Composition for secondary batteries, positive electrode, and positive electrode mixture layer

Abstract

The present invention provides a secondary battery with excellent rate characteristics and cycle characteristics, a positive electrode capable of manufacturing a secondary battery, and a composition for a positive electrode composite layer. [Solution] A secondary battery comprising a positive electrode having 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 having 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 outer casing material housing the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode have a structure in which multiple layers are stacked in the 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 termination voltage with a current of 5C, the standard deviation of the temperature rise at the temperature measurement point on the outer casing material is σ (°C), and the rated capacity of the battery is A (Ah), and σ / A is 0.35 (°C / Ah) or less.

Description

Technical Field

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

Background Art

[0002] Secondary batteries take advantage of the characteristics of being small, lightweight, and having a high voltage, and are widely used in electronic devices such as notebook PCs, mobile phones, smartphones, and tablet PCs. In recent years, against the backdrop of environmental issues, 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 and a battery.

[0003] Active materials expected to be used in the future include positive electrode active materials with a high nickel ratio among so-called ternary compounds or olivine-type phosphates, and negative electrode active materials are silicon. The positive electrode active materials are represented by the composition formulas Li a (Ni x Mn y Co z )O2 (0 ≤ a ≤ 1, x + y + z = 1) or Li a MPO4 (0 ≤ a ≤ 1, M is one or more selected from Fe, Co, Mn, Ni, and Cu).

[0004] There are drawbacks to using these positive electrode active materials. That is, compared with conventional active materials such as Li a CoO2, Li a (Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2 (0 ≤ a ≤ 1), etc., it is difficult to achieve both cycle characteristics and rate characteristics.

[0005] As an example of trying to solve such problems, there is disclosed a positive electrode for a lithium-ion secondary battery characterized by containing a carbon black composite in which fibrous carbon and carbon black are connected and the ash content defined in JIS K 1469 is 1.0 mass% or less, and olivine-type lithium iron phosphate (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2010-108889 [Overview of the Initiative] [Problems that the invention aims to solve]

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

[0008] This disclosure is made in view of the above-mentioned conventional circumstances and aims to provide a secondary battery having a structure in which multiple 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 manufacture a secondary battery having a structure in which multiple positive electrodes and negative electrodes are stacked in the thickness direction and having excellent rate characteristics and cycle characteristics. [Means for solving the problem]

[0009] The specific means for achieving the aforementioned objectives are as follows: <1> A secondary battery comprising a positive electrode having 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 having 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 outer casing material housing the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, The positive electrode mixture layer contains fibrous carbon, and when the secondary battery is discharged from a fully charged state to the discharge termination voltage with a current of 5C, the standard deviation of the temperature rise at the temperature measurement point on the outer material is σ (°C), and the rated capacity of the battery is A (Ah), such that σ / A is 0.35 (°C / Ah) or less. <2> When the aforementioned secondary battery is discharged from a fully charged state to the discharge termination voltage with a current of 5C, the σ / A ratio is 0.30 (°C / Ah) or less. <1> A rechargeable battery. <3> When the aforementioned secondary battery is charged with a current of 5C from a completely discharged state to the charge termination voltage, the σ / A ratio is 0.25 (°C / Ah) or less. <1> or <2> The secondary battery described above. <4> The average electrode area per positive electrode is 20 cm². 2 ~10,000cm 2 That is <1> ~ <3> A rechargeable battery as described in one of the following. <5> The aforementioned secondary battery is of the laminated type. <1> ~ <4> A rechargeable battery as described in one of the following. <6> The number of stacked positive electrodes and negative electrodes is 4 or more. <5> The secondary battery described above. <7> The aforementioned secondary battery is of the wound type. <1> ~ <4> A rechargeable battery as described in one of the following. <8> The number of turns is 4 or more. <7> The secondary battery described above. <9> The cathode mixture layer further contains carbon black <1> ~ <8> A rechargeable battery as described in one of the following. <10> The thickness of the positive electrode mixture layer and the thickness of the negative electrode mixture layer are each independently 30 μm or more. <1> ~ <9> A rechargeable battery as described in one of the following. <11> The density of the positive electrode mixture layer is 3.0 g / cm³ 3 As stated above, the density of the negative electrode mixture layer is 1.3 g / cm³. 3 That's all. <1> ~ <10> A rechargeable battery as described in one of the following. <12> The basis weight of the positive electrode mixture layer is 10.0 mg / cm³. 2 In summary, the basis weight of the negative electrode mixture layer is 5.0 mg / cm³. 2 That's all. <1> ~ <11> A rechargeable battery as described in one of the following. <13> The positive electrode active material is LiNi x Mn y Co z Al w O2 (x, y, z, w≧0, x+y+z+w=1), LiMPO4 (M is one or more selected from Fe, Co, Mn and Ni), and LiMn a Ni bThe secondary battery according to any one of <1> to <12>, comprising at least one selected from O4 (a, b ≧ 0, a + b = 2). <14> The positive electrode active material is LiNi x Mn y Co z Al w The secondary battery according to any one of <1> to <13>, comprising O2 (x, y, z, w ≧ 0, x + y + z + w = 1). <15> The positive electrode active material contains LiMPO4 (M is one or more selected from Fe, Co, Mn, and Ni), and the secondary battery according to any one of <1> to <14>. <16> The density of the positive electrode mixture layer is 2.0 g / cm 3 or more, and the density of the negative electrode mixture layer is 1.3 g / cm 3 or more. The secondary battery according to <15>. <17> The negative electrode active material contains at least one selected from Si, SiO x (0 < x ≦ 2), soft carbon, hard carbon, graphite, a composite of silicon and carbon, Li4Ti5O 12 and metallic Li, and the secondary battery according to any one of <1> to <16>. <18> The secondary battery according to any one of <1> to <17>, wherein the negative electrode active material contains graphite. <19> A positive electrode comprising a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode mixture layer containing fibrous carbon. When the positive electrode, the negative electrode, and an exterior member that houses the positive electrode and the negative electrode are provided, and the secondary battery has a structure in which a plurality of the positive electrode and the negative electrode are laminated in the thickness direction, and the secondary battery is discharged at a current of 5C from a fully charged state to a discharge cut-off voltage, the standard deviation of the temperature rise at the temperature measurement point on the exterior member is σ (°C), and the rated capacity of the battery is A (Ah). The positive electrode for which σ / A is 0.35 (°C / Ah) or less. <20> A composition for a positive electrode mixture layer used for forming a positive electrode mixture layer containing a positive electrode active material. <20> A composition for a positive electrode mixture layer used for forming a positive electrode mixture layer containing a positive electrode active material. The composition for a positive electrode mixture layer contains fibrous carbon. A positive electrode composition for a positive electrode layer, comprising a positive electrode having a positive electrode current collector and a positive electrode mixture layer formed from the positive electrode mixture composition disposed on the positive electrode current collector, a negative electrode, and an outer casing material housing the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, and when the secondary battery is discharged from a fully charged state to the discharge termination voltage with a current of 5C, the standard deviation of the temperature rise at the temperature measurement point on the outer casing material is σ (°C), and the rated capacity of the battery is A (Ah), such that σ / A is 0.35 (°C / Ah) or less. [Effects of the Invention]

[0010] According to this disclosure, it is possible to provide a secondary battery having a structure in which multiple positive electrodes and negative electrodes are stacked in the thickness direction and having excellent rate characteristics and cycle characteristics, as well as a positive electrode and positive electrode mixture layer composition that can be used to manufacture a secondary battery having a structure in which multiple positive electrodes and negative electrodes are stacked in the thickness direction and having excellent rate characteristics and cycle characteristics. [Brief explanation of the drawing]

[0011] [Figure 1] This is a cross-sectional view of a laminate-type rechargeable battery in the thickness direction. [Figure 2] This is a perspective view of a laminated rechargeable battery. [Figure 3] This is a perspective view of the positive electrode, negative electrode, and separator housed in the casing of a laminate-type secondary battery. [Figure 4] This graph shows the measurement results of the initial DCR in Example 1 and Comparative Example 1. [Figure 5] This graph shows the rate of change in DCR when the positive electrode base weight is 17.0 mg / cm2 in Example 1 and Comparative Example 1. [Figure 6] This graph shows the relationship between the current value and the charging capacity ratio when the positive electrode basis weight is 17.0 mg / cm2 in Example 1 and Comparative Example 1. [Figure 7] This graph shows the relationship between the current value and the charging capacity ratio when the positive electrode weight is 24.5 mg / cm2 in Example 1 and Comparative Example 1. [Figure 8] This graph shows the discharge capacity retention rate when high-rate charge-discharge tests were performed in Example 2 and Comparative Example 2. [Figure 9] This graph shows the ratio of the discharge capacity at 5C charge / discharge to the discharge capacity at 0.2C charge / discharge, with the discharge capacity at 0.2C being set to 100% in Example 2 and Comparative Example 2. [Figure 10] This graph shows the change in the voltage curve over time in Comparative Example 2. [Figure 11] This graph shows the change in the voltage curve over time in Example 2. [Figure 12] This graph shows the voltage curves during charging and discharging after 400 cycles of high-rate charge-discharging tests in Example 2 and Comparative Example 2. [Modes for carrying out the invention]

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

[0013] In this 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, provided that the purpose of such process is achieved. In this disclosure, the numerical range indicated using "~" includes the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described in stages within this disclosure, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in numerical ranges described within this disclosure, the upper or lower limit of that range may be replaced with the values ​​shown in the examples. In this disclosure, each component may contain multiple types of the corresponding substance. If multiple types of the substance 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 types of substances present in the composition, unless otherwise specified. In this disclosure, each component may contain multiple types of particles. If multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value for a mixture of such multiple types of particles present in the composition, unless otherwise specified. In this disclosure, the terms “layer” or “film” include cases where, when the region in which the layer or film exists is observed, it is formed not only over the entire region but also over only a portion of the region. In this disclosure, the term "lamination" refers to stacking layers, and two or more layers may be bonded together or detachable. When specific examples of secondary batteries are described in this disclosure with reference to the drawings, this disclosure is not limited to the forms shown in the drawings. Furthermore, the sizes of the components in each figure are conceptual, and the relative relationships between the sizes of the components are not limited thereto.

[0014] [Secondary battery] The secondary battery of this disclosure comprises a positive electrode having 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 having 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 outer casing 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 layers are stacked in the 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 termination voltage with a current of 5C, the standard deviation of the temperature rise at the temperature measurement point on the outer casing material is σ (°C), and the rated capacity of the battery is A (Ah), then σ / A is 0.35 (°C / Ah) or less.

[0015] The secondary battery of this disclosure has a structure in which multiple positive and negative electrodes are stacked in the thickness direction, which allows for measurement of the temperature rise at temperature measurement points on the outer casing material, and enables evaluation of the standard deviation of the temperature rise, which is the variation in temperature rise at multiple temperature measurement points. Furthermore, by employing a positive electrode mixture layer containing fibrous carbon, it is easier 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, the current flows more uniformly, the increase in DC resistance after cycling decreases, and the cycle characteristics of the secondary battery are expected to improve. In addition, by setting σ / A to 0.35 (°C / Ah) or less during high-rate discharge, localized temperature rises are suppressed, which is related to the suppression of gas generation, electrolyte decomposition, and positive electrode active material decomposition. As a result, it is expected that the capacity retention rate during high-rate charge-discharge is higher than that during low-rate charge-discharge (excellent rate characteristics). In this disclosure, "fully charged state" refers to the state after charging a secondary battery at 0.2C with a cutoff voltage of 4.2V or higher. In this disclosure, "completely discharged state" refers to the state after a secondary battery has been discharged at 0.2C with a termination voltage of 2.8V or less. As a general rule, the charging termination voltage is set to 4.2V or higher, and the discharging termination voltage is set to 2.8V or lower.

[0016] The secondary battery can take any form as long as it has a structure in which multiple positive and negative electrodes, housed in an outer casing, are stacked in the thickness direction. It may be a laminate-type secondary battery or a wound-type secondary battery. As an example of a wound-type secondary battery, it may be a cylindrical secondary battery in which an electrode pair and electrolyte obtained by winding a laminate in which positive and negative electrodes are stacked with a separator in between are sealed inside a cylindrical outer casing, or it may be a cylindrical secondary battery in which a cell obtained by winding a laminate in which positive and negative electrodes are stacked with a solid electrolyte is sealed inside a cylindrical outer casing.

[0017] A secondary battery may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a separator in between, and an electrolyte are housed in an outer casing, or it may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a solid electrolyte in between is housed in an outer casing.

[0018] The number of stacked positive and negative electrodes may be two or more, three or more, or four or more, respectively. In this disclosure, the stacking of positive and negative electrodes allows for measurement of the temperature rise at the temperature measurement point on the outer casing 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 to the number of stacked positive and negative electrodes; for example, it may be 100 or less, or 50 or less. In the case of wound-type secondary batteries, the number of layers mentioned above refers to the number of turns.

[0019] Figures 1 to 3 show examples of laminate-type secondary batteries. Figure 1 is a cross-sectional view of the laminate-type secondary battery in the thickness direction, Figure 2 is a perspective view of the laminate-type secondary battery, and Figure 3 is a perspective view of the positive electrode, negative electrode, and separator housed in the outer casing of the laminate-type secondary battery.

[0020] In Figure 1, the secondary battery 10 has a laminate formed by stacking a positive electrode 1 and a negative electrode 3 with a separator 5 in between, which is housed in an outer casing 6. The positive electrode tab 2A and the negative electrode tab 2B extend outside the outer casing 6. Figure 1 illustrates a secondary battery with 6 stacks of negative electrodes and 5 stacks of positive electrodes, but the number of stacks of negative and positive electrodes is not limited to these in this disclosure.

[0021] Figure 3 shows a configuration in which positive electrodes 1 and negative electrodes 3 are arranged alternately in the thickness direction. Furthermore, a separator 5 is placed between the positive electrode 1 and the negative electrode 3. When the components are stacked, multiple positive electrode 1s and negative electrode 3s are arranged so that multiple positive electrode tabs 2A overlap each other and multiple negative electrode tabs 2B overlap each other. The positive electrode 1s, negative electrode 3s and separator 5 arranged in this positional relationship are housed in an outer casing material 6, and after the electrolyte is housed in the outer casing material 6, the opening is sealed by heat fusion or the like to obtain the laminate-type secondary battery 10 shown in Figure 2.

[0022] The standard deviation of the temperature rise at temperature measurement points on the outer casing material can be determined, for example, as follows. First, the secondary battery is charged with a constant current of 0.2C with a charge termination voltage of 4.2V, and then discharged with a constant current of 5C with a discharge termination voltage of 2.5V. During this constant current discharge, the temperature is measured at seven points on the outer casing material. For example, when measuring the temperature using the laminated secondary battery 10 shown in Figure 2, the temperature should be measured in the seven regions indicated by the dotted lines. At this time, it is desirable to keep the distance between adjacent measurement points at 10 mm or more in order to grasp the temperature variation at each measurement point. Temperature measurement during charging is performed during constant current charging when a constant current discharge is performed with a discharge termination voltage of 2.5V and a current of 0.2C, and then a constant current charge is performed with a charge termination voltage of 4.2V and a current of 5C.

[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) are determined at each measurement point. Then, the standard deviation σ of the temperature rise during high-rate discharge can be determined from each measurement result of the temperature rise during discharge. Similarly, the standard deviation σ of the temperature rise during high-rate charging can be determined from each measurement result of the temperature rise during charging.

[0024] When a secondary battery is discharged with a current of 5C, if the standard deviation of the temperature rise at the temperature measurement point on the outer casing is σ (°C) and the rated capacity of the battery is A (Ah), then σ / A is 0.35 (°C / Ah) or less. It is preferable that the value is 0.33 (°C / Ah) or less, more preferably 0.32 (°C / Ah) or less, and even more preferably 0.30 (°C / Ah) or less, from the viewpoint of rate characteristics and cycle characteristics. The lower limit of the standard deviation of the temperature rise during the above discharge is not particularly limited and may be, for example, 0.10 (℃ / Ah) or higher.

[0025] When a secondary battery is charged with a current of 5C, the σ / A at the temperature measurement point on the outer casing 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, from the viewpoint of rate characteristics and cycle characteristics. The lower limit of the standard deviation of the temperature rise during charging described above is not particularly limited and may be, for example, 0.05 (°C / Ah) or higher.

[0026] The types of secondary batteries are not particularly limited, and 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, which can achieve high voltage and high energy density, and sodium-based secondary batteries, which can be cost-effective, are preferred. The following describes an example of a lithium-ion secondary battery, but the present invention is not limited to this example.

[0027] [Positive electrode] The secondary battery comprises a positive electrode having 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 does not oxidize and dissolve at high potential and is electrically conductive, 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 described later, a positive electrode mixture layer composition containing positive electrode active material and fibrous carbon, and optionally carbon black, binder, solvent, etc., is applied to the positive electrode current collector, the coated slurry is dried, and then pressed to form the positive electrode mixture layer on the positive electrode current collector.

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

[0031] The density of the positive electrode mixture layer is 2.0 g / cm³, from the viewpoint of energy density and safety. 3 It may be greater than or equal to 3.0 g / cm³. 3 It may be greater than or equal to 3.0 g / cm³. 3 ~4.0g / cm 3 That's fine.

[0032] The basis weight of the positive electrode mixture layer is 10.0 mg / cm³, considering energy density and safety. 2 It may be greater than or equal to 10.0 mg / cm³. 2 ~30.0 mg / cm³ 2 That's fine.

[0033] The average electrode area per sheet (average positive electrode area and average negative electrode area) is 20 cm². 2 ~10,000cm 2 It may also be 300cm 2 ~10,000cm 2 That's fine.

[0034] (Cathode 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 include compounds containing at least one of lithium, sodium, potassium, magnesium, and aluminum. Examples of positive electrode active materials include nickel-containing oxides and phosphates having an olivine-type structure. When the secondary battery is a lithium-based secondary battery, the positive electrode active material is LiNi x Mn y Coz Al w O2 (x, y, z, w≧0, x+y+z+w=1), LiMPO4 (M is one or more selected from Fe, Co, Mn and Ni), LiMn a Ni b Examples include O4 (a, b ≥ 0, a + b = 2), etc.

[0035] The positive electrode active material is LiNi x Mn y Co z Al w It is preferable that the mixture contains O2 (x, y, z, w≧0, x+y+z+w=1) or LiMPO4 (where M is one or more selected from Fe, Co, Mn, and Ni).

[0036] LiRing x Mn y Co z Al w For O2(x, y, z, w≧0, x+y+z+w=1), it is preferable that the nickel content is relatively high, for example, x≧0.5 or higher, and Li(Ni x Mn y Co z It is more preferable that O2(x≧0.5, y≦0.3, z≦0.3, x+y+z=1). Li(Ni x Mn y Co z )O2(x≧0.5, y≦0. 3. A positive electrode active material represented by (z ≤ 0.3, x + y + z = 1) is, for example, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, Li(Ni 0.7 Mn 0.2 Co 0.1 )O2, Li(Ni 0.7 Mn 0.1 Co 0.2 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2 and Li(Ni 0.5 Mn 0.2 Co 0.3)O2 is one example.

[0037] Examples of positive electrode active materials represented as LiMPO4 (where M is one or more selected from Fe, Co, Mn, and Ni) include LiFePO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.3 Mn 0.7 PO4, LiCoPO4 and LiCo 0.5 Mn 0.5 PO4 is one example.

[0038] At least a portion of the surface of the positive electrode active material may be coated with amorphous carbon. Known methods for coating with amorphous carbon include heating the positive electrode active material and flowing a hydrocarbon gas through it to perform CVD, and mixing a carbon precursor and the positive electrode active material, and then performing heat treatment with the carbon precursor attached to at least a portion of the surface of the positive electrode active material to carbonize the carbon precursor.

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

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

[0041] The 50% particle size (D) in the number-based cumulative particle size distribution of the primary particle size of the positive electrode active material. n50 This can be determined by selecting 100 primary particles of the positive electrode active material from SEM images of the electrode and cross-sectional SEM images, and then arithmetic mean the results of measuring the maximum length of the particles using image recognition software.

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

[0043] The 50% particle diameter (D n50 ) of the secondary particles of the positive electrode active material 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 hardly causing abnormal unevenness during electrode coating.

[0044] The 50% particle diameter (D n50 ) of the secondary particle diameter of the positive electrode active material can be obtained by selecting any 100 secondary particles of the positive electrode active material from the SEM photograph and the cross-sectional SEM photograph of the electrode and calculating the arithmetic mean of the measurement results of the maximum length of the particles by image recognition software.

[0045] In the present disclosure, the primary particles of the positive electrode active material refer to those that can be recognized as one grain in an SEM photograph at 3000 to 30000 times magnification. In this specification, the secondary particles of the positive electrode active material are agglomerates formed by the aggregation of primary particles. The secondary particles of the positive electrode active material can be confirmed from the SEM photograph. In this specification, when simply referring to the "particles" of the positive electrode active material, it refers to primary particles or secondary particles.

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

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

[0048] (Fibrous carbon) The positive electrode mixture layer contains fibrous carbon. Examples of fibrous carbon include carbon fibers, vapor-processed carbon fibers, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and carbon nanofibers. Vapor-processed carbon fibers are preferred from the viewpoint of excellent dispersibility, enabling low electrode resistance, and effectively suppressing temperature rise variations during high-rate discharge. Furthermore, gas-phase carbon fibers may be combined with other fibrous carbon, such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The positive electrode mixture layer may contain one type of fibrous carbon, or it 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 arbitrary fibers observed by SEM of the electrode. The diameter of a single fiber can be determined by measuring the width at any 10 points on a fiber from an SEM image and taking the arithmetic mean of these measurements. Here, 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 fibrous carbon can be measured as follows: First, the electrode is washed with a solvent to remove binders and other substances, and the resulting powder is dispersed in a dispersion medium. After spreading the dispersed powder on aluminum foil or similar material and drying, it is observed using a scanning electron microscope (SEM). The length of 300 arbitrary fibers along their fiber axes is measured, and the average fiber length is determined by taking the arithmetic mean of these measurements.

[0053] The fibrous carbon preferably includes 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 includes vapor-phase 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] Examples of fibrous carbon include VGCF(registered trademark)-X (manufactured by Showa Denko K.K.) and VGCF(registered trademark)-H (manufactured by Showa Denko K.K.).

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

[0056] In the positive electrode mixture layer, the fibrous carbon content 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 includes gas-phase carbon fibers and carbon nanotubes such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), the ratio of gas-phase carbon fibers to carbon nanotubes is not particularly limited. From the viewpoint of cycle characteristics in high-rate charge and discharge, for example, the ratio of gas-phase carbon fibers to 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 in secondary batteries. Examples of carbon black include Denka Black (registered trademark, manufactured by Denka Co., 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 size of carbon black may be between 10 nm and 100 nm. Here, primary particles refer to the part corresponding to a single bead in a bead-like structure called an aggregate. When the primary particle size of carbon black is within this range, it tends to disperse uniformly on the surface of the active material. From the viewpoint of improving dispersibility, the primary particle size of carbon black is preferably between 20 nm and 80 nm, and more preferably between 30 nm and 70 nm.

[0060] The primary particle size of carbon black can be determined by selecting 100 arbitrary primary carbon black particles from SEM images of the electrode and cross-sectional SEM images, and then taking the arithmetic mean of the maximum particle length measurements using image recognition software.

[0061] When the positive electrode mixture layer contains carbon black, the carbon black content is preferably 0.2% by mass or more, more preferably 0.6% by mass or more, and even more preferably 1.0% by 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% by mass or less, more preferably 4.0% by mass or less, and even more preferably 2.0% by 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 carbon black to 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 through the compounding of the two.

[0064] (binder) The positive electrode mixture layer may contain a binder. Suitable binders include those commonly used in positive electrode mixture layers for lithium-ion secondary batteries. Examples of binders include polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE).

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

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

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

[0068] [Negative electrode] The secondary battery comprises a negative electrode which includes 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 a material with electron conductivity, 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, a copper foil is used as the negative electrode current collector.

[0070] As described later, a composition for a negative electrode mixture layer containing a negative electrode active material and further containing a conductive assistant, a binder, a solvent, etc. as required is applied onto the negative electrode current collector, the applied slurry is dried, and then pressed to form 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] From the viewpoints of energy density and safety, the density of the negative electrode mixture layer may be 1.3 g / cm 3 or more, and may be 1.5 g / cm 3 to 2.0 g / cm 3 or more.

[0073] From the viewpoints of energy density and safety, the basis weight of the negative electrode mixture layer may be 5.0 mg / cm 2 or more, and may be from 10.0 mg / cm 2 to 20.0 mg / cm 2 or more.

[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, a composite of silicon and carbon, Li4Ti5O 12 , metallic Li, etc. Among them, it is preferable that the negative electrode active material contains graphite. Also, at least a part of the surface of the negative electrode active material may be coated with amorphous carbon.

[0075] (Conductive assistant) The negative electrode mixture layer may contain a conductive additive. Examples of conductive additives include carbon materials such as the aforementioned fibrous carbon, carbon black, and graphene.

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

[0077] (Exterior materials) The outer casing material for housing the positive and negative electrodes is not limited as long as it can accommodate the positive and negative electrodes, and optionally a separator and electrolyte, or a solid electrolyte. Examples of outer casing materials include commercially available battery packs, 18650 type cylindrical cells, and those packaged in aluminum foil, and the outer casing material can be freely designed and used.

[0078] (Separator) A secondary battery may include a separator between the positive and negative electrodes. The separator can be freely selected from those commonly used in secondary batteries, such as microporous films made of polyethylene or polypropylene. Separators containing particles such as SiO2 or Al2O3 as fillers, or separators with these particles attached to the surface, can also be used.

[0079] (electrolyte) In secondary batteries, the electrolyte may be contained within the outer casing. There are no particular restrictions on the electrolyte, and any electrolyte usable in a typical secondary battery can be suitably used. For example, an organic solvent in which a lithium salt is dissolved in a concentration of 0.5 mol / L to 2.0 mol / L can be used.

[0080] Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, and LiFSI.

[0081] Examples of organic solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DEC), diethyl carbonate (DEC), and propylene carbonate (PC). Organic solvents listed here and others may be appropriately selected and mixed. Examples of electrolyte additives include vinylene carbonate (VC), propane sultone (PS), and fluoroethylene carbonate (FEC). When additives are used, the additive content 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, based on 100% by mass of the organic solvent.

[0082] (Ionic liquid) Ionic liquids may be used as electrolytes, or ionic liquids may be used in combination with the aforementioned organic solvents. The ionic liquid is not particularly limited, and examples include combinations of cations such as imidazolium cations, pyrrolidinium cations, piperinidium cations, and ammonium cations with anions such as bis(trifluoromethane)sulfonamide anions.

[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 the positive electrode and negative electrode are sandwiched between solid electrolytes.

[0084] Examples of solid electrolytes include polymer electrolytes and inorganic solid electrolytes. Polymer electrolytes are not particularly limited, and examples include polymers such as polyethylene oxide impregnated with the above-mentioned lithium salt. Inorganic solid electrolytes are not particularly limited, and examples include Li 13 Ti 1.7 Al 0.3 Examples include (PO4)3 and Li2S-P2S5.

[0085] The positive electrode of the secondary battery of this disclosure is obtained by coating a positive electrode composite layer composition containing a positive electrode active material and fibrous carbon, and optionally containing carbon black, a binder, a solvent, etc., onto a positive electrode current collector, drying the coated slurry, and then pressing it to form a positive electrode composite layer on the positive electrode current collector.

[0086] [Composition for cathode composite layer] The positive electrode mixture layer composition of this disclosure is a positive electrode mixture layer composition used for forming a positive electrode mixture layer containing a positive electrode active material, and the positive electrode mixture layer composition contains fibrous carbon. Furthermore, when a secondary battery is made comprising a positive electrode having a positive electrode mixture layer formed using the positive electrode mixture layer composition of this disclosure, a negative electrode, and an outer casing material housing the positive electrode and the negative electrode, and the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, when the secondary battery is discharged from a fully charged state to the discharge termination voltage with a current of 5C, if the standard deviation of the temperature rise at the temperature measurement point on the outer casing material is σ (°C) and the rated capacity of the battery is A (Ah), then σ / A will be 0.35 (°C / Ah) or less. As a result, by using the cathode compound layer composition of this disclosure, it becomes possible to manufacture a secondary battery with excellent rate characteristics and cycle characteristics.

[0087] The preferred composition of the positive electrode active material and fibrous carbon contained in the positive electrode mixture layer composition of this disclosure, as well as the carbon black, binder, etc., which may be included as needed, is the same as the preferred composition described in the section on secondary batteries of this disclosure above.

[0088] The cathode mixture layer composition of this disclosure may contain a solvent. The solvent is not particularly limited, and examples include N-methyl-2-pyrrolidone (NMP).

[0089] The cathode mixture layer composition of this disclosure can be prepared, for example, as follows: (1) Weigh the positive electrode active material and fibrous carbon, and if necessary, weigh carbon black, binder, etc., and mix them while adding the solvent. (2) The mixture obtained in (1) above is kneaded while adding a solvent to obtain a positive electrode mixture layer composition of appropriate viscosity.

[0090] Furthermore, the positive electrode mixture layer composition may be applied to the positive electrode current collector using a doctor blade or coating machine, and the solvent may be dried by placing the coated positive electrode current collector on a heating device such as a hot plate. After that, vacuum drying is performed to remove any remaining solvent. The product obtained after removing the solvent is pressed and molded using a roll press, uniaxial press, or the like to obtain the positive electrode.

[0091] [Positive electrode] The positive electrode of this 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 comprising the positive electrode of this disclosure, a negative electrode, and an outer casing material housing the positive electrode and the negative electrode, and having a structure in which the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, is discharged from a fully charged state to the discharge termination voltage with a current of 5C, if the standard deviation of the temperature rise at the temperature measurement point on the outer casing material is σ (°C) and the rated capacity of the battery is A (Ah), then σ / A is 0.35 (°C / Ah). The following is the result. As a result, by using the positive electrode of this disclosure, it becomes possible to manufacture a secondary battery with excellent rate characteristics and cycle characteristics. The positive electrode of this disclosure can be prepared, for example, using the positive electrode mixture layer composition of this disclosure.

[0092] Each electrode can be stacked and packaged for use. Alternatively, single cells can be connected in series to form batteries, modules, etc. The secondary battery of this 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 such as power tools, vacuum cleaners, electric bicycles, drones, and electric vehicles; and for storing electricity obtained from fuel cells, solar power generation, wind power generation, etc. [Examples]

[0093] The present disclosure will be described in detail below with reference to examples, but the scope of the present disclosure is not limited to these examples.

[0094] [Example 1] (Fabrication of the positive electrode) As the positive electrode active material, NMC811(Li(Ni 0.8 Mn 0.1 Co 0.1 ) 96.5 parts by mass of O2, carbon black (primary particles D n50 An NMP solution containing 1.0 part by mass of (100 nm), 0.5 parts by mass of fibrous carbon with 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. Subsequently, NMP was added as needed while mixing in the kneader to prepare a cathode slurry with adjusted viscosity. Furthermore, the 50% particle size (D) in the number-based cumulative particle size distribution of the primary particles of the positive electrode active material NMC811. n50 ) is 0.5 μm, and the 50% particle size (D) in the number-based cumulative particle size distribution of secondary particles. n50 The diameter was 10.1 μm.

[0095] The aforementioned cathode slurry was coated onto a 20 μm thick aluminum foil using a roll coater, dried, and then vacuum-dried to obtain a cathode sheet. The cathode mixture layer was then roll-pressed to a thickness of 53 μm and to a density of 3.2 g / cm³. 3 That's what I decided.

[0096] (Fabrication of the negative electrode) 95.7 parts by mass of graphite particles as the negative electrode active material, and carbon black (primary particles D) as the conductive additive. n50 1.3 parts by mass of 50 nm (D) carboxymethylcellulose (CMC) as a binder, 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 Corporation) were weighed out and mixed in a kneader to obtain a slurry for the negative electrode. The graphite particles were defined as having a 50% particle size (D) in the volume-based cumulative particle size distribution. v50 A sample with a diameter of 14 μm was used.

[0097] The aforementioned negative electrode slurry was coated onto a 20 μm thick copper foil using a roll coater, dried, and then vacuum-dried to obtain a negative electrode sheet. The thickness of the negative electrode mixture layer was reduced to 88 μm by roll pressing, and the density of the negative electrode mixture layer was reduced to 1.4 g / cm³. 3 That's what I decided.

[0098] (Preparation of laminated full cells) A laminated full cell for evaluation in Example 1 was prepared in a glove box maintained in a dry argon gas atmosphere with a dew point of -80°C or lower, as described below. The positive electrode sheet and negative electrode sheet obtained from the above positive electrode and negative electrode fabrication process were punched out to form an area of ​​20 cm². 2 A 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, which served as a separator, was sandwiched between the negative electrode and the positive electrode to obtain a laminated cell with multiple negative electrodes and multiple positive electrodes, and a separator positioned between the negative electrode and the positive electrode. The laminated cell, except for the tabs, was wrapped in aluminum laminate packaging, and 2890 μL of electrolyte was injected inside. Then, the opening was sealed by heat fusion to fabricate an evaluation laminated full cell, which was a secondary battery. The electrolyte was a solvent prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2, adding vinylene carbonate (VC) to a concentration of 1% by mass, and then dissolving the electrolyte LiPF6 in this mixture to a concentration of 1.2 mol / L. The design capacity of the evaluation laminated full cell was 1000 mAh. Furthermore, as shown in Table 1, evaluation laminate-type full cells with different capacity ratios (negative electrode / positive electrode) and basis weight of the positive electrode mixture layer were prepared using laminate cells with different numbers of negative and positive electrode layers.

[0099] [Comparative Example 1] (Fabrication of the positive electrode) The same NMC811(Li(Ni)) used in Example 1 was used as the positive electrode active material. 0.8 Mn 0.1 Co 0.1An NMP solution containing 96.5 parts by mass of (O2), 1.5 parts by mass of the same carbon black used in Example 1, and 2.0 parts by mass of PVdF was weighed and mixed in a kneader. Then, NMP was added as needed while mixing in the kneader to prepare a cathode slurry with adjusted viscosity. A cathode sheet was prepared in the same manner as in Example 1 using the cathode slurry prepared in Comparative Example 1.

[0100] (Preparation of laminated full cells) A laminated full cell for evaluation was prepared in the same manner as in Example 1, using the positive electrode sheet prepared in Comparative Example 1 and the negative electrode sheet prepared in Example 1. Furthermore, similar to Example 1, as shown in Table 1, laminated full cells for evaluation were prepared using laminated cells with different numbers of negative and positive electrode layers, each with a different capacity ratio (negative electrode / positive electrode) and basis weight of the positive electrode mixture layer. The fabricated laminated full cells were subjected to an aging process.

[0101] (Charge and discharge evaluation of laminated full cells) The laminated full cell after the above aging (the basis weight of the positive electrode mixture layer is 17.0 mg / cm²) 2 For a battery with a capacitance ratio (negative / positive) of 1.1 (8 negative layers and 7 positive layers), constant current charging was performed with a cutoff voltage of 4.2V and a current of 0.2C. Next, constant current discharge was performed with a cutoff voltage of 2.5V and a current of 5C. During this constant current discharge, the temperature was measured at 7 points on the outer casing. Next, constant current discharge was performed with a cutoff voltage of 2.5V and a current of 0.2C. Next, constant current charging was performed with a cutoff voltage of 4.2V and a current of 5C. During this constant current charging, the temperature was measured at 7 points on the outer casing. The temperature measurement points were (1) the center, (2) the upper left center (below the left tab), (3) the upper right center (below the right tab), (4) the left center, (5) the right center, (6) the lower left center, and (7) the lower right center of the outer casing in a plan view of the laminated full cell. The temperature at seven points on the outer casing was measured during constant-current charging and constant-current discharging. The temperature rise during charging (maximum temperature - minimum temperature) and the temperature rise during discharging (maximum temperature - minimum temperature) were determined at each measurement point, and the standard deviation σ of the temperature rise at the temperature measurement points was calculated. Furthermore, σ / A (°C / Ah) was calculated, with the rated capacity of the battery being A (Ah). The results are shown in Tables 2 and 3.

[0102] (Initial DCR measurement) The laminated full cell after the above aging (the basis weight of the positive electrode mixture layer is 11.0 mg / cm²) 2 , 17.0 mg / cm³ 2 or 24.5 mg / cm³ 2 For batteries with a capacity ratio (negative electrode / positive electrode) of 1.1 (8 layers for the negative electrode and 7 layers for the positive electrode), one constant current charge-discharge cycle was performed at a current of 1C in a constant temperature bath at 25°C. After reaching a fully charged state, the batteries were discharged at 0.1C until they reached half the discharge capacity of the first cycle. After a 30-minute rest, a discharge was performed at 1000mA for 5 seconds. The internal resistance of the batteries (DCR at soc=50%) was calculated from the voltage drop during this time using Ohm's law (R=ΔV / 1). The results are shown in Table 3 and Figure 4.

[0103] (Evaluation of DCR changes) For the laminated full cells after the initial DCR measurement described above, the DCR was measured at cycles 100, 200, 300, 400, 500, and 600 as follows, and the change in DCR (%) at each cycle was determined, with the baseline (the initial DCR value of Comparative Example 1 in Figure 5) set to 100%. Specifically, the battery was subjected to constant current charge and discharge cycles a predetermined number of times at a current of 1C in a constant temperature bath at 25°C. After reaching a fully charged state, it was discharged at 0.1C until it reached half the discharge capacity of the first cycle. After a 30-minute rest, it was discharged at 1000mA for 5 seconds. The voltage drop at this time was used to determine the internal resistance of the battery (DCR at soc=50%) for each cycle using Ohm's law (R=ΔV / 1). The cycles were performed on the laminated full cell described above by constant current charging at a current of 1C with a cutoff voltage of 4.2V in a constant temperature bath at 25°C, followed by constant current discharge at a current of 1C with a cutoff voltage of 2.8V. The charging operation to bring the cell to a fully charged state and the discharge operation to bring the SoC to 50% immediately before the DCR measurement in each cycle were not counted in the cycle count. After the DCR measurement in each cycle, the charge / discharge operation was restarted with charging at 1C, and the number of cycles after the restart of charge / discharge was added to the previous cycle count. The results are shown in Table 3 and Figure 5. The DCR change (%) shown in Table 3 represents the percentage increase relative to the initial DCR value of Example 1, and the percentage increase relative to the initial DCR value of Comparative Example 1. It tastes good.

[0104] (Evaluation of rate characteristics) The basis weight of the positive electrode mixture layer is 17.0 mg / cm³. 2 or 24.5 mg / cm³ 2 For aged laminated full cells, the change in charge capacity ratio was evaluated when the current value was changed within the range of 0.1C to 1.0C in a constant temperature bath at 25°C. Specifically, constant current charge and discharge was performed under the condition of 0.1C, and then constant current charging was performed at each rate to determine the charge capacity. Then, the charge capacity ratio (%) at 0.2C, 0.5C, or 1.0C was calculated, with the charge capacity at 0.1C set as 100%. The results are shown in Table 3 and Figures 6 and 7. A higher charge capacity ratio indicates better rate characteristics. Table 3 shows the charge capacity ratio (%) at 1.0C relative to the charge capacity at 0.1C.

[0105] [Table 1]

[0106] [Table 2]

[0107] [Table 3]

[0108] As shown in Tables 2 and 3, it was confirmed that the initial DCR value decreases when the standard deviation of the temperature rise during high-rate discharge is reduced. Furthermore, as the standard deviation of temperature rise during high-rate discharge is reduced, the positive electrode surface area increases, as shown in Figure 4. In other words, the higher the energy density, the greater the reduction in DCR. A tendency to do so was observed. Furthermore, a reduction in the standard deviation of temperature rise during high-rate discharge resulted in a tendency to suppress the increase in DCR as the number of cycles increased, as shown in Figure 5. From these results, it can be inferred that other cycle characteristics, such as capacity after many cycles, are similarly superior.

[0109] As shown in Table 3 and Figures 6 and 7, a reduction in the standard deviation of temperature rise during high-rate charging suppressed the decrease in the charging capacity ratio at high rates, resulting in a tendency towards superior rate characteristics.

[0110] [Example 2] (Fabrication of the positive electrode) As the positive electrode active material, NMC811(Li(Ni 0.8 Mn 0.1 Co 0.1 ) 96.5 parts by mass of O2, carbon black (primary particles D n50 An NMP solution containing 1.1 parts by mass of (100 nm), 0.3 parts by mass of fibrous carbon with 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 (MWCNTs), and 2.0 parts by mass of PVdF was weighed and mixed in a kneader. Subsequently, NMP was added as needed while mixing in the kneader to prepare a cathode slurry with adjusted viscosity. Furthermore, the 50% particle size (D) in the number-based cumulative particle size distribution of the primary particles of the positive electrode active material NMC811. n50 ) is 0.5 μm, and the 50% particle size (D) in the number-based cumulative particle size distribution of secondary particles. n50 The diameter was 10.1 μm.

[0111] The aforementioned cathode slurry was coated onto a 20 μm thick aluminum foil using a roll coater, dried, and then vacuum-dried to obtain a cathode sheet. The cathode mixture layer was then roll-pressed to a thickness of 53 μm and the basis weight of the cathode mixture layer was 11.3 mg / cm². 2 The density of the positive electrode mixture layer is set to 3.2 g / cm³. 3 That's what I decided.

[0112] (Fabrication of the negative electrode) A slurry for the negative electrode was obtained using the same procedure as in Example 1. The aforementioned negative electrode slurry was coated onto a 20 μm thick copper foil using a roll coater, dried, and then vacuum-dried to obtain a negative electrode sheet. The thickness of the negative electrode mixture layer was reduced to 88 μm by roll pressing, and the basis weight of the negative electrode mixture layer was set to 8 mg / cm². 2 The density of the negative electrode mixture layer is set to 1.4 g / cm³. 3 That's what I decided.

[0113] (Preparation of laminated full cells) A laminated full cell for evaluation in Example 2 was prepared as follows in a glove box maintained in a dry argon gas atmosphere with a dew point of -80°C or lower. The positive electrode sheet and negative electrode sheet obtained from the above positive electrode and negative electrode fabrication process were punched out to form an area of ​​20 cm². 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, which served as a separator, was sandwiched between the negative and positive electrodes to obtain a laminated cell with five negative electrodes and four positive electrodes, with the separator positioned between the negative and positive electrodes. The laminated cell, except for the tabs, was wrapped in aluminum laminate packaging, and 1800 μL of electrolyte was injected inside. The opening was then sealed by heat fusion to create an evaluation laminated full cell, which served as a secondary battery. The electrolyte was prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2, adding vinylene carbonate (VC) to a mixture containing 1% by mass, and then dissolving the electrolyte LiPF6 in this mixture to a concentration of 1.2 mol / L. Furthermore, the design capacity of the laminated full cell used for evaluation was 570mAh, and the capacity ratio (negative electrode / positive electrode) was 1.1.

[0114] [Comparative Example 2] The basis weight of the positive electrode mixture layer is 11.3 mg / cm². 2 The basis weight of the negative electrode mixture layer is set to 8 mg / cm². 2 The positive electrode sheet and negative electrode sheet were manufactured in the same manner as in Comparative Example 1, except for the aforementioned difference.

[0115] (Preparation of laminated full cells) A laminated full cell for evaluation was prepared in the same manner as in Example 2, using the positive electrode sheet and the negative electrode sheet prepared 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. Pre-charging was performed by placing laminated full cells in a constant temperature bath set to 45°C under the conditions of CC / CV 0.03C 3.4V 0.01C Cutoff. Degassing was performed after pre-charging. The aging process was performed as follows: First, the initial charge was performed at 45°C under the conditions of CC / CV 0.1C 4.2V 0.02C Cutoff. After the initial charge, the laminated full cell was held for 12 hours to degass it. Next, the initial discharge was performed at 25°C under the conditions of CC 0.1C 2.8V. Then, the second charge was performed at 25°C under the conditions of CC / CV 0.2C 4.2V 0.05C Cutoff, and the second discharge was performed at 25°C under the conditions of CC 0.2C 2.8V.

[0117] (Charge and discharge evaluation of laminated full cells) For the laminated full cells after aging as described above, σ / A (°C / Ah) was determined in the same manner as in Example 1. In Example 2, it was 0.32 (°C / Ah), and in Comparative Example 2, it was 0.39 (°C / Ah).

[0118] The laminated full cells, after the above aging process, were repeatedly subjected to high-rate charge-discharge treatment: charging at 45°C under conditions of CC 6C and 4.2V cutoff, followed by discharging at 45°C under conditions of CC 5C and 2.8V cutoff. The results of the cycle test are shown in Figures 8 and 9. As shown in Figure 8, Example 2 exhibited superior high-rate cycle characteristics despite having the same cell capacity as Comparative Example 2. In Comparative Example 2, the discharge capacity retention rate decreased sharply from around 230 cycles, but in Example 2, the decrease in the discharge capacity retention rate was gradual even after 400 cycles of high-rate charging and discharging. Figure 9 shows the ratio of the discharge capacity at 5C charge / discharge to the discharge capacity at 0.2C charge / discharge, with the discharge capacity at 0.2C set to 100%. The decrease in discharge capacity at high charge / discharge rates was smaller in Example 2 than in Comparative Example 2, indicating superior discharge capacity at high rates.

[0119] Figures 10 and 11 show the voltage curves obtained when high-rate charge and discharge processing was performed in Example 2 and Comparative Example 2. As shown in Figure 10, Comparative Example 2 showed a voltage curve with a large IR drop from the beginning of the cycle, and the degradation with the progression of the cycle was large. On the other hand, as shown in Figure 11, Example 2 had a small IR drop, and the change in the voltage curve with the progression of the cycle was small.

[0120] For Example 2 and Comparative Example 2, after 400 cycles of high-rate charge / discharge, charging was performed at 25°C under conditions of CC / CV 0.2C, 4.2V, and 0.05C cutoff, followed by discharging at 25°C under conditions of CC 0.2C and 2.8V cutoff. The results are shown in Figure 12. As shown in Figure 12, even after 400 cycles, almost no capacitance degradation was observed in Example 2, and the voltage curve shape was also good. On the other hand, Comparative Example 2 showed a curve shape with low capacitance and high resistance, and cell degradation was confirmed.

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

Claims

1. A secondary battery comprising: a positive electrode having 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 having 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 outer casing material housing the positive electrode and the negative electrode, The positive electrode and the negative electrode have a structure in which multiple layers are stacked in the thickness direction. The positive electrode mixture layer contains fibrous carbon, and when the secondary battery is discharged from a fully charged state to the discharge termination voltage with a current of 5C, the standard deviation of the temperature rise at the temperature measurement point on the outer material is σ (°C), and the rated capacity of the battery is A (Ah), such that σ / A is 0.35 (°C / Ah) or less.

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

3. The secondary battery according to claim 1, wherein when the secondary battery is charged with a current of 5C from a completely discharged state to the charge termination voltage, the σ / A is 0.25 (°C / Ah) or less.

4. The average electrode area per positive electrode is 20 cm². ２ ~10,000 cm ２ The secondary battery according to claim 1.

5. The secondary battery according to claim 1, wherein the secondary battery is of the laminated type.

6. The secondary battery according to claim 5, wherein the number of stacked positive electrodes and negative electrodes is 4 or more.

7. The secondary battery according to claim 1, wherein the secondary battery is of the winding type.

8. The secondary battery according to claim 7, wherein the number of turns is four or more.

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

10. The secondary battery according to claim 1, 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 / cm³. ３ As stated above, the density of the negative electrode mixture layer is 1.3 g / cm³. ３ The secondary battery according to claim 1, wherein the above is true.

12. The basis weight of the positive electrode mixture layer is 10.0 mg / cm². ２ In summary, the basis weight of the negative electrode mixture layer is 5.0 mg / cm². ２ The secondary battery according to claim 1, wherein the above is true.

13. where 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), the secondary battery according to claim 1, comprising at least one selected therefrom.

14. The positive electrode active material is LiNi ｘ Mn ｙ Co ｚ Al ｗ O ２ A secondary battery according to claim 1, including (x, y, z, w ≥ 0, x + y + z + w = ​​1).

15. The positive electrode active material is LiMPO ４ The secondary battery according to claim 1, comprising (where M is one or more selected from Fe, Co, Mn, and Ni).

16. The density of the positive electrode mixture layer is 2.0 g / cm³. ３ As stated above, the density of the negative electrode mixture layer is 1.3 g / cm³. ３ The secondary battery according to claim 15.

17. 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 The secondary battery according to claim 1, comprising at least one selected from and metallic Li.

18. The secondary battery according to claim 1, wherein the negative electrode active material contains 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, The positive electrode mixture layer contains fibrous carbon, A positive electrode having a structure in which the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, when the secondary battery is discharged from a fully charged state to the discharge termination voltage with a current of 5C, and the standard deviation of the temperature rise at the temperature measurement point on the outer material is σ (°C) and the rated capacity of the battery is A (Ah), then σ / A is 0.35 (°C / Ah) or less.

20. A composition for a positive electrode mixture layer used to form a positive electrode mixture layer containing a positive electrode active material, The aforementioned positive electrode mixture layer composition contains fibrous carbon, A positive electrode composition for a positive electrode layer, comprising a positive electrode having a positive electrode current collector and a positive electrode mixture layer formed from the positive electrode mixture composition disposed on the positive electrode current collector, a negative electrode, and an outer casing material housing the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, and when the secondary battery is discharged from a fully charged state to the discharge termination voltage with a current of 5C, the standard deviation of the temperature rise at the temperature measurement point on the outer casing material is σ (°C), and the rated capacity of the battery is A (Ah), such that σ / A is 0.35 (°C / Ah) or less.

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