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 conductive aid 1; and a conductive aid 2, wherein the conductive aid 1 is a fibrous carbon material having an average fiber length of more than 1 μm, and the conductive aid 2 is a fibrous carbon material having an average fiber length of 1 μm or less or a particulate conductive aid. The conductive aid 1 is preferably a fibrous carbon material having a fiber length of 3 μm or more. The conductive aid 2 is preferably a particulate conductive aid.

Description

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

[0001] The present invention relates to a negative electrode composition, a negative electrode for a lithium secondary battery, and a lithium secondary battery. This application claims priority based on Japanese Patent Application No. 2024-012877, filed on January 31, 2024, the contents of which are incorporated herein by reference.

[0002] Lithium secondary batteries are used as power sources for small electronic devices such as mobile phones and laptops. In recent years, lithium secondary batteries have also been put to practical use as medium- to large-sized power sources for automobiles and power storage applications.

[0003] Carbon materials are known as negative electrode active materials for lithium secondary batteries. Conventionally, negative electrode active materials in which phosphorus is further added to a carbon material have been known for the purpose of improving battery performance (see, for example, Patent Document 1). The negative electrode active material described in Patent Document 1, which is a composite of a carbon material and phosphorus, can be used to provide a negative electrode with excellent charge / discharge capacity.

[0004] JP 2009-184861 A

[0005] In recent years, lithium secondary batteries with higher energy densities have been desired. One method for increasing the energy density of a battery is to thicken the electrode active material layer per electrode. By thickening the electrode active material layer, the volume ratio of the electrode active material layer that contributes to the battery reaction per unit volume of the battery increases, resulting in a battery with improved energy density.

[0006] However, thicker electrode active material layers tend to have lower electronic and ionic conductivity in the thickness direction. In other words, thicker electrode active material layers increase resistance to electron movement and also hinder lithium ion movement, resulting in high resistance to ion diffusion. This has led to the problem of a lower initial capacity.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a negative electrode composition that has a high initial capacity even when the electrode active material layer is made thick. Another object of the present invention is to provide a negative electrode for a lithium secondary battery and a lithium secondary battery that have a high initial capacity even when the electrode active material layer is made thick.

[0008] In order to solve the above-mentioned problems, one aspect of the present invention includes the following aspects. [1] A negative electrode composition including a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, a conductive additive 1, and a conductive additive 2, wherein the conductive additive 1 is a fibrous carbon material having an average fiber length of more than 1 μm, and the conductive additive 2 is a fibrous carbon material or a particulate conductive additive having an average fiber length of 1 μm or less. [2] The negative electrode composition according to [1], wherein the conductive additive 1 has an average fiber length of 3 μm or more. [3] The negative electrode composition according to [1] or [2], wherein a content of the conductive additive 1 with respect to the total amount of the negative electrode composition is 5% by mass or more and 20% by mass or less. [4] The negative electrode composition according to any one of [1] to [3], wherein the conductive additive 2 is a particulate conductive additive. [5] The negative electrode composition according to any one of [1] to [4], wherein the phosphorus-carbon composite negative electrode material has a peak indicative of a bond between a phosphorus atom and a carbon atom in an XPS spectrum. [6] The phosphorus-carbon composite negative electrode material has an intensity I at 2θ=20° in an XRD profile measured using CuKα radiation. ２０ , intensity I at 2θ=26.3° ２６．３ , and the intensity at 2θ=40° I ４０ The negative electrode composition according to any one of [1] to [5], wherein |P| / |B|<2 (1) satisfies the following formulas (1) to (3): ２６．３ -I ４０ ...(2) B = (I ２０ -I ４０)×(26.3−40) / (20−40) ... (3) [7] The anode composition according to any one of [1] to [6], wherein the phosphorus-carbon composite anode material has core particles containing phosphorus atoms and a carbon coating covering the surfaces of the core particles. [8] A lithium secondary battery anode comprising, on a current collector, an anode active material layer made from the anode composition according to any one of [1] to [7]. [9] The lithium secondary battery anode according to [8], wherein the anode active material layer has a film thickness of 20 μm or more and 150 μm or less.

[10] A lithium secondary battery comprising the lithium secondary battery anode according to [8] or [9].

[0009] According to the present invention, it is possible to provide a negative electrode composition having a high initial capacity even when the electrode active material layer is made thick. Furthermore, according to the present invention, it is possible 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.

[0010] FIG. 1 is an XPS spectrum of a PC negative electrode material. FIG. 2 is a transmission electron microscope (TEM) photograph of the PC negative electrode material. FIG. 3 is an XRD profile of the PC negative electrode material and the PC mixture. FIG. 4 is a schematic diagram showing an all-solid-state lithium secondary battery, which is an example of a battery. FIG. 5 is a schematic diagram showing an all-solid-state lithium secondary battery, which is an example of a battery.

[0011] <Negative electrode composition> The negative electrode composition of this embodiment includes a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, a conductive additive 1 which is a fibrous carbon material, and a conductive additive 2 which is different from the conductive additive 1. In the following description, the "phosphorus-carbon composite negative electrode material" may be simply abbreviated as a "P-C negative electrode material."

[0012] The negative electrode active material layer formed from the negative electrode composition of this embodiment contains a conductive additive 1, which is a fibrous carbon material. The conductive additive 1 forms a conductive network in the thickness direction in the negative electrode active material layer. This conductive network facilitates the movement of lithium ions even when the negative electrode active material layer is thickened, and the initial capacity is likely to be improved. A thick negative electrode active material layer means a negative electrode active material layer with a thickness of 30 μm or more.

[0013] The volume of a PC negative electrode material expands when lithium is inserted and contracts when lithium is desorbed. Compared to existing graphite negative electrode materials, the volume change associated with this expansion and contraction of a PC negative electrode material is greater. When expansion and contraction occur, the contact points between the PC negative electrode materials are easily broken, disrupting the conductive path. Lithium ions cannot move at the locations where the conductive path is disrupted, resulting in resistance. Because the negative electrode active material layer made from the negative electrode composition of this embodiment contains conductive additive 2, it is easy to maintain the conductive path even when volume changes associated with expansion and contraction occur, and the initial capacity is likely to improve. Hereinafter, each component will be described in the order of conductive additive 1, conductive additive 2, and PC negative electrode material.

[0014] Conductive additive 1: The conductive additive 1 is a fibrous carbon material having an average fiber length of more than 1 μm. Examples of the conductive additive 1 include carbon nanotubes, carbon nanofibers, vapor-grown carbon fibers, mesophase pitch-based carbon fibers, and isotropic pitch-based carbon fibers. Vapor-grown carbon fibers available from Showa Denko K.K. under the trademark VGCF can be used.

[0015] The average fiber length of the conductive additive 1 is preferably 1.1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more. The average fiber length of the conductive additive 1 is, for example, 20 μm or less, 15 μm or less, or 10 μm or less. The average fiber length of the conductive additive 1 is, for example, 1.1 μm or more and 20 μm or less, 3 μm or more and 15 μm or less, or 5 μm or more and 10 μm.

[0016] The average fiber diameter of the conductive additive 1 is preferably 10 nm or more, more preferably 100 nm or more, and even more preferably 150 nm or more. The average fiber diameter of the conductive additive 1 is, for example, 500 nm or less, 300 nm or less, or 200 nm or less. The average fiber diameter of the conductive additive 1 is, for example, 10 nm or more and 500 nm or less, 100 nm or more and 300 nm or less, or 150 nm or more and 200 nm or less.

[0017] The average fiber length and average fiber diameter of the conductive additive 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 taking the arithmetic average of the number of fibers. Here, one fibrous carbon material refers to a bundled fiber structure when a bundle structure is formed.

[0018] When the average fiber length of the conductive additive 1 is equal to or greater than the above-mentioned lower limit, a conductive network is easily formed in the thickness direction in the negative electrode active material layer.When the average fiber diameter of the conductive additive 1 is equal to or less than the above-mentioned upper limit, the fibrous carbon material is less likely to act as a resistance to ion diffusion, and the ion conductivity is excellent.

[0019] From the viewpoint of improving the conductivity-imparting effect, it is preferable that the compaction resistivity of the conductive additive 1 is small. Specifically, the compaction density of the conductive additive 1 is 0.8 g / cm ３ The compaction resistivity in 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.

[0020] The content of the conductive additive 1 relative to the total amount of the negative electrode composition is preferably 5% by mass or more, more preferably 5.5% by mass or more, and even more preferably 6% by mass or more. The content of the conductive additive 1 relative to the total amount of the negative electrode composition is preferably 20% by mass or less, more preferably 16% by mass or less, and even more preferably 14% by mass or more. The content of the conductive additive 1 relative to the total amount of the negative electrode composition is, for example, 5% by mass or more and 20% by mass or less, 5.5% by mass or more and 16% by mass or less, or 6% by mass or more and 14% by mass or less.

[0021] When the content of the conductive additive 1 is equal to or greater than the above lower limit, a conductive network is easily formed in the thickness direction in the negative electrode active material layer. When the content of the conductive additive 1 is equal to or less than the above upper limit, the fibrous carbon material is less likely to act as a resistance to ion diffusion, and the ion conductivity is excellent.

[0022] In this embodiment, the conductive additive 1 may be used alone or in combination of two or more kinds.

[0023] <Conductive Aid 2> The conductive aid 2 is a fibrous carbon material or a particulate conductive aid having an average fiber length of 1 μm or less. From the viewpoint of ensuring a conductive path between the P-C negative electrode materials, the conductive aid 2 is preferably a particulate conductive aid. The particulate conductive aid 2 is likely to form contact points with the P-C negative electrode material when interposed between the P-C negative electrode materials to ensure a conductive path. Furthermore, even if the position of the conductive aid 2 shifts due to expansion or contraction of the P-C negative electrode material, the contact points with the P-C negative electrode material are likely to be maintained. The particulate shape may be spherical or flaky.

[0024] Examples of the conductive additive 2 that can be used include natural graphite, artificial graphite such as mesocarbon microbeads, carbon black (e.g., acetylene black, ketjen black, and furnace black), graphite particles, graphene, fullerene, and other carbon materials. Also usable are metal powders such as copper, nickel, aluminum, silver, and gold, and conductive ceramic materials.

[0025] When carbon fibers having an average fiber length of 1 μm or less are used as the conductive additive 2, for example, vapor-grown carbon fibers or carbon nanotubes can be used.

[0026] The content of the conductive additive 2 relative to the total amount of the negative electrode composition is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. The content of the conductive additive 2 relative to the total amount of the negative electrode composition is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. The content of the conductive additive 2 relative to the total amount of the negative electrode composition is, for example, 0.1% by mass to 10% by mass, 0.5% by mass to 5% by mass, or 1% by mass to 3% by mass.

[0027] When the content of the conductive additive 2 is equal to or greater than the above-mentioned lower limit, a conductive path between the PC negative electrode materials is easily secured. When the content of the conductive additive 2 is equal to or less than the above-mentioned upper limit, the conductive additive 2 is less likely to act as a resistance, and a conductive network is easily formed.

[0028] In this embodiment, the conductive additive 2 may be used alone or in combination of two or more kinds.

[0029] <Phosphorus-Carbon Composite Negative Electrode Material> The PC negative electrode material used in this embodiment contains phosphorus atoms and carbon atoms. In one aspect of the present invention, the PC negative electrode material is in the form of a powder.

[0030] When the P-C negative electrode material contains components other than phosphorus atoms and carbon atoms, the content of the other components is preferably 1 mass % or more and 15 mass % or less, 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, based on the total amount of the components other than phosphorus atoms and carbon atoms.

[0031] When the content ratio of the other components is within the above range, the movement of lithium ions is less likely to be hindered, and the initial capacity of the secondary battery is more likely to be improved.

[0032] The PC negative electrode material is composed of 60 mass% or more of phosphorus atoms and carbon atoms, and may further contain elements other than phosphorus atoms and carbon atoms. Examples of elements contained in the PC negative electrode material other than phosphorus atoms and carbon atoms include lithium, silicon, germanium, tin, aluminum, zinc, magnesium, transition metals, nitrogen, oxygen, fluorine, silicon, titanium, niobium, sulfur, and chlorine. These can be obtained from metal oxides, composite metal oxides, metal fluorides, metal sulfides, metal chlorides, silicon oxides, silicates, titanates, and aluminates.

[0033] The content of phosphorus atoms, the content of carbon atoms, and the contents of other atoms that may be contained in the PC negative electrode material can be determined by known ICP analysis.

[0034] In the PC negative electrode material, the content of the phosphorus atoms is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 50% by mass or more. A high phosphorus content can increase the initial charge / discharge capacity of the PC negative electrode material. Furthermore, in the PC negative electrode material, the content of the phosphorus atoms is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 75% by mass or less. If the content of phosphorus atoms is too high, the cycle characteristics of the PC negative electrode material tend to deteriorate. The upper and lower limits of the content of phosphorus atoms can be arbitrarily combined. The content of phosphorus atoms is, for example, 20% by mass or more and 90% by mass or less, 30% by mass or more and 80% by mass or less, or 50% by mass or more and 75% by mass or less.

[0035] In the PC negative electrode material, the carbon atom content is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more. A high 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% by mass or less, more preferably 70% by mass or less, and even more preferably 50% by mass or less. If the carbon atom content is too high, the initial charge / discharge capacity of the PC negative electrode material is likely to decrease. The upper and lower limits of the carbon atom content can be arbitrarily combined. The carbon atom content is, for example, 10% by mass or more and 80% by mass or less, 20% by mass or more and 70% by mass or less, or 25% by mass or more and 50% by mass or less.

[0036] As will be described in detail later, the P-C negative electrode material can be produced by mixing a phosphorus material and a carbon material using a ball mill. The inventors conducted a detailed analysis and study of the obtained P-C negative electrode material and confirmed that the P-C negative electrode material is not a simple mixture of the raw materials, phosphorus and carbon material, but is a novel material in which phosphorus atoms and carbon atoms are chemically bonded (covalently bonded) to each other and have physical properties different from those of the raw materials, phosphorus and carbon materials.

[0037] The physical properties of the PC negative electrode material will be described in detail below, comparing it with a mixture of phosphorus and a carbon material (hereinafter sometimes referred to as a "PC mixture").

[0038] The following description compares an example of a PC negative electrode material with an example of a mixture obtained by mixing the same raw materials as the PC negative electrode material in a mortar. In the example shown below, the raw materials for the PC negative electrode material and the raw materials for the PC mixture both contain black phosphorus and a carbon material in a mass ratio of 6:4. In both the PC negative electrode material and the PC mixture, CSCNT (cup-stacked carbon nanotubes) are used as the carbon material.

[0039] [Presence or absence of P-C bond] P-C negative electrode materials have a peak indicative of a bond between a phosphorus atom and a carbon atom in an XPS spectrum. It is known that the "peak indicative of a bond between a phosphorus atom and a carbon atom" appears in the range of 132 to 136 eV in an XPS spectrum. In the following description, the bond between a phosphorus atom and a carbon atom may be simply referred to as a "P-C bond."

[0040] Figure 1 shows XPS spectra of the PC negative electrode material and the PC mixture. In Figure 1, the horizontal axis represents binding energy (eV) and the vertical axis represents the number of detected photoelectrons (cps (counts per second)). In Figure 1, symbol A represents the PC negative electrode material, and symbol X represents the PC mixture.

[0041] (XPS Spectrum Measurement Conditions) XPS spectra are measured under the following measurement conditions: Measuring instrument: XPS device, ESCA-3400 (manufactured by Shimadzu Corporation) Radiation source: Mg Kα radiation (20 mA, 10 kV) If a peak is detected in the range of 132 to 136 eV, it is determined that a P—C bond is present.

[0042] As shown in Figure 1, no peak indicating a P-C bond was detected in the range of 132 to 136 eV in the P-C mixture, whereas a peak indicating a P-C bond (denoted by the symbol α) was detected in the P-C negative electrode material.

[0043] From the results shown in FIG. 1, it is believed that the PC negative electrode material is a substance having a chemical bond between the phosphorus atom and the carbon atom.

[0044] [Appearance] The PC negative electrode material preferably has a core particle containing a phosphorus atom and a carbon coating covering the surface of the core particle.

[0045] Fig. 2 is a transmission electron microscope (TEM) photograph of the PC negative electrode material. As shown in Fig. 2, particles 50 of the PC negative electrode material have a core-shell structure in which the surface of a core particle 51 containing phosphorus atoms is covered with a carbon coating 52.

[0046] The presence of phosphorus atoms and carbon atoms in the PC negative electrode material can be confirmed by using EDX (Energy Dispersive X-ray Spectroscopy).

[0047] Furthermore, when the PC negative electrode material is composed only of phosphorus atoms and carbon atoms, the heavier phosphorus atoms appear darker in the TEM photograph, and therefore, based on the TEM photograph, it can be easily determined that the PC negative electrode material has a core-shell structure in which core particles 51 contain phosphorus atoms and are covered with carbon coatings 52 containing carbon atoms.

[0048] (TEM Photographing Conditions) TEM photographs are taken under the following photographing conditions: TEM device: Transmission electron microscope H-9000NAR (manufactured by Hitachi, Ltd.) Field of view: maximum 500 nm x 500 nm At least a portion of the particles of the PC negative electrode material is included in the field of view when photographing.

[0049] According to the above TEM photographing conditions, the core-shell structure of the PC negative electrode material can be confirmed.

[0050] In addition to satisfying the above-mentioned requirements for thermal behavior, the PC negative electrode material preferably has a core-shell structure.

[0051] [Crystalline state] Figure 3 shows 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 (a.u.). In Figure 3, symbol A represents the PC negative electrode material, and symbol X represents the PC mixture.

[0052] (XRD Profile Measurement Conditions) The XRD profile is measured under the following measurement conditions: Measurement equipment: horizontal sample multipurpose X-ray diffractometer Ultima IV (manufactured by Rigaku Corporation) Radiation source: Cu Kα radiation Measurement range (2θ): 10° to 90° Scan speed: 4° / min Sampling: 0.02° Voltage: 40 kV, current: 40 mA

[0053] As shown in Figure 3, in the PC mixture, the carbon material and black phosphorus each have a certain degree of crystallinity and exhibit diffraction peaks. In contrast, in the PC negative electrode material, the diffraction peaks seen in the PC mixture were not observed, and only noise was observed throughout the measurement range. In other words, from the results shown in Figure 3, it is believed that in the PC negative electrode material, the crystallinity of the carbon material used as a raw material has been lost and it has become amorphous, or the carbon material has become so fine that the crystalline state cannot be confirmed.

[0054] Here, when the XRD profile of the PC negative electrode material is measured under the above conditions, the intensity I at 2θ=20° in the XRD profile measured under the above conditions is ２０ , intensity I at 2θ=26.3° ２６．３ , and the intensity at 2θ=40° I ４０ It is preferable that |P| / |B|<2 (1) P=I ２６．３ -I ４０ ...(2) B = (I ２０ -I ４０ ) × (26.3 - 40) / (20 - 40) ... (3)

[0055] When the XRD profile of a raw carbon material is measured, a peak of the carbon material appears at 2θ = 26.3°, which corresponds to the graphite (002) peak. On the other hand, the XRD profile of a normal carbon material does not have peaks at 2θ = 20° or 2θ = 40°. Therefore, in the above formula (1), 2θ = 40°, where no peak of the carbon material exists, is set as the reference point, and the state of the carbon material can be determined by comparing the intensity of the reference point in the XRD profile with the intensity at the position (2θ = 26.3°) where the peak of the raw carbon material exists.

[0056] "P" expressed in the above formula (2) is the intensity I of the reference point ４０ and the intensity I at the position where the peak of the carbon material exists. ２６．３ and indicates the difference of the peak from the reference point.

[0057] "B" in the above formula (3) indicates the baseline intensity at 2θ=26.3°, estimated from two positions (2θ=20° and 2θ=40°) where there are no peaks.

[0058] The ratio of the baseline intensity from the reference point to the peak intensity can be calculated from the ratio of the absolute values ​​of P and B (|P| / |B|) expressed by the above formula (1). When |P| / |B| is 2 or more, it indicates the presence of a peak of the carbon material.

[0059] On the other hand, when |P| / |B| is less than 2 and satisfies formula (1), the peak of the carbon material is reduced, and it can be determined that the crystallinity of the carbon material is reduced and the carbon material is amorphous or so fine that the crystalline state cannot be confirmed. Therefore, it is preferable that the P-C negative electrode material satisfies the above-mentioned requirements for [thermal behavior] as well as the above-mentioned relational expressions (1) to (3).

[0060] As described above, the PC anode material is a novel substance that differs from the mixture of the raw material phosphorus (black phosphorus in Figures 1 to 3) and a carbon material in terms of its thermal behavior, bonding state, appearance, and crystalline state.

[0061] (Method for Producing PC Negative Electrode Material) The PC negative electrode material is obtained by mixing a phosphorus material and a carbon material. The phosphorus material and the carbon material are preferably mixed by a mixing and grinding process accompanied by compression.

[0062] The raw carbon materials include amorphous carbon, graphite, porous carbon, mesocarbon microbeads (MCMB), fullerene, carbon nanotubes, graphene, graphene oxide, and carbon nitrite (C ３ N ４), and laminated carbon nanofibers (Carbon nanofiber platelets). All of these carbon materials have a graphite structure.

[0063] Examples of amorphous carbon include carbon black (CB), acetylene black (AB), ketjen black, hard carbon, and soft carbon.

[0064] As fullerenes, C ６０ , C ７２ , C ８４ The following can be mentioned:

[0065] Examples of carbon nanotubes include single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), cup-stacked carbon nanotubes (CSCNTs), and vapor-grown carbon fibers (VGCFs).These have a structure extending in one axial direction (one-dimensional structure).

[0066] Graphene, graphene oxide, carbon nitrite (C ３ N ４ ), laminated carbon nanofibers (carbon nanofiber platelets) have a structure that extends in the plane direction (two-dimensional structure).

[0067] In carbon materials, the edges (edge ​​sites) of the graphite structure are more active than non-edge sites. Therefore, it is assumed that most of the P-C bonds are formed at the edge sites of the carbon material. Therefore, from the viewpoint of facilitating the formation of P-C bonds, it is preferable to use a carbon material that has many edge sites.

[0068] Carbon materials with many edge sites include CSCNT, graphite, mesocarbon microbeads (MCMB), graphene, graphene oxide, and carbon nitrite (C ３ N ４ Among them, CSCNT is preferred as the carbon material.

[0069] Any of the known phosphorus allotropes can be used as the raw material phosphorus, and black phosphorus is preferred as it is the most chemically stable of the phosphorus allotropes and is a good conductor of electricity.

[0070] "Mixed and crushed processing with compression" is a process in which a compressive force is applied to multiple types of powdered raw materials, mixing and crushing the raw materials. By subjecting the carbon material and phosphorus to mixed and crushed processing with compression, a strong impact force is applied to the carbon material and phosphorus, which is thought to cause a chemical change that cannot be achieved with a normal mixture while mixing and crushing the raw material powders. As a result, it is thought that a P-C bond that is not present in the raw materials is formed as a product, and a P-C negative electrode material that does not exhibit the crystalline state seen in the raw materials is obtained.

[0071] The "mixing and grinding treatment accompanied by compression" may be carried out until the peaks derived from the raw materials disappear in the XRD profile of the mixed material, or until the formation of P-C bonds can be confirmed in the XPS spectrum of the mixed material.

[0072] Examples of processing devices (pulverizers) capable of performing mixing and pulverization accompanied by compression include roller mills, jet mills, hammer mills, pin mills, disk mills, rod mills, ball mills, vibration mills, attritors, and bead mills. In the production of PC negative electrode materials, it is preferable to use an agitation type pulverizer equipped with a grinding vessel and a rotor, particularly because mixing and pulverization can be performed simultaneously. Examples of agitation type pulverizers include pin mills, disk mills, rod mills, ball mills, vibration mills, attritors, and bead mills.

[0073] Furthermore, as the processing device, a media-agitation type mill is preferred because it can mix and pulverize the raw material powder while simultaneously applying a strong impact force to the carbon material and phosphorus. Examples of media-agitation type mills include a ball mill, a vibration mill, an attritor, and a bead mill. Among these, a ball mill is particularly preferred from the viewpoint of easily controlling the property conditions.

[0074] It is believed that the ball mill mixing first causes atomization of phosphorus and decomposition of the carbon material, followed by the formation of P—C bonds. During this ball mill mixing process, phosphorus becomes the core particles described above.

[0075] It is believed that the carbon film is then formed around the core particles containing phosphorus atoms, resulting in a composite, thereby obtaining a PC negative electrode material.

[0076] During ball mill mixing, the production conditions can be controlled by adjusting the rotation speed of the ball mill, the amount of media (balls) relative to the raw materials (ball powder ratio), and the mixing time. That is, by adjusting the conditions, such as increasing the rotation speed of the ball mill, increasing the amount of media relative to the raw materials, or lengthening the mixing time, the mixing of phosphorus and the carbon material is promoted, making it easier to obtain a P-C negative electrode material.

[0077] Balls are grinding media for grinding metal materials. The diameter of the balls is the average particle size of the balls. The balls flow at high speed within the grinding container as the grinding container of the grinder rotates, colliding with the powder raw material containing the carbon material and phosphorus, thereby grinding the raw material into particles with a smaller average particle size. In the grinding process, it is preferable that the grinding container and beads are not excessively worn. Therefore, the shape of the balls is preferably spherical or ellipsoidal.

[0078] The diameter of the balls is preferably larger than the average particle size of the pulverized PC negative electrode material. By using such balls, a large amount of pulverization energy can be applied to the metal material, allowing metal particles to be obtained efficiently in a short time.

[0079] The diameter of the balls is preferably 0.1 to 10 mm, more preferably 1 to 10 mm. When the diameter of the balls is within this range, the formation of P—C bonds is promoted and re-agglomeration can be suppressed. The diameter of the balls placed in the grinding container may be uniform or may vary.

[0080] Examples of materials for the balls include glass, agate, alumina, zirconia, stainless steel, chrome steel, tungsten carbide, silicon carbide, and silicon nitride. Among these, zirconia is preferred because it has a relatively high hardness and is therefore resistant to wear, and its relatively high specific gravity allows for the generation of large pulverization energy. By using these balls, the raw material powder of the PC negative electrode material can be efficiently pulverized.

[0081] The weight ratio of the balls to the raw material powder of the PC negative electrode material is called the ball powder ratio. By increasing the ball powder ratio, a strong impact force can be applied to the raw material powder of the PC negative electrode material with high frequency, thereby further promoting the formation of PC bonds. If the ball powder ratio is too high, the production amount of PC negative electrode material per unit operation decreases. Therefore, the ball powder ratio is preferably 0.5 to 500, more preferably 1 to 200, and even more preferably 10 to 200.

[0082] After the ball mill mixing is completed, the balls are separated from the PC negative electrode material using a filter or the like.

[0083] The fact that the above-described PC negative electrode material has been obtained by ball mill mixing can be confirmed by measuring the XRD profile of the mixed material and checking that the peaks derived from the raw materials disappear, as shown in Figure 1. The duration of ball mill mixing can be determined by conducting a preliminary experiment to determine the correspondence between the mixing time and the time until the peaks derived from the raw materials disappear. In other words, ball mill mixing should be carried out until the peaks derived from the raw materials disappear in the XRD profile of the mixed material.

[0084] Alternatively, the formation of P-C bonds may be confirmed by measuring the XPS spectrum of the mixed material, as shown in Figure 1. In this case, the duration of ball mill mixing may be determined by conducting a preliminary experiment to determine the relationship between the mixing time and the time required for the formation of P-C bonds. In other words, ball mill mixing may be carried out until the formation of P-C bonds can be confirmed in the XPS spectrum of the mixed material.

[0085] During ball mill mixing, strong localized impacts (pressure) are applied to the carbon material and phosphorus due to collisions between media and between the media and the ball mill container. Although the details are unclear, it is believed that the application of such impacts causes chemical changes that cannot be achieved with ordinary mixtures, forming P-C bonds that are not present in the raw materials, resulting in a P-C negative electrode material that does not exhibit the crystalline state seen in the raw materials.

[0086] <<Method for Producing Negative Electrode Composition>> The negative electrode composition of this embodiment can be produced by mixing the PC negative electrode material obtained by the method described above, a binder, conductive additive 1, conductive additive 2, and optional components.

[0087] (Optional Components) In addition to the above-mentioned components, the negative electrode composition may contain known materials used as negative electrode active materials. Examples of such materials include (i) alloy-based negative electrode active materials that form a lithium alloy phase, (ii) transition metal oxides that undergo a decomposition / regeneration reaction (conversion reaction) with lithium ions, (iii) oxides or composite oxides that have activity as negative electrode active materials, and (iv) layered carbon.

[0088] (i) Examples of alloy-based negative electrode active materials include metals such as silicon, germanium, tin, aluminum, zinc, and magnesium, and alloys thereof.

[0089] (ii) Examples of transition metal oxides include manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, and magnesium oxide. These oxides may be composite metal oxides further containing other metals (e.g., lithium).

[0090] (iii) Examples of oxides or composite oxides include titanium oxide, lithium titanate, and silicon oxide.

[0091] (iv) Examples of layered carbon include graphite and hard carbon.

[0092] The negative electrode composition preferably contains a binder for binding the negative electrode active material to the current collector. The binder may have a known structure.

[0093] As the binder, one or a mixture of two or more of polyvinylidene fluoride (hereinafter sometimes referred to as PVdF), polyimide, polyamideimide, styrene butadiene rubber, carboxymethyl cellulose, and acrylic resin can be suitably used.

[0094] <Negative electrode for lithium secondary battery> The negative electrode for lithium secondary battery of this embodiment has an electrode active material layer made of the above-mentioned negative electrode composition on the surface of a current collector. As the material for the current collector, copper or a copper alloy can be suitably used.

[0095] The thickness of the negative electrode active material layer included in the negative electrode for a lithium secondary battery of this embodiment is preferably 20 μm or more and 200 μm or less, more preferably 50 μm or more and 150 μm or less. In the negative electrode active material layer formed from the negative electrode composition of this embodiment, the conductive additive 1 forms a conductive network in the thickness direction in the negative electrode active material layer. This conductive network facilitates the movement of lithium ions even when the negative electrode active material layer is made thick, and the initial capacity is likely to be improved.

[0096] <Lithium Secondary Battery> The lithium secondary battery of this embodiment includes the above-described negative electrode for lithium secondary batteries (hereinafter, negative electrode).

[0097] An example of a suitable lithium secondary battery when using the negative electrode of this embodiment has a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte solution disposed between the positive electrode and the negative electrode.

[0098] An example of a lithium secondary battery has a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte solution disposed between the positive electrode and the negative electrode.

[0099] 4 is a schematic diagram showing an example of a lithium secondary battery. For example, a cylindrical lithium secondary battery 10 is manufactured as follows.

[0100] First, as shown in the partially enlarged view of FIG. 4 , a pair of strip-shaped separators 1, 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 this order: separator 1, positive electrode 2, separator 1, negative electrode 3, and then wound to form an electrode group 4.

[0101] The positive electrode 2 includes, for example, a positive electrode active material layer 2 a containing a positive electrode active material (hereinafter, referred to as CAM) and a positive electrode current collector 2 b having the positive electrode active material layer 2 a formed on one surface thereof. The positive electrode 2 can be manufactured by first preparing a positive electrode mixture containing the CAM, a conductive material, and a binder, and then supporting the positive electrode mixture on one surface of the positive electrode current collector 2 b to form the positive electrode active material layer 2 a.

[0102] Examples of the negative electrode 3 include an electrode in which a negative electrode mixture containing a negative electrode active material (not shown) is supported on a negative electrode current collector, and an electrode made of a negative electrode active material alone, and can be manufactured in the same manner as the positive electrode 2.

[0103] Next, the electrode group 4 and an insulator (not shown) are housed in the battery can 5, the bottom of the can is sealed, the electrode group 4 is impregnated with an electrolyte solution 6, and the electrolyte is disposed between the positive electrode 2 and the negative electrode 3. Furthermore, the top of the battery can 5 is sealed with a top insulator 7 and a sealing member 8, whereby a lithium secondary battery 10 can be manufactured.

[0104] The shape of the electrode group 4 can be, for example, a columnar shape such that the cross-sectional shape when the electrode group 4 is cut perpendicular to the winding axis is a circle, an ellipse, a rectangle, or a rectangle with rounded corners.

[0105] The shape of a lithium secondary battery having such an electrode group 4 can be any shape specified in IEC 60086, which is a standard for batteries established by the International Electrotechnical Commission (IEC), or JIS C 8500. Examples of shapes include a cylindrical shape and a rectangular shape.

[0106] Furthermore, the lithium secondary battery is not limited to the above-mentioned wound type configuration, and may be a stacked type 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, and paper type (or sheet type) batteries.

[0107] The positive electrode, separator, negative electrode, and electrolyte constituting the lithium secondary battery can be those described in, for example,

[0113] to

[0140] of WO2022 / 113904A1, and the materials and manufacturing methods therefor.

[0108] <All-Solid-State Lithium Secondary Battery> Next, an all-solid-state lithium secondary battery having the above-described negative electrode will be described while explaining the configuration of the all-solid-state lithium secondary battery.

[0109] Fig. 5 is a schematic diagram showing an example of an all-solid-state lithium secondary battery. The all-solid-state lithium secondary battery 1000 shown in Fig. 5 includes a laminate 100 having a positive electrode 110, a negative electrode 120, and a solid electrolyte layer 130, and an exterior body 200 that houses the laminate 100. The all-solid-state lithium secondary battery 1000 may also have a bipolar structure in which a CAM and a negative electrode active material are disposed on both sides of a current collector. A specific example of a bipolar structure is the structure described in JP-A-2004-95400.

[0110] The positive electrode 110 includes a positive electrode active material layer 111 and a positive electrode current collector 112. The positive electrode active material layer 111 includes the above-described CAM and solid electrolyte. The positive electrode active material layer 111 may also include a conductive material and a binder.

[0111] The negative electrode 120 includes 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. The negative electrode active material layer 121 may also contain a solid electrolyte and a conductive material.

[0112] The laminate 100 may have an external terminal 113 connected to the positive electrode current collector 112 and an external terminal 123 connected to the negative electrode current collector 122. In addition, the all-solid-state lithium secondary battery 1000 may have a separator between the positive electrode 110 and the negative electrode 120.

[0113] The all-solid-state lithium secondary battery 1000 further includes an insulator (not shown) that insulates the laminate 100 from the exterior body 200 , and a sealing body (not shown) that seals the opening 200 a of the exterior body 200 .

[0114] The exterior body 200 may be a container molded from a highly corrosion-resistant metal material such as aluminum, stainless steel, or nickel-plated steel. Alternatively, the exterior body 200 may be a bag-shaped container made of a laminate film having corrosion resistance on at least one surface.

[0115] The shape of the all-solid-state lithium secondary battery 1000 may be, for example, a coin type, a button type, a paper type (or a sheet type), a cylindrical type, a square type, or a laminate type (pouch type).

[0116] Although the all-solid-state lithium secondary battery 1000 is illustrated as having one laminate 100 as an example, the present embodiment is not limited to this. The all-solid-state lithium secondary battery 1000 may have a configuration in which the laminate 100 is used as a unit cell, and a plurality of unit cells (laminated bodies 100) are sealed inside an exterior body 200.

[0117] For the all-solid-state lithium secondary battery, for example, the configuration, materials and manufacturing method described in paragraphs

[0141] to

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

[0118] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0119] Example 1 Production of PC Negative Electrode Material Black phosphorus manufactured by Rasa Industries and CSCNT (cup-stacked carbon nanotubes: manufactured by GSI Creos Co., Ltd.) were weighed out in a mass ratio of P:C = 6:4 (total amount 1.25 g) and mixed using a ball mill under the following conditions to obtain PC Negative Electrode Material 1. (Ball mill mixing conditions) Apparatus: Retsch PM-100 Container: ZrO ２ 125mL container Media: ZrO ２ 8 mm diameter ball manufactured by 150 g Atmosphere: Argon gas sealed Sample amount: 1.25 g as a mixture of raw materials Ball / powder ratio: 120 (weight ratio) Mixing time: 12 hours

[0120] A TEM photograph was taken of the obtained PC negative electrode material 1, and it was confirmed that the material had a core-shell structure in which the surface of a core particle containing phosphorus atoms was covered with a carbon film.

[0121] The obtained P-C negative electrode material 1 was charged at a charge rate of 0.2 C using lithium metal as the counter electrode. As a result, the average charge potential of P-C negative electrode material 1 relative to the capacity (mAh / g) during charging was 0.1 V or higher. It was confirmed that the obtained P-C negative electrode material 1 is a material that absorbs lithium ions when lithium (alkali metal) is used as the counter electrode and a voltage of 0.1 V or higher is applied.

[0122] The resulting P-C negative electrode material 1 had a phosphorus atom content of 60 mass % and a carbon atom content of 40 mass %. Because the container used in the above-mentioned production was extremely airtight, it was determined that the content of each atom in the P-C negative electrode material 1 was equal to the raw material ratio.

[0123] The obtained PC negative electrode material 1 was measured according to the above-mentioned (XPS spectrum measurement conditions), and a peak indicating a bond between a phosphorus atom and a carbon atom was confirmed.

[0124] The obtained PC negative electrode material 1 was measured according to the above-mentioned (XRD profile measurement conditions), and in the XRD profile, the intensity I at 2θ=20° ２０ is 3063, and the intensity at 2θ = 26.3° is I ２６．３ is 2603, and the intensity at 2θ = 40° is I ４０ was 1917. In addition, |P| / |B| was 0.87. PC negative electrode material 1 exhibiting these values ​​satisfies 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)

[0125] [Production of negative electrode composition] A PC negative electrode material 1 and VGCF (manufactured by Resonac, Inc., compressed density 0.8 g / cm 3 ) as a conductive additive 1 were mixed. ３A negative electrode composition 1 was obtained in which the content of VGCF as conductive additive 1 relative to the total amount of the negative electrode composition was 12 mass %, the content of SWCNT as conductive additive 2 was 1.2%, and 6 mass % of a binder (vinylidene fluoride PVdF Solef 5130 (manufactured by Solvay)).

[0126] (Preparation of Evaluation Battery) (1) Preparation of Negative Electrode 1 The negative electrode composition 1 and a solvent (NMP (N-methyl-2-pyrrolidone)) were kneaded in an agate mortar to prepare a negative electrode slurry. At this time, the slurry concentration was adjusted so that the content of the P-C negative electrode material 1 in the negative electrode slurry was 30 to 60 mass %.

[0127] The negative electrode slurry was applied to a copper foil current collector using a doctor blade, and then the coated material was vacuum-dried at 80°C for 1 hour to remove the solvent, yielding a laminate. The resulting laminate was pressed at 10 MPa for 10 seconds and then vacuum-dried at 150°C for 12 hours to obtain negative electrode 1.

[0128] The negative electrode 1 had a coating weight of 1.9 mg / cm ２ The thickness of the negative electrode active material layer was 20 μm.

[0129] The preparation of the negative electrode slurry and the preparation of the negative electrode were carried out in a glove box in an argon atmosphere.

[0130] (2) Preparation of Lithium-ion Secondary Battery The negative electrode, counter electrode, electrolyte, and separator prepared in (1) were combined to prepare a lithium-ion secondary battery (coin-type battery R2032). The battery was assembled in a glove box under an argon atmosphere.

[0131] Metallic lithium foil was used as the counter electrode.

[0132] As the electrolyte, LiPF ６ A mixed solution prepared by adding 10% by mass of fluoroethylene carbonate (FEC) to a solution (manufactured by Kishida Chemical Co., Ltd.) was used.

[0133] LiPF ６The solution was a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35, with LiPF ６ A solution in which the above was dissolved at a concentration of 1 mol / L was used.

[0134] As the separator, a polyethylene porous film separator (thickness: 12 μm) was used.

[0135] (Evaluation of Initial Capacity) Using the above evaluation battery, the initial capacity was evaluated by charging and discharging under the following conditions while maintaining the temperature at 25°C. A larger capacity at the first cycle indicates a higher initial discharge capacity, and therefore it can be determined that the material is superior as a negative electrode active material. Minimum charging voltage: 0.010 V Charging current: 1 C (1 C = 1800 mA / g) Maximum discharging voltage: 2.0 V Discharging current: 1 C (1 C = 1800 mA / g)

[0136] The capacity of the lithium secondary battery equipped with the negative electrode 1 made of the negative electrode composition 1 at the first cycle of 1C was 1464 mAh / g.

[0137] Example 2: The weight of the negative electrode composition 1 was 3.4 mg / cm ２ and the thickness of the negative electrode active material layer was changed to 37 μm.

[0138] The capacity of the lithium secondary battery equipped with the negative electrode 2 at the first cycle of 1C was 1427 mAh / g.

[0139] Example 3: The basis weight of the negative electrode composition 1 was 4.9 mg / cm ２ A negative electrode 3 was produced in the same manner as in Example 1, except that the thickness of the negative electrode active material layer was changed to 45 μm.

[0140] The capacity of the lithium secondary battery equipped with the negative electrode 3 at the first cycle of 1C was 1391 mAh / g.

[0141] Comparative Example 1 A negative electrode 11 was produced in the same manner as in Example 1, except that the conductive additive 1 was not used. The capacity of the lithium secondary battery including the negative electrode 11 at 1C in the first cycle was 1286 mAh / g.

[0142] Comparative Example 2 A negative electrode 12 was produced in the same manner as in Example 2, except that the conductive additive 1 was not used. The capacity of the lithium secondary battery including the negative electrode 12 at 1C in the first cycle was 1272 mAh / g.

[0143] Comparative Example 3 A negative electrode 13 was produced in the same manner as in Example 3, except that the conductive additive 1 was not used. The capacity of the lithium secondary battery including the negative electrode 13 at 1C in the first cycle was 1135 mAh / g.

[0144] As shown in the above results, the lithium secondary batteries using the negative electrode compositions containing conductive additive 1 and conductive additive 2 had initial capacities of 1390 mAh / g or more, even when the negative electrode thickness was 20 μm or more, and Examples 1 and 2 exceeded 1400 mAh / g. On the other hand, the lithium secondary batteries using the negative electrode composition not using conductive additive 1 had initial capacities below 1300 mAh / g. From the above, it was confirmed that the negative electrode composition of the present invention can provide a lithium secondary battery with a high initial capacity even when the negative electrode is made thick.

[0145] 50...particle, 51...core particle, 52...carbon film, 100...laminated body, 120...negative electrode, 121...negative electrode active material layer, 130...solid electrolyte layer

Claims

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

2. The negative electrode composition according to claim 1, wherein the conductive additive 1 has an average fiber length of 3 μm or more.

3. The negative electrode composition according to claim 1 or 2, wherein the content of said conductive additive 1 relative to the total amount of said negative electrode composition is 5 mass % or more and 20 mass % or less.

4. The negative electrode composition according to claim 1 or 2, wherein the conductive additive 2 is a particulate conductive additive.

5. The negative electrode composition according to claim 1 or 2, wherein the phosphorus-carbon composite negative electrode material has a peak indicative of a bond between a phosphorus atom and a carbon atom in an XPS spectrum.

6. The phosphorus-carbon composite negative electrode material has an intensity I at 2θ=20° in an XRD profile measured using CuKα radiation. ２０ , intensity I at 2θ=26.3° ２６．３ , and the intensity at 2θ=40° I ４０ The negative electrode composition according to claim 1 or 2, wherein |P| / |B|<2 (1) satisfies the following formulas (1) to (3): ２６．３ -I ４０ ...(2) B = (I ２０ -I ４０ ) × (26.3 - 40) / (20 - 40) ... (3) 7. The negative electrode composition according to claim 1 or 2, wherein the phosphorus-carbon composite negative electrode material comprises: a core particle containing a phosphorus atom; and a carbon coating covering the surface of the core particle.

8. A negative electrode for a lithium secondary battery, comprising a negative electrode active material layer made of the negative electrode composition according to claim 1 or 2 on the surface of a current collector.

9. The negative electrode for a lithium secondary battery according to claim 8, wherein the thickness of the negative electrode active material layer is 20 μm or more and 150 μm or less.

10. A lithium secondary battery comprising the negative electrode for lithium secondary batteries according to claim 8.

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