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

Abstract

To provide a negative electrode composition with a high initial capacity even in a case where the thickness of an electrode active material layer is made thicker.SOLUTION: A negative electrode composition includes: a phosphorus-carbon composite negative electrode material including phosphorus atoms and carbon atoms; a conductive additive 1; and a conductive additive 2. The conductive additive 1 is a fibrous carbon material having an average fiber length greater than 1 μm. The conductive additive 2 is either a fibrous carbon material having an average fiber length of 1 μm or less, or a particulate conductive additive. It is preferable that the conductive additive 1 is a fibrous carbon material having a fiber length of at least 3 μm. It is preferable that the conductive additive 2 is a particulate conductive additive.SELECTED DRAWING: None

Description

[Technical Field]

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

[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 contained in 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 is a composite of a carbon material and phosphorus, and by using the negative electrode active material, a negative electrode with excellent charge / discharge capacity can be obtained. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-184861 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, there has been a demand for lithium secondary batteries with higher energy densities. One way to increase 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. [Means for solving the problem]

[0008] In order to solve the above problems, one aspect of the present invention includes the following aspects. [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 [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 the content of the conductive additive 1 relative 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 in an XPS spectrum that indicates a bond between a phosphorus atom and a carbon atom. [6] The phosphorus-carbon composite negative electrode material has an intensity I at 2θ=20° in the XRD profile measured using CuKα radiation. 20 , intensity I at 2θ=26.3° 26.3 , and the intensity at 2θ=40° I40 The negative electrode composition according to any one of [1] to [5], wherein the following formulas (1) to (3) are satisfied: |P| / |B|<2 …(1) P=I 26.3 -I 40 …(2) B=(I 20 -I 40 ) × (26.3 - 40) / (20 - 40) … (3) [7] The negative electrode composition according to any one of [1] to [6], wherein the phosphorus-carbon composite negative electrode material has 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 any one of [1] to [7] on the surface of a current collector. [9] The negative electrode for a lithium secondary battery according to [8], wherein the film 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 [8] or [9]. [Effects of the Invention]

[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. [Brief explanation of the drawings]

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

[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 abbreviated simply as "PC negative electrode material."

[0012] The negative electrode active material layer made of 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 made thick, and the initial capacity is likely to be improved. The thickened negative electrode active material layer means a negative electrode active material layer having 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, and when expansion and contraction occur, 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 the conductive additive 2, the conductive path is easily maintained 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 that can be used include those commercially available under the trademark VGCF from Showa Denko K.K.

[0015] The conductive assistant 1 preferably has an average fiber length of 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 assistant 1 is, for example, 20 μm or less, 15 μm or less, or 10 μm or less. The average fiber length of the conductive assistant 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 or less.

[0016] The average fiber diameter of the conductive assistant 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 assistant 1 is, for example, 500 nm or less, 300 nm or less, or 200 nm or less. The average fiber diameter of the conductive assistant 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 if it forms a bundle structure.

[0018] When the average fiber length 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 average fiber diameter 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.

[0019] The conductive additive 1 preferably has a small compaction resistivity in order to improve the conductivity-imparting effect. Specifically, the compaction resistivity is preferably 0.8 g / cm. 3 The compaction resistivity 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 mass % or less, more preferably 16 mass % or less, and even more preferably 14 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 to 20% by mass, 5.5% by mass to 16% by mass, or 6% by mass to 14% by mass.

[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 additive 2> The conductive additive 2 is a fibrous carbon material or a particulate conductive additive having an average fiber length of 1 μm or less. The conductive additive 2 is preferably a particulate conductive additive from the viewpoint of ensuring a conductive path between the PC negative electrode materials. The particulate conductive additive 2 is likely to form contact points with the PC negative electrode materials when interposed between the PC negative electrode materials to ensure a conductive path. Furthermore, even if the position of the conductive additive 2 shifts due to expansion or contraction of the PC negative electrode material, the contact points with the PC 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 assistant 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 assistant 2 relative 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. 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 lower limit, a conductive path between the PC negative electrode materials can be easily secured. When the content of the conductive additive 2 is equal to or less than the upper limit, the conductive additive 2 is less likely to act as a resistance, and a conductive network is more likely to be 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 anode material> The PC negative electrode material used in this embodiment contains phosphorus atoms and carbon atoms. In one embodiment of the present invention, the PC negative electrode material is a powder.

[0030] When the PC negative electrode material contains components other than phosphorus atoms and carbon atoms, the content of the other components is preferably 1% by mass or more and 15% by mass or less, more preferably 2% by mass or more and 12% by mass or less, and even more preferably 2.5% by mass or more and 8% by 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 phosphorus atoms and carbon atoms at 60 mass % or more of the total, and may further contain elements other than phosphorus atoms and carbon atoms. Elements other than phosphorus and carbon atoms contained in PC anode materials 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 content 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 phosphorus atom content is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 50% by mass or more. A higher phosphorus content increases 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% by mass or less, more preferably 80% by mass or less, and even more preferably 75% by mass or less. This is because if the phosphorus atom content is too high, the cycle characteristics of the PC negative electrode material will deteriorate. The upper and lower limits of the phosphorus atom content can be arbitrarily combined.

[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 higher carbon atom content improves the cycle characteristics of the PC negative electrode material. In addition, 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. This is because if the carbon atom content is too high, the initial charge / discharge capacity of the PC negative electrode material decreases. The upper and lower limits of the carbon atom content can be combined as desired.

[0036] As will be described in detail later, the PC 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 PC negative electrode material and confirmed that the PC negative electrode material is not simply a 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 explanation 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 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] [PC bonding] The PC negative electrode material has 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 "PC 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) The XPS spectrum is measured under the following measurement conditions. Measurement equipment: XPS device, ESCA-3400 (Shimadzu Corporation) ·Radiation source: Mg Kα radiation (20mA, 10kV) When a peak is detected in the range of 132 to 136 eV, it is determined that a PC bond is present.

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

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

[0044] [exterior] 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 core particle 51 containing phosphorus atoms is covered with carbon film 52.

[0046] The presence of phosphorus atoms and carbon atoms in the PC negative electrode material can be confirmed 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 image, so it can be easily determined from the TEM image that the 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 conditions: TEM equipment: Transmission electron microscope H-9000NAR (Hitachi, Ltd.) Field of view: Maximum 500nm x 500nm. At least some of the particles of the PC negative electrode material must be included in the field of view.

[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 (au). 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 (Rigaku Corporation) ·Radiation source: Cu Kα radiation Measurement range (2θ): 10° to 90° Scan speed: 4° / min Sampling: 0.02° Voltage 40kV, current 40mA

[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 carbon material used as the raw material has lost its crystallinity and become amorphous, or the carbon material has become so fine that its 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 However, it is preferable that the following relational expressions (1) to (3) are satisfied. |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 typical 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 of the reference point I 40 and the intensity I at the position where the carbon material peak 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 a 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 the crystallinity of the carbon material is reduced, so that the carbon material is amorphous or so fine that the crystalline state cannot be confirmed. Therefore, it is preferable that the PC negative electrode material satisfies the above-mentioned requirements for [thermal behavior] as well as the above-mentioned relational expressions (1) to (3).

[0060] As shown above, the PC negative electrode 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] (Manufacturing method of PC negative electrode material) The PC negative electrode material is obtained by mixing a phosphorus material and a carbon material, which are preferably mixed by a mixing and grinding process accompanied by compression.

[0062] The carbon material used as the raw material can be at least one selected from the group consisting of amorphous carbon, graphite, porous carbon, mesocarbon microbeads (MCMB), fullerene, carbon nanotubes, graphene, graphene oxide, carbon nitrite (CN), 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 for fullerenes, C 60 , C 72 , C 84 The following can be mentioned:

[0065] Examples of carbon nanotubes include single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), cup-stacked carbon nanotubes (CSCNT), and carbon nanofibers VGCF (vapor grown carbon fiber). These have a structure that extends in one axial direction (one-dimensional structure).

[0066] Graphene, graphene oxide, carbon nitrite (C3N4), and 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 PC bonds are formed at the edge sites of carbon materials. Therefore, from the viewpoint of facilitating the formation of PC bonds, it is preferable to use a carbon material with many edge sites.

[0068] Examples of carbon materials with many edge sites include CSCNT, graphite, mesocarbon microbeads (MCMB), graphene, graphene oxide, carbon nitrite (CN), and laminated carbon nanofibers. Among these, 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 multiple types of powdered raw materials are mixed and crushed by applying a compressive force. By subjecting 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, PC bonds that are not present in the raw materials are formed as a product, and a PC negative electrode material that does not exhibit the crystalline state seen in the raw materials is thought to be obtained.

[0071] The "mixing and grinding process with compression" should 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 PC bonds can be confirmed in the XPS spectrum of the mixed material.

[0072] Processing devices (pulverizers) capable of mixing and pulverizing with 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 agitator-type pulverizer equipped with a grinding vessel and a rotor, particularly because it allows simultaneous mixing and pulverization. Examples of agitator-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 PC bonds. During this ball mill mixing process, phosphorus becomes the core particles mentioned above.

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

[0076] During ball mill mixing, the manufacturing 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. In other words, adjusting conditions such as increasing the rotation speed of the ball mill, increasing the amount of media relative to the raw materials, and extending the mixing time promotes mixing of the phosphorus and carbon material, making it easier to obtain PC negative electrode materials.

[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 crushed PC negative electrode material, as this allows for a large amount of crushing energy to be applied to the metal material, resulting in efficient production of metal particles 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 PC 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 ball materials 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, making it less susceptible to wear, and its relatively high specific gravity allows for large grinding energy to be obtained. Using these balls allows for efficient grinding of the raw material powder of PC negative electrode material.

[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-mentioned 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 should be determined by conducting a preliminary experiment to determine the relationship between the mixing time and the time it takes for the peaks derived from the raw materials to 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 XPS spectrum of the mixed material can be measured to confirm the formation of PC bonds, as shown in Figure 1. In this case, the duration of ball mill mixing should be determined by conducting a preliminary experiment to determine the relationship between mixing time and the time required for PC bonds to form. In other words, ball mill mixing should be continued until the formation of PC 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 unknown, it is thought that the application of such impacts causes chemical changes that cannot be achieved with normal mixtures, leading to the formation of PC bonds that are not present in the raw materials, resulting in the production of a PC 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 above-mentioned method, a binder, conductive additive 1, conductive additive 2, and optional components.

[0087] (optional ingredient) In addition to the above-mentioned components, the negative electrode composition may contain known materials used as negative electrode active materials, such as (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 are active 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] <Anode for lithium secondary batteries> The negative electrode for a 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 negative electrode active material layer included in the negative electrode for a lithium secondary battery of this embodiment preferably has a film thickness of 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-mentioned 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] For example, the positive electrode 2 includes a positive electrode active material layer 2a containing a positive electrode active material (hereinafter, referred to as CAM) and a positive electrode current collector 2b on one surface of which the positive electrode active material layer 2a is formed. Such a positive electrode 2 can be manufactured by first preparing a positive electrode mixture containing CAM, a conductive material, and a binder, and then supporting the positive electrode mixture on one surface of the positive electrode current collector 2b to form the positive electrode active material layer 2a.

[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 placed 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 section of the electrode group 4 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 by IEC60086, 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 laminated type configuration in which a laminated structure of a positive electrode, a separator, a negative electrode, and a separator is repeatedly stacked. Examples of laminated 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, for example, the configuration, materials, and manufacturing method described in

[0113] to

[0140] of WO2022 / 113904A1.

[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 has a positive electrode active material layer 111 and a positive electrode current collector 112. The positive electrode active material layer 111 contains the above-mentioned CAM and solid electrolyte. The positive electrode active material layer 111 may also contain a conductive material and a binder.

[0111] 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. 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] A container molded from a highly corrosion-resistant metal material such as aluminum, stainless steel, or nickel-plated steel can be used as exterior body 200. Alternatively, a container formed into a bag shape from a laminate film with corrosion resistance applied to at least one surface can also be used as exterior body 200.

[0115] The all-solid-state lithium secondary battery 1000 may have any shape, such as a coin shape, a button shape, a paper shape (or a sheet shape), a cylindrical shape, a square shape, or a laminate shape (pouch shape).

[0116] 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 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

[0141] to

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

[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 materials] Black phosphorus manufactured by Rasa Industries and CSCNT (cup-stacked carbon nanotubes: manufactured by GSI Creos Co., Ltd.) were weighed 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) Equipment:Retsch PM-100 Container: 125mL ZrO2 container Media: ZrO2 8mmφ ball, 150g Atmosphere: Argon gas filled Sample amount: 1.25g of raw material mixture Ball / powder ratio: 120 (by weight) 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 PC 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 PC negative electrode material 1 relative to the capacity (mAh / g) during charging was 0.1 V or more. 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 a voltage of 0.1 V or more is applied.

[0122] The obtained PC 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 PC 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° was 20 is 3063, and the intensity at 2θ=26.3° is I 26.3 is 2603, and the intensity at 2θ=40° is I 40was 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] PC negative electrode material 1 and conductive additive 1, VGCF (manufactured by Resonac, compressed density 0.8 g / cm 3 A negative electrode composition 1 was obtained in which the content of VGCF as conductive additive 1 was 12 mass% relative to the total amount of the negative electrode composition, 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 mixed in 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 %.

[0127] The 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, obtaining 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) Fabrication of lithium-ion secondary batteries 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, a mixed solution in which 10 mass % of fluoroethylene carbonate (FEC) was added to a LiPF6 solution (manufactured by Kishida Chemical Co., Ltd.) was used.

[0133] The LiPF6 solution used was a solution prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35.

[0134] A polyethylene porous film separator (12 μm thick) was used as the separator.

[0135] (Initial capacity evaluation) The above evaluation battery was charged and discharged under the following conditions at 25° C. to evaluate the initial capacity. A larger capacity at the first cycle indicates a higher initial discharge capacity, and therefore can be judged to be an excellent negative electrode active material. Minimum charging voltage: 0.010V Charging current: 1C (1C=1800mA / g) Maximum discharge voltage: 2.0V Discharge current: 1C (1C=1800mA / 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, and the negative electrode 2 was produced in the same manner as in Example 1.

[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 weight of negative electrode composition 1 was 4.9 mg / cm 2 and 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 1C in the first cycle 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 equipped with 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 equipped with 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 equipped with 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 thickness of the negative electrode was 20 μm or more, and Examples 1 and 2 exceeded 1400 mAh / g. On the other hand, the lithium secondary battery using the negative electrode composition that did not use the conductive additive 1 had an initial capacity below 1300 mAh / g. From the above, it was confirmed that the negative electrode composition of the present invention makes it possible to obtain a lithium secondary battery with a high initial capacity even when the negative electrode is made thick. [Explanation of symbols]

[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 a phosphorus atom and a carbon atom, a conductive additive 1, and a conductive additive 2, The negative electrode composition, 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 a content ratio of the first conductive additive 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 claim 1 or 2, wherein the conductive additive 2 is a particulate conductive additive.

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

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 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)

7. The phosphorus-carbon composite negative electrode material comprises core particles containing phosphorus atoms; The negative electrode composition according to claim 1 or 2, further comprising a carbon coating covering the surface of the core particle.

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

9. 9. The negative electrode for a lithium secondary battery according to claim 8, wherein the film 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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