weight increased as the standard deviation of the temperature rise during high-rate discharge increased, as shown in Figure 4, i.e., the DCR reduction rate tended to increase as the energy density increased. Furthermore, it was confirmed that the increase in DCR with an increase in the number of cycles tended to be suppressed as shown in Figure 5, as a result of reducing the standard deviation of the temperature rise during high-rate discharge. From these results, it is inferred that other cycle characteristics, such as capacity after multiple cycles, are also excellent.

[0109] As shown in Table 3 and Figures 6 and 7, it was confirmed that by reducing the standard deviation of the temperature rise during high-rate charging, the decrease in the charge capacity ratio during high-rate charging was suppressed, and the rate characteristics tended to be excellent.

[0110] [Example 2] (Preparation of Positive Electrode) NMC811 (Li(Ni) ０．８ Mn ０．１ Co ０．１ ) O ２ 96.5 parts by mass of carbon black (primary particle D ｎ５０An NMP solution containing 1.1 parts by mass of cellulose acetate (C100: 100 nm), 0.3 parts by mass of fibrous carbon having an average fiber diameter of 150 nm and an average fiber length of 6 μm, 0.1 parts by mass of multi-walled carbon nanotubes (MWCNT), and 2.0 parts by mass of PVdF was weighed and mixed in a kneader. Thereafter, the mixture was mixed in the kneader while appropriately adding NMP to prepare a positive electrode slurry with adjusted viscosity. The 50% particle diameter (D ｎ５０ ) is 0.5 μm, and the 50% particle diameter (D ｎ５０ ) was 10.1 μm.

[0111] The positive electrode slurry was applied to an aluminum foil having a thickness of 20 μm using a roll coater, dried, and then vacuum-dried to obtain a positive electrode sheet. The thickness of the positive electrode mixture layer was adjusted to 53 μm by roll pressing, and the basis weight of the positive electrode mixture layer was adjusted to 11.3 mg / cm. ２ and the density of the positive electrode mixture layer was 3.2 g / cm ３ It was decided.

[0112] (Fabrication of Negative Electrode) A negative electrode slurry was obtained in the same manner as in Example 1. The negative electrode slurry was applied to a copper foil having a thickness of 20 μm using a roll coater, dried, and then further vacuum-dried to obtain a negative electrode sheet. The thickness of the negative electrode mixture layer was adjusted to 88 μm by roll pressing, and the basis weight of the negative electrode mixture layer was adjusted to 8 mg / cm. ２ and the density of the negative electrode mixture layer was 1.4 g / cm ３ It was decided.

[0113] (Fabrication of Laminated Full Cell) A laminated full cell for evaluation in Example 2 was fabricated as follows in a glove box maintained in a dry argon gas atmosphere with a dew point of −80° C. or less. The positive electrode sheet and the negative electrode sheet obtained in the fabrication of the positive electrode and the negative electrode were punched out to form a sheet having an area of ​​20 cm. ２A positive electrode and a negative electrode were obtained. An Al tab was attached to the Al foil of the positive electrode, and a Ni tab was attached to the Cu foil of the negative electrode. A polypropylene microporous membrane as a separator was sandwiched between the negative electrode and the positive electrode, resulting in a stacked cell having five negative electrodes and four positive electrodes, with a separator disposed between the negative electrode and the positive electrode. The stacked cell was entirely wrapped in aluminum laminate packaging except for the tabs, and 1800 μL of electrolyte was injected into it. The opening was then sealed by heat sealing to prepare a laminated full cell for evaluation, which is a secondary battery. The electrolyte was prepared by mixing a solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2, with vinylene carbonate (VC) at 1 mass%, and this mixture was further mixed with the electrolyte LiPF ６ The liquid was prepared by dissolving the above in a concentration of 1.2 mol / L. The laminated full cell for evaluation had a design capacity of 570 mAh and a capacity ratio (negative electrode / positive electrode) of 1.1.

[0114] [Comparative Example 2] The basis weight of the positive electrode mixture layer was 11.3 mg / cm ２ The negative electrode mixture layer had a basis weight of 8 mg / cm ２ A positive electrode sheet and a negative electrode sheet were produced in the same manner as in Comparative Example 1, except that:

[0115] (Fabrication of Laminated Full Cell) A laminated full cell for evaluation was fabricated in the same manner as in Example 2 using the positive electrode sheet fabricated in Comparative Example 2 and the negative electrode sheet fabricated in Comparative Example 2.

[0116] The laminated full cells prepared in Example 2 and Comparative Example 2 were subjected to pre-charging and aging treatments. The laminated full cells were placed in a thermostatic chamber set at 45°C and pre-charged under the conditions of CC / CV 0.03C, 3.4V, 0.01C, and cut-off. After pre-charging, degassing was performed. The aging treatment was performed as follows. First, an initial charge was performed at 45°C under the conditions of CC / CV 0.1C, 4.2V, and 0.02C, and cut-off. After the initial charge, the laminated full cells were held for 12 hours and degassed. Then, an initial discharge was performed at 25°C under the conditions of CC 0.1C and 2.8V. Then, the second charge was performed at 25°C under the conditions of CC / CV 0.2C, 4.2V, 0.05C, cut off, and the second discharge was performed at 25°C under the conditions of CC 0.2C, 2.8V.

[0117] (Charge-Discharge Evaluation of Laminated Full Cells) For the laminated full cells after the aging, σ / A (°C / Ah) was determined in the same manner as in Example 1. The value was 0.32 (°C / Ah) for Example 2 and 0.39 (°C / Ah) for Comparative Example 2.

[0118] The aged laminated full cell was repeatedly charged at 45°C under CC 6C conditions with a 4.2V cutoff, followed by discharging at 45°C under CC 5C conditions with a 2.8V cutoff. The results of the cycle test are shown in Figures 8 and 9. As shown in Figure 8, Example 2 exhibited excellent high-rate cycle characteristics despite having the same cell capacity as Comparative Example 2. Comparative Example 2 showed a rapid decline in discharge capacity retention starting around 230 cycles, whereas Example 2 showed a gradual decline in discharge capacity retention even after 400 cycles of high-rate charge / discharge. Figure 9 shows the ratio of discharge capacity at 5C charge / discharge to the discharge capacity at 0.2C charge / discharge, taken as 100%. Example 2 showed a smaller decline in discharge capacity during high-rate charge / discharge than Comparative Example 2, demonstrating superior high-rate discharge capacity.

[0119] The voltage curves obtained when high-rate charge-discharge treatment was performed in Example 2 and Comparative Example 2 are shown in Figures 10 and 11. As shown in Figure 10, Comparative Example 2 showed a voltage curve with a large IR drop from the early stage of cycling, and deterioration with the progress of cycling was significant. On the other hand, as shown in Figure 11, Example 2 showed a small IR drop, and the change in the voltage curve with the progress of cycling was small.

[0120] After 400 cycles of high-rate charge and discharge, Example 2 and Comparative Example 2 were charged at 25°C under conditions of CC / CV 0.2C, 4.2V, and 0.05C cut-off, and then discharged at 25°C under conditions of CC 0.2C and 2.8V cut-off. The results are shown in Figure 12. As shown in Figure 12, even after 400 cycles, Example 2 showed almost no capacity degradation and the shape of the voltage curve was good. On the other hand, Comparative Example 2 showed a small capacity and a curve with a high resistance, confirming cell degradation.

[0121] The disclosure of Japanese Patent Application No. 2022-179914, filed on November 9, 2022, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A secondary battery comprising: a positive electrode including a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector; a negative electrode including a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector; and an exterior material that houses the positive electrode and the negative electrode, The positive electrode and the negative electrode have a structure in which a plurality of electrodes are laminated in a thickness direction, The positive electrode mixture layer contains fibrous carbon, and when the secondary battery is discharged from a fully charged state to a discharge end voltage at a current of 5 C, σ / A is 0.35 (° C. / Ah) or less, where σ (° C.) is a standard deviation of temperature rise at temperature measurement points on the exterior material, and A (Ah) is a rated capacity of the battery.

2. 2. The secondary battery according to claim 1, wherein when the secondary battery is discharged from a fully charged state to a discharge cut-off voltage at a current of 5 C, the σ / A is 0.30 (° C. / Ah) or less.

3. 2. The secondary battery according to claim 1, wherein said σ / A is 0.25 (° C. / Ah) or less when said secondary battery is charged at a current of 5 C from a completely discharged state to a charge cut-off voltage.

4. A secondary battery as described in claim 1, wherein the average fiber diameter of the fibrous carbon is 100 nm to 200 nm.

5. The average electrode area per positive electrode is 20 cm ２ ~10000cm ２ 2. The secondary battery according to claim 1 .

6. 2. The secondary battery according to claim 1, wherein the secondary battery is of a laminate type.

7. The secondary battery according to claim 6 , wherein the number of laminations of the positive electrodes and the negative electrodes is four or more.

8. The secondary battery according to claim 1 , wherein the secondary battery is of a wound type.

9. The secondary battery according to claim 8 , wherein the number of windings is 4 or more.

10. The secondary battery according to claim 1 , wherein the positive electrode mixture layer further contains carbon black.

11. A secondary battery as described in claim 10, wherein the mass ratio of carbon black to fibrous carbon (carbon black:fibrous carbon) in the positive electrode mixture layer is 1:1 to 3:

1.

12. A secondary battery as described in claim 1, wherein the fibrous carbon includes vapor-grown carbon fibers and carbon nanotubes.

13. 2. The secondary battery according to claim 1, wherein the positive electrode mixture layer and the negative electrode mixture layer each have a thickness of 30 μm or more.

14. The density of the positive electrode mixture layer is 3.0 g / cm ３ The density of the negative electrode mixture layer is 1.3 g / cm ３ The secondary battery according to claim 1 .

15. The positive electrode mixture layer has a basis weight of 10.0 mg / cm ２ The negative electrode mixture layer has a basis weight of 5.0 mg / cm ２ The secondary battery according to claim 1 .

16. The positive electrode active material is LiNi ｘ Mn ｙ Co ｚ A ｗ O ２ (x, y, z, w≧0, x+y+z+w=1), LiMPO ４ (M is at least one selected from Fe, Co, Mn and Ni) and LiMn ａ Ni ｂ O ４ The secondary battery according to claim 1 , comprising at least one selected from (a, b≧0, a+b=2).

17. The positive electrode active material is LiNi ｘ Mn ｙ Co ｚ A ｗ O ２ The secondary battery according to claim 1 , wherein (x, y, z, w≧0, x+y+z+w=1).

18. The positive electrode active material is LiMPO ４ 2. The secondary battery according to claim 1, wherein M is at least one selected from the group consisting of Fe, Co, Mn and Ni.

19. The density of the positive electrode mixture layer is 2.0 g / cm ３ The density of the negative electrode mixture layer is 1.3 g / cm ３ The secondary battery according to claim 18 .

20. The negative electrode active material is Si, SiO ｘ (0<x≦2), soft carbon, hard carbon, graphite, silicon-carbon composite, Li ４ Ti ５ O １２ and metallic Li.

21. The secondary battery according to claim 1 , wherein the negative electrode active material comprises graphite.

22. A positive electrode comprising a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector, The positive electrode mixture layer contains fibrous carbon, a positive electrode having a structure in which the positive electrode and the negative electrode are laminated in a thickness direction, and when the secondary battery is discharged at a current of 5 C from a fully charged state to a discharge end voltage, where σ (° C.) is a standard deviation of a temperature rise at temperature measurement points on the exterior material and A (Ah) is a rated capacity of the battery, the positive electrode having a structure in which the positive electrode and the negative electrode are laminated in a thickness direction and the secondary battery is discharged at a current of 5 C from a fully charged state to a discharge end voltage, the standard deviation of a temperature rise at temperature measurement points on the exterior material is σ (° C.), and A (Ah) is a rated capacity of the battery.

23. A composition for a positive electrode mixture layer used to form a positive electrode mixture layer containing a positive electrode active material, the composition for the positive electrode mixture layer contains fibrous carbon, a positive electrode mixture layer composition for a positive electrode mixture layer that is disposed on the positive electrode current collector, a negative electrode, and an exterior material that houses the positive electrode and the negative electrode, the positive electrode and the negative electrode being stacked in a thickness direction; a negative electrode; and a secondary battery having a structure in which the positive electrode and the negative electrode are stacked in a thickness direction, the secondary battery being discharged from a fully charged state to an end-of-discharge voltage at a current of 5 C, the standard deviation of temperature rise at temperature measurement points on the exterior material being σ (° C.), and the rated capacity of the battery being A (Ah), the positive electrode mixture layer composition for a positive electrode mixture layer having a structure in which the positive electrode mixture layer is stacked in a thickness direction, the positive electrode mixture layer composition for a positive electrode mixture layer having a positive electrode mixture layer that ...

24. A composition for a positive electrode mixture layer according to claim 23, prepared by the following operations (1) and (2): The composition for a positive electrode mixture layer of the present disclosure can be prepared, for example, as follows. (1) The positive electrode active material and fibrous carbon are weighed, and are mixed with the addition of a solvent. (2) The mixture obtained in (1) is kneaded while adding a solvent.