Secondary battery and electric apparatus

Abstract

The present disclosure provides a secondary battery and an electric apparatus. The secondary battery comprises a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on the surface of at least one side of the negative electrode current collector. The negative electrode film layer comprises a first negative electrode film layer and a second negative electrode film layer. The first negative electrode film layer is located between the negative electrode current collector and the second negative electrode film layer. The first negative electrode film layer comprises a first negative electrode active material, and the first negative electrode active material comprises a first silicon-based material. The second negative electrode film layer comprises a second negative electrode active material, and the second negative electrode active material comprises a second silicon-based material. The average particle size of the first silicon-based material is less than that of the second silicon-based material. The mass fraction of silicon element in the first negative electrode film layer is M1, the mass fraction of silicon element in the second negative electrode film layer is M2, and M1 and M2 satisfy the following relationship: M2-M1=16% to 45%.

Description

Secondary batteries and electrical devices

[0001] Cross-reference to related applications

[0002] This disclosure is based on and claims priority to Chinese patent applications No. 202411754608.3, filed on December 2, 2024, entitled "Secondary Battery and Electrical Device", and No. 202510330157.9, filed on March 19, 2025, entitled "Secondary Battery and Electrical Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0004] In recent years, with the increasingly wide application of rechargeable batteries, they 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 rechargeable batteries, higher requirements have been placed on their fast-charging performance and cycle life.

[0005] Therefore, improving the fast-charging performance and cycle performance of secondary batteries has become an urgent technical problem to be solved. Summary of the Invention

[0006] This disclosure is made in view of the above-mentioned problems, and its purpose is to provide a secondary battery and an electrical device, wherein the secondary battery of the present disclosure has excellent fast charging performance and cycle performance.

[0007] To achieve the above objectives, a first aspect of this disclosure provides a secondary battery, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one side surface of the negative current collector. The negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, with the first negative electrode film layer located between the negative current collector and the second negative electrode film layer. The first negative electrode film layer includes a first negative electrode active material, which includes a first silicon-based material. The second negative electrode film layer includes a second negative electrode active material, which includes a second silicon-based material. The number-average particle size of the first silicon-based material is smaller than the number-average particle size of the second silicon-based material. The mass percentage of silicon in the first negative electrode film layer is M1, and the mass percentage of silicon in the second negative electrode film layer is M2. M1 and M2 satisfy the following relationship: M2 - M1 = 16% to 45%. This is beneficial for reducing the polarization of the battery cell during charging, significantly improving the rate performance of the battery cell, and thus improving the fast-charging performance and cycle performance of the secondary battery.

[0008] In some implementations, M2-M1 is 18% to 35%. This is more conducive to reducing cell polarization during charging, significantly improving the cell's rate performance, and thus improving the cycle performance and fast charging performance of the secondary battery.

[0009] In some embodiments, the average particle size of the first silicon-based material in the first negative electrode film is 1 μm to 7 μm. This improves both the insufficient reaction rate at the interface between lithium ions and the first negative electrode film, thereby enhancing the fast-charging performance of the secondary battery, and reduces the rapid side reaction rate between the first silicon-based material and the electrolyte, which is beneficial for maintaining stable battery performance during long-term cycling.

[0010] In some embodiments, the average particle size of the second silicon-based material in the second negative electrode film is 7 μm to 20 μm. This allows for a relatively slower interfacial reaction rate, which is beneficial for maintaining stable battery performance during long-term cycling.

[0011] In some embodiments, the mass percentage (M1) of silicon in the first negative electrode film layer is 0.5% to 40%. This helps to reduce the polarization of the cell during charging, improve the rate performance of the cell, and thus enhance the fast-charging performance of the secondary battery.

[0012] In some embodiments, the mass percentage (M2) of silicon in the second negative electrode film is 18% to 60%. This helps to reduce the polarization of the cell during charging, improve the rate performance of the cell, and thus enhance the fast-charging performance of the secondary battery.

[0013] In some embodiments, the first negative electrode film layer further includes a first graphite material, and the second negative electrode film layer further includes a second graphite material, wherein the graphitization degree of the second graphite material is less than that of the first graphite material. This increases the number of lithium intercalation sites on the surface of the second graphite material, which is beneficial for improving the fast-charging performance of the secondary battery, and also improves the stability of the interface, thereby reducing the occurrence of side reactions during battery operation and improving the cycle stability and structural stability of the cell.

[0014] In some embodiments, the average particle size of the second graphite material is smaller than that of the first graphite material. This increases the number of lithium-intercalation active sites on the material surface, improves the lithium-ion diffusion rate, and thus enhances the fast-charging performance of the secondary battery.

[0015] In some embodiments, the average particle size of the first graphite material is 10 μm to 25 μm. In some embodiments, the specific surface area of ​​the first graphite material is 1.5 m². 2 / g~4.5m 2 / g. In some embodiments, the graphitization degree of the first graphite material is 92% to 97%. This is beneficial for improving the density of the surface structure of the first graphite material, enabling the battery to reduce the occurrence of side reactions during operation and improve the cycle stability and structural stability of the cell.

[0016] In some embodiments, the average particle size of the second graphite material is 8 μm to 20 μm. In some embodiments, the specific surface area of ​​the second graphite material is 2.0 m². 2 / g~5.0m 2 / g. In some embodiments, the graphitization degree of the second graphite material is 90% to 96%. This increases the number of lithium intercalation active sites on the surface of the second graphite material, thereby improving the fast-charging performance of the secondary battery.

[0017] In some embodiments, the mass percentage of the first graphite material in the first negative electrode film is greater than the mass percentage of the second graphite material in the second negative electrode film. In some embodiments, the mass percentage of the first graphite material in the first negative electrode film is 30% to 98%. This is beneficial for improving the cycle stability and structural stability of the battery cell, thereby helping to maintain stable performance of the battery throughout its entire life cycle. In some embodiments, the mass percentage of the second graphite material in the second negative electrode film is 10% to 80%. This is beneficial for improving the diffusion rate of lithium ions, thereby helping to improve the fast-charging performance of the secondary battery.

[0018] In some embodiments, the first silicon-based material and the second silicon-based material each independently include one or more of elemental silicon, silicon oxide, and silicon-carbon composites.

[0019] In some embodiments, the first silicon-based material and / or the second silicon-based material comprises a silicon-carbon composite, which satisfies one or more of the following characteristics:

[0020] (1) The silicon-carbon composite includes porous carbon and silicon-containing materials dispersed in the pores of the porous carbon; optionally, the porous carbon is hard carbon;

[0021] (2) The silicon-carbon composite also includes a carbon-containing coating layer located on the surface of porous carbon and / or silicon-containing materials;

[0022] (3) The silicon content in the silicon-carbon composite is 30% to 70% by mass;

[0023] (4) The powder resistivity of silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0024] (5) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g.

[0025] This is beneficial to improving the structural stability and specific capacity of silicon-carbon composites, thereby improving the energy density, lifespan, and fast-charging performance of secondary batteries.

[0026] In some embodiments, the first negative electrode film layer further includes a first binder, wherein the mass percentage of the first binder in the first negative electrode film layer is 1.0% to 3.0%, and / or, the second negative electrode film layer further includes a second binder, wherein the mass percentage of the second binder in the second negative electrode film layer is 1.5% to 4.0%. Thus, the binder ensures that the silicon-based materials can be tightly bound within the negative electrode film layer during battery charging and discharging, without losing electrical connections to form electrochemical islands that would cause the silicon-based materials to lose their electrochemical activity.

[0027] In some embodiments, the first negative electrode film layer further includes a first conductive agent, wherein the mass percentage of the first conductive agent in the first negative electrode film layer is 0.05% to 2%, and / or, the second negative electrode film layer further includes a second conductive agent, wherein the mass percentage of the second conductive agent in the second negative electrode film layer is 1.5% to 3.0%. Thus, the conductive agent ensures that the silicon-based material can be stably maintained in the conductive network during charging and discharging, reducing material loss due to the loss of electrical connection between silicon particles and graphite particles during significant contraction or expansion.

[0028] In some embodiments, the thickness of the first negative electrode film is 6 μm to 60 μm. This is beneficial for ensuring that the lithium ion transport distance is within a suitable range, thereby improving the energy density and rate performance of the secondary battery.

[0029] In some embodiments, the thickness of the second negative electrode film is 6 μm to 60 μm. This allows the lithium-ion transport distance to be within a suitable range, thereby improving the energy density and rate performance of the secondary battery.

[0030] In some embodiments, a buffer layer may be included between the negative electrode current collector and the first negative electrode film layer. The buffer layer includes a third binder and a third conductive agent. Thus, the buffer layer can enhance the adhesion between the first negative electrode film layer and the negative electrode current collector, and can also form a stable conductive network between the first negative electrode film layer and the negative electrode current collector.

[0031] In some embodiments, the third binder accounts for 10% to 70% of the mass of the buffer layer, and the third conductive agent accounts for 30% to 90% of the mass. This is beneficial for enhancing the adhesion between the first negative electrode film layer and the negative electrode current collector, and for forming a stable conductive network between the first negative electrode film layer and the negative electrode current collector.

[0032] In some embodiments, the thickness of the buffer layer is 0.2 μm to 1.5 μm. This helps to reduce the probability of the first negative electrode film layer detaching from the negative electrode current collector during battery charge-discharge cycles, and does not excessively affect the impedance between the cell and the negative electrode current collector, thus balancing the rate performance and cycle performance of the secondary battery.

[0033] A second aspect of this disclosure provides an electrical device including the secondary battery provided in the first aspect. Attached Figure Description

[0034] Figure 1 is a schematic diagram of the negative electrode sheet according to one embodiment of the present disclosure.

[0035] Figure 2 is a schematic diagram of a battery cell according to one embodiment of the present disclosure.

[0036] Figure 3 is an exploded view of a battery cell according to an embodiment of the present disclosure shown in Figure 2.

[0037] Figure 4 is a schematic diagram of a battery module according to one embodiment of the present disclosure.

[0038] Figure 5 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0039] Figure 6 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 5.

[0040] Figure 7 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0041] Explanation of reference numerals in the attached drawings: 10 Negative electrode; 101 Negative electrode current collector; 102 First negative electrode film layer; 103 Second negative electrode film layer; 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0042] Hereinafter, embodiments of the secondary battery and power-consuming device of this disclosure will be described in detail with appropriate reference to the accompanying drawings. 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 to enable those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter of the claims.

[0043] In this disclosure, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers from a to b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this document, and "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0044] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0045] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0046] 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.

[0047] Unless otherwise specified, the terminology used in this disclosure has the common meaning as commonly understood by those skilled in the art.

[0048] 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.

[0049] Currently, to improve battery energy density, silicon-based materials are typically added to the negative electrode. However, silicon-based materials have fewer lithium-ion insertion / extraction sites on their surface compared to graphite, resulting in a bottleneck in their overall rate performance. Furthermore, silicon-based materials react more strongly with the electrolyte than graphite, consuming more lithium ions and electrolyte, thus limiting their overall cycle life. As demands for faster charging and better cycle performance in rechargeable batteries increase, improvements are needed to overcome these limitations and enhance their fast-charging and cycle capabilities.

[0050] In existing technologies, improving the conductivity of the electrolyte enhances the diffusion ability of lithium ions within it, thereby improving the fast-charging performance of batteries. However, excessively high electrolyte conductivity significantly increases the reaction rate between the electrolyte and the anode and cathode, deteriorating the battery's cycle life. Furthermore, to improve cycle life, silicon-based materials are often coated, but thicker coatings increase interfacial impedance, further increasing the cell's DC resistance and worsening its fast-charging capability. Therefore, it is currently difficult to simultaneously achieve excellent fast-charging performance and long cycle life.

[0051] Based on this, the present disclosure provides a secondary battery and an electrical device, wherein the secondary battery of the present disclosure has excellent fast charging performance and cycle performance.

[0052] Secondary batteries

[0053] The first aspect of this disclosure provides a secondary battery, which includes a negative electrode sheet 10 as shown in FIG1. ​​The negative electrode sheet 10 includes a negative current collector 101 and a negative electrode film layer located on at least one side surface of the negative current collector. The negative electrode film layer includes a first negative electrode film layer 102 and a second negative electrode film layer 103. The first negative electrode film layer 102 is located between the negative current collector 101 and the second negative electrode film layer 103. The first negative electrode film layer 102 includes a first negative electrode active material, which includes a first silicon-based material. The second negative electrode film layer 103 includes a second negative electrode active material, which includes a second silicon-based material. The average particle size of the first silicon-based material is smaller than the average particle size of the second silicon-based material. The mass percentage of silicon in the first negative electrode film layer is M1, and the mass percentage of silicon in the second negative electrode film layer is M2. M1 and M2 satisfy the following relationship: M2-M1 = 16%~45%.

[0054] During battery charging, lithium ions gradually diffuse from the positive electrode to the negative electrode. The lithium ion concentration is relatively high in the second negative electrode film layer near the electrolyte, and relatively low in the first negative electrode film layer near the negative electrode current collector. In this disclosure, by keeping the difference in the mass ratio of silicon element in the first and second negative electrode films (M2-M1) within the aforementioned range, the content of silicon-based material in the second negative electrode film layer near the electrolyte is relatively high. Because the number of active sites per unit mass of silicon-based material is much higher than that of graphite material, by placing more silicon-based material in the second negative electrode film layer near the electrolyte, it is possible to promote the rapid embedding of lithium ions into the negative electrode material, significantly improve the rate performance of the cell, and enhance the fast charging performance of the rechargeable battery. On the other hand, since silicon-based materials have a large volume expansion, setting M2-M1 within the aforementioned range helps to reduce the probability of the negative electrode film layer delaminating due to binder failure caused by the volume expansion of silicon-based materials during charging and discharging.

[0055] Furthermore, since the lithium-ion transport time is inversely proportional to the particle size, shorter particle sizes are more conducive to shortening the transport time. Therefore, this disclosure significantly shortens the lithium-ion transport time in the negative electrode bulk structure and improves the fast-charging performance of the secondary battery by placing a first silicon-based material with a smaller average particle size in the first negative electrode film layer near the negative electrode current collector. In addition, silicon-based materials with a larger average particle size have a lower specific surface area and slower interfacial side reactions. Therefore, this disclosure slows down the interfacial side reaction rate and reduces the irreversible consumption of lithium ions and electrolyte by placing a second silicon-based material with a larger average particle size in the second negative electrode film layer near the electrolyte, thus maintaining battery stability during cycling and improving battery cycle performance.

[0056] In this disclosure, the difference M2-M1 between the mass percentage of silicon in the first negative electrode film M1 and the mass percentage of silicon in the second negative electrode film M2 is 16% to 45%. Exemplarily, M2-M1 is a value within a range of 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, or any combination thereof. In some embodiments, M2-M1 is 18% to 35%. Within the aforementioned range, M2-M1 is more conducive to promoting the rapid insertion of lithium ions into the second silicon-based material, reducing the polarization of the cell during charging, and improving cycle life. On the other hand, it is more conducive to reducing the probability of the negative electrode film delamination caused by the failure of the binder due to the volume expansion of the silicon-based material during charging and discharging, thereby significantly improving the rate performance of the cell and further improving the fast charging performance of the secondary battery.

[0057] In some embodiments, the mass percentage M1 of silicon in the first negative electrode film layer is 0.5% to 40%, optionally 0.5% to 20%. Having the mass percentage M1 of silicon in the first negative electrode film layer within this range is advantageous because it allows for the placement of less first silicon-based material in the first negative electrode film layer near the negative electrode current collector, ensuring that M2-M1 are within a suitable range. This promotes rapid lithium-ion insertion into the second silicon-based material, reduces polarization during charging, improves the rate performance of the cell, and thus enhances the fast-charging performance of the secondary battery. For example, the mass percentage M1 of silicon in the first negative electrode film layer is a value within a range of 0.5%, 1%, 5%, 10%, 20%, 30%, 40%, or any combination thereof.

[0058] In some embodiments, the mass percentage M2 of silicon in the second negative electrode film is 18% to 60%, optionally 18% to 40%. Having the mass percentage M2 of silicon in the second negative electrode film within this range facilitates the placement of more second silicon-based material in the second negative electrode film near the electrolyte, ensuring that M2-M1 are within a suitable range. This promotes rapid lithium-ion embedding into the second silicon-based material, reduces cell polarization during charging, improves the cell's rate performance, and thus enhances the fast-charging performance of the secondary battery. For example, the mass percentage M2 of silicon in the second negative electrode film is a value within a range of 18%, 20%, 30%, 40%, 50%, 60%, or any combination thereof.

[0059] In this disclosure, the type and mass percentage of elements (e.g., silicon) in the negative electrode film can be measured using equipment and methods known in the art. For example, the following method can be used: a certain amount of material is taken, and the type and mass of elements in the material are obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of elements in the material can then be calculated.

[0060] In this disclosure, the mass ratio of silicon in the first and second negative electrode films can be determined by using a combination of an ion polisher and an electron microscope. As an example, the test can be performed as follows: the negative electrode film is polished using an argon ion cross-section polisher (e.g., an IB-09010CP type argon ion cross-section polisher). After polishing, the cross-section is measured using a scanning electron microscope (e.g., a ZEISS Sigma 300). The mass ratio of silicon in the first and second negative electrode films is then measured using an EDS spectrometer.

[0061] In some embodiments, the average particle size of the first silicon-based material is 1 μm to 7 μm. By keeping the average particle size of the first silicon-based material within this range, it is beneficial to shorten the diffusion distance of lithium ions in the first silicon-based material, reduce the solid-phase transport time of lithium ions, and thereby improve the fast-charging performance of the battery. Exemplarily, the average particle size of the first silicon-based material is a value within a range of 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, or any combination thereof.

[0062] In some embodiments, the average particle size of the second silicon-based material is 7 μm to 20 μm. By keeping the average particle size of the second silicon-based material within this range, it is advantageous to have a smaller specific surface area, which reduces the reaction interface area between the second silicon-based material and the electrolyte, lowers the rate of side reactions between the second silicon-based material and the electrolyte, and helps the battery maintain stable performance during long-term cycling. Exemplarily, the average particle size of the second silicon-based material is a value within the range of 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or any combination thereof.

[0063] In this disclosure, the average particle size of the material can be tested using equipment and methods known in the art. For example, a scanning electron microscope (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain a scanning electron microscope (SEM) image of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample with a length × width of 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5 areas) within the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composites), read the particle size of each silicon-carbon composite particle in each test area (i.e., take the distance between the two farthest points on the silicon-carbon composite particle as the particle size). Count the number and particle size values ​​of silicon-carbon composite particles in each test area, and take the arithmetic mean of the silicon-carbon composite particles in each test area, which is the average particle size of the silicon-carbon composite particles in the test sample. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0064] In some embodiments, the first negative electrode film layer further includes a first graphite material, and the second negative electrode film layer further includes a second graphite material, wherein the graphitization degree of the second graphite material is lower than that of the first graphite material. Therefore, the second graphite material with a relatively lower graphitization degree has a higher specific surface area. By placing the second graphite material in the second negative electrode film layer, which has a higher lithium-ion concentration, the number of lithium intercalation active sites on the surface of the second graphite material can be increased, improving the lithium-ion diffusion rate and thus enhancing the fast-charging performance of the secondary battery. Conversely, the first graphite material with a relatively higher graphitization degree has a denser surface structure. Placing the first graphite material in the first negative electrode film layer near the current collector improves interface stability, reducing the occurrence of side reactions during battery operation and improving the cycle stability and structural stability of the cell.

[0065] In some embodiments, the average particle size of the second graphite material is smaller than that of the first graphite material. Therefore, the second graphite material has a relatively small average particle size and a large specific surface area, which increases the number of lithium intercalation sites on the material surface, improves the lithium-ion diffusion rate, and thus helps improve the fast-charging performance of the secondary battery.

[0066] In some embodiments, the average particle size of the first graphite material is 10 μm to 25 μm. This is beneficial for increasing the compaction density of the first negative electrode film, thereby increasing the energy density of the battery. Exemplarily, the average particle size of the first graphite material is a value within a range of 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, 23 μm, 25 μm, or any combination thereof.

[0067] In some embodiments, the BET specific surface area of ​​the first graphite material is 1.5 m². 2 / g~4.5m 2 / g. This helps reduce the number of defects on the surface of the first graphite material and decreases the rate of side reactions between the first graphite material and the electrolyte. For example, the BET specific surface area of ​​the first graphite material is 1.5m². 2 / g, 2.0m 2 / g, 2.5m 2 / g, 3.0m 2 / g, 3.5m 2 / g, 4.0m 2 / g, 4.5m 2 / g or a range between the values ​​of either / g or any two of them.

[0068] In some embodiments, the graphitization degree of the first graphite material is 92% to 97%. By keeping the graphitization degree of the first graphite material within this range, it is beneficial to improve the density of the surface structure of the first graphite material, thereby reducing the occurrence of side reactions during battery operation and improving the cycle stability and structural stability of the cell. Exemplarily, the graphitization degree of the first graphite material is a value within a range of 92%, 93%, 94%, 95%, 96%, 97%, or any combination thereof.

[0069] In some embodiments, the average particle size of the second graphite material is 8 μm to 20 μm. This is beneficial for increasing the compaction density of the second negative electrode film, thereby increasing the energy density of the battery. It also shortens the diffusion distance of lithium ions in the graphite bulk structure, improving the battery's fast-charging capability. Exemplarily, the average particle size of the second graphite material is a value within a range of 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, or any combination thereof.

[0070] In some embodiments, the BET specific surface area of ​​the second graphite material is 2.0 m². 2 / g~5.0m 2 / g. This helps reduce the number of defects on the surface of the second graphite material and decreases the rate of side reactions between the second graphite material and the electrolyte. For example, the BET specific surface area of ​​the second graphite material is 2.0 m². 2 / g, 2.5m 2 / g, 3.0m 2 / g, 3.5m 2 / g, 4.0m 2 / g, 4.5m 2 / g, 5.0m 2 / g or a range between the values ​​of either / g or any two of them.

[0071] In some embodiments, the graphitization degree of the second graphite material is 90% to 96%. By keeping the graphitization degree of the second graphite material within the above range, the second graphite material has a larger specific surface area, thereby increasing the number of lithium intercalation active sites on the surface of the second graphite material, improving the lithium intercalation capability of the material, and increasing the lithium ion diffusion rate, which is beneficial to improving the fast charging performance of the secondary battery. Exemplarily, the graphitization degree of the second graphite material is a value within a range of 90%, 91%, 92%, 93%, 94%, 95%, 96%, or any combination thereof.

[0072] In this disclosure, the specific surface area of ​​a material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0073] In this disclosure, the degree of graphitization of the material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using an X-ray diffractometer (such as a Bruker D8 Discover) according to JIS K 0131-1996 and JB / T 4220-2011. The measurement can be referenced to the determination of d... 002 The size is then determined according to the formula G = (0.344 - d). 002 The degree of graphitization is calculated by d / (0.344-0.3354)×100%, where d 002This refers to the interlayer spacing in the graphite crystal structure, measured in nm. In X-ray diffraction analysis, Cu Kα rays were used as the radiation source, with the ray wavelength scanning 2θ angle range of 20° to 80° and the scanning rate of 4° / min.

[0074] In some embodiments, the mass percentage of the first graphite material in the first negative electrode film layer is greater than the mass percentage of the second graphite material in the second negative electrode film layer.

[0075] In some embodiments, the mass percentage of the first graphite material in the first negative electrode film layer is 30% to 98%. A mass percentage of the first graphite material in the first negative electrode film layer within this range is beneficial for improving the cycle stability and structural stability of the battery cell, thereby helping to maintain stable performance of the battery throughout its entire lifespan. Exemplarily, the mass percentage of the first graphite material in the first negative electrode film layer is a value within a range of 30%, 40%, 50%, 70%, 90%, 95%, 98%, or any combination thereof.

[0076] In some embodiments, the mass percentage of the second graphite material in the second negative electrode film is 10% to 80%. A mass percentage of the second graphite material in the second negative electrode film within this range is beneficial for improving the diffusion rate of lithium ions, thereby improving the fast-charging performance of the secondary battery. Exemplarily, the mass percentage of the second graphite material in the second negative electrode film is a value within a range of 10%, 20%, 40%, 60%, 70%, 80%, or any combination thereof.

[0077] In some embodiments, the first silicon-based material and the second silicon-based material each independently include one or more of elemental silicon, silicon oxide, and silicon-carbon composites.

[0078] In some embodiments, the first silicon-based material and the second silicon-based material comprise a silicon-carbon composite that satisfies one or more of the following characteristics:

[0079] (1) The silicon-carbon composite includes porous carbon and silicon-containing materials dispersed in the pores of the porous carbon; optionally, the porous carbon is hard carbon;

[0080] (2) The silicon-carbon composite also includes a carbon-containing coating layer located on the surface of porous carbon and / or silicon-containing materials;

[0081] (3) The silicon content in the silicon-carbon composite is 30% to 70% by mass;

[0082] (4) The powder resistivity of silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0083] (5) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m2 / g.

[0084] In some embodiments, the silicon-carbon composite includes a core comprising porous carbon and silicon-containing material dispersed in the pores of the porous carbon. The porous carbon, acting as a carrier for the silicon-containing material, provides support for the nanoscale silicon-containing material and, simultaneously, provides expansion space for the expansion of the silicon nanoparticles, effectively mitigating the stress compression caused by expansion during charging. Especially when the silicon-containing particles are in the nanometer range, the specific capacity is higher and dispersion in the pores of the porous carbon is facilitated. Furthermore, the buffering effect of the porous carbon's pores on expansion can be more fully utilized. When this silicon-carbon composite is applied in a wound electrode assembly, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0085] In some embodiments, the porous carbon may optionally be hard carbon. When the porous carbon is hard carbon, it has stronger support, a more stable pore structure, and is harder, thus providing better porosity for the negative electrode active layer, providing a smoother path for active ion transport, and improving the charging capability of the battery cell.

[0086] In some embodiments, the silicon-containing material includes at least one of elemental silicon, silicon oxides, silicon nitrides, and silicon alloys. In some embodiments, the silicon-containing material includes crystalline silicon, thereby further improving the structural stability of the silicon-containing material and the energy density of the battery cell.

[0087] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer that coats the surface of the core. This can improve the conductivity of the silicon-carbon composite and reduce the internal impedance of the battery cell, while also effectively reducing the probability of direct contact between the silicon-containing material in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0088] In some embodiments, the silicon content in the silicon-carbon composite is 30% to 70% by mass. This approach, while maximizing the specific capacity of the negative electrode active material by utilizing silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0089] In this disclosure, the method for testing the silicon content in the silicon-carbon composite can be any method known in the art. As an example, the following method can be used: a certain amount of silicon-carbon composite is taken, and the mass of silicon in the silicon-carbon composite is obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of silicon in the silicon-carbon composite can then be calculated.

[0090] In some embodiments, the silicon-carbon composite powder resistivity at 8 MPa is 4 Ω·cm to 17 Ω·cm. Controlling the powder resistivity as described above improves the conductivity of the silicon-carbon composite, thereby increasing the charging rate of the battery cell. Exemplarily, the silicon-carbon composite powder resistivity at 8 MPa is a value within the range of 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm, 16 Ω·cm, 17 Ω·cm, or any combination thereof.

[0091] In this disclosure, the powder resistivity of silicon-carbon composites can be determined using methods known in the art. As an example, a four-probe method can be used, where two probes apply voltage and the other two probes measure current. The powder resistivity can be calculated by measuring the resistance value. Models of four-probe semiconductor powder resistivity testers include the ST-2722.

[0092] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g. For example, the specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g, 2.0m 2 / g, 3.0m 2 / g, 4.0m 2 / g, 5.0m 2 / g, 6.0m 2 / g, 6.7m 2 / g or a range between the values ​​of either / g or any two of them.

[0093] In some embodiments, the first negative electrode film layer further includes a first binder, the first binder comprising 1.0% to 3.0% of the mass of the first negative electrode film layer, and / or, the second negative electrode film layer further includes a second binder, the second binder comprising 1.5% to 4.0% of the mass of the second negative electrode film layer. Since both the first and second negative electrode film layers include silicon-based materials, the addition of a binder enables the silicon-based materials to be interconnected, ensuring that during battery charging and discharging, the silicon-based materials are tightly bound within the negative electrode film layer and do not lose their electrical connections to form electrochemical islands, thus preventing the silicon-based materials from losing their electrochemical activity. Furthermore, since the second silicon-based material comprises a relatively higher mass of the second negative electrode film layer, a relatively larger amount of binder needs to be added to the second negative electrode film layer. For example, the mass percentage of the first binder in the first negative electrode film layer is a value within a range of 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, or any two of these, and the mass percentage of the second binder in the second negative electrode film layer is a value within a range of 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, or any two of these.

[0094] In some embodiments, the first adhesive includes one or more of styrene-butadiene rubber, polyacrylic acid, polyacrylonitrile, carboxymethyl cellulose, sodium carboxymethyl cellulose, hydroxypropyl methylcellulose, methylcellulose, and their derivatives, and the second adhesive includes one or more of styrene-butadiene rubber, polyacrylic acid, polyacrylonitrile, carboxymethyl cellulose, sodium carboxymethyl cellulose, hydroxypropyl methylcellulose, methylcellulose, and their derivatives. The first adhesive and the second adhesive may be the same or different.

[0095] In some embodiments, the first negative electrode film layer further includes a first conductive agent, the mass percentage of which is 0.05% to 2%, and / or the second negative electrode film layer further includes a second conductive agent, the mass percentage of which is 1.5% to 3.0%. Since silicon-based materials undergo volume expansion during battery charging and discharging, adding a conductive agent can form a more stable conductive network, ensuring that the silicon-based material remains stably within the conductive network during charging and discharging, reducing material loss due to loss of electrical connection between silicon and graphite particles during significant contraction or expansion. Furthermore, because the particle size of the second silicon-based material in the second negative electrode film layer is relatively larger, the overall expansion of the larger-particle-size silicon-based material is relatively higher; therefore, a relatively larger amount of conductive agent needs to be added to the second negative electrode film layer. For example, the mass percentage of the first conductive agent in the first negative electrode film layer is a value within a range of 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, or any two of these, and the mass percentage of the second conductive agent in the second negative electrode film layer is a value within a range of 1.5%, 2.0%, 2.5%, 3.0%, or any two of these.

[0096] In some embodiments, the first conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, and the second conductive agent includes one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The first and second conductive agents may be the same or different.

[0097] In some embodiments, the thickness of the first negative electrode film is 6 μm to 60 μm, optionally 30 μm to 50 μm. A thickness within this range is advantageous for ensuring the lithium-ion transport distance is within a suitable range, thereby improving the energy density and rate performance of the secondary battery. Exemplarily, the thickness of the first negative electrode film is a value within a range of 6 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, or any combination thereof.

[0098] In some embodiments, the thickness of the second negative electrode film is 6 μm to 60 μm, optionally 30 μm to 50 μm. A thickness within this range is advantageous for ensuring that the lithium-ion transport distance is within a suitable range, thereby improving the energy density and rate performance of the secondary battery. Exemplarily, the thickness of the second negative electrode film is a value within the range of 6 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, or any combination thereof.

[0099] In some embodiments, a buffer layer may be included between the negative electrode current collector 101 and the first negative electrode film layer 102. The buffer layer includes a third adhesive and a third conductive agent. Thus, the buffer layer can enhance the adhesion between the first negative electrode film layer and the negative electrode current collector, thereby maintaining the adhesion between the first negative electrode film layer and the negative electrode current collector throughout the entire service life. On the other hand, it can form a stable conductive network between the first negative electrode film layer and the negative electrode current collector.

[0100] In some embodiments, the third binder includes one or more of styrene-butadiene rubber and carboxymethyl cellulose, and the third conductive agent includes one or more of acetylene black and conductive carbon black.

[0101] In some embodiments, the third binder comprises 10% to 70% by mass in the buffer layer, and the third conductive agent comprises 30% to 90% by mass. A mass percentage of the third binder within the aforementioned range is beneficial for enhancing the adhesion between the first negative electrode film layer and the negative electrode current collector. A mass percentage of the third conductive agent within the aforementioned range is beneficial for forming a stable conductive network between the first negative electrode film layer and the negative electrode current collector. For example, in the buffer layer, the mass percentage of the third binder is a value within the range of 10%, 20%, 40%, 50%, 60%, 70%, or any combination thereof, and the mass percentage of the third conductive agent is a value within the range of 30%, 40%, 50%, 60%, 70%, 80%, 90%, or any combination thereof.

[0102] In some embodiments, the thickness of the buffer layer is 0.2 μm to 1.5 μm. A buffer layer thickness within this range is beneficial for improving the adhesion between the negative electrode current collector and the first negative electrode film layer. This reduces the probability of the first negative electrode film layer detaching from the negative electrode current collector during battery charge-discharge cycles, and does not excessively affect the impedance between the cell and the negative electrode current collector, thus balancing the rate performance and cycle performance of the secondary battery. For example, the thickness of the buffer layer is a value within the range of 0.2 μm, 0.5 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, or any combination thereof.

[0103] In this disclosure, the thicknesses of the first negative electrode film layer, the buffer layer, and the second negative electrode film layer can be measured by the following method: The negative electrode film layer is peeled off from the negative electrode current collector; the peeled-off negative electrode film layer is fixed on the sample stage; the sample stage is installed in the sample holder and locked in place; the power of the argon ion cross-section polishing instrument (e.g., the IB-09010CP argon ion cross-section polishing instrument from JEOL Corporation, Japan) is turned on, and a vacuum is applied (e.g., 10...). -7 Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to rocking mode to begin polishing. After polishing, use a scanning electron microscope to measure the thickness of the cross-sections of the first negative electrode film, the buffer layer, and the second negative electrode film.

[0104] The secondary battery disclosed herein can be prepared using conventional methods. For example, it may include the following steps:

[0105] S1, a first negative electrode slurry and a second negative electrode slurry are coated on at least one side of the negative electrode current collector. After drying and cold pressing, a first negative electrode film layer and a second negative electrode film layer are formed respectively, thereby forming a negative electrode sheet. The first negative electrode film layer is located between the negative electrode current collector and the second negative electrode film layer. The first negative electrode film layer includes a first negative electrode active material, which includes a first silicon-based material. The second negative electrode film layer includes a second negative electrode active material, which includes a second silicon-based material. The average particle size of the first silicon-based material is smaller than the average particle size of the second silicon-based material. The mass percentage of silicon element in the first negative electrode film layer is M1, and the mass percentage of silicon element in the second negative electrode film layer is M2. M1 and M2 satisfy the following relationship: M2-M1=16%~45%;

[0106] S2, the above-mentioned negative electrode sheet, separator and positive electrode sheet are bent and wound to form an electrode assembly, and then injected with electrolyte to obtain a secondary battery.

[0107] In some embodiments, the coating weight ratio of the first negative electrode film layer and the second negative electrode film layer is (1:10) to (10:1), optionally (1:2) to (2:1). Controlling the coating weight ratio of the first negative electrode film layer and the second negative electrode film layer within the above range is beneficial for M1 and M2 to satisfy the aforementioned relationship, thereby promoting the rapid embedding of lithium ions into the second silicon-based material, reducing the polarization of the cell during charging, improving the rate performance of the cell, and thus improving the fast-charging performance of the secondary battery. For example, the coating weight ratio of the first negative electrode film layer and the second negative electrode film layer is a value within a range of 1:10, 1:5, 1:2, 1:1, 2:1, 5:1, 10:1, or any combination thereof.

[0108] In some embodiments, the coating weight of the first negative electrode film is 3 mg / cm³. 2 ~10mg / cm 2 The coating weight of the second negative electrode film is 3 mg / cm³. 2 ~10mg / cm 2 The coating weights of the first and second negative electrode films are within the aforementioned range, which helps to ensure that M1 and M2 satisfy the aforementioned relationship, thereby promoting the rapid insertion of lithium ions into the second silicon-based material, reducing polarization of the cell during charging, improving the rate performance of the cell, and thus enhancing the fast-charging performance of the secondary battery. For example, the coating weight of the first negative electrode film is 3 mg / cm³. 2 4mg / cm 2 6mg / cm 2 8mg / cm 2 10mg / cm 2 Or a value between any two of them, the coating weight of the second negative electrode film is 3 mg / cm³. 2 4mg / cm2 6mg / cm 2 8mg / cm 2 10mg / cm 2 Or the value between any two of them within a range.

[0109] In some embodiments, the first negative electrode slurry includes a first silicon-based material and a first graphite material, with a mass ratio of the first silicon-based material to the first graphite material of (1:120) to (1:4). A reasonable ratio of the first silicon-based material and the first graphite material helps to reserve space for the expansion of the silicon-based material within the first negative electrode film, thereby improving the battery's lifespan. For example, the mass ratio of the first silicon-based material to the first graphite material is a value within a range of 1:120, 1:100, 1:80, 1:60, 1:40, 1:20, 1:10, 1:4, or any combination thereof.

[0110] In some embodiments, the second negative electrode slurry includes a second silicon-based material and a second graphite material, with a mass ratio of the second silicon-based material to the second graphite material of (1:3) to (6:1). This appropriate ratio of the second silicon-based material to the second graphite material helps to reserve space for the expansion of the silicon-based material within the second negative electrode film, thereby improving the battery's lifespan. For example, the mass ratio of the second silicon-based material to the second graphite material is a value within a range of 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, or any combination thereof.

[0111] 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.

[0112] 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.

[0113] Negative electrode sheet

[0114] 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 the negative electrode active material of this disclosure.

[0115] 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.

[0116] 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.).

[0117] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0118] 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.

[0119] Positive electrode sheet

[0120] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0121] 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. 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum 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.).

[0122] 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 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0123] 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.

[0124] 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.

[0125] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0126] 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.

[0127] 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.

[0128] electrolytes

[0129] 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.

[0130] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] Separating membrane

[0135] 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.

[0136] 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.

[0137] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0138] 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.

[0139] 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.

[0140] 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 2 shows a square battery cell 5 as an example.

[0141] In some embodiments, referring to FIG3, 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.

[0142] 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.

[0143] Figure 4 shows a battery module 4 as an example. Referring to Figure 4, 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.

[0144] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0145] 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.

[0146] Figures 5 and 6 show a battery pack 1 as an example. Referring to Figures 5 and 6, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper 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.

[0147] Electrical appliances

[0148] A second aspect of this disclosure provides an electrical device that includes a secondary battery provided in the first aspect of this disclosure.

[0149] Secondary batteries can be used as a power source for electrical devices or as an energy storage unit for electrical devices. Electrical devices can include, but are not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0150] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0151] Figure 7 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.

[0152] 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.

[0153] Example

[0154] 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.

[0155] Example 1

[0156] Preparation of secondary batteries:

[0157] S1, Preparing the negative electrode sheet, specifically including the following steps:

[0158] a. Preparation of the first negative electrode slurry: The first graphite material, the first silicon-based material (silicon-carbon composite, including a core and a coating layer on the surface of the core, the core including porous carbon and silicon particles dispersed in the pores of the porous carbon, the silicon element in the silicon-carbon composite accounts for 60% by mass), binder (styrene-butadiene rubber), conductive agent (carbon black: conductive carbon nanotubes = 3:1), and dispersant (sodium carboxymethyl cellulose) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 76:20:1:2:1 to form the first negative electrode slurry (silicon element accounts for 12 wt%).

[0159] b. Preparation of the second negative electrode slurry: The second graphite material, the second silicon-based material (silicon-carbon composite, including a core and a coating layer on the surface of the core, the core including porous carbon and pores dispersed in the porous carbon, the silicon element in the silicon-carbon composite accounts for 60% by mass), binder (styrene-butadiene rubber), conductive agent (carbon black: conductive carbon nanotubes = 2:1), and dispersant (sodium carboxymethyl cellulose) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 35:60:1.7:2:1.3 to form the second negative electrode slurry (silicon element accounts for 36 wt%).

[0160] c. Preparation of buffer layer slurry: Polyvinylidene fluoride (PVDF) and conductive carbon black (SP) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 25:75 to form a buffer layer slurry;

[0161] d. Using an extrusion coating device, the above-mentioned buffer layer slurry is coated onto the negative electrode current collector copper foil to form a buffer layer with a thickness of 0.5 μm. The first negative electrode slurry and the second negative electrode slurry are simultaneously extruded. The first negative electrode slurry is coated onto the buffer layer to form a first negative electrode film, and the second negative electrode slurry is coated onto the first negative electrode film. The coating weight ratio of the first and second negative electrode slurries is 1:1. After drying and cold pressing, a second negative electrode film is formed. The first and second negative electrode film layers constitute the negative electrode film, resulting in a negative electrode sheet. The coating weight of the first negative electrode film is 7 mg / cm³. 2 The coating weight of the second negative electrode film is 7 mg / cm³. 2 The thickness of the first negative electrode film is 50 μm, and the thickness of the second negative electrode film is 50 μm.

[0162] S2, Preparing a secondary battery, specifically includes the following steps:

[0163] Preparation of the positive electrode sheet: Nickel-cobalt-manganese ternary positive electrode material (NCM811), conductive carbon black, and polyvinylidene fluoride are mixed in a weight ratio of 96.5:2.5:1 and then added to the solvent N-methylpyrrolidone. The mixture is stirred evenly to form a positive electrode slurry. The positive electrode slurry is coated onto the positive electrode current collector aluminum foil to form a positive electrode film layer. After drying and cold pressing, the positive electrode sheet is obtained.

[0164] Preparation of electrolyte: Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:2:1 to form an organic solvent. LiPF6 is dissolved in the organic solvent to obtain an electrolyte with a LiPF6 concentration of 1.2 mol / L.

[0165] Separating membrane: Using polyethylene microporous film as the separating membrane substrate, inorganic alumina powder, polyvinylpyrrolidone and acetone are mixed evenly in a weight ratio of 3:1.5:5.5 to form a slurry and coated on one side of the substrate. After drying, the separating membrane is obtained with a thickness of 13μm.

[0166] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is placed in an outer package, dried, and then injected with the prepared electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0167] Parameter testing:

[0168] (1) Average particle size test

[0169] Using a scanning electron microscope (ZEISS Sigma 300), and referring to JY / T010-1996, SEM images of the negative electrode sheet were obtained. A test sample with dimensions of 50mm x 100mm was randomly selected from the negative electrode sheet. Multiple test areas (5 in total) were randomly selected within the sample, and the particle size of each particle in each test area was read at a magnification of 1000x (i.e., the distance between the two furthest points on the particle was taken as the particle size). The number and particle size values ​​of particles in each test area were counted, and the arithmetic mean of the particles in each test area was taken as the average particle size of the test sample. To ensure the accuracy of the test results, the above test was repeated on multiple test samples (10 in total), and the average value of each test sample was taken as the final test result.

[0170] (2) Element content test

[0171] Disassemble the battery, peel the negative electrode film from the negative electrode current collector, fix the peeled negative electrode film onto the sample stage, install the sample stage into the sample holder and lock it in place, turn on the power of the argon ion cross-section polisher (IB-09010CP type argon ion cross-section polisher) and perform vacuuming (10 -7 Set the argon flow rate to 0.12 MPa and the polishing time to 90 min. Adjust the sample stage to rocking mode and begin polishing.

[0172] After polishing, the cross-section was measured using a scanning electron microscope S-4800, and the mass ratio of silicon in the first and second negative electrode films was tested using an energy dispersive spectroscopy (EDS) instrument.

[0173] (3) Specific surface area test

[0174] The specific surface area and pore size analyzer of the negative electrode material was tested using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA, following the standard procedure GB / T 19587-2017. The specific surface area was calculated using the BET method.

[0175] (4) Graphitization degree test

[0176] The degree of graphitization of the negative electrode material was tested using an X-ray diffractometer (Bruker D8 Discover) following standard procedures: JIS K 0131-1996 and JB / T4220-2011. In the X-ray diffraction analysis, Cu Kα rays were used as the radiation source, with the ray wavelength scanning 2θ angle range of 20°–80° and a scanning rate of 4° / min. The measured d... 002The size of is determined by the formula G = (0.344 - d). 002 The degree of graphitization is calculated by d / (0.344-0.3354)×100%, where d 002 It is the interlayer spacing in a graphite crystal structure measured in nanometers.

[0177] Performance testing of secondary batteries:

[0178] (1) Fast charging performance test

[0179] ① At 25℃, charge the secondary battery with a constant current of 0.33C to 4.25V, then charge it with a constant voltage to a current of 0.05C. After standing for 5 minutes, discharge the secondary battery with a constant current of 0.33C to 2.5V and record its actual capacity as C0.

[0180] ②Then charge the secondary battery sequentially with constant current at 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, and 3.0C0 to 4.25V or 0V negative electrode cutoff potential (whichever comes first). After each charge, discharge to 2.5V with 1C0. Record the negative electrode potential corresponding to 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% SOC (State of Charge) at different charging rates.

[0181] ③ Plot the charging rate-negative electrode potential curves under different SOC states. After linear fitting, obtain the charging rate corresponding to the negative electrode potential of 0V under different SOC states. This charging rate is the charging window under this SOC state, and is denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC, respectively.

[0182] ④ Calculate the charging time T (assuming no lithium plating) for the secondary battery from 10% SOC to 80% SOC using the following formula, in minutes. (60 / C) 20％SOC +60 / C 30％SOC +60 / C 40％SOC +60 / C 50％SOC +60 / C 60％SOC +60 / C 70％SOC +60 / C 80％SOC )×10%.

[0183] The test results are recorded in Table 3 below.

[0184] (2) Cyclic performance test

[0185] ① At 25℃, charge the secondary battery at a constant current of 0.33C to the charging cutoff voltage of 4.25V, then charge it at a constant voltage of 4.25V to a current of 0.05C, and then discharge it at a constant current of 0.33C to 2.5V. This is the first cycle, and the discharge capacity of the first cycle is recorded as C0.

[0186] ② Charge the secondary battery at a constant current of 0.5C to the charging cutoff voltage of 4.25V, then charge it at a constant voltage of 4.25V until the current is 0.05C, and then discharge it at a constant current of 0.5C to 2.5V. This completes the first charge-discharge cycle. Repeat the above cycle. The discharge capacity of the nth cycle is recorded as Cn. The capacity retention rate for each cycle is Cn / C0. Calculate the capacity retention rate after 1000 cycles. The test results are recorded in Table 3 below.

[0187] Examples 2 to 9

[0188] The secondary battery was prepared using the same method as in Example 1, except that the types and amounts of the first and second silicon-based materials were different. Please refer to Tables 1-1, 1-2, 2 and 3 below for details.

[0189] Comparative Examples 1 to 3

[0190] The secondary battery was prepared using the same method as in Example 1, except that the average particle size and amount of the first silicon-based material and the second silicon-based material were different. Please refer to Tables 1-1, 1-2, 2 and 3 below for details.

[0191] Examples 2 to 9 and Comparative Examples 1 to 3 were tested using the same testing method as in Example 1. The results are shown in Tables 1-1, 1-2, 2, and 3 below.

[0192] Table 1-1

[0193] Table 1-2

[0194] Table 2

[0195] Table 3

[0196] As can be seen from Tables 1-1, 1-2, 2 and 3, compared with Comparative Examples 1 to 3, Examples 1 to 9 significantly improved the cycle performance and fast charging performance of the secondary battery by placing the first negative electrode film layer between the negative electrode current collector and the second negative electrode film layer, controlling the average particle size of the first silicon-based material to be smaller than the average particle size of the second silicon-based material, and controlling M2-M1 to be in the range of 16% to 45%.

[0197] Examples 10 to 17

[0198] The secondary battery was prepared using the same method as in Example 1, except that the types of the first and second graphite materials were different. Please refer to Table 4 below for details.

[0199] The negative electrode films prepared in Examples 10 to 17 were subjected to parameter tests using the same test method as in Example 1, and the secondary batteries prepared in Examples 10 to 17 were subjected to performance tests using the same test method as in Example 1.

[0200] Table 4 below shows the relevant parameters of the negative electrode films prepared in Examples 10 to 17. Table 5 below shows the performance test results of the secondary batteries prepared in Examples 10 to 17.

[0201] Table 4

[0202] Table 5

[0203] As can be seen from Tables 4 and 5, Examples 10 to 17 significantly improved the cycle performance and fast charging performance of the secondary battery by controlling the graphitization degree of the second graphite material to be less than that of the first graphite material, and the average particle size of the second graphite material to be less than that of the first graphite material.

[0204] 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

1. A secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, The negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, wherein the first negative electrode film layer is located between the negative electrode current collector and the second negative electrode film layer. The first negative electrode film layer includes a first negative electrode active material, and the first negative electrode active material includes a first silicon-based material. The second negative electrode film layer includes a second negative electrode active material, and the second negative electrode active material includes a second silicon-based material. The average particle size of the first silicon-based material is smaller than the average particle size of the second silicon-based material. The mass percentage of silicon in the first negative electrode film is M1, and the mass percentage of silicon in the second negative electrode film is M2. M1 and M2 satisfy the following relationship: M2-M1 = 16%~45%.

2. The secondary battery according to claim 1, wherein, M2-M1 = 18%~35%.

3. The secondary battery according to claim 1 or 2, wherein, The average particle size of the first silicon-based material is 1 μm to 7 μm.

4. The secondary battery according to any one of claims 1 to 3, wherein, The average particle size of the second silicon-based material is 7 μm to 20 μm.

5. The secondary battery according to any one of claims 1 to 4, wherein, The mass percentage (M1) of silicon in the first negative electrode film layer is 0.5% to 40%.

6. In the secondary battery according to any one of claims 1 to 5, the mass percentage M2 of silicon element in the second negative electrode film layer is 18% to 60%.

7. The secondary battery according to any one of claims 1 to 6, wherein, The first negative electrode film layer further includes a first graphite material, and the second negative electrode film layer further includes a second graphite material, wherein the degree of graphitization of the second graphite material is less than that of the first graphite material.

8. The secondary battery according to claim 7, wherein, The average particle size of the second graphite material is smaller than that of the first graphite material.

9. The secondary battery according to claim 7 or 8, wherein, The first graphite material satisfies one or more of the following characteristics: (1) The average particle size is 10μm~25μm; (2) Specific surface area is 1.5m² 2 / g~4.5m 2 / g; (3) The degree of graphitization is 92% to 97%.

10. The secondary battery according to any one of claims 7 to 9, wherein, The second graphite material satisfies one or more of the following characteristics: (1) The average particle size is 8μm to 20μm; (2) Specific surface area is 2.0 m². 2 / g~5.0m 2 / g; (3) The degree of graphitization is 90% to 96%.

11. The secondary battery according to any one of claims 7 to 10, wherein, The mass percentage of the first graphite material in the first negative electrode film layer is greater than the mass percentage of the second graphite material in the second negative electrode film layer.

12. The secondary battery according to any one of claims 7 to 11, wherein, The mass percentage of the first graphite material in the first negative electrode film layer is 30% to 98%.

13. The secondary battery according to any one of claims 7 to 12, wherein, The second graphite material accounts for 10% to 80% of the mass of the second negative electrode film layer.

14. The secondary battery according to any one of claims 1 to 13, wherein, The first silicon-based material and the second silicon-based material each independently include one or more of elemental silicon, silicon oxide, and silicon-carbon composites.

15. The secondary battery according to any one of claims 1 to 14, wherein, The first silicon-based material and / or the second silicon-based material comprises a silicon-carbon composite, the silicon-carbon composite satisfying one or more of the following characteristics: (1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon, optionally, the porous carbon is hard carbon; (2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or the silicon-containing material; (3) The mass percentage of silicon in the silicon-carbon composite is 30% to 70%; (4) The powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm to 17 Ω·cm; (5) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g~6.7m 2 / g.

16. The secondary battery according to any one of claims 1 to 15, wherein, The first negative electrode film layer further includes a first binder, wherein the mass percentage of the first binder in the first negative electrode film layer is 1.0% to 3.0%, and / or, The second negative electrode film layer further includes a second binder, wherein the mass percentage of the second binder in the second negative electrode film layer is 1.5% to 4.0%.

17. The secondary battery according to any one of claims 1 to 16, wherein, The first negative electrode film layer further includes a first conductive agent, wherein the mass percentage of the first conductive agent in the first negative electrode film layer is 0.05% to 2%, and / or, The second negative electrode film layer also includes a second conductive agent, and the mass percentage of the second conductive agent in the second negative electrode film layer is 1.5% to 3%.

18. The secondary battery according to any one of claims 1 to 17, wherein, The thickness of the first negative electrode film is 6μm to 60μm.

19. The secondary battery according to any one of claims 1 to 18, wherein, The thickness of the second negative electrode film is 6μm to 60μm.

20. The secondary battery according to any one of claims 1 to 19, wherein, A buffer layer is further included between the negative electrode current collector and the first negative electrode film layer, the buffer layer comprising a third binder and a third conductive agent.

21. The secondary battery according to claim 20, wherein, In the buffer layer, the third adhesive accounts for 10% to 70% of the mass, and the third conductive agent accounts for 30% to 90% of the mass.

22. The secondary battery according to claim 20 or 21, wherein, The thickness of the buffer layer is 0.2μm to 1.5μm.

23. An electrical device comprising a secondary battery as described in any one of claims 1 to 22.

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