Secondary battery

Abstract

The disclosed secondary battery includes a positive electrode and a negative electrode. The negative electrode contains a first layer containing at least a negative electrode active material layer. The first layer also contains a flame retardant containing a halogen atom.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a secondary battery. BACKGROUND

[0002] A nonaqueous electrolyte secondary battery such as a lithium ion secondary battery has high power and high energy density. Therefore, the nonaqueous electrolyte secondary battery is utilized as a small-sized civilian use, a power storage device, and a power source for an electric automobile.

[0003] Various substances have been proposed as a negative electrode active material of a nonaqueous electrolyte secondary battery. As a negative electrode active material having high energy density, a silicon compound (for example, silicon oxide) alloyed with lithium, a silicon particle has been proposed. For example, Patent Literature 1 discloses, "A nonaqueous secondary battery, a positive electrode having a positive electrode mixture layer containing a specific Li-containing transition metal oxide having Ni, Mn, or the like as an essential constituent element, and a negative electrode having a negative electrode mixture layer containing a material containing Si and O among constituent elements (atomic ratio x of O to Si is 0.5≤x≤1.5) and graphite, and in the negative electrode mixture layer, when the total of the material containing Si and O among the constituent elements and the graphite is set to 100 mass%, the ratio of the material containing Si and O among the constituent elements is 3 to 20 mass%".

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2010-212228 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] In recent years, the demand for high energy density of a nonaqueous electrolyte secondary battery has been increasing. However, in the case where the energy density of the nonaqueous electrolyte secondary battery is improved, a safety countermeasure of the battery at the time of abnormality at a high level is required.

[0009] SOLUTION TO PROBLEM

[0010] An aspect of the present disclosure relates to a secondary battery. The secondary battery includes a positive electrode and a negative electrode, and the negative electrode contains a first layer containing a negative electrode active material, and the first layer further contains a flame retardant containing a halogen atom.

[0011] EFFECT OF THE INVENTION

[0012] According to the present disclosure, a secondary battery having high safety can be achieved.

[0013] The novel features of the application are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present application will be obtained by reference to the following detailed description that sets forth illustrative BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 A schematic cross-sectional view showing an example of a configuration of a negative electrode of a secondary battery according to an embodiment of the present disclosure.

[0015] Figure 2 A perspective view showing a part of a secondary battery according to an embodiment of the present disclosure cut away. DETAILED DESCRIPTION

[0016] Hereinafter, examples of embodiments of the present disclosure will be described. Note that in the following description of the embodiments of the present disclosure, examples will be described, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, and the like will be shown as examples, but other numerical values, other materials, and the like can also be used as long as the effects of the present disclosure can be obtained. In this specification, in the case where a range of values is referred to as "a range of value A to value B", value A and value B are included in the range.

[0017] (Secondary battery)

[0018] The secondary battery of the present embodiment includes a positive electrode and a negative electrode. The negative electrode contains a first layer containing a negative electrode active material. The first layer is typically provided on the surface of a negative electrode current collector. The first layer contains a flame retardant containing a halogen atom in addition to the negative electrode active material. The first layer can further contain carbon nanotubes. The flame retardant and the halogen atom will be sometimes referred to as "flame retardant (R)" and "halogen atom (Ha)", respectively, hereinafter. In addition, the secondary battery of the present embodiment will be sometimes referred to as "secondary battery (S)" hereinafter. The secondary battery (S) can be a nonaqueous electrolyte secondary battery.

[0019] In one embodiment, the negative electrode active material can contain particles (P) and graphite. Here, the particles (P) are at least one particle selected from the group consisting of a first particle containing silicon oxide represented by the formula SiO X (0.5≤X<1.6), a second particle containing a lithium silicate phase and silicon particles dispersed in the lithium silicate phase, and a third particle containing a carbon phase and silicon particles dispersed in the carbon phase. Note that in this specification, the silicon particles contained in the second particle can be alternatively referred to as a silicon phase, and the silicon particles contained in the third particle can be alternatively referred to as a silicon phase.

[0020] By using the particle (P) containing silicon (Si) as the negative electrode active material, it is possible to increase the capacity of the battery. On the other hand, as described in the examples, the present inventors and others have found that, in the case of using the particle (P), the temperature of the battery tends to increase at the time of an abnormality (for example, at the time of a nail penetration test). Furthermore, the present inventors and others have found that, by using a specific flame retardant, it is possible to suppress the increase in the temperature of the battery at the time of an abnormality without greatly reducing the characteristics of the battery. The present disclosure is based on these new insights.

[0021] The lithium silicate phase of the 2nd particle can include a lithium silicate of the formula Li 2Z SiO (2+Z) (0 < Z < 2). Details of the 2nd particle are described later.

[0022] The content ratio of the particle (P) in the negative electrode active material can be 1 mass% or more. According to this configuration, it is possible to achieve higher capacity compared to the case where the negative electrode active material is only graphite. The content ratio of the particle (P) in the negative electrode active material can be 3 mass% or more. The content ratio can be 50 mass% or less. These lower limits and upper limits can be arbitrarily combined as long as there is no contradiction.

[0023] The negative electrode active material can include a plurality of particles selected from the group consisting of the 1st particle, the 2nd particle, and the 3rd particle. For example, the particle (P) can be composed of two particles selected from among them, or can include all of the three particles. Specifically, the negative electrode active material can include the 1st particle and the 2nd particle, can include the 1st particle and the 3rd particle, or can include the 2nd particle and the 3rd particle. Alternatively, the negative electrode active material can include all of the 1st, 2nd, and 3rd particles.

[0024] The content ratio of graphite in the negative electrode active material can be in the range of 50 to 99 mass%. Note that, in the case where the particle (P) contains graphite on the surface and / or inside, this graphite is not included in the above-mentioned content ratio of graphite. The content ratio of graphite is the content ratio of graphite that is not contained in the particle (P).

[0025] When the mass ratio of the negative electrode active material to the flame retardant (R) in the 1st layer is represented by negative electrode active material:flame retardant (R)=100:a, a can be greater than 0 and less than 15. According to this configuration, it is possible to improve safety without greatly reducing the capacity of the battery. The value of a can be 0.1 or more, 0.5 or more, or 1 or more. The value of a can be 10 or less, 5 or less, less than 5, or 3 or less. These lower limits and upper limits can be arbitrarily combined as long as there is no contradiction. For example, the value of a can be in the range of 1 or more and 5 or less (or 1 or more and less than 5), and is more preferably in the range of 1 or more and 3 or less. In this case, the heat release is suppressed and it is possible to maintain high capacity, and it is possible to achieve a balance between high charge / discharge performance and high safety.

[0026] The contained ratio of the negative electrode active material in the first layer is found from a sample obtained by taking out only the negative electrode active material layer from the secondary battery in the discharged state. Specifically, first, the secondary battery in the discharged state is disassembled and the negative electrode is taken out. Next, the negative electrode is washed with an organic solvent, and further vacuum dried, and only the negative electrode active material layer is peeled off, thereby obtaining a sample. For the sample, thermal analysis such as TG-DTA is performed, and thereby the ratio of the binder component and the conductive material component other than the negative electrode active material can be calculated. In addition, the ratio of the flame retardant (R) contained in the negative electrode active material layer can be found by elemental analysis such as SEM-EDX (Energy Dispersive X-ray Spectroscopy) of the cross section of the negative electrode active material layer.

[0027] In one embodiment of the present disclosure, the flame retardant (R) can be unevenly present in the surface side of the first layer. In this case, the first layer contains, for example, a second layer containing at least the negative electrode active material, and a third layer disposed on the surface of the second layer and containing at least the flame retardant (R). The contained ratio of the flame retardant in the third layer is greater than the contained ratio of the flame retardant in the second layer. Here, the contained ratio of the flame retardant refers to the number of moles of the flame retardant contained in the unit volume (apparent volume) of the second layer or the third layer, and by performing elemental analysis such as SEM-EDX on the cross section of the first layer (the second layer and the third layer), the depth direction distribution of the flame retardant is found, and thereby it can be determined whether the flame retardant is unevenly present in the second layer side. In one embodiment, the second layer is a negative electrode active material layer (negative electrode mixture layer) containing at least the negative electrode active material, and the third layer can be a flame retardant layer containing at least the flame retardant (R). As the negative electrode active material layer of the second layer, carbon nanotubes can be contained as a conductive material.

[0028] The negative electrode active material contained in the second layer of the negative electrode active material layer preferably satisfies at least any one of the following conditions (i) and (ii).

[0029] Condition (i):

[0030] The negative electrode active material contains particles (P) and graphite. The particles (P) are at least one particle selected from the group consisting of the above-mentioned first particles, second particles, and third particles.

[0031] Condition (ii):

[0032] The negative electrode active material contains metallic lithium.

[0033] In the case where the condition (i) is satisfied, by using the particle (P) containing silicon (Si) as the negative electrode active material, it is possible to increase the capacity of the battery. On the other hand, in the case where the particle (P) is used, the temperature of the battery tends to increase at the time of abnormality (for example, at the time of a nail penetration test). Thus, it is an important subject to suppress the increase in the temperature of the battery at the time of abnormality.

[0034] In the case where the condition (ii) is satisfied, as with the condition (i), it is possible to achieve high capacity of the battery. However, in this case, lithium metal is deposited in the negative electrode at the time of charging, and thus, a safety countermeasure at the time of abnormality is required at a high level.

[0035] According to this embodiment, by disposing the flame retardant layer provided with a specific flame retardant on the surface of the negative electrode active material layer, it is possible to suppress the increase in the temperature of the battery at the time of abnormality without greatly reducing the characteristics of the battery.

[0036] The negative electrode active material can contain a plurality of particles selected from the group consisting of the first particle, the second particle, and the third particle. For example, the particle (P) can be composed of two particles selected from among them, or can contain all of the three particles. Specifically, the negative electrode active material can contain the first particle and the second particle, or can contain the first particle and the third particle, or can contain the second particle and the third particle. Alternatively, the negative electrode active material can contain all of the first, second, and third particles.

[0037] The content ratio of the particle (P) in the negative electrode active material can be 1% by mass or more. According to this configuration, it is possible to achieve higher capacity compared to the case where the negative electrode active material is only graphite. The content ratio of the particle (P) in the negative electrode active material can be 3% by mass or more. The content ratio can be 50% by mass or less. These lower limits and upper limits can be arbitrarily combined as long as there is no contradiction.

[0038] In the case where the particle (P) contains a plurality of particles selected from the group consisting of the first particle, the second particle, and the third particle, the content ratio of at least one of the plurality of particles in the negative electrode active material can be 1% by mass or more.

[0039] The third layer as the flame retardant layer contains a flame retardant (R) containing a halogen atom (Ha). By the negative electrode having the third layer, it is possible to suppress excessive heat release at the time of abnormality. In addition, the flame retardant (R) does not have electronic conductivity, and thus, in the secondary battery, the third layer functions as a resistance layer that suppresses short circuit in the case where a short circuit is likely to occur inside the battery by being interposed between the negative electrode active material layer and the separator. Thus, it is possible to effectively suppress heat release.

[0040] The third layer is preferably disposed on the surface of the second layer in such a manner as to be in contact with the surface of the negative electrode active material layer of the second layer as the negative electrode and cover at least a part of the negative electrode active material layer.

[0041] The third layer can contain a binder in addition to the flame retardant (R). The third layer can improve the adhesion of the particles of the flame retardant (R) to each other and to the second layer, which is the negative active material layer of the flame retardant (R), by containing the binder. That is, the third layer can be tightly adhered to the second layer. The binder is not particularly limited, and examples thereof include polyvinylidene fluoride (PVdF), ethylene dimethacrylate, allyl methacrylate, tert-dodecyl mercaptan, α-methylstyrene dimer, and methacrylic acid. Note that, in the case where polyvinylidene fluoride (PVdF), ethylene dimethacrylate, allyl methacrylate, tert-dodecyl mercaptan, α-methylstyrene dimer, or methacrylic acid is used as the binder, the negative electrode and the separator can be bonded by applying pressure and / or heat to the third layer.

[0042] The third layer can contain other particles in addition to the flame retardant (R) and the binder. Examples of the other particles include inorganic particles containing metal oxides such as alumina, boehmite, and titanium dioxide. The inorganic particles containing metal oxides function as spacers and can suppress the amount of the flame retardant to be added. The average particle diameter of the inorganic particles is preferably 0.01 μm or more and 5 μm or less, and more preferably 1 / 2 or less of the average particle diameter of the flame retardant (R).

[0043] In the third layer, the flame retardant (R) can exist in the form of an aggregate in which the particles of the flame retardant (R) are aggregated with each other or an aggregate in which the particles of the flame retardant (R) are aggregated with each other with the aid of the binder. The flame retardant layer (R) can partially cover the surface of the second layer, or the third layer can cover substantially the entire surface of the negative active material layer. The coverage ratio (area basis) of the third layer with respect to the surface of the second layer can be 5% or more, 10% or more, or 30% or more, and preferably 50% or more, in terms of suppressing the increase in the temperature of the battery at the time of abnormality.

[0044] Note that, in the case where the coverage ratio of the third layer with respect to the surface of the second layer is 100% and the surface of the second layer is completely covered with the third layer, the gap between the particles of the third layer is sufficiently large compared with the size of lithium ions, and thus the lithium ions can move through the gap without hindering charge and discharge. However, from the viewpoint of suppressing the increase in the resistance of the battery, the coverage ratio of the third layer with respect to the surface of the second layer can be 90% or less or 80% or less.

[0045] The coverage ratio of the third layer with respect to the surface of the second layer can be 5% or more and 90% or less, 10% or more and 90% or less, 30% or more and 90% or less, 50% or more and 90% or less, or 50% or more and 80% or less.

[0046] The coverage of the third layer is obtained by element mapping of the electrode surface by SEM-EDX or the like. For example, according to the element mapping, the particles of the flame retardant (R) and the negative electrode active material are element-mapped, and thus the coverage of the third layer with respect to the surface of the second layer can be calculated.

[0047] The average particle diameter of the particles of the flame retardant (R) in the third layer (when constituting an aggregate, the average particle diameter of the primary particles constituting the aggregate) can be 0.01 μm to 5 μm, and can be 0.05 μm to 3 μm. The average particle diameter of the flame retardant (R) is obtained as follows. First, 20 particles of the flame retardant (R) are randomly selected from the SEM image of the negative electrode surface. Next, the grain boundaries of the 20 selected particles are observed, and the outline of each particle is specified, and the average of the major axis of the 20 particles is obtained as the average particle diameter of the particles of the flame retardant (R). In the case where the third layer contains other particles than the flame retardant (R), the average particle diameter of the other particles is also obtained in the same manner.

[0048] In terms of suppressing the increase in the temperature of the battery at the time of abnormality, the weight per unit area of the third layer is preferably 0.1 g / m 2 More preferably, 0.3 g / m 2 or 1 g / m 2 or more. In terms of suppressing the increase in the resistance of the battery, the weight per unit area of the third layer is preferably 10 g / m 2 or more. These lower limits and upper limits can be arbitrarily combined as long as there is no contradiction. The weight per unit area of the third layer is a value obtained by dividing the mass (g) of the third layer by the area of the surface of the second layer (negative electrode active material layer) on which the third layer is disposed (when the coverage is less than 100%, the area including the region exposed by the second layer).

[0049] The third layer can be formed by depositing a mixture containing at least the third layer particles and a binder on the surface of the negative electrode active material layer. The mixture can be a slurry containing the particles of the flame retardant (R), the binder, and a solvent (dispersion medium). The third layer can be formed by spraying, dropping, or coating the slurry on the surface of the negative electrode active material layer and drying. The coverage and the weight per unit area (thickness) of the third layer can be controlled by adjusting the amount of the solvent in the slurry with respect to the amount of the particles of the flame retardant (R) and / or the coating amount of the slurry, and the like.

[0050] In the third layer, the ratio of the flame retardant (R) in the third layer as a whole can be 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more on a mass basis. The ratio of the flame retardant (R) in the third layer as a whole can be 100% or less, or 95% or less on a mass basis. These lower limits and upper limits can be arbitrarily combined as long as there is no contradiction. The ratio of the flame retardant (R) in the third layer can be found by elemental analysis such as SEM-EDX of the cross section of the flame retardant layer.

[0051] In terms of suppressing the rise in the battery temperature at the time of abnormality, 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 terms of suppressing the rise in the battery resistance, the thickness of the third layer is preferably 10 μm or less. These lower limits and upper limits can be arbitrarily combined 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 (negative electrode active material layer) is covered with the third layer, and is found from the SEM image of the cross section of the negative electrode.

[0052] The second layer containing at least the negative electrode active material can further contain carbon nanotubes. In one embodiment, the first layer includes: a second layer containing at least the negative electrode active material and the carbon nanotubes; and a third layer containing at least the flame retardant, the third layer being disposed between the second layer and the separator interposed between the positive electrode and the negative electrode. According to this embodiment, by disposing the flame retardant layer in which the specific flame retardant is disposed between the separator and the negative electrode active material layer, it is possible to suppress the rise in the battery temperature at the time of abnormality without greatly reducing the battery characteristics.

[0053] [Flame Retardant (R)]

[0054] The flame retardant (R) exhibits a flame retardant effect by releasing halogen atoms (Ha) at high temperatures. Therefore, according to the secondary battery (S), it is possible to suppress the excessive heat release at the time of abnormality.

[0055] The flame retardant (R) can satisfy at least one of the following conditions (1) and (2). However, the flame retardant (R) preferably satisfies both of the following conditions (1) and (2).

[0056] (1) The flame retardant (R) contains a ring structure to which halogen atoms (Ha) are bonded. The ring structure can be an aromatic ring or can not be an aromatic ring. In this case, all of the halogen atoms (Ha) can be bonded to the ring structure, or only a part of the halogen atoms (Ha) can be bonded to the ring structure. The structure in which the halogen atoms (Ha) are bonded to the ring structure is preferable in terms of easily increasing the content of the halogen atoms.

[0057] (2) The ratio of halogen atoms (Ha) in the flame retardant (R) is 45% by mass or more. The ratio can be 60% by mass or more (for example, 70% by mass or more). There is no particular limitation on the upper limit, and it can be 95% by mass or less (for example, 90% by mass or less). These lower limits and upper limits can be combined arbitrarily.

[0058] The following shows the structural formula of 1,2-bis(pentabromophenyl)ethane as an example of the flame retardant (R). The molecular weight of 1,2-bis(pentabromophenyl)ethane is 971.2, and it contains 10 bromine atoms (atomic weight: 79.9). Therefore, the ratio of halogen atoms (Ha) in 1,2-bis(pentabromophenyl)ethane is 100 x 10 x 79.9 / 971.2 = 82.3% by mass.

[0059]

[0060] There is no particular limitation on the halogen atoms (Ha), and examples of preferred halogen atoms (Ha) include bromine (Br), fluorine (F), and chlorine (Cl). In terms of the initial expected flame retardant effect from abnormal exothermic heat, the halogen atoms (Ha) can be bromine and / or fluorine, or can be bromine.

[0061] It is considered that the exothermic reaction at the time of abnormality is largely affected by the reaction in the initial negative electrode, and by adding a material that exerts a flame retardant effect in the negative electrode, the exothermic reaction can be effectively suppressed, and the safety can be improved. At the initial stage of the exothermic reaction, the negative electrode, which has a larger reaction area than the positive electrode, reacts preferentially with the electrolyte, and thereby H radicals are generated, and the H radicals repeatedly react with other products with acceleration. The flame retardant (R) containing halogen atoms (Ha) added to the negative electrode inactivates the H radicals, and thereby the exothermic reaction can be suppressed.

[0062] In addition, the specific gravity of such a flame retardant (R) containing halogen atoms (Ha) is large compared to the phosphorus-based flame retardants used in the past, and therefore, the volume can be reduced with respect to the added weight. Thus, a sufficient amount of the flame retardant can be added, and the mounting amount of the negative electrode active material can be maintained high, and a high capacity can be maintained. In terms of the large specific gravity, the flame retardant (R) preferably contains bromine (Br). In addition, the more the number of halogen atoms (Ha) bonded to the flame retardant (R), the better. The specific gravity of the flame retardant (R) can be increased easily by bonding halogen atoms (Ha) to the cyclic structure. The specific gravity of the flame retardant (R) can be 2.7 or more, and preferably 3.0 or more, for example.

[0063] The flame retardant (R) preferably does not contain a water generating moiety and / or a hydrophilic group in the structure of the compound. In this case, water is less likely to be mixed into the battery during the manufacturing process of the secondary battery, and a secondary battery with excellent reliability can be achieved. Note that examples of the water generating moiety include a hydroxyl group (-OH), a carboxyl group (-COOH), a carbonyl group (-CO-), and an oxygen-containing acid group such as a sulfo group and a phosphoric acid group. Examples of the hydrophilic group include an amino group, in addition to the above functional groups.

[0064] Further, in the case of using a negative electrode active material containing silicon (Si), the halogen atom (Ha) contained in the flame retardant (R) reacts with Si, and a stable coating film can be formed on the surface of the negative electrode active material. Thus, high cycle characteristics can be maintained, and high durability can be expected.

[0065] The flame retardant (R) can release the halogen atom (Ha) at a temperature of 180°C or higher (for example, 250°C or higher). If the flame retardant releases the halogen atom (Ha) at a relatively low temperature, the halogen atom (Ha) is released in a state that is not abnormal, and the characteristics of the battery sometimes decrease. Therefore, it is preferable that the flame retardant (R) substantially not release the halogen atom (Ha) at a temperature lower than 180°C.

[0066] The flame retardant (R) can 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-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) can use commercially available products. Alternatively, the flame retardant (R) can be synthesized by a publicly known synthesis method.

[0067] [First particle]

[0068] The first particle contains silicon oxide represented by the formula SiO X (0.5≤X<1.6). The first particle can contain a particle of silicon oxide, and a carbon layer disposed around the particle of silicon oxide.

[0069] The average particle diameter of the first particle can be in the range of 1 μm to 25 μm (for example, in the range of 4 μm to 15 μm).

[0070] [Second particle]

[0071] The second particle contains a lithium silicate phase and a silicon particle dispersed in the lithium silicate phase. As described above, the lithium silicate phase can contain lithium silicate represented by the formula Li 2ZSiO (2+Z) The lithium silicate phase can also be composed of the lithium silicate represented by Li2ZSiO4 (0 < Z < 2). Z preferably satisfies the relationship of 0 < Z < 1. The lithium silicate phase can be composed of 50% by mass or more (for example, 60% by mass or more) of the lithium silicate satisfying 0 < Z < 0.5.

[0072] The second particles can contain at least one element Me dispersed in the lithium silicate phase. The at least one element Me is at least one element selected from the group consisting of rare earth elements and alkaline earth metal elements. Examples of the alkaline earth metal elements include Mg, Ca, Sr, Ba, and the like.

[0073] The element Me can be dispersed in the lithium silicate phase as an Me oxide. The Me oxide can contain at least one selected from the group consisting of yttrium oxide, cerium oxide, calcium oxide, and magnesium oxide. The lithium silicate phase can contain zirconium oxide. Furthermore, the element Me can be dispersed in the zirconium oxide.

[0074] The amount of the element Me contained in the second particles can be an amount calculated assuming that the element Me forms an oxide in a stoichiometric amount (estimated Me oxide amount), regardless of the state of the element Me or the kind of the compound of the element Me. The estimated Me oxide amount can be in the range of 0.001% by mass to 1.0% by mass with respect to the total of the lithium silicate phase and the silicon particles. By making the estimated Me oxide amount 0.001% by mass or more, the effect of reducing the reaction area and improving the hardness of the lithium silicate phase becomes greater. On the other hand, by making the estimated Me oxide amount 1.0% by mass or less, the reduction in the initial capacity can be suppressed.

[0075] The lithium silicate phase can contain a metal compound such as a metal oxide, a metal carbide, a metal nitride, a metal boride, or the like. Suitable metal compounds are a metal oxide and a metal carbide. Among them, at least one selected from the group consisting of zirconium oxide (ZrO2), aluminum oxide (Al2O3), zirconium carbide (ZrC), tungsten carbide (WC), and silicon carbide (SiC) is preferably used. The amount of the compound of the metal element other than the element Me can be in the range of 0.005% by mass to 15% by mass (for example, in the range of 0.01% by mass to 10% by mass, in the range of 0.01% by mass to 1% by mass) with respect to the total of the lithium silicate phase and the silicon particles. The amount of the compound of the metal element can be calculated assuming that the metal element forms an oxide in a stoichiometric amount, like the content rate of the element Me.

[0076] The average particle diameter of the second particles can be in the range of 1 μm to 25 μm (for example, in the range of 4 μm to 15 μm). In this range, stress generated by volume change of the second particles accompanying charge and discharge is easily mitigated, and good cycle characteristics are easily obtained. Further, the surface area of the second particles also becomes moderate, and capacity reduction due to side reactions with the nonaqueous electrolyte can also be suppressed.

[0077] The crystallite size of the silicon particles dispersed in the lithium silicate phase is, for example, 10 nm or more. The silicon particles have a phase of particles of silicon (Si) simple substance. When the crystallite size of the silicon particles is 10 nm or more, the surface area of the silicon particles is less likely to increase, and thus degradation of the silicon particles accompanied by generation of irreversible capacity is less likely to occur. The crystallite size of the silicon particles is calculated from the half-value width of the diffraction peak attributed to the Si (111) plane in the X-ray diffraction (XRD) pattern of the silicon particles according to the Scherrer formula.

[0078] The average particle diameter of the silicon particles in the second particles before initial charge can be preferably 500 nm or less (more preferably 200 nm or less, and further preferably 50 nm or less). After initial charge, the average particle diameter of the silicon particles is preferably 400 nm or less (more preferably 100 nm or less). By making the silicon particles fine, the volume change during charge and discharge is reduced, and the structural stability of the second particles is further improved.

[0079] The content ratio of the silicon particles (simple substance Si) in the second particles is preferably in the range of 20 mass% to 95 mass% (for example, in the range of 35 mass% to 75 mass%) from the viewpoint of capacity increase and cycle characteristic improvement. According to this range, the diffusivity of lithium ions also becomes good, and excellent load characteristics are easily obtained. Further, the surface of the silicon particles exposed without being covered by the lithium silicate phase is reduced, and side reactions of the nonaqueous electrolyte with the silicon particles are suppressed.

[0080] The second particles can include an electrically conductive material that covers at least a part of the surface thereof. The lithium silicate phase lacks electron conductivity, and thus the electrical conductivity of the second particles is also likely to be low. By covering the surface with an electrically conductive material, the electrical conductivity can be dramatically improved. The electrically conductive layer is preferably thin to the extent that it does not substantially affect the average particle diameter of the second particles. For example, the thickness of the electrically conductive layer can be in the range of 1 nm to 200 nm (for example, in the range of 5 nm to 100 nm) from the viewpoint of ensuring electrical conductivity and diffusivity of lithium ions. Examples of the material of the electrically conductive layer and examples of the formation method thereof are described later.

[0081] [Third Particles]

[0082] The third particle contains a carbon phase and silicon particles dispersed in the carbon phase. The carbon phase of the third particle can be composed of amorphous carbon (i.e., non-crystalline carbon). The amorphous carbon can be hard carbon, soft carbon, or other. Amorphous carbon (non-crystalline carbon) generally refers to carbon material having an average interplanar spacing d002 of (002) plane of more than 0.34 nm as measured by X-ray diffraction.

[0083] The third particle contains a carbon phase and silicon particles dispersed in the carbon phase. The carbon phase of the third particle has electrical conductivity. Therefore, even if a pore is formed around the third particle, it is easy to maintain the junction of the third particle with its surroundings. As a result, it is easy to suppress the capacity reduction caused by repeated charge and discharge cycles.

[0084] The average particle diameter of the third particle can be 3 μm or more and 18 μm or less, can be 6 μm or more and 15 μm or less, or can be 8 μm or more and 12 μm or less.

[0085] The content rate of the silicon particles in the third particle can be 30 mass% or more and 80 mass% or less, or can be 40 mass% or more and 70 mass% or less. In this range, it is possible to achieve sufficient high capacity of the negative electrode, and it is also easy to improve the cycle characteristics.

[0086] The average particle diameter of the silicon particles in the third particle can be, for example, 1 nm or more. In addition, the average particle diameter of the silicon particles can be 1000 nm or less, can be 500 nm or less, can be 200 nm or less, or can be 100 nm or less (further 50 nm or less). The finer the silicon particles, the smaller the volume change of the third particle at the time of charge and discharge, and the structure stability of the third particle improves.

[0087] The composition and the content rate of the components of the second and third particles can be analyzed by the method described in International Publication No. 2018 / 179969.

[0088] The content rate of each element contained in the particle (P) can be measured by inductively coupled plasma emission spectrophotometry (ICP-AES), for example. Specifically, the particle (P) is dissolved in an acid solution after heating, the carbon of the solution residue is removed by filtration, and then the obtained filtrate is analyzed by ICP-AES to measure the spectral intensity of each element. Then, using a commercially available standard solution of each element, a standard curve is prepared, and the content rate of each element is calculated.

[0089] The second and third particles each have a so-called sea-island structure. The silicon particles (islands) in the second and third particles are dispersed in the matrix (sea) of the silicate phase and carbon phase, respectively, and are covered with the lithium ion-conducting phase (silicate phase and carbon phase). In the sea-island structure, the contact of the silicon particles with the electrolyte is limited, and thus, side reactions are suppressed. In addition, the stress generated in the expansion and contraction of the silicon particles is alleviated by the matrix of the lithium ion-conducting phase.

[0090] [Graphite]

[0091] Examples of the graphite include natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like. As the graphite used as the negative electrode active material, publicly known graphite can be used.

[0092] The graphite refers to a material in which a graphite-type crystal structure is developed, and generally refers to a carbon material in which the average interplanar spacing d002 of the (002) plane measured by an X-ray diffraction method is 0.340 nm or less.

[0093] The average particle diameter of the graphite (graphite particles) contained as the active material in the negative electrode can be 13 μm or more and 25 μm or less. The average particle diameter of the graphite is preferably larger than the average particle diameter of the particles (P). According to this configuration, pores are formed between the relatively large graphite particles, and the particles (P) are easily accommodated in the pores. Thus, the filling rate of the active material in the negative electrode is easily increased, and a negative electrode with a higher capacity is easily obtained. In addition, the particles (P) present in the pores are advantageous in maintaining the electronic contact between the graphite particles. On the other hand, the particles (P) present in the pores are less likely to cause expansion / contraction of the entire negative electrode even if they expand / contract, and thus, degradation caused by charge / discharge cycles is less likely to occur.

[0094] The average particle diameters of the particles (P), the silicon particles in the particles (P), and the graphite in the negative electrode active material layer can be measured by observing the cross section of the negative electrode active material layer with an SEM or a TEM. In this case, the average particle diameter is obtained by arithmetically averaging the maximum diameters of an arbitrary 100 particles.

[0095] For the average particle diameter of the particles (P) before the formation of the negative electrode mixture (negative electrode active material layer), the median particle diameter (D 50 ) at which the cumulative volume becomes 50% in the particle size distribution on a volume basis can be used. The median particle diameter can be obtained using, for example, a laser diffraction / scattering type particle size distribution measuring device.

[0096] [Method for producing the first particles]

[0097] The first particles can be produced, for example, in the following manner. First, particles composed of SiO (silicon monoxide) are pulverized / classified to adjust the particle size. Next, the surface of the obtained particles is covered with carbon by a CVD method under an argon atmosphere. Then, it is crushed / classified, and thus, particles composed of SiOX The first particle represented by Formula (1). Note that, for the SiO X The method for producing the particle can employ various publicly known methods. In addition, for the treatment of the particle covered with carbon, the treatment can be omitted. X The treatment of the particle can be omitted.

[0098] [Method for producing the second particle]

[0099] Next, an example of the method for producing the second particle will be described in detail. The second particle can be produced by a method other than the method described below. The second particle can be produced by the method described in International Publication No. 2018 / 179969.

[0100] The second particle is synthesized by roughly two processes of a pre-process in which a lithium silicate is obtained, and a post-process in which the second particle is obtained from the lithium silicate and a raw silicon. In the case where an element Me is added, the element Me can be added to the raw material of the lithium silicate in the pre-process, but it is preferable to add it in the post-process in a manner that does not affect the synthesis of the lithium silicate. More specifically, the method for producing the second particle preferably includes the following processes: process (i) in which silica and a lithium compound are mixed, and the obtained mixture is fired to obtain a lithium silicate; and process (ii) in which the lithium silicate and a raw silicon (and further an element Me, if necessary) are compounded, thereby obtaining the second particle containing a lithium silicate phase and a silicon particle dispersed in the lithium silicate phase.

[0101] [Process (i)]

[0102] Formula: Li 2Z SiO 2+Z The value of Z of the lithium silicate represented by Formula (1) can be controlled according to the atomic ratio of silicon to lithium: Li / Si in the mixture of silica and a lithium compound. In order to synthesize a high-quality lithium silicate in which the elution of an alkali component is small, it is preferable to make Li / Si less than 1.

[0103] Lithium compounds that can be used include lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride. They can be used alone as one kind, or in combination of two or more kinds.

[0104] The mixture containing silica and a lithium compound is preferably heated in air at 400°C to 1200°C, preferably 800°C to 1100°C, to cause the silica and the lithium compound to react.

[0105] [Process (ii)]

[0106] Next, the lithium silicate is compounded with the raw silicon. For example, the mixture of the lithium silicate and the raw silicon (which can also contain the element Me) is pulverized while imparting a shearing force to the mixture. Coarse particles of silicon having an average particle diameter of several μm to several tens of μm can be used as the raw silicon. The resulting silicon particles are preferably controlled so that the crystallite size calculated from the half-value width of the diffraction peak attributed to the Si (111) plane in the XRD pattern according to the Scherrer formula is 10 nm or more.

[0107] Among the materials of the element Me used in the process, oxides, oxalates, nitrates, sulfates, halides, carbonates, and the like of the element Me can be used. Among these, Me oxides are preferable in terms of stability and good ion conductivity. More specifically, CeO2, Sc2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, and the like can be given. In addition, compounds containing the element Me and other elements other than oxygen, such as yttria-stabilized zirconia, can also be used. They can be used alone or in combination with two or more.

[0108] For example, the lithium silicate and the raw silicon (and a compound of a further element Me, if necessary) can be mixed in a prescribed mass ratio, and a pulverizing device such as a ball mill can be used to granulate the mixture while stirring. However, the process of compounding is not limited to this. For example, silicon nanoparticles and lithium silicate nanoparticles (and a compound of the element Me, if necessary) can be synthesized without using a pulverizing device, and they can be mixed.

[0109] Next, the granulated mixture is heated and fired at 450°C to 1000°C, for example, in an inactive atmosphere (for example, an atmosphere of argon, nitrogen, or the like). At this time, the mixture can be fired while applying pressure to the mixture by hot pressing or the like, and a sintered body of the mixture can be produced. The lithium silicate is stable at 450°C to 1000°C and does not substantially react with silicon, and thus even if capacity reduction occurs, it is slight. The silicate softens at the time of firing and flows in a manner to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body in which the silicate phase serves as the sea and the silicon particles serve as the islands can be obtained.

[0110] The sintered body is then pulverized until it becomes a granular substance, and a second particle can be formed. At this time, by appropriately selecting the pulverizing conditions, a second particle having an average particle diameter in the above range can be obtained.

[0111] After the process (ii), a process (iii) of covering at least a part of the surface of the second particles with an electrically conductive material to form an electrically conductive layer can be performed. The electrically conductive material is preferably electrochemically stable, and is preferably a carbon material. As a method of covering the surface of the particulate material with the carbon material, a CVD method using a hydrocarbon gas such as acetylene, methane, or the like as a raw material can be used. Alternatively, a method of mixing coal pitch, petroleum pitch, phenol resin, or the like with the second particles and heating them can be used. Alternatively, carbon black can be attached to the surface of the second particles.

[0112] A process of washing the second particles with an acid can be performed. For example, the second particles can be washed with an acidic aqueous solution. By washing with an acid, a small amount of a component such as Li2SiO3 that can be generated when the raw material silicon is complexed with the lithium silicate can be dissolved and removed. As the acidic aqueous solution, an aqueous solution of an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, carbonic acid, or the like, or an aqueous solution of an organic acid such as citric acid, acetic acid, or the like can be used.

[0113] [Method of manufacturing third particles]

[0114] As an example of the method of manufacturing the third particles, the first and second methods are described below. The third particles can be manufactured by a method other than the manufacturing methods described below.

[0115] In the first method, first, the raw material silicon is mixed with a carbon source, and a pulverizing device such as a ball mill is used to pulverize and complex the mixture of the raw material silicon and the carbon source while micronizing the mixture. An organic solvent can be added to the mixture and wet pulverization can be performed. At this time, the raw material silicon is finely pulverized to generate silicon particles. The silicon particles are dispersed in the matrix of the carbon source.

[0116] As the carbon source, for example, a water-soluble resin such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, a polyacrylate, a polyacrylamide, a polyvinyl alcohol, a polyethylene oxide, a polyvinylpyrrolidone, or the like, a saccharide such as cellulose, sucrose, or the like, petroleum pitch, coal pitch, tall oil, or the like can be used, but is not particularly limited.

[0117] As the organic solvent, an alcohol, an ether, a fatty acid, an alkane, a cycloalkane, a silicate, a metal alkoxide, or the like can be used.

[0118] Next, the complex of the silicon particles and the carbon source is heated to 700°C to 1200°C in an atmosphere of an inactive gas such as an atmosphere of argon, nitrogen, or the like. The carbon source is carbonized by this heating to generate amorphous carbon. Thus, the third particles in which silicon particles are dispersed in a carbon phase containing amorphous carbon are obtained.

[0119] In the second method, first, the raw silicon is mixed with the carbon material, and using a pulverizing device such as a ball mill, the mixture of the raw silicon and the carbon material is micronized while being pulverized and compounded. Wet pulverization can be performed by adding an organic solvent to the mixture. At this time, the raw silicon is finely pulverized to produce silicon particles. The silicon particles are dispersed in the matrix of the carbon material.

[0120] By the above-described compounding of the raw silicon and the carbon material, the third particles in which the silicon particles are dispersed in the carbon phase of amorphous carbon are obtained. Thereafter, the third particles can be heated to 700°C to 1200°C in a non-reactive gas atmosphere.

[0121] As the carbon material, amorphous carbon is preferable, and easy graphitizable carbon (soft carbon), hard graphitizable carbon (hard carbon), carbon black, or the like can be used. As the carbon black, acetylene black, Ketjen black, or the like can be cited. In the case where graphite is used as the carbon material, the crystal structure of the graphite is also substantially not lost when the pulverizing device is used to obtain the compound of the silicon particles and the carbon material, and the carbon phase of amorphous carbon is formed.

[0122] Hereinafter, examples of the secondary battery (S) of the present embodiment and examples of the constituent elements thereof will be described in detail. Note that, for the constituent elements that are not characteristic parts in the present disclosure, publicly known constituent elements can be applied. The secondary battery (S) includes, for example, a housing (battery case), a positive electrode, a negative electrode, an electrolyte, and a separator disposed in the housing. The separator is disposed between the positive electrode and the negative electrode.

[0123] The shape of the secondary battery (S) is not limited, and can be cylindrical, prismatic, coin-shaped, button-shaped, or the like. The battery case can be selected in accordance with the shape of the secondary battery (S).

[0124] [Negative electrode]

[0125] The negative electrode contains the first layer containing the negative electrode active material. Typically, the negative electrode includes a negative electrode current collector and the first layer disposed on the surface of the negative electrode current collector. The first layer can be a negative electrode active material layer (negative electrode mixture layer). In this case, the first layer contains the negative electrode active material and the flame retardant (R), and other components other than the negative electrode active material and the flame retardant (R) as necessary. Examples of the other components include a binder, a conductive agent, a thickening agent, and the like. Among these other components, publicly known components used in secondary batteries can be used. The first layer can contain carbon nanotubes as a conductive material.

[0126] The first layer can be a layered structure of the second layer (negative electrode active material layer) containing at least a negative electrode active material and the 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 on the side other than the side facing the negative electrode current collector. The second layer contains a negative electrode active material and, as needed, other components. Examples of the other components include a conductive material, a binder, a thickening agent, and the like. Among these other components, components known to be used in secondary batteries can be used.

[0127] In the case where the first layer is a layered structure of the second layer (negative electrode active material layer) and the third layer (flame retardant layer), the flame retardant can be contained in both the second layer and the third layer. As the flame retardant contained in the second layer, a compound exemplified as the flame retardant (R) described above can be used, or another known flame retardant other than the flame retardant (R) can be used. However, the flame retardant contained in the second layer is preferably a flame retardant (R) containing a halogen atom, like the flame retardant contained in the third layer. In the case where the flame retardant (R) is contained in both the second layer and the third layer, the flame retardant (R) contained in the second layer can be a different compound from the flame retardant (R) contained in the third layer, or can be the same compound.

[0128] The first layer or the second layer as the negative electrode active material layer can be formed by, for example, applying a negative electrode slurry in which a negative electrode mixture is dispersed in a dispersion medium to the surface of a negative electrode current collector to form a coating film, and then drying the coating film. The coating film after drying can be calendered as needed. As the dispersion medium, water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof, or the like can be exemplified. The ratio of the components in the negative electrode mixture can be prepared by changing the mixing ratio of the materials of the negative electrode mixture.

[0129] Examples of the binder include a fluorine resin, a polyolefin resin, a polyamide resin, a polyimide resin, a vinyl resin, a styrene-butadiene copolymer rubber (SBR), a polyacrylic acid and a derivative thereof, and the like. Examples of the conductive agent include carbon black, a conductive fiber, a fluorinated carbon, an organic conductive material, and the like. Examples of the thickening agent include carboxymethyl cellulose (CMC), polyvinyl alcohol, and the like. Among these components, one material can be used alone, or two or more materials can be used in combination.

[0130] The ratio of the negative electrode active material in the negative electrode active material layer is determined from a sample obtained by removing only the negative electrode active material layer from the secondary battery in the discharged state. Specifically, first, the secondary battery in the discharged state is disassembled and the negative electrode is removed. Next, the negative electrode is washed with an organic solvent, and further vacuum dried, and only the negative electrode active material layer is peeled off to obtain a sample. Thermal analysis such as TG-DTA is performed on the sample, and thus the ratio of the binder component and the conductive material component other than the negative electrode active material can be calculated. Micro-Raman spectroscopy is performed on the cross section of the negative electrode active material layer, and thus the carbon species such as carbon nanotubes and acetylene black can be identified, and the ratio can be calculated from the thermal analysis such as TG-DTA of the peeled sample. In addition, the ratio of the flame retardant (R) in the negative electrode active material layer can be determined by elemental analysis such as fluorescent X-ray analysis (XRF) of the negative electrode active material layer.

[0131] Another aspect of the present disclosure relates to a negative electrode mixture that constitutes the first layer or the second layer of the negative electrode active material layer described above.

[0132] In addition, a further another aspect of the present disclosure relates to a negative electrode for a secondary battery having the above-described first layer containing at least a negative electrode active material and a flame retardant (R).

[0133] The first layer or the second layer of the negative electrode active material layer can further contain carbon nanotubes. The carbon nanotubes can be contained in the negative electrode active material layer as a conductive agent. The aspect ratio (ratio of length to diameter) of the carbon nanotubes is extremely large, and thus high conductivity can be exhibited even in a small amount. By using carbon nanotubes as a conductive material, the conductivity of the negative electrode active material layer can be maintained at a high level, and the ratio of the negative electrode active material in the negative electrode active material layer can be increased. Therefore, the secondary battery (S) can achieve high capacity.

[0134] The first layer or the second layer of the negative electrode active material layer can further contain at least one conductive carbon material selected from the group consisting of amorphous carbon and carbon fibers, in addition to the carbon nanotubes. The amorphous carbon includes hard carbon and soft carbon. Examples of the soft carbon include carbon black such as acetylene black and Ketjen black. These materials can be used in combination as a conductive material.

[0135] The negative electrode active material layer can contain a conductive material other than the carbon nanotubes, or can not contain a conductive material other than the carbon nanotubes. In addition to the carbon nanotubes, the negative electrode active material layer preferably contains carbon black as a conductive material other than the carbon nanotubes. However, if these are contained in a large amount, the ratio of the negative electrode active material in the negative electrode active material layer decreases. Therefore, the mass of the conductive material contained in the negative electrode active material layer, and the conductive material other than the carbon nanotubes, can be 10 times or less (for example, in the range of 0 to 5 times, 0 to 1 times, or 0 to 0.5 times) the mass of the carbon nanotubes contained in the negative electrode active material layer.

[0136] Examples of the carbon nanotube include carbon nanofibers. The carbon nanotube is commercially available in various products, and thus, a commercially available product can be used. Alternatively, the carbon nanotube can be synthesized by a publicly known synthesis method.

[0137] The carbon nanotube can be a single wall, a double wall, or a multi wall. In terms of obtaining a large effect with a small amount, a single wall carbon nanotube is preferable. Among carbon nanotubes having a diameter of 5 nm or less, a large amount of single wall carbon nanotubes are included. The single wall carbon nanotube can be 50% by mass or more of the entire carbon nanotube.

[0138] The diameter of the carbon nanotube is not particularly limited and can be in the range of 0.001 to 0.05 μm. The length of the carbon nanotube is not particularly limited and can be 0.5 μm or more from the viewpoint of ensuring electron conduction in the negative electrode active material layer. On the other hand, the length of the carbon nanotube has no upper limit as long as it is appropriately disposed inside the negative electrode. In view of the fact that the particle diameter of the negative electrode active material is usually 1 μm or more and 25 μm or less, the length of the carbon nanotube can be a length equivalent thereto. That is, the length of the carbon nanotube can be, for example, 1 μm or more and 25 μm or less. For example, when a plurality of (for example, 100 or more) carbon nanotubes are arbitrarily selected within the negative electrode active material layer, the length of 50% or more (number ratio) of the carbon nanotubes can be 1 μm or more and can be 1 μm or more and 25 μm or less. The length of 80% or more of the carbon nanotubes can be 1 μm or more and can be 1 μm or more and 20 μm or less.

[0139] The outer diameter and the length of the carbon nanotube can be found by image analysis using a scanning electron microscope (SEM). For example, the length is found by arbitrarily selecting a plurality of (for example, 100 to 1000) carbon nanotubes, measuring the length and the diameter, and averaging them.

[0140] As the negative electrode current collector, a non-porous conductive substrate (metal foil or the like), a porous conductive substrate (sieve body, mesh body, punched sheet, or the like) is used. As the material of the negative electrode current collector, stainless steel, nickel, nickel alloy, copper, copper alloy, or the like can be exemplified.

[0141] [Negative electrode active material]

[0142] As the negative electrode active material, a material that can electrochemically occlude and release lithium ions is suitably used. As such a material, a carbonaceous material, a Si-containing material, or the like can be exemplified. The negative electrode active material can be used alone by one kind, or two or more kinds in combination.

[0143] Examples of carbonaceous materials include graphite, easily graphitized carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). A single carbonaceous material can be used, or two or more can be used in combination. Graphite is preferred as a carbonaceous material due to its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Known graphite used as a negative electrode active material can be used among these.

[0144] Examples of silicon-containing materials include elemental silicon, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed within a lithium-ion conductive phase (matrix). Examples of silicon oxides include SiO₂. X Particles. X is, for example, 0.5 ≤ X < 2, or 0.5 ≤ X < 1.6 or 0.8 ≤ X ≤ 1.6. As the lithium-ion conducting phase, at least one can be selected from the group consisting of SiO2 phase, silicate phase and carbon phase.

[0145] As an example of Si-containing materials, SiO2 can be selected from the inclusion-type SiO2. X At least one particle (P) is formed from the group consisting of the first particle of silicon oxide shown in (0.5≤X<1.6), the second particle of silicon containing a lithium silicate phase and silicon particles dispersed in the lithium silicate phase, and the third particle of silicon containing a carbon phase and silicon particles dispersed in the carbon phase.

[0146] By using silicon (Si)-containing particles (P) as the negative electrode active material, the battery capacity can be increased. On the other hand, when using particles (P), the battery temperature is prone to rise under abnormal conditions (such as during nail penetration tests). However, by adding a flame retardant (R), the rise in battery temperature can be suppressed, resulting in high safety.

[0147] The negative electrode active material can comprise a variety of particles selected from the group consisting of a first particle, a second particle, and a third particle. For example, particle (P) can be composed of two types of particles selected therefrom, or it can comprise all three types of particles. Specifically, the negative electrode active material can comprise a first particle and a second particle, a first particle and a third particle, or a second particle and a third particle. Alternatively, the negative electrode active material can comprise all of the first, second, and third particles. Particle (P) is preferably used in combination with graphite as the negative electrode active material.

[0148] In the case where the negative electrode active material contains graphite and the particles (P), the content rate of the particles (P) in the negative electrode active material can be 1% by mass or more. According to this configuration, higher capacity can be achieved as compared with the case where the negative electrode active material is only graphite. The content rate of the particles (P) in the negative electrode active material can be 3% by mass or more. The content rate can be 50% by mass or less. These lower limits and upper limits can be arbitrarily combined as long as there is no contradiction.

[0149] The content rate of graphite in the negative electrode active material can be in the range of 50 to 99% by mass. Note that in the case where the particles (P) contain graphite on the surface and / or inside, the graphite is not included in the content rate of graphite described above. The content rate of graphite is the content rate of graphite not included in the particles (P).

[0150] [Positive electrode]

[0151] The positive electrode contains a positive electrode mixture. Typically, the positive electrode contains: a positive electrode current collector, and a positive electrode active material layer (positive electrode mixture layer) formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by coating a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium on the surface of the positive electrode current collector and drying, and thus can be formed. The coating film after drying can be calendered as needed. The positive electrode mixture contains a positive electrode active material as a necessary component, and can contain a binder, a conductive agent, and the like as arbitrary components.

[0152] As the positive electrode active material, a lithium complex metal oxide can be used. As the lithium complex metal oxide, for example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn 2- b M bO4, LiGPO4, Li2GPO4F. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. G contains at least a transition element (for example, contains at least one selected from the group consisting of Mn, Fe, Co, Ni). Here, 0 ≤ a ≤ 1.2, 0 ≤ b ≤ 0.9, 2.0 ≤ c ≤ 2.3. Note that the value of a, which represents the molar ratio of lithium, increases and decreases depending on charge and discharge.

[0153] As the binder and the conductive agent, the same substances as those for the negative electrode example can be used. As the conductive agent, graphite such as natural graphite, artificial graphite, or the like can be used.

[0154] The shape and thickness of the positive electrode current collector can be selected from those of the negative electrode current collector, respectively. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, or the like can be exemplified.

[0155] [Electrolyte]

[0156] In the electrolyte, an electrolyte solution containing a solvent and a solute dissolved in the solvent can be used. The solute is an electrolyte salt that undergoes ion dissociation in the electrolyte solution. The solute can contain, for example, a lithium salt. The component of the electrolyte solution other than the solvent and the solute is an additive. In the electrolyte solution, various additives can be contained.

[0157] The solvent can use a non-aqueous solvent. As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylate, a chain carboxylate, or the like can be used. As the cyclic carbonate, propylene carbonate (PC), ethylene carbonate (EC), vinylene carbonate (VC), or the like can be exemplified. As the chain carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or the like can be exemplified. In addition, as the cyclic carboxylate, γ-butyrolactone (GBL), γ-valerolactone (GVL), or the like can be exemplified. As the chain carboxylate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), or the like can be exemplified. The non-aqueous solvent can be used alone as one kind, or two or more kinds can be used in combination.

[0158] As the non-aqueous solvent, in addition, a cyclic ether, a chain ether, an acetonitrile, an amide such as dimethylformamide, or the like can be exemplified.

[0159] As the lithium salt, for example, a lithium salt containing a chloric acid (LiClO4, LiAlCl4, LiB 10 Cl 10A lithium salt can be used alone or in combination of two or more. Examples of the lithium salt include LiClO4, LiAsF6, LiPF6, LiBF4, LiSbF6, LiCF3SO3, LiCF3CO2, LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, LiCl, LiBr, LiI, and the like. The lithium salt can be used alone or in combination of two or more.

[0160] The concentration of the lithium salt in the electrolyte solution can be 1 mol / liter or more and 2 mol / liter or less, or 1 mol / liter or more and 1.5 mol / liter or less. By controlling the concentration of the lithium salt to be within the above range, an electrolyte solution having excellent ion conductivity and moderate viscosity can be obtained. However, the concentration of the lithium salt is not limited to the above.

[0161] The electrolyte solution can also contain other publicly known additives. As the additives, 1,3-propanesultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, and the like can be given.

[0162] [Separator]

[0163] A separator can be disposed between the positive electrode and the negative electrode. In the separator, a member having high ion permeability, moderate mechanical strength, and insulating properties can be used. As the separator, a microporous film, a woven fabric, a nonwoven fabric, or the like can be used. As the material of the separator, a polyolefin such as polypropylene or polyethylene is preferable.

[0164] One example of the secondary battery (S) includes a housing, an electrode group housed in the housing, and a nonaqueous electrolyte. The structure of the electrode group is not particularly limited. One example of the electrode group can be formed by winding a positive electrode, a negative electrode, and a separator in a manner that the separator is disposed between the positive electrode and the negative electrode. Another example of the electrode group can be formed by stacking a positive electrode, a negative electrode, and a separator in a manner that the separator is disposed between the positive electrode and the negative electrode. The shape of the secondary battery (S) is not limited, and can be cylindrical, square, coin-shaped, button-shaped, laminated, or the like.

[0165] The manufacturing method of the secondary battery (S) is not particularly limited, and a publicly known manufacturing method can be used, or at least a part of the publicly known manufacturing method can be changed and used.

[0166] Examples of the embodiments of the present disclosure will be described below with reference to the drawings. The above-described components can be used for the components of the examples described below. In addition, the examples described below can be changed based on the above description. In addition, the matters described below can be used for the above-described embodiments. In addition, in the embodiments described below, components that are not essential for the secondary battery of the present disclosure can be omitted.

[0167] Figure 1 A cross-sectional view showing a configuration example of a negative electrode (negative electrode for secondary battery) 2 of a secondary battery constituting an embodiment of the present disclosure. A negative electrode active material layer (2nd layer) 21 is disposed on the surface of a negative electrode current collector 20, and a flame retardant layer (3rd layer) 22 is disposed on the surface of the negative electrode active material layer 21. The flame retardant layer 22 contains a flame retardant (R). Figure 1 is an example in which the flame retardant layer 22 is formed so as to cover the entire surface of the negative electrode active material layer 21.

[0168] Figure 2 A perspective view showing a part of a prismatic secondary battery of an embodiment of the present disclosure cut away. Figure 2 The secondary battery 1 shown contains a bottomed square battery case 11, an electrode group 10 and an electrolyte (not shown) housed in the battery case 11. The electrode group 10 contains a long-size strip-shaped negative electrode, a long-size strip-shaped positive electrode, and a separator interposed therebetween for preventing direct contact. The electrode group 10 can be formed by winding the negative electrode, the positive electrode, and the separator around a flat plate-shaped core and pulling out the core.

[0169] One end of the negative electrode lead 15 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of the 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 the negative electrode terminal 13 provided to the sealing plate 12. A gasket 16 is disposed between the sealing plate 12 and the negative electrode terminal 13 to insulate them. 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 serves as a positive electrode terminal. A frame 18 made of resin is disposed at the upper portion of the electrode group 10. The frame 18 serves to separate the electrode group 10 from the sealing plate 12 and to separate the negative electrode lead 15 from the battery case 11. The opening portion of the battery case 11 is sealed by the sealing plate 12. The sealing plate 12 is formed with a liquid injection hole 17a. The electrolyte is injected into the battery case 11 from the liquid injection hole 17a. Thereafter, the liquid injection hole 17a is blocked by a sealing plug 17.

[0170] Embodiment

[0171] The secondary battery of the present disclosure is further described in detail according to the embodiment.

[0172] (Embodiment 1)

[0173] In this embodiment, a plurality of secondary batteries were produced and evaluated according to the following steps. The kind of the flame retardant and / or the ratio of the substances in the negative electrode mixture layer of the plurality of secondary batteries were different.

[0174] [Production of Negative Electrode]

[0175] Graphite is used as the negative active material. First, a negative slurry is prepared by mixing the negative active material with carboxymethylcellulose sodium (CMC-Na) and styrene-butadiene rubber (SBR) and water and a flame retardant as needed in a prescribed mass ratio. Next, the negative slurry is applied to the surface of a copper foil (negative current collector) to form a coating film. The coating film is dried and calendered to form a negative electrode mixture layer on both sides of the copper foil.

[0176] [Manufacture of the positive electrode]

[0177] As the positive active material, LiNi 0.88 Co 0.09 Al 0.03 O2is used. A positive slurry is prepared by mixing the positive active material, polyvinylidene fluoride, N-methyl-2-pyrrolidone (NMP), and acetylene black in a prescribed mass ratio.

[0178] Next, the positive slurry is applied to the surface of an aluminum foil (positive current collector) to form a coating film. The coating film is dried and calendered to form a positive electrode mixture layer on both sides of the aluminum foil.

[0179] [Preparation of the electrolyte]

[0180] An electrolyte is prepared by adding LiPF6as 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 LiPF6in the nonaqueous electrolyte is set to 1.3 mol / liter.

[0181] [Manufacture of the secondary battery]

[0182] Lead tabs are attached to each of the electrodes. Next, the positive electrode and the negative electrode are wound in a spiral shape with the lead tabs at the outermost periphery, with a separator interposed therebetween. In this way, an electrode group is manufactured. Next, the electrode group is inserted into a housing made of a laminate film having an aluminum foil as a barrier layer, and vacuum drying is performed. Next, an electrolyte is injected into the housing, and the opening of the housing is sealed. In this way, a secondary battery is obtained.

[0183] In this example, a plurality of secondary batteries (batteries Al to A12, Bl, B2, Cl) are manufactured by changing the kind of flame retardant and the ratio of the substances in the negative electrode mixture layer. Note that batteries Bl and B2 are comparative examples in which a flame retardant different from the flame retardant (R) containing a halogen atom is added. Battery Cl is a reference example in which no flame retardant is added. The ratio of the substances varies depending on the mixing ratio of the substances when the negative slurry is prepared. Some of these ratios are shown in Table 1 described later. The kind of flame retardant is described later.

[0184] For the manufactured secondary batteries, the following evaluations are performed.

[0185] (1) Measurement of capacity retention rate

[0186] The discharge capacity of the prepared secondary battery was measured in the following manner. First, the battery was charged at a constant current of 0.5 CA until the battery voltage became 4.2 V at 25°C, and then, constant voltage charging was continued until the current value became 0.02 CA. After the charged battery was left for 20 minutes, discharging was performed at a constant current of 1.0 CA until the battery voltage became 2.5 V. After that, the battery was left for 20 minutes. This operation (charge-discharge cycle) was repeated 100 times.

[0187] The discharge capacity at the initial discharging was taken as the initial capacity DC0, and the discharge capacity DC1 after the above charge-discharge cycle was repeated 100 times. Then, the capacity retention rate was calculated according to the following formula.

[0188] Capacity retention rate (%) = 100 x DC1 / DC0

[0189] (2) Nail penetration test

[0190] The nail penetration test was performed on the prepared secondary battery according to the following procedure.

[0191] (a) The battery was charged at a constant current of 0.5 CA until the battery voltage became 4.2 V at 25°C, and then, constant voltage charging was continued until the current value became 0.02 CA.

[0192] (b) The tip of a round nail (diameter 2.7 mm) was brought into contact with the central part of the battery charged in (a) at 25°C, and the nail was pierced at a speed of 1 mm / sec, and just after the battery voltage drop due to internal short circuit was detected, the piercing of the nail was stopped.

[0193] After the battery was short-circuited by the nail, the current value I of the short-circuit current and the voltage V of the battery were measured for 1 second. Then, the product (power) of the current value I and the voltage V was time-integrated to calculate the heat release amount for 1 second.

[0194] A part of the production conditions of the batteries and the evaluation results are shown in Table 1. In Table 1, the a value (mass ratio) indicating the content ratio of the flame retardant indicates the mass of the flame retardant when the mass of the negative electrode active material in the negative electrode active material layer is assumed to be 100. In the batteries Al to A12, Bl, and B2, the content ratio of the negative electrode active material and the flame retardant in the negative electrode active material layer was changed while the total content ratio of the negative electrode active material and the flame retardant in the negative electrode active material layer was constant. In Table 1, the flame retardant r1 indicates 1,2-bis(pentabromophenyl)ethane (SAYTEX (registered trademark)-8010 manufactured by Albemarle Japan K.K.). The flame retardant r2 indicates ethylene bis-tetrabromophthalimide (halogen atom content ratio: 67 mass%). The flame retardant r3 indicates potassium citrate.

[0195] [Table 1]

[0196]

[0197] According to Table 1, in the batteries Al to A12 to which the flame retardant (R) was added, the heat generation amount at the time of the nail penetration test was reduced, and in addition, the reduction in the initial capacity and the capacity maintenance rate was suppressed, as compared with the battery Cl. That is, in the batteries Al to A12, the heat generation amount can be reduced, and high charge-discharge performance and high safety can be achieved.

[0198] In the batteries Al to A5 and the batteries A7 to Al 1 in which the a value was 0.1 or more and less than 5, the initial capacity and the capacity maintenance rate equivalent to or higher than those of the battery Cl to which no flame retardant was added were obtained. In the batteries A3, A4, A9, and A10 in which the a value was 1 or more and less than 5, a significantly low heat generation amount was obtained, the initial capacity equivalent to that of the battery Cl was maintained, and the capacity maintenance rate was improved more than that of the battery Cl.

[0199] Note that in the batteries Bl and B2, the discharge capacity after 70 charge-discharge cycles was reduced to about 70% of the initial capacity, and the capacity maintenance rate after 100 charge-discharge cycles was almost reduced to 0%, and the charge-discharge could not be performed. In contrast, in the batteries Al to A12, the reduction in the initial capacity and the capacity maintenance rate was also suppressed as compared with the battery Cl. Therefore, in the case where the flame retardant (R) is included in the negative electrode active material layer, the function as a battery can be maintained even after 100 cycles of charge-discharge.

[0200] (Example 2)

[0201] In this example, a plurality of nonaqueous electrolyte secondary batteries were produced and evaluated. The nonaqueous electrolyte secondary batteries were produced according to the following procedure.

[0202] [Production of negative electrode]

[0203] Graphite or a mixture of graphite and the particles (P) is used in the negative active material. First, a negative slurry is prepared by mixing the negative active material with sodium carboxymethylcellulose (CMC-Na) and styrene-butadiene rubber (SBR) and water and a flame retardant as needed in a prescribed mass ratio. Next, the negative slurry is applied to the surface of a copper foil (negative current collector) to form a coating film. The coating film is dried and calendered. Thus, a negative electrode mixture layer is formed on both sides of the copper foil.

[0204] The first particles are produced in the following manner. First, particles composed of SiO (silicon monoxide) are pulverized / classified to adjust the particle size. Next, the surfaces of the resulting particles are covered with carbon according to the CVD method under an argon atmosphere. Then, they are crushed / classified to produce SiO X The first particles are produced in the following manner. First, particles composed of SiO (silicon monoxide) are pulverized / classified to adjust the particle size. Next, the surfaces of the resulting particles are covered with carbon according to the CVD method under an argon atmosphere. Then, they are crushed / classified to produce SiO

[0205] The second particles are produced in the following manner. First, silica and lithium carbonate are mixed in an atomic ratio of Si / Li of 1.05, and the mixture is fired at 950°C in air for 10 hours to obtain a lithium silicate represented by the formula: Li2Si2O5. The resulting lithium silicate is pulverized to have an average particle size of 10 μm.

[0206] Next, the resulting lithium silicate, raw silicon (3N, average particle size 10 μm), and yttria (Y2O3) are mixed in a mass ratio of 50:50:0.0005. The mixture is filled into a pot (SUS, volume: 500 mL) of a planetary ball mill (P-5, manufactured by Fritsch), 24 SUS balls (diameter 20 mm) are put in the pot, the lid is closed, and the mixture is pulverized for 50 hours at 200 rpm in an inactive atmosphere. Next, the powder-like mixture is taken out in an inactive atmosphere, and sintered at 800°C for 4 hours in an inactive atmosphere with a pressure applied using a hot press to obtain a sintered body of the mixture (parent particles).

[0207] After that, the sintered body is pulverized, passed through a 40-μm sieve, mixed with coal tar pitch (MCP250, manufactured by JFE chemical), and fired at 800°C in an inactive atmosphere to cover the surfaces of the pulverized particles with electrically conductive carbon to form an electrically conductive layer. The amount of the electrically conductive layer is set to 5% by mass of the total mass of the pulverized particles. After that, a sieve is used to obtain second particles having an average particle size of 5 μm with the electrically conductive layer.

[0208] In the same manner as in Example 1, a positive electrode was produced, and an electrolyte solution was prepared. Lead tabs were attached to each of the electrodes. Next, the positive electrode and the negative electrode were spirally wound with the lead tabs located at the outermost periphery, with a separator interposed therebetween, to produce an electrode group having a substantially elliptical cross section. Next, the electrode group was housed in an aluminum battery case having a bottomed square shape with an opening. A rectangular gasket having a negative terminal at the center was disposed on the opening of the battery case. The negative lead tab was connected to the negative terminal, and the positive lead tab was connected to the lower surface of the gasket. The end of the opening was laser-welded to the gasket to seal the opening of the battery case. Thereafter, a nonaqueous electrolyte was injected into the battery case from the injection hole of the gasket. Thus, a nonaqueous electrolyte secondary battery (theoretical capacity 3000 mAh) having a square shape as shown in FIG. 1 was produced. Figure 2

[0209] For the nonaqueous electrolyte secondary battery thus produced, the capacity retention rate was measured and the nail penetration test was performed in the same manner as in Example 1. However, in the nail penetration test, the surface temperature of the battery was measured instead of the heat release amount after 1 minute from the occurrence of internal short circuit of the battery.

[0210] In this example, a plurality of secondary batteries (batteries A13, A14, C2 to C5) were produced by changing the type of the flame retardant, the type of the negative electrode active material, and the ratio of the substances in the negative electrode mixture layer. Note that the batteries C2 to C5 are comparative batteries. The ratio of the substances was changed by changing the mixing ratio of the substances at the time of preparing the negative electrode slurry. Some of these ratios are shown in Table 2 described later. The type of the flame retardant is described later.

[0211] Some of the production conditions of the batteries and the evaluation results are shown in Table 2. The content ratio of the particles (P) in Table 2 is the content ratio of the particles (P) in the negative electrode active material. The content ratio of the flame retardant in Table 2 is the content ratio of the flame retardant in the negative electrode mixture layer. In Table 2, the flame retardant R1 represents 1,2-bis(pentabromophenyl)ethane (SAYTEX (registered trademark) - 8010 manufactured by Albemarle Japan Corporation). The flame retardant R2 represents ammonium polyphosphate.

[0212] [Table 2]

[0213]

[0214] The temperature of the battery shown in Table 1 is preferably low. In addition, the capacity retention rate is preferably high. As shown in the comparisons C2, C3, and C5, if the negative electrode active material contains the particles (P), the temperature of the battery during the nail penetration test greatly increases. On the other hand, in the case where the negative electrode active material contains the particles (P), the increase in the temperature of the battery during the nail penetration test can also be suppressed by adding the flame retardant to the negative electrode mixture layer.

[0215] ​However, in the battery C4 in which ammonium polyphosphate known as a flame retardant for a battery was added, the capacity retention rate was greatly reduced. In contrast, in the batteries A13 and A14 in which the above flame retardant (R) was used, the battery temperature at the time of the nail penetration test could be suppressed at a low level, and a high capacity retention rate could be achieved. It is considered that this is because the above flame retardant (R) has a high resistance to reduction as compared with ammonium polyphosphate, and therefore, unlike the battery C4, the capacity retention rate was not reduced even though the flame retardant was contained. Note that compounds generally containing halogen atoms such as bromine exhibit a high electron-withdrawing property, and if exposed to the negative electrode potential of a nonaqueous electrolyte secondary battery, a decomposition reaction occurs, and it is expected that the battery characteristics will be reduced. However, the above flame retardant (R) is a bromine compound, but has a unique stability inside the negative electrode mixture layer, and even if added to a nonaqueous electrolyte secondary battery, the battery characteristics are not reduced.

[0216] (Example 3)

[0217] In this example, a plurality of secondary batteries were produced and evaluated in the following steps.

[0218] [Production of negative electrode]

[0219] A mixture of graphite and the particles (P) was used as the negative electrode active material. First, a negative electrode slurry was prepared by mixing the negative electrode active material, sodium carboxymethylcellulose (CMC-Na), styrene-butadiene rubber (SBR), carbon nanotubes (CNT), and water at a prescribed mass ratio. Carbon nanotubes having an average diameter of about 1.5 nm and a length of about 1 μm to about 5 μm were used. Next, the negative electrode slurry was applied to the surface of a copper foil (negative electrode current collector), and a coating film was formed. The coating film was dried and calendered. In this way, a negative electrode active material layer (2nd layer) was formed on both sides of the copper foil.

[0220] Next, a slurry for the flame retardant layer was prepared by mixing the flame retardant (R), polyvinylidene fluoride (PVdF), and N-methyl-2-pyrrolidone (NMP) at a prescribed mass ratio. The resulting slurry was applied to the surface of the negative electrode active material layer and dried to form a flame retardant layer (3rd layer). As the flame retardant (R), 1,2-bis(pentabromophenyl)ethane (SAYTEX (registered trademark) -8010 manufactured by Albemarle Japan K.K.) was used. The adjustment was performed so that the weight per unit area of the flame retardant layer would be 3 g / m 2 In this way, a negative electrode in which the 1st layer having the 2nd layer and the 3rd layer was formed on the negative electrode current collector was obtained.

[0221] As the particles (P), the 1st particles and / or the 2nd particles produced in the same manner as in Example 2 were used.

[0222] A secondary battery was produced in the same manner as in Example 2, except that Figure 2The prismatic nonaqueous electrolyte secondary battery (theoretical capacity 3000 mAh) shown was fabricated to obtain secondary batteries A15, A16, C6, and C7.

[0223] In the secondary battery A15, the mixing ratio of each component in the negative electrode slurry was set to a mass ratio of graphite : particle (P) : total of CMC-Na and SBR : CNT = 91 : 6 : 2.9 : 0.1, and a negative electrode was fabricated to obtain a secondary battery. The particle (P) used was one in which the first particle and the second particle were mixed at a mass ratio of 1 : 1.

[0224] In the secondary battery A16, the mixing ratio of each component in the negative electrode slurry was set to a mass ratio of graphite : particle (P) : total of CMC-Na and SBR : CNT = 88 : 9 : 2.9 : 0.1, and a negative electrode was fabricated to obtain a secondary battery. The particle (P) used was only the second particle.

[0225] In the secondary battery C6, the flame retardant layer (the third layer) was not formed, and a negative electrode was fabricated to obtain a secondary battery. The fabrication was otherwise the same as in the secondary battery A15.

[0226] In the secondary battery C7, the flame retardant layer (the third layer) was not formed, and a negative electrode was fabricated to obtain a secondary battery. The fabrication was otherwise the same as in the secondary battery A16.

[0227] For the fabricated secondary batteries, the following evaluations were performed.

[0228] (1) Nail penetration test

[0229] For the fabricated secondary batteries, the battery temperature after the nail penetration test was measured according to the following procedure.

[0230] (a) At 25°C, the battery was charged at a constant current of 0.5 C until the battery voltage became 4.2 V, and then, the battery was charged at a constant voltage until the current value became 0.02 C.

[0231] (b) At 25°C, the tip of a nail (diameter 2.7 mm) was brought into contact with the central portion of the battery after the charge in (a), and the nail was penetrated at a speed of 1 mm / sec. Just after the battery voltage drop due to internal short-circuit was detected, the penetration of the nail was stopped. Then, the surface temperature of the battery was measured after the battery was short-circuited for 1 minute.

[0232] A part of the fabrication conditions of the batteries and the evaluation results are shown in Table 3. According to Table 3, the higher the content of the particle (P), the more easily the temperature rose after the nail penetration test. However, in the batteries A15 and A16 in which the third layer containing the above flame retardant (R) was provided on the surface of the negative electrode active material layer (the second layer), the temperature rose at 3 g / m 2The left and right relatively small unit area weight is sufficient to suppress the effect of temperature rise.

[0233] [Table 3]

[0234]

[0235] (Example 4)

[0236] In this example, a plurality of secondary batteries were produced and evaluated according to the following procedure.

[0237] [Production of negative electrode]

[0238] A mixture of graphite and particles (P) was used in the negative electrode active material. First, a negative electrode slurry was prepared by mixing the negative electrode active material, sodium carboxymethylcellulose (CMC-Na), styrene-butadiene rubber (SBR), carbon nanotubes (CNT), and water in a prescribed mass ratio. Carbon nanotubes having an average diameter of about 1.5 nm and a length of about 1 μm to 5 μm were used. Next, the negative electrode slurry was applied to the surface of a copper foil (negative electrode current collector) to form a coating film. The coating film was dried and calendered. In this way, a negative electrode active material layer (second layer) was formed on both sides of the copper foil. The mixing ratio of each component in the negative electrode slurry was set to a mass ratio of graphite:particles (P):total of CMC-Na and SBR:CNT = 94:5:2.9:0.1.

[0239] Next, a slurry for the flame retardant layer was prepared by mixing a flame retardant (R), polyvinylidene fluoride (PVdF), N-methyl-2-pyrrolidone (NMP), and alumina particles (AI2O3) as needed in a prescribed mass ratio. The resulting slurry was applied to the surface of the negative electrode active material layer and dried to form a flame retardant layer (third layer).

[0240] As the particles (P), the second particles produced in the same manner as in Example 2 were used.

[0241] A nonaqueous electrolyte secondary battery (theoretical capacity 100 mAh) was produced in the same manner as in Example 1, except as described below.

[0242] In this example, a plurality of secondary batteries (batteries A17 to A20, C8) were produced by changing the type of flame retardant contained in the flame retardant layer (third layer) and the ratio of the substances in the flame retardant layer. Note that battery C8 is a comparative example battery, and no flame retardant layer was provided. The ratio of the substances in the flame retardant layer was changed by changing the mixing ratio of the substances when preparing the slurry for the flame retardant layer. Some of these ratios are shown in Table 4 described later. The type of flame retardant is described later.

[0243] For the secondary batteries produced, the battery temperature after the nail penetration test was measured in the same manner as in Example 3.

[0244] A part of the production conditions of the battery and the evaluation results are shown in Table 4. The content ratios of the flame retardant and the binder in Table 4 respectively show the content ratios of the flame retardant and the binder (PVdF) in the slurry for the flame retardant layer. In Table 4, the flame retardant r1 shows 1,2-bis(pentabromophenoxy)ethane (SAYTEX (registered trademark)-8010 manufactured by Albemarle Japan K.K.). The flame retardant r2 shows ethylene bis-tetrachlorophthalimide.

[0245] [Table 4]

[0246]

[0247] According to Table 4, in the batteries A17 to A20 in which the third layer containing the above flame retardant (R) is provided on the surface of the negative electrode active material layer (the second layer), the temperature rise after the nail penetration test can be suppressed. According to Table 4, a sufficient temperature rise suppression effect can be obtained with a relatively thin film thickness of about 3 μm for the third layer.

[0248] The battery A19 corresponds to a battery in which a part of the flame retardant contained in the flame retardant layer in the battery A17 is replaced with aluminum oxide particles to reduce the content ratio of the flame retardant, and a sufficient temperature rise suppression effect can be obtained even with a flame retardant content ratio of 60%.

[0249] Industrial applicability

[0250] The present disclosure can be used for a secondary battery.

[0251] While the present application has been described in connection with the presently preferred embodiments thereof, it is to be understood that no limitation of the disclosure is thereby intended. It will be apparent to those skilled in the art that various modifications and variations can be made in the present application without departing from the spirit or scope of the application. Thus, it is intended that the present application covers the modifications and variations of this application provided they come within the scope of the appended claims and their equivalents.

[0252] Explanation of reference numerals

[0253] 1: nonaqueous electrolyte secondary battery, 10: electrode group, 11: battery case, 12: sealing plate, 13: negative electrode terminal, 14: positive electrode lead wire, 15: negative electrode lead wire, 16: gasket, 17: sealing plug, 17a: liquid injection hole, 18: frame, 20: negative electrode current collector, 21: negative electrode active material layer (second layer), 22: flame retardant layer (third layer).

Claims

1. A secondary battery comprising a positive electrode and a negative electrode, The negative electrode contains a first layer comprising a negative electrode active material. The first layer also contains a flame retardant comprising halogen atoms. in, The flame retardant comprises a cyclic structure bonded with the halogen atoms, wherein the halogen atoms account for more than 45% by mass in the flame retardant. The negative electrode active material comprises: Select free inclusion SiO X The particle represents at least one particle from the group consisting of a first silicon oxide particle, a second particle comprising a lithium silicate phase and silicon particles dispersed in the lithium silicate phase, and a third particle comprising a carbon phase and silicon particles dispersed in the carbon phase, wherein 0.5 ≤ X < 1.6; and, graphite, Wherein, when the ratio of negative electrode active material to flame retardant in the first layer is expressed as 100:a, the mass ratio of negative electrode active material to flame retardant is greater than 1 and less than 5.

2. The secondary battery according to claim 1, wherein, The lithium silicate phase includes the formula Li 2Z SiO （2+Z） The lithium silicate shown, 0 <Z<2。 3. The secondary battery according to claim 1, wherein, The content of at least one particle in the negative electrode active material is more than 1% by mass.

4. The secondary battery according to claim 1, wherein, The negative electrode active material comprises a variety of particles selected from the group consisting of the first particle, the second particle, and the third particle.

5. The secondary battery according to any one of claims 1 to 4, wherein, The first layer comprises: a second layer containing at least the negative electrode active material; and a third layer disposed on the surface of the second layer and containing at least the flame retardant.

6. The secondary battery according to claim 5, wherein, The weight per unit area of ​​the third layer is 0.1 g / m². 2 Above and 10g / m 2 the following.

7. The secondary battery according to claim 5, further comprising a separator sandwiched between the positive electrode and the negative electrode. The third layer is disposed between the second layer and the separator.

8. The secondary battery according to claim 5, wherein, The thickness of the third layer is greater than 0.1 μm and less than 10 μm.

9. The secondary battery according to claim 5, wherein, The flame retardant content in the third layer is greater than that in the second layer.

10. The secondary battery according to claim 5, wherein, The flame retardant contained in the third layer accounts for more than 50% of the total content of the third layer by mass.

11. The secondary battery according to claim 1, wherein, The first layer contains carbon nanotubes.

12. The secondary battery according to claim 1, wherein, The flame retardant releases the halogen atoms at temperatures above 180°C.

13. The secondary battery according to claim 1, wherein, The flame retardant is selected from at least one of the following groups: 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, and tri(2,2,2-trifluoroethyl) phosphate.

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