Secondary batteries

Abstract

A secondary battery disclosed herein is a nonaqueous electrolyte secondary battery that includes a positive electrode and a negative electrode. The positive electrode includes a first layer including a positive electrode active material. The first layer further includes a flame-retardant including a halogen atom, and carbon nanotubes.

Description

[Technical Field]

[0001] The present disclosure relates to secondary batteries. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, have high power output and high energy density, and are therefore used as power sources for small consumer devices, power storage devices, and electric vehicles.

[0003] Various additives have conventionally been added to the positive electrode active material layer of non-aqueous electrolyte secondary batteries. For example, Patent Document 1 discloses "a non-aqueous electrolyte secondary battery having a positive electrode in which a halogen-substituted cyclic organic compound substituted with one or more chlorine or bromine is added to a positive electrode active material mainly composed of a lithium-transition metal composite oxide containing lithium and at least one of cobalt (Co), nickel (Ni), iron (Fe), manganese (Mn), and copper (Cu); a negative electrode made of a compound mainly composed of lithium metal, a lithium alloy, or a material capable of absorbing and desorbing lithium; and a non-aqueous electrolyte."

[0004] Patent Document 2 proposes a composite electrode plate for a lithium-ion battery, characterized in that it "includes a battery electrode plate and a functional coating layer composited on the surface of the battery electrode plate, the functional coating layer being made of a functional substance and a binder, the functional substance being one or more selected from a phosphorus-containing compound, a nitrogen-containing compound, and an inorganic silicon compound, and the battery electrode plate being a battery positive electrode and / or a battery negative electrode." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-212228 [Patent Document 2] Special Publication No. 2017-534138 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, there has been an increasing demand for higher energy densities in non-aqueous electrolyte secondary batteries. However, increasing the energy density of lithium-ion secondary batteries requires high levels of safety measures for the batteries in the event of an abnormality. [Means for solving the problem]

[0007] One aspect of the present disclosure relates to a secondary battery including a positive electrode and a negative electrode, wherein the positive electrode includes a first layer including a positive electrode active material, and the first layer further includes a flame retardant including a halogen atom and carbon nanotubes. [Effects of the Invention]

[0008] According to the present disclosure, a highly safe secondary battery can be realized. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away; [Figure 2] FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a positive electrode that constitutes a secondary battery according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] Examples of embodiments according to the present disclosure will be described below. Note that, although examples of embodiments according to the present disclosure will be described below, the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and other materials may be applied as long as the effects of the present disclosure can be obtained. In this specification, when a "range between numerical value A and numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0011] (Secondary battery) The secondary battery according to this embodiment includes a positive electrode and a negative electrode. The positive electrode includes a first layer including a positive electrode active material. The first layer includes a flame retardant including a halogen atom. The first layer may further include carbon nanotubes. The flame retardant and the halogen atom may be hereinafter referred to as a "flame retardant (R)" and a "halogen atom (X)," respectively. The secondary battery according to this embodiment may be hereinafter referred to as a "secondary battery (S)." In one embodiment, the first layer may be a positive electrode active material layer (positive electrode mixture layer) including a positive electrode active material, a flame retardant (R), and carbon nanotubes as a conductive material.

[0012] As a result of their investigation, the inventors have newly discovered that by using a specific flame retardant in combination with carbon nanotubes, it is possible to achieve both high capacity and safety, and to obtain a secondary battery that is also excellent in other properties (capacity retention rate during charge-discharge cycles). The present disclosure is based on this new finding.

[0013] (Flame retardant(R)) The flame retardant (R) exhibits a flame retardant effect by releasing halogen atoms (X) at high temperatures, and therefore the secondary battery (S) can suppress excessive heat generation and fire in abnormal conditions.

[0014] The flame retardant (R) may satisfy at least one of the following conditions (1) and (2): The flame retardant (R) preferably satisfies both of the following conditions (1) and (2). (1) The flame retardant (R) contains a cyclic structure to which a halogen atom (X) is bonded. The cyclic structure may or may not be an aromatic ring. In this case, all of the halogen atoms (X) may be bonded to the cyclic structure, or only some of the halogen atoms (X) may be bonded to the cyclic structure. A structure in which the halogen atoms (X) are bonded to a cyclic structure is preferred because it is easy to increase the content of halogen atoms. (2) The proportion of halogen atoms (X) in the flame retardant (R) is 45% by mass or more. This proportion may be 60% by mass or more (e.g., 70% by mass or more). There is no particular upper limit, but it may be 95% by mass or less (e.g., 90% by mass or more). These lower and upper limits can be combined in any way.

[0015] The structural formula of ethylene-1,2-bispentabromophenyl, an example of a flame retardant (R), is shown below. Ethylene-1,2-bispentabromophenyl has a molecular weight of 971.2 and contains 10 bromine atoms (atomic weight: 79.9). Therefore, the proportion of halogen atoms (X) in ethylene-1,2-bispentabromophenyl is 100 x 10 x 79.9 / 971.2 = 82.3% by mass.

[0016] [ka]

[0017] Although the halogen atom (X) is not particularly limited, preferred examples of the halogen atom (X) include bromine (Br), chlorine (F), and fluorine (F). The halogen atom (X) may be bromine and / or chlorine, or may be bromine, since a flame retardant effect can be expected from the early stage of abnormal heat generation.

[0018] Flame retardants (R) containing such halogen atoms (X) have a higher specific gravity than conventionally used phosphorus-based flame retardants, allowing for a smaller volume relative to the mass added. This allows for a sufficient heat generation suppression effect while reducing the thickness of the flame retardant layer. Therefore, the thickness of the active material layer is not limited by the flame retardant layer, allowing for a high capacity to be achieved using a thick active material layer. In terms of its high specific gravity, the flame retardant (R) preferably contains bromine (Br). Furthermore, the more halogen atoms (X) bonded to the flame retardant (R), the better. The specific gravity of the flame retardant (R) can be easily increased by bonding halogen atoms (X) to a cyclic structure. The specific gravity of the flame retardant (R) may be, for example, 2.7 or more, preferably 3.0 or more.

[0019] It is preferable that the flame retardant (R) does not contain a moiety that generates moisture and / or a hydrophilic group in the compound structure. In this case, moisture is less likely to be mixed into the battery during the manufacturing process of the secondary battery, and a highly reliable secondary battery can be realized. Examples of moieties that generate moisture include a hydroxyl group (-OH), a carboxyl group (-COOH), a carbonyl group (-CO-), and an oxo acid group such as a sulfo group or a phosphate group. Examples of hydrophilic groups include the above functional groups as well as an amino group.

[0020] The flame retardant (R) may release halogen atoms (X) at temperatures of 180°C or higher (e.g., 250°C or higher). If the flame retardant releases halogen atoms (X) at relatively low temperatures, the halogen atoms (X) may be released under normal conditions, resulting in a deterioration in battery performance. Therefore, it is preferable that the flame retardant (R) does not substantially release halogen atoms (X) at temperatures below 180°C.

[0021] The flame retardant (R) may be at least one selected from the group consisting of ethylene-1,2-bispentabromophenyl, ethylenebistetrabromophthalimide, tetrabisbromobisphenol A, hexabromocyclododecane, 2,4,6-tribromophenol, 1,6,7,8,9,14,15,16,17,17,18,18-dodecachloropentacyclo(12.2.1.16,9.02,13.05,10)octadeca-7,15-diene (trade name: Dechlorane Plus), and tris(2,2,2-trifluoroethyl)phosphate. These flame retardants (R) may be commercially available. Alternatively, the flame retardant (R) may be synthesized using known synthesis methods.

[0022] When the mass ratio of the positive electrode active material to the flame retardant (R) in the first layer is expressed as positive electrode active material:flame retardant (R)=100:a, a may be greater than 0 and less than 7. This configuration can improve safety without significantly reducing battery capacity. The value of a may be 0.1 or greater, 0.3 or greater, 0.5 or greater, or 1.0 or greater. The value of a may be less than 7.0, less than 4.5, 3.0 or less, 2.0 or less, 1.5 or less, or 1.0 or less. These lower and upper limits can be arbitrarily combined as long as there is no contradiction. For example, the value of a may be in the range of 0.1 or greater and less than 7 (e.g., in the range of 0.1 or greater and less than 4.5, 0.1 to 3.0, 0.1 to 2.0, 0.1 to 1.0, 0.5 to 2.0, or 0.5 to 1.0).

[0023] The first layer may or may not contain acetylene black. When the mass ratio of the positive electrode active material, acetylene black, and carbon nanotubes in the first layer is expressed as positive electrode active material:acetylene black:carbon nanotubes = 100:b:c, b and c may satisfy 0≦b<5 and b + c<10. According to this configuration, high capacity and high cycle performance can be achieved. b and c may satisfy 0≦b<3 and b + c<5, or 0≦b<1 and 0.02 < b + c<5 (for example, 0.1 < b + c<1). The value of c may be in the range of 0.02 to 3.0 (for example, in the range of 0.02 to 2.0, in the range of 0.05 to 1.0, in the range of 0.05 to 0.5, or in the range of 0.1 to 0.5). The value of b may be in the range of 0 to 3.0 (for example, in the range of 0 to 2.0, in the range of 0 to 1.0, or in the range of 0 to 0.5).

[0024] In a preferred example of the secondary battery (S), the value of a above is in the range of 0.5 to 1.0, b above is in the range of 0 to 0.5, and c above is in the range of 0.02 to 0.5 (for example, 0.1 to 0.5). The flame retardant (R) in this example may be ethylene-1,2-bis(pentabromophenyl) and / or ethylenebistetrabromophthalimide.

[0025] (Carbon nanotubes) The carbon nanotubes form a conductive path between the particles of the positive electrode active material and function as a conductive material for enhancing the conductivity of the positive electrode active material layer (for example, the first layer or the second layer described later) containing the positive electrode active material. The aspect ratio (the ratio of the length to the diameter) of the carbon nanotubes is extremely large. Therefore, the carbon nanotubes exhibit high conductivity even in a small amount. Also, by using carbon nanotubes as the conductive material, it is possible to increase the ratio of the positive electrode active material in the positive electrode active material layer. Therefore, the secondary battery (S) can have a higher capacity.

[0026] The carbon nanotube content in the positive electrode active material layer may be 0.01% by mass or more, 0.3% by mass or more, or 0.1% by mass or more in order to reduce battery resistance. On the other hand, the carbon nanotube content may be 10% by mass or less, 3% by mass or less, or 1% by mass or less in order to achieve high capacity and suppress an increase in battery temperature in the event of an abnormality. These lower and upper limits can be combined arbitrarily as long as there is no contradiction.

[0027] The percentage of the positive electrode active material in the positive electrode active material layer can be determined from a sample obtained by removing only the positive electrode active material layer from a discharged secondary battery. Specifically, the discharged secondary battery is first disassembled to remove the positive electrode. Next, the positive electrode is washed with an organic solvent and further vacuum-dried, and then the positive electrode active material layer is removed to obtain a sample. Thermal analysis such as TG-DTA can be performed on the sample to calculate the percentage of binder components and conductive material components other than the positive electrode active material. When the binder component and conductive material component contain multiple types of carbon materials, the percentage of carbon nanotubes can be calculated by performing microscopic Raman spectroscopy on a cross-section of the positive electrode active material layer. In addition, the percentage of the flame retardant (R) in the positive electrode active material layer can be determined by elemental analysis such as EDS on a cross-section of the positive electrode active material layer.

[0028] The outer diameter and length of carbon nanotubes can be determined by image analysis using a scanning electron microscope (SEM). For example, the length can be determined by measuring the lengths and diameters of a number of randomly selected carbon nanotubes (e.g., 100 to 1000), and averaging these measurements.

[0029] Examples of carbon nanotubes include carbon nanofibers. Various types of carbon nanotubes are commercially available, so commercially available ones may be used. Alternatively, carbon nanotubes may be synthesized by known synthesis methods.

[0030] The carbon nanotubes may be single-walled, double-walled, or multi-walled. Single-walled carbon nanotubes are preferred because a small amount can produce a significant effect. Carbon nanotubes with a diameter of 5 nm or less contain a large amount of single-walled carbon nanotubes. The single-walled carbon nanotubes may account for 50% by mass or more of the total carbon nanotubes.

[0031] The diameter of the carbon nanotubes is not particularly limited and may be in the range of 0.001 to 0.05 μm. The length of the carbon nanotubes is not particularly limited, but may be 0.5 μm or more from the viewpoint of ensuring electronic conduction in the positive electrode active material layer. On the other hand, there is no upper limit to the length of the carbon nanotubes as long as they are properly arranged inside the positive electrode. Considering that the particle diameter of the positive electrode active material is generally 1 μm to 20 μm, the length of the carbon nanotubes may be approximately the same. That is, the length of the carbon nanotubes may be, for example, 1 μm to 20 μm. For example, when a plurality of carbon nanotubes (e.g., 100 or more) are arbitrarily selected in the positive electrode active material layer, the length of 50% or more (by number) of the carbon nanotubes may be 1 μm or more, or may be 1 μm to 20 μm. The length of 80% or more of the carbon nanotubes may be 1 μm or more, or may be 1 μm to 20 μm.

[0032] In one embodiment of the present disclosure, the flame retardant (R) may be unevenly distributed on the surface side of the first layer. In this case, the first layer may include, for example, a second layer containing at least a positive electrode active material and carbon nanotubes, and a third layer located closer to the surface of the positive electrode than the second layer and containing at least the flame retardant (R). The content of the flame retardant in the third layer is greater than the content of the flame retardant in the second layer. Here, the flame retardant content refers to the number of moles of the flame retardant contained in a unit volume (apparent volume) of the second or third layer. Whether the flame retardant is unevenly distributed on the second layer side can be determined by performing elemental analysis such as EDS on a cross section of the first layer (second and third layers) to determine the depth distribution of the flame retardant. In one embodiment, the second layer may be a positive electrode active material layer (positive electrode mixture layer) containing at least a positive electrode active material and carbon nanotubes as a conductive material, and the third layer may be a flame retardant layer containing at least the flame retardant (R).

[0033] The second layer may further contain carbon nanotubes. Adding carbon nanotubes to the second layer containing the positive electrode active material reduces the battery's resistance and suppresses deterioration due to repeated charge and discharge. However, secondary batteries containing carbon nanotubes in the positive electrode active material layer are more susceptible to abnormal events accompanied by heat generation, such as internal short circuits, compared to secondary batteries containing the same amount of conductive material, such as acetylene black. However, by disposing a third layer containing a flame retardant (R) between the separator and the second layer, which is the positive electrode active material layer, and adding carbon nanotubes to the second layer, high battery performance can be maintained and an increase in battery temperature during abnormal conditions can be suppressed. In this case, the second layer does not necessarily need to contain substantially any flame retardant (R).

[0034] The third layer, which serves as a flame retardant layer, contains a flame retardant (R) containing a halogen atom (X) and exhibits a flame retardant effect by releasing the halogen atom (X) at high temperatures. Therefore, the secondary battery (S) can suppress excessive heat generation in abnormal conditions. Furthermore, since the third layer, which serves as a flame retardant layer, does not have electronic conductivity, when interposed between the second layer, which serves as a positive electrode active material layer, and the separator, it also functions as a resistive layer that suppresses short circuits even in situations where a short circuit may occur inside the battery. This effectively suppresses heat generation.

[0035] A secondary battery according to another embodiment of the present disclosure is a secondary battery including a positive electrode and a negative electrode. The positive electrode includes a first layer containing a positive electrode active material. The first layer includes at least a positive electrode active material and a flame retardant (R) containing a halogen atom (X), and the flame retardant (R) is concentrated on the surface side of the first layer. For example, the first layer includes a second layer containing at least a positive electrode active material and a flame retardant (R), and a third layer located closer to the surface of the positive electrode than the second layer and containing at least the flame retardant (R). The content of the flame retardant in the third layer is greater than the content of the flame retardant in the second layer. Here, the content of the flame retardant refers to the number of moles of the flame retardant contained in a unit volume (apparent volume) of the second or third layer, and is measured, for example, by elemental analysis such as EDS.

[0036] By making the content of flame retardant in the second layer located on the current collector side of the positive electrode smaller than the content of flame retardant (R) in the third layer located on the surface side of the positive electrode, an increase in battery resistance in the second layer is suppressed, and deterioration due to repeated charge and discharge can be suppressed. Furthermore, a third layer with a high content of flame retardant can suppress an increase in battery temperature in abnormal conditions. Therefore, a secondary battery that combines high battery characteristics with suppression of an increase in battery temperature in abnormal conditions can be easily realized. In this case, it is not essential to add carbon nanotubes to the second layer (and third layer), and a material commonly used as a conductive material, such as carbon black, may be added. The third layer may contain a positive electrode active material. The mass-based content of the positive electrode active material in the third layer is preferably smaller than the mass-based content of the positive electrode active material in the second layer.

[0037] The third layer is preferably disposed on the surface of the second layer so as to contact the surface of the second layer containing the positive electrode active material and cover at least a portion of the second layer.

[0038] The third layer may contain a binder in addition to the flame retardant (R). The inclusion of a binder in the third layer can enhance the adhesion between the flame retardant (R) particles and the adhesion of the flame retardant (R) to the second layer, which is the positive electrode active material layer. In other words, the third layer can be tightly attached to the second layer. The binder is not particularly limited, but examples include polyvinylidene fluoride (PVdF), ethylene dimethacrylate, allyl methacrylate, t-dodecyl mercaptan, α-methylstyrene dimer, and methacrylic acid. When polyvinylidene fluoride (PVdF), ethylene dimethacrylate, allyl methacrylate, t-dodecyl mercaptan, α-methylstyrene dimer, and methacrylic acid are used as the binder, the application of pressure and / or heat to the third layer can bond the positive electrode to the separator.

[0039] The third layer may contain particles other than the flame retardant (R) and binder. Examples of other particles include inorganic particles containing metal oxides such as alumina, boehmite, and titania. The inorganic particles containing metal oxides function as spacers, reducing the amount of flame retardant added. The average particle size of the inorganic particles is preferably 0.01 μm to 5 μm, and more preferably 1 / 2 or less of the average particle size of the flame retardant (R).

[0040] In the third layer, the flame retardant (R) may exist in the form of an aggregate in which particles of the flame retardant (R) aggregate together, or in the form of an aggregate in which particles of the flame retardant (R) aggregate together via a binder. The third layer may partially cover the surface of the second layer, or the third layer may cover almost the entire surface of the second layer. The coverage rate (area basis) of the third layer with respect to the surface of the second layer may be 5% or more, 10% or more, or 30% or more, and preferably 50% or more, in order to suppress an increase in battery temperature in abnormal conditions.

[0041] Even if the coverage of the surface of the second layer with the third layer is 100% and the surface of the second layer is completely covered with the third layer, the gaps between the particles of the third layer are large enough compared to the size of the lithium ions, allowing the lithium ions to move through the gaps and not hindering charging and discharging. However, from the viewpoint of suppressing an increase in battery resistance, the coverage of the surface of the second layer with the third layer may be 90% or less or 80% or less.

[0042] The coverage of the third layer relative to the surface of the second layer may be 5% to 90%, 10% to 90%, 30% to 90%, 50% to 90%, or 50% to 80%.

[0043] The coverage of the third layer can be determined by elemental mapping of the electrode surface using SEM-EDX (Energy Dispersive X-ray spectrometry), etc. For example, by elemental mapping the flame retardant (R) particles and the positive electrode active material, the coverage of the third layer relative to the surface of the second layer can be calculated.

[0044] The average particle diameter of the flame retardant (R) particles in the third layer (when forming an aggregate, the average particle diameter of the primary particles forming the aggregate) may be 0.01 μm to 5 μm, or 0.05 μm to 3 μm. The average particle diameter of the flame retardant (R) is determined as follows. First, 20 particles of the flame retardant (R) are randomly selected from an SEM image of the positive electrode surface. Next, the grain boundaries of the selected 20 particles are observed to identify the particle shapes, and the major axis of each of the 20 particles is determined, and the average value of these is taken as the average particle diameter of the flame retardant (R) particles. If the third layer contains particles other than the flame retardant (R), the average particle diameter of the other particles is determined in the same manner.

[0045] The thickness of the third layer is preferably 0.1 μm or more, more preferably 1 μm or more or 3 μm or more, in order to suppress an increase in battery temperature under abnormal conditions. The thickness of the third layer is preferably 10 μm or less in order to suppress an increase in battery resistance. These lower and upper limits can be combined arbitrarily as long as there is no contradiction. The thickness of the third layer is the average thickness in the region where the surface of the second layer is covered with the third layer, and is determined from an SEM image of the cross section of the positive electrode.

[0046] The third layer can be formed by depositing a mixture containing at least particles of the flame retardant (R) and a binder on the surface of the second layer. The mixture can be a slurry containing particles of the flame retardant (R), a binder, and a solvent (dispersion medium). The third layer can be formed by spraying, dripping, or applying the slurry to the surface of the second layer and drying it. The coverage and thickness of the third layer can be controlled by adjusting the amount of solvent relative to the amount of flame retardant (R) particles in the slurry and / or the amount of slurry applied.

[0047] In the third layer, the content of the flame retardant (R) in the entire third layer may be 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. The content of the flame retardant (R) in the entire third layer may be 100% by mass or less, or 95% by mass or less. These lower and upper limits can be combined arbitrarily as long as there is no contradiction. The proportion of the flame retardant (R) in the third layer can be determined by elemental analysis such as EDS of a cross section of the third layer.

[0048] When the second layer contains a flame retardant (R), the content of the flame retardant (R) in the entire second layer may be 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more. The content of the flame retardant (R) in the entire second layer may be 5% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, or 0.5% by mass or less. These lower and upper limits can be combined arbitrarily as long as there is no contradiction. The proportion of the flame retardant (R) in the second layer can be determined by elemental analysis such as EDS of a cross section of the second layer.

[0049] To achieve high capacity, the amount of positive electrode active material layer per unit area (coating amount) provided on the surface of the positive electrode current collector is set to 250 g / m 2 It may be more than that.

[0050] An example of a secondary battery (S) according to this embodiment and examples of its components are described below. Note that known components may be applied to components that are not characteristic of the present disclosure. The secondary battery (S) includes, for example, an exterior body (battery case), a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator disposed within the exterior body. The separator is disposed between the positive electrode and the negative electrode.

[0051] The shape of the secondary battery (S) is not limited, and may be cylindrical, rectangular, coin-shaped, button-shaped, etc. The battery case is selected according to the shape of the secondary battery (S).

[0052] [Positive electrode] The positive electrode includes a first layer containing a positive electrode active material, and further includes a positive electrode current collector as needed. Typically, the positive electrode includes a positive electrode current collector and a first layer disposed on the surface of the positive electrode current collector. The first layer may be a positive electrode active material layer (positive electrode mixture layer). In this case, the first layer includes a positive electrode active material, a flame retardant (R), and other substances (e.g., conductive material, binder, thickener) as needed. Known substances may be used as the other substances (e.g., conductive material, binder, thickener). The first layer preferably includes carbon nanotubes as a conductive material.

[0053] The first layer may have a laminated structure of a second layer (positive electrode active material layer) containing at least a positive electrode active material and carbon nanotubes, and a third layer (flame retardant layer) containing at least a flame retardant (R). In this case, the third layer is disposed on the surface of the second layer that does not face the positive electrode current collector. The second layer contains a positive electrode active material, carbon nanotubes, and, if necessary, other components. Examples of other components include a conductive material, a binder, a thickener, etc. These other components may be components used in known secondary batteries.

[0054] As another example, the first layer may have a laminated structure including a second layer containing at least a positive electrode active material and a flame retardant (R) and a third layer containing at least a positive electrode active material and a flame retardant (R), and the content of the flame retardant (R) in the third layer on the surface side of the positive electrode (the side not facing the positive electrode current collector) may be higher than the content of the flame retardant (R) in the second layer. The second and third layers contain the positive electrode active material, the flame retardant (R), and, if necessary, other substances (such as conductive materials, binders, thickeners, etc.). Known substances may be used as the other substances (such as conductive materials, binders, thickeners, etc.). In this case, the second and third layers do not necessarily contain carbon nanotubes as conductive materials.

[0055] Examples of binders include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives, etc. Examples of thickeners include carboxymethyl cellulose (CMC), polyvinyl alcohol, etc. These components may be used alone or in combination of two or more materials.

[0056] When the first layer (or second layer) contains carbon nanotubes, the first layer (or second layer) may or may not further contain a conductive material other than carbon nanotubes. The first layer (or third layer) may or may not contain a flame retardant other than the flame retardant (R). However, if a large amount of such a flame retardant is contained, the proportion of the positive electrode active material decreases. Therefore, when the first layer (or second layer) contains carbon nanotubes, the mass of the conductive material contained in the first layer (or second layer) other than carbon nanotubes may be 10 times or less (for example, in a range of 0 to 5 times, 0 to 1 time, or 0 to 0.5 times) the mass of the carbon nanotubes contained in the first layer (or second layer). An example of a conductive material other than carbon nanotubes includes acetylene black. Furthermore, the mass of the flame retardant contained in the first layer (or the third layer) other than the flame retardant (R) may be in the range of 2 times or less (for example, 0 to 1 time, 0 to 0.5 time, or 0 to 0.1 time) the mass of the flame retardant (R) contained in the first layer (or the third layer).

[0057] In one example of a method for producing a positive electrode, first, a positive electrode slurry is prepared by dispersing the materials for the first layer in a dispersion medium. The ratio of the positive electrode active material, flame retardant (R), and carbon nanotubes in the positive electrode slurry is selected to correspond to the ratios in the first layer to be produced. Next, the positive electrode slurry is applied to the surface of a positive electrode current collector and dried. The dried coating may be rolled as necessary. In this manner, a positive electrode can be produced. The positive electrode active material layer may be formed on only one surface of the positive electrode current collector, or on both surfaces.

[0058] When forming the first layer, including the second and third layers, on the surface of a positive electrode current collector, a positive electrode slurry is first prepared by dispersing the materials for the second layer in a dispersion medium. The ratio of the positive electrode active material to the carbon nanotubes in the positive electrode slurry is selected to correspond to the ratio in the second layer to be prepared. Next, the positive electrode slurry is applied to the surface of the positive electrode current collector and dried. The dried coating may be rolled as needed. In this manner, the second layer as a positive electrode active material layer can be formed on the surface of the positive electrode current collector. The positive electrode active material layer may be formed on only one surface of the positive electrode current collector or on both surfaces. Next, the third layer is formed on the surface of the second layer that does not face the positive electrode current collector.

[0059] The second layer may contain a flame retardant. The second layer may be a layer of a mixture containing a positive electrode active material and a flame retardant. The flame retardant contained in the second layer may be any of the compounds listed as the flame retardant (R) above, or may be any known flame retardant other than the flame retardant (R). The flame retardant contained in the second layer is preferably a flame retardant containing a halogen atom, similar to the flame retardant (R). However, when the flame retardant contained in the second layer is a flame retardant containing a halogen atom, it may be a compound different from or the same as the flame retardant (R). The proportion of the flame retardant (R) in the second layer can be determined by elemental analysis such as X-ray fluorescence analysis (XRF) of a cross section of the second layer.

[0060] (Cathode active material) As the positive electrode active material, a lithium-containing composite oxide having a layered structure (for example, a rock salt type crystal structure) containing lithium and a transition metal can be used. The lithium-containing composite oxide is, for example, Li a Ni x M 1-x O2 (where 0 < a ≤ 1.2, 0.8 ≤ x < 1, and M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B).) It may be a lithium-nickel composite oxide represented by. Among them, M preferably contains at least one selected from the group consisting of Co, Mn, and Fe. From the viewpoint of the stability of the crystal structure, M may contain Al. The a value indicating the molar ratio of lithium increases or decreases by charge and discharge. Specific examples of such a composite oxide include lithium-nickel-cobalt-aluminum composite oxide (LiNi 0.9 Co 0.05 Al 0.05 O2, etc.).

[0061] In the above lithium-nickel composite oxide, the higher the Ni ratio x, the more lithium ions can be extracted from the lithium-nickel composite oxide during charging, and the capacity can be increased. However, Ni in the lithium-nickel composite oxide with increased capacity in this way tends to have a higher valence. As a result, the crystal structure becomes particularly unstable in the fully charged state, and it easily changes (inactivates) to a crystal structure in which reversible insertion and extraction of lithium ions are difficult by repeated charge and discharge. As a result, the cycle characteristics are likely to deteriorate. In particular, when a configuration is adopted in which the thickness of the positive electrode active material layer is increased and / or the positive electrode active material layer is compressed to increase the amount of positive electrode active material per unit area, the flow of lithium ions and / or electrons is easily inhibited during the charge and discharge reaction, and unevenness is likely to occur in the charge and discharge reaction. When unevenness occurs in the charge and discharge reaction, inactivation of the crystal structure progresses in some regions where the amount of lithium ions extracted by the over-progressed charge reaction is large, and the cycle characteristics may deteriorate.

[0062] However, since the positive electrode active material layer of the secondary battery (S) contains carbon nanotubes, unevenness in the charge / discharge reaction is suppressed even when the loading amount (application amount) per unit area of ​​the positive electrode active material layer is increased. Therefore, even when a lithium-containing composite oxide with a large Ni ratio x is used, deterioration in cycle characteristics is suppressed. Therefore, a secondary battery with excellent cycle characteristics and high energy density can be realized.

[0063] From the viewpoint of obtaining a high capacity, the Ni ratio x in the lithium-containing composite oxide may be 0.85 or more (x≧0.85), or may be 0.9 or more (x≧0.9).

[0064] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0065] Another aspect of the present disclosure relates to a positive electrode having a first layer including the flame retardant, the carbon nanotubes, and a positive electrode active material.

[0066] [Negative electrode] The negative electrode includes a negative electrode active material layer and, if necessary, further includes a negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and, if necessary, further includes other materials (such as a binder). In one example of a method for producing a negative electrode, first, a negative electrode slurry is prepared by dispersing materials for the negative electrode active material layer in a dispersion medium. Next, the negative electrode slurry is applied to the surface of the negative electrode current collector and dried. The dried coating may be rolled if necessary. Examples of dispersion mediums include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or a mixture thereof. The ratio of components in the negative electrode active material layer can be adjusted by changing the mixing ratio of the materials for the negative electrode active material. In this manner, a negative electrode can be produced. The negative electrode active material layer may be formed on only one surface of the negative electrode current collector or on both surfaces.

[0067] The negative electrode active material layer contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive material, a thickener, etc. Known materials can be used as the binder, conductive material, and thickener.

[0068] (Negative electrode active material) The negative electrode active material can be at least one selected from materials that electrochemically absorb and release lithium ions, lithium metal, and lithium alloys. Examples of materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxides and tin oxides formed by bonding these with oxygen may also be used.

[0069] Examples of alloy materials containing silicon include a lithium ion conductive phase and a silicon composite material in which silicon particles are dispersed in the lithium ion conductive phase. Examples of the lithium ion conductive phase include a silicon oxide phase, a silicate phase, and / or a carbon phase. The main component of the silicon oxide phase (e.g., 95 to 100% by mass) may be silicon dioxide. Among these, composite materials composed of a silicate phase and silicon particles dispersed in the silicate phase are preferred because of their high capacity and low irreversible capacity.

[0070] The silicate phase may contain, for example, at least one selected from the group consisting of Group 1 elements and Group 2 elements of the long-period periodic table. As the Group 1 elements and Group 2 elements of the long-period periodic table, for example, lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. can be used. Other elements may include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), titanium (Ti), etc. Among them, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferable because it has a small irreversible capacity and high initial charge-discharge efficiency.

[0071] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li 2z SiO 2+z (0 < z < 2). z preferably satisfies the relationship 0 < z < 1, and z = 1 / 2 is more preferable. Examples of elements other than Li, Si, and O that may be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), etc.

[0072] The carbon phase can be composed of, for example, low-crystalline amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or others.

[0073] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil), a porous conductive substrate (such as a mesh body, a net body, a punching sheet), etc. are used. As the material of the negative electrode current collector , stainless steel, nickel, nickel alloys, copper, copper alloys, and the like.

[0074] [Electrolyte] The electrolyte may be an electrolytic solution containing a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that ionically dissociates in the electrolytic solution. The solute may include, for example, a lithium salt. Components of the electrolytic solution other than the solvent and the solute are additives. The electrolytic solution may contain various additives.

[0075] A non-aqueous solvent is used as the solvent. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of the cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of the chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of the chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0076] Other examples of the non-aqueous solvent include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0077] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0078] Examples of chain ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o- Examples include dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0079] These solvents may be fluorinated solvents in which some of the hydrogen atoms are substituted with fluorine atoms. Fluoroethylene carbonate (FEC) may be used as the fluorinated solvent.

[0080] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0081] The concentration of the lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0082] The electrolyte may contain other known additives, such as 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0083] [Separator] A separator may be disposed between the positive electrode and the negative electrode. A material having high ion permeability and adequate mechanical strength and insulating properties may be used as the separator. A microporous thin film, woven fabric, nonwoven fabric, or the like may be used as the separator. Polyolefins such as polypropylene and polyethylene are preferred as the separator material. Aramid fibers may also be used to increase mechanical strength.

[0084] An example of the secondary battery (S) includes an outer casing, an electrode group housed in the outer casing, and a non-aqueous electrolyte. There is no particular limitation on the structure of the electrode group. One example of the electrode group is formed by winding a positive electrode, a negative electrode, and a separator so that the separator is disposed between the positive electrode and the negative electrode. Another example of the electrode group is formed by stacking a positive electrode, a negative electrode, and a separator so that the separator is disposed between the positive electrode and the negative electrode. There is no limitation on the shape of the secondary battery (S), and it may be cylindrical, prismatic, coin-shaped, button-shaped, laminate-shaped, or the like.

[0085] An example of the secondary battery (S) includes an outer casing, an electrode group housed in the outer casing, and a non-aqueous electrolyte. There is no particular limitation on the structure of the electrode group. One example of the electrode group is formed by winding a positive electrode, a negative electrode, and a separator so that the separator is disposed between the positive electrode and the negative electrode. Another example of the electrode group is formed by stacking a positive electrode, a negative electrode, and a separator so that the separator is disposed between the positive electrode and the negative electrode. There is no limitation on the shape of the secondary battery (S), and it may be cylindrical, prismatic, coin-shaped, button-shaped, laminate-shaped, or the like.

[0086] There is no particular limitation on the method for producing the secondary battery (S), and a known production method may be applied, or a known production method may be applied with at least a part thereof modified.

[0087] Examples of embodiments according to the present disclosure will be specifically described below with reference to the drawings. The components described above can be applied to the components of the examples described below. Furthermore, the examples described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiments. Furthermore, in the embodiments described below, components that are not essential for the secondary battery according to the present disclosure may be omitted.

[0088] FIG. 1 is a schematic perspective view, with a portion cut away, of a prismatic non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. The secondary battery 1 shown in FIG. 1 includes a bottomed prismatic battery case 11, and an electrode group 10 and a non-aqueous electrolyte (not shown) housed within the battery case 11. The electrode group 10 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween to prevent direct contact. The electrode group 10 is formed by winding the negative electrode, the positive electrode, and the separator around a flat core and then removing the core. As described above, the positive electrode includes a first layer according to the present disclosure. The first layer includes a positive electrode active material, a flame retardant (R), and carbon nanotubes.

[0089] One end of a negative electrode lead 15 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of a positive electrode lead 14 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the negative electrode lead 15 is electrically connected to a negative electrode terminal 13 provided on the sealing plate 12. A gasket 16 is disposed between the sealing plate 12 and the negative electrode terminal 13 to insulate them from each other. The other end of the positive electrode lead 14 is connected to the sealing plate 12 and is electrically connected to the battery case 11, which also serves as the positive electrode terminal. A resin frame 18 is disposed above the electrode group 10. The frame 18 separates the electrode group 10 from the sealing plate 12 and also separates the negative electrode lead 15 from the battery case 11. The opening of the battery case 11 is sealed with the sealing plate 12. The sealing plate 12 has a liquid injection hole 17a formed therein. The electrolyte is poured into the battery case 11 through the pouring hole 17a. The pouring hole 17a is then closed with the sealing plug 17.

[0090] 2 is a cross-sectional view showing an example of the configuration of a positive electrode 3 constituting a secondary battery according to an embodiment of the present disclosure. A positive electrode active material layer (second layer) 31 is disposed on the surface of a positive electrode current collector 30, and a flame retardant layer (third layer) 32 is disposed on the surface of the positive electrode active material layer 31. The flame retardant layer 32 contains a flame retardant (R). The positive electrode active material layer 31 and the flame retardant layer 32 constitute a first layer. FIG. 2 shows an example in which the flame retardant layer 32 is formed so as to cover the entire surface of the positive electrode active material layer 31. [Example]

[0091] The secondary battery according to the present disclosure will be described in further detail with reference to examples.

[0092] Example 1 A plurality of secondary batteries were fabricated and evaluated in Example 1. The secondary batteries were fabricated in the following manner.

[0093] [Preparation of negative electrode] The negative electrode active material was a mixture of silicon composite material and graphite in a mass ratio of silicon composite material:graphite = 5:95. The negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and water were mixed in a predetermined mass ratio to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of copper foil (negative electrode current collector) to form a coating film. The coating film was dried and then rolled to form a negative electrode active material layer on both sides of the copper foil.

[0094] [Preparation of positive electrode] As the positive electrode active material, LiNi 0.88 Co 0.09 Al 0.03 O2 was used. Positive electrode slurry was prepared by mixing the positive electrode active material, polyvinylidene fluoride, N-methyl-2-pyrrolidone (NMP), and, if necessary, a flame retardant, acetylene black, and carbon nanotubes (CNTs) in a predetermined mass ratio. The carbon nanotubes used had an average diameter of approximately 1.5 nm and a length of approximately 1 μm to 5 μm.

[0095] Next, the positive electrode slurry was applied to the surface of an aluminum foil (positive electrode current collector) to form a coating film, which was then dried and rolled to form a first layer on both sides of the aluminum foil.

[0096] [Preparation of electrolyte] An electrolyte solution was prepared by adding LiPF6 as a lithium salt to a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. The concentration of LiPF6 in the non-aqueous electrolyte solution was 1.0 mol / L.

[0097] [Secondary battery production] A lead tab was attached to each electrode. Next, the positive electrode and negative electrode were spirally wound with the separator interposed between them so that the lead was located at the outermost periphery. In this way, an electrode group was produced. Next, the electrode group was inserted into an outer casing made of a laminate film with an aluminum foil barrier layer, and vacuum dried. Next, an electrolyte solution was injected into the outer casing, and the opening of the outer casing was sealed. In this way, a secondary battery was obtained.

[0098] In this example, several secondary batteries (batteries A1 to A8, C1 to C3) were fabricated by varying the type of flame retardant used in the first layer and the ratio of materials in the first layer. Specifically, the ratios of the positive electrode active material, flame retardant, acetylene black, and carbon nanotubes in the positive electrode active material layer were varied. These ratios were changed by changing the mixing ratio when preparing the positive electrode slurry. These ratios are shown in Table 1 below. Ethylene-1,2-bispentabromophenyl or ethylenebistetrabromophthalimide was used as the flame retardant.

[0099] The first layer of each battery was formed to have the same thickness. Therefore, if the proportion of flame retardant and conductive material in the first layer increases, the amount of positive electrode active material contained in the first layer decreases, resulting in a decrease in capacity.

[0100] The fabricated secondary batteries were evaluated as follows. (1) Measurement of initial discharge capacity and capacity retention rate The discharge capacity of the fabricated secondary battery was measured using the following method. First, the battery was charged at a constant current of 40 mA in a 25°C environment until the battery voltage reached 4.2 V, and then continued to be charged at a constant voltage until the current reached 10 mA. After leaving the charged battery for 20 minutes, it was discharged at a constant current of 60 mA until the battery voltage reached 2.5 V. It was then left for 20 minutes. This operation (charge / discharge cycle) was repeated 100 times.

[0101] The discharge capacity DC0 at the first discharge and the discharge capacity DC1 after repeating the above charge-discharge cycle 100 times were measured, and the capacity retention rate was calculated using the following formula. Capacity maintenance rate (%)=100×DC1 / DC0

[0102] (2) Nail penetration test The fabricated secondary batteries were subjected to a nail penetration test according to the following procedure. (a) In an environment of 25°C, the battery was charged at a constant current of 60 mA until the battery voltage reached 4.2 V, and then continued to be charged at a constant voltage until the current value reached 10 mA. (b) In a 25°C environment, the tip of a round nail (2.7 mm in diameter) was brought into contact with the center of the battery charged in (a). The round nail was then thrust into the battery in the stacking direction of the electrode plate group. The nail was thrust at a speed of 1 mm / sec. The thrusting of the round nail was stopped immediately after detecting a drop in the battery voltage due to an internal short circuit. (c) After the battery was short-circuited by the round nail, the short-circuit current value I and the battery voltage V were measured for one second. The heat generation amount for one second was calculated by integrating the product (power) of the current value I and the voltage V over time.

[0103] Some of the battery fabrication conditions and evaluation results are shown in Table 1. The amounts a, b, and c in Table 1 represent the mass ratios of the positive electrode active material, flame retardant, acetylene black (AB), and carbon nanotubes (CNT) in the first layer, expressed as positive electrode active material:flame retardant:AB:CNT=100:a:b:c. (*1) In Tables 1 and 2, flame retardant R1 represents ethylene-1,2-bispentabromophenyl. (*2) In Tables 1 and 2, flame retardant R2 represents ethylenebistetrabromophthalimide.

[0104] [Table 1]

[0105] The initial discharge capacity and capacity retention shown in Table 1 are preferably high, and the heat generation rate is preferably low. As shown in Table 1, the positive electrode active material layer (first layer) of batteries A1 to A8 contains a flame retardant (R) and carbon nanotubes. On the other hand, the positive electrode active material layer (first layer) of batteries C1 to C3 does not contain at least one of a flame retardant (R) and carbon nanotubes. Batteries A1 to A8 had higher initial discharge capacities and lower heat generation rates than battery C1. When batteries A2, A4, and C2, which contain the same type and amount of flame retardant (R), were compared, batteries A2 and A4 had higher initial discharge capacities, lower heat generation rates, and higher capacity retention rates than battery C2. As such, according to this embodiment, a battery capable of achieving both high capacity and safety can be obtained.

[0106] The positive electrode active material layer (first layer) of Battery A2 was configured with carbon nanotubes replacing the acetylene black in the positive electrode active material layer (first layer) of Battery A4. The heat generation of Battery A2 was lower than that of Battery A4. Similarly, the positive electrode active material layer (first layer) of Battery A6 was configured with carbon nanotubes replacing the acetylene black in the positive electrode active material layer (first layer) of Battery A8. The heat generation of Battery A6 was lower than that of Battery A8. The carbon nanotubes were arranged in a network on the surface of the positive electrode active material. Battery A2 contained more carbon nanotubes than Battery A4, and therefore the carbon nanotubes formed a more comprehensive network-like conductive network on the surface of the positive electrode active material in Battery A2 than in Battery A4. It is believed that the comprehensive arrangement of the carbon nanotubes in Battery A2 also resulted in a more uniform distribution of the flame retardant. It is believed that the more uniform distribution of the flame retardant resulted in a lower heat generation of Battery A2 than in Battery A4. The reason why the heat generation amount of Battery A6 was lower than that of Battery A8 is also thought to be due to the uniform distribution of the flame retardant accompanying the comprehensive distribution of carbon nanotubes on the surface of the positive electrode active material.

[0107] Furthermore, the capacity retention rates of batteries A1 to A8 were equal to or higher than those of batteries C1 to C3. Carbon nanotubes have a large aspect ratio and excellent electrical conductivity. By disposing such carbon nanotubes between particles of positive electrode active material, the potential variation between the positive electrode active material particles is reduced, and non-uniformity in the charge / discharge reaction is suppressed. Furthermore, carbon nanotubes with a large aspect ratio occupy only a small volume in the positive electrode active material layer. Therefore, the carbon nanotubes are also suppressed from reducing the liquid circulation of the electrolyte. Furthermore, because carbon nanotubes are fibrous, gaps for the electrolyte are easily secured even when the positive electrode active material is densely arranged in the positive electrode active material layer. Therefore, it is believed that the addition of carbon nanotubes improves the capacity retention rate.

[0108] On the other hand, when comparing batteries A1, A2, A5, and A6 with battery C3, the capacity retention rates of batteries A1, A2, A5, and A6 were higher than that of battery C3, even though they contained the same amount of carbon nanotubes. The reason for this is unclear, but it may be due to a synergistic effect of adding both flame retardant (R) and carbon nanotubes. Flame retardants (R) containing halogen atoms are low-dielectric materials and have good wettability with electrolyte components (e.g., chain carbonates). Therefore, it is thought that adding flame retardant (R) improves the liquid permeability of the electrolyte. This improved liquid permeability of the electrolyte is thought to be one factor behind the improved capacity retention rate.

[0109] Furthermore, because the flame retardant (R) and carbon nanotubes each have poor dispersibility, adding them alone to the positive electrode slurry tends to reduce the uniformity of the positive electrode active material layer. On the other hand, adding both of them to the positive electrode slurry may result in easier dispersion, for reasons unknown. Therefore, using both of them can shorten the time required to prepare the positive electrode slurry and facilitate the production of a highly uniform positive electrode active material layer. One of the reasons why Batteries A1 to A8 exhibit good characteristics may be the improved uniformity of the positive electrode active material layer (first layer) due to the use of both the flame retardant (R) and carbon nanotubes. For example, using both may improve the dispersibility of the flame retardant (R), thereby achieving a high flame retardant effect.

[0110] Example 2 In Example 2, multiple secondary batteries were fabricated and evaluated. In Example 2, multiple secondary batteries were fabricated under the same conditions and by the same method as the batteries fabricated in Example 1, except that the amount of flame retardant was increased. The fabricated batteries were subjected to a nail penetration test by the method described above. Table 2 shows some of the fabrication conditions and the heat generation during the nail penetration test.

[0111] [Table 2]

[0112] As shown in the results in Tables 1 and 2, the amount of flame retardant was greater, resulting in a lower heat release. On the other hand, too much flame retardant may result in a decrease in the initial discharge capacity and capacity retention rate.

[0113] Example 3 In the production of the positive electrode, LiNi is used as the positive electrode active material. 0.88 Co 0.09 Al 0.03A positive electrode slurry was prepared by mixing the positive electrode active material, polyvinylidene fluoride (PVdF), N-methyl-2-pyrrolidone (NMP), acetylene black (AB), and, if necessary, carbon nanotubes (CNTs) in a predetermined mass ratio using O2. The carbon nanotubes used had an average diameter of approximately 1.5 nm and a length of approximately 1 μm to 5 μm.

[0114] Next, the positive electrode slurry was applied to the surface of an aluminum foil (positive electrode current collector) to form a coating film, which was then dried and rolled to form a second layer, which was a positive electrode active material layer, on both sides of the aluminum foil.

[0115] Next, a flame retardant (R), polyvinylidene fluoride (PVdF), N-methyl-2-pyrrolidone (NMP), and optionally alumina particles (Al2O3) were mixed in a predetermined mass ratio to prepare a slurry for the third layer. The resulting slurry was applied to the surface of the second layer and dried to form the third layer, which is a flame retardant layer. In this way, a first layer comprising the second and third layers was formed on the surface of the positive electrode current collector.

[0116] Except for this, a plurality of secondary batteries were fabricated in the same manner as in Examples 1 and 2, and the following evaluations were carried out.

[0117] In this example, multiple secondary batteries (batteries A9 to A13, C4, and C5) were fabricated by varying the ratio of materials in the second layer, the type of flame retardant contained in the third layer, and the ratio of materials in the third layer. Specifically, the ratios of the positive electrode active material, flame retardant, acetylene black, and carbon nanotubes in the second layer were varied. These ratios were changed by changing the mixing ratio when preparing the positive electrode slurry. Furthermore, the ratio of the flame retardant (R) to the binder (PVdF) in the third layer was changed by changing the mixing ratio when preparing the slurry for the third layer. Some of these ratios are shown in Table 3 below. The type of flame retardant will be described later.

[0118] (1) Battery resistance The battery was charged at a constant current of 40 mA in a 25°C environment until the battery voltage reached 4.2 V, and then continued to be charged at a constant voltage until the current value reached 10 mA. After charging, the battery was connected to a tester and the internal resistance was measured.

[0119] (2) Nail penetration test The temperature of the produced secondary batteries was measured after the nail penetration test according to the following procedure. (a) In an environment of 25°C, the battery was charged at a constant current of 0.5C until the battery voltage reached 4.2V, and then continued to be charged at a constant voltage until the current value reached 0.02C. (b) In a 25°C environment, the tip of a round nail (2.7 mm in diameter) was brought into contact with the center of the battery charged in (a) and pierced at a speed of 1 mm / sec. Immediately after detecting a drop in battery voltage due to an internal short circuit, the nail was stopped piercing. The surface temperature of the battery was then measured one minute after the battery shorted.

[0120] Table 3 shows some of the battery fabrication conditions, and Table 4 shows the evaluation results. The flame retardant layer ratios in Table 3 indicate the content of the flame retardant and binder (PVdF) in the slurry for the flame retardant layer. In Table 3, flame retardant r1 represents ethylene-1,2-bispentabromophenyl (SAYTEX®-8010, manufactured by Albemarle Japan Co., Ltd.). Flame retardant r2 represents ethylene bistetraphthalimide.

[0121] [Table 3]

[0122] [Table 4]

[0123] Comparing batteries C4 and C5 from Tables 3 and 4, battery C4, which does not contain carbon nanotubes in the second layer (positive electrode active material layer), has a high battery resistance. In contrast, battery C5, which contains carbon nanotubes in the second layer (positive electrode active material layer), can reduce the battery resistance, but the battery temperature after the nail penetration test increases. The battery temperature of battery C5 after the nail penetration test is significantly higher than the battery temperature of battery C4, which does not contain carbon nanotubes.

[0124] However, in batteries A9 to A13, which added carbon nanotubes to the second layer (positive electrode active material layer) and provided a third layer containing the flame retardant (R) on the surface of the second layer, the battery resistance was reduced and the temperature rise after the nail penetration test was suppressed. Table 3 shows that a relatively thin third layer (approximately 3 μm) was enough to suppress the temperature rise.

[0125] Battery A12 is equivalent to Battery A9, in which a portion of the flame retardant (R) contained in the third layer has been replaced with alumina particles, thereby reducing the content of flame retardant (R). In this case, the alumina particles function as spacers, increasing the gaps through which lithium ions can move, and the amount of flame retardant added is reduced, which is thought to have suppressed the increase in battery resistance compared to Battery A9. [Industrial Applicability]

[0126] The present disclosure can be used in secondary batteries. While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention. [Explanation of symbols]

[0127] 1: secondary battery, 3: positive electrode, 10: electrode group, 11: battery case, 12: sealing plate, 13: negative electrode terminal, 14: positive electrode lead, 15: negative electrode lead, 16: gasket, 17: sealing plug, 17a: liquid injection hole, 18: frame, 30: positive electrode current collector, 31: positive electrode active material layer, 32: flame retardant layer

Claims

1. A secondary battery including a positive electrode and a negative electrode, the positive electrode includes a first layer including a positive electrode active material, a flame retardant including a halogen atom, and carbon nanotubes; the first layer includes a second layer containing the positive electrode active material, the carbon nanotubes, and the flame retardant, and a third layer located closer to a surface of the positive electrode than the second layer and containing at least the flame retardant, a content of the flame retardant in the second layer as a whole of 0.1% by mass or more and 5% by mass or less; A secondary battery, wherein the content of the flame retardant in the entire third layer is 50 mass % or more and 100 mass % or less.

2. The flame retardant contains a cyclic structure to which the halogen atoms are bonded, 2. The secondary battery according to claim 1, wherein the proportion of the halogen atoms in the flame retardant is 45 mass % or more.

3. The secondary battery according to claim 1 , wherein the flame retardant releases the halogen atoms at a temperature of 180° C. or higher.

4. 2. The secondary battery according to claim 1, wherein the flame retardant is at least one selected from the group consisting of ethylene-1,2-bispentabromophenyl, ethylenebistetrabromophthalimide, tetrabisbromobisphenol A, hexabromocyclododecane, 2,4,6-tribromophenol, 1,6,7,8,9,14,15,16,17,17,18,18-dodecachloropentacyclo(12.2.1.16,9.02,13.05,10)octadeca-7,15-diene, and tris(2,2,2-trifluoroethyl)phosphate.

5. 5. The secondary battery according to claim 1, wherein a mass ratio of the positive electrode active material to the flame retardant in the first layer is expressed as positive electrode active material:flame retardant=100:a, where a is greater than 0 and less than 7.

6. the first layer contains acetylene black; 6. The secondary battery according to claim 1, wherein when a mass ratio of the positive electrode active material, the acetylene black, and the carbon nanotubes in the first layer is expressed as positive electrode active material:acetylene black:carbon nanotubes=100:b:c, b and c satisfy 0≦b<3 and b+c<5.

7. 7. The secondary battery according to claim 1, wherein the third layer is disposed on a surface of the second layer.

8. 8. The secondary battery according to claim 1, wherein the content of the carbon nanotubes in the second layer is 0.01% by mass or more and 10% by mass or less.

9. 9. The secondary battery according to claim 1, wherein the third layer has a thickness of 0.1 μm or more and 10 μm or less.

10. A secondary battery including a positive electrode and a negative electrode, the positive electrode includes a first layer including a positive electrode active material, the first layer includes a second layer containing at least the positive electrode active material and a flame retardant containing a halogen atom, and a third layer located closer to a surface of the positive electrode than the second layer and containing at least the flame retardant, a content of the flame retardant in the second layer as a whole of 0.1% by mass or more and 5% by mass or less; A secondary battery, wherein the content of the flame retardant in the entire third layer is 50 mass % or more and 100 mass % or less.

11. The flame retardant contains a cyclic structure to which the halogen atoms are bonded, The secondary battery according to claim 10 , wherein the proportion of the halogen atoms in the flame retardant is 45 mass % or more.

12. The secondary battery according to claim 10 or 11, wherein the flame retardant releases the halogen atoms at a temperature of 180° C. or higher.

13. 11. The secondary battery according to claim 10, wherein the flame retardant is at least one selected from the group consisting of ethylene-1,2-bispentabromophenyl, ethylenebistetrabromophthalimide, tetrabisbromobisphenol A, hexabromocyclododecane, 2,4,6-tribromophenol, 1,6,7,8,9,14,15,16,17,17,18,18-dodecachloropentacyclo(12.2.1.16,9.02,13.05,10)octadeca-7,15-diene, and tris(2,2,2-trifluoroethyl)phosphate.

14. 14. The secondary battery according to claim 10, wherein when a mass ratio of the positive electrode active material to the flame retardant in the first layer is expressed as the positive electrode active material: the flame retardant = 100: a, the a is greater than 0 and less than 7.

15. 15. The secondary battery according to claim 10, wherein the third layer is disposed on a surface of the second layer.

16. 16. The secondary battery according to claim 10, wherein the third layer has a thickness of 0.1 μm or more and 10 μm or less.

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