Electrodes for secondary batteries and secondary batteries

Abstract

This disclosed electrode for secondary batteries comprises a current collector and an active material layer disposed on the surface of the current collector. The active material layer contains at least an active material, and an organic compound containing bromine. When the active material layer is divided into a first region and a second region having the same thickness, and the first region is closer to the current collector side than the second region, mass-based content A1 of the organic compound in the active material in the first region is greater than mass-based content A2 of the organic compound in the active material in the second region.　According to the present disclosure, a deterioration of cycle characteristics at low temperatures in secondary batteries is suppressed.

Description

[Technical Field]

[0001] This disclosure relates to electrodes for secondary batteries and secondary batteries using the same. [Background technology]

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

[0003] Patent Document 1 proposes using a positive electrode in which a halogen-substituted cyclic organic compound, substituted with one or more chlorine or bromine atoms, is added to a positive electrode active material mainly composed of a lithium-transition metal composite oxide, thereby suppressing oxidative decomposition of the electrolyte at the interface with the lithium-transition metal composite oxide in the charged state and reducing the increase in internal resistance during high-temperature storage.

[0004] Patent Document 2 proposes a positive electrode body having a positive electrode current collector and a positive electrode layer containing a positive electrode active material formed on the positive electrode current collector, in which a decomposition inhibitor that suppresses the decomposition of organic electrolytes such as metal oxides is added to the positive electrode layer, and the concentration of the decomposition inhibitor on the current collector side surface of the positive electrode layer is made higher than the concentration of the decomposition inhibitor on the opposite side surface, thereby efficiently suppressing the decomposition of organic electrolytes and improving cycle characteristics. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2000-58068 [Patent Document 2] Japanese Patent Publication No. 2009-64715 [Overview of the project] [Problems that the invention aims to solve]

[0006] Meanwhile, it is known that the cycle characteristics of a non-aqueous electrolyte secondary battery are also affected by the temperature at which the secondary battery is used. Although the technology disclosed in Patent Document 2 mentions the improvement of cycle characteristics, there is still room for improvement in terms of suppressing the deterioration of the cycle characteristics of the secondary battery at low temperatures. [Means for Solving the Problem]

[0007] One aspect according to the present disclosure relates to an electrode for a secondary battery. The electrode for a secondary battery includes a current collector and an active material layer disposed on a surface of the current collector. The active material layer contains at least an active material and an organic compound containing bromine. When the active material layer is divided into a first region and a second region having the same thickness, and the first region is located closer to the current collector than the second region, the mass-based content A1 of the organic compound relative to the active material in the first region is greater than the mass-based content A2 of the organic compound relative to the active material in the second region.

[0008] Another aspect according to the present disclosure relates to a secondary battery including a first electrode that is the aforementioned electrode for a secondary battery, a second electrode, and a separator interposed between the first electrode and the second electrode. [Advantageous Effects of the Invention]

[0009] According to the present disclosure, the deterioration of the low-temperature cycle characteristics of a secondary battery is suppressed.

[0010] The novel features of the present invention are set forth in the appended claims, and the present invention will be better understood with respect to both the constitution and content thereof, together with other objects and features of the present invention, through the following detailed description with reference to the accompanying drawings. [Brief Description of Drawings]

[0011] [Figure 1] FIG. 1 is a schematic perspective view with a part cut away of the secondary battery according to an embodiment of the present disclosure. [Mode for Carrying Out the Invention]

[0012] Examples of embodiments relating to this disclosure are described below. While examples of embodiments relating to this disclosure are given below, this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are achieved. In this specification, when referring to a "range of numerical values ​​A to numerical values ​​B," the range includes numerical values ​​A and B, and can be interpreted as "greater than or equal to numerical value A and less than or equal to numerical value B." In the following description, when exemplified lower and upper limits of numerical values ​​relating to specific physical properties or conditions, either of the exemplified lower limits and either of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than or equal to the upper limit. When multiple materials are exemplified, one may be selected and used alone, or two or more may be used in combination.

[0013] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0014] In the following explanation, the terms "contains" or "includes" encompass expressions such as "contains (or includes)," "substantially consists of," and "consists of."

[0015] (Electrode for secondary batteries) The electrode for a secondary battery according to this embodiment comprises a current collector and an active material layer disposed on the surface of the current collector. The active material layer comprises at least an active material and an organic compound containing bromine. Hereinafter, the organic compound containing bromine may be referred to as "bromine compound (B)".

[0016] The inclusion of bromine compound (B) in the active material layer improves the wettability of the electrolyte with the active material. This enhances the cycle characteristics of secondary batteries using electrodes designed for secondary batteries.

[0017] On the other hand, in order to increase the energy density of secondary batteries, attempts have been made to increase the density of the active material in the active material layer by compressing the electrode containing the active material layer (for example, by rolling). In this case, since the bromine compound (B) is softer than the active material, the bromine compound (B) present on the electrode surface is easily compressed during the electrode compression process. As a result, the voids on the electrode surface can be blocked by the bromine compound (B), which can hinder the flow of the electrolyte within the electrode. In low-temperature environments, the fluidity of the electrolyte itself may decrease compared to room temperature, and as a result, the charge-discharge reaction is more easily inhibited in regions closer to the current collector than to the surface in the thickness direction of the electrode, and the cycle characteristics at low temperatures tend to deteriorate.

[0018] In the secondary battery electrode according to this embodiment, the active material layer is divided into a first region and a second region having the same thickness, and assuming that the first region is closer to the current collector than the second region, the content A1 of bromine compound (B) in the first region is greater than the content A2 of bromine compound (B) in the second region. That is, in the secondary battery electrode according to this embodiment, the content A3 of bromine compound (B) observed on the surface of the electrode on the current collector side is greater than the content A4 of bromine compound (B) observed on the surface of the electrode on the separator side. The content A1 to A4 of bromine compound (B) are expressed as a mass-based content ratio relative to the active material. By reducing the amount of bromine compound (B) contained in the second region on the surface side of the electrode, the decrease in the fluidity of the electrolyte within the electrode can be suppressed. Ensuring the fluidity of the electrolyte within the electrode makes it possible to enjoy the effect of bromine compound (B) in improving cycle characteristics, even when using secondary batteries in low-temperature environments where the fluidity of the electrolyte itself decreases.

[0019] The electrode for the secondary battery is obtained, for example, by forming a first layer containing an active material and a bromine compound (B) on a current collector, and then forming a second layer containing at least an active material on the first layer. The first and second layers do not have to be the same thickness. That is, the boundary between the first and second layers does not have to coincide with the boundary between the first and second regions. If the content of bromine compound (B) in the first layer is higher than the content of bromine compound (B) in the second layer, then when the entire first and second layers are divided into first and second regions of the same thickness, the content of bromine compound (B) A1 in the first region will be greater than the content of bromine compound (B) A2 in the second region. The electrode for the secondary battery has a laminated structure of three or more active material layers with different bromine compound (B) content, and the bromine compound (B) content may be lower in the layers further away from the current collector. Furthermore, the concentration of bromine compound (B) within the active material layer may continuously change such that it decreases as it moves further away from the current collector.

[0020] The content A2 of bromine compound (B) in the second region may be 0.5 times or less (A2 / A1 is 1 / 2 or less) of the content A1 of bromine compound (B) in the first region, and more preferably 0.25 times or less (A2 / A1 is 1 / 4 or less). The content A4 of bromine compound (B) confirmed on the surface of the electrode on the separator side may be 0.5 times or less (A4 / A3 is 1 / 2 or less) of the content A3 of bromine compound (B) confirmed on the surface of the electrode on the current collector side, and more preferably 0.25 times or less (A4 / A3 is 1 / 4 or less), and bromine compound (B) may not be confirmed on the surface of the electrode on the separator side. When A1 and A2 satisfy this relationship, the inhibition of the charge-discharge reaction in the first region is effectively suppressed, and high cycle characteristics can be maintained even at low temperatures.

[0021] The first region is a surface layer region extending to a predetermined depth from the surface of the secondary battery electrode, and may not substantially contain bromine compound (B). For example, the secondary battery electrode may not contain bromine compound (B) in the region from the surface of the active material layer that does not face the current collector to a depth of at least 1 / 4 of the total thickness of the active material layer. In this case, the inhibition of the charge-discharge reaction in the first region is significantly suppressed, and high cycle characteristics can be maintained even at low temperatures. The surface layer region that does not contain bromine compound (B) may have a thickness of 1 / 3 or more of the total thickness of the active material layer, or it may have a thickness of 1 / 2 or more. Here, "a region that does not contain bromine compound (B)" means that the proportion of bromine compound (B) in that region is 0.02 parts by mass or less per 100 parts by mass of active material.

[0022] When the mass ratio of the active material to the bromine compound (B) in the active material layer is expressed as active material:bromine compound (B) = 100:a, it is preferable that a is between 0.1 and 1.5. More preferably, a may be between 0.1 and 1.0, between 0.1 and 0.75, between 0.15 and 0.75, or between 0.35 and 0.75. a represents the proportion of the bromine compound (B) to the active material in the entire active material layer, and is the average value of the bromine compound (B) content in the first region and the bromine compound (B) content in the second region. When a is 0.1 or higher, the effect of improving cycle characteristics by the bromine compound (B) is sufficiently obtained, and the decrease in the bromine compound (B) content in the second region can suppress the deterioration of cycle characteristics at low temperatures. Furthermore, when a is 1.5 or lower, the increase in internal resistance due to the addition of the bromine compound (B) can be suppressed.

[0023] The electrode for the secondary battery may be either a positive electrode or a negative electrode. When the electrode for the secondary battery is a positive electrode, the active material is a positive electrode active material, and the active material layer is a positive electrode active material layer. When the electrode for the secondary battery is a negative electrode, the active material is a negative electrode active material, and the active material layer is a negative electrode active material layer. According to this embodiment, by adding a bromine compound (B) together with the positive electrode active material or the negative electrode active material to either the positive electrode active material layer or the negative electrode active material layer, and by reducing the content of the bromine compound (B) on the electrode surface side compared to the current collector side, the effect of improving cycle characteristics by the bromine compound (B) can be maximized, and the deterioration of cycle characteristics at low temperatures can be suppressed.

[0024] (Manufacturing method for electrodes for secondary batteries) The electrode for a secondary battery according to this embodiment can be manufactured by a method comprising, for example, the steps of preparing a current collector and arranging an active material layer on the surface of the current collector. The step of arranging the active material layer includes the steps of forming a first layer on the surface of the current collector which comprises at least an active material and an organic compound containing bromine, and forming a second layer on the first layer which comprises at least the active material.

[0025] The first layer is formed, for example, by coating a slurry containing an active material and a bromine compound (B) onto a current collector and drying it. The second layer is formed, for example, by coating a slurry containing at least an active material on top of the first layer and drying it. The slurry used to form the second layer may also contain a bromine compound (B). However, the content of the bromine compound (B) in the slurry used to form the second layer is less than the content of the bromine compound (B) in the slurry used to form the first layer, so that the content of the bromine compound (B) in the second layer is less than the content of the bromine compound (B) in the first layer.

[0026] A slurry that does not contain bromine compound (B) may be used to form the second layer. This makes it possible to form a second layer on the surface of the electrode that is substantially free of bromine compound (B).

[0027] The process may further include a step of compressing the active material layer after the formation of the second layer. Compression can be performed, for example, by rolling the electrode on which the active material layer is arranged. Compression increases the density of the active material in the electrode, making it easier to achieve high capacity.

[0028] During compression, the surface of the active material layer that does not face the current collector is more susceptible to large compressive forces, while the current collector side of the active material layer is more easily relieved of compressive forces. As a result, the second layer of the active material layer is easily compressed, while the first layer is less so. Furthermore, if the active material layer contains a bromine compound (B), compression selectively deforms the bromine compound (B), making it easier to block voids within the active material layer.

[0029] However, according to the manufacturing method of this embodiment, the content of bromine compound (B) in the second layer is reduced, so even when the active material layer is compressed, blockage of voids in the second layer is suppressed. Therefore, the obstruction of electrolyte flow into the first layer is suppressed, and high cycle characteristics (especially cycle characteristics at low temperatures) can be maintained.

[0030] An active material layer may be formed by stacking three or more layers with different bromine compound (B) content using three or more types of electrode slurries. In this case, the bromine compound (B) content should be reduced in the layer further away from the current collector in the stacked structure.

[0031] (Bromine compound (B)) Bromine compound (B) enhances the wettability of the active material with the electrolyte, thereby improving its cycle characteristics. Bromine compound (B) adheres to the particles of the active material. The particle size of bromine compound (B) may be 0.5 μm to 10 μm. The particle size of bromine compound (B) is calculated by image analysis of the cross-section of the active material layer, as described later.

[0032] The bromine compound (B) may contain a cyclic structure to which bromine atoms (Br) are bonded. The cyclic structure may or may not be an aromatic ring. In this case, all bromine atoms (Br) may be bonded to the cyclic structure, or only some of the bromine atoms (Br) may be bonded to the cyclic structure. A structure in which bromine atoms (Br) are bonded to the cyclic structure is preferred because it makes it easier to increase the bromine atom content.

[0033] The proportion of bromine atoms (Br) in the bromine compound (B) is preferably 45% by mass or more. This proportion may be 60% by mass or more (for example, 70% by mass or more). There is no particular upper limit, but it may be 95% by mass or less (for example, 90% by mass or more). These lower and upper limits can be combined arbitrarily. Such a bromine compound (B) has a large number of bromine atoms, and a bromine compound (B) with a large number of bromine atoms further improves the wettability of the electrolyte to the active material, thereby enhancing the effect of improving the cycle characteristics.

[0034] Furthermore, such bromine compounds (B) exhibit flame-retardant effects by releasing bromine atoms (Br) at high temperatures, and may also have the effect of suppressing excessive heat generation in secondary batteries. By using secondary battery electrodes containing bromine compounds (B) in the active material layer, excessive heat generation in the event of a secondary battery malfunction can be suppressed.

[0035] Examples of bromine compounds (B) include at least one selected from the group consisting of ethylene-1,2-bispentabromophenyl, ethylenebistetrabromophthalimide, tetrabromobisphenol A, hexabromocyclododecane, and 2,4,6-tribromophenol. These bromine compounds (B) may be commercially available, or they may be synthesized by known synthetic methods.

[0036] The structural formula of ethylene-1,2-bispentabromophenyl, an example of a bromine compound (B), is shown below. The molecular weight of ethylene-1,2-bispentabromophenyl is 971.2, and it contains 10 bromine atoms (atomic weight: 79.9). Therefore, the proportion of bromine atoms (Br) in ethylene-1,2-bispentabromophenyl is 100 × 10 × 79.9 / 971.2 = 82.3% by mass.

[0037] [ka]

[0038] In addition, because bromine compounds (B) containing such bromine atoms (Br) have a high specific gravity, their volume can be reduced relative to their added weight. This allows for the addition of a sufficient amount of bromine compound (B) while maintaining a high amount of active material, thus enabling the maintenance of high capacity. Bromine compounds (B) tend to have a high specific gravity due to the bonding of halogen atoms (Ha) to their cyclic structure. The specific gravity of bromine compound (B) may be, for example, 2.7 or higher, and preferably 3.0 or higher.

[0039] It is preferable that the bromine compound (B) does not contain any water-generating moieties and / or hydrophilic groups in its structure. In this case, water is less likely to enter the battery during the manufacturing process of the secondary battery, and a highly reliable secondary battery can be realized. Examples of water-generating moieties include hydroxyl groups (-OH), carboxyl groups (-COOH), carbonyl groups (-CO-), and oxoacid groups such as sulfo groups and phosphate groups. Examples of hydrophilic groups include the above functional groups as well as amino groups.

[0040] Furthermore, when using a secondary battery electrode as the negative electrode and employing a negative electrode active material containing silicon (Si), as described later, the bromine atoms (Br) in the bromine compound (B) react with Si, forming a stable film on the surface of the negative electrode active material. This allows for the maintenance of higher cycle characteristics and the expectation of high durability.

[0041] The amount of active material in the active material layer can be determined from a sample obtained by removing only the active material layer from a discharged secondary battery. Specifically, first, the discharged secondary battery is disassembled and the electrodes are removed. Next, the electrodes are cleaned with an organic solvent, then vacuum-dried, and finally the active material layer is peeled off to obtain a sample. By performing thermal analysis such as TG-DTA on this sample, the ratio of binder components and conductive material components other than the active material can be calculated. In addition, the amount of bromine compound (B) in the active material layer can be determined by elemental analysis such as EDS on a cross-section of the active material layer.

[0042] The particle size of bromine compound (B) is determined by observing the cross-section of the active material layer and obtaining the average particle diameter through image processing. The cross-section may be formed using a cross-section polisher (CP). The diameter of a circle having the same area as the cross-sectional area of ​​the bromine compound (B) particles (the area of ​​the particles observed in the cross-section of the active material layer) is determined, and the average value of the equivalent circle diameters is taken as the particle size of bromine compound (B). Ten or more particles are observed, and the maximum diameter is determined.

[0043] (Secondary battery) The secondary battery according to this embodiment comprises a first electrode, a second electrode, and a separator interposed between the first and second electrodes, which are electrodes for a secondary battery as described above. Hereinafter, the secondary battery according to this embodiment may be referred to as "secondary battery (S)". One of the first and second electrodes is the positive electrode, and the other is the negative electrode.

[0044] At least the first electrode of the two electrodes is a secondary battery electrode containing the bromine compound (B) described above. The first electrode may be the positive electrode and the second electrode the negative electrode, or the first electrode may be the negative electrode and the second electrode the positive electrode. That is, the positive electrode of the secondary battery (S) may be composed of the above secondary battery electrode, and the negative electrode of the secondary battery (S) may be composed of the above secondary battery electrode. Both the positive and negative electrodes may be composed of the above secondary battery electrode. Below, the configuration of the secondary battery (S) will be described using the case where the positive electrode of the secondary battery (S) is the above secondary battery electrode as an example.

[0045] Below, an example of a secondary battery (S) according to this embodiment and examples of its components will be described in detail. Note that known components may be used for components that are not characteristic of this disclosure. The secondary battery (S) includes, for example, an outer casing (battery case), and a positive electrode, a negative electrode, an electrolyte, and a separator arranged inside the outer casing. The separator is positioned between the positive electrode and the negative electrode. The secondary battery may be a non-aqueous electrolyte secondary battery.

[0046] The shape of the secondary battery (S) is not limited and may be cylindrical, rectangular, coin-shaped, button-shaped, etc. The battery case is selected according to the shape of the secondary battery (S).

[0047] [Negative electrode] The negative electrode includes a negative electrode mixture. Typically, the negative electrode includes a negative electrode current collector and a negative electrode active material layer (negative electrode mixture layer) disposed on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and, optionally, a bromine compound (B) and other components other than the negative electrode active material and bromine compound (B). Examples of other components include binders, conductive agents, and thickeners. These other components may be those used in known secondary batteries.

[0048] The negative electrode active material layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector to form a coating, and then drying the coating. The dried coating may be rolled if necessary. Examples of dispersion media include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or mixed solvents thereof. The ratio of components in the negative electrode mixture can be adjusted by changing the mixing ratio of the materials in the negative electrode mixture.

[0049] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) and porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0050] [Negative electrode active material] As the negative electrode active material, a material capable of electrochemically intercalating and releasing lithium ions is preferably used. Examples of such materials include carbonaceous materials and Si-containing materials. The negative electrode active material may be used alone or in combination of two or more types.

[0051] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). A single carbonaceous material may be used, or two or more may be used in combination. Among these, graphite is preferred as the 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 may also be used.

[0052] Graphite refers to a material in which a graphite-type crystal structure has developed, and generally refers to the average interplanar spacing d of the (002) plane, which is measured by X-ray diffraction. 002 This refers to carbon materials with a wavelength of 0.340 nm or less.

[0053] Si-containing materials include elemental Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which the silicon phase is dispersed within the lithium-ion conductive phase (matrix). Examples of silicon oxides include SiO2. X Particles are an example. X is, for example, 0.5 ≤ X < 2, and may also be 0.5 ≤ X < 1.6 or 0.8 ≤ X ≤ 1.6. As the lithium ion conducting phase, at least one selected from the group consisting of SiO2 phase, silicate phase, and carbon phase may be used.

[0054] Examples of Si-containing materials include SiO XAt least one particle (P) selected from the group consisting of a first particle containing silicon oxide represented by the formula (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 may be used. In this specification, the silicon particles contained in the second particle can be read as the silicon phase, and the silicon particles contained in the third particle can be read as the silicon phase.

[0055] By using silicon (Si)-containing particles (P) as the negative electrode active material, it is possible to increase the capacity of the battery.

[0056] The negative electrode active material may contain multiple types of particles selected from the group consisting of first particles, second particles, and third particles. For example, particle (P) may consist of two types of particles selected from these, or it may contain all three types of particles. Specifically, the negative electrode active material may contain first particles and second particles, first particles and third particles, or second particles and third particles. Alternatively, the negative electrode active material may contain all of the first, second, and third particles. It is preferable to use particle (P) in combination with graphite as the negative electrode active material.

[0057] When the negative electrode active material contains graphite and particles (P), the particle (P) content in the negative electrode active material may be 1% by mass or more. This configuration allows for higher capacity compared to when the negative electrode active material is graphite alone. The particle (P) content in the negative electrode active material may be 3% by mass or more. This content may also be 50% by mass or less. These lower and upper limits can be combined arbitrarily as long as they do not contradict each other.

[0058] The graphite content in the negative electrode active material may be in the range of 50 to 99% by mass. However, if particles (P) contain graphite on their surface and / or internally, that graphite is not included in the above graphite content. The graphite content is the content of graphite not contained in the particles (P).

[0059] [First particles] The first particles are SiOX X comprising silicon oxide represented by the formula (0.5≦X<1.6). The first particles may include silicon oxide particles and a carbon layer disposed around the silicon oxide particles.

[0060] The average particle diameter of the first particles may be in the range of 1 μm to 25 μm (for example, in the range of 4 μm to 15 μm).

[0061] [Second particles] The second particles include a lithium silicate phase and silicon particles dispersed in the lithium silicate phase. The lithium silicate phase is Li2Z 2Z SiO(2-Z) (2+Z) which may include lithium silicate represented by the formula (0<Z<2), and may be composed of the lithium silicate. Z preferably satisfies the relationship of 0<Z<1. 50% by mass or more (for example, 60% by mass or more) of the lithium silicate phase may be composed of lithium silicate satisfying 0<Z≦0.5.

[0062] The second particles may include 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 alkaline earth metal elements include Mg, Ca, Sr, Ba and the like.

[0063] The element Me may be dispersed as an Me oxide in the lithium silicate phase. The Me oxide may include at least one selected from the group consisting of yttrium oxide, cerium oxide, calcium oxide, and magnesium oxide. The lithium silicate phase may include zirconium oxide. Further, the element Me may be dispersed in the zirconium oxide.

[0064] The amount of elemental Me contained in the second particle can be indicated by the amount calculated assuming that elemental Me forms a stoichiometric oxide, regardless of the state of elemental Me or the type of compound of elemental Me (estimated Me oxide amount). The estimated Me oxide amount may be in the range of 0.001 mass% to 1.0 mass% relative to the total of the lithium silicate phase and silicon particles. By setting the estimated Me oxide amount to 0.001 mass% or more, the effect of reducing the reaction area and improving the hardness of the lithium silicate phase is greatly increased. On the other hand, by setting the estimated Me oxide amount to 1.0 mass% or less, the decrease in initial volume can be suppressed.

[0065] The lithium silicate phase may contain metal compounds such as metal oxides, metal carbides, metal nitrides, and metal borides. Preferred metal compounds are metal oxides and metal carbides. In particular, it is preferable to use 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). The amount of metal element compounds other than element Me may be in the range of 0.005 mass% to 15 mass% (for example, in the range of 0.01 mass% to 10 mass% or 0.01 mass% to 1 mass%) relative to the total of the lithium silicate phase and silicon particles. The amount of metal element compounds can be determined by assuming that the metal elements form stoichiometric oxides, similar to the content of element Me.

[0066] The average particle size of the second particle may be in the range of 1 μm to 25 μm (for example, 4 μm to 15 μm). In this range, stress due to volume changes of the second particle during charging and discharging is easily relieved, making it easier to obtain good cycle characteristics. Furthermore, the surface area of ​​the second particle becomes appropriate, and capacity reduction due to side reactions with non-aqueous electrolytes is suppressed.

[0067] The crystallite size of silicon particles dispersed within the lithium silicate phase is, for example, 10 nm or larger. The silicon particles have a particulate phase of elemental silicon (Si). When the crystallite size of the silicon particles is 10 nm or larger, the surface area of ​​the silicon particles can be kept small, making it less likely for the silicon particles to degrade, which is associated with the generation of irreversible capacitance. The crystallite size of the silicon particles is calculated using Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon particles.

[0068] The average particle size of the silicon particles in the second particle is preferably 500 nm or less (more preferably 200 nm or less, and even more preferably 50 nm or less) before the first charge. After the first charge, the average particle size of the silicon particles is preferably 400 nm or less (more preferably 100 nm or less). By miniaturizing the silicon particles, the volume change during charging and discharging is reduced, and the structural stability of the second particle is further improved.

[0069] The content of silicon particles (elementary Si) in the second particle is preferably in the range of 20% to 95% by mass (for example, 35% to 75% by mass) from the viewpoint of increasing capacity and improving cycle characteristics. Within this range, lithium ion diffusion is also good, making it easier to obtain excellent loading characteristics. Furthermore, the surface area of ​​silicon particles that is not covered by the lithium silicate phase is reduced, and side reactions between the non-aqueous electrolyte and silicon particles are suppressed.

[0070] The second particle may include a conductive material covering at least a portion of its surface. Since the lithium silicate phase has poor electronic conductivity, the conductivity of the second particle tends to be low as well. By covering the surface with a conductive material, the conductivity can be dramatically increased. Preferably, the conductive layer is thin enough not to substantially affect the average particle size of the second particle. For example, from the viewpoint of ensuring conductivity and lithium ion diffusion, the thickness of the conductive layer may be in the range of 1 nm to 200 nm (e.g., 5 nm to 100 nm). Examples of materials for the conductive layer and examples of methods for forming it will be described later.

[0071] [The third particle] The third particle comprises a carbon phase and silicon particles dispersed within the carbon phase. The carbon phase of the third particle may consist of amorphous carbon. Amorphous carbon may be hard carbon, soft carbon, or something else. Amorphous carbon is generally defined as the average interplanar spacing d of the (002) plane as measured by X-ray diffraction. 002 This refers to carbon materials with a wavelength exceeding 0.34 nm.

[0072] The third particle consists of a carbon phase and silicon particles dispersed within the carbon phase. The carbon phase of the third particle is conductive. Therefore, even if voids form around the third particle, contact between the third particle and its surroundings is easily maintained. As a result, capacity degradation due to repeated charge-discharge cycles is easily suppressed.

[0073] The average particle size of the third particle may be 3 μm or more and 18 μm or less, 6 μm or more and 15 μm or less, or 8 μm or more and 12 μm or less.

[0074] The silicon particle content in the third particle may be 30% by mass or more and 80% by mass or less, or 40% by mass or more and 70% by mass or less. Within this range, a sufficiently high capacity of the negative electrode can be achieved, and the cycle characteristics are also easily improved.

[0075] The average particle size of the silicon particles in the third particle can be, for example, 1 nm or larger. Alternatively, the average particle size of the silicon particles may be 1000 nm or less, 500 nm or less, 200 nm or less, or 100 nm or less (and even 50 nm or less). The finer the silicon particles, the smaller the volume change of the third particle during charging and discharging, and the improved structural stability of the third particle.

[0076] The composition and component content of the second and third particles can be analyzed by the method described in International Publication No. 2018 / 179969.

[0077] The content of each element in the particles (P) may be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, the particles (P) are dissolved in a heated acid solution, the carbon in 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. Subsequently, a calibration curve is created using commercially available standard solutions of each element, and the content of each element is calculated.

[0078] The second and third particles each possess a so-called sea-island structure. The silicon particles (islands) in the second and third particles are dispersed in a matrix (sea) of silicate and carbon phases, respectively, and covered by a lithium-ion conducting phase (silicate and carbon phases). In the sea-island structure, contact between the silicon particles and the electrolyte is limited, thus suppressing side reactions. Furthermore, stresses generated by the expansion and contraction of the silicon particles are relieved by the lithium-ion conducting phase matrix.

[0079] The average particle size of the graphite (graphite particles) included as the active material in the negative electrode may be between 13 μm and 25 μm. Preferably, the average particle size of the graphite is larger than the average particle size of the particles (P). With this configuration, voids are formed between the relatively large graphite particles, and the particles (P) are easily accommodated in these voids. Therefore, it is easier to increase the packing density of the active material in the negative electrode and to obtain a higher-capacity negative electrode. In addition, the particles (P) present in the voids contribute to maintaining electronic contact between the graphite particles. On the other hand, even if the particles (P) present in the voids expand and contract, expansion and contraction of the entire negative electrode are unlikely to occur, thus reducing degradation due to charge-discharge cycles.

[0080] The average particle sizes of particles (P), silicon particles within particles (P), and graphite in the negative electrode mixture may be measured by observing a cross-section of the layer formed by the negative electrode mixture using SEM or TEM. In this case, the average particle size is obtained by arithmetic mean of the maximum diameters of any 100 particles.

[0081] For the average particle diameter of the particles (P) before forming the negative electrode mixture, the median diameter (D 50 ) can be used. The median diameter can be determined, for example, using a laser diffraction / scattering particle size distribution analyzer.

[0082] [Method for Producing First Particles] The first particles may be produced, for example, by the following method. First, particles having a composition of SiO (silicon monoxide) are pulverized and classified to adjust the particle size. Next, the surface of the obtained particles is coated with carbon by a CVD method under an argon atmosphere. Then, by crushing and classifying the resulting product, SiO X the first particles represented by are prepared. Note that SiO X Various well-known methods can be employed for coating particles with carbon. Further, the treatment of coating SiO X particles with carbon may be omitted.

[0083] [Method for Producing Second Particles] Next, an example of a method for producing the second particles will be described in detail. The second particles may be produced by a method other than the production method described below. The second particles may be produced by the method described in International Publication No. 2018 / 179969.

[0084] The second particles are generally synthesized through two processes: a pre-process for obtaining lithium silicate, and a post-process for obtaining the second particles from lithium silicate and raw material silicon. When the element Me is added, the element Me may be added to the raw material of lithium silicate in the pre-process, but it is preferably added in the post-process so as not to affect the synthesis of lithium silicate. More specifically, the method for producing the second particles includes: step (i) of mixing silicon dioxide and a lithium compound, and firing the obtained mixture to obtain lithium silicate; and step (ii) of forming a composite of lithium silicate and raw material silicon (and further the element Me if necessary) to obtain second particles including a lithium silicate phase and silicon particles dispersed in the lithium silicate phase. It is preferable to include this step (ii).

[0085] [Step (i)] Formula: Li 2Z SiO 2+Z The Z value of lithium silicate, represented by [formula], can be controlled by the atomic ratio of silicon to lithium in the mixture of silicon dioxide and lithium compound: Li / Si. To synthesize high-quality lithium silicate with minimal leaching of alkaline components, it is preferable to make Li / Si less than 1.

[0086] Lithium compounds such as lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride can be used. These may be used individually or in combination of two or more.

[0087] It is preferable to heat the mixture containing silicon dioxide and a lithium compound in air at 400°C to 1200°C, preferably 800°C to 1100°C, to react the silicon dioxide and the lithium compound.

[0088] [Step (ii)] Next, the lithium silicate is compounded with the raw silicon. For example, the mixture of lithium silicate and raw silicon (which may also contain the element Me) can be pulverized while applying shear force. For the raw silicon, coarse silicon particles with an average particle size of several μm to several tens of μm can be used. It is preferable that the silicon particles obtained in the end have a crystallite size of 10 nm or more, calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane of the XRD pattern.

[0089] For the element Me used in the preparation, any oxide, oxalate, nitrate, sulfate, halide, or carbonate of element Me may be used. Among these, Me oxides are preferred because they are stable and have good ionic conductivity. More specifically, examples include CeO2, Sc2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. Compounds containing elements other than Me and oxygen, such as yttria-stabilized zirconia, may also be used. These may be used individually or in combination of two or more.

[0090] For example, lithium silicate and raw silicon (and optionally a compound of element Me) can be mixed in a predetermined mass ratio, and the mixture can be stirred while being atomized using a grinding device such as a ball mill. However, the compounding process is not limited to this. For example, silicon nanoparticles and lithium silicate nanoparticles (and optionally a compound of element Me) can be synthesized without using a grinding device, and these can then be mixed.

[0091] Next, the particulated mixture is heated and sintered at 450°C to 1000°C in an inert atmosphere (e.g., an atmosphere such as argon or nitrogen). At this time, pressure may be applied to the mixture using a hot press or the like during sintering to produce a sintered body of the mixture. Lithium silicate is stable at 450°C to 1000°C and hardly reacts with silicon, so any decrease in capacity is minor. During sintering, the silicate softens and flows to fill the gaps between silicon particles. As a result, a dense, block-shaped sintered body can be obtained in which the silicate phase forms the "sea" and the silicon particles form the "islands".

[0092] The sintered body can then be crushed into granular material to obtain second particles. By appropriately selecting the crushing conditions, second particles with an average particle size within the range described above can be obtained.

[0093] After step (ii), step (iii) may be performed to form a conductive layer by coating at least a portion of the surface of the second particles with a conductive material. The conductive material is preferably electrochemically stable, and carbon material is preferred. As a method for coating the surface of the particulate material with carbon material, a CVD method using hydrocarbon gases such as acetylene and methane as raw materials may be used. Alternatively, a method may be used in which coal pitch, petroleum pitch, phenolic resin, etc., are mixed with the second particles and then heated. In addition, carbon black may be attached to the surface of the second particles.

[0094] A step of washing the second particles with acid may be performed. For example, the second particles may be washed with an acidic aqueous solution. Washing with acid can dissolve and remove trace amounts of components such as Li2SiO3 that may be generated when the raw silicon and lithium silicate are compounded. As the acidic aqueous solution, aqueous solutions of inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, or aqueous solutions of organic acids such as citric acid and acetic acid can be used.

[0095] [Method for producing the third particle] The first and second methods are described below as examples of methods for producing the third particle. The third particle may also be produced by methods other than those described below.

[0096] In the first method, the raw silicon and carbon source are first mixed, and the mixture of raw silicon and carbon source is pulverized and compounded using a pulverizing device such as a ball mill, while simultaneously reducing it to fine particles. Alternatively, an organic solvent may be added to the mixture for wet pulverization. At this stage, the raw silicon is finely pulverized to produce silicon particles. These silicon particles are then dispersed in the matrix of the carbon source.

[0097] As a carbon source, examples of water-soluble resins such as carboxymethylcellulose (CMC), hydroxyethylcellulose, polyacrylates, polyacrylamide, polyvinyl alcohol, polyethylene oxide, and polyvinylpyrrolidone, as well as sugars such as cellulose and sucrose, petroleum pitch, coal pitch, and tar may be used, but are not particularly limited.

[0098] Suitable organic solvents include alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, and metal alkoxides.

[0099] Next, the composite of silicon particles and a carbon source is heated to 700°C to 1200°C in an inert gas atmosphere (e.g., argon or nitrogen). This heating carbonizes the carbon source, generating amorphous carbon. This yields a third particle in which silicon particles are dispersed in a carbon phase containing amorphous carbon.

[0100] In the second method, first, the raw silicon and carbon material are mixed, and the mixture of raw silicon and carbon material is pulverized and compounded using a pulverizing device such as a ball mill, while simultaneously reducing it to fine particles. Alternatively, an organic solvent may be added to the mixture for wet pulverization. At this stage, the raw silicon is finely pulverized to produce silicon particles. These silicon particles are then dispersed in a matrix of carbon material.

[0101] By compounding the raw silicon and carbon material as described above, a third particle is obtained in which silicon particles are dispersed in the carbon phase of amorphous carbon. Subsequently, the third particle may be heated to 700°C to 1200°C in an inert gas atmosphere.

[0102] As the carbon material, amorphous carbon is preferred, and easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), and carbon black can be used. Examples of carbon black include acetylene black and Ketjen black. Even when graphite is used as the carbon material, when a composite of silicon particles and the carbon material is obtained using a grinding device, the crystalline structure of the graphite is almost completely lost, and an amorphous carbon phase is formed.

[0103] Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives. Examples of conductive agents include carbon black, conductive fibers, fluorinated carbon, and organic conductive materials. Examples of thickeners include carboxymethylcellulose (CMC) and polyvinyl alcohol. These components may be used individually or in combination of two or more materials.

[0104] [Positive electrode] The positive electrode includes a positive electrode mixture. Typically, the positive electrode includes 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 active material layer includes a positive electrode active material and a bromine compound (B), and optionally includes other components other than the positive electrode active material and bromine compound (B). Examples of other components include binders, conductive agents, and thickeners. These other components may be components used in known secondary batteries.

[0105] The positive electrode active material layer can be formed by applying a positive electrode slurry, which is obtained by dispersing a positive electrode mixture in a dispersion medium, to the surface of the positive electrode current collector to form a coating, and then drying the coating. The dried coating may be rolled if necessary.

[0106] The positive electrode active material layer is formed such that, when the positive electrode active material layer is divided into a first region on the positive electrode current collector side and a second region on the positive electrode surface side, both having the same thickness, the content of bromine compound (B) in the first region is greater than the content of bromine compound (B) in the second region. Such a positive electrode active material layer can be realized, for example, by applying the positive electrode slurry in multiple stages using two or more positive electrode slurries with different mixing ratios of bromine compound (B). By unevenly distributing the content of bromine compound (B) in the thickness direction of the positive electrode active material layer so that the content of bromine compound (B) decreases as it approaches the positive electrode surface, the deterioration of cycle characteristics at low temperatures can be suppressed even when bromine compound (B) is included in the positive electrode active material layer.

[0107] A lithium composite metal oxide can be used as the positive electrode active material. For example, Li a CoO2, Li a KiO2, 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 b O 4、 LiGPO 4、 Li2GPO4F is one example. 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 includes at least one transition element (for example, at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. Note that the value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0108] The same binders and conductive agents as those exemplified for the negative electrode can be used. Graphite such as natural graphite or artificial graphite may be used as the conductive agent.

[0109] The shape and thickness of the positive electrode current collector can be selected from the same shape and range as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0110] [Electrolytes] The electrolyte can be an electrolyte solution containing a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that undergoes ion dissociation in the electrolyte solution. The solute may include, for example, a lithium salt. Components of the electrolyte solution other than the solvent and solute are additives. Various additives may be included in the electrolyte solution.

[0111] Non-aqueous solvents are used as solvents. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). Non-aqueous solvents may be used individually or in combination of two or more.

[0112] Other non-aqueous solvents include cyclic ethers, linear ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0113] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts of fluorine-containing acids (such as LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0114] The lithium salt concentration in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0115] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0116] [Separator] A separator may be placed between the positive and negative electrodes. The separator can be made of a material with high ion permeability, appropriate mechanical strength, and insulating properties. Examples of separators include microporous thin films, woven fabrics, and nonwoven fabrics. Polyolefins such as polypropylene and polyethylene are preferred as the material for the separator.

[0117] An example of a secondary battery (S) includes an outer casing, an electrode group housed within the casing, and a non-aqueous electrolyte. The structure of the electrode group is not particularly limited. One example of an electrode group is formed by winding a positive electrode, a negative electrode, and a separator such that a separator is placed between the positive electrode and the negative electrode. Another example of an electrode group is formed by stacking a positive electrode, a negative electrode, and a separator such that a separator is placed between the positive electrode and the negative electrode. The form of the secondary battery (S) is not limited and may be cylindrical, prismatic, coin-shaped, button-shaped, laminate-shaped, etc.

[0118] There are no particular limitations on the method of manufacturing the secondary battery (S); known manufacturing methods may be applied, or known manufacturing methods may be applied with at least a part of them modified.

[0119] Examples of embodiments relating to this disclosure will be specifically described below with reference to the drawings. The components of the examples described below can be the components described above. Furthermore, the examples described below can be modified based on the above description. In addition, the matters described below may be applied to the embodiments described above. Furthermore, in the embodiments described below, components that are not essential to the secondary battery relating to this disclosure may be omitted.

[0120] Figure 1 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present disclosure. The secondary battery 1 shown in Figure 1 includes a bottomed rectangular battery case 11, an electrode group 10 and an electrolyte (not shown) housed within the battery case 11. The electrode group 10 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 10 is formed by winding the negative electrode, positive electrode, and separator around a flat core and then removing the core.

[0121] 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 on the sealing plate 12. A gasket 16 is placed 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 electrically connected to the battery case 11, which also serves as the positive electrode terminal. A resin frame 18 is placed on top of the electrode group 10. The frame 18 isolates the electrode group 10 from the sealing plate 12 and also isolates the negative electrode lead 15 from the battery case 11. The opening of the battery case 11 is sealed by the sealing plate 12. An injection hole 17a is formed in the sealing plate 12. The electrolyte is injected into the battery case 11 through the injection port 17a. After that, the injection port 17a is sealed by the seal 17. [Examples]

[0122] The secondary battery relating to this disclosure will be described in more detail by reference to examples, but this disclosure is not limited to the following examples.

[0123] In this embodiment, several types of secondary batteries were fabricated and evaluated using the following procedure. The several types of secondary batteries use the above-mentioned secondary battery electrode containing a bromine compound (B) as the positive electrode, and differ in the content of the bromine compound (B) in the positive electrode active material layer and / or the distribution of the bromine compound (B) in the thickness direction.

[0124] [Fabrication of the negative electrode] Graphite was used as the negative electrode active material. First, the negative electrode active material, sodium carboxymethylcellulose (CMC-Na), styrene-butadiene rubber (SBR), and water were mixed in a predetermined mass ratio to prepare a negative electrode slurry. Next, a coating film was formed by applying the negative electrode slurry to the surface of the copper foil (negative electrode current collector). After drying the coating film, it was rolled. In this way, negative electrode mixture layers were formed on both sides of the copper foil.

[0125] [Fabrication of the positive electrode] As the positive electrode active material, LiNi 0.88 Co 0.09 Al 0.03 O2 was used. A first cathode slurry was prepared by mixing the cathode active material, polyvinylidene fluoride, N-methyl-2-pyrrolidone (NMP), acetylene black, and, if necessary, ethylene-1,2-bispentabromophenyl (SAYTEX®-8010, manufactured by Albemarle Japan Co., Ltd.) as a bromine compound (B) in a predetermined mass ratio.

[0126] Next, a second positive electrode slurry was prepared by mixing the above positive electrode active material, polyvinylidene fluoride, N-methyl-2-pyrrolidone (NMP), acetylene black, and, if necessary, ethylene-1,2-bispentabromophenyl (SAYTEX®-8010, manufactured by Albemarle Japan Co., Ltd.) as a bromine compound (B) in a predetermined mass ratio.

[0127] A first positive electrode slurry was applied to the surface of an aluminum foil (positive electrode current collector) to form a coating film, which was then dried to form the first layer of positive electrode active material. Subsequently, a second positive electrode slurry was applied on the first layer to form a coating film, which was then dried to form the second layer of positive electrode active material. The application amounts of the first and second positive electrode slurries were set so that the total mass of positive electrode active material in the first layer and the total mass of positive electrode active material in the second layer were the same.

[0128] Subsequently, the positive electrode active material layer was rolled to obtain a positive electrode in which a positive electrode active material layer having two layers, a first layer (lower layer) and a second layer (upper layer), was formed on both sides of the aluminum foil.

[0129] [Preparation of electrolyte solution] An electrolyte was prepared by adding LiPF6 as a lithium salt to a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. The concentration of LiPF6 in the non-aqueous electrolyte was 1.3 mol / liter.

[0130] [Manufacturing of secondary batteries] Lead tabs were attached to each electrode. Next, the positive and negative electrodes were wound in a spiral pattern via a separator so that the leads were located on the outermost edge. The electrode group was thus fabricated. Next, the electrode group was inserted into an outer casing made of laminate film with aluminum foil as a barrier layer and vacuum dried. Then, a non-aqueous electrolyte was injected into the outer casing and the opening of the casing was sealed. In this way, a secondary battery was obtained.

[0131] In this embodiment, multiple secondary batteries (batteries A1-A8, B1-B4) were fabricated by varying the content X1 of bromine compound (B) in the first layer and the content X2 of bromine compound (B) in the second layer. The content X1 of bromine compound (B) in the first layer was varied by changing the mixing ratio of bromine compound (B) during the preparation of the first positive electrode slurry. The content X2 of bromine compound (B) in the second layer was varied by changing the mixing ratio of bromine compound (B) during the preparation of the second positive electrode slurry.

[0132] The following evaluations were performed on the fabricated secondary batteries. (1) Measurement of capacity retention rate The discharge capacity of the fabricated secondary battery was measured using the following method. First, the battery was charged at a constant current of 0.5C in an environment of 10°C until the battery voltage reached 4.2V, and then charging was continued at a constant voltage until the current value reached 0.02C. After the charged battery was left for 20 minutes, it was discharged at a constant current of 0.5C until the battery voltage reached 2.5V. After that, it was left for another 20 minutes. This operation (charge-discharge cycle) was repeated 50 times.

[0133] The discharge capacity during the first discharge was defined as the initial capacity DC0, and the discharge capacity DC1 was measured after repeating the above charge-discharge cycle 50 times. The capacity retention rate was then calculated using the following formula. Capacity maintenance rate (%)=100×DC1 / DC0

[0134] The evaluation results are shown in Table 1. Table 1 also shows the content X1 of bromine compound (B) in the first layer, the content X2 of bromine compound (B) in the second layer, the ratio of content X2 to X1 (X2 / X1), and the total content of bromine compound (B) in the positive electrode active material layer ((X1+X2) / 2). In Table 1, content X1 and X2 are expressed in parts by mass per 100 parts by mass of positive electrode active material. In this example, since the thickness of the first layer and the thickness of the second layer are approximately equal, content X1 and X2 are approximately equal to the content A1 and A2 when the entire positive electrode active material layer is divided into a first region and a second region with the same thickness, respectively.

[0135] Batteries B1 to B4 are comparative examples, and the content of bromine compound (B) in the first and second layers was not changed (X1=X2). In battery B1, neither the first nor the second layer contains bromine compound (B) (X1=X2=0). In batteries B2 to B4, an improvement in capacity retention rate is observed compared to battery B1 due to the addition of bromine compound (B). However, the capacity retention rate is 75% or less, indicating a small improvement.

[0136] In contrast, in batteries A1 to A8, the content X1 of bromine compound (B) in the first layer is higher than the content X2 of bromine compound (B) in the second layer (X1 > X2). In this case, the capacity retention rate was maintained at a high level of 82% or more. In batteries A5 to A8, the second layer does not contain bromine compound (B) (X2 = 0). In this case, the capacity retention rate was 87% or more, and the cycle characteristics were significantly improved.

[0137] [Table 1] [Industrial applicability]

[0138] This disclosure can be used in secondary batteries, but its applications are not limited to this. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of Symbols]

[0139] 1: Non-aqueous electrolyte secondary battery, 10: Electrode group, 11: Battery case, 12: Sealing plate, 13: Negative electrode terminal, 14: Positive electrode lead, 15: Negative electrode lead, 16: Gasket, 17: Sealing plug, 17a: Injection hole, 18: Frame

Claims

1. Current collector and, It has an active material layer arranged on the surface of the current collector, The active material layer comprises at least an active material and an organic compound containing bromine, When the active material layer is divided into a first region and a second region having the same thickness, and the first region is located closer to the current collector than the second region, the mass-based content A of the organic compound relative to the active material in the first region is １ However, the mass-based content A of the organic compound relative to the active material in the second region ２ Larger than, The aforementioned organic compound includes a cyclic structure to which a bromine atom is bonded, An electrode for a secondary battery, wherein the proportion of bromine atoms in the aforementioned organic compound is 45% by mass or more.

2. A mass-based content of the organic compound relative to the active material in the second region ２ However, the mass-based content A of the organic compound relative to the active material in the first region １ The electrode for a secondary battery according to claim 1, wherein it is 0.5 times or less.

3. The electrode for a secondary battery according to claim 1, wherein the organic compound is not contained in the region of the active material layer from the surface of the active material layer not facing the current collector to a depth of at least one-quarter of the active material layer.

4. The electrode for a secondary battery according to any one of claims 1 to 3, wherein when the mass ratio of the active material to the organic compound in the active material layer is expressed as active material:organic compound = 100:a, a is 0.1 or more and 1.5 or less.

5. The electrode for a secondary battery according to any one of claims 1 to 3, wherein the organic compound is at least one selected from the group consisting of ethylene-1,2-bispentabromophenyl, ethylenebistetrabromophthalimide, tetrabromobisphenol A, hexabromocyclododecane, and 2,4,6-tribromophenol.

6. A first electrode which is an electrode for a secondary battery according to any one of claims 1 to 3, The second electrode and A secondary battery comprising a separator interposed between the first electrode and the second electrode.

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