Negative active material, method for preparing the same, secondary battery, and electric device

Abstract

This application provides a negative electrode active material and its preparation method, a secondary battery, and an electrical device, relating to the field of lithium battery technology. The secondary battery includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode film layer located on at least one side of the current collector. The negative electrode film layer includes a negative electrode active material, which includes graphite particles. The graphite particles have an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. The roundness of the graphite particles is represented by ri, which indicates the radius of curvature of the surface protrusions in the two-dimensional projection of the graphite particles, and R, which indicates the radius of the circumcircle of the two-dimensional projection of the graphite particles. This configuration effectively reduces the expansion of the secondary battery during cycling.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to a negative electrode active material and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] The development of electric vehicles has placed higher demands on battery range. As battery energy density increases, battery expansion also increases, leading to safety issues such as battery electrode breakage. In addition, as battery life requirements increase, the reliability requirements for batteries are also constantly rising.

[0003] Currently, for batteries with graphite-containing negative electrode sheets, the expansion needs to be further reduced. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a negative electrode active material and its preparation method, a secondary battery and an electrical device, so as to alleviate the expansion of graphite-containing negative electrode sheets and batteries during cycling. The first aspect of this application provides a secondary battery including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one side of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0005] The negative electrode active material includes graphite particles, with an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65.

[0006] Among them, the roundness of graphite particles ri represents the radius of curvature of the surface protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

[0007] Therefore, in this application, the average sphericity of the graphite particles is controlled to be 0.65-0.95. High overall sphericity of the graphite particles, i.e., using high-sphericity graphite particles, can, to a certain extent, convert the volume expansion caused by lithium intercalation into spatial positioning adjustment and pore filling of the graphite particles within the electrode due to their isotropic expansion characteristics and point contact between particles. This helps reduce the macroscopic thickness expansion rate of the electrode, especially in the early stages of cycling. A low average sphericity indicates more protrusions on the graphite particle surface, higher protrusion sharpness, stress concentration, and interlocking between particles, hindering the orderly arrangement of graphite particles during cold pressing, thus worsening the accumulation of force within the negative electrode and exacerbating its expansion. Conversely, a high average sphericity indicates a larger protrusion curvature and a smoother, flatter surface structure of the graphite particles. Combined with the aforementioned high sphericity, this causes the contact mode between particles to change from surface contact to point contact, thereby reducing the binder bridging area and worsening the forces between graphite particles and between graphite particles and the binder, thus worsening the negative electrode's expansion. The cohesive force of the negative electrode sheet leads to intensified expansion. Therefore, while controlling the average sphericity of graphite particles to 0.65-0.95, the average roundness of graphite particles should also be controlled to 0.42-0.65. Within this range, the overall sphericity of graphite particles is high and the surface has suitable micro-protrusions, which can promote the orderly arrangement of particles during the cold pressing process. This not only slows down the rate of stress release and reduces the stress accumulation during cold pressing, but also facilitates good contact between graphite particles and between graphite particles and binder, thereby improving the cohesive force of the negative electrode sheet and suppressing the irreversible expansion of the negative electrode sheet.

[0008] In any embodiment, the average sphericity of the graphite particles is 0.71-0.85. Sphericity determines the macroscopic arrangement of graphite particles. When the sphericity is within the above range, the shape of the graphite particles deviates slightly from a sphere, which can promote the orderly arrangement of particles during cold pressing, while maintaining sufficient contact area. This is beneficial for good bonding between particles and between particles and binder, improving the cohesion of the negative electrode sheet, thereby effectively reducing battery expansion.

[0009] In any embodiment, the average sphericity of the graphite particles is 0.42-0.55. By controlling the sphericity of the graphite particles within the above range and optimizing the average sphericity to 0.42-0.55, the two reinforce each other, optimizing the arrangement of graphite particles during cold pressing, thereby effectively reducing the expansion of the negative electrode sheet during cycling.

[0010] In any embodiment, at least 50% of the graphite particles have a roundness Y ≥ 0.38. Controlling the roundness of most or all of the graphite particles within the above range, and controlling the surface protrusions of most or all of the graphite particles within a suitable range, is more conducive to promoting the orderly arrangement of graphite particles during cold pressing, reducing stress accumulation during cold pressing, and also more conducive to good contact between graphite particles and between graphite particles and binder, improving the cohesion of the negative electrode sheet, and thus effectively reducing expansion.

[0011] In any embodiment, at least 50% of the graphite particles have a sphericity ≥ 0.65. Controlling the sphericity of most or all graphite particles within the above range, i.e., controlling the sphericity of most or all graphite particles to be high, utilizes the isotropic expansion characteristics of most or all graphite particles and the point contact between particles, which is beneficial to improving the cohesion of the negative electrode sheet, thereby effectively reducing expansion.

[0012] In any embodiment, the graphite particles include secondary particles with a carbon coating layer on their surface. This secondary particle structure effectively reduces material volume expansion during lithium intercalation, thereby improving the maximum expansion force. Furthermore, the carbon coating on the secondary particle surface acts as a protective layer, firmly binding the primary particles together, increasing the overall mechanical strength of the particles, effectively buffering volume change stress, suppressing particle breakage, and enhancing structural and interfacial stability.

[0013] In any embodiment, the tap density of the secondary particles is 1.25 g / cc to 1.45 g / cc. Controlling the tap density of the secondary particles to reach the above-mentioned relatively high and suitable range is beneficial for the close arrangement of graphite particles and the increase of contact area. Each particle is supported by the surrounding particles, so local stress concentration can be reduced during cold pressing and the particles are under confined pressure. This helps to suppress lateral slippage and cracking of the particles, and allows the graphite particles to maintain mechanical integrity and structural stability after cold pressing. This results in more effectively resisting and dispersing internal stress during battery cycling. At the same time, this dense structure also helps to suppress electrode expansion during cycling, thereby improving the trend of expansion force growth.

[0014] In any embodiment, the graphite particles include secondary particles with an OI value of 0.5-5. Controlling the OI value of the secondary particles within the aforementioned low range indicates that the orientation of the primary particles tends to be random, and the secondary particles as a whole possess good isotropic characteristics. Combined with the aforementioned sphericity and average roundness, the secondary particles are easy to rotate and slide during cold pressing, which can promote the orderly arrangement of particles during the cold pressing process, reduce the stress accumulation during cold pressing, and at the same time ensure good contact between graphite particles and between graphite particles and binder, thereby improving the cohesion of the negative electrode sheet and effectively reducing the expansion of the negative electrode sheet during cycling.

[0015] In any embodiment, the graphite particles comprise single particles with a tap density of 1.30 g / cc to 1.50 g / cc. Controlling the tap density of the single particles to within this relatively high and suitable range facilitates a tighter arrangement of the graphite particles and increases the contact area. Each particle is supported by surrounding particles, thus reducing local stress concentration during cold pressing and maintaining a confined pressure state. This helps suppress lateral slippage and cracking of the particles, ensuring the graphite particles maintain mechanical integrity and structural stability after cold pressing. Consequently, they more effectively resist and disperse internal stress during battery cycling. Simultaneously, this dense structure also helps suppress electrode expansion during cycling, thereby improving the trend of increasing expansion force.

[0016] In any embodiment, the graphite particles include single particles with an OI value of 2-8. By controlling the OI value of the single particles within the above range, the single particles possess a certain degree of anisotropy. Combined with the aforementioned sphericity and roundness, the particle structure remains stable during cold pressing, facilitating rotation and sliding. This promotes the orderly arrangement of particles during cold pressing, reduces stress accumulation during cold pressing, and ensures good contact between graphite particles and between graphite particles and the binder. This enhances the cohesion of the negative electrode sheet, thereby effectively reducing the expansion of the negative electrode sheet during cycling.

[0017] In any embodiment, the volumetric particle size distribution Dv50 of the graphite particles is 4μm-20μm. Within this range, the particles are less prone to breakage during rolling, thus suppressing the risk of accelerated electrode expansion due to breakage.

[0018] In some embodiments, the ratio of (Dv90-Dv10) / Dv50 of the graphite particles is 1.1-1.5. Controlling the ratio of (Dv90-Dv10) / Dv50 of the graphite particles within the above range results in a more concentrated particle size distribution and similar particle sizes, which reduces the absolute expansion and is beneficial for reducing battery expansion.

[0019] In any embodiment, the liquid absorption rate of the negative electrode sheet with a compaction density of 1.60 g / cc is ≥1.8 mg / s. 0.5 Controlling the liquid absorption rate within the above range indicates that during the initial formation, the electrolyte can rapidly and synchronously wet the surface of all negative electrode active materials, forming a uniform and stable SEI film. This can suppress the expansion of the negative electrode caused by uneven volume changes due to excessively rapid local lithium intercalation or excessive stress concentration. It also indicates that the negative electrode has a suitable pore structure and wettability, which can provide a buffer space for the volume expansion of the negative electrode material during charging and discharging, absorb some mechanical stress, and reduce electrode expansion.

[0020] A second aspect of this application also provides a negative electrode active material comprising graphite particles, wherein the average sphericity of the graphite particles is 0.65-0.95 and the average roundness of the graphite particles is 0.42-0.65.

[0021] Among them, the roundness of graphite particles ri represents the radius of curvature of the surface protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

[0022] Therefore, the negative electrode active material provided in this application, by controlling the average sphericity of the graphite particles to be 0.65-0.95 and the average roundness to be 0.42-0.65, can reduce both the volume expansion of the electrode during the lithium intercalation process and the expansion throughout the battery's life cycle when applied to secondary batteries. Specifically, controlling the average sphericity of the graphite particles to 0.65-0.95 results in high overall sphericity, which can, to a certain extent, transform the volume expansion caused by lithium intercalation into spatial positioning adjustment and pore filling of the graphite particles within the electrode, thus reducing lithium intercalation expansion. A low average sphericity indicates numerous protrusions on the graphite particle surface, hindering the orderly arrangement of graphite particles during cold pressing. This worsens the accumulation of forces within the negative electrode, leading to increased expansion of the negative electrode. Conversely, excessive sphericity indicates a smooth and flat surface structure of the graphite particles. Combined with the aforementioned high sphericity characteristic, this causes the contact mode between particles to change from surface contact to point contact, thus worsening the forces between graphite particles and between graphite particles and the binder. This deteriorates the cohesive force of the negative electrode, further exacerbating its expansion during cycling. Therefore, controlling the average sphericity of the graphite particles is crucial. Based on a sphericity of 0.65-0.95, the average sphericity of graphite particles is controlled at 0.42-0.65. Within this range, the graphite particles have high overall sphericity and a relatively smooth and rounded surface. The graphite particles have suitable friction, which can promote the orderly arrangement of particles during the cold pressing process. This not only slows down the stress release rate and reduces the stress accumulation during cold pressing, but also facilitates good contact between graphite particles and between graphite particles and binder. This enhances the cohesion of the negative electrode sheet, effectively reduces the expansion of the negative electrode sheet during cycling, and reduces the expansion throughout the battery's entire life cycle.

[0023] In any embodiment, the graphite particles include secondary particles, the surface of which has a carbon coating layer; And / or, the tap density of the secondary particles is 1.25 g / cc - 1.45 g / cc; And / or, the graphite particles include secondary particles with an OI value of 0.5-5; And / or, the graphite particles include single particles with a tap density of 1.30 g / cc to 1.50 g / cc; And / or, the OI value of a single particle is 2-8; And / or, of the multiple graphite particles, at least 50% of the graphite particles by number have a sphericity ≥ 0.65; And / or, of the multiple graphite particles, at least 50% of the graphite particles have a sphericity Y ≥ 0.38. By controlling the graphite particles to meet any of the above conditions, when applied to secondary batteries, it can simultaneously reduce the volume expansion of the electrode during the lithium intercalation process and the expansion throughout the battery's entire life cycle.

[0024] A third aspect of this application provides a method for preparing a negative electrode active material, comprising: The first raw material is deeply shaped to obtain the second raw material; wherein, the first raw material includes a first aggregate and / or granules, the first aggregate is obtained by crushing and pre-shaping the raw coke, and the granules are obtained by crushing and pre-shaping the raw coke, mixing the obtained second aggregate with a binder, and granulating; the volatile matter of the raw coke is 10wt%-18wt%, and the Hayek grindability coefficient is 60-150, the Dv50 of the first raw material is D1, the Dv50 of the second raw material is D2, and the D2 / D1 ratio is 0.68-0.90; The second raw material is pre-carbonized and graphitized to obtain graphite particles that serve as the negative electrode active material. The negative electrode active material includes graphite particles, with an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. Among them, the roundness of graphite particles ri represents the radius of curvature of the protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

[0025] The preparation method provided in this application, in the process of preparing the negative electrode active material, selects the volatile matter content of the raw material coke to be 10 wt%-18 wt%. The high D2 / D1 ratio (0.65-0.95) and the Härtel grindability coefficient (60-150) facilitate better rounding and spheroidization during subsequent deep shaping processes. This also helps reduce the expansion performance of the prepared graphite particles. Furthermore, controlling D2 / D1 to 0.68-0.90 enables deep shaping of the first raw material, further improving its rounding and spheroidization. This significantly reduces surface protrusions in the first raw material particles, thereby improving the sphericity and reducing the roundness of the second raw material particles. This results in graphite particles with an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. At this point, the graphite particles have high overall sphericity and suitable micro-protrusions on the surface, promoting orderly particle arrangement during cold pressing. This not only slows down the stress release rate and reduces stress accumulation during cold pressing but also facilitates good contact between graphite particles and between graphite particles and the binder, enhancing the cohesion of the negative electrode sheet and effectively reducing expansion throughout the battery's lifespan.

[0026] In any embodiment, granulation includes: heating to 290°C-310°C, holding at that temperature for 50 min-70 min while stirring, and then heating to 690°C-710°C and holding at that temperature for 100 min-140 min while stirring. By improving the above-mentioned process, the granulation strength is enhanced, which is beneficial to improving the density of the first raw material obtained after granulation. This is conducive to obtaining graphite particles with high tap density and an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. This not only helps to improve the tap density of the negative electrode active material, reduce particle rearrangement caused by repeated particle expansion, and slow down stress release, thereby improving the trend of expansion force growth, but also improves the structural stability of secondary particles. This is also conducive to obtaining graphite particles with high tap density and an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. It can promote the orderly arrangement of particles during cold pressing, which can not only slow down the rate of stress release and reduce the stress accumulation during cold pressing, but also facilitate good contact between graphite particles and between graphite particles and binders, improve the cohesion of the negative electrode sheet, and thus effectively reduce the expansion throughout the battery's entire life cycle.

[0027] In any embodiment, the amount of binder added is 12%-20% of the mass of the second aggregate. Controlling the amount of binder added within the above-mentioned higher range is beneficial to improving the density of the first raw material obtained after granulation. This not only helps to increase the tap density of the negative electrode active material, reduce particle rearrangement caused by repeated particle expansion, and slow down stress release, thereby improving the trend of expansion force growth, but also enhances the structural stability of the secondary particles.

[0028] The fourth aspect of this application provides an electrical device, including a secondary battery of the first aspect of this application, a negative electrode active material of the second aspect, or a negative electrode active material prepared by the preparation method of the third aspect of this application. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating an example of calculating the roundness of graphite particles.

[0030] Figure 2 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0031] Figure 3 yes Figure 2 An exploded view of a secondary battery according to an embodiment of this application is shown.

[0032] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application.

[0033] Figure 5This is a schematic diagram of a battery pack according to one embodiment of this application.

[0034] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0035] Figure 7 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0036] Explanation of reference numerals in the attached figures: 1-Battery pack; 2-Upper housing; 3-Lower housing; 4-Battery module; 5-Battery cell; 51-Housing; 52-Electrode assembly; 53-Top cover assembly. Detailed Implementation

[0037] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the negative electrode active material, its preparation method, secondary battery, and power application device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0038] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0041] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0042] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0043] Unless otherwise specified, the numerical range "AB" mentioned in this application includes both numerical values ​​A and B.

[0044] The development of electric vehicles has placed higher demands on battery range. As battery energy density increases, battery expansion also increases, leading to safety issues such as battery electrode breakage. In addition, as battery life requirements increase, battery reliability requirements are also constantly rising. Therefore, graphite is usually used as the negative electrode. However, for high energy density batteries, the problem of large expansion still exists.

[0045] Currently, to reduce expansion, the orientation of graphite particles is usually reduced to make the volume expansion of graphite particles anisotropic, thereby achieving the expansion of the electrode, and / or the particle size of graphite particles is reduced, thereby reducing the expansion by reducing the absolute amount of particle expansion.

[0046] Analysis of the causes of battery expansion revealed that the increase in cell expansion force primarily originates from the expansion of the negative electrode. Further analysis of the negative electrode expansion showed that its sustained rebound is due to the continuous irreversible expansion of the electrode, while the reversible expansion of the cell changes relatively little during cycling. In other words, reducing material orientation, decreasing material expansion, and improving the intrinsic physical properties of the active material can only reduce the volume expansion of the electrode during lithium intercalation; however, it has no significant effect on improving expansion throughout the battery's entire lifespan.

[0047] Experiments and simulations revealed that the irreversible thickness increase of the electrode sheet is mainly due to stress release during cycling. After the rolling process, the negative electrode active material particles in the electrode sheet undergo orientational arrangement, resulting in residual stress inside the electrode sheet. This stress is gradually released during cycling, altering the arrangement of the negative electrode active material particles and eventually restoring them to their pre-rolling arrangement, thus causing an irreversible increase in electrode thickness. Further analysis of changes throughout the material's life cycle revealed a strong correlation between the change in the arrangement of negative electrode active material particles and their surface properties; the smoothness of the particle surface significantly affects particle accumulation.

[0048] Based on this, the first aspect of the present application provides a secondary battery, which includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one side of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0049] The negative electrode active material includes graphite particles, with an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65.

[0050] Among them, the roundness of graphite particles ri represents the radius of curvature of the surface protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

[0051] "At least one side of the negative electrode film layer" means that the negative electrode current collector has two opposite sides in its own thickness direction. A negative electrode film layer can be formed on one side of the negative electrode current collector, or a negative electrode film layer can be formed on both sides of the negative electrode current collector. In the same electrode assembly, a negative electrode film layer can be formed on one side of some negative electrode current collectors, and a negative electrode film layer can be formed on both sides of some negative electrode current collectors. Alternatively, in the same electrode assembly, a negative electrode film layer can be formed on one side of all negative electrode current collectors; or a negative electrode film layer can be formed on both sides of all negative electrode current collectors in the same electrode assembly. This application does not limit this.

[0052] "Sphericity" refers to the macroscopic sphericity of graphite particles. Sphericity can be defined as the value obtained by dividing the circumference of a circle having the same area as the projected image of the graphite particles for secondary batteries by the circumference of the projected image of the graphite particles for secondary batteries. Specifically, sphericity = (circumference of a circle having the same area as the projected image of the graphite particles for secondary batteries) / (circumference of the projected image of the graphite particles for secondary batteries). The closer the sphericity is to 1, the closer the graphite particles are to being spherical.

[0053] "Average sphericity" can be defined as the average of the sphericity of 50 randomly selected graphite particles, reflecting the degree of deviation of the overall shape.

[0054] Sphericity can be measured using a particle shape analyzer such as the Sysmex FPIA3000 (manufactured by Mavern).

[0055] "Roundness" refers to the surface micromorphology of graphite particles, reflecting the smoothness of the particle surface and the distribution of protrusions. Its sensitive element is the surface micro-protrusions, reflecting the edge sharpness. Roundness is defined as the average ratio of the radii of curvature of n protrusions on the contour line of the two-dimensional projection of the graphite particle to the radius of the circumcircle of the corresponding two-dimensional projection. This ratio characterizes the roundness of the graphite particle. Here, ri represents the radius of curvature of the protrusions on the two-dimensional projection of the graphite particle, R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle, and n represents the number of protrusions on the contour of the two-dimensional projection of a single graphite particle. It can be understood that the smaller the roundness, the smoother the surface of the graphite particle. The above formula has been normalized, so regardless of the particle size, it does not affect the test result Y, and the roundness can be directly compared using the above roundness formula. It can be understood that: 1. A small ri radius of curvature corresponds to sharp protrusions, stress concentration, and easy expansion; a large ri radius of curvature corresponds to gentle protrusions and a smooth surface. 2. The spreading of the binder on the particle surface is affected by the curvature; small ri radii correspond to sharp protrusions with poor wetting and weak adhesion, while large ri radii correspond to smooth areas with good wetting and strong adhesion. 3. The rotational / slipping behavior of graphite particles during cold pressing is determined by the coefficient of friction at the contact points. μ ≈tan( θ ),in The angle of the protrusion is related to the radius of curvature. Therefore, at the sharp points of the protrusions corresponding to a small radius of curvature, the mechanical interlocking force between particles is large, making rearrangement difficult during cold pressing. Conversely, at the sharp points of the protrusions corresponding to a large radius of curvature, the mechanical interlocking force between particles is small, making rearrangement easier during cold pressing. Thus, the above formula can accurately predict the expansion of graphite particles.

[0056] "Contour line protrusion in a two-dimensional projection" refers to the local deviation of the contour line of a two-dimensional projection from the center of the projection, which is a protrusion relative to the ideal smooth curve of a circle or ellipse.

[0057] The roundness testing method includes: using a microscope to image the sample (graphite particles from a secondary battery). As fluid flows through the sample cell, a high-speed camera captures the particle projection and transmits it to software. Image processing technology is then used to determine the graphite particle size and quantitatively analyze its morphology, thereby obtaining the radius of curvature *ri* of the protrusion of the two-dimensional projection of the graphite particle and the radius of the circumcircle of the two-dimensional projection. The roundness *Y* of the graphite particle is then obtained using the aforementioned formula. An example of roundness calculation is provided. Figure 1 As shown, R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle. It can be seen that the outline of the two-dimensional projection of the graphite particle has nine protrusions. The radii of curvature of each of the nine protrusions are obtained using the formula described above. Calculate roundness, Figure 1 In Chinese, "roundness" is used to represent roundness, according to... Figure 1 It can be seen that the roundness of the graphite particle is Y=0.1.

[0058] "Average roundness" can be defined as the number-average of the roundness of 50 randomly selected graphite particles.

[0059] It should be noted that when conducting tests on the sphericity, roundness, and tap density and OI of the graphite particles in the negative electrode sheet of the secondary battery provided in the first aspect of this application, the corresponding method for obtaining the graphite particles includes: disassembling the secondary battery after full discharge, cleaning the negative electrode sheet three times with DMC for 2 hours each time to remove residual electrolyte on the surface, drying it, cleaning the negative electrode sheet with a hot solution of 2M HCl to separate the graphite particles from the negative electrode current collector, cleaning it with deionized water and drying it to obtain black powder, and then carbonizing the black powder at high temperature to obtain the final reduced graphite particle powder.

[0060] In summary, in this application, the average sphericity of the graphite particles is controlled to be 0.65-0.95. The overall sphericity of the graphite particles is high, but they are not perfect spheres. That is, high-sphericity graphite particles are used. Due to their isotropic expansion characteristics and the point contact between particles, the volume expansion caused by lithium intercalation can be converted to spatial position adjustment and pore filling of the graphite particles inside the electrode to a certain extent. This is beneficial to reducing the macroscopic thickness expansion rate of the electrode. If the average roundness is too small, it indicates that there are more protrusions on the surface of the graphite particles, and the sharper the protrusions are, the stress is concentrated and the particles are interlocked, which hinders the orderly arrangement of graphite particles during the cold pressing process, thereby worsening the accumulation of force in the negative electrode and causing the expansion of the negative electrode to intensify. On the other hand, if the average roundness is too large, it indicates that the curvature of the protrusions is larger and the surface structure of the graphite particles is smooth and flat. At this time, combined with the above-mentioned characteristics of high sphericity, This causes the contact mode between particles to change from surface contact to point contact, thereby reducing the bridging area of ​​the binder and worsening the interaction forces between graphite particles and between graphite particles and the binder. This worsens the cohesive force of the negative electrode sheet and leads to increased expansion. Therefore, while controlling the average sphericity of graphite particles to 0.65-0.95, the average roundness of graphite particles should also be controlled to 0.42-0.65. Within this range, the overall sphericity of graphite particles is high and the surface has suitable micro-protrusions, which can promote the orderly arrangement of particles during the cold pressing process. This not only slows down the stress release rate and reduces the stress accumulation during cold pressing, but also facilitates good contact between graphite particles and between graphite particles and the binder, improving the cohesive force of the negative electrode sheet and effectively reducing the expansion of the negative electrode sheet during cycling, thus reducing the expansion throughout the battery's entire life cycle.

[0061] In other words, the above settings can reduce both the volume expansion of the electrode during the lithium intercalation process and the expansion throughout the battery's entire life cycle.

[0062] For example, the average sphericity of the graphite particles is any one of 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95 or between any two values.

[0063] For example, the average roundness of the graphite particles is any one of 0.42, 0.45, 0.47, 0.50, 0.53, 0.55, 0.57, 0.60, 0.63, 0.65 or between any two values.

[0064] In some embodiments, the average sphericity of the graphite particles is 0.71-0.85. Sphericity determines the macroscopic arrangement of graphite particles. When the sphericity is within the above range, the shape of the graphite particles deviates slightly from a sphere, which can promote the orderly arrangement of particles during cold pressing while maintaining sufficient contact area. This is beneficial for good bonding between particles and between particles and binder, improving the cohesion of the negative electrode sheet, thereby effectively reducing battery expansion.

[0065] For example, the average sphericity of the graphite particles is any one of 0.71, 0.72, 0.73, 0.74, 0.75, 0.78, 0.79, 0.80, 0.82, 0.83, 0.84, 0.85 or between any two values.

[0066] In some embodiments, the average sphericity of the graphite particles is 0.42-0.55. By controlling the sphericity of the graphite particles within the above range and optimizing the average sphericity to 0.42-0.55, the two reinforce each other, optimizing the arrangement of graphite particles during cold pressing, thereby effectively reducing the expansion of the negative electrode sheet during cycling.

[0067] For example, the average roundness of the graphite particles is any one of 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55 or between any two values. In some embodiments, at least 50% of the graphite particles have a roundness Y ≥ 0.38. Controlling the roundness of most or all of the graphite particles within the above range, and controlling the surface protrusions of most or all of the graphite particles within a suitable range, is more conducive to promoting the orderly arrangement of graphite particles during cold pressing, reducing stress accumulation during cold pressing, and also more conducive to good contact between graphite particles and between graphite particles and binder, improving the cohesion of the negative electrode sheet, thereby effectively reducing the expansion of the negative electrode sheet during cycling.

[0068] For example, the statement that at least 50% of the graphite particles in the plurality of graphite particles have a roundness Y ≥ 0.38 means that: when imaging graphite particles for secondary batteries, as fluid flows through the sample cell, a high-speed camera captures the projection of the particles and transmits it to the software. Then, image processing technology is used to determine the particle size and perform quantitative analysis of the morphology of the graphite particles. Among 100 randomly selected graphite particles, 50 or more of them have a roundness Y ≥ 0.38.

[0069] For example, the proportion of graphite particles with a roundness Y ≥ 0.38 in a plurality of graphite particles is any one of 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100% or between any two values.

[0070] For example, among a plurality of graphite particles, at least 60% of the graphite particles have a roundness Y ≥ 0.38.

[0071] In some embodiments, at least 50% of the graphite particles have a sphericity ≥ 0.65. By controlling the sphericity of most or all graphite particles within the above range, and utilizing the isotropic expansion characteristics and point contact between the particles, the volume expansion caused by lithium intercalation can be converted to spatial positioning adjustment and pore filling of the graphite particles within the electrode to a certain extent, thereby effectively reducing battery expansion.

[0072] For example, in the multiple graphite particles mentioned here, at least 50% of the graphite particles are graphite particles that meet the following criteria: when 100 graphite particles are randomly selected, 50 or more of them meet the requirement that the sphericity of the graphite particles is ≥0.65.

[0073] For example, in a plurality of graphite particles, at least 60% of the graphite particles have a sphericity ≥ 0.65.

[0074] In some embodiments, the graphite particles include secondary particles with a carbon coating on their surface. This secondary particle structure effectively reduces material volume expansion during lithium intercalation, thereby improving the maximum expansion force. Furthermore, the carbon coating on the secondary particles acts as a protective layer, firmly binding the primary particles together, increasing the overall mechanical strength of the particles, effectively buffering volume change stress, suppressing particle breakage, and enhancing structural and interfacial stability.

[0075] "Secondary particles" refer to larger particles formed by the aggregation of multiple primary particles. Primary particles refer to individual fine grains, which are the most basic units in negative electrode active materials and the initial fine particles formed during the crystallization process. Under an electron microscope, they appear as individual particles with complete boundaries. Defects may exist inside the particles, but there are no complete boundaries within the particles sufficient to divide them into two or more particles. Secondary particles typically have relatively regular shapes, such as spherical or near-spherical.

[0076] The graphite particles and carbon coating can be distinguished by: ultrasonically dispersing the graphite particles in ethanol, dropping them onto a copper mesh carbon support film, drying them, and then testing them. TEM is then turned on, and high-resolution TEM (HRTEM) mode is selected. The highly crystalline graphite particle region will show clear parallel lattice fringes (corresponding to the (002) crystal plane of the graphite particles, with a spacing of approximately 0.335 nm), while the outer carbon coating layer will not show long-range ordered lattice fringes.

[0077] It should be noted that the surface of the secondary particles has a carbon coating layer. In this case, the carbon coating layer is actually part of the secondary particles. That is, the secondary particles include a core composed of primary graphite particles and a carbon coating layer covering the core.

[0078] The thickness of the carbon coating layer can be determined with reference to relevant technologies. For example, the thickness of the carbon coating layer can be, but is not limited to, 0.5 nm-4 nm, and for example, the thickness of the carbon coating layer is 0.5 nm-3 nm.

[0079] In some embodiments, the tap density of the secondary particles is 1.25 g / cc to 1.45 g / cc. A tap density within this relatively high and suitable range is beneficial for the close arrangement of graphite particles and increases the contact area. Each particle is supported by surrounding particles, thus reducing local stress concentration and maintaining a confined pressure state during cold pressing. This helps suppress lateral slippage and cracking of the particles, allowing the graphite particles to maintain mechanical integrity and structural stability after cold pressing. This results in more effectively resisting and dispersing internal stress during battery cycling. Simultaneously, this dense structure also helps suppress electrode expansion during cycling, thereby improving the trend of increasing expansion force.

[0080] Tap density can be measured by filling a container with the negative electrode active material of a secondary battery, measuring the final volume obtained by vibrating the container a specific number of times, and calculating the apparent density based on the final volume.

[0081] For example, the tap density of the secondary particles is any one of 1.25 g / cc, 1.26 g / cc, 1.27 g / cc, 1.28 g / cc, 1.29 g / cc, 1.30 g / cc, 1.31 g / cc, 1.32 g / cc, 1.33 g / cc, 1.34 g / cc, 1.35 g / cc, 1.36 g / cc, 1.37 g / cc, 1.38 g / cc, 1.39 g / cc, 1.40 g / cc, 1.41 g / cc, 1.42 g / cc, 1.43 g / cc, 1.44 g / cc, or 1.45 g / cc, or between any two of these values.

[0082] Optionally, the tap density of the secondary particles here refers to the tap density of secondary particles with a carbon coating on the surface.

[0083] In some embodiments, the graphite particles include secondary particles with an OI value of 0.5-5. Controlling the OI value of the secondary particles within this low range indicates that the orientation of the primary particles tends to be random, and the secondary particles as a whole possess good isotropic characteristics. Combined with the aforementioned sphericity and average roundness, the secondary particles are easy to rotate and slide during cold pressing, which can promote the orderly arrangement of particles during the cold pressing process, reduce the stress accumulation during cold pressing, and at the same time ensure good contact between graphite particles and between graphite particles and binder, improve the cohesion of the negative electrode sheet, and thus effectively reduce the expansion of the negative electrode sheet during cycling.

[0084] "OI value" refers to the orientation degree of graphite particles, describing the uniformity of crystal orientation within the graphite particle material. The OI value is the ratio of the peak area of ​​plane (004) to the peak area of ​​plane (110) obtained by X-ray diffraction (XRD). In this application, the powder OI value of the material can be determined using instruments and methods known in the art. For example, an X-ray diffractometer (such as a Bruker D8 Discover) can be used for testing, and the testing can be performed with reference to JIS K 0131-1996 and JB / T4220-2011. The X-ray diffraction pattern of the powder sample is obtained, and the powder OI value is calculated according to OI value = I004 / I110. I004 is the integrated area of ​​the diffraction peak of the 004 crystal plane of crystalline carbon in the powder sample, and I110 is the integrated area of ​​the diffraction peak of the 110 crystal plane of crystalline carbon in the powder sample. In the X-ray diffraction analysis test of this application, a copper target can be used as the anode target, CuKα rays can be used as the radiation source, the ray wavelength scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min.

[0085] It is understandable that the OI value for secondary particles referred to here includes both secondary particles without a carbon coating on their surface and those with a carbon coating on their surface.

[0086] For example, the OI value of the secondary particles is any one of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5, or between any two values.

[0087] In some embodiments, the graphite particles comprise single particles with a tap density of 1.30 g / cc to 1.50 g / cc. Controlling the tap density of the single particles to within this relatively high and suitable range facilitates a tighter arrangement of the graphite particles and increases the contact area. Each particle is supported by surrounding particles, thus reducing local stress concentration during cold pressing and maintaining a confined pressure state. This helps suppress lateral slippage and cracking of the particles, allowing the graphite particles to maintain mechanical integrity and structural stability after cold pressing. Consequently, it more effectively resists and disperses internal stress during battery cycling. Simultaneously, this dense structure also helps suppress electrode expansion during cycling, thereby improving the trend of increasing expansion force.

[0088] A "single particle" refers to a particle consisting of a single primary particle. Single particles typically have an irregular shape.

[0089] The determination of "single particles" and "secondary particles" can be achieved by: ultrasonically dispersing graphite particles in ethanol, dropping the mixture onto a copper mesh carbon support film, and drying it before analysis. Then, using TEM (High Resolution Transformer) in high-resolution mode (HRTEM), images are acquired at, for example, 3000x magnification. The particles in the electron microscope images are then analyzed using ImageJ software (1.46r, Win64 version) to determine whether they are single or secondary particles.

[0090] For example, the tap density of a single particle is any one of 1.30 g / cc, 1.31 g / cc, 1.32 g / cc, 1.33 g / cc, 1.34 g / cc, 1.35 g / cc, 1.36 g / cc, 1.37 g / cc, 1.38 g / cc, 1.39 g / cc, 1.40 g / cc, 1.41 g / cc, 1.42 g / cc, 1.43 g / cc, 1.44 g / cc, 1.45 g / cc, 1.46 g / cc, 1.47 g / cc, 1.48 g / cc, 1.49 g / cc, or 1.50 g / cc, or between any two of these values.

[0091] In some embodiments, the graphite particles include single particles with an OI value of 2-8. By controlling the OI value of the single particles within the aforementioned lower range, the single particles exhibit a certain degree of anisotropy. Combined with the aforementioned sphericity and roundness, this maintains the stability of the particle structure during cold pressing, facilitates rotation and sliding, promotes the orderly arrangement of particles during cold pressing, reduces stress accumulation during cold pressing, and ensures good contact between graphite particles and between graphite particles and the binder. This enhances the cohesion of the negative electrode sheet, thereby effectively reducing the expansion of the negative electrode sheet during cycling.

[0092] For example, the OI value of a single particle is any one of 2, 3, 4, 5, 6, 7, 8 or between any two values.

[0093] It should be noted that in this application, the graphite particles in the negative electrode film layer that serve as the negative electrode active material can be single particles, secondary particles, or a combination of single and secondary particles.

[0094] In some embodiments, the volumetric particle size distribution Dv50 of the graphite particles is 4 μm - 20 μm. Within this range, the particles are less prone to breakage during rolling, suppressing the risk of accelerated electrode expansion due to breakage.

[0095] Volumetric particle size distribution (Dv50) refers to the particle size at which the cumulative volumetric particle size distribution reaches 50%. Dv50 represents the median particle size of the powder. The test method for Dv50 is laser particle size analysis, referring to GB / T 19077-2016, using a laser particle size analyzer. The testing instrument can be the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0096] For example, the Dv50 of the graphite particles is any value of 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm or between any two values.

[0097] In some embodiments, the ratio of (Dv90-Dv10) / Dv50 of the graphite particles is 1.1-1.5. Controlling the ratio of (Dv90-Dv10) / Dv50 of the graphite particles within the above range results in a more concentrated particle size distribution and similar particle sizes, which reduces the absolute expansion and is beneficial for reducing battery expansion.

[0098] The volumetric particle size distribution Dv10 refers to the particle size when the cumulative volumetric particle size distribution reaches 10%, and the volumetric particle size distribution Dv90 refers to the particle size when the cumulative volumetric particle size distribution reaches 90%.

[0099] For example, the (Dv90-Dv10) / Dv50 of the graphite particles is any value of 1.1, 1.2, 1.3, 1.4, 1.5 or between any two values.

[0100] In some embodiments, the liquid absorption rate of the negative electrode sheet with a compaction density of 1.60 g / cc is ≥1.8 mg / s. 0.5Controlling the liquid absorption rate within the above range indicates that during the initial formation, the electrolyte can rapidly and synchronously wet the surface of all negative electrode active materials, forming a uniform and stable SEI film. This can suppress the expansion of the negative electrode caused by uneven volume changes due to excessively rapid local lithium intercalation or excessive stress concentration. It also indicates that the negative electrode has a suitable pore structure and wettability, which can provide a buffer space for the volume expansion of the negative electrode material during charging and discharging, absorb some mechanical stress, and reduce electrode expansion.

[0101] The electrolyte absorption rate refers to the speed at which the electrolyte wets and fills the pores inside the dry negative electrode sheet.

[0102] The aspiration rate is derived from the Washburn equation / Lucas-Washburn equation, and is used here to illustrate that the Washburn equation / Lucas-Washburn equation is simplified as follows: m = k t 0.5 Where m is the mass of liquid absorbed, t is time, and k is the absorption rate constant, with units of mg / s. 0.5 .

[0103] The second aspect of this application provides a negative electrode active material, which includes graphite particles with an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65.

[0104] Among them, the roundness of a single graphite particle ri represents the radius of curvature of the surface protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

[0105] The testing methods for average sphericity and average roundness can refer to the testing methods provided in the first aspect of this application, and will not be repeated here. It is understood that the testing method provided in the second aspect of this application is for directly prepared negative electrode active materials.

[0106] In summary, this application controls the average sphericity of the graphite particles to be 0.65-0.95. The high overall sphericity of the graphite particles, meaning the use of high-sphericity graphite particles, allows the volume expansion caused by lithium intercalation to be converted to spatial adjustment and pore filling within the electrode due to their isotropic expansion characteristics and point contact between particles. This helps reduce the macroscopic thickness expansion rate of the electrode, especially in the early stages of cycling. A low average sphericity indicates more protrusions on the graphite particle surface, with higher protrusion sharpness, leading to stress concentration and interlocking between particles. This hinders the orderly arrangement of graphite particles during cold pressing, further worsening the accumulation of forces within the negative electrode and exacerbating its expansion. Conversely, a high average sphericity indicates greater protrusion curvature and a smoother, flatter surface structure. Combined with the aforementioned high sphericity, this changes the contact mode between particles from surface contact to point contact, reducing the binder bridging area and worsening the forces between graphite particles and between graphite particles and the binder, thus deteriorating the negative electrode. Cohesion leads to increased expansion; therefore, while controlling the average sphericity of graphite particles to 0.65-0.95, the average roundness of graphite particles should also be controlled to 0.42-0.65. Within this range, the overall sphericity of graphite particles is high and the surface has suitable micro-protrusions, which can promote the orderly arrangement of particles during the cold pressing process. This not only slows down the rate of stress release and reduces the stress accumulation during cold pressing, but also facilitates good contact between graphite particles and between graphite particles and binder, thereby improving the cohesion of the negative electrode sheet and effectively reducing the expansion during the entire battery life cycle.

[0107] For example, the average sphericity of the graphite particles is any one of 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95 or between any two values.

[0108] For example, the average roundness of the graphite particles is any one of 0.42, 0.45, 0.47, 0.50, 0.53, 0.55, 0.57, 0.60, 0.63, 0.65 or between any two values.

[0109] In some embodiments, the graphite particles include secondary particles with a carbon coating on their surface. The carbon coating acts as a protective layer, firmly binding the primary particles within the secondary particles together, thus improving the overall mechanical strength of the particles, effectively buffering volumetric stress, suppressing particle breakage, and enhancing structural and interfacial stability.

[0110] In some embodiments, the tap density of the secondary particles is 1.25 g / cc to 1.45 g / cc. The tap density of the secondary particles is within the above-mentioned relatively high and suitable range, which is conducive to the close arrangement of graphite particles and the increase of contact area. Each particle is supported by the surrounding particles, so local stress concentration can be reduced during cold pressing and the particles are under confined pressure. This helps to suppress the lateral slippage and cracking of the particles, and allows the graphite particles to maintain mechanical integrity and structural stability after cold pressing. This enables them to more effectively resist and disperse internal stress during battery cycling. At the same time, this dense structure also helps to suppress electrode expansion during cycling, thereby improving the trend of expansion force growth.

[0111] In some embodiments, the graphite particles include secondary particles with an OI value of 0.5-5.

[0112] Controlling the OI value of the secondary particles within the aforementioned low range indicates that the orientation of the primary particles tends to be random, and the secondary particles as a whole possess good isotropic characteristics. Combined with the aforementioned sphericity and average roundness, the secondary particles are easy to rotate and slide during cold pressing, which can promote the orderly arrangement of particles during the cold pressing process, reduce the stress accumulation during cold pressing, and at the same time ensure good contact between graphite particles and between graphite particles and binder, thereby improving the cohesion of the negative electrode sheet and effectively reducing the expansion of the negative electrode sheet during cycling.

[0113] In some embodiments, the graphite particles include single particles with a tap density of 1.30 g / cc to 1.50 g / cc. Controlling the tap density of the single particles to reach the above-mentioned high and suitable range is beneficial for the graphite particles to be closely arranged and increase the contact area. Each particle is supported by the surrounding particles, so local stress concentration can be reduced during cold pressing and the particles are under confined pressure. This helps to suppress the lateral slippage and cracking of the particles, and allows the graphite particles to maintain mechanical integrity and structural stability after cold pressing. This enables them to more effectively resist and disperse internal stress during battery cycling. At the same time, this dense structure also helps to suppress electrode expansion during cycling, thereby improving the trend of expansion force growth.

[0114] In some embodiments, the graphite particles include single particles with an OI value of 2-8. Controlling the OI value of the single particles within this range allows for a certain degree of anisotropy. Combined with the aforementioned sphericity and roundness, this maintains the stability of the particle structure during cold pressing, facilitates rotation and sliding, promotes orderly particle arrangement during cold pressing, reduces stress accumulation, and ensures good contact between graphite particles and between graphite particles and the binder. This enhances the cohesion of the negative electrode sheet, thereby effectively reducing its expansion during cycling. In some embodiments, at least 50% of the graphite particles have a roundness Y ≥ 0.38. Controlling the roundness of most or all graphite particles within the above-mentioned range, and controlling the surface protrusions of most or all graphite particles within a suitable range, is more conducive to promoting the orderly arrangement of graphite particles during the cold pressing process, reducing the stress accumulation during cold pressing, and is also more conducive to good contact between graphite particles and between graphite particles and binder, thereby improving the cohesion of the negative electrode sheet and effectively reducing the expansion of the negative electrode sheet during cycling.

[0115] In some embodiments, at least 50% of the graphite particles have a sphericity ≥ 0.65. By controlling the sphericity of most or all graphite particles within the above range, and utilizing the isotropic expansion characteristics and point contact between the particles, the volume expansion caused by lithium intercalation can be converted to spatial positioning adjustment and pore filling of the graphite particles within the electrode to a certain extent, thereby effectively reducing battery expansion.

[0116] In some embodiments, the negative electrode active material provided in the second aspect of this application serves as the negative electrode active material in the negative electrode sheet of the first aspect of this application.

[0117] A third aspect of this application provides a method for preparing a negative electrode active material, comprising: The first raw material is deeply shaped to obtain the second raw material; wherein, the first raw material includes a first aggregate and / or granulated material, the first aggregate is obtained by crushing and pre-shaping the raw coke, and the granulated material is obtained by crushing and pre-shaping the raw coke, mixing the obtained second aggregate with a binder, and granulating; the volatile matter of the raw coke is 10 wt%-18 wt%, and the Haas grindability coefficient is 60-150, the Dv50 of the first raw material is D1, the Dv50 of the second raw material is D2, and the D2 / D1 ratio is 0.68-0.90; The second raw material is pre-carbonized and graphitized to obtain graphite particles, which serve as the negative electrode active material.

[0118] The negative electrode active material includes graphite particles, with an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. Among them, the roundness of graphite particles ri represents the radius of curvature of the protrusion of the outline of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

[0119] It is understandable that when the first raw material is the first aggregate, the corresponding graphite particles prepared are single-particle graphite, and when the first raw material is granulation material, the corresponding graphite particles prepared are secondary particles.

[0120] The deep shaping of the first raw material results in a more rounded shape, more uniform particle size, and less fine powder, which is beneficial for improving the sphericity and roundness of the final graphite particles. Furthermore, this application employs a process sequence of granulation followed by deep shaping for the secondary particles, which, compared to deep shaping followed by granulation, results in a more rounded second raw material.

[0121] Granulation is the process of agglomerating primary particles into secondary particles. The secondary aggregate before granulation is a primary particle. The first material obtained by granulating these secondary aggregates includes the secondary particles formed by the agglomeration of the secondary aggregate.

[0122] It is understandable that the preparation of the first raw material involves the crushing and preliminary shaping of the raw coke. The preliminary shaping here can refer to relevant processes.

[0123] The volatile matter content of the raw coke is 10 wt%-18 wt%, and the Hay Group grindability is 80-150. Controlling the raw coke to meet these parameters is beneficial for better rounding and spheroidization during subsequent deep shaping processes, and also facilitates the preparation and acquisition of low-expansion graphite particles. The raw coke includes, but is not limited to, at least one of petroleum coke, pitch coke, and needle coke.

[0124] The volatile matter was determined according to SH / T0026-1990, the method for determining volatile matter in petroleum coke.

[0125] The Hardy Grindability Index (HGA) is a relative indicator that measures the ease with which coal can be ground into powder. It can be tested using a Hardy Grindability Index tester. Controlling the HGA to be ≥80 is beneficial for obtaining aggregates with concentrated particle size distribution and good sphericity or near-sphericity. Controlling the raw coke to meet the above requirements is also beneficial for preparing low-expansion graphite particles, which helps to improve the expansion of graphite particles.

[0126] By controlling D2 / D1 to be 0.68-0.90, the first raw material is deeply shaped, and the first raw material is rounded and spheroidized, which significantly reduces the surface protrusion of the first raw material particles. This is beneficial to improving the sphericity of the second raw material particles and reducing their roundness, and is conducive to preparing graphite particles with an average sphericity of 0.7-1.0 and an average roundness of 0.42-0.65.

[0127] The preparation method provided in this application, in the process of preparing the negative electrode active material, selects the volatile matter content of the raw material coke to be 10wt%-18wt%. The high D2 / D1 ratio (0.65-0.95) and the Härtel grindability coefficient (60-150) facilitate better rounding and spheroidization during subsequent deep shaping processes. This also helps reduce the expansion performance of the prepared graphite particles. Furthermore, controlling the D2 / D1 ratio to 0.68-0.90 enables deep shaping of the first raw material, achieving better rounding and spheroidization and significantly reducing surface protrusions. This, in turn, improves the sphericity and reduces the roundness of the second raw material particles, resulting in graphite particles with an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. At this point, the graphite particles exhibit high overall sphericity and suitable micro-protrusions on the surface, promoting orderly particle arrangement during cold pressing. This not only slows down the stress release rate and reduces stress accumulation during cold pressing but also facilitates good contact between graphite particles and between graphite particles and the binder, enhancing the cohesion of the negative electrode sheet and effectively reducing expansion throughout the battery's lifespan.

[0128] For example, the volatile matter content of the raw coke is any one of 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, or between any two of these values.

[0129] For example, the Hay Group grindability coefficient of the raw coke is any value of 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 105, 145, 106, 136, 85, 105, 115, 145 or between any two values.

[0130] For example, D2 / D1 is any value among 0.68, 0.70, 0.75, 0.80, 0.85, 0.90, 0.773, 0.739, 0.850, 0.895, 0.895, 0.850, 0.789, 0.824, and 0.708, or between any two values.

[0131] In some embodiments, the Dv50 of the first aggregate and the second aggregate are 6.0 μm-10 μm, respectively. This readily yields single-particle and secondary-particle graphite products with high mechanical strength and good structural integrity.

[0132] For example, the Dv50 of the first aggregate and the second aggregate are any value of 6.0μm, 7μm, 8μm, 9.0μm, 9.5μm, 10μm or between any two values.

[0133] In some embodiments, when the first raw material is obtained by crushing and pre-shaping raw coke to obtain the second aggregate, and then mixing the second aggregate with a binder and granulating it, the Dv50 (D1) of the first raw material is 14μm-25μm. Controlling the Dv50 of the granulated first raw material within the above range helps to ensure that the obtained secondary particles have good structural stability and particle size, so as to cooperate with subsequent shaping to obtain a suitable second raw material.

[0134] For example, the Dv50 of the first raw material obtained by granulation is any value of 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm or between any two values.

[0135] In some embodiments, granulation includes: heating to 290°C-310°C, holding at that temperature for 50 min-70 min while stirring, and then heating to 690°C-710°C and holding at that temperature for 100 min-140 min while stirring.

[0136] By improving the above-mentioned process, the granulation strength is enhanced, which is beneficial to improving the density of the first raw material obtained after granulation. This is conducive to obtaining graphite particles with high tap density and an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65. This not only helps to improve the tap density of the negative electrode active material, reduce particle rearrangement caused by repeated particle expansion, and slow down stress release, thereby improving the trend of expansion force growth, but also improves the structural stability of secondary particles. It can promote the orderly arrangement of particles during cold pressing, which can not only slow down the rate of stress release and reduce the stress accumulation during cold pressing, but also facilitate good contact between graphite particles and between graphite particles and binders, improve the cohesion of the negative electrode sheet, and thus effectively reduce the expansion during the entire battery life cycle.

[0137] In some embodiments, the amount of binder added is 12%-20% of the mass of the second aggregate. Controlling the amount of binder added within the above-mentioned higher range is beneficial to improving the density of the first raw material obtained after granulation. This not only helps to increase the tap density of the negative electrode active material, reduce particle rearrangement caused by repeated particle expansion, and slow down stress release, thereby improving the trend of expansion force growth, but also enhances the structural stability of the secondary particles.

[0138] For example, the amount of binder added is any one of 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20% of the mass of the second aggregate, or between any two values.

[0139] In some embodiments, the softening point of the binder is 150°C-250°C. Selecting a binder with a suitable softening point facilitates the formation of a uniform, continuous, and robust carbonaceous binder phase between and on the surface of the primary particles, thereby improving the structural stability of the secondary particles and enhancing their density.

[0140] For the testing of softening point, refer to GB / T 4507, and use the ring and ball method to determine the softening point of the adhesive.

[0141] For example, the softening point of the adhesive is any value of 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, or between any two values.

[0142] The binder includes, but is not limited to, asphalt. Optionally, the asphalt has a softening point of 150℃-250℃ and a coking value of 50%-67%.

[0143] In some embodiments, the second raw material is pre-carbonized, the obtained carbide is mixed with an antioxidant, and graphitized to obtain graphite particles as a negative electrode active material. The antioxidant includes boron-based compounds, and the mass ratio of antioxidant to carbide is 0.5-1.5:100.

[0144] Boron compounds include, but are not limited to, boron carbide or boron.

[0145] By introducing boron compounds as antioxidants during the pre-carbonization process, oxidative ablation can be inhibited. At the same time, controlling the mass ratio of antioxidant to carbide to be 0.5-1.5:100 can not only maintain the integrity of the particles and avoid the formation of surface depressions and pores, but also promote the graphite particles to obtain an ordered structure and improve the density of the structure. The combined effect makes the particles maintain a high degree of roundness.

[0146] For example, the mass ratio of antioxidant to carbide is any one of 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.9:100, 1.0:100, 1.1:100, 1.2:100, 1.3:100, 1.4:100, 1.5:100, or between any two of these values.

[0147] In some embodiments, carbonization includes heat treatment at a temperature of 700°C-1500°C for 8-36 hours under an inert atmosphere. The carbonization temperature and time can be selected according to the type of raw coke and the type of the second raw material.

[0148] In some embodiments, graphitization includes holding at 2000℃-3000℃ for 24h-72h. The graphitization temperature and time can be selected according to the type of raw coke and the type of the second raw material.

[0149] Alternatively, graphitization can be carried out in an inert atmosphere.

[0150] In some embodiments, the product may also be shaped.

[0151] The fourth aspect of this application provides an electrical device, including secondary particles provided in the first aspect of this application, negative electrode active material provided in the second aspect, or negative electrode active material prepared by the preparation method provided in the third aspect.

[0152] In addition, the secondary battery, battery module, battery pack and power device of this application will be described below with appropriate reference to the accompanying drawings.

[0153] [Rechargeable Battery] The second aspect of this application provides a secondary battery. This application does not particularly limit the type of secondary battery; for example, the secondary battery can be a lithium-ion battery, etc.

[0154] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0155] This application does not impose any particular restriction on the type of electrolyte, which can be selected according to actual needs. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions). This applies to secondary batteries using electrolyte solutions, as well as some secondary batteries using solid electrolytes.

[0156] [Positive electrode plate] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive active material. As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0157] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0158] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include, but is not limited to, at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.8 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0159] In some embodiments, to further improve the energy density of secondary batteries, the positive electrode active material for lithium-ion batteries may include materials with the general formula Li. a Ni b Co c M d Oe A f One or more of lithium transition metal oxides and their modified compounds. 0.8 ≤ a ≤ 1.2, 0.5 ≤ b < 1, 0 < c < 1, 0 < d < 1, 1 ≤ e ≤ 2, 0 ≤ f ≤ 1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A is selected from one or more of N, F, S, and Cl.

[0160] In some embodiments, by way of example, the positive electrode active material for a lithium ion battery may include one or more of LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.85 Co 0.15 Al 0.05 O2, LiFePO4, and LiMnPO4.

[0161] In the present application, the modified compounds of the above positive electrode active materials may be doping modification and / or surface coating modification of the positive electrode active materials.

[0162] It should be noted that during the charge and discharge process of the battery, the insertion and extraction and consumption of Li will occur, and the molar content of Li is different when the battery is discharged to different states. In the listing of the above positive electrode active materials in the present application, the molar content of Li is the initial state of the material, that is, the state before feeding. When the positive electrode material is applied to the battery system, after charge and discharge cycling, the molar content of Li will change. In the listing of the above positive electrode active materials in the present application, the molar content of O is only the theoretical state value, and lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of O will show fluctuations.

[0163] Non-limitingly, the weight percentage of the positive electrode active material in the positive electrode active material layer may be greater than or equal to 80 wt%, and further may be greater than or equal to 90 wt%. In the present application, unless otherwise specified, "wt%" represents weight percentage.

[0164] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a terpolymer of PVDF-tetrafluoroethylene-propylene, a terpolymer of PVDF-hexafluoropropylene-tetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorinated acrylate resin. Non-limitingly, the weight percentage of the binder in the positive electrode active material layer may be 0.5wt%-15wt%, more further 0.5wt%-10wt%, even further 0.5-5wt%, even further 1wt%-5wt%, and even more preferably 1wt%-3wt%.

[0165] In some embodiments, the positive electrode film layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Without limitation, based on the total weight of the positive electrode active material layer, the weight percentage of the conductive agent in the positive electrode active material layer may be 0-10 wt%, more further 0-8 wt%, and even more further 0-5 wt%.

[0166] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0167] [Negative electrode plate] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector. The negative electrode film layer includes a negative electrode active material; the negative electrode active material includes graphite particles, the graphite particles having an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65; wherein, the roundness of the graphite particles... ri represents the radius of curvature of the protrusion of the outline of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

[0168] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0169] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0170] In some embodiments, the negative electrode active material may be solely the negative electrode active material shown in the second aspect of this application or the negative electrode active material prepared by the preparation method provided in the third aspect of this application, or it may be used in combination with negative electrode active materials for batteries known in the art. As an example, negative electrode active materials for batteries known in the art include at least one of the following materials: natural graphite, soft carbon, hard carbon, silicon-based materials, and artificial graphite whose parameters do not meet the relevant requirements of this application. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

[0171] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). Non-limitingly, the weight percentage of the binder in the negative electrode active layer may be 0 wt%-20 wt%, more further 0 wt%-10 wt%, even further 0-5 wt%, even further 1 wt%-5 wt%, and even more preferably 1 wt%-3 wt%.

[0172] In some embodiments, the negative electrode film layer may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Non-limitingly, the weight percentage of the conductive agent in the negative electrode active layer may be 0wt%-15wt%, more preferably 0wt%-10wt%, and even more preferably 0wt%-5wt%.

[0173] In some embodiments, the negative electrode active material layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)). Non-limitingly, the weight percentage of other additives in the negative electrode active layer may be 0wt%-15wt%, further preferably 0wt%-10wt%, even more preferably 0wt%-5wt%, even more preferably 0wt%-3wt%, and even more preferably 0wt%-2wt%. In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then performing processes such as drying and cold pressing to obtain the negative electrode sheet.

[0174] In other embodiments, the negative electrode sheet further includes a base coating layer disposed on the negative current collector. The base coating layer is located between the negative current collector and the negative electrode film layer. The base coating layer basically does not contain negative electrode active material, but may contain a small amount of carbon material. However, the carbon material forms a thin coating and cannot function as a negative electrode active material. In some embodiments, the base coating layer may also include a binder. The type of binder is not particularly limited, and those skilled in the art can choose flexibly according to actual needs.

[0175] [Electrolytes] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements.

[0176] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0177] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0178] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0179] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0180] For example, the additive may include, but is not limited to, at least one of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), propylene sulfate, vinyl sulfite (ES), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), sulfonate cyclic quaternary ammonium salts, succinic anhydride, succinic anhydride (SN), adiponitrile (AND), tris(trimethylsilane) phosphate (TMSP), or tris(trimethylsilane) borate (TMSB).

[0181] [Isolation membrane] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0182] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0183] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0184] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0185] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0186] A secondary battery is a battery cell that can be recharged after it has been discharged, allowing the active materials to be activated and the battery to continue to be used.

[0187] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 The example shown is a square-structured battery cell 5.

[0188] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0189] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0190] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0191] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0192] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0193] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0194] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0195] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0196] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0197] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0198] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0199] Example 1 Preparation of negative electrode active materials 1. Petroleum coke with 12 wt% volatile matter and a HAF grindability coefficient of 105 is crushed, shaped and classified to obtain aggregate with a Dv50 of 9.5 μm.

[0200] 2. Aggregate and asphalt (softening point 195℃-200℃, coking value 54%-56%) are mixed at a mass ratio of 100:14 (based on aggregate as 100% and asphalt addition as 14%). Granulation is carried out using a horizontal granulation reactor. The granulation reactor is heated from room temperature to 300℃ in a nitrogen atmosphere and kept at this temperature for 1 hour while stirring. The temperature is then further increased to 700℃ and kept at this temperature for 2 hours while stirring. After granulation, the first raw material is obtained. The Dv50 of the first raw material is D1, and D1=22μm.

[0201] The first raw material is subjected to deep shaping to obtain the second raw material. The Dv50 of the second raw material is D2, D2=17μm, and the ratio of D2 / D1 is 0.773.

[0202] 3. The second raw material is carbonized in a carbonization kiln at 1150℃ for 8 hours under a nitrogen atmosphere to obtain carbonized material.

[0203] 4. Mechanically mix the antioxidant, boron carbide, and carbide material at a mass ratio of 1:100. Perform high-temperature graphitization treatment at 3000℃ for 48 hours using an Atchison furnace. Shape, sieve, and demagnetize the resulting material to obtain the graphite material. The graphite material has a Dv50 of 16μm.

[0204] Preparation of the negative electrode sheet The above-mentioned graphite material, conductive carbon black (Super P), thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 96.4:1:1.2:1.4 to form a negative electrode slurry. The negative electrode slurry was coated onto a copper foil negative electrode current collector using an extrusion coating method. After drying, cold pressing, edge trimming, cutting, and slitting, a negative electrode sheet was obtained. The negative electrode film layer includes the negative electrode current collector and negative electrode film layers disposed on both sides of the negative electrode current collector along its thickness direction. The compaction density of a single-sided negative electrode film layer is 1.60 g / cc.

[0205] Preparation of Electrolyte The electrolyte was prepared in an argon atmosphere glove box with a water content of <10ppm. Ethyl carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1mol / L.

[0206]

Isolation Film

[0207] Button cell batteries Using a lithium metal sheet as the counter electrode, the aforementioned negative electrode, separator, counter electrode, and electrolyte are assembled into a CR2430 coin cell in an argon-protected glove box.

[0208] The following tests were performed on Example 1: 1. Negative electrode active material: The battery was fully discharged and disassembled. After disassembly, the negative electrode sheet was cleaned three times with DMC for 2 hours each time to remove residual electrolyte on the surface. After drying, the negative electrode sheet was cleaned with a hot solution of 2M HCl to separate the graphite particles from the negative electrode current collector. The graphite was then cleaned with deionized water and dried to obtain black powder. The black powder was then carbonized at high temperature to obtain the final reduced graphite powder for relevant testing.

[0209] (1) Roundness testing methods include: Graphite powder is imaged using the principle of microscopy. As fluid flows through the sample cell, a high-speed camera captures the projection of the particles and transmits the image to software. Image processing techniques are then used to determine the particle size and quantitatively analyze the morphology of the graphite particles. This yields the radius of curvature *ri* of the protrusions of the two-dimensional projection of the graphite particles and the radius of the circumcircle of the two-dimensional projection. The formula is then used to... ri represents the radius of curvature of the protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle. The roundness Y of the graphite particle is obtained. The average roundness of 50 randomly selected graphite particles is obtained. At the same time, the number of graphite particles with roundness Y≥0.38 is counted from 100 randomly selected graphite particles.

[0210] Test results: The average roundness of the graphite particles is 0.51. Among 100 randomly selected graphite particles, 84 have a roundness Y ≥ 0.38. That is, among multiple graphite particles, 84% of the graphite particles have a roundness Y ≥ 0.38.

[0211] (2) Sphericity test: The sphericity of graphite powder was measured using a particle shape analyzer such as the Sysmex FPIA3000 (manufactured by Mavern). Sphericity was calculated as: (circumference of a circle with the same area as the projected image of the graphite particle) / (circumference of the projected image of the graphite particle). The average sphericity was calculated as the numerical average of the sphericity of 50 randomly selected graphite particles. Simultaneously, 100 graphite particles were randomly selected, and the number of particles with a sphericity ≥ 0.65 was counted.

[0212] Test results: The average sphericity of the graphite particles is 0.83. When 100 graphite particles are randomly selected, 88 of them have a sphericity ≥ 0.65. That is, among multiple graphite particles, 88% of the graphite particles have a sphericity ≥ 0.65.

[0213] (3) Testing of powder OI value: Following the testing method in JIS K 0131-1996, a Bruker D8 Discover X-ray diffractometer was used for testing. A copper target was used as the anode target, and CuKα rays were used as the radiation source. The X-ray wavelength scan ranged from 20° to 80° at a scanning rate of 4° / min to obtain the X-ray diffraction pattern of the powder sample. Based on the X-ray diffraction pattern, I004 and I110 were calculated, where I004 is the integrated area of ​​the diffraction peak of the 004 crystal plane of crystalline carbon in the powder sample, and I110 is the integrated area of ​​the diffraction peak of the 110 crystal plane of crystalline carbon in the powder sample. The OI value of the powder sample was calculated according to OI value = I004 / I110.

[0214] Test results: The OI value of the graphite particles is 1.5.

[0215] (4) Volumetric particle size distribution: Referring to GB / T19077-2016, the graphite powder was tested using a laser particle size distribution diffraction instrument (Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd.), and the values ​​of Dv10, Dv50, and Dv90 were obtained, and the value of (Dv90-Dv10) / Dv50 was calculated.

[0216] Test results: The graphite particles have a Dv10 of 5.8 μm, a Dv50 of 16.0 μm, and a Dv90 of 24.3 μm. The calculated value of (Dv90-Dv10) / Dv50 is 1.17.

[0217] (5) Tap density: The density of the powder was determined using a powder tap density tester in accordance with GB / T 5162-2006. The testing instrument can be Dandong Baite BT-301, with the following parameters: vibration frequency 250±15 times / minute, amplitude 3±0.2mm, number of vibrations 5000, and a measuring cylinder of 25mL.

[0218] Test results: The tap density of the graphite particles is 1.35 g / cc.

[0219] 2. Negative electrode plate: Cut a negative electrode sheet to obtain a test sample with a dry weight of W1. Place the test sample horizontally and drop 50 μL of electrolyte onto the surface of the test sample using a dropper. Record the time it takes for the liquid to be completely absorbed. (When the liquid mirror disappears visually), obtain the weight W2 of the test sample. Calculate the liquid absorption rate using the Washburn equation with W2 and W1. The specific electrolyte formulation can be found in Example 1.

[0220] Test results: The liquid absorption rate of the negative electrode film with a single-sided compaction density of 1.60 g / cc was 1.84 mg / s. 0.5 .

[0221] 3. Rebound rate of the negative electrode after 600 cycles of full charge: The design thickness of the negative electrode sheet after cold pressing is denoted as h1. The electrode sheet thickness after the battery is fully charged (100% SOC) is denoted as h2. The rebound rate of the negative electrode sheet after full charge is denoted as (h2-h1) / h1. The electrode sheet thickness after the battery is fully charged after 600 cycles at 25℃ in the mode of 0.33C charging / 0.5C discharging is denoted as h3. The rebound rate of the negative electrode sheet after 600 cycles of full charge is denoted as (h3-h1) / h1.

[0222] Test results: The rebound rate of the negative electrode after 600 cycles of full charge is 20%.

[0223] Comparative Example 1 The differences between Comparative Example 1 and Example 1 are shown in Table 1, except that in the preparation of the negative electrode active material, the volatile matter content of petroleum coke is 10 wt%, and the Hay Group grindability coefficient is 65. Aggregate and asphalt were mixed at a mass ratio of 100:10 and granulated using a horizontal granulation reactor. The granulation reactor was heated from room temperature to 300°C in a nitrogen atmosphere and held at this temperature for 1 hour while stirring. The temperature was then further increased to 700°C and held at this temperature for 2 hours while stirring. The granulated material yielded the first raw material, with a Dv50 of D1 and a Dv50 of 18 μm. The first raw material was then shaped to obtain the second raw material, with a Dv50 of D2 and a Dv50 of 17 μm. The D2 / D1 ratio was 0.944.

[0224] Comparative Example 2 The differences between Comparative Example 2 and Example 1 are shown in Table 1, except that in the preparation of the negative electrode active material, the volatile matter of petroleum coke is 10 wt%, and the Hay Group grindability coefficient is 57. Aggregate and asphalt were mixed at a mass ratio of 100:10 and granulated using a horizontal granulation reactor. The granulation reactor was heated from room temperature to 300°C in a nitrogen atmosphere and held at this temperature for 1 hour while stirring. The temperature was then further increased to 700°C and held at this temperature for 2 hours while stirring. The granulated material yielded the first raw material, with a Dv50 of D1 and a Dv50 of 19 μm. The first raw material was then shaped to obtain the second raw material, with a Dv50 of D2 and a Dv50 of 17 μm. The D2 / D1 ratio was 0.895.

[0225] Comparative Examples 1 and 2 were tested as shown in Example 1, and the results are shown in Table 2.

[0226] Table 1

[0227] Table 2

[0228] As can be seen from Tables 1 and 2 above, Example 1, compared with Comparative Examples 1-2, can effectively reduce the rebound rate of the negative electrode sheet after 600 cycles and reduce the irreversible expansion during battery cycling.

[0229] In Comparative Examples 1 and 2, the average roundness was too small, resulting in a significantly larger rebound rate of the negative electrode sheet after 600 cycles compared to Example 1. This indicates that simply meeting the range requirements of this application for average sphericity is insufficient to effectively reduce the rebound rate of the negative electrode sheet after 600 cycles.

[0230] According to Comparative Examples 1 and 2, the average sphericity of Comparative Example 1 is less than that of Comparative Example 2. Although better sphericity results in better suppression of expansion, the rebound rate of Comparative Example 2 is actually greater than that of Comparative Example 1 because the average roundness of Comparative Example 1 is greater than that of Comparative Example 2. This indicates that roundness has a greater impact on the rebound rate of the electrode.

[0231] Example 2-14 The differences between each embodiment and Embodiment 1 are shown in Table 3. Table 3

[0232] The tests performed on each embodiment as shown in Embodiment 1 are shown in Table 4. Table 4

[0233] As can be seen from Examples 1-14, the average sphericity of the graphite particles is 0.65-0.95, and the average roundness is 0.42-0.64, which can reduce expansion throughout the battery's life cycle and reduce the rebound rate of the negative electrode sheet after 600 cycles. Optionally, if the average sphericity of the graphite particles is 0.71-0.85 and the average roundness is 0.42-0.55, expansion can be further reduced.

[0234] As can be seen from Examples 1-3, 6-8 and 12, even with very small differences in average sphericity, average roundness significantly affects the rebound rate of the negative electrode sheet after 600 cycles.

[0235] As can be seen from Examples 1 and 11, the introduction of antioxidants is beneficial to further improve the average sphericity and average roundness. Meanwhile, based on the graphite particles of Examples 1 and 11, the OI values ​​are the same. Since the average sphericity and average roundness of Example 11 are lower than those of Example 1, the rebound rate is increased, indicating that the key factors affecting the rebound rate are the average sphericity and average roundness.

[0236] As can be seen from Examples 1 and 13-14, regardless of whether the graphite particles are secondary or single particles, as long as the average sphericity of the graphite particles is 0.65-0.95 and the average roundness of the graphite particles is 0.42-0.65, the thickness change rate of the negative electrode sheet can be effectively reduced after 600 cycles, thus reducing the expansion during the battery cycle. It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, It includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one side of the negative current collector, the negative electrode film layer including a negative electrode active material; The negative electrode active material includes graphite particles, the graphite particles having an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65; Among them, the roundness of the graphite particles ri represents the radius of curvature of the protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

2. The secondary battery according to claim 1, characterized in that, The average sphericity of the graphite particles is 0.71-0.

85.

3. The secondary battery according to claim 2, characterized in that, The average roundness of the graphite particles is 0.42-0.

55.

4. The secondary battery according to claim 1, characterized in that, Of the plurality of graphite particles, at least 50% of the graphite particles have a roundness Y ≥ 0.38; And / or, of the plurality of graphite particles, at least 50% of the graphite particles have a sphericity ≥ 0.

65.

5. The secondary battery according to claim 1, characterized in that, The graphite particles include secondary particles, and the surface of the secondary particles has a carbon coating layer.

6. The secondary battery according to claim 5, characterized in that, The OI value of the secondary particles is 0.5-5; And / or, the tap density of the secondary particles is 1.25 g / cc - 1.45 g / cc.

7. The secondary battery according to claim 1, characterized in that, The graphite particles include single particles, and the tap density of the single particles is 1.30 g / cc to 1.50 g / cc.

8. The secondary battery according to claim 1, characterized in that, The graphite particles include single particles, and the OI value of the single particles is 2-8.

9. The secondary battery according to any one of claims 1-8, characterized in that, The volumetric particle size distribution Dv50 of the graphite particles is 4μm-20μm; And / or, the (Dv90-Dv10) / Dv50 of the graphite particles is 1.1-1.

5.

10. The secondary battery according to any one of claims 1-8, characterized in that, The liquid absorption rate of the negative electrode film layer with a compaction density of 1.60 g / cc is ≥1.8 mg / s. 0.5 .

11. A negative electrode active material, characterized in that, The negative electrode active material includes graphite particles, the graphite particles having an average sphericity of 0.65-0.95 and an average roundness of 0.42-0.65; Among them, the roundness of the graphite particles ri represents the radius of curvature of the surface protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

12. The negative electrode active material according to claim 11, characterized in that, The graphite particles include secondary particles; The secondary particles have a carbon coating layer on their surface; And / or, the tap density of the secondary particles is 1.25 g / cc - 1.45 g / cc; And / or, the graphite particles include secondary particles, the OI value of which is 0.5-5; And / or, the graphite particles include single particles, the tap density of which is 1.30 g / cc to 1.50 g / cc; And / or, the graphite particles include single particles, the single particles having an OI value of 2-8; And / or, of the plurality of graphite particles, at least 50% of the graphite particles have a sphericity ≥ 0.65; And / or, of the plurality of graphite particles, at least 50% of the graphite particles have a roundness Y ≥ 0.

38.

13. A method for preparing a negative electrode active material, characterized in that, include: The first raw material is deeply shaped to obtain the second raw material; wherein the first raw material includes a first aggregate and / or granules, the first aggregate is obtained by crushing and pre-shaping the raw coke, and the granules are obtained by crushing and pre-shaping the raw coke, mixing the obtained second aggregate with a binder, and granulating; the volatile matter of the raw coke is 10 wt%-18 wt%, and the Haas grindability coefficient is 60-150, the Dv50 of the first raw material is D1, the Dv50 of the second raw material is D2, and the D2 / D1 ratio is 0.68-0.90; The second raw material is pre-carbonized and graphitized to obtain graphite particles, which serve as the negative electrode active material. The negative electrode active material includes graphite particles, wherein the average sphericity of the graphite particles is 0.65-0.95 and the average roundness of the graphite particles is 0.42-0.

65. Among them, the roundness of the graphite particles ri represents the radius of curvature of the protrusion of the two-dimensional projection of the graphite particle, and R represents the radius of the circumcircle of the two-dimensional projection of the graphite particle.

14. The preparation method according to claim 13, characterized in that, The granulation process includes: heating to 290℃-310℃, maintaining the temperature for 50 min-70 min while stirring, and then heating to 690℃-710℃ while stirring for 100 min-140 min. And / or, the amount of binder added is 12%-20% of the mass of the second aggregate.

15. An electrical appliance, characterized in that, It includes the secondary battery according to any one of claims 1-10, the negative electrode active material according to any one of claims 11-12, or the negative electrode active material prepared by the preparation method according to any one of claims 13-14.

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