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

Abstract

A secondary battery provided with a positive electrode provided with a positive electrode current collector and a positive electrode mixture layer that is arranged on the positive electrode current collector and contains a positive electrode active material, a negative electrode provided with a negative electrode current collector and a negative electrode mixture layer that is arranged on the negative electrode current collector and contains a negative electrode active material, and an exterior material, the positive electrode and the negative electrode 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 an end-of-discharge voltage at a current of 5C, the end-of-discharge voltage of the secondary battery is greater than the end-of-discharge voltage of the secondary battery, and the end-of-discharge voltage of the secondary battery is greater than the end-of-discharge voltage of the secondary battery. When the standard deviation of the temperature rise at the temperature measurement point on the exterior material is set as sigma (DEG C) and the rated capacity of the battery is set as A (Ah), sigma / A is 0.35 (DEG 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 effectively utilize the characteristics of being small, lightweight, and having a high voltage, and are widely used in electronic devices such as notebook personal computers, mobile phones, smart phones, and tablet personal computers. In recent years, against the backdrop of environmental issues, secondary batteries such as lithium-based secondary batteries have become popular in electric vehicles (EVs) that run solely on batteries, and hybrid electric vehicles (HEVs) that combine gasoline engines and batteries.

[0003] As active materials expected to be used in the future, the positive electrode active material is a material with a high nickel ratio in the so-called ternary compound, or an olivine-type phosphate, and the negative electrode active material is silicon. The positive electrode active material is composed of Li a (Ni x Mn y Co z )O2 (0≤a≤1, x+y+z=1) or Li a The composition formula of MPO4 (0≤a≤1, M is one or more selected from Fe, Co, Mn, Ni and Cu) is shown.

[0004] There are disadvantages to using these positive electrode active materials. a CoO2、Li a (Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2(0≤a≤1) etc., it is difficult to take into account both the cycle characteristics and the rate characteristics.

[0005] As an example of solving such a problem, a positive electrode for a lithium ion secondary battery is disclosed, characterized in that it contains a carbon black composite formed by connecting fibrous carbon and carbon black and having an ash content of less than 1.0 mass% as specified in JIS K 1469, and olivine-type lithium iron phosphate (for example, refer to Patent Document 1).

[0006] Prior art literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2010-108889 Summary of the invention

[0008] Patent Document 1 does not discuss a method for improving 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 laminated secondary battery.

[0009] The present disclosure is completed in view of the above-mentioned previous situation, and its purpose is to provide a secondary battery having a structure in which a positive electrode and a negative electrode are stacked in multiple layers in the thickness direction and have excellent rate characteristics and cycle characteristics, and a composition for a positive electrode and a positive electrode mixture layer that can be used to produce a secondary battery having a structure in which a positive electrode and a negative electrode are stacked in multiple layers in the thickness direction and have excellent rate characteristics and cycle characteristics.

[0010] The specific means for achieving the above-mentioned objects are as follows.

[0011] <1> A secondary battery comprises a positive electrode, a negative electrode and an outer packaging material for accommodating the positive electrode and the negative electrode, wherein the positive electrode comprises a positive electrode collector and a positive electrode mixture layer disposed on the positive electrode collector and containing a positive electrode active material, and the negative electrode comprises a negative electrode collector and a negative electrode mixture layer disposed on the negative electrode collector and containing a negative electrode active material, and the secondary battery has a structure in which the positive electrode and the negative electrode are stacked in multiple layers in a thickness direction.

[0012] The positive electrode mixture layer contains fibrous carbon. When the secondary battery is discharged from a fully charged state to a discharge termination voltage at a current of 5C, the standard deviation of the temperature rise at the temperature measuring point on the outer packaging material is set to σ (°C), and the rated capacity of the battery is set to A (Ah), and σ / A is less than 0.35 (°C / Ah).

[0013] <2> according to <1> In the secondary battery, when the secondary battery is discharged from a fully charged state to a discharge termination voltage at a current of 5 C, the σ / A is less than or equal to 0.30 (° C. / Ah).

[0014] <3> according to <1> or <2> When the secondary battery is charged from a fully discharged state to a charge termination voltage at a current of 5 C, the σ / A is less than or equal to 0.25 (° C. / Ah).

[0015] <4> according to <1> ~ <3> The secondary battery according to any one of the preceding claims, wherein the average electrode area per positive electrode is 20 cm 2 ~10000cm 2 .

[0016] <5> according to <1> ~ <4> The secondary battery described above is a laminated type secondary battery.

[0017] <6> according to <5> In the secondary battery, the stacking numbers of the positive electrode and the negative electrode are respectively 4 or more.

[0018] <7> according to <1> ~ <4> The secondary battery described above is a wound type secondary battery.

[0019] <8> according to <7> The secondary battery has a winding number of 4 or more.

[0020] <9> according to <1> ~ <8> In the secondary battery described above, the positive electrode mixture layer further contains carbon black.

[0021] <10> according to <1> ~ <9> In the secondary battery described in any one of the above aspects, the thickness of the positive electrode mixture layer and the thickness of the negative electrode mixture layer are each independently 30 μm or more.

[0022] <11> according to <1> ~ <10> The secondary battery according to any one of the preceding claims, wherein 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 above.

[0023] <12> according to <1> ~ <11> The secondary battery according to any one of the preceding claims, wherein the positive electrode mixture layer has a weight per unit area of ​​10.0 mg / cm 2 The negative electrode mixture layer has a unit area weight of 5.0 mg / cm 2 above.

[0024] <13> according to <1> ~ <12> The secondary battery according to any one of the preceding claims, wherein the positive electrode active material contains a selected from LiNi x Mn y Co z Al w O2 (x, y, z and w ≥ 0, x + y + z + w = ​​1), LiMPO4 (M is one or more selected from Fe, Co, Mn and Ni) and LiMn a Ni b At least one of O4 (a and b are both greater than or equal to 0, a+b=2).

[0025] <14> according to <1> ~ <13> The secondary battery according to any one of the preceding claims, wherein the positive electrode active material contains LiNi x Mn y Co z Al w O2 (x, y, z and w≥0, x+y+z+w=1).

[0026] <15> according to <1> ~ <14> In any one of the secondary batteries, the positive electrode active material contains LiMPO4 (M is one or more selected from Fe, Co, Mn and Ni).

[0027] <16> according to <15> The density of the positive electrode mixture layer of the secondary battery is 2.0 g / cm 3 The density of the negative electrode mixture layer is 1.3 g / cm 3 above.

[0028] <17>The secondary battery according to any one of <1> to <16>, wherein 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.

[0029] <18>The secondary battery according to any one of <1> to <17>, wherein the negative electrode active material contains graphite.

[0030] <19>A positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material,

[0031] the positive electrode mixture layer contains fibrous carbon,

[0032] When a secondary battery is fabricated including the positive electrode, the negative electrode, and an exterior material housing the positive electrode and the negative electrode, and the secondary battery is discharged from a fully charged state to a discharge cut-off voltage at a current of 5C, if the standard deviation of the temperature rise at a temperature measurement point on the exterior material is set to σ (°C) and the rated capacity of the battery is set to A (Ah), σ / A is 0.35 °C / Ah or less, the secondary battery has a structure in which the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction.

[0033] <20>A composition for a positive electrode mixture layer, which is used to form a positive electrode mixture layer containing a positive electrode active material,

[0034] the composition for a positive electrode mixture layer contains fibrous carbon,

[0035] When a secondary battery is fabricated including a positive electrode, a negative electrode, and an exterior material housing the positive electrode and the negative electrode, and the secondary battery is discharged from a fully charged state to a discharge cut-off voltage at a current of 5C, if the standard deviation of the temperature rise at a temperature measurement point on the exterior material is set to σ (°C) and the rated capacity of the battery is set to A (Ah), σ / A is 0.35 °C / Ah or less, the secondary battery has a structure in which the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, the positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, and the positive electrode mixture layer is formed from the composition for a positive electrode mixture layer.

[0036] According to the present disclosure, it is possible to provide a secondary battery having a structure in which a positive electrode and a negative electrode are stacked in multiple layers 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 capable of fabricating a secondary battery having a structure in which a positive electrode and a negative electrode are stacked in multiple layers in the thickness direction and having excellent rate characteristics and cycle characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a cross-sectional view in the thickness direction of a laminated secondary battery.

[0038] Figure 2 It is a perspective view of a laminated secondary battery.

[0039] Figure 3 This is a perspective view of a positive electrode, a negative electrode, and a separator housed in an exterior material of a laminated secondary battery.

[0040] Figure 4 It is a graph showing the measurement results of the initial DCR of Example 1 and Comparative Example 1.

[0041] Figure 5 It means that the positive electrode unit area weight in Example 1 and Comparative Example 1 is 17.0 mg / cm 2 Graph of the DCR change rate when .

[0042] Figure 6 It means that the positive electrode unit area weight in Example 1 and Comparative Example 1 is 17.0 mg / cm 2 A coordinate diagram showing the relationship between the current value and the charging capacity ratio when .

[0043] Figure 7 It means that the positive electrode unit area weight in Example 1 and Comparative Example 1 is 24.5 mg / cm 2 A coordinate diagram showing the relationship between the current value and the charging capacity ratio when .

[0044] Figure 8 This is a graph showing the discharge capacity retention rate when a charge and discharge test at a high rate was performed in Example 2 and Comparative Example 2.

[0045] Fig. 9 This is a graph showing the ratio of the discharge capacity at 5C charge and discharge when the discharge capacity at 0.2C charge and discharge in Example 2 and Comparative Example 2 is set to 100%.

[0046] Fig.10 This is a graph showing changes in the voltage curve as cycles pass in Comparative Example 2.

[0047] Fig.11 This is a graph showing changes in the voltage curve as cycles pass in Example 2.

[0048] Fig.12 This is a graph showing voltage curves during charge and discharge after 400 cycles of high-rate charge and discharge tests were performed in Example 2 and Comparative Example 2. DETAILED DESCRIPTION

[0049] Hereinafter, the embodiments of the present disclosure are described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, the constituent elements (including element steps, etc.) are not necessary except for the cases specifically indicated. The same is true for numerical values ​​and their ranges, which do not limit the present disclosure.

[0050] In the present disclosure, the term "process" includes not only a process independent of other processes but also a process that cannot be clearly distinguished from other processes as long as the purpose of the process can be achieved.

[0051] In the present disclosure, a numerical range expressed using "to" includes the numerical values ​​described before and after "to" as a minimum value and a maximum value, respectively.

[0052] In the numerical range recorded in stages in the present disclosure, the upper limit or lower limit recorded in one numerical range may also be replaced by the upper limit or lower limit of the numerical range recorded in other stages. In addition, in the numerical range recorded in the present disclosure, the upper limit or lower limit of the numerical range may also be replaced by the value shown in the embodiment.

[0053] In the present disclosure, each component may contain a plurality of corresponding substances. When a plurality of substances corresponding to each component are present in the composition, the content or content of each component refers to the total content or content of the plurality of substances present in the composition unless otherwise specified.

[0054] In the present disclosure, multiple types of particles corresponding to each component may be contained. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value of the mixture of the multiple types of particles present in the composition unless otherwise specified.

[0055] In the present disclosure, the term "layer" or "film" includes not only the case where it is formed on the entire region when a region where the layer or film exists is observed, but also the case where it is formed on only a partial region.

[0056] In the present disclosure, the term “laminated” means overlapping of layers, and two or more layers may be combined together, or two or more layers may be separable.

[0057] In the present disclosure, when specific examples of secondary batteries are described with reference to the drawings, the present disclosure is not limited to the embodiments shown in the drawings. In addition, the sizes of components in each figure are conceptual, and the relative relationship between the sizes of the components is not limited to this.

[0058] [Secondary battery]

[0059] The secondary battery disclosed in the present invention comprises a positive electrode, a negative electrode and an outer packaging material, wherein the positive electrode comprises a positive electrode collector and a positive electrode mixture layer which is arranged on the positive electrode collector and contains a positive electrode active material, the negative electrode comprises a negative electrode collector and a negative electrode mixture layer which is arranged on the negative electrode collector and contains a negative electrode active material, the outer packaging material accommodates the positive electrode and the negative electrode, the secondary battery has a structure in which the positive electrode and the negative electrode are stacked in a plurality 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 termination voltage at a current of 5C, the standard deviation of the temperature rise at the temperature measuring point on the outer packaging material is set to σ (°C), and when the rated capacity of the battery is set to A (Ah), σ / A is less than 0.35 (°C / Ah).

[0060] In the secondary battery disclosed in the present invention, by having a structure in which a plurality of layers of positive and negative electrodes are stacked in the thickness direction, the temperature rise at the temperature measuring point on the outer packaging material can be measured, and the fluctuation of the temperature rise at multiple temperature measuring points, that is, the standard deviation of the temperature rise, can be evaluated. Furthermore, it is speculated that by adopting 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, the current is easy to flow evenly, the increase in the DC resistance after the cycle is reduced, and the cycle characteristics of the secondary battery are improved. Furthermore, by making σ / A at high-rate discharge less than 0.35 (°C / Ah), the local temperature rise is suppressed, which is related to suppressing gas generation, decomposition of the electrolyte, decomposition of the positive electrode active material, etc. As a result, it is speculated that the capacity retention rate of high-rate charge and discharge relative to low-rate charge and discharge is improved (excellent rate characteristics).

[0061] In the present disclosure, in the case of a secondary battery, a “fully charged state” is a state after the battery is charged at 0.2C with a final voltage of 4.2V or more.

[0062] In the present disclosure, in the case of a secondary battery, a “fully discharged state” is a state after the battery is discharged at 0.2C with a cut-off voltage of 2.8 V or less.

[0063] As a general example, the charge end voltage is set to 4.2V or higher, and the discharge end voltage is set to 2.8V or lower.

[0064] As a secondary battery, as long as it has a structure in which a plurality of positive electrodes and negative electrodes are stacked in the thickness direction and housed in an outer packaging material, it can be a laminated secondary battery or a wound secondary battery. As a wound secondary battery, for example, it can be a cylindrical secondary battery in which an electrode pair and an electrolyte are sealed in a cylindrical outer packaging body by winding a stacked body in which a positive electrode and a negative electrode are stacked with a separator, or it can be a cylindrical secondary battery in which a unit is sealed in a cylindrical outer packaging body by winding a stacked body in which a positive electrode and a negative electrode are stacked with a solid electrolyte.

[0065] The secondary battery may be a battery in which a positive electrode and a negative electrode are stacked with a separator and an electrolyte is contained in an outer casing, or a battery in which a positive electrode and a negative electrode are stacked with a solid electrolyte and a stack is contained in an outer casing.

[0066] The number of stacking of the positive electrode and the negative electrode can be 2 or more, 3 or more, or 4 or more. In the present disclosure, by stacking the positive electrode and the negative electrode, the temperature rise at the temperature measuring point on the outer packaging material can be measured, and by making the standard deviation of the temperature rise within an appropriate range, a secondary battery with excellent rate characteristics and cycle characteristics can be obtained.

[0067] The upper limit of the number of stacked positive electrodes and negative electrodes is not particularly limited, and may be, for example, 100 or less, or 50 or less.

[0068] In the case of a wound type secondary battery, the above-mentioned number of stacked layers refers to the number of windings.

[0069] Figure 1 to Figure 3 An example of a laminated secondary battery is shown. Figure 1 is a cross-sectional view of a laminated secondary battery in the thickness direction, Figure 2 is a perspective view of a laminated secondary battery. Figure 3 This is a perspective view of a positive electrode, a negative electrode, and a separator housed in an exterior material of a laminated secondary battery.

[0070] exist Figure 1 In the secondary battery 10, a stacked body in which a positive electrode 1 and a negative electrode 3 are stacked with a separator 5 interposed therebetween is housed in an outer packaging material 6. In addition, a positive electrode tab 2A and a negative electrode tab 2B are extended to the outside of the outer packaging material 6. Figure 1 , a secondary battery in which the number of stacked negative electrodes is 6 and the number of stacked positive electrodes is 5 is illustrated, but in the present disclosure, the number of stacked negative electrodes and positive electrodes is not limited thereto.

[0071] exist Figure 3, a structure in which positive electrodes 1 and negative electrodes 3 are alternately arranged in the thickness direction is shown. Furthermore, a separator 5 is arranged between the positive electrode 1 and the negative electrode 3. When stacking the components, a plurality of positive electrodes 1 and negative electrodes 3 are arranged in such a manner that a plurality of positive electrode tabs 2A overlap each other and a plurality of negative electrode tabs 2B overlap each other. The positive electrode 1, the negative electrode 3, and the separator 5 arranged in such a positional relationship are housed in an outer packaging material 6, and an electrolyte is housed in the outer packaging material 6, and then the opening is sealed by heat welding or the like, thereby obtaining Figure 2 A laminated secondary battery 10 is shown.

[0072] The standard deviation of the temperature rise at the temperature measurement points on the outer packaging material can be calculated, for example, as follows. First, the secondary battery is charged at a constant current of 0.2C with a charge end voltage of 4.2V. Next, the discharge end voltage is set to 2.5V and a constant current discharge is performed at a current of 5C. During this constant current discharge, the temperature of seven points on the outer packaging material is measured. For example, when using Figure 2 When the temperature of the laminated secondary battery 10 shown is measured, the temperature of the seven areas indicated by the dotted lines can be measured. At this time, from the perspective of understanding the temperature fluctuation of each measurement point, it is desirable to set the distance between adjacent measurement points to be 10 mm or more. The temperature measurement during charging is performed during constant current charging when the discharge end voltage is 2.5V and the current is 0.2C, and then the charge end voltage is set to 4.2V and the current is 5C.

[0073] For example, during constant current charging and constant current discharging, the temperature of 7 areas is measured, and the temperature rise range during charging (maximum temperature - minimum temperature) and the temperature rise range during discharging (maximum temperature - minimum temperature) of each measurement point are calculated. Then, the standard deviation σ of the temperature rise during high-rate discharge can be calculated based on the measurement results of the temperature rise range during discharge. Similarly, the standard deviation σ of the temperature rise during high-rate charging can be calculated based on the measurement results of the temperature rise range during charging.

[0074] When the secondary battery is discharged at a current of 5C, if the standard deviation of the temperature rise at the temperature measuring point on the outer packaging material is set to σ (°C) and the rated capacity of the battery is set to A (Ah), then σ / A is less than 0.35 (°C / Ah). From the viewpoint of rate characteristics and cycle characteristics, it is preferably less than 0.33 (°C / Ah), more preferably less than 0.32 (°C / Ah), and further preferably less than 0.30 (°C / Ah).

[0075] 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.

[0076] When the secondary battery is charged at a current of 5 C, from the viewpoint of rate characteristics and cycle characteristics, σ / A at the temperature measurement point on the outer packaging material is preferably less than 0.28 (°C / Ah), more preferably less than 0.25 (°C / Ah), and further preferably less than 0.20 (°C / Ah).

[0077] 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.

[0078] The type of secondary battery is not particularly limited, and examples thereof include lithium secondary batteries, sodium secondary batteries, potassium secondary batteries, magnesium secondary batteries, aluminum secondary batteries, etc. Among them, lithium secondary batteries that can achieve high energy density at high voltage and sodium secondary batteries that can achieve low cost are preferred.

[0079] Hereinafter, a lithium-based secondary battery will be described as an example, but the present disclosure is not limited thereto.

[0080] [positive electrode]

[0081] The secondary battery includes a positive electrode including a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material.

[0082] The material of the positive electrode current collector is not particularly limited as long as it is a material that does not oxidize and dissolve at high potential and has electronic conductivity, 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.

[0083] As an example, aluminum foil is used as the positive electrode current collector.

[0084] As described later, a positive electrode mixture layer composition containing a positive electrode active material and fibrous carbon and, if necessary, carbon black, a binder, a solvent, etc. is applied to the positive electrode collector, the applied slurry is dried, and then pressed to form a positive electrode mixture layer on the positive electrode collector.

[0085] From the viewpoint of energy density and safety, 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.

[0086] From the perspective of energy density and safety, the density of the positive electrode mixture layer can be 2.0 g / cm 3 Above, it can also be 3.0g / cm 3 Above, can also be 3.0g / cm 3 ~4.0g / cm 3 .

[0087] From the perspective of energy density and safety, the weight per unit area of ​​the positive electrode mixture layer can be 10.0 mg / cm 2 Above, it can also be 10.0mg / cm 2 ~30.0mg / cm 2 .

[0088] The average electrode area (average positive electrode area and average negative electrode area) of each piece can be 20cm 2 ~10000cm 2 , can also be 300cm 2 ~10000cm 2 .

[0089] (Positive electrode active material)

[0090] The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material can be appropriately selected according to the type of secondary battery, and for example, a compound containing at least one of lithium, sodium, potassium, magnesium and aluminum can be cited. Examples of the positive electrode active material include oxides containing nickel, phosphates having an olivine structure, and the like. In the case where the secondary battery is a lithium-based secondary battery, examples of the positive electrode active material include 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), LiMn a Ni b O4 (a, b≥0, a+b=2), etc.

[0091] The positive electrode active material preferably contains LiNi x Mn y Co z Al w O2 (x, y, z, w≥0, x+y+z+w=1), or contains LiMPO4 (M is one or more selected from Fe, Co, Mn and Ni).

[0092] As LiNi x Mn y Co z Al w O2 (x, y, z, w ≥ 0, x + y + z + w = ​​1), preferably with a higher nickel ratio, for example, x ≥ 0.5 or more, more preferably Li (Ni x Mn y Co z )O2(x≥0.5, y≤0.3, z≤0.3, x+y+z=1). x Mn yCo z )O2 (x ≥ 0.5, y ≤ 0.3, z ≤ 0.3, x + y + z = 1) represented by a positive electrode active material, 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.

[0093] Examples of the positive electrode active material represented by LiMPO4 (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.

[0094] At least part of the surface of the positive electrode active material may also be covered with amorphous carbon. As a method of covering with amorphous carbon, the following known methods can be cited: heating the positive electrode active material and performing CVD by flowing hydrocarbon gas therein; mixing a carbon precursor and the positive electrode active material, and heat treating the carbon precursor while it is attached to at least part of the surface of the positive electrode active material, thereby carbonizing the carbon precursor.

[0095] From the viewpoint of obtaining a sufficient capacity ratio, the 50% particle size (D n50 ) is preferably 0.01 μm or more, more preferably 0.02 μm or more, and further preferably 0.03 μm or more.

[0096] From the viewpoint of obtaining sufficient rate characteristics, the 50% particle size (D n50 ) is preferably 2.0 μm or less, more preferably 1.7 μm or less, and further preferably 1.5 μm or less.

[0097] The 50% particle size (D n50 ) can be obtained by selecting any 100 primary particles of the positive electrode active material from the SEM photograph and the cross-sectional SEM photograph of the electrode and taking the arithmetic average of the measurement results of the maximum length of the particles obtained by image recognition software.

[0098] From the viewpoint of excellent powder handling properties and easy increase in electrode density, the 50% particle size (D n50 ) is preferably 0.5 μm or more, D n50 The thickness is more preferably 1.0 μm or more, and even more preferably 2.0 μm or more.

[0099] From the viewpoint of obtaining sufficient rate characteristics and preventing abnormal irregularities from occurring during electrode coating, the 50% particle size (D n50 ) is preferably 30.0 μm or less, more preferably 25.0 μm or less, and further preferably 20.0 μm or less.

[0100] The 50% particle size (D n50 ) 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 taking the arithmetic average of the measurement results of the maximum length of the particles obtained by image recognition software.

[0101] In the present disclosure, the primary particle of the positive electrode active material refers to a particle that can be identified as one particle in a SEM photograph at 3000 to 30000 times magnification.

[0102] In this specification, the secondary particles of the positive electrode active material are aggregates of primary particles. The secondary particles of the positive electrode active material can be confirmed from a SEM photograph.

[0103] In this specification, when the positive electrode active material is simply referred to as a "particle", it means a primary particle or a secondary particle.

[0104] In the positive electrode mixture 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 positive electrode capacity.

[0105] In the positive electrode mixture layer, from the viewpoint of ensuring the amounts of other components, 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.

[0106] (Fibrous Carbon)

[0107] The positive electrode mixture layer contains fibrous carbon. As fibrous carbon, carbon fiber, vapor-grown carbon fiber, carbon nanotubes such as single-walled carbon nanotubes (SWCNT), and multi-walled carbon nanotubes (MWCNT), carbon nanofibers, etc. can be cited. From the viewpoint of excellent dispersibility, low resistance of the electrode, and suitable suppression of temperature rise fluctuations during high-rate discharge, vapor-grown carbon fiber is preferred.

[0108] Furthermore, the vapor-grown carbon fiber may be combined with other fibrous carbon, for example, carbon nanotubes such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).

[0109] The positive electrode mixture layer may contain one type of fibrous carbon, or may contain two or more types of fibrous carbon.

[0110] 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.

[0111] The average fiber diameter of the fibrous carbon can be obtained by taking the arithmetic mean of the diameters of 300 arbitrary fibers observed by SEM using an electrode. The diameter of one fiber can be obtained by measuring the widths of arbitrary 10 locations of a certain fiber from an SEM photograph and taking the arithmetic mean of the widths.

[0112] Here, the width of a fiber refers to the dimension of the fiber in a direction perpendicular to the longitudinal direction.

[0113] 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.

[0114] The average fiber length of fibrous carbon can be measured as follows: First, the electrode is washed with a solvent, the powder after the binder etc. is removed is dispersed in a dispersion medium, spread on an aluminum foil etc. and dried, and then SEM observation is performed to measure the length of 300 random fibers along the fiber axis, and the average fiber length can be determined by taking the arithmetic mean thereof.

[0115] 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.

[0116] Specific examples of the fibrous carbon include VGCF (registered trademark)-X (manufactured by Showa Denko K.K.) and VGCF (registered trademark)-H (manufactured by Showa Denko K.K.).

[0117] In the positive electrode mixture layer, the content of the 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.

[0118] In the positive electrode mixture layer, the content of the fibrous carbon is preferably 3.0 mass % or less, more preferably 2.0 mass % or less, and even more preferably 1.0 mass % or less, from the viewpoint of ensuring the positive electrode capacity.

[0119] In the positive electrode mixture layer, when the fibrous carbon contains gas-phase carbon fibers and carbon nanotubes such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), the ratio of the gas-phase carbon fibers to the carbon nanotubes is not particularly limited. From the perspective of cycle characteristics during high-rate charge and discharge, for example, the ratio of the gas-phase carbon fibers to the carbon nanotubes may be 20:1 to 1:1, 10:1 to 1:1, or 5:1 to 2:1.

[0120] (Carbon Black)

[0121] The positive electrode mixture layer may also 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 Co., Ltd.), C-NERGY (registered trademark) Super C45, C65 (manufactured by Imeligraw Instruments & Co.), and Ketjen Black (manufactured by Ketchen Black).

[0122] The primary particle size of carbon black may also be 10 nm to 100 nm. The primary particle referred to herein refers to the portion of a bead in the bead-like structure called an aggregate, which is equivalent to a bead of a rosary. If the primary particle size of carbon black is 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 size of carbon black is preferably 20 nm to 80 nm, and more preferably 30 nm to 70 nm.

[0123] The primary particle size of carbon black can be determined by selecting any 100 primary particles of carbon black from the SEM photograph and the cross-sectional SEM photograph of the electrode and taking the arithmetic average of the maximum length measurement results of the particles obtained by image recognition software.

[0124] When the positive electrode mixture layer contains carbon black, the content of carbon black is preferably 0.2 mass % or more, more preferably 0.6 mass % or more, and further preferably 1.0 mass % or more, from the viewpoint of obtaining excellent cycle characteristics and rate characteristics.

[0125] In the positive electrode mixture layer, the content of carbon black 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 positive electrode capacity.

[0126] When the positive electrode mixture layer contains carbon black, from the viewpoint of further reducing the resistance of the positive electrode mixture layer by compounding the two, 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 further preferably 1:1 to 3:1.

[0127] (Adhesive)

[0128] The positive electrode mixture layer may also contain a binder. As the binder, a binder used in a positive electrode mixture layer of a conventional lithium ion secondary battery may be suitably used. As the binder, for example, polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE) may be cited.

[0129] 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 a function as a binder.

[0130] In the positive electrode mixture layer, the content of the binder 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.

[0131] (Other ingredients)

[0132] The positive electrode mixture layer may contain other components besides the above, 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.

[0133] [negative electrode]

[0134] The secondary battery includes a negative electrode including a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material.

[0135] The material of the negative electrode current collector is not particularly limited as long as it has 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.

[0136] As an example, a copper foil is used as the negative electrode current collector.

[0137] As described later, a composition for a negative electrode mixture layer containing a negative electrode active material and, if necessary, a conductive assistant, a binder, a solvent, etc. is applied to the negative electrode current collector, and the applied slurry is dried and then pressed, whereby a negative electrode mixture layer is formed on the negative electrode current collector.

[0138] From the viewpoints of energy density and safety, the thickness of the negative electrode mixture layer can be 30 μm or more, can be 50 μm to 100 μm, or can be 100 μm to 150 μm.

[0139] From the viewpoints of energy density and safety, the density of the negative electrode mixture layer can be 1.3 g / cm 3 or more, or can be 1.5 g / cm 3 to 2.0 g / cm 3 .

[0140] From the viewpoints of energy density and safety, the weight per unit area of the negative electrode mixture layer can be 5.0 mg / cm 2 or more, or can be 10.0 mg / cm 2 to 20.0 mg / cm 2 .

[0141] (Negative electrode active material)

[0142] 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, the negative electrode active material preferably contains graphite. In addition, at least part of the surface of the negative electrode active material may be covered with amorphous carbon.

[0143] (Conductive assistant)

[0144] The negative electrode mixture layer may also contain a conductive assistant. Examples of the conductive assistant include the aforementioned carbon materials such as fibrous carbon, carbon black, and graphene.

[0145] (Binder)

[0146] The negative electrode mixture layer may also contain a binder. As examples of the binder, in addition to PVdF, PTFE, etc. that can be used in the same manner as the positive electrode mixture layer, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), etc. can also be used.

[0147] (Outer packaging material)

[0148] As the outer packaging material for housing the positive electrode and the negative electrode, there is no limitation as long as it can house the positive electrode and the negative electrode and house the separator and the electrolyte, or the solid electrolyte as required. As the outer packaging material, for example, a commonly commercially available battery pack, a 18650-type cylindrical unit, a material in the form of a packaging material, etc. can be cited, and the outer packaging material can be freely designed and used.

[0149] (Diaphragm)

[0150] The secondary battery may also have a separator between the positive electrode and the negative electrode. As the separator, it can be freely selected from separators that can be used in general secondary batteries, for example, microporous membranes made of polyethylene or polypropylene can be cited. A separator mixed with particles such as SiO2 and Al2O3 as fillers, a separator with these particles attached to the surface, etc. can also be used.

[0151] (Electrolyte)

[0152] In the secondary battery, the electrolyte may also be contained in the outer packaging material. The electrolyte is not particularly limited, and an electrolyte that can be used in a conventional secondary battery can be preferably used. For example, an organic solvent in which 0.5 mol / L to 2.0 mol / L of a lithium salt is dissolved can be used.

[0153] Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, and LiFSI.

[0154] As organic solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DEC), diethyl carbonate (DEC), propylene carbonate (PC) etc. can be enumerated. Organic solvent can also appropriately select the material enumerated here and the material in addition to mix them for use. In addition, as the additive of electrolyte, vinylene carbonate (VC), propane sultone (PS), fluoroethylene carbonate (FEC) can be enumerated. When using additive, relative to 100 mass % of organic solvent, the content of additive is preferably 0.01 mass %~20 mass %, more preferably 0.1 mass %~10 mass %, further preferably 0.5 mass %~5 mass %.

[0155] (Ionic Liquids)

[0156] An ionic liquid may be used as an electrolyte, or the ionic liquid may be used in combination with the above-mentioned organic solvent. The ionic liquid is not particularly limited, and examples thereof include imidazolium cations, pyrrolidine cations, Cation, piperidine A combination of a cation such as a cation, an ammonium cation, and an anion such as a bis(trifluoromethane)sulfonamide anion.

[0157] (Solid Electrolyte)

[0158] A solid electrolyte may also be used as the electrolyte. When a solid electrolyte is used, a separator is not required, and a battery in which a positive electrode and a negative electrode are sandwiched by a solid electrolyte (for example, an all-solid lithium ion secondary battery) can be formed.

[0159] Examples of the solid electrolyte include polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte is not particularly limited, and examples thereof include polymer electrolytes in which the above-mentioned lithium salt is impregnated in a polymer such as polyethylene oxide. The inorganic solid electrolyte is not particularly limited, and examples thereof include Li 13 Ti 1.7 Al 0.3 (PO4)3, Li2S-P2S5, etc.

[0160] The positive electrode of the secondary battery disclosed in the present invention is obtained by coating a positive electrode mixture layer composition containing a positive electrode active material and fibrous carbon and further containing carbon black, a binder, a solvent, etc. as needed on a positive electrode collector, drying the coated slurry, and then pressing to form a positive electrode mixture layer on the positive electrode collector.

[0161] [Composition for positive electrode mixture layer]

[0162] The positive electrode mixture layer composition disclosed in the present invention is a positive electrode mixture layer composition for forming a positive electrode mixture layer containing a positive electrode active material, and the positive electrode mixture layer composition contains fibrous carbon. In addition, a secondary battery having a positive electrode, a negative electrode, and an outer packaging material for accommodating the positive electrode and the negative electrode is formed, the positive electrode having a positive electrode mixture layer formed using the positive electrode mixture layer composition disclosed in the present invention, the secondary battery having a structure in which the positive electrode and the negative electrode are stacked in a plurality in the thickness direction, when forming such a secondary battery, when the secondary battery is discharged from a fully charged state to a discharge termination voltage at a current of 5C, the standard deviation of the temperature rise at the temperature measuring point on the outer packaging material is set to σ (°C), and when the rated capacity of the battery is set to A (Ah), σ / A is less than 0.35 (°C / Ah).

[0163] Thus, by using the positive electrode mixture layer composition disclosed herein, a secondary battery having excellent rate characteristics and cycle characteristics can be produced.

[0164] Preferred configurations of the positive electrode active material and fibrous carbon contained in the positive electrode mixture layer composition of the present disclosure, and carbon black and a binder contained as necessary, are the same as those described in the above-mentioned section of the secondary battery of the present disclosure.

[0165] The positive electrode material mixture layer composition of the present disclosure may contain a solvent. The solvent is not particularly limited, and an example thereof includes N-methyl-2-pyrrolidone (NMP).

[0166] The positive electrode material mixture layer composition disclosed in the present invention can be prepared, for example, as follows.

[0167] Step (1): A positive electrode active material and fibrous carbon are weighed, and further, if necessary, carbon black, a binder, etc. are weighed, and these are mixed while adding a solvent.

[0168] Step (2): A solvent is added to the mixture obtained in the above step (1) and kneaded to obtain a positive electrode material mixture layer composition having an appropriate viscosity.

[0169] Furthermore, a scraper or a coating machine may be used to apply the positive electrode mixture layer composition to the positive electrode current collector, and the positive electrode current collector coated with the positive electrode mixture layer composition is placed on a heating device such as a hot plate to dry the solvent. Then, vacuum drying is performed to remove the residual solvent. The product obtained by removing the solvent is pressed and formed using a roller press, a uniaxial press, etc., to obtain a positive electrode.

[0170] [positive electrode]

[0171] The positive electrode disclosed in the present invention comprises a positive electrode collector and a positive electrode mixture layer disposed on the positive electrode collector and containing a positive electrode active material, wherein the positive electrode mixture layer contains fibrous carbon. Moreover, a secondary battery comprising the positive electrode disclosed in the present invention, a negative electrode, and an outer packaging material for accommodating the positive electrode and the negative electrode is formed, wherein the secondary battery has a structure in which the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, and when such a secondary battery is formed, when the secondary battery is discharged from a fully charged state to a discharge termination voltage at a current of 5C, the standard deviation of the temperature rise at the temperature measuring point on the outer packaging material is set to σ (°C), and when the rated capacity of the battery is set to A (Ah), σ / A is less than 0.35 (°C / Ah). Thus, by using the positive electrode disclosed in the present invention, a secondary battery with excellent rate characteristics and cycle characteristics can be manufactured.

[0172] The positive electrode of the present disclosure can be prepared using, for example, the composition for a positive electrode material mixture layer of the present disclosure.

[0173] Each electrode can be packaged after being stacked. In addition, the single cells can be connected in series and used as a storage battery, module, etc.

[0174] The secondary battery disclosed herein can be used as a power source for electronic devices such as smart phones, tablet PCs, portable information terminals, etc.; as a power source for motors of power tools, vacuum cleaners, electric bicycles, drones, electric vehicles, etc.; and for storing electricity obtained through fuel cells, solar power generation, wind power generation, etc.

[0175] Example

[0176] Hereinafter, the present disclosure will be specifically described by way of examples, but the scope of the present disclosure is not limited to these examples.

[0177] [Example 1]

[0178] (Production of positive electrode)

[0179] The NMP solution containing 96.5 parts by mass of NMC811 (Li (Ni 0.8 Mn 0.1 Co 0.1 )O2), 1.0 mass part of carbon black (primary particles D n50 : 100 nm), 0.5 parts 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. Then, NMP was appropriately added and mixed in a kneader to prepare a positive electrode slurry having an adjusted viscosity.

[0180] Furthermore, the 50% particle size (D n50 ) is 0.5 μm, and the 50% particle size (D n50 ) is 10.1μm.

[0181] The positive electrode slurry was coated on an aluminum foil with a thickness of 20 μm using a roll coater and dried, and then vacuum dried to obtain a positive electrode sheet. The thickness of the positive electrode mixture layer was 53 μm and the density of the positive electrode mixture layer was 3.2 g / cm 3 .

[0182] (Production of negative electrode)

[0183] 95.7 parts by mass of graphite particles as a negative electrode active material and 1.3 parts by mass of carbon black (primary particles D n50 : 50nm), a 2 mass % CMC aqueous solution containing 1.5 mass parts of carboxymethyl cellulose (CMC) as a binder and 1.5 mass parts of POLYSOL (registered trademark, manufactured by Showa Denko K.K.), which were mixed in a mixer to obtain a negative electrode slurry. As graphite particles, the 50% particle size (D v50 ) are graphite particles of 14 μm.

[0184] The negative electrode slurry was applied to a copper foil with a thickness of 20 μm using a roll coater and dried, and then vacuum dried to obtain a negative electrode sheet. The thickness of the negative electrode mixture layer was 88 μm and the density of the negative electrode mixture layer was 1.4 g / cm 3 .

[0185] (Fabrication of laminated full cell)

[0186] In a glove box maintained in a dry argon atmosphere having a dew point of -80°C or less, a laminated full cell for evaluation of Example 1 was produced as follows.

[0187] The positive electrode sheet and the negative electrode sheet obtained in the above-mentioned preparation of the positive electrode and the preparation of the negative electrode were punched out to obtain a sheet with an area of ​​20 cm 2 The positive electrode and negative electrode are provided. The Al tabs are installed on the Al foil of the positive electrode, and the Ni tabs are installed on the Cu foil of the negative electrode. A polypropylene microporous membrane as a separator is sandwiched between the negative electrode and the positive electrode to obtain a stacked unit having multiple negative electrodes and multiple positive electrodes and a separator arranged between the negative electrode and the positive electrode. The stacked unit is wrapped with an aluminum laminate packaging material except for the tabs, and 2890 μL of electrolyte is injected into it. Then, the opening is sealed by heat welding to prepare a laminated full unit for evaluation as a secondary battery. Furthermore, the electrolyte is a liquid in which vinylene carbonate (VC) is mixed in a solvent in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate are mixed in a volume ratio of 3:5:2 to make it reach 1% by mass, and the electrolyte LiPF6 is further dissolved in the mixed solution to make its concentration reach 1.2 mol / L. In addition, the design capacity of the laminated full unit for evaluation is 1000mAh.

[0188] As shown in Table 1, laminated cells having different numbers of negative and positive electrodes were used to prepare evaluation laminated full cells having different capacity ratios (negative electrode / positive electrode) and weights per unit area of ​​positive electrode mixture layers.

[0189] [Comparative Example 1]

[0190] (Production of positive electrode)

[0191] 96.5 parts by mass of NMC811 (Li (Ni 0.8 Mn 0.1 Co 0.1 )O2), 1.5 parts by mass of carbon black and 2.0 parts by mass of PVdF in NMP solution were mixed with a mixer. Then, NMP was appropriately added while mixing with a mixer to prepare a positive electrode slurry with adjusted viscosity. The positive electrode slurry prepared in Comparative Example 1 was used to prepare a positive electrode sheet in the same manner as in Example 1.

[0192] (Fabrication of laminated full unit)

[0193] Using the positive electrode sheet prepared in Comparative Example 1 and the negative electrode sheet prepared in Example 1, a laminated full cell for evaluation was prepared in the same manner as in Example 1.

[0194] In addition, similarly to Example 1, as shown in Table 1, laminated cells with different numbers of negative and positive electrodes were used to prepare evaluation laminated full cells with different capacity ratios (negative electrode / positive electrode) and weights per unit area of ​​positive electrode mixture layers.

[0195] The produced laminated full unit was subjected to aging treatment.

[0196] (Charge and discharge evaluation of laminated full cell)

[0197] For the laminated full cell after aging (weight per unit area of ​​the positive electrode mixture layer is 17.0 mg / cm 2 , capacity ratio (negative electrode / positive electrode) is 1.1, 8 layers of negative electrode and 7 layers of positive electrode), the end voltage is set to 4.2V, and constant current charging is performed at a current of 0.2C. Next, the end voltage is set to 2.5V, and constant current discharge is performed at a current of 5C. During this constant current discharge, the temperature of 7 points on the outer packaging material is measured. Next, the end voltage is set to 2.5V, and constant current discharge is performed at a current of 0.2C. Next, the end voltage is set to 4.2V, and constant current charging is performed at a current of 5C. During this constant current charging, the temperature of 7 points on the outer packaging material is measured. When looking down at the entire laminated unit, the temperature measurement locations are the (1) central part of the outer packaging material, (2) the upper left central part (lower part of the left pole ear), (3) the upper right central part (lower part of the right pole ear), (4) the left central part, (5) the right central part, (6) the lower left central part, and (7) the lower right central part.

[0198] The temperature of 7 points on the outer material during constant current charging and constant current discharging was measured, and the temperature rise range during charging (maximum temperature - minimum temperature) and the temperature rise range during discharging (maximum temperature - minimum temperature) of each measuring point were calculated, and the standard deviation σ of the temperature rise of the temperature measuring point was calculated. Furthermore, the rated capacity of the battery was set to A (Ah), and σ / A (℃ / Ah) was calculated. The results are shown in Tables 2 and 3.

[0199] (Determination of initial DCR)

[0200] For the laminated full cell after aging (weight per unit area of ​​the positive electrode mixture layer was 11.0 mg / cm 2 、17.0mg / cm 2 or 24.5 mg / cm 2, capacity ratio (negative electrode / positive electrode) is 1.1, negative electrode has 8 layers and positive electrode has 7 layers), after one constant current charge and discharge at 1C in a constant temperature chamber at 25°C, it becomes fully charged, and then discharged at 0.1C to half the discharge capacity of the first cycle. After pausing for 30 minutes, discharge at 1000mA for 5 seconds. According to Ohm's law (R=ΔV / 1), the internal resistance of the battery (DCR at soc=50%) is calculated according to the voltage drop at this time. The results are shown in Tables 3 and Figure 4 .

[0201] (Evaluation of DCR changes)

[0202] For the laminated full cell after the above initial DCR measurement, the DCR at the 100th, 200th, 300th, 400th, 500th and 600th cycles were measured as follows, and the DCR at the reference ( Figure 5 In the above example, the initial DCR value of Comparative Example 1 was taken as 100%, and the DCR change (%) in each cycle was calculated.

[0203] Specifically, after repeated constant current charge and discharge at 1C for a predetermined number of times in a thermostatic chamber at 25°C, the battery is fully charged and then discharged at 0.1C to half the discharge capacity of the first cycle. After pausing for 30 minutes, the battery is discharged at 1000mA for 5 seconds. According to Ohm's law (R = ΔV / 1), the internal resistance of the battery (DCR at soc = 50%) is taken as the DCR in each cycle based on the voltage drop at this time.

[0204] The cycle is performed as follows: for the above laminated full cell, the end voltage is set to 4.2V in a constant temperature chamber at 25°C, and constant current charging is performed at a current of 1C. Then, the end voltage is set to 2.8V, and constant current discharge is performed at a current of 1C. The charging operation for reaching the full charge state and the discharging operation for reaching the soc of 50% before the DCR measurement in each of the above cycles are not counted in the number of cycles. After the DCR measurement of each cycle, the charging and discharging operation is restarted from the charging of 1C, and the number of cycles after the restart of charging and discharging is added to the number of cycles so far for counting.

[0205] The results are shown in Table 3 and Figure 5 .

[0206] The DCR changes (%) shown in Table 3 refer to the increase rate relative to the initial DCR value of Example 1 for Example 1, and refer to the increase rate relative to the initial DCR value of Comparative Example 1 for Comparative Example 1.

[0207] (Evaluation of rate characteristics)

[0208] The unit area weight of the positive electrode mixture layer is 17.0 mg / cm 2or 24.5 mg / cm 2 The aged laminated full unit was evaluated for the change in charge capacity ratio when the current value was changed in the range of 0.1C to 1.0C in a constant temperature chamber at 25°C. Specifically, after constant current charge and discharge at 0.1C, constant current charging at various rates was performed to determine the charge capacity. Then, the charge capacity ratio (%) at 0.2C, 0.5C or 1.0C was determined when the charge capacity at 0.1C was set to 100%. The results are shown in Table 3 and Figure 6 and Figure 7 The higher the value of the charge capacity ratio, the better the rate characteristics. Table 3 shows the ratio (%) of the charge capacity at 1.0 C to the charge capacity at 0.1 C.

[0209] Table 1

[0210]

[0211] Table 2

[0212]

[0213] Table 3

[0214]

[0215] 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.

[0216] Furthermore, it was confirmed that by reducing the standard deviation of the temperature rise during high rate discharge, Figure 4 As shown, the weight per unit area of ​​the positive electrode increases, that is, there is a tendency that the reduction ratio of DCR increases as the energy density increases.

[0217] In addition, it was confirmed that by reducing the standard deviation of the temperature rise during high-rate discharge, Figure 5 This shows that the DCR increase tends to be suppressed when the number of cycles increases. From this result, it is estimated that other cycle characteristics such as capacity after multiple cycles are also excellent.

[0218] As shown in Table 3 and Figure 6 and Figure 7 As shown, 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 there was a tendency for the rate characteristics to be excellent.

[0219] [Example 2]

[0220] (Production of positive electrode)

[0221] The NMP solution containing 96.5 parts by mass of NMC811 (Li (Ni 0.8 Mn 0.1 Co 0.1 )O2), 1.1 parts by mass of carbon black (primary particles D n50 : 100nm), 0.3 parts by mass of fibrous carbon having an average fiber diameter of 150nm and an average fiber length of 6μm, 0.1 parts by mass of multi-walled carbon nanotubes (MWCNTs), and 2.0 parts by mass of PVdF. Then, NMP was appropriately added while mixing in a mixer to prepare a positive electrode slurry with adjusted viscosity.

[0222] Furthermore, the 50% particle size (D n50 ) is 0.5 μm, and the 50% particle size (D n50 ) is 10.1μm.

[0223] The positive electrode slurry was coated on an aluminum foil with a thickness of 20 μm using a roll coater and dried, and then vacuum dried to obtain a positive electrode sheet. The thickness of the positive electrode mixture layer was 53 μm by roll pressing, and the unit area weight of the positive electrode mixture layer was 11.3 mg / cm 2 The density of the positive electrode mixture layer is 3.2 g / cm 3 .

[0224] (Production of negative electrode)

[0225] According to the same procedure as in Example 1, a negative electrode slurry was obtained.

[0226] The negative electrode slurry was applied to a copper foil with a thickness of 20 μm using a roll coater and dried, and then vacuum dried to obtain a negative electrode sheet. The negative electrode mixture layer was rolled to a thickness of 88 μm and a unit area weight of 8 mg / cm 2 The density of the negative electrode mixture layer is 1.4 g / cm 3 .

[0227] (Fabrication of laminated full unit)

[0228] In a glove box maintained in a dry argon atmosphere having a dew point of -80°C or less, a laminated full cell for evaluation of Example 2 was produced as follows.

[0229] The positive electrode sheet and the negative electrode sheet obtained in the above-mentioned preparation of the positive electrode and the preparation of the negative electrode were punched out to obtain a sheet with an area of ​​20 cm 2The positive and negative electrodes are respectively provided with Al tabs on the Al foil of the positive electrode, and Ni tabs on the Cu foil of the negative electrode. A polypropylene microporous membrane as a separator is sandwiched between the negative electrode and the positive electrode to obtain a stacked unit having 5 negative electrodes and 4 positive electrodes and a separator arranged between the negative electrode and the positive electrode. The stacked unit is wrapped with an aluminum laminate packaging material except for the tabs, and 1800 μL of electrolyte is injected into it. Then, the opening is sealed by thermal welding to prepare a laminated full unit for evaluation as a secondary battery. Furthermore, the electrolyte is a liquid in which vinylene carbonate (VC) is mixed in a solvent formed by mixing ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 3:5:2 to make it 1% by mass, and the electrolyte LiPF6 is further dissolved in the mixed solution to make its concentration reach 1.2 mol / L. In addition, the design capacity of the laminated full unit for evaluation is 570 mAh, and the capacity ratio (negative electrode / positive electrode) is 1.1.

[0230] [Comparative Example 2]

[0231] In addition to setting the unit area weight of the positive electrode mixture layer to 11.3 mg / cm 2 and the unit area weight of the negative electrode mixture layer is 8 mg / cm 2 A positive electrode sheet and a negative electrode sheet were prepared in the same manner as in Comparative Example 1 except for the above.

[0232] (Fabrication of laminated full unit)

[0233] A laminated full cell for evaluation was prepared in the same manner as in Example 2 using the positive electrode sheet prepared in Comparative Example 2 and the negative electrode sheet prepared in Comparative Example 2.

[0234] The laminated full cells prepared in Example 2 and Comparative Example 2 were subjected to pre-charging and aging treatments.

[0235] The laminated full cell was placed in a thermostatic chamber set at 45°C and pre-charged under CC / CV conditions of 0.03C 3.4V 0.01C cutoff. After pre-charging, exhaust was performed.

[0236] The aging treatment is performed as follows. First, the initial charge is performed at 45°C and CC / CV 0.1C 4.2V 0.02C cutoff. After the initial charge, the laminated full cell is kept for 12 hours and vented. Next, the initial discharge is performed at 25°C and CC 0.1C, 2.8V. Then, the second charge is performed at 25°C and CC / CV 0.2C 4.2V 0.05C cutoff, and the second discharge is performed at 25°C and CC 0.2C, 2.8V.

[0237] (Charge and discharge evaluation of laminated full cell)

[0238] For the laminated full cell after aging, σ / A (°C / Ah) was determined in the same manner as in Example 1. The result was 0.32 (°C / Ah) in Example 2 and 0.39 (°C / Ah) in Comparative Example 2.

[0239] The aged laminated full cell was repeatedly subjected to high rate charge and discharge treatments, in which the charge was carried out at 45°C, CC 6C, and 4.2V cut-off, and then the discharge was carried out at 45°C, CC 5C, and 2.8V cut-off. The results of the cycle test are shown in FIG. Figure 8 and Fig. 9 .

[0240] like Figure 8 As shown, in Example 2, although the cell capacity is the same as that of Comparative Example 2, the cycle characteristics at a high rate are excellent. In Comparative Example 2, the discharge capacity retention rate begins to drop sharply from about 230 cycles, while in Example 2, the discharge capacity retention rate drops slowly even when 400 cycles of charge and discharge are performed at a high rate.

[0241] Fig. 9 The ratio of the discharge capacity at 5C charge and discharge is shown in the figure, when the discharge capacity at 0.2C charge and discharge is 100%. In Example 2, the discharge capacity drop at high rate charge and discharge is smaller than that in Comparative Example 2, and the discharge capacity at high rate is excellent.

[0242] Fig.10 and Fig.11 : represents the voltage curves when the charge and discharge treatment at a high rate is performed in Example 2 and Comparative Example 2. Fig.10 As shown in FIG. 1 , in Comparative Example 2, a voltage curve showing a large IR drop is shown from the initial stage of the cycle, and the degradation accompanying the cycle is large. Fig.11 As shown, in Example 2, the IR drop is small and the change in the voltage curve accompanying the cycle is small.

[0243] After 400 cycles of high rate charge and discharge, Example 2 and Comparative Example 2 were charged at 25°C, CC / CV 0.2C, 4.2V, 0.05C cut-off, and then discharged at 25°C, CC 0.2C, 2.8V cut-off. The results are shown in Fig.12 .

[0244] like Fig.12 As shown, even after 400 cycles, almost no capacity degradation was observed in Example 2, and the shape of the voltage curve was good. On the other hand, in Comparative Example 2, a curve shape with small capacity and high resistance was shown, and cell degradation was observed.

[0245] The disclosure of Japanese Patent Application No. 2022-179914 filed on November 9, 2022 is incorporated into this specification by reference in its entirety.

[0246] All documents, patent applications, and technical standards described in this specification are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually described as being incorporated by reference.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode and an outer packaging material for accommodating the positive electrode and the negative electrode, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material, and the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material, The secondary battery has a structure in which the positive electrode and the negative electrode are stacked in multiple layers in a thickness direction. The positive electrode mixture layer contains fibrous carbon. When the secondary battery is discharged from a fully charged state to a discharge termination voltage at a current of 5 C, the standard deviation of the temperature rise at the temperature measuring point on the outer packaging material is set to σ in ° C, and the rated capacity of the battery is set to A in Ah, and σ / A is less than 0.35 ° C / Ah. 2 . The secondary battery according to claim 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 claim 1 , wherein when the secondary battery is charged from a fully discharged state to a charge end voltage at a current of 5 C, the σ / A is 0.25° C. / Ah or less.

4. The secondary battery according to claim 1, wherein the average electrode area of ​​each positive electrode is 20 cm 2 ~10000cm 2 . The secondary battery according to claim 1 , which is a laminate type. 6 . The secondary battery according to claim 5 , wherein the number of stacked layers of the positive electrode and the negative electrode is 4 or more. 7 . The secondary battery according to claim 1 , which is a wound type secondary battery. 8 . The secondary battery according to claim 7 , wherein the number of windings is 4 or more. 9 . The secondary battery according to claim 1 , wherein the positive electrode mixture layer further contains 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 secondary battery according to claim 1, wherein 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 above.

12. The secondary battery according to claim 1, wherein the positive electrode mixture layer has a weight per unit area of ​​10.0 mg / cm 2 The negative electrode mixture layer has a unit area weight of 5.0 mg / cm2 above.

13. The secondary battery according to claim 1, wherein the positive electrode active material contains a selected from LiNi x Mn y Co z Al w O2, LiMPO4 and LiMn a Ni b At least one of O4, wherein x, y, z and w are all greater than or equal to 0, x+y+z+w=1, M is one or more selected from Fe, Co, Mn and Ni, a and b are both greater than or equal to 0, a+b=2.

14. The secondary battery according to claim 1, wherein the positive electrode active material contains LiNi x Mn y Co z Al w O2, where x, y, z and w are all greater than or equal to 0, x+y+z+w=1.

15. The secondary battery according to claim 1, wherein the positive electrode active material contains LiMPO4, wherein M is one or more selected from the group consisting of Fe, Co, Mn and Ni.

16. The secondary battery according to claim 15, wherein 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 above.

17. The secondary battery according to claim 1, wherein the negative electrode active material contains a compound selected from Si, SiO x , soft carbon, hard carbon, graphite, silicon-carbon composite, Li4Ti5O 12 and at least one of metal Li, wherein 0<x≤2。 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 disposed on the positive electrode current collector and containing a positive electrode active material, The positive electrode mixture layer contains fibrous carbon, When a secondary battery having the positive electrode, the negative electrode and an outer casing material for accommodating the positive electrode and the negative electrode is made, and the secondary battery is discharged from a fully charged state to a discharge termination voltage at a current of 5C, the standard deviation of the temperature rise at the temperature measuring point on the outer casing is set to σ in °C, and the rated capacity of the battery is set to A in Ah, σ / A is less than 0.35 °C / Ah, and the secondary battery has a structure in which the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction.

20. A positive electrode mixture layer composition for forming a positive electrode mixture layer containing a positive electrode active material. The positive electrode material layer composition contains fibrous carbon. When a secondary battery is made into a positive electrode, a negative electrode and an outer casing material for accommodating the positive electrode and the negative electrode, and the secondary battery is discharged from a fully charged state to a discharge termination voltage at a current of 5C, the standard deviation of the temperature rise at the temperature measuring point on the outer casing is set to σ in units of °C, and the rated capacity of the battery is set to A in units of Ah, σ / A is less than 0.35 °C / Ah, the secondary battery has a structure in which the positive electrode and the negative electrode are stacked in multiple layers in the thickness direction, the positive electrode has a positive electrode collector and a positive electrode mixture layer arranged on the positive electrode collector, and the positive electrode mixture layer is formed by a composition for the positive electrode mixture layer.

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