Negative electrode composition, negative electrode for lithium secondary battery and lithium secondary battery

Abstract

A negative electrode composition comprising a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, a conductivity aid 1, and a conductivity aid 2, wherein the conductivity aid 1 is a fibrous carbon material having an average fiber length exceeding 1 μm, and the conductivity aid 2 is a fibrous carbon material having an average fiber length of 1 μm or less or a particulate conductivity aid. Preferably, the conductivity aid 1 is a fibrous carbon material having a fiber length of 3 μm or more. Preferably, the conductivity aid 2 is a particulate conductivity aid.

Description

Technology Field

[0001] The present invention relates to a negative electrode composition, a negative electrode for a lithium secondary battery, and a lithium secondary battery.

[0002] The present application claims priority based on Japanese Patent Application No. 2024-012877 filed in Japan on January 31, 2024, and incorporates the contents thereof herein. Background Technology

[0003] Lithium-ion batteries are used as power sources for small electronic devices such as mobile phones and laptop personal computers. In recent years, lithium-ion batteries have also been commercialized for medium or large-scale power sources, such as for automobiles and power storage.

[0004] Carbon materials are known as negative electrode active materials for lithium secondary batteries. Conventionally, negative electrode active materials containing phosphorus in addition to carbon materials are known for the purpose of improving battery performance (see, for example, Patent Document 1). In the negative electrode active material described in Patent Document 1, by using a negative electrode active material that is a composite of carbon materials and phosphorus, a negative electrode with excellent charge / discharge capacity can be produced. Prior art literature

[0005] Japanese Patent Publication No. 2009-184861 The problem to be solved

[0006] In recent years, there has been a demand for lithium secondary batteries with higher energy density.

[0007] One method to increase the energy density of a battery is to thicken the electrode active material layer per electrode. By making the electrode active material layer a thick film, the volume ratio of the electrode active material layer contributing to the battery reaction per unit volume of the battery increases, thereby enabling the production of a battery with improved energy density.

[0008] However, thickened electrode active material layers are prone to reduced electronic and ion conductivity in the thickness direction. In other words, thickened electrode active material layers increase resistance to electron movement, and simultaneously, make it difficult for lithium ions to move, resulting in high resistance to ion diffusion. Consequently, there has been a challenge in that the initial capacity is prone to degradation.

[0009] The present invention has been made in consideration of the above circumstances and aims to provide a negative electrode composition having a high initial capacity even when the electrode active material layer is made into a thick film.

[0010] In addition, according to the present invention, the purpose is to provide a negative electrode for a lithium secondary battery and a lithium secondary battery having a high initial capacity even when the electrode active material layer is made thick. means of solving the problem

[0011] In order to solve the above problem, one aspect of the present invention includes the following aspects.

[0012] [1] A negative electrode composition comprising a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, a conductive aid 1, and a conductive aid 2, wherein the conductive aid 1 is a fibrous carbon material with an average fiber length of more than 1 μm, and the conductive aid 2 is a fibrous carbon material or a particulate conductive aid with an average fiber length of 1 μm or less.

[0013] [2] The negative electrode composition described in [1], wherein the average fiber length of the challenge aid 1 is 3 μm or more.

[0014] [3] The negative electrode composition described in [1] or [2], wherein the content ratio of the conductive aid 1 to the total amount of the negative electrode composition is 5 mass% or more and 20 mass% or less.

[0015] [4] The above-mentioned conductive aid 2 is a particulate conductive aid, a negative electrode composition described in any one of [1] to [3].

[0016] [5] The above phosphorus-carbon composite negative electrode material is a negative electrode composition described in any one of [1] to [4], having a peak in the XPS spectrum indicating the bonding of phosphorus atoms and carbon atoms.

[0017] [6] The above phosphorus-carbon composite negative electrode material has an intensity I at 2θ=20° in the XRD profile measured using CuKα rays. 20 , intensity I at 2θ=26.3° 26.3 , and intensity I at 2θ=40° 40 A negative electrode composition described in any one of [1] to [5] satisfying the following formulas (1) to (3).

[0018] |P| / |B|<2 … (1)

[0019] P=I 26.3 -I 40 … (2)

[0020] B=( I 20 -I 40 )×(26.3-40) / (20-40) … (3)

[0021] [7] The above phosphorus-carbon composite negative electrode material is a negative electrode composition described in any one of [1] to [6], having a core particle containing phosphorus atoms and a carbon film covering the surface of the core particle.

[0022] [8] A negative electrode for a lithium secondary battery having a negative electrode active material layer composed of a negative electrode composition described in any one of [1] to [7] on the surface of a current collector.

[0023] [9] A negative electrode for a lithium secondary battery as described in [8], wherein the thickness of the negative electrode active material layer is 20㎛ or more and 150㎛ or less.

[0024]

[10] A lithium secondary battery comprising a negative electrode for a lithium secondary battery as described in [8] or [9]. Effects of the invention

[0025] According to the present invention, a negative electrode composition having a high initial capacity can be provided even when the electrode active material layer is made thick. Furthermore, according to the present invention, a negative electrode for a lithium secondary battery and a lithium secondary battery having a high initial capacity can be provided even when the electrode active material layer is made thick. Brief explanation of the drawing

[0026] Figure 1 is the XPS spectrum of the PC negative electrode material. Figure 2 is a transmission electron microscope (TEM) image of the PC negative electrode material. Figure 3 is the XRD profile of the PC negative electrode material and the PC mixture. Figure 4 is a schematic diagram illustrating an all-solid-state lithium secondary battery, which is an example of a battery. Figure 5 is a schematic diagram illustrating an all-solid-state lithium secondary battery, which is an example of a battery. Specific details for implementing the invention

[0027] <Negative composition>

[0028] The negative electrode composition of the present embodiment comprises a phosphorus-carbon composite negative electrode material comprising phosphorus atoms and carbon atoms, a conductive aid 1 which is a fibrous carbon material, and a conductive aid 2 which is different from conductive aid 1. In the following description, the "phosphorus-carbon composite negative electrode material" may be abbreviated simply as "PC negative electrode material."

[0029] The negative electrode active material layer composed of the negative electrode composition of the present embodiment includes a conductive aid 1, which is a fibrous carbon material. The conductive aid 1 forms a conductive network in the thickness direction within the negative electrode active material layer. Due to this conductive network, lithium ions can move easily even when the negative electrode active material layer is made thick, making it easy to improve the initial capacity.

[0030] A thick negative electrode active material layer refers to a negative electrode active material layer with a film thickness of 30㎛ or more.

[0031] The PC negative electrode material expands in volume when lithium is inserted and contracts in volume when lithium is removed. Compared to conventional graphite negative electrode materials, the volume change accompanying this expansion and contraction of the PC negative electrode material is large, and when expansion and contraction occur, the contact points between PC negative electrode materials are easily broken, causing the conductive path to be interrupted. At locations where the conductive path is interrupted, lithium ions cannot move, resulting in resistance. Since the negative electrode active material layer composed of the negative electrode composition of the present embodiment includes a conductive aid 2, it is easy to maintain the conductive path even when volume changes accompanying expansion and contraction occur, making it easy to improve the initial capacity.

[0032] Below, each component will be described in the order of conductivity aid 1, conductivity aid 2, and PC negative electrode material.

[0033] ≪Challenge Aid 1≫

[0034] Conductivity aid 1 is a fibrous carbon material with an average fiber length exceeding 1 μm. Conductivity aid 1 is, for example, carbon nanotubes, carbon nanofibers, vapor-grown carbon fibers, mesophase pitch-based carbon fibers, and isotropic pitch-based carbon fibers. For vapor-grown carbon fibers, materials commercially available as VGCF (registered trademark) of Showa Denko Co., Ltd. may be used.

[0035] The average fiber length of the conduction aid 1 is preferably 1.1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more.

[0036] The average fiber length of the challenge aid 1 is, for example, 20㎛ or less, 15㎛ or less, or 10㎛ or less.

[0037] The average fiber length of the challenge aid 1 is, for example, 1.1 μm or more and 20 μm or less, 3 μm or more and 15 μm or less, and 5 μm or more and 10 μm.

[0038] The average fiber diameter of the conductive aid 1 is preferably 10 nm or more, more preferably 100 nm or more, and even more preferably 150 nm or more.

[0039] The average fiber diameter of the conduction aid 1 is, for example, 500 nm or less, 300 nm or less, or 200 nm or less.

[0040] The average fiber diameter of the conductivity aid 1 is, for example, 10 nm or more and 500 nm or less, 100 nm or more and 300 nm or less, and 150 nm or more and 200 nm or less.

[0041] The average fiber length and average fiber diameter of the conductivity aid 1 can be calculated by observing five fibrous carbon materials within the field of view using a transmission electron microscope or a scanning electron microscope and averaging the number. In this context, one fibrous carbon material refers to a bundled fiber structure when it forms a bundle structure.

[0042] If the average fiber length of the conductivity aid 1 is greater than or equal to the lower limit value, it is easy to form a conductive network in the thickness direction within the negative electrode active material layer.

[0043] If the average fiber diameter of the conductive aid 1 is less than or equal to the upper limit value, the fibrous carbon material is less likely to resist ion diffusion, and thus has excellent ion conductivity.

[0044] From the perspective of improving the conductivity imparting effect, it is desirable that the consolidation resistivity of conductivity aid 1 be small. Specifically, the consolidation resistivity of conductivity aid 1 at a compressive density of 0.8 g / cm³ is preferably 0.03 Ω·cm or less, more preferably 0.02 Ω·cm or less, and even more preferably 0.18 Ω·cm or less.

[0045] The content ratio of conductive aid 1 to the total amount of the negative electrode composition is preferably 5 mass% or more, more preferably 5.5 mass% or more, and even more preferably 6 mass% or more.

[0046] The content ratio of conductive aid 1 to the total amount of the negative electrode composition is preferably 20 mass% or less, more preferably 16 mass% or less, and even more preferably 14 mass% or more.

[0047] The content ratio of conductive aid 1 to the total amount of the negative electrode composition is, for example, 5 mass% or more and 20 mass% or less, 5.5 mass% or more and 16 mass% or less, and 6 mass% or more and 14 mass% or less.

[0048] If the content ratio of conductivity aid 1 is greater than or equal to the lower limit value above, a conductive network in the thickness direction is easily formed in the negative electrode active material layer.

[0049] If the content ratio of the conductivity aid 1 is below the upper limit value, the fibrous carbon material is less likely to resist ion diffusion, and thus has excellent ion conductivity.

[0050] In this embodiment, the conductive aid 1 may be used alone or in combination with multiple types.

[0051] ≪Challenge Aid 2≫

[0052] Conductive aid 2 is a fibrous carbon material or a particulate conductive aid with an average fiber length of 1 μm or less.

[0053] From the perspective of securing a conductive path between PC negative electrode materials, it is preferable that the conductive aid 2 be a particulate conductive aid. When the particulate conductive aid 2 is interposed between PC negative electrode materials to secure a conductive path, it is easy to form contact points with the PC negative electrode materials. Furthermore, even if the position of the conductive aid 2 is misaligned due to the expansion or contraction of the PC negative electrode materials, it is easy to maintain contact points with the PC negative electrode materials. The particles may be spherical or flake-shaped.

[0054] Conductivity aid 2 may use carbon materials such as, for example, natural graphite, artificial graphite such as mesocarbon micro beads, carbon black (e.g., acetylene black, Ketjen black, and furnace black), graphite particles, graphene, and fullerene. In addition, metal powders such as copper, nickel, aluminum, silver, and gold, and conductive ceramic materials may be used.

[0055] When using carbon fibers with an average fiber length of 1 μm or less as the conduction aid 2, for example, vapor-grown carbon fibers or carbon nanotubes may be used.

[0056] The content ratio of conductive aid 2 to the total amount of negative electrode composition is preferably 10 mass% or less, more preferably 5 mass% or less, and even more preferably 3 mass% or less.

[0057] The content ratio of conductive aid 2 to the total amount of the negative electrode composition is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and even more preferably 1 mass% or more.

[0058] The content ratio of conductive aid 2 to the total amount of the negative electrode composition is, for example, 0.1 mass% or more and 10 mass% or less, 0.5 mass% or more and 5 mass% or less, and 1 mass% or more and 3 mass% or less.

[0059] If the content ratio of conductivity aid 2 is greater than or equal to the lower limit value above, it is easy to secure a conductivity path between PC negative electrode materials.

[0060] If the content ratio of conductivity aid 2 is below the upper limit value, conductivity aid 2 is less likely to become resistant, making it easier to form a conductive network.

[0061] In this embodiment, the conductive aid 2 may be used alone or in combination with multiple types.

[0062] Phosphorus-Carbon Composite Negative Electrode Materials

[0063] The PC negative electrode material used in this embodiment includes phosphorus atoms and carbon atoms.

[0064] In one embodiment of the present invention, the PC negative electrode material is a powder.

[0065] When the PC negative electrode material contains components other than phosphorus atoms and carbon atoms, the content ratio of the other components is preferably 1 mass% or more and 15 mass% or less with respect to the total amount of components other than phosphorus atoms and carbon atoms, more preferably 2 mass% or more and 12 mass% or less, and even more preferably 2.5 mass% or more and 8 mass% or less.

[0066] If the content ratio of other components is within the above range, the movement of lithium ions is less likely to be inhibited, making it easier to improve the initial capacity of the secondary battery.

[0067] The PC negative electrode material consists of more than 60 mass% of phosphorus atoms and carbon atoms, and may additionally include elements other than phosphorus atoms and carbon atoms.

[0068] Examples of elements other than phosphorus and carbon atoms contained in PC negative electrode materials include lithium, silicon, germanium, tin, aluminum, zinc, magnesium, transition metals, nitrogen, oxygen, fluorine, silicon, titanium, niobium, sulfur, and chlorine. These can be used as raw materials for metal oxides, complex metal oxides, metal fluorides, metal sulfides, metal chlorides, silicon oxide, silicates, titaniumates, and aluminates.

[0069] The phosphorus atom content, carbon atom content, and other atoms that may be additionally included in the PC negative electrode material can be determined by known ICP analysis.

[0070] In the PC negative electrode material, the phosphorus atom content is preferably 20 mass% or more, more preferably 30 mass% or more, and even more preferably 50 mass% or more. A higher phosphorus content can increase the initial charge-discharge capacity of the PC negative electrode material. Furthermore, in the PC negative electrode material, the phosphorus atom content is preferably 90 mass% or less, more preferably 80 mass% or less, and even more preferably 75 mass% or less. If the phosphorus atom content is too high, the cycle characteristics of the PC negative electrode material are prone to deterioration.

[0071] The upper and lower limits of the phosphorus atom content can be arbitrarily combined.

[0072] The phosphorus content is, for example, 20 mass% or more and 90 mass% or less, 30 mass% or more and 80 mass% or less, and 50 mass% or more and 75 mass% or less.

[0073] In the PC negative electrode material, the carbon atom content is preferably 10 mass% or more, more preferably 20 mass% or more, and even more preferably 25 mass% or more. A higher carbon atom content can improve the cycle characteristics of the PC negative electrode material. Furthermore, in the PC negative electrode material, the carbon atom content is preferably 80 mass% or less, more preferably 70 mass% or less, and even more preferably 50 mass% or less. If the carbon atom content is too high, the initial charge-discharge capacity of the PC negative electrode material is prone to decreasing.

[0074] The upper and lower limits of the carbon atom content can be arbitrarily combined.

[0075] The carbon atom content is, for example, 10 mass% or more and 80 mass% or less, 20 mass% or more and 70 mass% or less, and 25 mass% or more and 50 mass% or less.

[0076] As will be described in detail later, the PC negative electrode material can be manufactured by ball mill mixing phosphorus and carbon materials. The inventors conducted a detailed analysis and examination of the obtained PC negative electrode material and confirmed that the PC negative electrode material is not merely a mixture of the raw materials, phosphorus and carbon materials, but rather a novel material in which phosphorus atoms and carbon atoms form chemical bonds (covalent bonds) within the PC negative electrode material, possessing physical properties different from those of the raw materials, phosphorus and carbon materials.

[0077] Below, the physical properties of the PC negative electrode material will be explained in detail in comparison with the mixture of phosphorus and carbon materials (hereinafter referred to as the “PC mixture”).

[0078] In the following description, an example of a PC negative electrode material and an example of a mixture obtained by mixing the same raw materials as the PC negative electrode material in a mortar and pestle are compared. In the examples shown below, the raw materials of the PC negative electrode material and the PC mixture both contain black phosphorus and carbon materials in a ratio of 6:4 (mass ratio). In the PC negative electrode material and the PC mixture, the carbon material used is CSCNT (cup-stacked carbon nanotube).

[0079] [Presence or absence of PC connection]

[0080] PC negative electrode materials have a peak in the XPS spectrum indicating the bond between phosphorus atoms and carbon atoms. It is known that the "peak indicating the bond between phosphorus atoms and carbon atoms" appears in the XPS spectrum in the range of 132 to 136 eV. Additionally, in the following description, the bond between phosphorus atoms and carbon atoms may be simply referred to as a "PC bond."

[0081] Figure 1 shows the XPS spectra of a PC negative electrode material and a PC mixture. In Figure 1, the horizontal axis represents the binding energy (eV), and the vertical axis represents the number of detected photoelectrons (cps (count per second)). In Figure 1, symbol A represents the PC negative electrode material, and symbol X represents the PC mixture.

[0082] (XPS Spectrum Measurement Conditions)

[0083] The XPS spectrum is measured under the following measurement conditions.

[0084] · Measuring instrument: XPS device, ESCA-3400 (manufactured by Shimadzu Seisakusho Co., Ltd.)

[0085] · Source: Mg Kα line (20mA, 10kV)

[0086] If a peak is detected in the range of 132 to 136 eV, it is determined that it has PC coupling.

[0087] As shown in Figure 1, no peaks indicating PC bonding in the range of 132 to 136 eV were detected in the PC mixture. On the other hand, it can be seen that peaks indicating PC bonding (indicated by the symbol α) are detected in the PC negative electrode material.

[0088] From the results shown in Figure 1, it is believed that the PC negative electrode material is a material having chemical bonds between phosphorus atoms and carbon atoms.

[0089] [appearance]

[0090] The PC negative electrode material preferably has core particles containing phosphorus atoms and a carbon film covering the surface of the core particles.

[0091] FIG. 2 is a transmission electron microscope (TEM) image of a PC negative electrode material. As shown in FIG. 2, the particles (50) of the PC negative electrode material exhibit a core-shell structure in which a carbon film (52) covers the surface of a core particle (51) containing phosphorus atoms.

[0092] The inclusion of phosphorus and carbon atoms in the PC negative electrode material can be confirmed by using EDX (Energy Dispersive X-ray Spectroscopy).

[0093] In addition, when the PC negative electrode material contains only phosphorus atoms and carbon atoms, the phosphorus atoms, which are heavier atoms, are captured more intensely in the TEM image. Therefore, based on the TEM image, it can be roughly determined that the core particle (51) contains phosphorus atoms and is covered with a carbon film (52) containing carbon atoms, forming a core-shell structure.

[0094] (Imaging conditions for TEM photos)

[0095] TEM photos are taken under the following imaging conditions.

[0096] ·TEM device: Transmission electron microscope H-9000NAR (manufactured by Hitachi Seisakusho Co., Ltd.)

[0097] · Field of view: Max. 500 nm × 500 nm. At least some of the particles of the PC negative electrode material are placed within the field of view for imaging.

[0098] According to the imaging conditions of the above TEM photo, the core-shell structure of the PC negative electrode material can be confirmed.

[0099] In addition to satisfying the requirements regarding the [behavior with respect to heat] described above, it is preferable that the PC negative electrode material has a core-shell structure.

[0100] [Decision State]

[0101] Figure 3 shows the XRD profiles of the PC negative electrode material and the PC mixture. In Figure 3, the horizontal axis represents the diffraction angle (2θ, °), and the vertical axis represents the diffracted X-ray intensity (au). In Figure 3, symbol A represents the PC negative electrode material, and symbol X represents the PC mixture.

[0102] (XRD profile measurement conditions)

[0103] The XRD profile is measured under the following measurement conditions.

[0104] · Measuring instrument: Sample horizontal multi-purpose X-ray diffraction apparatus Ultima IV (Manufactured by Rigaku Co., Ltd.)

[0105] · Source: Cu Kα line

[0106] · Measurement range (2θ): 10° to 90°

[0107] · Scan speed: 4° / min

[0108] · Sampling: 0.02°

[0109] · Voltage 40 kV, Current 40 mA

[0110] As shown in Fig. 3, in the PC mixture, the carbon material and black phosphorus each have a certain crystallinity and exhibit diffraction peaks. In contrast, in the PC negative electrode material, the diffraction peaks observed in the PC mixture were not visible, and only noise could be observed across the entire measurement range. That is, from the results shown in Fig. 3, it is thought that in the PC negative electrode material, the crystallinity of the carbon material used as a raw material is lost and it becomes amorphous, or that it consists of a carbon material so fine that the crystal state cannot be confirmed.

[0111] Here, the PC negative electrode material is the intensity I at 2θ=20° in the XRD profile measured under the above conditions when the XRD profile is measured under the above conditions. 20 , intensity I at 2θ=26.3° 26.3 , and intensity I at 2θ=40° 40 It is desirable to satisfy the following relationship equations (1) to (3).

[0112] |P| / |B|<2 … (1)

[0113] P=I 26.3 -I 40 … (2)

[0114] B=( I20 -I 40 )×(26.3-40) / (20-40) … (3)

[0115] When the XRD profile is measured for the raw carbon material, the peak of the carbon material appears at 2θ=26.3°, which corresponds to the peak of graphite (002). On the other hand, in the XRD profile of a normal carbon material, there is no peak at 2θ=20° or 2θ=40°. Therefore, in the above equation (1), the state of the carbon material can be determined by comparing the intensity of the reference point in the XRD profile with the intensity of the location (2θ=26.3°) where the peak of the raw carbon material exists, using 2θ=40°, where the peak of the carbon material does not exist, as a reference point.

[0116] "P" represented by the above equation (2) is the strength I of the reference point 40 and, intensity I at the location where the carbon material peak exists 26.3 It is the difference and represents the difference in peaks from the reference point.

[0117] "B" represented by the above equation (3) represents the baseline intensity of 2θ=26.3°, estimated from two peak-free positions (2θ=20° and 2θ=40°).

[0118] From the ratio of the absolute values ​​of P and B (|P| / |B|) represented by the above equation (1), the ratio of baseline intensity to peak intensity from the reference point is obtained. When |P| / |B| is 2 or greater, it indicates that a peak of carbon material exists.

[0119] On the other hand, when |P| / |B| is less than 2 and satisfies Equation (1), the peak of the carbon material is reduced, so it can be determined that the carbon material is amorphous or a carbon material so fine that the crystalline state cannot be confirmed. Therefore, in addition to satisfying the requirements regarding [behavior with respect to heat] described above, it is desirable for the PC negative electrode material to satisfy the above-mentioned relationships (1) to (3).

[0120] As shown above, the PC negative electrode material is a novel material that differs in its behavior with respect to heat, bonding state, appearance, and crystal state compared to a mixture of raw material phosphorus (black phosphorus in FIGS. 1 to 3) and carbon material.

[0121] (Method for manufacturing PC negative electrode material)

[0122] The PC negative electrode material is obtained by mixing a phosphorus material and a carbon material. It is preferable to mix the phosphorus material and the carbon material by a mixing and grinding process involving compression.

[0123] As a raw carbon material, at least one type selected from the group consisting of amorphous carbon, graphite, porous carbon, mesocarbon micro beads (MCMB), fullerene, carbon nanotubes, graphene, graphene oxide, carbon nitrite (C3N4), and stacked carbon nanofibers (carbon nanofiber platelets) may be used. All of these carbon materials have a graphite structure.

[0124] Examples of amorphous carbons include carbon black (CB), acetylene black (AB), Ketjen black, hard carbon, and soft carbon.

[0125] As a fullerene, C 60 , C 72 , C 84 You can lift the back.

[0126] Examples of carbon nanotubes include single-layer carbon nanotubes (SWCNT), multilayer carbon nanotubes (MWCNT), cup-stacked carbon nanotubes (CSCNT), and carbon nanofibers VGCF (vapor-grown carbon fiber).

[0127] These have a structure that extends in a 1-axis direction (1-dimensional structure).

[0128] Graphene, graphene oxide, carbon nitrite (C3N4), and stacked carbon nanofibers (carbon nanofiber platelets) have a structure that expands in the planar direction (two-dimensional structure).

[0129] In carbon materials, the ends of the graphite structure (edge ​​sites) are more active compared to locations other than the edges. Therefore, it is assumed that most PC bonds are formed at the edge sites of the carbon material. Accordingly, from the perspective of facilitating the formation of PC bonds, it is desirable to use a carbon material that has many edge sites.

[0130] Examples of carbon materials with many edge sites include CSCNT, graphite, mesocarbon micro beads (MCMB), graphene, graphene oxide, carbon nitrite (C3N4), and stacked carbon nanofibers. Among these, CSCNT is preferred as a carbon material.

[0131] In addition, all known phosphorus allotropes can be used as the raw material phosphorus. Among the phosphorus allotropes, black phosphorus is chemically the most stable and is also a good conductor of electricity.

[0132] "Mixing and grinding treatment accompanied by compression" is a process in which a compressive force is applied to multiple types of powder raw materials to mix and grind the raw materials. It is believed that by performing a mixing and grinding treatment accompanied by compression on carbon materials and phosphorus, a strong impact force is applied to the carbon materials and phosphorus, thereby simultaneously mixing and grinding the raw material powders and causing chemical changes that cannot be obtained with a normal mixture. As a result, it is believed that PC bonds not present in the raw materials are formed as a product, and a PC negative electrode material is obtained in which the crystalline state observed in the raw materials is not visible.

[0133] The "mixing and grinding treatment involving compression" should be performed until peaks originating from the raw materials disappear in the XRD profile of the mixed materials. Alternatively, it should be performed until the occurrence of PC bonding can be confirmed in the XPS spectrum of the mixed materials.

[0134] Examples of processing devices (grinders) capable of performing mixing and grinding processes accompanied by compression include roller mills, jet mills, hammer mills, fin mills, disc mills, rod mills, ball mills, vibratory mills, attritors, and bead mills. In the manufacture of PC negative electrode materials, it is particularly desirable to use a stirred grinder equipped with a grinding vessel and a rotating body, as mixing and grinding can be performed simultaneously. Examples of stirred grinders include fin mills, disc mills, rod mills, ball mills, vibratory mills, attritors, and bead mills.

[0135] In addition, as the processing device, a medium-stirring type grinder is preferred in that it can simultaneously mix and grind the raw material powder and apply a strong impact force to the carbon material and phosphorus. Examples of medium-stirring type grinders include ball mills, vibratory mills, attritors, and bead mills. Among these, a ball mill is particularly preferred as a grinder in terms of being able to easily control the properties.

[0136] According to ball mill mixing, it is thought that first, the micronization of phosphorus and the decomposition of carbon materials occur, followed by the formation of PC bonds. During this ball mill mixing process, phosphorus becomes the core particle described above.

[0137] Subsequently, it is believed that a PC negative electrode material is obtained by forming a composite while forming the aforementioned carbon film around a core particle containing phosphorus atoms.

[0138] When mixing in a ball mill, manufacturing conditions can be controlled by adjusting the rotational speed of the ball mill device, the amount of media (balls) relative to the raw material (ball powder ratio), and the mixing time. That is, by adjusting conditions such as increasing the rotational speed of the ball mill device, increasing the amount of media relative to the raw material, or extending the mixing time, the mixing of phosphorus and carbon materials is promoted, making it easier to obtain PC negative electrode materials.

[0139] Balls are a grinding medium for grinding metal materials. The diameter of a ball refers to the average particle size of the ball. The ball flows at high speed within the grinding vessel due to the rotation of the grinding vessel itself and collides with powder raw materials containing carbon materials and phosphorus, thereby grinding them into particles with a smaller average particle size. In the grinding process, it is desirable that the grinding vessel and beads not be excessively worn. For this reason, the shape of the ball is preferably spherical or ellipsoidal.

[0140] It is preferable that the diameter of the ball be larger than the average particle size of the PC negative electrode material after grinding. By using such a ball, a large grinding energy can be applied to the metal material, allowing metal particles to be obtained efficiently in a short time.

[0141] The diameter of the ball is preferably 0.1 to 10 mm, and more preferably 1 to 10 mm. When the diameter of the ball is within this range, it is possible to promote the formation of PC bonds while suppressing re-aggregation. The diameter of the ball placed in the grinding container may be uniform or may vary.

[0142] Examples of materials for the balls include glass, agate, alumina, zirconia, stainless steel, chrome steel, tungsten carbide, silicon carbide, and silicon nitride. Among these, it is preferable to use zirconia because it has relatively high hardness, making it resistant to wear, and also has a relatively high specific gravity, which allows for obtaining large grinding energy. By using these balls, the raw powder of the PC negative electrode material can be efficiently ground.

[0143] The weight ratio of balls to raw material powder of PC negative electrode material is set as the ball powder ratio. By increasing the ball powder ratio, strong impact force can be applied to the raw material powder of PC negative electrode material at a high frequency, thereby further promoting the formation of PC bonds. If the ball powder ratio is too high, the amount of PC negative electrode material produced per unit operation decreases. Therefore, the preferred ball powder ratio is 0.5 to 500, more preferably 1 to 200, and even more preferably 10 to 200.

[0144] In addition, after the ball mill mixing is finished, the balls are separated from the PC negative electrode material using a filter or the like.

[0145] It can be confirmed that the PC negative electrode material described above is obtained by ball mill mixing by measuring the XRD profile of the mixed material and observing that the peak originating from the raw material has disappeared, as shown in FIG. 1. The duration of ball mill mixing can be determined by conducting preliminary experiments on the relationship between the mixing time and the time until the peak originating from the raw material disappears. In other words, ball mill mixing should be performed until the peak originating from the raw material disappears in the XRD profile of the mixed material.

[0146] Alternatively, the occurrence of PC bonding may be confirmed by measuring the XPS spectrum of the mixed material and observing that PC bonding is occurring as shown in Fig. 1. In this case, the duration of ball mill mixing can be determined by conducting preliminary experiments on the relationship between the mixing time and the time until PC bonding occurs. In other words, ball mill mixing should be performed until PC bonding can be confirmed in the XPS spectrum of the mixed material.

[0147] In ball mill mixing, carbon materials or phosphorus are interposed during collisions between media or between media and the ball mill container, thereby applying a localized strong impact (pressure) to the carbon materials or phosphorus. Although the details are unclear, it is thought that when such impact is applied, chemical changes that cannot be obtained with a normal mixture occur, PC bonds that are not present in the raw materials are formed, and a PC negative electrode material is obtained in which the crystalline state that was visible in the raw materials is not visible.

[0148] ≪Method for Manufacturing a Negative Electrode Composition≫

[0149] The negative electrode composition of the present embodiment can be prepared by mixing the PC negative electrode material obtained by the method described above, a binder, a conductivity aid 1, a conductivity aid 2, and an optional component.

[0150] (Optional component)

[0151] In addition to the components described above, the negative electrode composition may include known materials used as negative electrode active materials. Examples of such materials include (i) alloy-based negative electrode active material materials that involve the formation of a lithium alloy phase, (ii) transition metal oxides that undergo a decomposition and regeneration reaction (conversion reaction) with lithium ions, (iii) oxides or composite oxides that have activity as negative electrode active materials, and (iV) layered carbon.

[0152] (i) As alloy-based negative electrode active materials, metals such as silicon, germanium, tin, aluminum, zinc, and magnesium, or alloys thereof may be used.

[0153] (ii) Examples of transition metal oxides include manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, and magnesium oxide. These oxides may also be complex metal oxides containing other metals (e.g., lithium).

[0154] (iii) Examples of oxides or complex oxides include titanium oxide, lithium titanium oxide, and silicon oxide.

[0155] (iV) Examples of layered carbon include graphite and hard carbon.

[0156] The negative electrode composition preferably has a binder for holding and fixing the negative electrode active material to the current collector. The binder may employ a known composition.

[0157] As a binder, one or more mixtures of polyvinylidene fluoride (hereinafter referred to as PVdF), polyimide, polyamideimide, styrene-butadiene rubber, carboxymethylcellulose, and acrylic resin may be suitably used.

[0158] Negative electrode for lithium secondary batteries

[0159] The negative electrode for a lithium secondary battery according to the present embodiment is provided with an electrode active material layer composed of the negative electrode composition described above on the surface of a current collector. Copper or a copper alloy may be suitably used as the material of the current collector.

[0160] The thickness of the negative electrode active material layer of the negative electrode for a lithium secondary battery of the present embodiment is preferably 20㎛ or more and 200㎛ or less, and more preferably 50㎛ or more and 150㎛ or less.

[0161] In the negative electrode active material layer composed of the negative electrode composition of the present embodiment, the conductive aid 1 forms a conductive network in the thickness direction within the negative electrode active material layer. Due to this conductive network, lithium ions can move easily even when the negative electrode active material layer is made thick, making it easy to improve the initial capacity.

[0162] Lithium secondary battery

[0163] The lithium secondary battery of the present embodiment includes the negative electrode for the lithium secondary battery (hereinafter, negative electrode) described above.

[0164] An example of a lithium secondary battery suitable for using the negative electrode of the present embodiment has a positive electrode and a negative electrode, a separator placed between the positive electrode and the negative electrode, and an electrolyte placed between the positive electrode and the negative electrode.

[0165] An example of a lithium secondary battery has a positive electrode and a negative electrode, a separator placed between the positive electrode and the negative electrode, and an electrolyte placed between the positive electrode and the negative electrode.

[0166] FIG. 4 is a schematic diagram illustrating an example of a lithium secondary battery. For example, a cylindrical lithium secondary battery (10) is manufactured as follows.

[0167] First, as shown in the partial enlarged view of FIG. 4, a pair of separators (1) having a strip shape, a strip-shaped positive electrode (2) having a positive electrode lead (21) at one end, and a strip-shaped negative electrode (3) having a negative electrode lead (31) at one end are stacked in the order of separator (1), positive electrode (2), separator (1), and negative electrode (3) and wound to form an electrode group (4).

[0168] The positive electrode (2) has, for example, a positive electrode active material layer (2a) comprising a positive electrode active material (hereinafter, CAM) and a positive electrode current collector (2b) on which the positive electrode active material layer (2a) is formed on one side. This positive electrode (2) can be manufactured by first preparing a positive electrode composite material comprising CAM, a conductive material, and a binder, and then supporting the positive electrode composite material on one side of the positive electrode current collector (2b) to form the positive electrode active material layer (2a).

[0169] The negative electrode (3) can be an electrode formed by supporting a negative electrode composite containing a negative electrode active material, which is not shown as an example, on a negative electrode current collector, and an electrode formed by the negative electrode active material alone, and can be manufactured in the same way as the positive electrode (2).

[0170] Next, after accommodating an electrode group (4) and an insulator not shown in the battery can (5), the bottom of the can is sealed, and the electrolyte (6) is impregnated into the electrode group (4) to place the electrolyte between the positive electrode (2) and the negative electrode (3). Additionally, by sealing the top of the battery can (5) with a top insulator (7) and a sealing body (8), a lithium secondary battery (10) can be manufactured.

[0171] As for the shape of the electrode group (4), for example, when the electrode group (4) is cut perpendicular to the axis of the winding, the cross-sectional shape can be a circle, an ellipse, a rectangle, or a rectangular shape with rounded corners.

[0172] In addition, as for the shape of the lithium secondary battery having such electrode group (4), the shape specified in IEC60086, a battery standard set by the International Electrotechnical Commission (IEC), or JIS C 8500 may be adopted. For example, shapes such as cylindrical or prismatic shapes may be used.

[0173] In addition, the lithium secondary battery is not limited to the above-mentioned wound configuration, but may also have a stacked configuration in which a stacked structure of a positive electrode, a separator, a negative electrode, and a separator is repeatedly stacked. Examples of stacked lithium secondary batteries include so-called coin-type batteries, button-type batteries, or paper-type (or sheet-type) batteries.

[0174] For the positive electrode, separator, negative electrode, and electrolyte constituting the lithium secondary battery, for example, the configuration, materials, and manufacturing methods described in

[0113] to

[0140] of WO2022 / 113904A1 may be used.

[0175] All-solid-state lithium secondary battery

[0176] Next, while explaining the composition of the all-solid-state lithium secondary battery, an all-solid-state lithium secondary battery having the aforementioned negative electrode will be described.

[0177] FIG. 5 is a schematic diagram illustrating an example of an all-solid-state lithium secondary battery. The all-solid-state lithium secondary battery (1000) illustrated in FIG. 5 has a laminate (100) having a positive electrode (110), a negative electrode (120), and a solid electrolyte layer (130), and an outer body (200) that accommodates the laminate (100). Additionally, the all-solid-state lithium secondary battery (1000) may have a bipolar structure in which a CAM and a negative electrode active material are arranged on both sides of a current collector. As a specific example of a bipolar structure, the structure described in JP-A-2004-95400 may be cited.

[0178] The positive electrode (110) has a positive electrode active material layer (111) and a positive electrode current collector (112). The positive electrode active material layer (111) includes the CAM and solid electrolyte described above. Additionally, the positive electrode active material layer (111) may include a conductive material and a binder.

[0179] The negative electrode (120) has a negative electrode active material layer (121) and a negative electrode current collector (122). The negative electrode active material layer (121) contains a negative electrode active material. Additionally, the negative electrode active material layer (121) may contain a solid electrolyte and a conductive material.

[0180] The laminate (100) may have an external terminal (113) connected to a positive electrode current collector (112) and an external terminal (123) connected to a negative electrode current collector (122). Additionally, the all-solid-state lithium secondary battery (1000) may have a separator between the positive electrode (110) and the negative electrode (120).

[0181] The all-solid-state lithium secondary battery (1000) additionally has an insulator not shown that insulates the laminate (100) and the outer body (200), and a seal not shown that seals the opening (200a) of the outer body (200).

[0182] The outer body (200) may be a container formed from a metal material with high corrosion resistance, such as aluminum, stainless steel, or nickel-plated steel. Additionally, as the outer body (200), a container formed by processing a laminate film with corrosion resistance treatment on at least one side into a pouch shape may be used.

[0183] As for the shape of the all-solid-state lithium secondary battery (1000), examples include coin type, button type, paper type (or sheet type), cylindrical type, prismatic type, or laminate type (pouch type).

[0184] The all-solid-state lithium secondary battery (1000) is illustrated as having one laminate (100) as an example, but the present embodiment is not limited thereto. The all-solid-state lithium secondary battery (1000) may be configured such that a plurality of unit cells (laminated (100)) are sealed inside an outer body (200) using the laminate (100) as a unit cell.

[0185] For all-solid-state lithium secondary batteries, for example, the configuration, materials, and manufacturing methods described in

[0141] to

[0181] of WO2022 / 113904A1 may be used.

[0186] Examples

[0187] The present invention is described below by way of examples, but the present invention is not limited to these examples.

[0188] ≪Example 1≫

[0189] [Manufacture of PC negative electrode material]

[0190] Black phosphorus manufactured by Lhasa High School and CSCNT (cup-stacked carbon nanotubes: manufactured by GSI Creos Co., Ltd.) were weighed in a mass ratio of P:C=6:4 (total weight 1.25g), and mixed using a ball mill under the following conditions to obtain PC negative electrode material 1.

[0191] (Ball mill mixing conditions)

[0192] Device: Retsch PM-100

[0193] Container: 125mL ZrO2 container

[0194] Media: ZrO2 8mmφ ball, 150g

[0195] Atmosphere: Argon gas seal

[0196] Sample amount: 1.25g as a mixture of raw materials

[0197] Ball / Powder Ratio: 120 (by weight)

[0198] Mixing time: 12 hours

[0199] TEM images were taken of the obtained PC negative electrode material 1, and it was confirmed that it has a core-shell structure in which a carbon film covers the surface of a core particle containing phosphorus atoms.

[0200] For the obtained PC negative electrode material 1, lithium metal was used as the counter electrode and charged at a charging rate of 0.2C. As a result, the average charging potential of PC negative electrode material 1 with respect to the capacity (mAh / g) during charging was 0.1V or higher. It was confirmed that the obtained PC negative electrode material 1 is a material that absorbs lithium ions when lithium (alkali metal) is used as the counter electrode and 0.1V or higher is applied.

[0201] The obtained PC negative electrode material 1 had a phosphorus atom content of 60 mass% and a carbon atom content of 40 mass%. Given that the airtightness of the container used during the above-described manufacturing process is very high, the content of each atom in PC negative electrode material 1 is determined to be the same as the raw material ratio.

[0202] For the obtained PC negative electrode material 1, a peak indicating the bonding of phosphorus atoms and carbon atoms was confirmed by measuring according to the above-described (XPS spectrum measurement conditions).

[0203] For the obtained PC negative electrode material 1, measurement was performed according to the above-described (XRD profile measurement conditions), and in the XRD profile, intensity I at 2θ=20° 20 intensity I at 3063, 2θ=26.3° 26.3 2603, and intensity I at 2θ=40° 40 It was 1917. Also, |P| / |B| was 0.87. PC negative electrode material 1, which represents these values, satisfies the following formulas (1) to (3).

[0204] |P| / |B|<2 … (1)

[0205] P=I 26.3 -I 40 … (2)

[0206] B=( I 20 -I 40 )×(26.3-40) / (20-40) … (3)

[0207] [Preparation of negative electrode composition]

[0208] By mixing PC negative electrode material 1, VGCF which is conductivity aid 1 (manufactured by Resonac, with a compacted resistivity of 0.017 Ω·cm at a compressive density of 0.8 g / cm³ and an average fiber length of 6 μm), and SWCNT which is conductivity aid 2 (average fiber length of several hundred nm to 1 μm), a negative electrode composition 1 is obtained in which the content ratio of VGCF which is conductivity aid 1 is 12 mass% and the content ratio of SWCNT which is conductivity aid 2 is 1.2% relative to the total amount of the negative electrode composition, and the composition comprises 6 mass% of a binder (vinylidene fluoride PVdF Solleph 5130 (manufactured by Solvay)).

[0209] (Production of evaluation batteries)

[0210] (1) Production of Bu-geuk 1

[0211] The above negative electrode composition 1 and a solvent (NMP (N-methyl-2-pyrrolidone)) were mixed using an agate mortar to prepare a negative electrode slurry. At this time, the slurry concentration was adjusted so that the content of PC negative electrode material 1 in the negative electrode slurry was 30 to 60 mass%.

[0212] A negative electrode slurry was applied to a copper foil current collector using a doctor blade, and then vacuum dried at 80°C for 1 hour to remove the solvent, thereby obtaining a laminate. The obtained laminate was pressurized at 10 MPa for 10 seconds, and then vacuum dried at 150°C for 12 hours to obtain negative electrode 1.

[0213] The negative electrode 1 had a weight per unit area of ​​1.9 mg / cm², which is the coating amount of negative electrode composition 1, and the thickness of the negative electrode active material layer was 20 μm.

[0214] In addition, the preparation of the aforementioned negative electrode slurry and the preparation of the negative electrode were carried out in a glove box under an argon atmosphere.

[0215] (2) Production of lithium-ion secondary batteries

[0216] A lithium-ion secondary battery (coin battery R2032) was fabricated by combining the negative electrode, counter electrode, electrolyte, and separator prepared in (1). The assembly of the battery was carried out in a glove box under an argon atmosphere.

[0217] A metal lithium foil was used as the counter electrode.

[0218] As an electrolyte, a mixture of 10 mass% of fluoroethylene carbonate (FEC) was added to a LiPF6 solution (manufactured by Kishida Chemical Co., Ltd.).

[0219] As for the LiPF6 solution, a solution was used in which LiPF6 was dissolved at a ratio of 1 mol / L in a mixed solvent in which ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) were each in a volume ratio of 30:35:35.

[0220] As a separator, a polyethylene porous film separator (12 μm thickness) was used.

[0221] (Evaluation of initial capacity)

[0222] Using the above-mentioned evaluation battery, the initial capacity was evaluated by charging and discharging under the conditions shown below while maintaining a temperature of 25°C. Since a larger capacity in the first cycle indicates a higher initial discharge capacity, it can be judged to be excellent as a negative electrode active material.

[0223] Minimum charging voltage: 0.010V

[0224] Charging current: 1C (1C = 1800mA / g)

[0225] Maximum discharge voltage: 2.0V

[0226] Discharge current: 1C (1C = 1800mA / g)

[0227] The capacity of the first cycle of the 1C of a lithium secondary battery equipped with a negative electrode 1 composed of negative electrode composition 1 was 1464mAh / g.

[0228] ≪Example 2≫

[0229] A negative electrode 2 was prepared in the same manner as in Example 1, except that the weight per unit area of ​​the negative electrode composition 1 was changed to 3.4 mg / cm² and the thickness of the negative electrode active material layer was changed to 37 μm.

[0230] The capacity of the first cycle of the 1C of the lithium secondary battery equipped with negative electrode 2 was 1427mAh / g.

[0231] ≪Example 3≫

[0232] A negative electrode 3 was prepared in the same manner as in Example 1, except that the weight per unit area of ​​the negative electrode composition 1 was changed to 4.9 mg / cm² and the thickness of the negative electrode active material layer was changed to 45 μm.

[0233] The capacity of the first cycle of the 1C lithium secondary battery equipped with negative electrode 3 was 1391mAh / g.

[0234] ≪Comparative Example 1≫

[0235] A negative electrode 11 was prepared in the same manner as in Example 1, except that conductive aid 1 was not used.

[0236] The capacity of the first cycle of the 1C of the lithium secondary battery equipped with the negative electrode 11 was 1286mAh / g.

[0237] ≪Comparative Example 2≫

[0238] A negative electrode 12 was prepared in the same manner as in Example 2, except that conductive aid 1 was not used.

[0239] The capacity of the first cycle of the 1C of the lithium secondary battery equipped with the negative electrode 12 was 1272 mAh / g.

[0240] ≪Comparative Example 3≫

[0241] A negative electrode 13 was prepared in the same manner as in Example 3, except that conductive aid 1 was not used.

[0242] The capacity of the first cycle of the 1C of the lithium secondary battery equipped with the negative electrode 13 was 1135mAh / g.

[0243] As shown in the results above, a lithium secondary battery using a negative electrode composition containing conductivity aid 1 and conductivity aid 2 has an initial capacity of 1390 mAh / g or more even when the thickness of the negative electrode is 20 μm or more, and Examples 1 and 2 exceeded 1400 mAh / g.

[0244] Meanwhile, a lithium secondary battery using a negative electrode composition that did not use conduction aid 1 had an initial capacity of less than 1300 mAh / g.

[0245] From the above, it was confirmed that according to the negative electrode composition of the present invention, a lithium secondary battery with a high initial capacity is obtained even when the negative electrode is made into a thick film. Explanation of the symbols

[0246] 50: Particle, 51: Core particle, 52: Carbon film, 100: Laminate, 120: Negative electrode, 121: Negative electrode active material layer, 130: Solid electrolyte layer

Claims

Claim 1 A negative electrode composition comprising a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, a conductivity aid 1, and a conductivity aid 2, wherein the conductivity aid 1 is a fibrous carbon material having an average fiber length exceeding 1 μm, and the conductivity aid 2 is a fibrous carbon material or a particulate conductivity aid having an average fiber length of 1 μm or less. Claim 2 A negative electrode composition according to claim 1, wherein the average fiber length of the conductivity aid 1 is 3 μm or more. Claim 3 A negative electrode composition according to claim 1 or 2, wherein the content ratio of the conductivity aid 1 to the total amount of the negative electrode composition is 5 mass% or more and 20 mass% or less. Claim 4 A negative electrode composition according to claim 1 or 2, wherein the conductivity aid 2 is a particulate conductivity aid. Claim 5 The negative electrode composition according to claim 1 or 2, wherein the phosphorus-carbon composite negative electrode material has a peak indicating the bonding of phosphorus atoms and carbon atoms in the XPS spectrum. Claim 6 In claim 1 or 2, the phosphorus-carbon composite negative electrode material has an intensity I at 2θ=20° in the XRD profile measured using CuKα rays. 20 , intensity I at 2θ=26.3° 26.3 , and intensity I at 2θ=40° 40 This is a negative electrode composition satisfying the following formulas (1) to (3). |P| / |B|<2 … (1)P=I 26.3 -I 40 … (2)B=(I 20 -I 40 )×(26.3-40) / (20-40) … (3) Claim 7 In claim 1 or 2, the phosphorus-carbon composite negative electrode material is a negative electrode composition having a core particle containing phosphorus atoms and a carbon film covering the surface of the core particle. Claim 8 A negative electrode for a lithium secondary battery having a negative electrode active material layer composed of the negative electrode composition described in claim 1 or 2 on the surface of a current collector. Claim 9 A negative electrode for a lithium secondary battery according to claim 8, wherein the film thickness of the negative electrode active material layer is 20㎛ or more and 150㎛ or less. Claim 10 A lithium secondary battery comprising a negative electrode for a lithium secondary battery as described in claim 8.

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