Phosphorus-carbon composite negative electrode material, negative electrode active material, negative electrode for lithium secondary battery, lithium secondary battery, and method for producing phosphorus-carbon composite negative electrode material

Abstract

Provided is a phosphorus-carbon composite negative electrode material comprising phosphorus atoms and carbon atoms. The phosphorus-carbon composite negative electrode material has at least a peak having a line width of 0.1-5 mT in the range of 250-390 mT in a spectrum measured at a temperature of 297 K by means of an electron spin resonance method that uses the X band. Also provided is a method for producing a phosphorus-carbon composite negative electrode material that comprises: a step for compounding a carbon material and phosphorus by performing mixing and grinding in association with compression, thereby obtaining a precursor of the phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms; and a step for firing the precursor in an inert atmosphere.

Description

Phosphorus-carbon composite negative electrode material, negative electrode active material, negative electrode for lithium secondary battery, lithium secondary battery, and method for producing phosphorus-carbon composite negative electrode material

[0001] The present invention relates to a phosphorus-carbon composite negative electrode material, a negative electrode active material, a negative electrode for a lithium secondary battery, a lithium secondary battery, and a method for producing the phosphorus-carbon composite negative electrode material. This application claims priority based on Japanese Patent Application No. 2024-104948, filed on June 28, 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 uses a black phosphorus carbon composite, which is a composite of a carbon material and phosphorus, as the negative electrode active material, thereby achieving a negative electrode with excellent charge / discharge capacity.

[0004] JP 2009-184861 A

[0005] Lithium secondary batteries require further safety measures, and the negative electrode active material described in Patent Document 1 has ample room for improvement in terms of safety measures. The black phosphorus used in Patent Document 1 has high chemical stability among phosphorus allotropes, and black phosphorus alone is excellent in safety. However, the present inventors have discovered that when black phosphorus is combined with carbon, the chemical stability and safety tend to decrease.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a highly safe phosphorus-carbon composite negative electrode material, a negative electrode active material, a negative electrode for a lithium secondary battery, a lithium secondary battery, and a method for producing the phosphorus-carbon composite negative electrode material.

[0007] In order to solve the above problems, one aspect of the present invention includes the following: [1] A phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, the phosphorus-carbon composite negative electrode material having at least a peak with a line width of 0.1 mT or more and 5 mT or less in the range of 250 mT or more and 390 mT or less in a spectrum measured at a temperature of 297 K by electron spin resonance using X-band. [2] A negative electrode active material comprising the phosphorus-carbon composite negative electrode material according to [1]. [3] A negative electrode for a lithium secondary battery comprising the negative electrode active material according to [2]. [4] A lithium secondary battery comprising the negative electrode active material according to [2]. [5] A method for producing a phosphorus-carbon composite negative electrode material, comprising the steps of: forming a composite of a carbon material and phosphorus by a mixing and grinding process involving compression to obtain a precursor of the phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms; and firing the precursor in an inert atmosphere. [6] The method for producing a phosphorus-carbon composite negative electrode material according to [5], wherein the firing step is carried out at a firing temperature of 250° C. or higher and lower than the decomposition temperature of the precursor.

[0008] According to the present invention, it is possible to provide a highly safe phosphorus-carbon composite negative electrode material, a negative electrode active material, a negative electrode for a lithium secondary battery, a lithium secondary battery, and a method for producing the phosphorus-carbon composite negative electrode material.

[0009] FIG. 1 is a diagram showing the ESR spectrum of a PC negative electrode material. FIG. 2 is an XPS spectrum of a PC negative electrode material and a PC mixture. FIG. 3 is a transmission electron microscope (TEM) photograph of the PC material. FIG. 4 is an XRD profile of a PC negative electrode material and a PC mixture. FIG. 5 is a schematic diagram showing an all-solid-state lithium secondary battery, which is an example of a battery. FIG. 6 is a schematic diagram showing an all-solid-state lithium secondary battery, which is an example of a battery. A diagram for explaining the line width of an ESR spectrum.

[0010] <Phosphorus-carbon composite negative electrode material> The present invention relates to a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms. In the following description, the "phosphorus-carbon composite negative electrode material" may be simply abbreviated as "P-C negative electrode material." The P-C negative electrode material of this embodiment has at least a peak with a line width of 0.1 mT or more and 5 mT or less in the range of 250 mT or more and 390 mT or less in a spectrum measured at a temperature of 297 K by electron spin resonance using X-band.

[0011] The PC negative electrode material of this embodiment is highly safe. In this embodiment, whether or not a PC negative electrode material is highly safe is evaluated by repeatedly applying a spark to the material using the cerium-iron spark method described in the Examples below, and checking the number of times the material is exposed to sparks before ignition. The greater the number of times the material is exposed to sparks before ignition, the higher the safety is evaluated to be.

[0012] As described below, the P-C negative electrode material is produced by producing a precursor by subjecting a phosphorus raw material and a carbon raw material to a mixing and grinding process accompanied by compression, and then firing the precursor. In the following description, the term "precursor" refers to a composite containing phosphorus atoms and carbon atoms, which is produced by combining a carbon material and phosphorus by a mixing and grinding process accompanied by compression. The P-C negative electrode material is a fired product obtained by firing this precursor.

[0013] Black phosphorus, a phosphorus raw material, is chemically stable among phosphorus allotropes, and black phosphorus alone is unlikely to generate heat or ignite in the atmosphere. In addition, carbon raw materials alone are unlikely to generate heat or ignite in the atmosphere.

[0014] However, the present inventors have confirmed that the above precursors tend to have reduced chemical stability and safety, and speculate as follows about the reason why the precursors become unstable.

[0015] First, the precursor has an "isolated" carbon radical (unpaired electron). An "isolated" carbon radical is an unstable carbon radical present in the precursor. The fact that the carbon radical is "isolated" can be confirmed from the shape of the peak obtained by measuring electron spin resonance (ESR). It has been confirmed by ESR that the carbon raw material does not contain isolated carbon radicals.

[0016] When an ESR spectrum obtained by measuring ESR has a broad peak, it means that an "isolated" carbon radical is present. In contrast, when the "isolated" carbon radical is reduced, a spectrum having a sharp peak is observed. Here, in this specification, a "sharp peak" means a peak with a line width of 5 mT or less, and a "broad peak" means a peak with a line width of more than 5 mT.

[0017] The isolated carbon radicals contained in the precursor are thought to be generated by cleavage of carbon-carbon bonds contained in the carbon material due to mechanical stimulation during the process of mixing and grinding the phosphorus raw material and the carbon raw material, which involves compression. The isolated carbon radicals generated are highly reactive (active) and easily bond with oxygen. Therefore, the precursor is thought to be prone to heat generation and ignition.

[0018] Based on the above measurement results and considerations, the present inventors believed that a highly safe P-C negative electrode material could be obtained by reducing the "isolated" carbon radicals contained in the precursor of the P-C negative electrode material, and thus completed the present invention.

[0019] A PC negative electrode material having at least a peak with a line width of 0.1 mT to 5 mT in the range of 250 mT to 390 mT in a spectrum measured at a temperature of 297 K by electron spin resonance using X-band is highly safe due to the reduced amount of "isolated" carbon radicals. FIG. 1 shows an example of a spectrum obtained as a result of measurement of the PC negative electrode material of this embodiment. The ESR spectrum shown in FIG. 1 is a spectrum measured at a temperature of 297 K by electron spin resonance using X-band. The ESR spectrum shown in FIG. 1 has at least a peak with a line width of 1.2 mT in the range of 250 mT to 390 mT. In the ESR spectrum shown in FIG. 1, the vertical axis represents "signal intensity" and the horizontal axis represents "magnetic field (mT)." The ESR spectrum can be measured by the following method.

[0020] [ESR Spectrum Measurement Method] The ESR spectrum is measured using an "X-band ESR measurement device" manufactured by JEOL Ltd. The line width of the ESR signal is measured using a standard sample (Mn ２＋ / MgO).

[0021] For the obtained ESR spectrum, the line width of the peak is calculated from the difference in magnetic field values ​​at which the fitting curve takes a peak. The line width means the distance between peaks in the ESR spectrum, as shown in FIG.

[0022] The PC negative electrode material of this embodiment includes phosphorus atoms and carbon atoms. In one aspect of the present invention, the PC negative electrode material is a powder.

[0023] 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.

[0024] 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.

[0025] 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 transition metals such as lithium, silicon, germanium, tin, aluminum, magnesium, zinc, niobium, and titanium, as well as nitrogen, oxygen, fluorine, silicon, 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.

[0026] 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.

[0027] 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 higher phosphorus content increases 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. This is because if the content of phosphorus atoms is too high, the cycle characteristics of the PC negative electrode material will deteriorate. The upper and lower limits of the content of phosphorus atoms can be combined arbitrarily.

[0028] 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. 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. 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 arbitrarily.

[0029] As will be described in detail later, the P-C negative electrode material is produced by calcining a precursor produced by ball milling a phosphorus material and a carbon material. 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 rather that the phosphorus atoms and carbon atoms in the P-C negative electrode material are chemically bonded (covalently bonded), resulting in a novel material with physical properties different from those of the raw materials, phosphorus and carbon materials.

[0030] 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").

[0031] 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.

[0032] [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."

[0033] Figure 2 shows XPS spectra of the PC negative electrode material and the PC mixture. In Figure 2, the horizontal axis represents binding energy (eV) and the vertical axis represents the number of detected photoelectrons (cps (counts per second)). In Figure 2, symbol A represents the PC negative electrode material, and symbol X represents the PC mixture.

[0034] (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.

[0035] As shown in FIG. 2, no peak indicating a P—C bond was detected in the range of 132 to 136 eV in the P—C mixture, but a peak indicating a P—C bond (denoted by the symbol α) was detected in the P—C negative electrode material.

[0036] From the results shown in FIG. 2, it is believed that the PC negative electrode material is a substance having a chemical bond between the phosphorus atom and the carbon atom.

[0037] [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.

[0038] Fig. 3 is a transmission electron microscope (TEM) photograph of the PC negative electrode material. As shown in Fig. 3, 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.

[0039] 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).

[0040] 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.

[0041] (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.

[0042] According to the above TEM photographing conditions, the core-shell structure of the PC negative electrode material can be confirmed.

[0043] In addition to satisfying the above-mentioned requirements for thermal behavior, the PC negative electrode material preferably has a core-shell structure.

[0044] [Crystalline state] Figure 4 shows XRD profiles of the PC negative electrode material and the PC mixture. In Figure 4, the horizontal axis represents the diffraction angle (2θ, °) and the vertical axis represents the diffracted X-ray intensity (a.u.). In Figure 4, symbol A represents the PC negative electrode material, and symbol X represents the PC mixture.

[0045] (XRD Profile Measurement Conditions) The XRD profile is measured under the following measurement conditions: Measurement equipment: horizontal sample type 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

[0046] As shown in Figure 4, 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 4, 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.

[0047] 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)

[0048] 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.

[0049] "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.

[0050] "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.

[0051] 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.

[0052] 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).

[0053] As described above, the P-C negative electrode material is a novel substance that differs from the mixture of the raw material phosphorus (black phosphorus in Figures 2 to 4) and a carbon material in terms of its thermal behavior, bonding state, appearance, and crystalline state.

[0054] <Method for Producing P-C Negative Electrode Material> The method for producing a P-C negative electrode material of the present invention includes the steps of: compounding a carbon material and phosphorus by a mixing and grinding process involving compression to obtain a precursor of a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms; and firing the precursor in an inert atmosphere.

[0055] [Step of Obtaining Precursor] The precursor is obtained by mixing a phosphorus material and a carbon material, which are preferably mixed by a mixing and grinding process accompanied by compression.

[0056] 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.

[0057] Examples of amorphous carbon include carbon black (CB), acetylene black (AB), ketjen black, hard carbon, and soft carbon.

[0058] As fullerenes, C ６０ , C ７２ , C ８４ The following can be mentioned:

[0059] 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).

[0060] 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).

[0061] 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.

[0062] 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.

[0063] 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.

[0064] "Mixed and crushed processing with compression" is a process in which a compressive force is applied to multiple types of powdered raw materials, resulting in mixing and crushing the raw materials. By subjecting carbon materials and phosphorus to mixed and crushed processing with compression, a strong impact force is applied to the carbon materials 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 precursor that does not exhibit the crystalline state seen in the raw materials is obtained.

[0065] 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.

[0066] 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 precursors, it is preferable to use an agitation type pulverizer equipped with a pulverization container 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.

[0067] 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.

[0068] 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.

[0069] It is believed that the precursor is then obtained by forming the carbon film around the core particle containing the phosphorus atom and forming a composite.

[0070] 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 precursor.

[0071] 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.

[0072] The diameter of the balls is preferably larger than the average particle size of the precursor after pulverization, since the use of such balls allows a large amount of pulverization energy to be applied to the metal material, thereby enabling metal particles to be obtained efficiently in a short time.

[0073] 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.

[0074] 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 precursor raw material powder can be pulverized efficiently.

[0075] The weight ratio of balls to precursor raw material powder is called the ball powder ratio. By increasing the ball powder ratio, strong impact force can be applied to the precursor raw material powder with high frequency, which further promotes the formation of P-C bonds. If the ball powder ratio is too high, the amount of precursor produced 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.

[0076] After the ball mill mixing is completed, the balls are separated from the precursor using a filter or the like.

[0077] The fact that the above-described precursor 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 2. 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.

[0078] Alternatively, the formation of P-C bonds can be confirmed by measuring the XPS spectrum of the mixed material, as shown in Figure 2. In this case, the duration of ball mill mixing can 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 should be carried out until the formation of P-C bonds can be confirmed in the XPS spectrum of the mixed material.

[0079] 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, forming P-C bonds that are not present in the raw materials, resulting in the production of precursors that do not exhibit the crystalline state seen in the raw materials.

[0080] [Step of Calcining Precursor] The step of calcining the precursor can be carried out in an inert atmosphere. The inert atmosphere used in the step of calcining the precursor may contain a small amount of oxygen as long as it does not impair the effects of the present invention. The inert atmosphere may be an atmosphere filled with one or more inert gases selected from the group consisting of nitrogen, argon, and carbon dioxide.

[0081] The step of calcining the precursor is preferably carried out at a temperature of 80°C or higher but lower than the decomposition temperature of the precursor, more preferably 90°C or higher but 400°C or lower, and even more preferably 250°C or higher but 350°C or lower. When the calcination temperature is equal to or higher than the lower limit, a PC negative electrode material is obtained that has at least a peak with a line width of 0.1 mT or higher but 5 mT or lower in the range of 250 mT or higher but 390 mT or lower in a spectrum measured at a temperature of 297 K by electron spin resonance using X-band. When the calcination temperature exceeds the upper limit, the precursor is more likely to decompose upon calcination, so calcination at a temperature equal to or lower than the upper limit is preferred.

[0082] The step of calcining the precursor is preferably performed for 5 hours or more and 20 hours or less, more preferably 10 hours or more and 18 hours or less.

[0083] <Negative Electrode Active Material> The negative electrode active material of this embodiment includes the above-described PC negative electrode material.

[0084] The negative electrode active material may be the above-described P-C negative electrode material or composition alone, or may contain, in addition to the P-C negative electrode material or composition, a known material used as a negative electrode active material. Examples of such materials include (i) carbon materials capable of inserting and desorbing lithium ions, (ii) alloy-based negative electrode active materials that form a lithium alloy phase, (iii) transition metal oxides that undergo a decomposition and regeneration reaction (conversion reaction) with lithium ions, and (iv) oxides or composite oxides that have activity as a negative electrode active material.

[0085] (i) Examples of carbon materials include graphite, hard carbon, soft carbon, and carbon nanotubes.

[0086] (ii) Examples of alloy-based negative electrode active materials include metals such as silicon, germanium, tin, aluminum, zinc, and magnesium, and alloys thereof.

[0087] (iii) 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).

[0088] (iv) Examples of oxides or composite oxides include titanium oxide, lithium titanate, and silicon oxide.

[0089] <Negative electrode for lithium secondary battery> The negative electrode for lithium secondary battery of this embodiment contains the above-mentioned negative electrode active material. The negative electrode for lithium secondary battery has a current collector in addition to the negative electrode active material. Known materials can be used as the current collector material, and copper or a copper alloy can be preferably used.

[0090] The negative electrode for a lithium secondary battery of this embodiment may or may not have a binder for binding the negative electrode active material to the current collector. When a binder is used, a known structure can be adopted, and 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.

[0091] <Lithium Secondary Battery> The lithium secondary battery of this embodiment includes the above-described negative electrode for lithium secondary batteries (hereinafter, negative electrode).

[0092] An example of a suitable lithium secondary battery when using the negative electrode active material 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.

[0093] 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.

[0094] 5 is a schematic diagram showing an example of a lithium secondary battery. For example, a cylindrical lithium secondary battery 10 is manufactured as follows.

[0095] First, as shown in the partially enlarged view of FIG. 5 , 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] <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.

[0104] Fig. 6 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. 6 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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 .

[0109] 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.

[0110] 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).

[0111] 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.

[0112] 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.

[0113] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0114] Example 1 [Manufacturing Process of PC Negative Electrode Material Precursor] 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 Precursor 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

[0115] PC negative electrode material precursor 1 was baked at 300° C. for 12 hours in a nitrogen atmosphere to obtain PC negative electrode material 1.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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)

[0121] Example 2 A PC negative electrode material precursor was produced in the same manner as in Example 1, to obtain a PC negative electrode material precursor 1. PC negative electrode material precursor 1 was baked in a nitrogen atmosphere at 200°C for 12 hours, to obtain a PC negative electrode material 2.

[0122] Example 3 A PC negative electrode material precursor was produced in the same manner as in Example 1, to obtain a PC negative electrode material precursor 1. PC negative electrode material precursor 1 was baked in a nitrogen atmosphere at 100°C for 12 hours, to obtain a PC negative electrode material 3.

[0123] Comparative Example 1 A PC negative electrode material was produced by the same method as in Example 1, to obtain a PC negative electrode material precursor 1. The PC negative electrode material precursor 1 was designated as a PC negative electrode material 11.

[0124] [ESR spectrum measurement method] The ESR spectrum was measured using an "X-band ESR measurement device" manufactured by JEOL Ltd. Approximately 20 mg of powder of the PC negative electrode material was placed in a sample tube for ESR spectrum measurement under an argon atmosphere. There were no particular restrictions on other measurement conditions as long as the ESR spectrum could be accurately measured. The line width of the ESR signal was measured using the standard sample (Mn ２＋ / MgO).

[0125] The line width of the peaks in the obtained ESR spectrum was calculated from the difference in magnetic field values ​​at which the fitting curves reached their peaks. The line width was calculated as the distance between peaks in the ESR spectrum, as shown in FIG.

[0126] [Ignition Test] Ignition tests were carried out on the PC negative electrode materials 1 to 3 produced in Examples 1 to 3 and the PC negative electrode material 11 produced in Comparative Example 1 by the cerium-iron spark method described below.

[0127] (Cerium-iron spark method) 0.3 g of each composition was placed in a cone shape on an insulating board (Isowool board 1300S), and a spark was applied to each composition from a distance of about 1 cm using a rock lighter (cerium-iron) manufactured by Shoei Sangyo, and the number of times it took for the composition to ignite was counted. For example, if the composition was ignited by a single spark, the number of times it ignited was recorded as "1," and if the composition was ignited by 10 sparks, the number of times it ignited was recorded as "10," and these are recorded in Table 1. The higher the number of times it ignited, the more difficult it was to burn and the higher its stability was evaluated to be.

[0128] [Evaluation of Discharge Capacity] (Preparation of Evaluation Battery) (1) Preparation of Negative Electrode Each composition, conductive material, and binder were weighed to prepare a mixture of [each composition]:[conductive material]:[binder] = 80:10:10 (mass ratio). The materials used were as follows. Binder: vinylidene fluoride PVdF Kureha KF Polymer #1100 (manufactured by Kureha Corporation) Conductive material: acetylene black, Denka Black HS100 (manufactured by Denka Company Limited).

[0129] The mixture was kneaded with a solvent (NMP (N-methyl-2-pyrrolidone)) using an agate mortar to prepare a negative electrode slurry. At this time, the slurry concentration was adjusted so that the content of the negative electrode active material (the total of the P-C negative electrode material or the P-C mixture, the conductive material, and the binder) in the negative electrode slurry was 30 to 60 mass%.

[0130] The negative electrode slurry was applied to a copper foil current collector using a doctor blade, and then the resulting laminate was dried with air at 120°C for 1 hour to remove the solvent, yielding a laminate. The resulting laminate was pressed at 10 MPa for 10 seconds and then further dried in vacuum at 120°C for 12 hours to obtain a negative electrode.

[0131] 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.

[0132] (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.

[0133] Metallic lithium foil was used as the counter electrode.

[0134] 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.

[0135] LiPF ６ The 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 ６A solution in which the above was dissolved at a concentration of 1 mol / L was used.

[0136] As the separator, a laminated film separator (16 μm thick) was used, in which a heat-resistant porous layer was laminated on a polyethylene porous film.

[0137] (Discharge Capacity) Using the above evaluation battery, a charge / discharge test was carried out under the following conditions while maintaining the temperature at 25°C. Here, the theoretical capacity of phosphorus was 2600 mAh / g, the actual capacity of the P-C negative electrode material was 1800 mAh / g, and the 1C current for charging or discharging the electrode capacity in 1 hour was 1800 mA / g. The definition of 1C in the examples is the same hereinafter. Minimum charge voltage: 0.005 V Charging current: 0.1 C (1C = 1800 mA / g) Maximum discharge voltage: 2.0 V Discharge current: 0.1 C (1C = 1800 mA / g)

[0138]

[0139] As shown in Table 1, the P-C negative electrode materials of Examples 1 to 3, which were produced by calcining the precursor, had a line width of 1.5 mT, confirming that "isolated" carbon radicals were reduced. The P-C negative electrode material of Comparative Example 1, which was produced without calcining the precursor, had a line width of 6.5 mT, confirming the presence of "isolated" carbon radicals.

[0140] The PC negative electrode materials of Examples 1 to 3 were confirmed to be PC negative electrode materials with higher safety, having a higher number of ignitions than the composition of Comparative Example 1. Furthermore, it was confirmed that Example 1 was safer than Comparative Example 1 and had a discharge capacity equivalent to that of Comparative Example 1.

[0141] From the above, it was confirmed that the present invention is useful.

[0142] 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 phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, which has at least a peak with a line width of 0.1 mT or more and 5 mT or less in the range of 250 mT or more and 390 mT or less in a spectrum measured at a temperature of 297 K by an electron spin resonance method using X-band.

2. A negative electrode active material comprising the phosphorus-carbon composite negative electrode material according to claim 1.

3. A negative electrode for a lithium secondary battery, comprising the negative electrode active material according to claim 2.

4. A lithium secondary battery comprising the negative electrode active material according to claim 2.

5. A method for producing a phosphorus-carbon composite negative electrode material, comprising: a step of compounding a carbon material and phosphorus by a mixing and grinding process involving compression to obtain a precursor of the phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms; and a step of firing the precursor in an inert atmosphere.

6. The method for producing a phosphorus-carbon composite negative electrode material according to claim 5, wherein the firing step is carried out at a firing temperature of 250° C. or higher and lower than the decomposition temperature of the precursor.

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