Secondary battery

Abstract

Disclosed is a nonaqueous electrolyte secondary battery including a positive electrode and a negative electrode. The positive electrode contains a first layer containing a positive electrode active material, and the first layer further contains a halogen atom-containing flame retardant. The first layer can contain carbon nanotubes. The first layer can contain a second layer and a third layer, the third layer is disposed closer to the surface of the positive electrode than the second layer, and the content rate of the flame retardant in the third layer is higher than the content rate of the flame retardant in the second layer.

Description

Technical Field

[0001] This disclosure relates to secondary batteries. Background Technology

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, possess high power and high energy density. Therefore, they are utilized as power sources for small-scale consumer applications, energy storage devices, and electric vehicles.

[0003] Various additives have 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 comprising: a positive electrode, which is formed by adding a halogen-substituted cyclic organic compound replaced by one or more chlorine or bromine to a positive electrode active material comprising lithium and at least one lithium-transition metal composite oxide selected from cobalt (Co), nickel (Ni), iron (Fe), manganese (Mn), and copper (Cu); a negative electrode, which is formed by a compound comprising lithium metal, lithium alloy, or a material capable of absorbing / releasing lithium as the main material; and a non-aqueous electrolyte."

[0004] Patent document 2 proposes a composite electrode plate for lithium-ion batteries, characterized in that it "comprising a battery electrode plate and a functional coating layer composited on the surface of the aforementioned battery electrode plate, wherein the aforementioned functional coating layer is made of a functional substance and a binder, wherein the aforementioned functional substance is one or more selected from phosphorus-containing compounds, nitrogen-containing compounds and inorganic silicon compounds, and the aforementioned battery electrode plate is the positive electrode and / or the negative electrode of the battery."

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2000-058068

[0008] Patent Document 2: Japanese Patent Publication No. 2017-534138 Summary of the Invention

[0009] The problem the invention aims to solve

[0010] In recent years, the demand for higher energy density in non-aqueous electrolyte secondary batteries has been increasing. However, while increasing the energy density of lithium-ion secondary batteries, it is also necessary to have high-level safety measures in place to address battery malfunctions.

[0011] Solution for solving the problem

[0012] One aspect of this disclosure relates to a secondary battery. The secondary battery includes a positive electrode and a negative electrode, wherein the positive electrode contains a first layer comprising a positive electrode active material, and the first layer further comprises a flame retardant containing halogen atoms and carbon nanotubes.

[0013] The effects of the invention

[0014] According to this disclosure, a secondary battery with high safety can be realized.

[0015] The novel features of the invention are set forth in the appended claims, but both the composition and content of the invention, together with its other objects and features, will be further better understood from the following detailed description with reference to the accompanying drawings. Attached Figure Description

[0016] Figure 1 A simplified perspective view of a secondary battery according to one embodiment of the present disclosure, with a portion cut off.

[0017] Figure 2 A schematic cross-sectional view showing an example of the configuration of the positive electrode of a secondary battery constituting an embodiment of the present disclosure. Detailed Implementation

[0018] Hereinafter, examples of embodiments of the present disclosure will be described. It should be noted that the following descriptions of embodiments of the present disclosure are examples, but the present disclosure is not limited to the examples described below. In the following descriptions, specific numerical values ​​and materials are sometimes cited, but other numerical values ​​and materials can also be applied as long as the effects of the present disclosure can be obtained. In this specification, when referred to as the "range of numerical value A to numerical value B," this range includes both numerical value A and numerical value B.

[0019] (Secondary battery)

[0020] The secondary battery of this embodiment includes a positive electrode and a negative electrode. The positive electrode contains a first layer comprising a positive electrode active material. The first layer contains a flame retardant containing halogen atoms. The first layer may also contain carbon nanotubes. Hereinafter, the flame retardant and halogen atoms are sometimes referred to as "flame retardant (R)" and "halogen atom (X)," respectively. In addition, the secondary battery of this embodiment is sometimes referred to as "secondary battery (S)." In one embodiment, the first layer may be a positive electrode active material layer (positive electrode compound layer) comprising a positive electrode active material, a flame retardant (R), and carbon nanotubes as a conductive material.

[0021] Research was conducted, and the inventors made a new discovery: by combining specific flame retardants with carbon nanotubes, a secondary battery that can achieve both high capacity and safety, as well as excellent performance in other characteristics (capacity retention during charge-discharge cycles), can be obtained. This disclosure is based on this new insight.

[0022] (Flame retardant(R))

[0023] Flame retardants (R) exhibit their flame-retardant effect by releasing halogen atoms (X) at high temperatures. Therefore, based on the secondary battery (S), excessive heat release and ignition can be suppressed in abnormal situations.

[0024] The flame retardant (R) can 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).

[0025] (1) The flame retardant (R) comprises a cyclic structure in which halogen atoms (X) are bonded. This cyclic structure may or may not be an aromatic ring. In this case, all halogen atoms (X) may be bonded to the cyclic structure, or only a portion of the halogen atoms (X) may be bonded to the cyclic structure. A structure in which halogen atoms (X) are bonded to the cyclic structure is preferred in terms of facilitating an increase in the halogen atom content.

[0026] (2) The proportion of halogen atoms (X) in the flame retardant (R) is 45% by mass or more. This proportion can be 60% by mass or more (e.g., 70% by mass or more). There is no particular upper limit, and it can be 95% by mass or less (e.g., 90% by mass or less). These lower and upper limits can be combined arbitrarily.

[0027] The following shows the structural formula of 1,2-bis(pentabromophenyl)ethane as an example of a flame retardant (R). 1,2-bis(pentabromophenyl)ethane has a molecular weight of 971.2, containing 10 bromine atoms (atomic weight: 79.9). Therefore, the proportion of halogen atoms (X) in 1,2-bis(pentabromophenyl)ethane is 100 × 10 × 79.9 / 971.2 = 82.3% by mass.

[0028]

[0029] The halogen atom (X) is not particularly limited, but preferred examples of halogen atoms (X) include bromine (Br), chlorine (F), and fluorine (F). In respect of expecting a flame-retardant effect from the initial stage of abnormal exothermic heating, the halogen atom (X) can be bromine and / or chlorine, or simply bromine.

[0030] This halogen-containing flame retardant (R) has a higher specific gravity than conventional phosphorus-based flame retardants, thus allowing for a smaller volume relative to the added mass. This enables the flame retardant layer to be thinner while simultaneously achieving sufficient exothermic suppression. Consequently, the limitation of the active material layer thickness by the flame retardant layer is prevented, allowing for the use of a thicker active material layer to achieve high capacity. Regarding 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 halogen atoms (X) in the flame retardant (R) are bonded to a cyclic structure, which facilitates an increase in specific gravity. The specific gravity of the flame retardant (R) can be, for example, 2.7 or more, preferably 3.0 or more.

[0031] The flame retardant (R) preferably does not contain any water-generating components and / or hydrophilic groups in its structure. In this case, moisture is less likely to enter the battery during the secondary battery manufacturing process, resulting in a secondary battery with excellent reliability. It should be noted that examples of water-generating components include hydroxyl (-OH), carboxyl (-COOH), carbonyl (-CO-), and oxyacid groups such as sulfonyl and phosphate groups. Examples of hydrophilic groups, in addition to the aforementioned functional groups, also include amino groups.

[0032] Flame retardant (R) can release halogen atoms (X) at temperatures above 180°C (e.g., above 250°C). If the flame retardant releases halogen atoms (X) at lower temperatures, the battery characteristics may sometimes be degraded under non-abnormal conditions. Therefore, flame retardant (R) preferably does not substantially release halogen atoms (X) at temperatures below 180°C.

[0033] The flame retardant (R) may be at least one selected from the group consisting of 1,2-bis(pentabromophenyl)ethane, ethylenebistetrabromophthalimide, tetrabromobisphenol A, hexabromocyclododecane, 2,4,6-tribromophenol, 1,6,7,8,9,14,15,16,17,17,18,18-dodecylpentane (12.2.1.16,9.02,13.05,10)octadec-7,15-diene (trade name: Dechlorane Plus), and tri(2,2,2-trifluoroethyl) phosphate. These flame retardants (R) may be commercially available products. Alternatively, the flame retardant (R) may be synthesized using known synthetic methods.

[0034] When the mass ratio of positive electrode active material to flame retardant (R) in the first layer is expressed as positive electrode active material:flame retardant (R) = 100:a, a can be greater than 0 and less than 7. This configuration improves safety without significantly reducing battery capacity. The value of a can be greater than 0.1, greater than 0.3, greater than 0.5, or greater than 1.0. The value of a can also be less than 7.0, less than 4.5, less than 3.0, less than 2.0, less than 1.5, or less than 1.0. These lower and upper limits can be combined arbitrarily as long as there is no contradiction. For example, the value of a can be in the range of greater than 0.1 and less than 7 (e.g., the range of greater than 0.1 and less than 4.5, the range of 0.1 to 3.0, the range of 0.1 to 2.0, the range of 0.1 to 1.0, the range of 0.5 to 2.0, and the range of 0.5 to 1.0).

[0035] The first layer may or may not contain acetylene black. When the mass ratio of the positive electrode active material: acetylene black: carbon nanotubes in the first layer is represented as 100:b:c, b and c may satisfy 0≤b<5 and b + c<10. With 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, 0.05 to 1.0, 0.05 to 0.5, or 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, 0 to 1.0, or 0 to 0.5).

[0036] In a preferred example of the secondary battery (S), the value of a is in the range of 0.5 to 1.0, the value of b is in the range of 0 to 0.5, and the value of c 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 1,2-bis(pentabromophenyl)ethane and / or ethylene bis(tetrabromophthalimide).

[0037] (carbon nanotubes)

[0038] Carbon nanotubes function as a conductive material for forming a conductive path between the particles of the positive electrode active material and improving the conductivity of the positive electrode active material layer (such as the first layer or the second layer described later) containing the positive electrode active material. The aspect ratio (length to diameter ratio) of carbon nanotubes is extremely large. Therefore, carbon nanotubes exhibit high conductivity even in small amounts. In addition, by using carbon nanotubes as the conductive material, the ratio of the positive electrode active material in the positive electrode active material layer can be increased. Therefore, the secondary battery (S) can achieve high capacity.

[0039] From the aspect of reducing the battery resistance, the content rate of carbon nanotubes 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. On the other hand, from the aspect of achieving high capacity and suppressing the rise in battery temperature during abnormal conditions, the content rate of carbon nanotubes may be 10% by mass or less, 3% by mass or less, or 1% by mass or less. These lower limits and upper limits can be arbitrarily combined as long as there is no contradiction.

[0040] The percentage of 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, firstly, the discharged secondary battery is disassembled and the positive electrode is removed. Next, the positive electrode is cleaned with an organic solvent, further vacuum dried, and then only the positive electrode active material layer is peeled off to obtain the sample. Thermal analysis such as TG-DTA is performed on this sample to calculate the ratio of binder components and conductive material components other than the positive electrode active material. If the binder and conductive material components contain multiple carbon materials, the percentage of carbon nanotubes can be calculated by performing micro Raman spectroscopy on a cross-section of the positive electrode active material layer. Furthermore, the percentage of 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.

[0041] The outer diameter and length of carbon nanotubes can be determined by analyzing images obtained using a scanning electron microscope (SEM). For example, the length can be calculated by arbitrarily selecting approximately 100 to 1000 carbon nanotubes, measuring their length and diameter, and averaging them.

[0042] Examples of carbon nanotubes include carbon nanofibers. Various commercially available carbon nanotube products are available, so commercially available products can be used. Alternatively, carbon nanotubes can be synthesized using well-known synthetic methods.

[0043] Carbon nanotubes can be single-walled, double-walled, or multi-walled. Single-walled carbon nanotubes are preferred for achieving a large effect with a small amount of material. Carbon nanotubes with a diameter of 5 nm or less contain a large number of single-walled carbon nanotubes. Single-walled carbon nanotubes can account for more than 50% of the total mass of carbon nanotubes.

[0044] There is no particular limitation on the diameter of carbon nanotubes, which can be in the range of 0.001 to 0.05 μm. There is also no particular limitation on the length of carbon nanotubes; from the viewpoint of ensuring electron conduction in the positive electrode active material layer, it can be 0.5 μm or longer. On the other hand, as long as they are appropriately disposed within the positive electrode, there is no upper limit to the length of the carbon nanotubes. Given that the particle size of the positive electrode active material is typically 1 μm or larger and 20 μm or smaller, the length of the carbon nanotubes can be of a similar magnitude. That is, the length of the carbon nanotubes can, for example, be 1 μm or larger and 20 μm or smaller. For example, when arbitrarily selecting multiple (e.g., more than 100) carbon nanotubes within the positive electrode active material layer, more than 50% (by number) of the carbon nanotubes can have a length of 1 μm or larger, or 1 μm or larger and 20 μm or smaller. More than 80% of the carbon nanotubes can have a length of 1 μm or larger, or 1 μm or larger and 20 μm or smaller.

[0045] In one embodiment of this disclosure, the flame retardant (R) may be non-uniformly distributed on the surface side of the first layer. In this case, the first layer may, for example, contain: a second layer comprising at least a positive electrode active material and carbon nanotubes; and a third layer located further on the surface side of the positive electrode than the second layer and comprising at least the flame retardant (R). The flame retardant content in the third layer is greater than that in the second layer. Here, the flame retardant content refers to the number of moles of flame retardant contained in a unit volume (apparent volume) of the second or third layer, and the distribution of the flame retardant in the depth direction can be determined by performing elemental analysis, such as EDS, on the cross-section of the first layer (the second and third layers), thereby determining whether the flame retardant is non-uniformly distributed on the third layer side. In one embodiment, the second layer may be a positive electrode active material layer (positive electrode compound layer) comprising at least a positive electrode active material and carbon nanotubes as a conductive material, and the third layer may be a flame retardant layer comprising at least the flame retardant (R).

[0046] The second layer may also contain carbon nanotubes. By adding carbon nanotubes to the second layer containing the positive electrode active material, the battery resistance can be reduced, and degradation caused by repeated charge-discharge cycles can be suppressed. On the other hand, a secondary battery with carbon nanotubes added to the positive electrode active material layer is more prone to abnormal events such as internal short circuits and heat generation compared to a secondary battery with an equal amount of conductive materials such as acetylene black added. However, by placing 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 the rise in battery temperature during abnormal events can be suppressed. In this case, the second layer may substantially not contain the flame retardant (R).

[0047] The third layer, serving as the flame retardant layer, contains a flame retardant (R) comprising halogen atoms (X). At high temperatures, it releases halogen atoms (X), thus exhibiting a flame-retardant effect. Therefore, according to the secondary battery (S), excessive heat release during abnormal conditions can be suppressed. Furthermore, the third layer, as the flame retardant layer, lacks electronic conductivity; therefore, when sandwiched between the second layer (serving as the positive electrode active material layer) and the separator, it can also function as a resistive layer to suppress short circuits in the event of a potential internal short circuit within the battery. This effectively suppresses heat release.

[0048] In another embodiment of this disclosure, the secondary battery is a secondary battery comprising a positive electrode and a negative electrode, wherein the positive electrode comprises a first layer containing a positive electrode active material. The first layer comprises at least the positive electrode active material and a flame retardant (R) containing halogen atoms (X), and the flame retardant (R) is unevenly distributed on the surface side of the first layer. For example, the first layer comprises: a second layer comprising at least the positive electrode active material and the flame retardant (R); and a third layer located on the surface side of the positive electrode, which also comprises at least the flame retardant (R). The flame retardant content in the third layer is greater than that in the second layer. Here, the flame retardant content refers to the number of moles of flame retardant contained in a unit volume (apparent volume) of the second or third layer, which can be determined, for example, by elemental analysis such as EDS.

[0049] By making the flame retardant content in the second layer located on the current collector side of the positive electrode less than the flame retardant (R) content in the third layer located on the surface side of the positive electrode, the increase in battery resistance in the second layer can be suppressed, thus suppressing degradation caused by repeated charge-discharge cycles. Furthermore, the third layer, with its higher flame retardant content, can suppress the rise in battery temperature during abnormal conditions. Therefore, a rechargeable battery that balances high battery performance with suppression of battery temperature rise during abnormal conditions can be easily achieved. In this case, the addition of carbon nanotubes to the second (and third) layers is not necessary; materials commonly used as conductive materials, such as carbon black, can also be added. The third layer may contain positive electrode active material. Preferably, the mass-based content of the positive electrode active material in the third layer is less than the mass-based content of the positive electrode active material in the second layer.

[0050] The third layer is preferably disposed on the surface of the second layer in such a way that it contacts the surface of the second layer containing the positive electrode active material and covers at least a portion of the second layer.

[0051] The third layer may also contain a binder in addition to the flame retardant (R). By including a binder, the third layer can improve the adhesion between the flame retardant (R) particles and the adhesion of the flame retardant (R) to the second layer, which serves as the positive electrode active material layer. That is, the third layer can be tightly bonded to the second layer. The binder is not particularly limited, and examples include polyvinylidene fluoride (PVdF), dimethyl ethylene acrylate, allyl methacrylate, tert-dodecyl mercaptan, α-methylstyrene dimer, and methacrylic acid. It should be noted that when polyvinylidene fluoride (PVdF), dimethyl ethylene acrylate, allyl methacrylate, tert-dodecyl mercaptan, α-methylstyrene dimer, and methacrylic acid are used as binders, the positive electrode can be bonded to the separator by applying pressure and / or heat to the third layer.

[0052] 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 titanium dioxide. These inorganic particles containing metal oxides function as spacers, suppressing the amount of flame retardant added. The average particle size of the inorganic particles is preferably 0.01 μm to 5 μm, more preferably less than half the average particle size of the flame retardant (R).

[0053] In the third layer, the flame retardant (R) can exist as an aggregate of flame retardant (R) particles, or as an aggregate of flame retardant (R) particles formed by a binder. The third layer can partially cover the surface of the second layer, or it can substantially completely cover the surface of the second layer. The coverage rate (area basis) of the third layer relative to the surface of the second layer can be 5% or more, 10% or more, or 30% or more, preferably 50% or more, in terms of suppressing the rise in battery temperature during abnormal conditions.

[0054] It should be noted that even when the third layer covers 100% of the surface of the second layer, and the surface of the second layer is completely covered by the third layer, the gaps between the particles in the third layer are much larger than the size of lithium ions. Therefore, lithium ions can move through these gaps without hindering charging and discharging. However, from the viewpoint of suppressing the increase in battery resistance, the coverage of the third layer relative to the surface of the second layer can be kept to below 90% or 80%.

[0055] The coverage of the third layer relative to the second layer can be 5% or more and less than 90%, 10% or more and less than 90%, 30% or more and less than 90%, 50% or more and less than 90%, or 50% or more and less than 80%.

[0056] The coverage of the third layer can be determined by elemental mapping of the electrode surface using methods such as SEM-EDX (energy-dispersive X-ray spectroscopy). For example, by performing elemental mapping on the flame retardant (R) particles and the positive electrode active material, the coverage of the third layer relative to the second layer can be calculated.

[0057] The average particle size of the flame retardant (R) particles in the third layer (when forming an aggregate, the average particle size of the primary particles constituting the aggregate) can be 0.01 μm to 5 μm, or 0.05 μm to 3 μm. The average particle size of the flame retardant (R) is calculated as follows: First, 20 flame retardant (R) particles are randomly selected from the SEM image of the positive electrode surface. Then, the grain boundaries and specific particle shapes of the 20 selected particles are observed, and the major and minor diameters of each of the 20 particles are calculated. Their average value is taken as the average particle size of the flame retardant (R). If the third layer contains other particles besides the flame retardant (R), the average particle size of the other particles is calculated in the same way.

[0058] 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 the rise in battery temperature during abnormal conditions. The thickness of the third layer is preferably 10 μm or less, in order to suppress the rise 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 of the area covered by the third layer on the surface of the second layer, determined from an SEM image of the cross-section of the positive electrode.

[0059] The third layer can be formed by depositing a mixture of particles containing at least flame retardant (R) and a binder onto the surface of the second layer. The mixture can be a slurry containing flame retardant (R) particles, a binder, and a solvent (dispersion medium). The third layer can be formed by spraying, dripping, or coating the slurry onto the surface of the second layer and then 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 by adjusting the coating amount of the slurry.

[0060] In the third layer, the content of flame retardant (R) in the entire third layer can be 50% or more by mass, 60% or more by mass, 70% or more by mass, 80% or more by mass, or 90% or more by mass. The content of flame retardant (R) in the entire third layer can be less than 100% by mass or less than 95% by mass. These lower and upper limits can be combined arbitrarily as long as there is no contradiction. The proportion of flame retardant (R) in the third layer can be determined by performing elemental analysis such as EDS on the cross-section of the third layer.

[0061] When the second layer contains a flame retardant (R), the content of the flame retardant (R) in the second layer can 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 second layer can be less than 5% by mass, less than 3% by mass, less than 2% by mass, less than 1% by mass, or less than 0.5% by mass. 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 performing elemental analysis such as EDS on the cross-section of the second layer.

[0062] To achieve high capacity, the amount of positive electrode active material layer per unit area (coating amount) applied to the surface of the positive electrode current collector can be 250 g / m². 2 above.

[0063] Hereinafter, examples of the secondary battery (S) of this embodiment and examples of its constituent elements will be described. It should be noted that known constituent elements may also be used for constituent elements that are not characteristic of this disclosure. The secondary battery (S) includes, for example, a casing (battery housing), a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator disposed within the casing. The separator is disposed between the positive electrode and the negative electrode.

[0064] The shape of the secondary battery (S) is not limited and can be cylindrical, square, coin-shaped, button-shaped, etc. The battery casing can be selected according to the shape of the secondary battery (S).

[0065] [positive electrode]

[0066] The positive electrode contains a first layer comprising a positive electrode active material, and may also include a positive electrode current collector if necessary. Typically, the positive electrode comprises 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 binder layer). In this case, the first layer comprises the positive electrode active material, a flame retardant (R), and other substances (conductive material, binder, thickener, etc.) as needed. Other substances (conductive material, binder, thickener, etc.) may also use known materials. The first layer preferably comprises carbon nanotubes as a conductive material.

[0067] The first layer can be a stacked structure consisting of a second layer (positive electrode active material layer) comprising at least a positive electrode active material and carbon nanotubes, and a third layer (flame retardant layer) comprising at least a flame retardant (R). In this case, the third layer is disposed on the surface of the side not opposite the positive electrode current collector of the second layer. The second layer comprises the positive electrode active material, carbon nanotubes, and other components as needed. Examples of other components include conductive materials, binders, thickeners, etc. These other components can also be components used in known secondary batteries.

[0068] As another example, the first layer is a stacked structure consisting of 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). The flame retardant (R) content in the third layer (the side not opposite the positive electrode current collector) can be higher than that in the second layer. The second and third layers contain the positive electrode active material, the flame retardant (R), and other substances (conductive materials, binders, thickeners, etc.) as needed. Other substances (conductive materials, binders, thickeners, etc.) can also be known materials. In this case, the second and third layers may not contain carbon nanotubes as conductive materials.

[0069] Examples of adhesives include fluoropolymers, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene copolymer rubber (SBR), polyacrylic acid, and their derivatives. Examples of thickeners include carboxymethyl cellulose (CMC) and polyvinyl alcohol. These components can be used individually or in combination of two or more materials.

[0070] When the first (or second) layer contains carbon nanotubes, it may also contain conductive materials other than carbon nanotubes, or it may not contain conductive materials other than carbon nanotubes. The first (or third) layer may contain flame retardants other than flame retardant (R), or it may not contain flame retardants other than flame retardant (R). If a large amount of these are included, the ratio of the positive electrode active material decreases. Therefore, when the first (or second) layer contains carbon nanotubes, the mass of conductive materials other than carbon nanotubes contained in the first (or second) layer can be less than 10 times the mass of carbon nanotubes contained in the first (or second) layer (e.g., 0 to 5 times, 0 to 1 times, or 0 to 0.5 times). Examples of conductive materials other than carbon nanotubes include acetylene black. In addition, the mass of flame retardant other than flame retardant (R) contained in the first layer (or the third layer) may be less than twice the mass of flame retardant (R) contained in the first layer (or the third layer) (e.g., 0 to 1 times, 0 to 0.5 times, or 0 to 0.1 times).

[0071] In one example of a method for fabricating the positive electrode, firstly, the material of the first layer is dispersed in a dispersion medium to prepare a positive electrode slurry. The ratio of the positive electrode active material to the flame retardant (R) to the carbon nanotubes in the positive electrode slurry is selected in a manner corresponding to the ratio of these components in the fabricated first layer. Next, the positive electrode slurry is coated onto the surface of the positive electrode current collector and dried. The dried coating can be calendered as needed. This process can then be used to fabricate the positive electrode. The positive electrode active material layer can be formed on only one surface of the positive electrode current collector, or it can be formed on both surfaces.

[0072] When forming the first layer including the second layer and the third layer on the surface of the positive electrode current collector, first, a material for the second layer is dispersed in a dispersion medium to prepare a positive electrode paste. The ratio of the positive electrode active material to the carbon nanotubes in the positive electrode paste is selected corresponding to their ratio in the produced second layer. Then, the positive electrode paste is coated on the surface of the positive electrode current collector and dried. The dried coating film can be calendered as needed. Thus, the second layer as the positive electrode active material layer can be formed on the surface of the positive electrode current collector. The positive electrode active material layer can be formed only on one surface of the positive electrode current collector or on both surfaces. Then, the third layer is formed on the surface of the second layer that does not face the positive electrode current collector.

[0073] 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 a compound listed in the above flame retardant (R), or other known flame retardants other than the flame retardant (R). Similar to the flame retardant (R), the flame retardant contained in the second layer is preferably a halogen atom-containing flame retardant. However, when the flame retardant contained in the second layer is a halogen atom-containing flame retardant, it may be a compound different from the flame retardant (R) or the same compound. The ratio of the flame retardant (R) in the second layer can be determined by elemental analysis such as fluorescence X-ray analysis (XRF) of the cross-section of the second layer.

[0074] (Positive electrode active material)

[0075] As the positive electrode active material, a lithium-containing composite oxide having a layered structure (such as a rock salt-type crystal structure) containing lithium and a transition metal can be used. The lithium-containing composite oxide can be, 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). 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, Al can be included as M. It should be noted that the value of a representing the molar ratio of lithium increases or decreases during charge and discharge. As a specific example of such a composite oxide, a lithium-nickel-cobalt-aluminum composite oxide (LiNi 0.9 Co 0.05 Al 0.05 O2, etc.) can be cited.

[0076] A higher Ni ratio (x) in the aforementioned lithium-nickel composite oxide allows for the capture of more lithium ions during charging, thus increasing capacity. However, this increased capacity in the lithium-nickel composite oxide tends to result in a higher Ni valence. Consequently, especially under full charge, the crystal structure becomes unstable and prone to inertization, becoming difficult to reversibly absorb and release lithium ions due to repeated charge-discharge cycles. This leads to decreased cycle performance. Particularly when increasing the thickness and / or compression of the positive electrode active material layer to improve the amount of positive electrode active material per unit area, the flow of lithium ions and / or electrons during charge-discharge reactions becomes more easily hindered, leading to unevenness in the charge-discharge process. If unevenness occurs during charge-discharge reactions, inertization of the crystal structure occurs in regions where excessive lithium ion capture occurs during overcharging, sometimes resulting in decreased cycle performance.

[0077] However, since the positive electrode active material layer of the secondary battery (S) contains carbon nanotubes, even when increasing the amount of positive electrode active material per unit area (coating amount), unevenness in the charge-discharge reaction can be suppressed. Therefore, even when using lithium-containing composite oxides with a high Ni ratio x, the reduction in cycle characteristics can be suppressed. Thus, a secondary battery with excellent cycle characteristics and high energy density can be achieved.

[0078] From the perspective of obtaining high capacity, the Ni ratio x in lithium-containing composite oxides can be 0.85 or higher (x≥0.85) or 0.9 or higher (x≥0.9).

[0079] The shape and thickness of the positive current collector can be selected from those of the negative current collector. Examples of materials that can be used for the positive current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0080] Another aspect of this disclosure relates to a positive electrode having the aforementioned flame retardant, the aforementioned carbon nanotubes, and a first layer comprising a positive electrode active material.

[0081] [negative electrode]

[0082] The negative electrode comprises a negative electrode active material layer and, if necessary, a negative electrode current collector. The negative electrode active material layer comprises a negative electrode active material and, if necessary, other substances (binders, etc.). In one example of a method for manufacturing the negative electrode, firstly, the material of the negative electrode active material layer is dispersed in a dispersion medium to prepare a negative electrode slurry. Next, the negative electrode slurry is coated onto the surface of the negative electrode current collector and dried. The dried coating can be calendered if necessary. Examples of dispersion media include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or mixtures thereof. The ratio of components in the negative electrode active material layer can be adjusted by changing the mixing ratio of the negative electrode active material. This allows the fabrication of a negative electrode. The negative electrode active material layer can be formed on only one surface of the negative electrode current collector, or on both surfaces.

[0083] The negative electrode active material layer contains a negative electrode active material as an essential component, and may include binders, conductive materials, thickeners, etc., as arbitrary components. Commonly known materials can be used as binders, conductive materials, and thickeners.

[0084] (Negative electrode active material)

[0085] 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. As materials that electrochemically absorb and release lithium ions, carbon materials, alloy materials, etc., can be used. Examples of carbon materials include graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among these, graphite, which exhibits excellent charge-discharge stability and low irreversible capacity, is preferred. As alloy materials, those containing at least one metal capable of forming an alloy with lithium can be used, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide, tin oxide, etc., formed by combining these with oxygen, can also be used.

[0086] As an alloying material containing silicon, examples include lithium-ion conductive phases and silicon composite materials in which silicon particles are dispersed in the lithium-ion conductive phase. As the lithium-ion conductive phase, examples include silicon oxide phases, silicate phases, and / or carbon phases. The main component of the silicon oxide phase (e.g., 95–100% by mass) can be silicon dioxide. From the perspective of high capacity and low irreversible capacity, composite materials composed of a silicate phase and silicon particles dispersed in that silicate phase are preferred.

[0087] The silicate phase may, for example, contain at least one selected from the group consisting of Group 1 elements and Group 2 elements of the long-period form of the periodic table. As the Group 1 elements and Group 2 elements of the long-period form of the periodic table, for example, lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. can be used. As other elements, aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), titanium (Ti), etc. can be contained. Among them, from the aspect of small irreversible capacity and high initial charge-discharge efficiency, a silicate phase containing lithium (hereinafter, also referred to as a lithium silicate phase) is preferred.

[0088] The lithium silicate phase only needs to be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and other elements can also be contained. 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 can have a composition represented by the formula: Li 2z SiO 2+z (0 < z < 2). z preferably satisfies the relationship of 0 < z < 1, and more preferably z = 1 / 2. As elements other than Li, Si, and O that can be contained in the lithium silicate phase, for example, iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), etc. can be cited.

[0089] The carbon phase may, for example, be composed of amorphous carbon with low crystallinity (i.e., non-crystalline carbon). The amorphous carbon may, for example, be hard carbon, may be soft carbon, or may be others.

[0090] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a sieve, a net, a punched sheet, etc.) can be used. As the material of the negative electrode current collector, stainless steel, nickel, nickel alloy, copper, copper alloy, etc. can be exemplified.

[0091] [Electrolyte]

[0092] The electrolyte can use an electrolytic solution containing a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that dissociates into ions in the electrolytic solution. The solute may, for example, contain a lithium salt. The components of the electrolytic solution other than the solvent and the solute are additives. Various additives can be contained in the electrolytic solution.

[0093] Non-aqueous solvents can be used. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One non-aqueous solvent can be used alone, or two or more can be used in combination.

[0094] Other examples of non-aqueous solvents include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0095] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-eucalyptol, crown ethers, etc.

[0096] 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-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, tetraethylene glycol dimethyl ether, etc.

[0097] These solvents can also be fluorinated solvents obtained by replacing some of the hydrogen atoms with fluorine atoms. Ethyl fluorocarbonate (FEC) can also be used as a fluorinated solvent.

[0098] As lithium salts, lithium salts containing chloric acid (LiClO4, LiAlCl4, LiB) can be used, for example. 10 Cl 10Lithium salts containing fluorine acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium imide salts containing fluorine acids (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), and lithium halides (LiCl, LiBr, LiI, etc.) are examples of lithium salts. A single lithium salt can be used, or two or more can be used in combination.

[0099] The concentration of lithium salt in the electrolyte can be above 1 mol / L and below 2 mol / L, or above 1 mol / L and below 1.5 mol / L. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0100] The electrolyte may also contain other known additives. Examples of additives include 1,3-propanesulfonate lactone, methylbenzene sulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0101] [Separator]

[0102] A separator can be disposed between the positive and negative electrodes. The separator can be a component with high ion permeability, moderate mechanical strength, and insulation. Microporous membranes, woven fabrics, and nonwoven fabrics can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred as materials for the separator. Alternatively, aromatic polyamide fibers can be used to improve mechanical strength.

[0103] An example of a secondary battery (S) includes: a housing, an electrode assembly housed within the housing, and a non-aqueous electrolyte. The structure of the electrode assembly is not particularly limited. One example of the electrode assembly is formed by winding the positive electrode, negative electrode, and separator together, with a separator placed between the positive and negative electrodes. Another example of the electrode assembly is formed by stacking the positive electrode, negative electrode, and separator together, with a separator placed between the positive and negative electrodes. The shape of the secondary battery (S) is not limited and can be cylindrical, square, coin-shaped, button-shaped, laminated, etc.

[0104] There is no particular limitation on the manufacturing method of the secondary battery (S). A known manufacturing method can be used, or at least a part of a known manufacturing method can be modified.

[0105] Examples of embodiments of this disclosure will be specifically described below with reference to the accompanying drawings. The aforementioned constituent elements can be used as constituent elements in the examples described below. Furthermore, the examples described below can be modified based on the above description. Additionally, the matters described below can be used in the above embodiments. Furthermore, in the embodiments described below, constituent elements that are not essential to the secondary battery of this disclosure can be omitted.

[0106] Figure 1 A simplified perspective view of a square non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut off. Figure 1 The secondary battery 1 shown includes: a square-shaped battery casing 11 with a base, an electrode assembly 10 housed within the battery casing 11, and a non-aqueous electrolyte (not shown). The electrode assembly 10 includes: an elongated strip-shaped negative electrode, an elongated strip-shaped positive electrode, and a separator sandwiched between them to prevent direct contact. The electrode assembly 10 is formed by winding the negative electrode, positive electrode, and separator around a flat core as a center and then pulling out the core. As described above, the positive electrode includes the first layer of this disclosure. The first layer includes a positive electrode active material, a flame retardant (R), and carbon nanotubes.

[0107] One end of the negative electrode lead 15 is mounted to a negative current collector at the negative electrode via welding or the like. One end of the positive electrode lead 14 is mounted to a positive current collector at the positive electrode via welding or the like. The other end of the negative electrode lead 15 is electrically connected to a negative terminal 13 disposed on the sealing plate 12. A gasket 16 is disposed between the sealing plate 12 and the negative terminal 13 to insulate them. The other end of the positive electrode lead 14 is connected to the sealing plate 12 and electrically connected to the battery housing 11, which also serves as a positive terminal. A resin frame 18 is disposed on the upper part of the electrode assembly 10. The frame 18 serves to separate the electrode assembly 10 from the sealing plate 12 and also separates the negative electrode lead 15 from the battery housing 11. The opening of the battery housing 11 is sealed by the sealing plate 12. An injection hole 17a is formed on the sealing plate 12. Electrolyte is injected into the battery housing 11 through the injection hole 17a. Afterward, the injection hole 17a is blocked by a sealing plug 17.

[0108] Figure 2 This is a cross-sectional view showing an example of the configuration of the positive electrode 3 of a secondary battery constituting an embodiment of the present disclosure. A positive electrode active material layer (second layer) 31 is disposed on the surface of the 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 the first layer. Figure 2 An example is a flame retardant layer 32 formed in such a way that it covers the entire surface of the positive electrode active material layer 31.

[0109] Example

[0110] The secondary battery of this disclosure will be further described in detail with reference to embodiments.

[0111] (Example 1)

[0112] In Example 1, multiple secondary batteries were fabricated and evaluated. The secondary batteries were fabricated according to the following steps.

[0113] [Making the negative electrode]

[0114] In the negative electrode active material, a mixture of silicon composite material and graphite in a mass ratio of 5:95 is used. The negative electrode active material is mixed with sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and water in a specified mass ratio to prepare a negative electrode slurry. Next, the negative electrode slurry is coated onto the surface of a copper foil (negative electrode current collector) to form a coating film. After the coating film is dried, it is calendered to form negative electrode active material layers on both sides of the copper foil.

[0115] [The production of the positive electrode]

[0116] LiNi is used as the positive electrode active material. 0.88 Co 0.09 Al 0.03 O2. A positive electrode slurry is prepared by mixing the positive electrode active material, polyvinylidene fluoride, N-methyl-2-pyrrolidone (NMP), and, as needed, a flame retardant, acetylene black, and carbon nanotubes (CNTs) in a specified mass ratio. The carbon nanotubes used have an average diameter of approximately 1.5 nm and a length of approximately 1 μm to 5 μm.

[0117] Next, a positive electrode paste is coated onto the surface of the aluminum foil (positive current collector) to form a coating film. After the coating film dries, it is rolled to form the first layer on both sides of the aluminum foil.

[0118] [Preparation of Electrolyte]

[0119] An electrolyte 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 was set to 1.0 mol / L.

[0120] [Making a Second-hand Battery]

[0121] Lead tabs are attached to each electrode. Then, with the leads at the outermost periphery, the positive and negative electrodes are wound in a spiral pattern over a separator. This forms the electrode assembly. Next, the electrode assembly is embedded in a laminated film casing with aluminum foil as a barrier layer and vacuum dried. Then, electrolyte is injected into the casing, and the opening of the casing is sealed. This yields a secondary battery.

[0122] In this embodiment, multiple secondary batteries (batteries A1-A8, C1-C3) are fabricated by varying the type of flame retardant used in the first layer and the ratio of substances in the first layer. Specifically, the ratio of positive electrode active material, flame retardant, acetylene black, and carbon nanotubes in the positive electrode active material layer is changed. Their ratios vary depending on the mixing ratio during the preparation of the positive electrode slurry. These ratios are shown in Table 1, described later. The flame retardant used is 1,2-bis(pentabromophenyl)ethane or ethylenebistetrabromophthalimide.

[0123] It should be noted that the first layer of each battery is formed in such a way that the thickness of the first layer is the same. 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.

[0124] The following evaluation is made of the secondary battery produced.

[0125] (1) Determination of initial discharge capacity and capacity maintenance rate

[0126] The discharge capacity of the fabricated secondary battery was determined using the following method. First, the battery was charged at a constant current of 40mA at 25°C until the battery voltage reached 4.2V. Then, it was charged continuously at a constant voltage until the current reached 10mA. After the battery was left to stand for 20 minutes, it was discharged at a constant current of 60mA until the battery voltage reached 2.5V. This process was then repeated for another 20 minutes. This charge-discharge cycle was repeated 100 times.

[0127] The discharge capacity DC0 at the initial discharge and the discharge capacity DC1 after 100 repeated charge-discharge cycles were measured. Then, the capacity retention rate was calculated according to the following formula.

[0128] Capacity maintenance rate (%) = 100 × DC1 / DC0

[0129] (2) Piercing test

[0130] For the manufactured secondary battery, perform a nail puncture test according to the following steps.

[0131] (a) At 25°C, charge the battery with a constant current of 60mA until the battery voltage reaches 4.2V, and then charge it with a constant voltage until the current reaches 10mA.

[0132] (b) At 25°C, the tip of a round nail (2.7 mm in diameter) is brought into contact with the center of the charged battery from (a). Then, the round nail is pierced along the stacking direction of the battery's electrode assembly. The round nail is pierced at a speed of 1 mm / s. The piercing is stopped just after a drop in battery voltage caused by an internal short circuit is detected.

[0133] (c) After the battery is short-circuited by the round nail, the short-circuit current value I and the battery voltage V are measured for 1 second. Then, the product of the current value I and the voltage V (electricity) is accumulated over time to calculate the heat released in 1 second.

[0134] A portion of the battery manufacturing conditions and evaluation results are shown in Table 1. Quantities a, b, and c in Table 1 are values ​​representing the mass ratio of positive electrode active material: flame retardant: AB: CNT = 100: a: b: c, indicating the mass ratio of positive electrode active material, flame retardant, acetylene black (AB), and carbon nanotubes (CNT) in the first layer.

[0135] (*1) In Tables 1 and 2, flame retardant R1 represents 1,2-bis(pentabromophenyl)ethane.

[0136] (*2) In Tables 1 and 2, flame retardant R2 represents ethylene bis(tetrabromophthalimide).

[0137] [Table 1]

[0138]

[0139] Table 1 shows that the initial discharge capacity and capacity retention rate are preferably high, and the heat release 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 the flame retardant (R) and carbon nanotubes. Compared with battery C1, batteries A1 to A8 have higher initial discharge capacity and lower heat release. If batteries A2, A4, and C2 with the same type and amount of flame retardant (R) are compared, batteries A2 and A4 have higher initial discharge capacity, lower heat release, and higher capacity retention rate than battery C2. Thus, according to this embodiment, a battery that can balance high capacity and safety is obtained.

[0140] The positive electrode active material layer (first layer) of battery A2 has a structure in which the acetylene black of the positive electrode active material layer (first layer) of battery A4 is replaced with carbon nanotubes. The heat release of battery A2 is lower than that of battery A4. Similarly, the positive electrode active material layer (first layer) of battery A6 has a structure in which the acetylene black of the positive electrode active material layer (first layer) of battery A8 is replaced with carbon nanotubes. The heat release of battery A6 is lower than that of battery A8. The carbon nanotubes are arranged in a network on the surface of the positive electrode active material. It is believed that since battery A2 contains more carbon nanotubes than battery A4, in battery A2, compared with battery A4, the carbon nanotubes form a network of conductive mesh on the surface of the positive electrode active material. In battery A2, with the network arrangement of carbon nanotubes, the distribution of flame retardant is also more uniform. It is believed that by further homogenizing the distribution of flame retardant, the heat release of battery A2 is suppressed compared with battery A4. The reason why battery A6 has a lower heat release than battery A8 is also believed to be that the distribution of flame retardant is more uniform due to the network distribution of carbon nanotubes on the surface of the positive electrode active material.

[0141] Furthermore, the capacity retention rates of batteries A1 to A8 are equal to or better than those of batteries C1 to C3. Carbon nanotubes have a large aspect ratio and excellent conductivity. By placing such carbon nanotubes between the particles of the positive electrode active material, potential fluctuations between the particles are reduced, and unevenness in the charge-discharge reaction is suppressed. In addition, the large aspect ratio of carbon nanotubes occupies a small volume within the positive electrode active material layer. Therefore, the reduction of electrolyte fluid flow caused by carbon nanotubes can also be suppressed. Furthermore, since carbon nanotubes are fibrous, even when the positive electrode active material is densely arranged within the positive electrode active material layer, the interstitial space of the electrolyte is easily maintained. Therefore, it is believed that the addition of carbon nanotubes improves the capacity retention rate.

[0142] On the other hand, when comparing batteries A1, A2, A5, and A6 with battery C3, although the amount of carbon nanotubes was the same, the capacity retention rates of batteries A1, A2, A5, and A6 were higher than that of battery C3. The reason for this is not yet clear, but it may be due to the synergistic effect of adding flame retardant (R) and carbon nanotubes. The halogen-containing flame retardant (R) is a low dielectric constant material with good wettability to electrolyte components (such as chain carbonates). Therefore, it is believed that the addition of flame retardant (R) improves the fluidity of the electrolyte. This improved fluidity is considered one of the factors contributing to the improved capacity retention rate.

[0143] Furthermore, both the flame retardant (R) and carbon nanotubes exhibit poor dispersibility, thus the uniformity of the positive electrode active material layer tends to decrease when they are added to the positive electrode slurry alone. On the other hand, when both are added to the positive electrode slurry, although the reason is unclear, they sometimes disperse easily. Therefore, using both of them can shorten the time required to prepare the positive electrode slurry and make it easier to produce a highly uniform positive electrode active material layer. One reason why batteries A1 to A8 exhibit good characteristics is that the uniformity of the positive electrode active material layer (first layer) may be improved by using both the flame retardant (R) and carbon nanotubes. For example, by using both, the dispersibility of the flame retardant (R) is improved, thereby potentially achieving a high flame retardant effect.

[0144] (Example 2)

[0145] In Example 2, multiple secondary batteries were fabricated and evaluated. In Example 2, the amount of flame retardant was increased; otherwise, multiple secondary batteries were fabricated under the same conditions and methods as those fabricated in Example 1. The fabricated batteries were subjected to nail penetration tests as described above. A portion of the fabrication conditions and the heat release during the nail penetration tests are shown in Table 2.

[0146] [Table 2]

[0147]

[0148] As shown in Tables 1 and 2, a higher amount of flame retardant results in lower heat release. On the other hand, a higher amount of flame retardant can sometimes lead to lower initial discharge capacity and capacity retention.

[0149] (Example 3)

[0150] In the fabrication of the positive electrode, LiNi is used as the positive electrode active material. 0.88 Co 0.09 Al 0.03 O2 is used to prepare the positive electrode slurry by mixing the positive electrode active material, polyvinylidene fluoride (PVdF), N-methyl-2-pyrrolidone (NMP), acetylene black (AB), and carbon nanotubes (CNTs) as needed in a specified mass ratio. The carbon nanotubes used have an average diameter of approximately 1.5 nm and a length of approximately 1 μm to 5 μm.

[0151] Next, a positive electrode paste is coated onto the surface of the aluminum foil (positive current collector) to form a coating film. After the coating film dries, it is calendered to form a second layer, which serves as the positive electrode active material layer, on both sides of the aluminum foil.

[0152] Then, a slurry for the third layer is prepared by mixing flame retardant (R) with polyvinylidene fluoride (PVdF), N-methyl-2-pyrrolidone (NMP), and alumina particles (Al2O3) as needed in a specified mass ratio. The resulting slurry is coated onto the surface of the second layer and dried to form the third layer as a flame retardant layer. Thus, a first layer having the second and third layers is formed on the surface of the positive electrode current collector.

[0153] In addition, multiple secondary batteries were manufactured in the same manner as in Examples 1 and 2, and the following evaluations were conducted.

[0154] In this embodiment, multiple secondary batteries (batteries A9-A13, C4, C5) are fabricated by varying the ratios of the substances in the second layer, the type of flame retardant contained in the third layer, and the ratios of the substances in the third layer. Specifically, the ratios of the positive electrode active material, flame retardant, acetylene black, and carbon nanotubes in the second layer are varied. These ratios vary depending on the mixing ratio of these substances during the preparation of the positive electrode slurry. Furthermore, the ratio of the flame retardant (R) to the binder (PVdF) in the third layer is changed by varying the mixing ratio of these substances during the preparation of the slurry for the third layer. A portion of these ratios are shown in Table 3, which is described later. The types of flame retardants are as described later.

[0155] (1) Battery resistance

[0156] At 25°C, the battery was charged with a constant current of 40mA until the battery voltage reached 4.2V. Then, the battery was charged continuously with a constant voltage until the current reached 10mA. The charged battery was then connected to a testing instrument to measure its internal resistance.

[0157] (2) Piercing test

[0158] For the manufactured secondary battery, the battery temperature after the nail puncture test is measured according to the following steps.

[0159] (a) At 25°C, charge the battery at a constant current of 0.5C until the battery voltage becomes 4.2V, and then charge it at a constant voltage until the current value becomes 0.02C.

[0160] (b) At 25°C, the tip of a round nail (2.7 mm in diameter) was brought into contact with the center of the charged battery in (a) and pierced at a speed of 1 mm / s. The piercing was stopped just after a voltage drop caused by an internal short circuit was detected. Then, the battery was short-circuited, and the surface temperature of the battery was measured after 1 minute.

[0161] A portion of the battery manufacturing conditions are shown in Table 3, and the evaluation results are shown in Table 4. The flame retardant layer ratios in Table 3 represent the content of flame retardant and binder (PVdF) in the slurry used for the flame retardant layer. In Table 3, flame retardant r1 represents 1,2-bis(pentabromophenyl)ethane (SAYTEX (registered trademark)-8010 manufactured by Albemarle Japan Co., Ltd.). Flame retardant r2 represents ethylene bis(tetraphthalimide).

[0162] [Table 3]

[0163]

[0164] [Table 4]

[0165]

[0166] According to Tables 3 and 4, when comparing batteries C4 and C5, battery C4, which does not have carbon nanotubes added to the second layer (the positive electrode active material layer), exhibits higher battery resistance. Conversely, by adding carbon nanotubes to the second layer of battery C5, the battery resistance can be reduced, 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 that of battery C4 without added carbon nanotubes.

[0167] However, in batteries A9 to A13, where carbon nanotubes are added to the second layer (serving as the positive electrode active material) and a third layer containing the aforementioned flame retardant (R) is provided on the surface of the second layer, battery resistance can be reduced and temperature rise after a nail puncture test can be suppressed. According to Table 3, a relatively thin film thickness of approximately 3 μm for the third layer can achieve a sufficient effect in suppressing temperature rise.

[0168] Battery A12 is equivalent to reducing the content of flame retardant (R) by replacing a portion of the flame retardant (R) contained in the third layer of battery A9 with alumina particles. It is believed that in this case, the alumina particles function as spacers, increasing the gaps where lithium ions can move, and suppressing the amount of flame retardant added. Therefore, compared with battery A9, the increase in battery resistance is suppressed.

[0169] Industrial availability

[0170] This disclosure can be used for secondary batteries.

[0171] Although the invention has been described in conjunction with presently preferred embodiments, such disclosure is not intended to be limiting. Various modifications and alterations will be readily apparent to those skilled in the art from the foregoing disclosure. Therefore, the appended claims should be construed as encompassing all modifications and alterations without departing from the spirit and scope of the invention.

[0172] Explanation of reference numerals in the attached figures

[0173] 1: Secondary battery; 3: Positive electrode; 10: Electrode assembly; 11: Battery casing; 12: Sealing plate; 13: Negative 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 comprising a positive electrode and a negative electrode, the positive electrode contains a first layer containing a positive electrode active material, the first layer further contains a halogen atom-containing flame retardant and a carbon nanotube, the first layer contains: a second layer containing at least the positive electrode active material and the carbon nanotube; and a third layer located further on a surface side of the positive electrode than the second layer and containing at least the flame retardant, a content ratio of the flame retardant in the entire second layer is 0.1 mass% or more and 5 mass% or less, and a content ratio of the flame retardant in the entire third layer is 50 mass% or more and 100 mass% or less, the halogen atom is bromine.

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

3. The secondary battery according to claim 1 or 2, wherein the flame retardant releases the halogen atom at a temperature of 180°C or higher.

4. The secondary battery according to claim 1, wherein the flame retardant is at least one selected from the group consisting of 1,2-bis(pentabromophenyl)ethane, ethylenebistetrabromophthalimide, tetrabromobisphenol A, hexabromocyclododecane, and 2,4,6-tribromophenol.

5. The secondary battery according to any one of claims 1 to 4, wherein when a mass ratio of the positive electrode active material to the flame retardant in the first layer is represented by the positive electrode active material: the flame retardant = 100: a, the a is greater than 0 and lower than 7.

6. The secondary battery according to any one of claims 1 to 5, wherein the first layer contains acetylene black, when a mass ratio of the positive electrode active material to the acetylene black to the carbon nanotube in the first layer is represented by the positive electrode active material: the acetylene black: the carbon nanotube = 100: b: c, the b and the c satisfy 0 < b < 3, b + c < 5.

7. The secondary battery according to any one of claims 1 to 6, wherein the third layer is disposed on a surface of the second layer.

8. The secondary battery according to any one of claims 1 to 7, wherein a content ratio of the carbon nanotube in the second layer is 0.01 mass% or more and 10 mass% or less.

9. The secondary battery according to any one of claims 1 to 8, wherein a thickness of the third layer is 0.1 μm or more and 10 μm or less.

10. A secondary battery comprising a positive electrode and a negative electrode, the positive electrode contains a first layer containing a positive electrode active material, the first layer contains: a second layer containing at least the positive electrode active material and a halogen atom-containing flame retardant; and a third layer located further on a surface side of the positive electrode than the second layer and containing at least the flame retardant, a content ratio of the flame retardant in the third layer is greater than a content ratio of the flame retardant in the second layer, a content ratio of the flame retardant in the entire third layer is 50 mass% or more and 100 mass% or less, a content ratio of the flame retardant in the entire second layer is 0.1 mass% or more and 5 mass% or less.

11. The secondary battery according to claim 10, wherein the flame retardant contains a ring structure to which the halogen atom is bonded, a ratio of the halogen atom 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 atom at a temperature of 180°C or higher.

13. The secondary battery according to claim 10, wherein The flame retardant is at least one selected from the group consisting of 1,2-bis(pentabromophenyl)ethane, ethylene-bistetra-bromophthalimide, tetrabromobisphenol 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 phosphoric acid tris(2,2,2-trifluoroethyl) ester.

14. The secondary battery according to any one of claims 10 to 13, wherein When the mass ratio of the positive electrode active material to the flame retardant in the first layer is represented by the positive electrode active material: the flame retardant = 100: a, the a is greater than 0 and lower than 7.

15. The secondary battery according to any one of claims 10 to 14, wherein The third layer is disposed on a surface of the second layer.

16. The secondary battery according to any one of claims 10 to 15, wherein The thickness of the third layer is 0.1 μm or more and 10 μm or less.

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