Negative electrode for lithium secondary battery, and lithium secondary battery

Abstract

To provide a negative electrode for a lithium secondary battery, having a high capacity retention rate, and a lithium secondary battery using the same.SOLUTION: There is provided 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 includes a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms. In the negative electrode active material layer, an area ratio of black portions is 0.05 or more and 12% or less 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.SELECTED DRAWING: None

Description

[Technical Field]

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

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

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

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

[0005] As the range of applications for lithium secondary batteries expands, there is a demand for further improvements in battery characteristics. 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. [Means for solving the problem]

[0006] In order to solve the above problems, one aspect of the present invention includes the following aspects. [1] A negative electrode for a lithium secondary battery, comprising a negative electrode active material layer 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 having a black area ratio of 0.05% to 12% relative to the entire area of ​​a binarized image obtained by the following method: [How to obtain a binary image] The surface of the negative electrode active material layer is imaged with a scanning electron microscope at a magnification of 40 to obtain a negative electrode surface image, which is then 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 0.03mm 2 The negative electrode for a lithium secondary battery according to [1], which is: [3] The negative electrode according to [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 the 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 the following formulas (1) to (3) are satisfied: |P| / |B|<2 …(1) P=I 26.3 -I 40 …(2) B=(I 20 -I 40 ) × (26.3 - 40) / (20 - 40) … (3) [5] The negative electrode according to any one of [1] to [4], wherein the phosphorus-carbon composite negative electrode material has a core particle containing a phosphorus atom and a carbon coating covering the surface of the core particle. [6] A lithium secondary battery comprising the negative electrode 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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

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

[0009] <Anode for lithium secondary batteries> 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, as a negative electrode active material, a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms. As the material for the current collector, copper or a copper alloy can be suitably used. In the following description, the "phosphorus-carbon composite negative electrode material" may be abbreviated simply as "PC 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 for the negative electrode active material layer (PC negative electrode material) may contain coarse particles. Therefore, it is expected that the surface of the coating film obtained by applying and drying the above-mentioned slurry will have parts where the coarse particles are present that are higher than other parts. When such a coating film is pressed as described above, it is thought that the coarse particles that are higher than other parts are more susceptible to pressure and are more easily compressed than other parts. As a result, the resulting negative electrode active material layer will have regions formed by the coarse particles derived from the electrode material being compressed during the pressing process, as well as regions where the pressure is relatively lower than these regions.

[0012] It is thought that the region formed by compacting coarse particles is less likely to expand during charging and less likely to contract during discharging than other regions where the pressure is lower. In other words, the two regions have different volumetric change behaviors (expansion and contraction behaviors) associated with charging and discharging. For this reason, it is thought that when charging and discharging are repeated, the volumetric changes of the two regions do not link with each other, making it easy for cracks to occur between the two regions. At the cracks that occur, the contact points between the PC negative electrode materials are broken, making the conductive path prone to being interrupted. At the points where the conductive path is interrupted, lithium ions cannot move, resulting in resistance.

[0013] The inventors hypothesized the degradation mechanism of electrodes containing PC negative electrode materials as described above. According to the degradation mechanism, a negative electrode active material layer with fewer "areas formed by compacting coarse particles" would reduce the areas "between two areas" where cracks are likely to occur, thereby suppressing negative electrode degradation.

[0014] In verifying the degradation mechanism using the above 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 area was observed in the SEM image. This "darkened area" is thought to be an area that had been compressed by strong pressing pressure, and is thought to correspond to the "area formed by the compression of coarse particles."

[0015] In the SEM images of the negative electrode active material layer after charging and discharging, cracks were observed around the darkened areas. It was also confirmed 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 way in which pressure is applied changes continuously in the planar direction of the layer surface, making it impossible to clearly distinguish between the two regions as in the model assumed above. Therefore, it is difficult to evaluate the amount of the "region formed by compacting coarse particles" in an actual negative electrode active material layer.

[0017] As a result of intensive research based on the above confirmation results, the inventors came up with the idea that the amount of the "region formed by compacting coarse particles" 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] [How to obtain a binary image] To obtain a binarized image, the surface of the negative electrode active material layer is photographed with a scanning electron microscope at a magnification of 40 times to obtain an image of the negative electrode surface. 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 the negative electrode active material layer is removed from the lithium secondary battery, it is preferable to remove it after discharging the lithium secondary battery.

[0020] The scanning electron microscope used was a JEOL 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, a negative electrode surface image having a pixel count of 2560 pixels x 1920 pixels is obtained as a negative electrode surface image.

[0021] The obtained negative electrode surface image is input into image processing software, and adaptive binarization processing is performed according to the following procedure to obtain a binarized image. The image analysis software used is the Python library openCV and scikit-image (module: threshold_sauvola). First, the following processes (A) and (B) are performed on all pixels of the negative electrode surface image. (A): The threshold is calculated using the pixels in the local area (specifically, 251 x 251 pixels) surrounding the pixel of interest. The threshold is calculated using the pixel values ​​of the pixels in the local area 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 value is calculated for each pixel of interest. (B): If the pixel value of the pixel of interest is greater than the threshold value obtained in (A) above, it is set to white, and if it is less than that, it is set to 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 areas formed by compacting coarse particles, are small, and resistance is less likely to increase even when charging and discharging are repeated, so the capacity retention rate is less likely to decrease. The smaller the area ratio of the black portion, the more preferable, but the above lower limit inevitably exists 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 0.03mm 2 Less than 0.002mm is preferable 2 More than 0.02mm 2 Less than 0.003 mm is preferable. 2 More than 0.01mm 2 The following is even more preferred:

[0027] When 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 has fewer "regions formed by compacting coarse particles." In other words, when the area of ​​the islands in the sea-island structure is small, the number of locations where cracks are likely to occur is 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 more preferable it is, but the above lower limit inevitably exists and is an allowable area.

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

[0029] <Phosphorus-carbon composite anode material> The PC negative electrode material used in this embodiment contains phosphorus atoms and carbon atoms. In one embodiment of the present invention, the PC negative electrode material is a powder.

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

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

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

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

[0034] In the PC negative electrode material, the phosphorus atom content is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 50% by mass or more. A higher phosphorus content increases the initial charge / discharge capacity of the PC negative electrode material. Furthermore, in the PC negative electrode material, the phosphorus atom content is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 75% by mass or less. This is because if the phosphorus atom content is too high, the cycle characteristics of the PC negative electrode material will deteriorate. The upper and lower limits of the phosphorus atom content can be arbitrarily combined.

[0035] In the PC negative electrode material, the carbon atom content is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more. A higher carbon atom content improves the cycle characteristics of the PC negative electrode material. In addition, in the PC negative electrode material, the carbon atom content is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 50% by mass or less. This is because if the carbon atom content is too high, the initial charge / discharge capacity of the PC negative electrode material decreases. The upper and lower limits of the carbon atom content can be combined as desired.

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

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

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

[0039] [PC bonding] The PC negative electrode material has a peak indicative of a bond between a phosphorus atom and a carbon atom in an XPS spectrum. It is known that the "peak indicative of a bond between a phosphorus atom and a carbon atom" appears in the range of 132 to 136 eV in an XPS spectrum. In the following description, the bond between a phosphorus atom and a carbon atom may be simply referred to as a "PC bond."

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

[0041] (XPS spectrum measurement conditions) The XPS spectrum is measured under the following measurement conditions. Measurement equipment: XPS device, ESCA-3400 (Shimadzu Corporation) ·Radiation source: Mg Kα radiation (20mA, 10kV) When a peak is detected in the range of 132 to 136 eV, it is determined that a PC bond is present.

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

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

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

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

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

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

[0048] (TEM photographing conditions) TEM photographs are taken under the following conditions: TEM equipment: Transmission electron microscope H-9000NAR (Hitachi, Ltd.) Field of view: Maximum 500nm x 500nm. At least some of the particles of the PC negative electrode material must be included in the field of view.

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

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

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

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

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

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

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

[0056] "P" expressed in the above formula (2) is the intensity of the reference point I 40 and the intensity I at the position where the carbon material peak exists. 26.3 and indicates the difference of the peak from the reference point.

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

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

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

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

[0061] (Manufacturing method of PC negative electrode material) The PC negative electrode material is obtained by mixing a phosphorus material and a carbon material, which are preferably mixed by a mixing and grinding process accompanied by compression.

[0062] The carbon material used as the raw material can be at least one selected from the group consisting of amorphous carbon, graphite, porous carbon, mesocarbon microbeads (MCMB), fullerene, carbon nanotubes, graphene, graphene oxide, carbon nitrite (CN), and laminated carbon nanofibers (carbon nanofiber platelets). All of these carbon materials have a graphite structure.

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

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

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

[0066] Graphene, graphene oxide, carbon nitrite (C3N4), and laminated carbon nanofibers (carbon nanofiber platelets) have a structure that extends in the plane direction (two-dimensional structure).

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

[0068] Examples of carbon materials with many edge sites include CSCNT, graphite, mesocarbon microbeads (MCMB), graphene, graphene oxide, carbon nitrite (CN), and laminated carbon nanofibers. Among these, CSCNT is preferred as the carbon material.

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

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

[0071] The "mixing and grinding process with compression" should be carried out until the peaks derived from the raw materials disappear in the XRD profile of the mixed material, or until the formation of PC bonds can be confirmed in the XPS spectrum of the mixed material.

[0072] Processing devices (pulverizers) capable of mixing and pulverizing with compression include roller mills, jet mills, hammer mills, pin mills, disk mills, rod mills, ball mills, vibration mills, attritors, and bead mills. In the production of PC negative electrode materials, it is preferable to use an agitator-type pulverizer equipped with a grinding vessel and a rotor, particularly because it allows simultaneous mixing and pulverization. Examples of agitator-type pulverizers include pin mills, disk mills, rod mills, ball mills, vibration mills, attritors, and bead mills.

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

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

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

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

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

[0078] The diameter of the balls is preferably larger than the average particle size of the crushed PC negative electrode material, as this allows for a large amount of crushing energy to be applied to the metal material, resulting in efficient production of metal particles in a short time.

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

[0080] Examples of ball materials include glass, agate, alumina, zirconia, stainless steel, chrome steel, tungsten carbide, silicon carbide, and silicon nitride. Among these, zirconia is preferred because it has a relatively high hardness, making it less susceptible to wear, and its relatively high specific gravity allows for large grinding energy to be obtained. Using these balls allows for efficient grinding of the raw material powder of PC negative electrode material.

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

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

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

[0084] Alternatively, the XPS spectrum of the mixed material can be measured to confirm the formation of PC bonds, as shown in Figure 1. In this case, the duration of ball mill mixing should be determined by conducting a preliminary experiment to determine the relationship between mixing time and the time required for PC bonds to form. In other words, ball mill mixing should be continued until the formation of PC bonds can be confirmed in the XPS spectrum of the mixed material.

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

[0086] <Method for manufacturing negative electrode for lithium secondary battery> A 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 applying the negative electrode composition to the current collector and drying it. If the pressing step is not performed, the coarse particles will not be compressed, and the black portion will not increase. When the pressing step is carried out, by setting the pressing pressure to less than 0.3 MPa, coarse particles are less likely to be compressed and the number of black portions is less likely to increase. Furthermore, by using a negative electrode active material that contains few coarse particles with particle sizes of 10 μm or more, it becomes 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. Other examples of the conductive additive include metal powders such as copper, nickel, aluminum, silver, and gold, and conductive ceramic materials.

[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-mentioned negative electrode for lithium secondary batteries (hereinafter, negative electrode).

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

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

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

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

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

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

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

[0104] The 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 section of the electrode group 4 cut perpendicular to the winding axis is a circle, an ellipse, a rectangle, or a rectangle with rounded corners.

[0106] The shape of a lithium secondary battery having such an electrode group 4 can be any shape specified by IEC60086, a standard for batteries established by the International Electrotechnical Commission (IEC), or JIS C 8500. Examples of shapes include a cylindrical shape and a rectangular shape.

[0107] Furthermore, the lithium secondary battery is not limited to the above-mentioned wound type configuration, and may be a laminated type configuration in which a laminated structure of a positive electrode, a separator, a negative electrode, and a separator is repeatedly stacked. Examples of laminated lithium secondary batteries include so-called coin type batteries, button type batteries, and paper type (or sheet type) batteries.

[0108] The positive electrode, separator, negative electrode, and electrolyte constituting the lithium secondary battery can be, for example, the configuration, materials, and manufacturing method described in

[0113] to

[0140] of WO2022 / 113904A1.

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

[0112] The negative electrode 120 has a negative electrode active material layer 121 and a negative electrode current collector 122. The negative electrode active material layer 121 contains a negative electrode active material. The negative electrode active material layer 121 may also contain a solid electrolyte and a conductive material.

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

[0116] The all-solid-state lithium secondary battery 1000 may have any shape, such as a coin shape, a button shape, a paper shape (or a sheet shape), a cylindrical shape, a square shape, or a laminate shape (pouch shape).

[0117] The all-solid-state lithium secondary battery 1000 is illustrated as having one laminate 100 as an example, but the present embodiment is not limited to this. The all-solid-state lithium secondary battery 1000 may have a configuration in which the laminate 100 is used as a unit cell, and a plurality of unit cells (laminated bodies 100) are sealed inside an exterior body 200.

[0118] For the all-solid-state lithium secondary battery, for example, the configuration, materials, and manufacturing method described in

[0141] to

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

[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 materials] Black phosphorus manufactured by Rasa Industries and CSCNT (cup-stacked carbon nanotubes: manufactured by GSI Creos Co., Ltd.) were weighed in a mass ratio of P:C = 6:4 (total amount 1.25 g) and mixed using a ball mill under the following conditions to obtain PC negative electrode material 1. (Ball mill mixing conditions) Equipment:Retsch PM-100 Container: 125mL ZrO2 container Media: ZrO2 8mmφ ball, 150g Atmosphere: Argon gas filled Sample amount: 1.25g of raw material mixture Ball / powder ratio: 120 (by weight) Mixing time: 12 hours

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

[0123] The obtained PC negative electrode material 1 had a phosphorus atom content of 60 mass % and a carbon atom content of 40 mass %. Because the container used in the above-mentioned production was extremely airtight, it was determined that the content of each atom in PC negative electrode material 1 was equal to the raw material ratio.

[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° was 20 is 3063, and the intensity at 2θ=26.3° is I 26.3 is 2603, and the intensity at 2θ=40° is I 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] PC negative electrode material 1 was mixed with vapor-grown carbon fiber (manufactured by Resonac, product name VGCF) and single-walled carbon nanotubes as a conductive additive, and a binder (vinylidene fluoride PVdF Solef 5130 (manufactured by Solvay)) to obtain 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 mixed in an agate mortar to prepare a negative electrode slurry. At this time, the slurry concentration was adjusted so that the content of PC negative electrode material 1 in the negative electrode slurry was 30 to 60 mass %.

[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 accelerating voltage of 15 kV, a working distance of 13 mm, and a magnification of 40x to obtain a negative electrode surface image with a pixel count of 2560 pixels x 1920 pixels. The obtained negative electrode surface image is shown in Figure 6(a). The obtained negative electrode surface image was imported into image processing software and subjected to adaptive binarization processing according to the following procedure to obtain a binarized image. The image analysis software used was the Python library openCV and scikit-image (module threshold_sauvola). First, the following processes (A) and (B) were performed on all pixels of the negative electrode surface image. (A): The threshold was calculated using the pixels in a local area (specifically, 251 x 251 pixels) surrounding the pixel of interest. The threshold was calculated using the pixel values ​​of the pixels in the local area 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, binarization was performed with W=251, k=0.1, and R=256.

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

[0134] (B): When the pixel value of the pixel of interest is larger than the threshold value obtained in (A) above, it is represented as white, and when it is smaller, it is represented as black, and the pixel is binarized. 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 areas to the entire negative electrode surface image was calculated. For Negative Electrode 1, the area ratio of the black areas to the entire negative electrode surface image was 2.0%. As shown in Figure 6(b), the obtained binarized image had a sea-island structure with black areas scattered like islands. The average area of ​​all islands in the sea-island structure, calculated using Python image analysis software, was 0.0038 mm 2 It was.

[0136] (Fabrication of a lithium-ion secondary battery for evaluation) The negative electrode, counter electrode, electrolyte, and separator prepared above 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.

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

[0138] As the electrolyte, a mixed solution in which 10 mass % of fluoroethylene carbonate (FEC) was added to a LiPF6 solution (manufactured by Kishida Chemical Co., Ltd.) was used.

[0139] The LiPF6 solution used was a solution prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35.

[0140] A polyethylene porous film separator (12 μm thick) was used as the separator.

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

[0142] The capacity retention rate of the lithium secondary battery equipped with 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 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% of the total negative electrode surface image. Furthermore, as shown in Figure 7(b), the obtained binarized image had a sea-island structure with black areas 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 under pressure 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 equipped 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 results above, in Examples 1 and 2, in which the area ratio of the black portion was 2.0% or 5.6%, the capacity retention rate exceeded 90%. This is thought to be because there were few 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, where the area ratio of the black portions exceeded 12%. This is thought to be due to the occurrence of cracks around the areas formed by the compaction of coarse particles during repeated charging and discharging, which increased resistance and reduced the capacity retention rate. [Explanation of symbols]

[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, the negative electrode active material layer contains a phosphorus-carbon composite negative electrode material containing phosphorus atoms and carbon atoms, The negative electrode for a lithium secondary battery, wherein the negative electrode active material layer has an area ratio of black portions to the entirety of a binarized image obtained by the following method of 0.05% to 12%. [Method for obtaining binarized images] The surface of the negative electrode active material layer is photographed with a scanning electron microscope at a magnification of 40 times to obtain an image of the negative electrode surface. 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. 3. The negative electrode according to claim 1, wherein the phosphorus-carbon composite negative electrode material has a peak in an XPS spectrum that indicates a bond between a phosphorus atom and a carbon atom.

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 the following formulas (1) to (3) are satisfied: |P| / |B|<2...(1) P=I ２６．３ -I ４０ …(2) B=(I ２０ -I ４０ )×(26.3-40) / (20-40) …(3)

5. The phosphorus-carbon composite negative electrode material comprises core particles containing phosphorus atoms; The negative electrode according to claim 1 or 2, further comprising 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. 7. The lithium secondary battery of claim 6, wherein the electrolyte comprises a solid electrolyte interface-forming agent.

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