Negative electrode sheet, secondary battery, electric device, and negative electrode active material and preparation method therefor

Abstract

Provided in the present disclosure are a negative electrode sheet, a secondary battery, an electric device, and a negative electrode active material and a preparation method therefor. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a negative electrode active material. The negative electrode active material comprises first artificial graphite and second artificial graphite, wherein the first artificial graphite comprises primary particles, and the second artificial graphite comprises secondary particles. The powder OI value of the negative electrode active material is 2.5-6.5; and the powder compaction density of the negative electrode active material under a pressure of 50,000 N is 1.75-1.94 g / cc.

Description

Negative electrode sheet, secondary battery, electrical device, negative electrode active material and its preparation method

[0001] Cross-reference to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202411214813.0, filed on August 30, 2024, entitled “Negative Electrode Sheet, Secondary Battery, Electrical Device, Negative Electrode Active Material and Preparation Method Thereof”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of lithium battery technology, and in particular to a negative electrode sheet, a secondary battery, an electrical device, a negative electrode active material, and a method for preparing the same. Background Technology

[0004] In recent years, with the increasingly wide range of applications, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and many other fields. With the rapid development of secondary batteries, higher requirements have been placed on their energy density and kinetic performance.

[0005] Therefore, how to make secondary batteries have both high energy density and excellent kinetic performance has become an urgent problem to be solved in this field. Summary of the Invention

[0006] This disclosure is made in view of the above-mentioned issues, and its purpose is to provide a negative electrode sheet, a secondary battery, an electrical device, a negative electrode active material and a method for preparing the same, thereby enabling the secondary battery to have both high energy density and excellent kinetic performance.

[0007] To achieve the above objectives, a first aspect of this disclosure provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer comprises a negative electrode active material, wherein the negative electrode active material comprises a first artificial graphite and a second artificial graphite, the first artificial graphite comprising primary particles and the second artificial graphite comprising secondary particles, the powder OI value of the negative electrode active material being 2.5-6.5; and the powder compaction density of the negative electrode active material under a pressure of 50000N being 1.75 g / cc-1.94 g / cc. This disclosure, by employing a negative electrode active material comprising a first artificial graphite and a second artificial graphite, enables the secondary battery to achieve both excellent kinetic performance and high energy density.

[0008] In some embodiments, the OI value of the negative electrode active material powder is 2.8-6.3. This is beneficial for improving the kinetic performance of the secondary battery.

[0009] In some embodiments, the powder compaction density of the negative electrode active material at a pressure of 50,000 N is 1.79 g / cc to 1.85 g / cc. This is beneficial for improving the energy density of the secondary battery.

[0010] In some embodiments, the specific capacity of the negative electrode active material is 353 mAh / g to 359 mAh / g. Maintaining the specific capacity of the negative electrode active material within this range is beneficial for improving the energy density of the secondary battery.

[0011] In some embodiments, the specific capacity of the negative electrode active material is 354.6 mAh / g-356.8 mAh / g.

[0012] In some embodiments, the degree of graphitization of the negative electrode active material is 93.5%-95.5%. By keeping the degree of graphitization of the negative electrode active material within the above range, it is beneficial to improve the energy density of the negative electrode active material.

[0013] In some embodiments, the degree of graphitization of the negative electrode active material is 93.7%-95.3%.

[0014] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.3; and / or, the volume distribution particle size Dv50 of the negative electrode active material is 10.0 μm-15.5 μm. This is beneficial for the negative electrode film to have a suitable pore distribution, thereby improving the kinetic performance of the secondary battery.

[0015] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.25; and / or, the volume distribution particle size Dv50 of the negative electrode active material is 11.0 μm-14.5 μm. This is beneficial for further improving the kinetic performance of the secondary battery.

[0016] In some embodiments, the volume distribution of particles with a diameter greater than 6.5 μm and less than 22.5 μm in the negative electrode active material is greater than or equal to 75%. Maintaining the volume distribution of the negative electrode active material within this range is beneficial for improving the cycle performance of the secondary battery. In some embodiments, the volume distribution of particles with a diameter greater than 6.5 μm and less than 22.5 μm in the negative electrode active material is 75%-90%.

[0017] In some embodiments, the mass ratio of the first artificial graphite to the second artificial graphite in the negative electrode active material is from 1:9 to 3:7. Maintaining the mass ratio of the first artificial graphite to the second artificial graphite within this range allows the negative electrode active material to achieve a balance between good kinetic performance, high energy density, and good cycle performance. In some embodiments, the mass ratio of the first artificial graphite to the second artificial graphite in the negative electrode active material is from 1:9 to 1:3.

[0018] In some embodiments, the volumetric particle size distribution (Dv50) of the first artificial graphite is 7 μm-9 μm, and the volumetric particle size distribution (Dv1) is 3 μm-5.5 μm. By ensuring that the volumetric particle size distribution of the first artificial graphite is within the above range, it is beneficial to improve the kinetic performance of the secondary battery and increase its energy density.

[0019] In some embodiments, the OI value of the first artificial graphite powder is 7-10. When the OI of the first artificial graphite powder is within the above range, it is beneficial to improve the kinetic performance of the secondary battery.

[0020] In some embodiments, the specific capacity of the first artificial graphite is 354.1 mAh / g to 359.6 mAh / g. Maintaining the specific capacity of the first artificial graphite within this range is beneficial for improving the energy density of the secondary battery.

[0021] In some embodiments, the compacted density of the first artificial graphite powder under a pressure of 50,000 N is 1.72 g / cc to 1.92 g / cc. Maintaining the compacted density of the first artificial graphite powder within this range is beneficial for improving the energy density of the secondary battery.

[0022] In some embodiments, the volumetric particle size distribution (Dv50) of the second artificial graphite is 13 μm-18 μm. This is beneficial to the kinetic performance of the secondary battery.

[0023] In some embodiments, the OI value of the second artificial graphite powder is 2-5. By keeping the OI value of the second artificial graphite powder within the above range, it is beneficial to improve the kinetic performance of the secondary battery.

[0024] In some embodiments, the specific capacity of the second artificial graphite is 353.0 mAh / g to 358.2 mAh / g. Maintaining the specific capacity of the second artificial graphite within this range is beneficial for improving the energy density of the secondary battery.

[0025] In some embodiments, the powder compaction density of the second artificial graphite at a pressure of 50,000 N is 1.75 g / cc to 1.95 g / cc. By keeping the powder compaction density of the second artificial graphite within the above range, it is beneficial to increase the compaction density of the negative electrode film layer, thereby improving the energy density of the secondary battery.

[0026] The second aspect of this disclosure also provides a secondary battery, including the negative electrode provided in the first aspect of this disclosure. Therefore, the secondary battery of this disclosure exhibits excellent kinetic performance and high energy density.

[0027] A third aspect of this disclosure also provides an electrical device including the secondary battery provided in the second aspect of this disclosure. Since the electrical device of this disclosure includes the secondary battery provided in this disclosure, it has at least the same advantages as the secondary battery.

[0028] The fourth aspect of this disclosure also provides a negative electrode active material, comprising a first artificial graphite and a second artificial graphite. The first artificial graphite comprises primary particles, and the second artificial graphite comprises secondary particles. The powder OI value of the negative electrode active material is 2.5-6.5; the powder compaction density of the negative electrode active material under 50000N pressure is 1.75 g / cc-1.94 g / cc. This disclosure, by employing a negative electrode active material comprising both first and second artificial graphite, enables the negative electrode film to achieve both excellent kinetic performance and high energy density.

[0029] In some embodiments, the degree of graphitization of the negative electrode active material is 93.5%-95.5%. By keeping the degree of graphitization of the negative electrode active material within the above range, it is beneficial to improve the energy density of the negative electrode active material.

[0030] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.3. Maintaining the particle size distribution of the negative electrode active material within this range facilitates a suitable pore distribution in the negative electrode film, thereby improving the kinetic performance of the secondary battery. In some embodiments, the volumetric particle size distribution Dv50 of the negative electrode active material is 10.0 μm-15.5 μm. Maintaining the volumetric particle size distribution Dv50 of the negative electrode active material within this range also facilitates improved kinetic performance of the secondary battery.

[0031] The fifth aspect of this disclosure also provides a method for preparing a negative electrode active material, comprising the following steps:

[0032] The first artificial graphite preparation steps are as follows: the first raw material with a sulfur content of less than or equal to 1.5% by mass is crushed, shaped, graded and screened, and then graphitized at 3000℃-3200℃ to make primary particles, which are used as the first artificial graphite.

[0033] The second artificial graphite preparation steps are as follows: the second raw material with a sulfur content of less than or equal to 1.5% by mass is crushed and shaped to obtain primary particles, then a binder is added for granulation, and graphitization is carried out at 3000℃-3200℃ to produce secondary particles, which are used as the second artificial graphite.

[0034] Mixing step: The first artificial graphite and the second artificial graphite are mixed to obtain the negative electrode active material; wherein, the powder OI value of the negative electrode active material is 2.5-6.5, and the powder compaction density of the negative electrode active material under 50000N pressure is 1.75g / cc-1.94g / cc.

[0035] The negative electrode active material prepared by the method disclosed herein is beneficial to improving the kinetic performance of secondary batteries and increasing their energy density.

[0036] In some embodiments, during the first artificial graphite preparation step, a first graphitization process is performed to produce primary particles with a volume distribution particle size Dv50 of 7 μm-9 μm and Dv1 of 3 μm-5.5 μm, which serve as the first artificial graphite. This is beneficial for improving the kinetic performance of the secondary battery and simultaneously increasing its energy density.

[0037] In some embodiments, in the preparation step of the second artificial graphite, the second raw material is crushed and shaped to obtain primary particles with a volume distribution particle size Dv50 of 7μm-9μm. Then, a binder is added for granulation, and secondary graphitization is performed at 3000℃-3200℃ to produce secondary particles with a (Dv90-Dv10) / Dv50 ratio of 1.0-1.25 and a Dv50 of 13μm-18μm, thus obtaining the second artificial graphite. This is beneficial for improving the kinetic performance of secondary batteries.

[0038] In some implementations, the first raw material and the second raw material may be the same or different.

[0039] In some embodiments, the mass percentage of sulfur in the first raw material and / or the second raw material is ≤0.6%, and the mass percentage of volatile components is ≤7%. By keeping the sulfur content and volatile components of the raw materials within the above ranges, it is beneficial to increase the graphitization degree of the negative electrode active material, thereby improving the energy density of the secondary battery.

[0040] In some embodiments, during the second artificial graphite preparation step, the softening point of the binder is ≥150°C; in other embodiments, the softening point of the binder is 150°C-280°C. Using a binder with a softening point within the above range is beneficial for improving the specific capacity of the negative electrode active material and increasing the energy density of the secondary battery.

[0041] In some embodiments, during the second artificial graphite preparation step, the amount of binder added is 8%-14% relative to the mass of the primary particles. By keeping the amount of binder added within the above range, it is beneficial to achieve better adhesion and improve the isotropy of the secondary particles.

[0042] In some embodiments, the ratio of the first artificial graphite to the second artificial graphite in the mixing step is from 1:9 to 3:7. By keeping the ratio of the first artificial graphite to the second artificial graphite within the above range, it is beneficial for the negative electrode active material to achieve both good kinetic performance, high energy density, and good cycle performance. Attached Figure Description

[0043] Figure 1 is a schematic diagram of a battery cell according to an embodiment of the present disclosure.

[0044] Figure 2 is an exploded view of a battery cell according to an embodiment of the present disclosure shown in Figure 1.

[0045] Figure 3 is a schematic diagram of a battery module according to one embodiment of the present disclosure.

[0046] Figure 4 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0047] Figure 5 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 4.

[0048] Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0049] Figure 7 is a scanning electron microscope image of the negative electrode active material of Example 1 of this disclosure.

[0050] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 6 Primary battery cells; 7 Secondary battery cells Detailed Implementation

[0051] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the negative electrode sheet, secondary battery, electrical device, negative electrode active material, and preparation method thereof. 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 a full understanding of this disclosure by those skilled in the art and are not intended to limit the subject matter of the claims.

[0052] The "range" disclosed in this disclosure is defined by a lower limit and an upper limit, whereby a given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the 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 expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​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 disclosure, 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.

[0053] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0054] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0055] Unless otherwise specified, all steps of this disclosure may be performed sequentially or randomly, preferably sequentially. For example, if a 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 it is mentioned that 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.

[0056] Unless otherwise specified, the terminology used in this disclosure has the common meaning as commonly understood by those skilled in the art.

[0057] Unless otherwise specified, the values ​​of the parameters mentioned in this disclosure can be determined using various test methods commonly used in the art, for example, according to the test methods given in this disclosure.

[0058] Unless otherwise specified, in this disclosure, the term "active ion" refers to ions that can be inserted and extracted back and forth between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions.

[0059] Currently, to obtain rechargeable batteries with high kinetics, some reports have proposed surface coating of graphite materials. However, surface coating reduces the specific capacity and electrode compaction density, affecting the energy density of the rechargeable battery. Additionally, some reports have suggested using hybrid silicon materials (including nano-silicon, silicon suboxide, and silicon-carbon composites) as the negative electrode active material to improve kinetics. However, silicon materials have a large volume expansion rate, which easily leads to the pulverization of the negative electrode active material particles, deteriorating the performance of the rechargeable battery. Therefore, current rechargeable batteries struggle to simultaneously achieve high energy density and good kinetic performance.

[0060] In view of the above, this disclosure provides a negative electrode sheet, a secondary battery, an electrical device, a negative electrode active material, and a method for preparing the same, thereby enabling the secondary battery to achieve both high energy density and excellent kinetic performance.

[0061] Negative electrode sheet

[0062] This disclosure provides a negative electrode sheet, including a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer includes a negative electrode active material, which includes a first artificial graphite and a second artificial graphite. The first artificial graphite includes primary particles, and the second artificial graphite includes secondary particles. The powder OI value of the negative electrode active material is 2.5-6.5; the powder compaction density of the negative electrode active material under a pressure of 50000N is 1.75g / cc-1.94g / cc.

[0063] In this disclosure, the OI value of the negative electrode active material powder is 2.5-6.5, which is beneficial to improving the isotropy of the negative electrode active material. Active ions can be inserted and extracted from various directions, thereby increasing the number of insertion and extraction channels for active ions in the negative electrode active material particles, which is beneficial to improving the kinetic performance of the secondary battery. In addition, the powder compaction density of the negative electrode active material at 50000N pressure is 1.75 g / cc-1.94 g / cc, which is beneficial to improving the compaction density of the negative electrode film and increasing the energy density of the secondary battery.

[0064] This disclosure utilizes a negative electrode active material comprising a first artificial graphite and a second artificial graphite, which facilitates the adjustment of the powder OI value and the powder compaction density under 50,000 N pressure of the negative electrode active material, thereby enabling the negative electrode film to achieve both excellent kinetic performance and high energy density.

[0065] In this disclosure, the negative electrode active material includes a first artificial graphite and a second artificial graphite, wherein the first artificial graphite comprises primary particles and the second artificial graphite comprises secondary particles. By including primary particles of the first artificial graphite and secondary particles of the second artificial graphite in the negative electrode active material, it is advantageous to achieve both excellent kinetic performance and high energy density in the negative electrode active material.

[0066] In some embodiments, the first artificial graphite is predominantly composed of primary particles; for example, more than 80% of the first artificial graphite particles are primary particles. Exemplarily, 80%, 85%, or 90% of the first artificial graphite particles are primary particles. The second artificial graphite is predominantly composed of secondary particles; for example, more than 80% of the second artificial graphite particles are secondary particles. Exemplarily, 80%, 85%, or 90% of the second artificial graphite particles are secondary particles.

[0067] In some embodiments, the OI value of the negative electrode active material powder can be, for example, a value between 2.5, 2.6, 2.7, 3.0, 3.2, 3.5, 3.7, 4.0, 4.2, 4.5, 4.7, 5.0, 5.2, 5.5, 5.7, 6.0, 6.2, 6.5, or any two of these values. In some alternative embodiments, the OI value of the negative electrode active material powder is 2.8-6.3.

[0068] In some embodiments, the compacted powder density of the negative electrode active material at 50,000 N pressure can be, for example, 1.75 g / cc, 1.76 g / cc, 1.77 g / cc, 1.78 g / cc, 1.79 g / cc, 1.80 g / cc, 1.81 g / cc, 1.83 g / cc, 1.85 g / cc, 1.87 g / cc, 1.89 g / cc, 1.90 g / cc, 1.92 g / cc, 1.94 g / cc, or a value within a range of any two of these values. In some alternative embodiments, the compacted powder density of the negative electrode active material at 50,000 N pressure is 1.79 g / cc to 1.85 g / cc.

[0069] In some embodiments, the specific capacity of the negative electrode active material is 353 mAh / g to 359 mAh / g, for example, it can be 353 mAh / g, 354 mAh / g, 355 mAh / g, 356 mAh / g, 357 mAh / g, 358 mAh / g, 359 mAh / g, or a value within a range of any two of these values. By keeping the specific capacity of the negative electrode active material within the above range, it is beneficial to improve the energy density of the secondary battery. In some optional embodiments, the specific capacity of the negative electrode active material is 354.6 mAh / g to 356.8 mAh / g.

[0070] In some embodiments, the degree of graphitization of the negative electrode active material is 93.5%-95.5%. By keeping the degree of graphitization of the negative electrode active material within this range, it is beneficial for the negative electrode active material to have a high compaction density and specific capacity, thereby improving the energy density of the negative electrode active material. Exemplarily, the degree of graphitization of the negative electrode active material can be 93.5%, 93.6%, 93.8%, 94.0%, 94.2%, 94.5%, 94.8%, 95.0%, 95.2%, 95.5%, or a value within a range of any two of these values. In some alternative embodiments, the degree of graphitization of the negative electrode active material is 93.7%-95.3%.

[0071] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.3, for example, it can be 0.95, 0.96, 0.97, 0.98, 0.99, 0.10, 0.11, 0.12, 0.13, or any two values ​​thereof. By keeping the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material within the above range, its particle uniformity is better, which is beneficial for the negative electrode film to have a suitable pore distribution, thereby improving the kinetic performance of the secondary battery. In some optional embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.25.

[0072] In some embodiments, the volumetric particle size distribution Dv50 of the negative electrode active material is 10.0-15.5 μm, optionally 10.5-15.0 μm, and further optionally 11.0-15.0 μm. By keeping the volumetric particle size distribution Dv50 of the negative electrode active material within the above range, it is beneficial to improve the transport performance of active ions and electrons, thereby further improving the kinetic performance of the secondary battery. Exemplarily, the volumetric particle size distribution Dv50 of the negative electrode active material can be 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12.0 μm, 12.5 μm, 13.0 μm, 13.5 μm, 14.0 μm, 14.5 μm, 15.0 μm, 15.5 μm, or a value within a range consisting of any two of these values. In some optional embodiments, the volumetric particle size distribution Dv50 of the negative electrode active material is 11.0 μm-14.5 μm.

[0073] In some embodiments, the volume distribution of particles with a diameter greater than 6.5 μm and less than 22.5 μm in the negative electrode active material is greater than or equal to 75%. By ensuring that the volume distribution of the negative electrode active material is within the above range, the particle size distribution of the negative electrode active material is uniform, which is beneficial to improving the uniformity of lithium intercalation in the negative electrode film, thereby improving the cycle performance of the secondary battery. Exemplarily, the volume distribution of particles with a diameter greater than 6.5 μm and less than 22.5 μm in the negative electrode active material can be 75%, 76%, 77%, 78%, 80%, 82%, 84%, 86%, 88%, 89%, 90%, 91%, 92%, or a value within a range of any two of these values. In some optional embodiments, the volume distribution of particles with a diameter greater than 6.5 μm and less than 22.5 μm in the negative electrode active material is 75%-90%, optionally 78%-88%.

[0074] In some embodiments, the mass ratio of the first artificial graphite to the second artificial graphite in the negative electrode active material is from 1:9 to 3:7. Maintaining the mass ratio of the first artificial graphite to the second artificial graphite within this range facilitates the adjustment of the powder OI value and powder compaction density of the negative electrode active material, thereby enabling the negative electrode active material to achieve a balance between good kinetic performance, high energy density, and good cycle performance. Exemplarily, the mass ratio of the first artificial graphite to the second artificial graphite can be 3:7, 1:3, 1:4, 5:17, 1:9, or a value within a range of any two of these ratios. In some optional embodiments, the mass ratio of the first artificial graphite to the second artificial graphite is from 1:9 to 1:3. In one optional embodiment, the mass ratio of the first artificial graphite to the second artificial graphite is 1:3.

[0075] In some embodiments, the volumetric particle size distribution (Dv50) of the first artificial graphite is 7.0 μm-9.0 μm, and the Dv1 is 3 μm-5.5 μm. By setting the Dv50 of the first artificial graphite to 7.0 μm-9.0 μm, the smaller particle size helps reduce the distance of the solid-phase diffusion path of active ions in the first artificial graphite, increasing the insertion / extraction rate of active ions and thus improving the kinetic performance of the secondary battery. Setting the Dv1 of the first artificial graphite to 3 μm-5.5 μm helps increase the number of active ions inserted into the first artificial graphite, thereby increasing the specific capacity of the negative electrode active material and improving the energy density of the secondary battery. Furthermore, the first artificial graphite helps improve the stability and processing performance of the negative electrode active material, increasing the proportion of negative electrode active material in the negative electrode film layer, thereby further improving the energy density of the negative electrode film layer. For example, the volume distribution particle size Dv50 of the first artificial graphite is 7 μm, 7.5 μm, 8 μm, 8.5 μm, and 9.0 μm, and the Dv1 is 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, and 5.5 μm.

[0076] In some embodiments, the OI value of the first artificial graphite powder is 7-10, for example, it can be a value between 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10, or any two of these values. When the OI of the first artificial graphite powder is within the above range, it is beneficial to improve the kinetic performance of the secondary battery.

[0077] In some embodiments, the specific capacity of the first artificial graphite is 354.1 mAh / g to 359.6 mAh / g, optionally 354.5 mAh / g to 359.0 mAh / g. By keeping the specific capacity of the first artificial graphite within the above range, it is beneficial to improve the energy density of the secondary battery. Exemplarily, the specific capacity of the first artificial graphite can be 354.1 mAh / g, 354.5 mAh / g, 355.5 mAh / g, 356.0 mAh / g, 356.5 mAh / g, 357.0 mAh / g, 357.5 mAh / g, 358.5 mAh / g, 359.0 mAh / g, 359.6 mAh / g, or a value within a range consisting of any two of these values.

[0078] In some embodiments, the compaction density of the first artificial graphite powder at a pressure of 50,000 N is 1.72 g / cc to 1.92 g / cc, for example, it can be a value between 1.72 g / cc, 1.75 g / cc, 1.76 g / cc, 1.77 g / cc, 1.78 g / cc, 1.79 g / cc, 1.80 g / cc, 1.81 g / cc, 1.83 g / cc, 1.85 g / cc, 1.87 g / cc, 1.89 g / cc, 1.90 g / cc, 1.92 g / cc, or any two of these values. By ensuring the compaction density of the first artificial graphite powder is within the above range, it is beneficial to increase the compaction density of the negative electrode film layer and improve the energy density of the secondary battery.

[0079] In some embodiments, the volume distribution particle size Dv50 of the second artificial graphite is 13.0 μm-18.0 μm, optionally 13.5 μm-17.0 μm. By ensuring the volume distribution particle size of the second artificial graphite is within the aforementioned range, it is beneficial to improve the isotropy of the negative electrode active material, increase the number of insertion / extraction channels for active ions in the negative electrode active material, and improve the transport efficiency of active ions and electrons, thereby enhancing the kinetic performance of the secondary battery. Exemplarily, the volume distribution particle size Dv50 of the second artificial graphite can be 13.0 μm, 13.5 μm, 14.0 μm, 14.5 μm, 15.0 μm, 15.5 μm, 16.0 μm, 16.5 μm, 17.0 μm, 17.5 μm, 18.0 μm, or a value within a range consisting of any two of these values.

[0080] In some embodiments, the OI value of the second artificial graphite powder is 2-5, for example, it can be 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, or any two of these values. By ensuring that the OI value of the second artificial graphite powder is within the above range, it is beneficial to improve the isotropy of the secondary particles, increase the number of active ion insertion / extraction channels, and increase the active sites of the negative electrode active material, thereby improving the kinetic performance of the secondary battery.

[0081] In some embodiments, the specific capacity of the second artificial graphite is 353.0 mAh / g to 358.2 mAh / g, optionally 353.0 mAh / g to 358.0 mAh / g. By keeping the specific capacity of the second artificial graphite within the above range, it is beneficial to increase the specific capacity of the negative electrode active material, thereby increasing the energy density of the secondary battery. Exemplarily, the specific capacity of the second artificial graphite can be 353.0 mAh / g, 353.5 mAh / g, 354.0 mAh / g, 354.5 mAh / g, 355.0 mAh / g, 355.5 mAh / g, 356.0 mAh / g, 356.5 mAh / g, 357.0 mAh / g, 357.5 mAh / g, 358.0 mAh / g, 358.2 mAh / g, or a value within a range consisting of any two of these values.

[0082] In some embodiments, the powder compaction density of the second artificial graphite at a pressure of 50,000 N is 1.75 g / cc to 1.95 g / cc, for example, it can be a value between 1.75 g / cc, 1.76 g / cc, 1.77 g / cc, 1.78 g / cc, 1.79 g / cc, 1.80 g / cc, 1.81 g / cc, 1.83 g / cc, 1.85 g / cc, 1.87 g / cc, 1.89 g / cc, 1.90 g / cc, 1.92 g / cc, 1.95 g / cc, or any two of these values. By ensuring that the powder compaction density of the second artificial graphite is within the above range, it is beneficial to increase the compaction density of the negative electrode film layer, thereby improving the energy density of the secondary battery.

[0083] In this disclosure, by simultaneously including the aforementioned smaller-diameter first artificial graphite and the aforementioned highly isotropic second artificial graphite in the negative electrode active material, the negative electrode active material can exhibit high capacity, high compaction density, and excellent kinetic performance, resulting in a secondary battery with high energy density and excellent kinetic performance. Furthermore, the smaller-diameter first artificial graphite can fill the gaps between the secondary particles of the second artificial graphite in the negative electrode active material, serving to support and disperse the secondary particles, improving the structural stability and processing performance of the negative electrode active material, reducing the amount of dispersant and suspending agent in the negative electrode film, increasing the active sites of the negative electrode active material, thereby improving the kinetic performance of the secondary battery and increasing the proportion of the negative electrode active material in the negative electrode film, thus increasing the energy density of the secondary battery.

[0084] In this disclosure, the OI value of a material (e.g., anode active material, first artificial graphite, second artificial graphite, etc.) refers to the graphite orientation degree of the material, describing the uniformity of crystal orientation within the graphite material particles. It can be tested 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. The testing can be performed according to JIS K 0131-1996 and JB / T 4220-2011, obtaining the X-ray diffraction pattern of the powder sample. Based on the formula OI value = I... 004 / I 110 The OI value of the powder in the sample was calculated. 004 I is the integrated area of ​​the diffraction peak of the crystalline carbon 004 crystal plane in the powder sample. 110 This represents the integrated area of ​​the diffraction peaks on the crystalline carbon-110 plane in the powder sample. In the X-ray diffraction analysis of this disclosure, a copper target can be used as the anode target, with CuKα rays as the radiation source, and the ray wavelength... The scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min.

[0085] In this disclosure, the powder compaction density of materials (negative electrode active materials, first artificial graphite, second artificial graphite, etc.) is the mass per unit volume of the powder under specified conditions. The powder compaction density can be determined using instruments and methods known in the art. For example, it can be determined using an electronic pressure testing machine (e.g., a UTM7305 type electronic pressure testing machine) according to GB / T 24533-2009. An exemplary test method is as follows: Weigh 1g of sample powder and add it to a container with a bottom area of ​​1.327cm². 2 In the mold, the pressure is increased to 50,000 N, held for 30 seconds, then the pressure is released and held for 10 seconds. The compaction density of the powder under 50,000 N pressure is then recorded and calculated.

[0086] In this disclosure, the volume distribution particle sizes Dv1, Dv10, Dv50, and Dv90 of materials (e.g., negative electrode active materials, first artificial graphite, second artificial graphite, etc.) represent the particle sizes corresponding to a cumulative volume distribution percentage of 1%, 10%, 50%, and 90%, respectively, and can be determined using instruments and methods known in the art. For example, they can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0087] In this disclosure, the volume distribution percentage of particles with a diameter greater than 6.5 μm and less than 22.5 μm can be determined using the following method: Particle size can be determined using instruments and methods known in the art. For example, it can be determined using a laser particle size analyzer, referring to GB / T19077-2016. The volume distribution percentage X corresponding to a particle size less than or equal to 6.5 μm and the volume distribution percentage Y corresponding to a particle size less than 22.5 μm are tested. The volume distribution percentage of particles with a diameter greater than 6.5 μm and less than 22.5 μm is calculated using the formula (YX)*100%.

[0088] In this disclosure, the specific capacity of the materials (negative electrode active material, first artificial graphite, second artificial graphite, etc.) is the ratio of the electrical capacity that the active material can release to the mass of the active material, and can be tested using methods known in the art. An exemplary test method is as follows: The sample powder is mixed evenly with a conductive agent, a binder, and optional other additives in a certain mass ratio with a solvent to form a slurry; the prepared slurry is coated on the surface of the negative electrode current collector copper foil and dried in an oven for later use; the electrolytic salt is dissolved in an organic solvent to prepare an electrolyte of a certain concentration; then, using a lithium metal sheet as the counter electrode and a polyethylene (PE) film as the separator, the CR2430 coin cell is assembled with the electrolyte in an argon-protected glove box. After the obtained coin cells were allowed to stand for 12 hours, they were discharged at 25°C with a constant current of 0.15 mA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 50 μA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 10 μA to 0.005 V. Then, they were charged with a constant current of 0.3 mA to 2.0 V, and the charging capacity was recorded. The ratio of charging capacity to sample mass is the specific capacity of the corresponding material (e.g., negative electrode active material, first artificial graphite, second artificial graphite, etc.).

[0089] In this disclosure, the degree of graphitization of materials (e.g., negative electrode active materials, first artificial graphite, second artificial graphite, etc.) and the proportion of carbon elements existing in the form of graphite structures can be tested 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 / T 4220-2011, to obtain the average interlayer spacing d of the C(002) crystal plane in the material's crystal structure. 002 Then, according to the formula g = (0.344 - d) 002 The degree of graphitization is calculated as d / (0.344-0.3354)×100%. In the above formula, d... 002 It is the average interlayer spacing of the C(002) crystal plane in the material crystal structure, expressed in nanometers (nm).

[0090] In this disclosure, primary particles and secondary particles have meanings known in the art. Primary particles refer to non-agglomerated particles. Secondary particles refer to agglomerated particles formed by the aggregation of two or more primary particles. Primary and secondary particles can be distinguished using scanning electron microscopy (SEM) images. An exemplary test method is as follows: The secondary battery is discharged at a current of 0.33C to the lower cutoff voltage, left to stand for 30 minutes, and then discharged at a current of 0.1C to the lower cutoff voltage. The secondary battery is then disassembled, the negative electrode sheet is removed, and the negative electrode sheet is cleaned with DMC to remove the lithium salt and dried. The cleaned negative electrode sheet is then immersed in NMP and ultrasonically treated to separate the copper foil, obtaining the negative electrode material. After drying the negative electrode material, it is calcined at 350℃-500℃, washed multiple times with water, and dried at 80℃ to obtain the negative electrode active material. The negative electrode active material was adhered to the conductive adhesive, and the particle morphology was tested using a scanning electron microscope. The primary particles were non-agglomerated particles, and the secondary particles were agglomerated particles formed by the aggregation of two or more primary particles.

[0091] Negative electrode active materials and their preparation methods

[0092] This disclosure also provides a negative electrode active material, which has the same characteristics as the negative electrode active materials included in the above-mentioned negative electrode sheet, and will not be described in detail here.

[0093] The method for preparing the negative electrode active material disclosed herein includes the following steps:

[0094] The first artificial graphite preparation steps are as follows: the first raw material with a sulfur content of less than or equal to 1.5% by mass is crushed, shaped, graded and screened, and then graphitized at 3000℃-3200℃ to produce primary particles, which are used as the first artificial graphite.

[0095] The second artificial graphite preparation steps are as follows: the second raw material with a sulfur content of less than or equal to 1.5% by mass is crushed and shaped to obtain primary particles, then a binder is added for granulation, and secondary graphitization is carried out at 3000℃-3200℃ to produce secondary particles, which are used as the second artificial graphite.

[0096] Mixing step: The first artificial graphite and the second artificial graphite are mixed to obtain the negative electrode active material; wherein, the powder OI value of the negative electrode active material is 2.5-6.5, and the powder compaction density of the negative electrode active material under 50000N pressure is 1.75g / cc-1.94g / cc.

[0097] In this disclosure, in the aforementioned first and / or second artificial graphite preparation steps, the first graphitization and / or second graphitization are performed at 3000°C-3200°C. Exemplarily, the first and / or second graphitization can be performed at temperatures between 3000°C, 3050°C, 3100°C, 3150°C, 3200°C, or any two of these values.

[0098] In this disclosure, the processing time for the first graphitization and / or the second graphitization can be 4-60 hours. For example, when the raw material is low-sulfur needle coke, the graphitization temperature is 3100°C, and the graphitization time can be 12 hours, 24 hours, or 36 hours.

[0099] By mixing first and second artificial graphite, the primary particles of the first artificial graphite have a smaller particle size and higher powder compaction density, while the secondary particles of the second artificial graphite have higher isotropy. This results in an OI value of 2.5-6.5 for the negative electrode active material and a powder compaction density of 1.75 g / cc-1.94 g / cc under 50,000 N pressure. This facilitates the utilization of the high capacity, high compaction density, and excellent kinetic performance of the negative electrode active material, leading to high energy density and excellent kinetic performance in the secondary battery. The smaller particle size of the first artificial graphite fills the gaps between the secondary particles of the second artificial graphite in the negative electrode active material, supporting and dispersing the secondary particles. This improves the structural stability and processing performance of the negative electrode active material, reduces the amount of dispersant and suspending agent in the negative electrode film, increases the active sites of the negative electrode active material, thereby enhancing the kinetic performance of the secondary battery and increasing the proportion of negative electrode active material in the negative electrode film, thus improving the energy density of the secondary battery.

[0100] In some embodiments, in the first artificial graphite preparation step, a first graphitization is performed to produce primary particles with a volume distribution particle size Dv50 of 7 μm-9 μm and Dv1 of 3 μm-5.5 μm, which serve as the first artificial graphite. In the first artificial graphite preparation step, a classifier is used to grade and prepare primary particles with a volume distribution particle size Dv50 of 7 μm-9 μm and Dv1 of 3 μm-5.5 μm. The smaller particle size of the primary particles helps to reduce the distance of the solid-phase diffusion path of active ions within the primary particles, increasing the intercalation / deintercalation rate of active ions and thus improving the kinetic performance of the secondary battery. Furthermore, making the volume distribution particle size Dv1 of the primary particles 3 μm-5.5 μm helps to increase the number of intercalated active ions in the primary particles, thereby increasing the specific capacity of the negative electrode active material and improving the energy density of the secondary battery.

[0101] In some embodiments, in the preparation step of the second artificial graphite, the second raw material is crushed and shaped to obtain primary particles with a volume distribution particle size Dv50 of 7μm-9μm. Then, a binder is added for granulation, and secondary graphitization is performed at 3000℃-3200℃ to produce secondary particles with a (Dv90-Dv10) / Dv50 ratio of 1.0-1.25 and a Dv50 of 13μm-18μm, thus obtaining the second artificial graphite. In the preparation step of the second artificial graphite, preparing secondary particles with a Dv50 of 13μm-18μm is beneficial to improving the isotropy of the negative electrode active material, increasing the number of active ion insertion / extraction channels in the negative electrode active material, and improving the transport efficiency of active ions and electrons, thereby enhancing the kinetic performance of the secondary battery.

[0102] In this disclosure, the first raw material and / or the second raw material can be any raw material known in the art that can be used to prepare artificial graphite, without particular limitation. Exemplarily, the first raw material and / or the second raw material may include one or more of petroleum coke, calcined petroleum coke, needle coke, calcined needle coke, pitch coke, and metallurgical coke; optionally, the first raw material and / or the second raw material may include one or more of petroleum coke, needle coke, and calcined needle coke. In one embodiment, the first raw material and / or the second raw material is needle coke.

[0103] In some embodiments, the sulfur content in the first and / or second raw materials is ≤0.6% by mass, and the volatile components are ≤7% by mass. Here, volatile components refer to volatile organic compounds, such as alkanes, aromatics, and lipids. Having the sulfur content and volatile components of the first and / or second raw materials within the above-mentioned ranges is beneficial for reducing impurities in the resulting first and second artificial graphite, increasing the graphitization degree of the negative electrode active material, and thus improving the energy density of the secondary battery.

[0104] In this disclosure, grading and screening can be performed using equipment known in the art, without particular limitation. In some embodiments, grading and screening can be performed using a classifier. Exemplarily, the classifier is selected from cyclone classifiers and airflow classifiers.

[0105] In this disclosure, the binder in the second artificial graphite preparation step can be any binder known in the art, without particular limitation. Exemplarily, the binder may include one or more of oil-based asphalt, coal-based pitch, polymers, etc.

[0106] In some embodiments, in the second artificial graphite preparation step, the softening point of the adhesive is ≥150°C. The softening point of the adhesive refers to the temperature at which the adhesive changes from a solid state to a softened state with a certain degree of fluidity; in this disclosure, the softening point can be tested using instruments and methods known in the art, for example, it can be determined with reference to GB / T4507-2014.

[0107] Using a binder with a softening point within the aforementioned range results in a high coking value, facilitates graphitization, and is beneficial for increasing the powder compaction density of the second artificial graphite, thereby improving the specific capacity of the negative electrode active material and enhancing the energy density of the secondary battery. In some optional embodiments, the softening point of the binder is 150°C-280°C. Exemplarily, the softening point of the binder can be a value between 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, or any two of these values.

[0108] In some embodiments, during the second artificial graphite preparation step, the binder mass percentage is 8%-14% relative to the mass of the primary particles. Maintaining the binder mass within this range facilitates better adhesion and improves the isotropy of the secondary particles. Exemplarily, the binder mass percentage in the secondary particles can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, or a value within a range of any two of these values.

[0109] In some embodiments, the ratio of the first artificial graphite to the second artificial graphite in the mixing step is from 1:9 to 3:7. Maintaining the mass ratio of the first artificial graphite to the second artificial graphite within this range facilitates the adjustment of the powder OI value and powder compaction density of the negative electrode active material, thereby enabling the negative electrode active material to achieve a balance between good kinetic performance, high energy density, and good cycle performance. Exemplarily, the mass ratio of the first artificial graphite to the second artificial graphite can be 3:7, 1:3, 1:4, 5:17, 1:9, or a value within a range of any two of these ratios. In some alternative embodiments, the mass ratio of the first artificial graphite to the second artificial graphite is from 1:9 to 1:3. In one alternative embodiment, the mass ratio of the first artificial graphite to the second artificial graphite is 1:3.

[0110] Secondary batteries

[0111] This disclosure also provides a secondary battery.

[0112] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0113] Typically, a single secondary battery cell includes 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.

[0114] In addition, the secondary battery and electrical device of this disclosure will be described below with appropriate reference to the accompanying drawings.

[0115] Negative electrode sheet

[0116] The secondary battery disclosed herein includes the negative electrode plate as described above.

[0117] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0118] 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.

[0119] 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.).

[0120] 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).

[0121] In some embodiments, the negative electrode film 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.

[0122] 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 the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0123] Positive electrode sheet

[0124] 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 the positive electrode active material of the first aspect of the present disclosure.

[0125] 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.

[0126] 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.).

[0127] In some embodiments, when the battery cell 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 at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this disclosure 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 / 3 Co 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 LiNi0.5 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) 0.85 Co 0.1 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.

[0128] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this disclosure, the molar Li content refers to the initial state of the material, i.e., the state before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0129] In the examples of positive electrode active materials in this disclosure, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0130] 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), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0131] In some embodiments, the positive electrode film 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.

[0132] 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.

[0133] electrolytes

[0134] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This disclosure does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0135] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] Separating membrane

[0140] In some embodiments, the battery cell also includes a separator. This disclosure does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0141] 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.

[0142] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0143] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0144] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0145] This disclosure does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery cell 5 as an example.

[0146] In some embodiments, referring to FIG2, 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 cover 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.

[0147] 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.

[0148] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0149] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0150] 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.

[0151] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, 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 box 2 and a lower box 3, with the upper box 2 covering the lower box 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.

[0152] Electrical appliances

[0153] In addition, this disclosure also provides an electrical device, which includes a secondary battery provided by this disclosure. The secondary battery can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0154] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0155] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. 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.

[0156] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0157] Example

[0158] The following describes embodiments of this disclosure. The embodiments described below are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure. 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 the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0159] Preparation of the first artificial graphite

[0160] Material 1-1

[0161] Step 1: The needle-shaped raw coke (sulfur content of 0.5% and volatile matter of 5%) is mechanically crushed and shaped, and then classified by an air classifier to remove small particles, thus obtaining the shaped material.

[0162] Step 2: The above-mentioned shaped material is graphitized in an Atchison graphitization furnace at a high temperature of 3100℃ for 36 hours; the graphitized particles are sieved and demagnetized to obtain primary particles with Dv50 of 7.0μm and Dv1 of 3.5μm as the first artificial graphite (material 1-1).

[0163] Materials 1-2 to 1-4 and Materials 1-1' to 1-3'

[0164] The preparation methods of materials 1-2 to 1-4 and materials 1-1' to 1-3' are similar to those of material 1-1, except that the types of raw materials and graphitization temperature are adjusted. The Dv1, Dv50, powder OI value, specific capacity and compacted density of materials 1-2 to 1-4 and materials 1-1' to 1-3' are shown in Table 1.

[0165] The above materials were tested according to the following methods, and the results are shown in Table 1.

[0166] Testing of particle size distribution in materials:

[0167] Dv50 test: The test was conducted using a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK, in accordance with GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0168] Dv1: Particle size distribution was tested using a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK, in accordance with GB / T 19077-2016 Laser Diffraction Method.

[0169] (Dv90-Dv10) / Dv50: Referring to GB / T 19077-2016 Particle size distribution laser diffraction method, the values ​​of Dv90, Dv50 and Dv10 were measured by a Mastersizer 3000 laser particle size analyzer of Malvern Instruments Ltd., UK, and the value of (Dv90-Dv10) / Dv50 was calculated.

[0170] Testing of the powder OI value of the material:

[0171] Following the test method for lattice parameters of artificial graphite in JB / T 4220-2011, 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 wavelength of the rays was... The scanning angle range was 20°–80°, and the scanning rate was 4° / min. The integrated area I of the diffraction peaks of the crystalline carbon 004 crystal plane in the material was obtained. 004 The integral area I of the diffraction peak of the 110 crystal plane of crystalline carbon in the material. 110 The OI of powder is calculated using the formula OI value = I 004 / I 110 To calculate.

[0172] Testing of material specific capacity:

[0173] The prepared materials, conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed uniformly with solvent N-methylpyrrolidone (NMP) at a mass ratio of 91.6:1.8:6.6 to form a slurry. The slurry was coated onto the surface of the negative electrode current collector copper foil and dried in an oven for later use. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in this organic solvent to prepare an electrolyte with a concentration of 1 mol / L. A CR2430 coin cell was assembled with a lithium metal sheet as the counter electrode and a polyethylene (PE) film as the separator, along with the electrolyte, in an argon-protected glove box. The resulting coin cell was allowed to stand for 12 hours.

[0174] At 25°C, the prepared coin cell was first discharged to 0.005V with a constant current of 0.15mA, allowed to stand for 10 minutes, and then discharged to 0.005V with a constant current of 50μA. After standing for 10 minutes, it was discharged to 0.005V with a constant current of 10μA. The first discharge capacity of the coin cell was recorded. Then, it was charged to 2.0V with a constant current of 0.3mA, and the first charge capacity of the coin cell was recorded. The ratio of the charge capacity to the mass of the material is the material's specific capacity.

[0175] Testing the compacted density of powder materials:

[0176] A certain amount of powder is placed in a compaction mold, and then the mold is placed on a compaction density instrument. The pressure is set to 50000N. The thickness of the powder under this pressure (the thickness after depressurization) can be read on the instrument. The compaction density is calculated by ρ = m / v.

[0177] Table 1

[0178] Preparation of the second artificial graphite

[0179] Material 2-1

[0180] Step 1: The needle-shaped raw coke (sulfur content 0.5%, volatile matter 5%) is mechanically crushed and shaped to obtain primary particles with a Dv50 of 8.0 μm.

[0181] Step 2: Using oily asphalt with a softening point of 180℃ as a binder, the above-mentioned primary particles are granulated together in a granulation reactor to obtain granulated material. The amount of binder added is 10% relative to the total weight of the primary particles.

[0182] Step 3: Graphitize the above granulated material at 3100℃ for 36 hours; sieve and demagnetize the graphitized particles to obtain secondary particles with a Dv50 of 14.5μm, which are used as the second artificial graphite (material 2-1).

[0183] Materials 2-2 to 2-3 and Materials 2-1' to 2-2'

[0184] The preparation methods of materials 2-2 to 2-3 and materials 2-1' to 2-2' are similar to those of material 2-1. The difference lies in that by adjusting the selection of raw materials, the crushed particle size of raw materials, the granulation particle size and the graphitization temperature, the Dv50, powder OI value, specific capacity and powder compaction density under 50000N pressure of materials 2-2 to 2-3 are obtained as shown in Table 2.

[0185] The particle size, OI value, specific gravity, and powder compaction density of the material were tested using the same method as the first artificial graphite described above, and the test results are recorded in Table 2.

[0186] Table 2

[0187] Example 1

[0188] (1) Preparation of negative electrode active materials

[0189] The first artificial graphite 1-1 and the second artificial graphite 2-1 are mixed at a mass ratio of 25:75 to obtain the negative electrode active material.

[0190] The particle size, OI value, specific capacity, and powder compaction density of the negative electrode active material were tested using the same method as the first artificial graphite described above.

[0191] The obtained negative electrode active material was imaged using a scanning electron microscope (SEM), as shown in Figure 7. Figure 7 shows that the negative electrode active material comprises primary particles 6 and secondary particles 7. The unaggregated monolithic graphite material is the primary particle, such as primary particle 6 marked in Figure 7; the particles formed by the aggregation of multiple monolithic graphite materials are the secondary particles, such as secondary particles 7 marked in Figure 7.

[0192] (2) Preparation of negative electrode sheet

[0193] The prepared negative electrode active material, conductive agent 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 97.4:1:0.7:0.9 to form a negative electrode slurry. The negative electrode slurry was coated onto the negative electrode current collector copper foil by extrusion coating. After drying and cold pressing, the negative electrode sheet was obtained.

[0194] (3) Preparation of positive electrode sheet

[0195] Lithium iron phosphate, conductive carbon black, and PVDF were mixed in a mass ratio of 96.2:2:1.8, and then N-methylpyrrolidone solvent was added and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was coated onto both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0196] (4) Preparation of lithium-ion batteries

[0197] A 12μm polyethylene film was used as the separator.

[0198] Ethylene 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 1 mol / L.

[0199] The positive and negative electrode sheets prepared above are placed in sequence, with the separator placed between the positive and negative electrode sheets to provide isolation. Then, the electrode assembly is wound up to obtain the electrode assembly. The electrode assembly is placed in the outer packaging, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping, a secondary battery is obtained.

[0200] Examples 2-9

[0201] The battery preparation methods in Examples 2-9 are similar to those in Example 1, except that, in the preparation steps of the negative electrode active material, the types of the first and second artificial graphite and the mass ratio of the first and second artificial graphite are adjusted as shown in Table 3.

[0202] Comparative Example 1

[0203] The battery preparation method of Comparative Example 1 is similar to that of Example 1, except that only the first artificial graphite is used.

[0204] Comparative Example 2

[0205] The battery preparation method of Comparative Example 2 is similar to that of Example 1, except that only the second artificial graphite is used as the negative electrode active material in the negative electrode preparation step. The inventors discovered that when using only the second artificial graphite as the negative electrode active material according to the method of Example 1, and preparing the negative electrode sheet with the negative electrode active material, conductive agent carbon black (Super P), thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber in a weight ratio of 97.4:1:0.7:0.9, the second artificial graphite agglomerates and cannot be fully dispersed to form a uniform negative electrode slurry, thus it cannot be used to prepare the negative electrode sheet. To obtain a uniform negative electrode slurry, the inventors adjusted the weight ratio of the negative electrode active material, conductive agent carbon black (Super P), thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber to 96.1:1:1.6:1.3.

[0206] Performance testing

[0207] (1) Energy density

[0208] At 25℃, the secondary battery was charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 1 / 3C to 2.5V. The battery discharge energy was recorded. The battery discharge energy divided by the battery weight is the battery's gravimetric energy density, expressed in Wh / kg. Detailed measurement data are shown in Table 3.

[0209] (2) Fast charging performance test of secondary batteries

[0210] At 25°C, the secondary battery was charged to 3.65V at a constant current of 0.33C, then charged to 0.05C at a constant voltage. After standing for 5 minutes, the secondary battery was discharged to 2.5V at a constant current of 0.33C, and its actual capacity was recorded as C0.

[0211] Then, the secondary battery was sequentially charged at constant current rates of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, and 3.0C0 until it reached 3.65V or 0V negative terminal cutoff potential (whichever comes first). After each charge, it was discharged to 2.5V at 1C0. The state of charge (SOC) was recorded at different charging rates until it reached 10%, 20%, 30%, ..., 80%. The negative electrode potential corresponding to the state of charge (SOC) is plotted, and the charge rate-negative electrode potential curves are obtained for different SOC states. After linear fitting, the charge rate corresponding to the negative electrode potential of 0V under different SOC states is obtained. This charge rate is the charging window for that SOC state, denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC, respectively. The charging time T (assuming no lithium plating in the secondary battery) from 10%SOC to 80%SOC is calculated using the following formula, in minutes. The shorter the charging time, the better the kinetic performance of the secondary battery.

[0212] T=(60 / C10%SOC+60 / C20%SOC+60 / C30%SOC+60 / C40%SOC+60 / C50%SOC+60 / C60%SOC+60 / C70%SOC+60 / C80%SOC)×10%

[0213] The performance of the secondary batteries prepared in Examples 1-9 and Comparative Examples 1-2 was tested, and the results are shown in Table 3.

[0214] Table 3

[0215] As can be seen from the data in Table 3, compared with the secondary batteries prepared in Comparative Examples 1 and 2, the secondary batteries prepared in Examples 1 to 6, with the negative electrode active material including first artificial graphite and second artificial graphite, show a significant increase in energy density and a significant reduction in charging time. The secondary batteries provided by the embodiments of this disclosure can achieve both high energy density and excellent kinetic performance. In Comparative Example 1, the powder OI of the negative electrode active material was too high, resulting in a decrease in battery kinetic performance. In Comparative Example 2, only second artificial graphite with secondary particles was used as the negative electrode active material. The second artificial graphite agglomerated and could not be fully dispersed. In order to form a uniform negative electrode slurry, the amount of dispersant was increased, which led to a decrease in the energy density of the secondary battery. In addition, due to the increased proportion of dispersant, more active sites on the surface of the negative electrode active material were occupied by the dispersant, which also had an adverse effect on the kinetic performance.

[0216] Examples 10-14

[0217] The battery preparation methods in Examples 10-14 are similar to those in Example 1, except that, in the preparation steps of the negative electrode active material, the types of the first and second artificial graphite and the mass ratio of the first and second artificial graphite are adjusted as shown in Table 4.

[0218] Table 4

[0219] As can be seen from the data in Table 4, compared with Comparative Examples 1 and 2 in Table 3, the secondary batteries prepared in Examples 10-14, in which the negative electrode active material includes primary particles of first artificial graphite and secondary particles of second artificial graphite, have improved energy density, shortened charging time, and can balance high energy density and excellent kinetic performance.

[0220] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. 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, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

A negative electrode sheet includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a negative electrode active material. The negative electrode active material includes a first type of artificial graphite and a second type of artificial graphite. The first artificial graphite comprises primary particles, and the second artificial graphite comprises secondary particles. The OI value of the powder of the negative electrode active material is 2.5-6.5; The powder compaction density of the negative electrode active material under 50,000 N pressure is 1.75 g / cc-1.94 g / cc. According to claim 1, the negative electrode sheet, wherein, The OI value of the negative electrode active material powder is 2.8-6.

3. The negative electrode sheet according to claim 1 or 2, wherein, The compacted density of the negative electrode active material under 50,000 N pressure is 1.79 g / cc to 1.85 g / cc. The negative electrode sheet according to any one of claims 1-3, wherein, The specific capacity of the negative electrode active material is 353mAh / g-359mAh / g. According to claim 4, the negative electrode sheet, wherein, The specific capacity of the negative electrode active material is 354.6 mAh / g-356.8 mAh / g. The negative electrode sheet according to any one of claims 1-5, wherein, The degree of graphitization of the negative electrode active material is 93.5%-95.5%. According to the negative electrode sheet of claim 6, wherein, The degree of graphitization of the negative electrode active material is 93.7%-95.3%. The negative electrode sheet according to any one of claims 1-7, wherein, The particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.3; and / or The volume distribution particle size Dv50 of the negative electrode active material is 10.0 μm-15.5 μm. According to claim 8, the negative electrode sheet, wherein, The particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.25; and / or The volume distribution particle size Dv50 of the negative electrode active material is 11.0 μm-14.5 μm. The negative electrode sheet according to any one of claims 1-9, wherein, In the negative electrode active material, the volume distribution ratio of particles with a particle size greater than 6.5 μm and less than 22.5 μm is greater than or equal to 75%. According to claim 10, the negative electrode sheet, wherein, In the negative electrode active material, the volume distribution ratio of particles with a particle size greater than 6.5 μm and less than 22.5 μm is 75%-90%. The negative electrode sheet according to any one of claims 1-11, wherein, In the negative electrode active material, the mass ratio of the first artificial graphite to the second artificial graphite is 1:9 to 3:

7. According to claim 12, the negative electrode sheet, wherein, In the negative electrode active material, the mass ratio of the first artificial graphite to the second artificial graphite is 1:9 to 1:

3. The negative electrode sheet according to any one of claims 1-13, wherein, The first artificial graphite has a volume distribution particle size Dv50 of 7μm-9μm and a Dv1 of 3μm-5.5μm. The negative electrode sheet according to any one of claims 1-14, wherein, The first artificial graphite satisfies at least one of the following characteristics: (1) The powder OI value of the first artificial graphite is 7-10; (2) The specific capacity of the first artificial graphite is 354.1 mAh / g-359.6 mAh / g; (3) The compaction density of the first artificial graphite powder under 50000N pressure is 1.72g / cc-1.92g / cc. The negative electrode sheet according to any one of claims 1-15, wherein, The volumetric particle size Dv50 of the second artificial graphite is 13μm-18μm. The negative electrode sheet according to any one of claims 1-16, wherein, The second artificial graphite satisfies at least one of the following characteristics: (1) The OI value of the second artificial graphite powder is 2-5; (2) The specific capacity of the second artificial graphite is 353.0 mAh / g-358.2 mAh / g; (3) The compaction density of the second artificial graphite powder under 50000N pressure is 1.75g / cc-1.95g / cc. A secondary battery comprising the negative electrode sheet according to any one of claims 1-17. An electrical device comprising the secondary battery as described in claim 18. A negative electrode active material, said negative electrode active material comprising a first artificial graphite and a second artificial graphite, The first artificial graphite comprises primary particles, and the second artificial graphite comprises secondary particles. The OI value of the negative electrode active material is 2.5-6.5; the compaction density of the negative electrode active material under 50000N pressure is 1.75g / cc-1.94g / cc. According to claim 20, the negative electrode active material, wherein, The degree of graphitization of the negative electrode active material is 93.5%-95.5%. The negative electrode active material according to claim 20 or 21, wherein, The particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 0.95-1.3; and / or The volume distribution particle size Dv50 of the negative electrode active material is 10.0 μm-15.5 μm. A method for preparing a negative electrode active material includes the following steps: The first artificial graphite preparation steps are as follows: the first raw material with a sulfur content of less than or equal to 1.5% by mass is crushed, shaped, graded and screened, and then graphitized at 3000℃-3200℃ to produce primary particles, which are used as the first artificial graphite. The second artificial graphite preparation steps are as follows: the second raw material with a sulfur content of less than or equal to 1.5% by mass is crushed and shaped to obtain primary particles, then a binder is added for granulation, and secondary graphitization is carried out at 3000℃-3200℃ to produce secondary particles, which are used as the second artificial graphite. Mixing step: The first artificial graphite and the second artificial graphite are mixed to obtain the negative electrode active material; wherein, the powder OI value of the negative electrode active material is 2.5-6.5, and the powder compaction density of the negative electrode active material under 50000N pressure is 1.75g / cc-1.94g / cc. According to the preparation method of claim 23, wherein, In the first artificial graphite preparation step, a first graphitization is performed to produce primary particles with a volume distribution particle size Dv50 of 7μm-9μm and Dv1 of 3μm-5.5μm, which are used as the first artificial graphite. The preparation method according to claim 23 or 24, wherein, In the preparation step of the second artificial graphite, the second raw material is crushed and shaped to obtain primary particles with a volume distribution particle size Dv50 of 7μm-9μm. Then, a binder is added for granulation, and secondary graphitization is carried out at 3000℃-3200℃ to produce secondary particles with (Dv90-Dv10) / Dv50 of 1.0-1.25 and Dv50 of 13μm-18μm, which are used as the second artificial graphite. The preparation method according to any one of claims 23-25, wherein, The mass percentage of sulfur in the first raw material and / or the second raw material is ≤0.6%, and the mass percentage of volatile components is ≤7%. The preparation method according to any one of claims 23-26, wherein, In the second artificial graphite preparation step, the softening point of the adhesive is ≥150℃. According to the preparation method of claim 27, wherein, In the second artificial graphite preparation step, the softening point of the adhesive is 150℃-280℃. The preparation method according to any one of claims 23-28, wherein, In the second artificial graphite preparation step, the amount of binder added is 8%-14% relative to the mass of the primary particles. The preparation method according to any one of claims 23-29, wherein, In the mixing step, the ratio of the first artificial graphite to the second artificial graphite is 1:9 to 3:7.

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