Negative electrode for lithium secondary battery, and lithium secondary battery

Abstract

The present invention provides a negative electrode, for a lithium secondary battery, in which a negative electrode active material layer is formed on a current collector, wherein a negative electrode active material includes a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, and the negative electrode active material layer has an area ratio of 0.05-12% of black portions with respect to the entirety of a binary image acquired through the following method. [Binary image acquisition method] A surface of the negative electrode active material layer is imaged by a scanning electron microscope at an imaging magnification of 40 times to obtain a negative electrode surface image. The negative electrode surface image is subjected to adaptive binarization processing to acquire a binary image.

Description

Negative electrode for lithium secondary battery and lithium secondary battery

[0001] The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery. This application claims priority to Japanese Patent Application No. 2024-027419, filed on February 27, 2024, the contents of which are incorporated herein by reference.

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

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

[0004] JP 2009-184861 A

[0005] As the application fields of lithium secondary batteries expand, further improvements in battery characteristics are required. Among battery characteristics, the capacity retention rate is an indicator of deterioration due to repeated charging and discharging, so improving the capacity retention rate is a major challenge. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode for a lithium secondary battery having a high capacity retention rate and a lithium secondary battery using the same.

[0006] In order to solve the above problems, one aspect of the present invention includes the following. [1] A negative electrode for a lithium secondary battery, in which a negative electrode active material layer is formed on a current collector, the negative electrode active material layer containing a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, and the negative electrode active material layer has an area ratio of black portions to the entire binarized image obtained by the following method: 0.05% to 12%. [Method for obtaining a binarized image] The surface of the negative electrode active material layer is imaged with a scanning electron microscope at a magnification of 40 times to obtain a negative electrode surface image. The negative electrode surface image is subjected to adaptive binarization processing to obtain a binarized image. [2] The binarized image has a sea-island structure in which the black portions are scattered like islands, and the average area of ​​the islands in the sea-island structure is 0.001 mm ２ More than 0.03 mm ２ [3] The negative electrode for a lithium secondary battery according to [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. [4] 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 according to any one of [1] to [3], wherein |P| / |B|<2 (1) satisfies the following formulas (1) to (3): ２６．３ -I ４０ ...(2) B = (I ２０ -I ４０ )×(26.3−40) / (20−40) ... (3) [5] The anode according to any one of [1] to [4], wherein the phosphorus-carbon composite anode material has a core particle containing a phosphorus atom and a carbon coating covering a surface of the core particle. [6] A lithium secondary battery comprising the anode according to any one of [1] to [5]. [7] The lithium secondary battery according to claim 6, wherein the electrolyte contains a solid electrolyte interface forming agent.

[0007] According to the present invention, it is possible to provide a negative electrode for a lithium secondary battery having a high capacity retention rate and a lithium secondary battery using the same.

[0008] FIG. 1 is an XPS spectrum of the PC negative electrode material. FIG. 2 is a transmission electron microscope (TEM) photograph of the PC negative electrode material. FIG. 3 is an XRD profile of the PC negative electrode material and the PC mixture. FIG. 4 is a schematic diagram showing an all-solid-state lithium secondary battery, which is an example of a battery. FIG. 5 is a schematic diagram showing an all-solid-state lithium secondary battery, which is an example of a battery. FIG. 6 is a negative electrode surface image and a binarized image of the negative electrode for the lithium secondary battery produced in Example 1. FIG. 7 is a negative electrode surface image and a binarized image of the negative electrode for the lithium secondary battery produced in Example 2. FIG. 8 is a negative electrode surface image and a binarized image of the negative electrode for the lithium secondary battery produced in Comparative Example 1. FIG. 9 is a negative electrode surface image and a binarized image of the negative electrode for the lithium secondary battery produced in Comparative Example 2.

[0009] <Negative electrode for lithium secondary battery> This embodiment is a negative electrode for a lithium secondary battery in which a negative electrode active material layer is formed on a current collector. The negative electrode active material layer contains a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms as the negative electrode active material. Copper or a copper alloy can be suitably used as the material for the current collector. In the following description, the "phosphorus-carbon composite negative electrode material" may be simply abbreviated as "P-C negative electrode material."

[0010] Generally, electrodes are manufactured by applying a slurry containing the electrode material onto a current collector foil, drying it, and then pressing it. The pressing process is carried out to make the electrode thickness uniform and to compact it.

[0011] The raw material (P-C negative electrode material) of the negative electrode active material layer, which is the electrode material, may contain coarse particles. Therefore, it is expected that the surface of the coating film obtained by applying and drying a slurry containing the electrode material will have portions where the coarse particles are present that are higher than other portions. When such a coating film is pressed, it is presumed that the coarse particles that are relatively higher than other portions are more susceptible to pressure and are more easily compressed than other portions. As a result, the resulting negative electrode active material layer will have a region formed by compressing the coarse particles derived from the electrode material in the pressing process, and a region where the pressing pressure is relatively lower than that region. Hereinafter, the "region formed by compressing the coarse particles" will be referred to as "region P," and the "region where the pressing pressure is relatively lower than that of region P" will be referred to as "region L."

[0012] It is thought that region P is less likely to expand during charging and less likely to contract during discharging than region L. In other words, region P and region L have different volumetric change behaviors (expansion and contraction behaviors) associated with charging and discharging. For this reason, it is presumed that when charging and discharging are repeated, the volumetric changes of region P and region L do not link with each other, making it easy for cracks to occur between region P and region L. At the cracked portion, the contact points between the P-C negative electrode materials are broken, and the conductive path is easily interrupted. At the location where the conductive path is interrupted, lithium ions cannot move, resulting in resistance.

[0013] The inventors hypothesized the deterioration mechanism of an electrode containing a PC negative electrode material as described above. According to the deterioration mechanism, it is thought that in a negative electrode active material layer with a small amount of region P, the area between region P and region L, where cracks are likely to occur, is reduced, making the negative electrode less susceptible to deterioration.

[0014] In examining the degradation mechanism using the model, the inventors focused on the color of the surface of the negative electrode active material layer. When the surface of the negative electrode active material layer after charge / discharge was observed with an SEM, a relatively dark region was observed in the SEM image. This "darkened region" is thought to be a region that was compressed by a strong pressing pressure, and is considered to correspond to region P.

[0015] The present inventors confirmed that cracks were generated around the darkened areas in SEM images of the negative electrode active material layer after charge and discharge, and that there was a negative correlation between the size of the darkened areas in the SEM images and the capacity retention rate.

[0016] On the other hand, in an actual negative electrode active material layer, the application of pressure changes continuously in the planar direction of the layer surface, making it impossible to clearly distinguish between "region P" and "region L" as in the model assumed above. Therefore, it is difficult to evaluate the amount of region P in an actual negative electrode active material layer.

[0017] As a result of extensive investigation based on the above confirmation results, the inventors came up with the idea that the amount of region P can be evaluated by binarizing the color tone of the surface of the negative electrode active material layer.

[0018] That is, in the negative electrode active material layer, black and white portions are observed in a binarized image obtained by the following method, and the area ratio of the black portions to the entire binarized image is 0.05% to 12%.

[0019] [Method of Obtaining a Binary Image] To obtain a binary image, a negative electrode surface image is obtained by photographing the surface of the negative electrode active material layer with a scanning electron microscope at a magnification of 40x. The surface of the negative electrode active material layer may be observed after the pressing step, or the assembled lithium secondary battery may be disassembled and the negative electrode active material layer removed for observation. When removing the negative electrode active material layer from the lithium secondary battery, it is preferable to remove it after discharging the lithium secondary battery.

[0020] The scanning electron microscope is a JEOL Ltd. scanning electron microscope (product name: JCM-7000 NeoScope) with an acceleration voltage of 15.0 kV and a working distance of 12.6 mm. In addition, as the negative electrode surface image, a negative electrode surface image with a pixel count of 2560 pixels x 1920 pixels is obtained.

[0021] The obtained negative electrode surface image is imported into image processing software, and adaptive binarization processing is performed according to the following procedure to obtain a binarized image. As image analysis software, the Python library openCV or scikit-image (threshold_sauvola as a module) is used. First, the following processes (A) and (B) are performed for all pixels of the negative electrode surface image. (A): A threshold is calculated using pixels in a local region (specifically, 251 x 251 pixels in length x width) surrounding the pixel of interest. The threshold is calculated using the pixel values ​​of the pixels in the local region using the following formula.

[0022] T = m(x, y) * (1 + k * ((s(x, y) / R) - 1)) In the above formula, m(x, y) is the average value of a square area with one side equal to the window size W. s(x, y) is the standard deviation of a square area with one side equal to the window size W. k is a parameter that weights the standard deviation. R is the maximum standard deviation of the grayscale image.

[0023] The threshold is calculated for each pixel of interest. (B): When the pixel value of the pixel of interest is greater than the threshold calculated in (A) above, it is determined as white, and when it is less than that, it is determined as black, and the pixel is binarized.

[0024] In the negative electrode active material layer used in this embodiment, the area ratio of the black portion to the entire binarized image is preferably 0.1% to 10%, more preferably 1.5% to 8%.

[0025] When the area ratio of the black parts is equal to or less than the upper limit, the black parts that cause cracks, i.e., the regions P, are small, and the resistance is less likely to increase even when charging and discharging are repeated, so that the capacity retention rate is less likely to decrease. The smaller the area ratio of the black parts, the better, but the lower limit is unavoidably present and is an allowable ratio.

[0026] One aspect of the binarized image obtained by the above method has a sea-island structure in which black portions are scattered like islands. The average area of ​​the islands in the sea-island structure is 0.001 mm ２ More than 0.03 mm ２ Preferably, 0.002 mm or less ２ More than 0.02 mm ２More preferably, 0.003 mm or less ２ More than 0.01 mm ２ The following is even more preferred:

[0027] If the average area of ​​the islands in the sea-island structure is equal to or less than the upper limit, the negative electrode active material layer will have fewer regions P. In other words, if the area of ​​the islands in the sea-island structure is small, the number of locations where cracks are likely to occur will be reduced, making it easier to obtain a negative electrode that is less susceptible to deterioration. The smaller the average area of ​​the islands in the sea-island structure, the better, but the lower limit is unavoidably present and is an allowable area.

[0028] The average island area of ​​the sea-island structure is the number average of the areas of all islands obtained by image analysis of the binarized image obtained by the above method. Image analysis software that can be used to calculate the number average can be the Python library openCV or scikit-image (threshold_sauvola as a module).

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

[0030] When the P-C negative electrode material contains components other than phosphorus atoms and carbon atoms, the content of the other components is preferably 1 mass % or more and 15 mass % or less, more preferably 2 mass % or more and 12 mass % or less, and even more preferably 2.5 mass % or more and 8 mass % or less, based on the total amount of the components other than phosphorus atoms and carbon atoms.

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

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

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

[0034] In the PC negative electrode material, the content of the phosphorus atoms is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 50% by mass or more. A 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.

[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. 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.

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

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

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

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

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

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

[0042] As shown in FIG. 1, 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.

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

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

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

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

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

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

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

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

[0051] [Crystalline state] Figure 3 shows XRD profiles of the PC negative electrode material and the PC mixture. In Figure 3, the horizontal axis represents the diffraction angle (2θ, °) and the vertical axis represents the diffracted X-ray intensity (a.u.). In Figure 3, symbol A represents the PC negative electrode material, and symbol X represents the PC mixture.

[0052] (XRD Profile Measurement Conditions) The XRD profile is measured under the following measurement conditions: Measurement equipment: horizontal sample 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0077] The balls are grinding media for grinding phosphorus and carbon. The ball diameter is the average diameter of the balls. The balls flow at high speed within the grinding container due to the rotation of the grinding container itself of the grinder, and collide with the powder raw material containing the carbon material and phosphorus, grinding it 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 P-C negative electrode material. By using such balls, large crushing energy can be applied to the metal material, allowing the P-C negative electrode material particles to be obtained efficiently in a short time.

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

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

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

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

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

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

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

[0086] <<Method for Producing Negative Electrode for Lithium Secondary Battery>> The negative electrode for a lithium secondary battery is produced by applying a negative electrode composition containing a negative electrode active material to a current collector, drying the applied composition, and optionally performing a pressing step.

[0087] In order to keep the area ratio of the black portion within the above range, it is preferable not to perform a pressing step after coating the negative electrode composition on the current collector and drying it. If the pressing step is not performed, the coarse particles are not compressed, and the black portion is less likely to increase. If the pressing step is performed, setting the pressing pressure to less than 0.3 MPa makes it difficult for the coarse particles to be compressed, and the black portion is less likely to increase. Furthermore, by using a negative electrode active material with few coarse particles having a particle size of 10 μm or more, it is easier to control the particle size of the black portion within the above range.

[0088] 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.

[0089] 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.

[0090] The negative electrode composition preferably contains a conductive additive. Examples of the conductive additive include carbon nanotubes, carbon nanofibers, vapor-grown carbon fibers, mesophase pitch-based carbon fibers, isotropic pitch-based carbon fibers, natural graphite, artificial graphite such as mesocarbon microbeads, carbon black (e.g., acetylene black, ketjen black, and furnace black), graphite particles, graphene, and fullerene. Metal powders such as copper, nickel, aluminum, silver, and gold, and conductive ceramic materials can also be used.

[0091] 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.

[0092] (i) Examples of alloy-based negative electrode active materials include metals such as silicon, germanium, tin, aluminum, zinc, and magnesium, and alloys thereof.

[0093] (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).

[0094] (iii) Examples of oxides or composite oxides include titanium oxide, lithium titanate, and silicon oxide.

[0095] (iv) Examples of layered carbon include graphite and hard carbon.

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

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

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

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

[0100] First, as shown in the partially enlarged view of FIG. 4 , a pair of strip-shaped separators 1, a strip-shaped positive electrode 2 having a positive electrode lead 21 at one end, and a strip-shaped negative electrode 3 having a negative electrode lead 31 at one end are stacked in this order: separator 1, positive electrode 2, separator 1, negative electrode 3, and then wound to form an electrode group 4.

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

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

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

[0104] The electrolyte preferably contains a solid electrolyte interface-forming agent, such as a carbonate-based solvent, a phosphate ester-based solvent, or a sulfolane-based solvent.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] <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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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 .

[0115] 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.

[0116] 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).

[0117] 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.

[0118] 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.

[0119] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0120] Example 1 Production of PC Negative Electrode Material Black phosphorus manufactured by Rasa Industries and CSCNT (cup-stacked carbon nanotubes: manufactured by GSI Creos Co., Ltd.) were weighed out in a mass ratio of P:C = 6:4 (total amount 1.25 g) and mixed using a ball mill under the following conditions to obtain PC Negative Electrode Material 1. (Ball mill mixing conditions) Apparatus: Retsch PM-100 Container: ZrO ２ 125mL container Media: ZrO ２ 8 mm diameter ball manufactured by 150 g Atmosphere: Argon gas sealed Sample amount: 1.25 g as a mixture of raw materials Ball / powder ratio: 120 (weight ratio) Mixing time: 12 hours

[0121] 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.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] 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)

[0126] [Production of Negative Electrode Composition] A PC negative electrode material 1, vapor-grown carbon fiber (product name: VGCF, manufactured by Resonac) as a conductive additive, single-walled carbon nanotubes, and a binder (vinylidene fluoride PVdF Solef 5130 (manufactured by Solvay)) were mixed together to obtain a negative electrode composition 1 containing 6 mass % of the binder.

[0127] [Production of Negative Electrode 1] The negative electrode composition 1 and a solvent (NMP (N-methyl-2-pyrrolidone)) were kneaded in an agate mortar to prepare a negative electrode slurry. At this time, the slurry concentration was adjusted so that the content of the PC negative electrode material 1 in the negative electrode slurry was 30 to 60 mass %.

[0128] The negative electrode slurry was applied to a copper foil current collector using a doctor blade, and then vacuum dried at 60°C for 1 hour to remove the solvent, obtaining a laminate. The obtained laminate was further vacuum dried at 150°C for 8 hours to obtain negative electrode 1.

[0129] The negative electrode 1 had a coating weight of 1.9 mg / cm ２ It was.

[0130] 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.

[0131] The surface of the negative electrode 1 was imaged using a scanning electron microscope (product name JCM-7000) manufactured by JEOL Ltd. at an acceleration voltage of 15 kV, a working distance of 13 mm, and a 40x magnification, and a negative electrode surface image with a pixel count of 2560 pixels x 1920 pixels was obtained. The negative electrode surface image obtained at this time is shown in FIG. 6(a). The obtained negative electrode surface image was imported into image processing software, and adaptive binarization processing was performed according to the following procedure to obtain a binarized image. As image analysis software, openCV and scikit-image (module threshold_sauvola), which are Python libraries, were used. First, the following (A) and (B) processes were performed on all pixels of the negative electrode surface image. (A): The threshold was calculated using the pixels of a local region (specifically, vertical x horizontal = 251 x 251 pixels) that are pixels surrounding the pixel of interest. The threshold was calculated using the pixel values ​​of the pixels in the local region using the following formula. T = m(x, y) * (1 + k * ((s(x, y) / R) - 1)) In the above formula, m(x, y) is the average value of a square area with one side equal to the window size W. s(x, y) is the standard deviation of a square area with one side equal to the window size W. k is a parameter that weights the standard deviation. R is the maximum standard deviation of the grayscale image.

[0132] More specifically, the binarization process was performed with W=251, k=0.1, and R=256.

[0133] The threshold value was calculated for each pixel of interest.

[0134] (B): The pixel value of the pixel of interest was binarized by representing it as white if it was greater than the threshold value calculated in (A) above, and black if it was smaller. The resulting binarized image is shown in Figure 6(b).

[0135] In the binarized image obtained by the above method, the area ratio of the black portions to the entire negative electrode surface image was determined. For Negative Electrode 1, the area ratio of the black portions to the entire negative electrode surface image was 2.0%. As shown in FIG. 6(b), the obtained binarized image had a sea-island structure in which the black portions were scattered like islands. The average area of ​​all islands in the sea-island structure, calculated using Python as image analysis software, was 0.0038 mm ２ It was.

[0136] (Fabrication of a Lithium-Ion Secondary Battery as an Evaluation Battery) The negative electrode, counter electrode, electrolyte, and separator fabricated above were combined to fabricate a lithium-ion secondary battery (coin-type battery R2032). The battery was assembled in a glove box under an argon atmosphere.

[0137] Metallic lithium foil was used as the counter electrode.

[0138] 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.

[0139] 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 ６ A solution in which the above was dissolved at a concentration of 1 mol / L was used.

[0140] As the separator, a polyethylene porous film separator (thickness: 12 μm) was used.

[0141] (Evaluation of Capacity Retention Rate) Using the above-described evaluation battery, constant current / constant voltage charging and constant current discharging were performed at a current setting of 1 CA at 25°C. The maximum charging voltage was 2.5 V, and the minimum discharging voltage was 0.01 V. Subsequently, constant current / constant voltage charging and constant current discharging were repeated at a test temperature of 25°C under the following conditions. The charge / discharge cycle was repeated 100 times. Charging: Current setting of 1 CA, maximum voltage of 2.5 V, constant voltage / constant current charging. Discharging: Battery setting of 1 CA, minimum voltage of 0.01 V, constant current discharging. The capacity retention rate was calculated from the discharge capacity at the first cycle and the discharge capacity at the 100th cycle using the following formula. A higher capacity retention rate indicates that the battery does not deteriorate and maintains its capacity even after repeated charging and discharging, which is desirable in terms of battery performance. Capacity retention rate (%) = Discharge capacity at 100th cycle (mAh / g) / Discharge capacity at 1st cycle (mAh / g) × 100

[0142] The capacity retention rate of the lithium secondary battery equipped with the negative electrode 1 was 93%.

[0143] Example 2 Negative electrode 2 was obtained in the same manner as in Example 1, except that the negative electrode slurry was applied to a copper foil current collector using a doctor blade, followed by vacuum drying at 60°C for 1 hour to remove the solvent, and the laminate was pressed under pressure at 0.1 MPa for 10 seconds, and then further vacuum dried at 150°C for 8 hours. An image of the negative electrode surface of negative electrode 2 is shown in Figure 7(a), and a binarized image is shown in Figure 7(b). Negative electrode 2 had a black area ratio of 5.6% to the entire negative electrode surface image. Furthermore, as shown in Figure 7(b), the obtained binarized image had a sea-island structure in which black areas were scattered like islands. The average area of ​​all islands in the sea-island structure was 0.0225 mm ２ It was.

[0144] The capacity retention rate of the lithium secondary battery equipped with the negative electrode 2 was 91%.

[0145] Comparative Example 1 A negative electrode 11 was obtained in the same manner as in Example 2, except that the laminate was pressed at 0.3 MPa for 10 seconds. An image of the negative electrode surface of the negative electrode 11 is shown in FIG. 8(a) and a binarized image is shown in FIG. 8(b). The negative electrode 11 had a black area ratio of 15.9% to the entire negative electrode surface image. As shown in FIG. 8(b), the obtained binarized image had a sea-island structure in which the black areas were scattered like islands. The average area of ​​all the islands in the sea-island structure was 0.0455 mm ２ It was.

[0146] The capacity retention rate of the lithium secondary battery provided with the negative electrode 11 was 59%.

[0147] Comparative Example 2 A negative electrode 12 was obtained in the same manner as in Example 2, except that the laminate was pressed at 0.5 MPa for 10 seconds. An image of the negative electrode surface of the negative electrode 12 is shown in FIG. 9( a), and a binarized image is shown in FIG. 9( b). The negative electrode 12 had a black area ratio of 17.8% to the entire negative electrode surface image. As shown in FIG. 9( b), the obtained binarized image had a sea-island structure in which the black areas were scattered like islands. The average area of ​​all the islands in the sea-island structure was 0.0487 mm ２ It was.

[0148] The capacity retention rate of the lithium secondary battery equipped with the negative electrode 12 was 88%.

[0149] As shown in the above results, the capacity retention rates exceeded 90% in Examples 1 and 2, in which the area ratio of the black portion was 2.0% or 5.6%. This is thought to be because there were fewer areas formed by compacting coarse particles around which cracks are likely to occur, so resistance was less likely to increase even when charging and discharging were repeated, and a high capacity retention rate was maintained.

[0150] In contrast, the capacity retention rates were less than 90% in Comparative Examples 1 and 2, in which the area ratio of the black portions exceeded 12%. This is thought to be because cracks occurred around the areas formed by compacting the coarse particles during repeated charging and discharging, increasing the resistance and resulting in a decrease in the capacity retention rate.

[0151] 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 for a lithium secondary battery, comprising a negative electrode active material layer formed on a current collector, wherein the negative electrode active material layer comprises a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, and wherein the negative electrode active material layer has a black area ratio of 0.05% to 12% of the entire binarized image obtained by the following method: [Method for obtaining a binarized image] The surface of the negative electrode active material layer is imaged with a scanning electron microscope at a magnification of 40x to obtain a negative electrode surface image. The negative electrode surface image is subjected to adaptive binarization processing to obtain a binarized image.

2. The binarized image has a sea-island structure in which the black portions are scattered like islands, and the average area of ​​the islands in the sea-island structure is 0.001 mm ２ More than 0.03 mm ２ 2. The negative electrode for a lithium secondary battery according to claim 1, wherein:

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

4. 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 according to claim 1 or 2, wherein |P| / |B|<2 (1) satisfies the following formulas (1) to (3): ２６．３ -I ４０ ...(2) B = (I ２０ -I ４０ ) × (26.3 - 40) / (20 - 40) ... (3) 5. The negative electrode according to claim 1 or 2, wherein the phosphorus-carbon composite negative electrode material comprises a core particle containing a phosphorus atom, and a carbon coating covering the surface of the core particle.

6. A lithium secondary battery comprising the negative electrode according to claim 1 or 2.

7. The lithium secondary battery according to claim 6, wherein the electrolyte comprises a solid electrolyte interface-forming agent.

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