Secondary battery and electrical apparatus

Abstract

Provided are a secondary battery and an electrical apparatus. The secondary battery comprises a negative electrode plate; the negative electrode plate comprises 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 silicon-carbon composite material; the negative electrode plate extends in a first direction, the length of the negative electrode plate in the first direction being greater than the length thereof in a second direction, and the second direction being perpendicular to the first direction; when the state of charge of the secondary battery is 100% SOC, in a cross section of the negative electrode plate in the second direction, the thickness of the negative electrode plate gradually decreases from two ends to the middle.

Description

Secondary batteries and electrical devices

[0001] Cross-references to related applications

[0002] This disclosure is based on and claims priority to Chinese patent applications No. 202411754625.7, filed on December 2, 2024, entitled "Secondary Battery and Electrical Device", and No. 202510330141.8, 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 secondary 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 secondary batteries, higher requirements have been placed on their energy density and cycle life.

[0005] Therefore, how to improve the cycle life of secondary batteries while taking into account energy density 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 can improve cycle performance while taking into account energy density.

[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 surface of the negative current collector. The negative electrode film layer comprises a silicon-carbon composite material. The negative electrode sheet extends along a first direction, and its length in the first direction is greater than its length in a second direction. The second direction is perpendicular to the first direction. When the secondary battery is at 100% SOC, in a cross-section of the negative electrode sheet along the second direction, the thickness of the negative electrode sheet decreases from both ends towards the center. Therefore, the cycle performance of the secondary battery can be improved without affecting its energy density.

[0008] In some embodiments, when the secondary battery is at 100% SOC, the minimum thickness X1 and maximum thickness Y1 of the negative electrode film in the cross-section of the negative electrode sheet in the second direction satisfy the following relationship: 1.0 < Y1 / X1 ≤ 1.2. This results in the negative electrode sheet being thicker on both sides in the second direction than in the middle, which helps reduce cell polarization, improve lithium plating, and enhance the cycle performance of the secondary battery.

[0009] In some embodiments, when the secondary battery is at 100% SOC, the minimum thickness X1 and maximum thickness Y1 of the negative electrode film in the cross-section of the negative electrode sheet in the second direction satisfy the following relationship: 1.01 ≤ Y1 / X1 ≤ 1.1. This is more conducive to reducing cell polarization, improving lithium plating, and enhancing the cycle performance of the secondary battery.

[0010] In some embodiments, when the secondary battery is at 0% SOC, the minimum thickness X2 and maximum thickness Y2 of the negative electrode film in the cross-section of the negative electrode sheet in the second direction satisfy the following relationship: 1.0 ≤ Y2 / X2 ≤ 1.1. This helps to reduce cell polarization and improve the cycle performance of the secondary battery.

[0011] In some embodiments, when the secondary battery is at 0% SOC, the minimum thickness X2 and maximum thickness Y2 of the negative electrode film in the cross-section of the negative electrode sheet in the second direction satisfy the following relationship: 1.0 ≤ Y2 / X2 ≤ 1.05. This is more conducive to reducing cell polarization and improving the cycle performance of the secondary battery.

[0012] In some implementations, when the secondary battery is at 0% SOC, the minimum thickness X2 and maximum thickness Y2 of the negative electrode film satisfy the following relationship: 0 μm ≤ Y2 - X2 ≤ 6 μm. This helps to reduce cell polarization and improve the cycle performance of the secondary battery.

[0013] In some embodiments, in the second direction of the negative electrode sheet, the mass percentage of silicon in the negative electrode film layer decreases from both ends to the center. This allows the negative electrode sheet to form a structure where the thickness decreases from both ends to the center in the second direction after battery cycling, which helps reduce cell polarization, decrease intermediate lithium plating, and improve the cycle performance of the secondary battery.

[0014] In some embodiments, the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, which are stacked in a direction away from the negative electrode current collector. The first negative electrode film layer comprises a silicon-carbon composite material, and the second negative electrode film layer comprises a graphite material. This helps to reduce the expansion of the silicon-carbon composite material and improve the cycle performance of the secondary battery.

[0015] In some embodiments, when the secondary battery is at 0% SOC, in the cross-section of the negative electrode sheet in the second direction, the minimum thickness of the first negative electrode film layer in the middle region is d1, and the maximum thickness of the first negative electrode film layer in the two side regions is d2, where d2 > d1. This facilitates the formation of a structure with greater thickness on both sides and less thickness in the middle after the battery is charged, thereby reducing cell polarization and improving the cycle performance of the secondary battery.

[0016] In some implementations, d1 = 0. This is advantageous for forming a structure with greater thickness on both sides and less thickness in the middle after the battery is charged.

[0017] In some implementations, when the secondary battery is at 0% SOC, d2 and d1 satisfy the following relationship: d2 / d1 ≥ 1.01. This helps to reduce cell polarization and improve the cycle performance of the secondary battery.

[0018] In some embodiments, in the cross-section of the negative electrode sheet in the second direction, the maximum thickness of the second negative electrode film layer in the middle region is d3, then d3 / d1 ≥ 0.2. This is beneficial for reducing cell polarization and improving the cycle performance of the secondary battery.

[0019] In some embodiments, in the cross-section of the negative electrode sheet in the second direction, the maximum thickness of the second negative electrode film layer in the middle region is d3, then d3 / d1 ≥ 0.5. This is more conducive to reducing cell polarization and improving the cycle performance of the secondary battery.

[0020] In some embodiments, the mass percentage M1 of silicon in the first negative electrode film is 2.5% to 68%. This allows for the effective utilization of silicon to significantly increase the specific capacity of the negative electrode active material, while also helping to control the expansion of silicon during charging and improving the stability of the first negative electrode film.

[0021] In some embodiments, the second negative electrode film layer further comprises silicon, with the mass percentage (M2) of silicon in the second negative electrode film layer being 0% to 40%. This allows for the effective utilization of silicon to significantly increase the specific capacity of the negative electrode active material, while also facilitating the control of silicon expansion during charging and improving the stability of the second negative electrode film layer.

[0022] In some embodiments, 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, then 2.5% ≤ M1 - M2 ≤ 68.0%. This is beneficial for increasing the number of lithium intercalation active sites in the first negative electrode film and reducing expansion, thereby improving the cycle performance of the secondary battery.

[0023] In some embodiments, the second negative electrode film is located between the first negative electrode film and the negative electrode current collector, and the graphite material in the second negative electrode film accounts for 50% to 99% of the mass. This helps to reduce the polarization of the cell during charging and improve the cycle performance of the secondary battery.

[0024] In some embodiments, the mass percentage of carbon in the silicon-carbon composite material is 30% to 70%. This improves the conductivity of the silicon-carbon composite material and allows space for the expansion of silicon particles, thus extending the battery's lifespan.

[0025] In some embodiments, the silicon-carbon composite material satisfies one or more of the following characteristics:

[0026] (1) The silicon-carbon composite material includes a core, which includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon. Optionally, the porous carbon is hard carbon.

[0027] (2) Silicon-carbon composite materials also include a carbon-containing coating layer, which coats the surface of the core;

[0028] (3) The silicon content in the silicon-carbon composite material is 30% to 70% by mass;

[0029] (4) The average particle size of the silicon-carbon composite material is 2μm to 15μm, and optionally, it is 5μm to 11μm;

[0030] (5) The powder resistivity of silicon-carbon composite material at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0031] (6) The BET specific surface area of ​​the silicon-carbon composite material is 1.0 m². 2 / g~6.7m 2 / g.

[0032] Silicon-carbon composite materials meet one or more of the above characteristics, which is beneficial to improving the structural stability and specific capacity of the material, thereby improving the energy density, service life and fast charging performance of secondary batteries.

[0033] In some implementations, the secondary battery has a wound structure. This helps to reduce the amount of electrolyte squeezed out from the middle of the cell, reduce lithium plating, thereby reducing cell polarization and improving the cycle performance and fast-charging performance of the secondary battery.

[0034] The second aspect of this disclosure provides an electrical device, which includes the secondary battery of the first aspect. Attached Figure Description

[0035] Figure 1 is a schematic diagram of the negative electrode in the second direction of a secondary battery according to an embodiment of the present disclosure when the battery is at 100% SOC.

[0036] Figure 2 is a schematic diagram of the negative electrode in the second direction of a secondary battery according to an embodiment of the present disclosure when the battery is in a state of charge of 0% (SOC).

[0037] Figure 3 is a schematic diagram of the negative electrode in the second direction of a secondary battery with a state of charge of 100% (SOC) according to another embodiment of the present disclosure.

[0038] Figure 4 is a schematic diagram of the negative electrode in the second direction of a secondary battery with a state of charge of 0% (SOC) according to another embodiment of the present disclosure.

[0039] Figure 5 is a schematic diagram of a battery cell according to one embodiment of the present disclosure.

[0040] Figure 6 is an exploded view of a battery cell according to an embodiment of the present disclosure shown in Figure 5.

[0041] Figure 7 is a schematic diagram of a battery module according to one embodiment of the present disclosure.

[0042] Figure 8 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0043] Figure 9 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 8.

[0044] Figure 10 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0045] Figure 11 is a cross-sectional view of the negative electrode sheet in the second direction during the preparation process of Embodiment 1 of this disclosure.

[0046] Explanation of reference numerals in the attached drawings: 10 Negative electrode sheet; 101 Negative current collector; 102 Second negative electrode film layer; 103 First negative electrode film layer; 104 Gasket; 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

[0047] 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.

[0048] 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.

[0049] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0050] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0051] 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.

[0052] Unless otherwise specified, the terminology used in this disclosure has the common meaning as commonly understood by those skilled in the art.

[0053] 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.

[0054] Currently, in order to improve the energy density of batteries, silicon-based materials are usually added to the negative electrode. However, silicon-based materials have a high expansion rate, which can easily squeeze the electrolyte out of the cell, thereby causing cell polarization and affecting the cycle life of the secondary battery.

[0055] To address these issues, current methods often employ techniques such as reducing the compaction density of the negative electrode, creating pores in the negative electrode, or adding pressure-resistant particles to the separator coating to resist the volume expansion of silicon-based materials. However, these methods often negatively impact the volumetric energy density of the secondary battery.

[0056] Based on this, the present disclosure provides a secondary battery and an electrical device, wherein the secondary battery of the present disclosure has excellent cycle performance while taking into account the battery energy density.

[0057] Secondary batteries

[0058] The first aspect of this disclosure provides a secondary battery including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the negative electrode film layer comprising a silicon-carbon composite material, the negative electrode sheet extending along a first direction, the length of the negative electrode sheet in the first direction being greater than the length in a second direction, the second direction being perpendicular to the first direction, and the secondary battery being in a state of charge of 100%, in a cross-section of the negative electrode sheet in the second direction, the thickness of the negative electrode sheet decreasing from both ends towards the center.

[0059] The inventors discovered that silicon-containing batteries always preferentially exhibit lithium plating in the center after expansion, and the thickness of the negative electrode sheet in the second direction increases more significantly in the center compared to the sides after charging. This disclosure addresses this by making the thickness of the negative electrode sheet in the second direction decrease from both ends towards the center when the secondary battery is at 100% SOC. This allows space to be reserved for the increased thickness in the center due to silicon expansion, reducing the amount of electrolyte squeezed out from the center of the cell, thus achieving electrolyte retention. This ensures electrolyte is present in the center, reducing cell polarization, improving lithium plating, and enhancing the cycle performance of the secondary battery. Furthermore, this disclosure sets the thickness of the negative electrode sheet in the second direction to decrease from both ends towards the center when the SOC is 100%, without changing the amount of negative electrode active material added. Therefore, it can improve the cycle performance of the secondary battery while maintaining energy density.

[0060] In addition, the silicon-carbon composite material contained in the negative electrode film has strong support and can form a top-to-bottom through-structure in the negative electrode film, thereby ensuring the structural stability of the negative electrode sheet when lithium ions are inserted at high concentrations. This can reduce the electrolyte being squeezed out from the middle of the cell, which is beneficial to reduce cell polarization and improve the cycle performance of the secondary battery.

[0061] In this disclosure, a state of charge (SOC) of 100% refers to a secondary battery that is fully charged.

[0062] In this disclosure, the first direction is the extension direction of the negative electrode sheet, representing the length direction of the electrode sheet. The second direction is perpendicular to the first direction, representing the width direction of the electrode sheet.

[0063] In this disclosure, "decreasing from both ends to the center" means that the overall trend is decreasing, including various methods such as step-like decreasing, linear decreasing, and arc-shaped decreasing.

[0064] In some embodiments, when the secondary battery is at 100% SOC, the minimum thickness X1 and maximum thickness Y1 of the negative electrode film in the cross-section of the negative electrode sheet in the second direction satisfy the following relationship: 1.0 < Y1 / X1 ≤ 1.2, optionally, 1.01 ≤ Y1 / X1 ≤ 1.1. By keeping the ratio of Y1 to X1 within the above range, the thickness of the negative electrode sheet on both sides in the second direction is greater than the thickness in the middle, which helps to reduce the electrolyte being squeezed out from the middle of the cell, reduce cell polarization, and improve the cycle performance of the secondary battery. For example, Y1 / X1 is a value within a range of 1.01, 1.05, 1.1, 1.15, 1.2, or any two of these values.

[0065] In some embodiments, when the secondary battery is at 0% SOC, the minimum thickness X2 and maximum thickness Y2 of the negative electrode film in the cross-section of the negative electrode sheet in the second direction satisfy the following relationship: 1.0≤Y2 / X2≤1.1, optionally, 1.0≤Y2 / X2≤1.05.

[0066] In this disclosure, a state of charge (SOC) of 0% refers to the state of a fully discharged secondary battery.

[0067] When the secondary battery is at 0% SOC, by keeping the ratio of Y2 to X2 within the aforementioned range, it is beneficial to ensure that the negative electrode sheet forms a structure in the second direction with a thickness decreasing from both ends to the center after the battery is charged. This reduces the amount of electrolyte squeezed out from the middle of the cell, which helps to reduce cell polarization and improve the cycle performance of the secondary battery. For example, Y2 / X2 is a value within a range of 1.0, 1.03, 1.05, 1.07, 1.1, or any combination thereof.

[0068] In some embodiments, when the secondary battery is at 0% SOC, the minimum thickness X2 and maximum thickness Y2 of the negative electrode film satisfy the following relationship: 0 μm ≤ Y2 - X2 ≤ 6 μm. When X2 and Y2 satisfy this relationship, it is beneficial to form a structure in which the thickness of the negative electrode sheet decreases from both ends to the center in the second direction after the battery is charged. This reduces the amount of electrolyte squeezed out from the middle of the cell, which helps to reduce cell polarization and improve the cycle performance of the secondary battery. For example, Y2 - X2 is a value within a range of 0 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, or any combination thereof.

[0069] In some embodiments, in the second direction of the negative electrode sheet, the mass percentage of silicon in the negative electrode film layer decreases from both ends to the center. This can be achieved by using a partitioned coating method, for example, coating different concentrations of slurry in the middle and side regions respectively, to make the mass percentage of silicon in the negative electrode film layer decrease from both ends to the center. During battery charging and discharging, the silicon in the negative electrode film layer undergoes volume expansion. Therefore, by setting the mass percentage of silicon in the negative electrode film layer to decrease from both ends to the center in the second direction, the expansion degree of the negative electrode film layer in the middle region can be made lower than that in the side regions. This results in the negative electrode sheet forming a structure where the thickness decreases from both ends to the center in the second direction after battery cycling. This reduces the amount of electrolyte squeezed out from the middle of the cell, thereby achieving the purpose of electrolyte locking, which is beneficial for reducing cell polarization, reducing intermediate lithium plating, and improving the cycle life of the secondary battery.

[0070] In some embodiments, the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, which are stacked in a direction away from the negative electrode current collector. The first negative electrode film layer comprises a silicon-carbon composite material, and the second negative electrode film layer comprises a graphite material. This helps to reduce the expansion of the silicon-carbon composite material and improve the cycle life of the secondary battery.

[0071] In some embodiments, the mass percentage of carbon in the silicon-carbon composite material is 30% to 70%. A mass percentage of carbon in the silicon-carbon composite material within this range can improve its electrical conductivity, adequately accommodate silicon particles, and fully utilize the advantages of silicon. Exemplarily, the mass percentage of carbon in the silicon-carbon composite material is a value within a range of 30%, 40%, 50%, 60%, 70%, or any combination thereof. The mass percentage of silicon in the silicon-carbon composite material is 30% to 70%.

[0072] In some embodiments, as shown in Figures 1 and 2, the second negative electrode film layer 102 is located between the first negative electrode film layer 103 and the negative electrode current collector 101. In some embodiments, as shown in Figures 3 and 4, the first negative electrode film layer 103 is located between the second negative electrode film layer 102 and the negative electrode current collector 101.

[0073] In some embodiments, when the secondary battery is at 0% SOC, in the cross-section of the negative electrode sheet in the second direction, the minimum thickness of the first negative electrode film layer in the middle region is d1, and the maximum thickness of the first negative electrode film layer in the two side regions is d2, where d2 > d1. Therefore, when the secondary battery is at 0% SOC, the maximum thickness of the first negative electrode film layer in the second direction is greater on both sides than in the middle region. This is beneficial for forming a structure with greater thickness on both sides and less thickness in the middle after charging, increasing the space for electrolyte in the middle region, thereby reducing cell polarization and improving the cycle life and fast-charging performance of the secondary battery.

[0074] In this disclosure, the negative electrode film layer includes a central region and two side regions in its cross-section in the second direction. When the length from one end of the electrode to the center of the negative electrode film layer in the cross-section in the second direction is L, the central region refers to the region formed by extending 80%L from the center of the negative electrode film layer to both ends, and the other part is the side regions.

[0075] In some embodiments, the minimum thickness d1 of the first negative electrode film layer in the middle region can be 0. This is beneficial for forming a structure with greater thickness on both sides and less thickness in the middle after the battery is charged.

[0076] In some implementations, when d1 is not 0, d2 and d1 satisfy the following relationship: d2 / d1 ≥ 1.01. When d2 and d1 satisfy this relationship, it is beneficial to form a structure with greater thickness on both sides and less thickness in the middle after the battery is charged. This increases the space in the middle region to accommodate the electrolyte, reduces cell polarization, and improves the cycle life of the secondary battery. For example, d2 / d1 is a value within a range of 1.01, 1.03, 1.05, 1.07, 1.1, or any two of these values.

[0077] In some embodiments, in the cross-section of the negative electrode sheet in the second direction, the maximum thickness of the second negative electrode film layer in the middle region is d3. When d1 is not 0, then d3 / d1 ≥ 0.2, and optionally, d3 / d1 ≥ 0.5. When d1 and d3 satisfy the above relationship, it is beneficial to form a structure with a larger thickness on both sides and a smaller thickness in the middle after the battery is charged, thereby increasing the space for electrolyte in the middle region, reducing cell polarization, and improving the cycle life of the secondary battery. For example, d3 / d1 is a value within a range of 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or any two of them. The minimum thickness of the second negative electrode film layer in the two side regions is d4, and d4 can be 0.

[0078] In this disclosure, the thicknesses of the negative electrode sheet, the first negative electrode film layer, and the second negative electrode film layer in the cross-section of the negative electrode sheet in the second direction can be observed and measured in the following manner: Specifically, the battery is disassembled to obtain the negative electrode sheet, which is then fixed on a sample stage; the sample stage is installed into a sample holder and locked in place; the power of an argon ion cross-section polishing instrument (e.g., the IB-09010CP argon ion cross-section polishing instrument from JEOL Corporation of 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 swing mode to begin polishing. After polishing, perform ion polishing cross-section elemental analysis to distinguish the first negative electrode film layer and the second negative electrode film layer, thereby obtaining the thickness of the negative electrode sheet, the first negative electrode film layer, and the second negative electrode film layer.

[0079] In some embodiments, the mass percentage of silicon-carbon composite material in the first negative electrode film layer is 4% to 97.5%, and the mass percentage M1 of silicon element in the first negative electrode film layer is 2.5% to 68%. This effectively increases the specific capacity of the negative electrode active material through silicon element and helps control the expansion of silicon element during charging, thereby improving the stability of the first negative electrode film layer. Exemplarily, the mass percentage of silicon-carbon composite material in the first negative electrode film layer is a value within the range of 4%, 6%, 8%, 10%, 30%, 50%, 70%, 90%, 97%, 97.5%, or any combination thereof. Exemplarily, the mass percentage M1 of silicon element in the first negative electrode film layer is a value within the range of 2.5%, 5%, 10%, 30%, 50%, 60%, 68%, or any combination thereof.

[0080] In some embodiments, the second negative electrode film layer may further comprise a silicon material, such as a silicon-oxygen material or the aforementioned silicon-carbon composite material. The mass percentage M2 of silicon in the second negative electrode film layer is 0% to 40%. This sufficiently improves the specific capacity of the negative electrode active material and facilitates control of silicon expansion during charging, thereby enhancing the stability of the second negative electrode film layer. Exemplarily, the mass percentage of silicon in the second negative electrode film layer is a value within the range of 0%, 10%, 20%, 30%, 40%, or any combination thereof.

[0081] In some embodiments, 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, then 2.5% ≤ M1 - M2 ≤ 68.0%. When M1 and M2 satisfy the above relationship, the mass percentage of silicon in the first negative electrode film is higher, which can create a gradient distribution characteristic with high capacity in the first negative electrode film and low capacity in the second negative electrode film. This is beneficial for increasing the number of lithium intercalation active sites in the first negative electrode film and reducing expansion, thereby improving the cycle life of the secondary battery. For example, M1-M2 is a value within a range of 2.5%, 5.0%, 6.0%, 10.0%, 30.0%, 38.0%, 50.0%, 60.0%, 68.0%, or any combination thereof.

[0082] In some embodiments, as shown in Figures 1 and 2, the second negative electrode film layer 102 is located between the first negative electrode film layer 103 and the negative electrode current collector 101, and the graphite material in the second negative electrode film layer accounts for 50% to 99% of the total mass. A higher proportion of graphite material in the second negative electrode film layer, especially when the second negative electrode film layer consists solely of graphite as the active material and the silicon-carbon composite material is entirely located in the first negative electrode film layer, is beneficial for improving the contact between the silicon-carbon composite material and the electrolyte, promoting rapid lithium-ion insertion into the silicon-carbon composite material, thereby reducing the polarization of the cell during charging and improving the cycle performance of the secondary battery.

[0083] In some embodiments, as shown in Figures 3 and 4, the first negative electrode film layer 103 may be located between the second negative electrode film layer 102 and the negative electrode current collector 101.

[0084] In the cross-section along the second direction of the negative electrode, the interface between the first and second negative electrode films can be an arc shape as shown in Figures 1-4, or it can be other shapes, such as a trapezoid or a horizontal straight line. When it is a straight line, the mass percentage of silicon in the negative electrode film can be reduced from both ends to the center by coating different concentrations of slurry in the middle and side regions respectively.

[0085] The silicon-carbon composite material disclosed herein can be prepared using conventional preparation methods, such as depositing nano-silicon materials on porous carbon by chemical vapor deposition, and can be further coated with carbon, such as using amorphous carbon coating.

[0086] In some embodiments, the silicon-carbon composite material satisfies one or more of the following characteristics:

[0087] (1) The silicon-carbon composite material includes a core, which includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon, and optionally the porous carbon is hard carbon;

[0088] (2) Silicon-carbon composite materials also include a carbon-containing coating layer, which coats the surface of the core;

[0089] (3) The silicon content in the silicon-carbon composite material is 30% to 70% by mass;

[0090] (4) The average particle size of the silicon-carbon composite material is 2μm to 15μm, and optionally 5μm to 11μm;

[0091] (5) The powder resistivity of silicon-carbon composite material at 8 MPa is 4 Ω·cm to 17 Ω·cm;

[0092] (6) The BET specific surface area of ​​the silicon-carbon composite material is 1.0 m². 2 / g~6.7m 2 / g.

[0093] In some embodiments, the silicon-carbon composite material 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 nanoscale silicon particles, effectively mitigating the stress compression caused by expansion during charging. Especially when the silicon-containing particles are nanoscale in size, the specific capacity is higher and they are better dispersed in the pores of the porous carbon. Furthermore, the buffering effect of the porous carbon's pores on expansion can be more fully utilized. When this silicon-carbon composite material is used in wound electrode assemblies, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0094] 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.

[0095] 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.

[0096] In some embodiments, the silicon-carbon composite material further includes a carbon-containing coating layer that coats the surface of the core. This can improve the conductivity of the silicon-carbon composite material and reduce the internal impedance of the battery cell. At the same time, it can effectively reduce 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 material.

[0097] In some embodiments, the silicon content in the silicon-carbon composite material 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.

[0098] In this disclosure, the method for testing the silicon content in silicon-carbon composite materials can be any method known in the art. As an example, the following method can be used: a certain amount of silicon-carbon composite material is taken, and the mass of silicon in the silicon-carbon composite material is obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of silicon in the silicon-carbon composite material can then be calculated.

[0099] In addition to providing structural support and buffering for the expansion of silicon materials, the pores in silicon-carbon composite materials also form between the particles. To further improve the flow of lithium ions through the intraparticle and interparticle pores, in some embodiments, the average particle size of the silicon-carbon composite material is 2 μm to 15 μm. Optionally, the average particle size of the silicon-carbon composite material is 5 μm to 11 μm, and more preferably, it is 7 μm to 10 μm. This creates a particle size distribution between the average particle size of the silicon-carbon composite material and the average particle size of the graphite material, thus facilitating the use of interparticle gaps to increase the compaction of the negative electrode active layer, thereby further improving the energy density of the battery cell.

[0100] The average particle size of the aforementioned silicon-carbon composite material can be tested using equipment and methods known in the art. For example, a scanning electron microscope (SEM) can be used (e.g., ZEISS Sigma 300), referring to JY / T010-1996, to obtain SEM images of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample of length × width = 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5) within the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composite materials), 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.

[0101] In some embodiments, the powder resistivity of the silicon-carbon composite material at 8 MPa is 4 Ω·cm to 17 Ω·cm. Controlling the powder resistivity improves the conductivity of the silicon-carbon composite material, thereby increasing the charging rate of the battery cell. Exemplarily, the powder resistivity of the silicon-carbon composite material at 8 MPa is a value within the range of 4 Ω·cm, 5 Ω·cm, 7 Ω·cm, 10 Ω·cm, 13 Ω·cm, 15 Ω·cm, 17 Ω·cm, or any combination thereof.

[0102] In this disclosure, the powder resistivity of silicon-carbon composite materials 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.

[0103] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite material is 1.0 m². 2 / g~6.7m 2 / g. For example, the specific surface area of ​​the silicon-carbon composite material 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.

[0104] In this disclosure, the method for testing the BET specific surface area of ​​silicon-carbon composite materials can employ methods known in the art. As an example, referring to GB / T 19587-2017, a nitrogen adsorption specific surface area analysis method can be used. The sample tube containing the first graphite material sample is immersed in liquid nitrogen at -196°C. The amount of nitrogen adsorbed on the surface of the solid sample at different pressures (0.05–0.30) is measured. Based on the BET multilayer adsorption theory and calculation formula, the amount of monolayer adsorption of the sample is obtained, and thus the BET specific surface area is calculated. This test can be performed using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0105] In some embodiments, the secondary battery has a wound structure. Therefore, a wound secondary battery can form a structure where the thickness of the negative electrode sheet decreases from both ends to the center in the second direction when the state of charge is 100%. This helps to reduce the amount of electrolyte squeezed out from the center of the cell, achieving electrolyte retention, thereby reducing cell polarization, reducing lithium plating, and improving the cycle life of the secondary battery.

[0106] 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.).

[0107] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0108] 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.

[0109] In some embodiments, a double-layer coating extrusion head can be used to simultaneously coat and form the negative electrode sheet. By designing the shape of the spacer, the interface between the first and second negative electrode film layers can be made into different shapes. For example, by using an arc-shaped spacer, the second negative electrode film layer can be made thicker in the middle and thinner on both sides, while the first negative electrode film layer can be made thinner in the middle and thicker on both sides.

[0110] In some implementations, the negative electrode sheet can also be formed through multiple coating processes. For example, thin coating areas and thick coating areas can be pre-designed and coated separately. Alternatively, a conventional coating method (with uniform thickness) can be used, in which different concentrations of slurry are applied to ensure that the silicon content in the middle region of the negative electrode film is lower than that in the two outer regions.

[0111] In some embodiments, the coating weight of the first negative electrode film layer is 15 mg / 1540.25 mm. 2 ~100mg / 1540.25mm 2 Optionally, it is 20mg / 1540.25mm. 2 ~90mg / 1540.25mm 2 The coating weight of the first negative electrode film is within the above-mentioned range, which is beneficial for improving the cycle life and fast-charging performance of the secondary battery while maintaining energy density. For example, the coating weight of the first negative electrode film is 15mg / 1540.25mm². 2 20mg / 1540.25mm2 40mg / 1540.25mm 2 60mg / 1540.25mm 2 90mg / 1540.25mm 2 100mg / 1540.25mm 2 Or the value between any two of them within a range.

[0112] In some embodiments, the coating weight of the second negative electrode film is 15 mg / 1540.25 mm. 2 ~100mg / 1540.25mm 2 Optionally, it is 20mg / 1540.25mm. 2 ~90mg / 1540.25mm 2 The coating weight of the second negative electrode film is within the above-mentioned range, which is beneficial for improving the cycle life and fast-charging performance of the secondary battery while maintaining energy density. For example, the coating weight of the second negative electrode film is 15mg / 1540.25mm. 2 20mg / 1540.25mm 2 40mg / 1540.25mm 2 60mg / 1540.25mm 2 90mg / 1540.25mm 2 100mg / 1540.25mm 2 Or the value between any two of them within a range.

[0113] 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.

[0114] 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.

[0115] Positive electrode sheet

[0116] 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 includes a positive electrode active material, which may be a positive electrode active material known in the art and is not particularly limited.

[0117] 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.).

[0118] 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 0.05At 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] electrolytes

[0125] 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.

[0126] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] Separating membrane

[0131] 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.

[0132] 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.

[0133] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0134] 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.

[0135] 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.

[0136] 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 5 shows a square battery cell 5 as an example.

[0137] In some embodiments, referring to FIG6, 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.

[0138] 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.

[0139] Figure 7 shows a battery module 4 as an example. Referring to Figure 7, 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.

[0140] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0141] 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.

[0142] Figures 8 and 9 illustrate a battery pack 1 as an example. Referring to Figures 8 and 9, the battery pack 1 may include a battery compartment and multiple battery modules 4 disposed within the battery compartment. The battery compartment includes an upper compartment 2 and a lower compartment 3, with the upper compartment 2 covering the lower compartment 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 compartment.

[0143] Electrical appliances

[0144] A second aspect of this disclosure provides an electrical device that includes the secondary battery provided in the first aspect of this disclosure.

[0145] 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.

[0146] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0147] Figure 10 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.

[0148] 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.

[0149] Example

[0150] 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.

[0151] Example 1

[0152] Preparation of secondary batteries:

[0153] S1, Preparing the negative electrode sheet, specifically including the following steps:

[0154] a. Preparation of the first negative electrode slurry: Graphite material and silicon-carbon composite material (the silicon-carbon composite material includes a core and a coating layer located on the surface of the core; the core includes a porous carbon matrix and silicon particles dispersed in the pores of the porous carbon matrix; the average particle size of the silicon-carbon composite material is 2 μm, and the BET specific surface area is 5.5 m²) are prepared. 2 The powder resistivity at 8MPa is 10Ω·cm, the mass percentage of silicon in the silicon-carbon composite material is 45%), binder (styrene-butadiene rubber), conductive agent (carbon black: carbon nanotubes = 8.5:1.5), and dispersant (carboxymethyl cellulose) are mixed thoroughly in an appropriate amount of deionized water at a weight ratio of 55:40:3:1:1 to form the first negative electrode slurry.

[0155] b. Preparation of the second negative electrode slurry: Graphite material, acetylene black, sodium carboxymethyl cellulose, and styrene-butadiene rubber are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 96:2:1:1 to form the second negative electrode slurry;

[0156] c. A double-layer coating extrusion head is used, employing an arc-shaped gasket, as shown in Figure 11. The maximum distance d3' from the center of the gasket to the current collector is 920 μm, and the minimum distance d4' from both ends of the gasket to the current collector is 850 μm. The first negative electrode slurry and the second negative electrode slurry are extruded simultaneously. The second electrode slurry is coated onto the copper foil of the negative electrode current collector, as shown in Figure 11, located below the arc-shaped gasket, forming a convex second negative electrode film layer. The first negative electrode slurry is coated onto the second negative electrode slurry, as shown in Figure 11, located above the arc-shaped gasket. After drying and cold pressing, a concave first negative electrode film layer is formed. The thinnest thickness d1' in the middle of the first negative electrode film layer is 80 μm, and the thickest thickness d2' at both ends is 150 μm. The coating weight ratio of the first negative electrode slurry to the second negative electrode slurry is 1:5, and the coating weight of the first negative electrode film layer is 18.0 mg / 1540.25 mm. 2 The coating weight of the second negative electrode film is 90.0 mg / 1540.25 mm. 2 ;

[0157] S2, Preparing a secondary battery, specifically includes the following steps:

[0158] Preparation of the positive electrode: The nickel-cobalt-manganese ternary positive electrode material (NCM) is prepared... 811 Acetylene black and polyvinylidene fluoride are mixed in a weight ratio of 97:2:1 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, a positive electrode sheet is obtained.

[0159] 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 1.0 mol / L.

[0160] Separating membrane: Polyethylene microporous film is used 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 7μm.

[0161] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then formed by bending and winding. The electrode assembly is placed in outer packaging, dried, and then injected with the prepared electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0162] Multiple secondary batteries were prepared according to the above method and used for the following tests.

[0163] Parameter testing:

[0164] (1) Thickness test

[0165] The secondary battery prepared above was charged to 4.3V and reached 100% SOC. The secondary battery was disassembled, and the negative electrode sheet was extended along the first direction. In the cross section of the negative electrode sheet in the second direction, the thickness distribution of the negative electrode film was observed, and the minimum thickness X1 and maximum thickness Y1 of the negative electrode film were measured.

[0166] The secondary battery prepared above was discharged to 2.5V, reaching 0% SOC (State of Charge). The secondary battery was disassembled, and the negative electrode sheet was extended along the first direction. In the cross-section of the negative electrode sheet along the second direction, the minimum thickness X2, the maximum thickness Y2, the minimum thickness d1 of the first negative electrode sheet in the middle region, the maximum thickness d2 of the first negative electrode sheet in both side regions, the maximum thickness d3 of the second negative electrode sheet in the middle region, and the minimum thickness d4 of the second negative electrode sheet in both side regions were measured. The results are shown in Table 1.

[0167] (2) Element content test

[0168] The mass percentage of silicon in the negative electrode film was measured using an inductively coupled plasma optical emission spectrometer (EXPEC 6000). The ICP results showed that the mass percentage of silicon (M1) in the first negative electrode film of Example 1 was 18%, and the mass percentage of silicon (M2) in the second negative electrode film was 0%.

[0169] Performance testing of secondary batteries:

[0170] (1) Battery capacity test

[0171] At 25℃, the secondary battery was charged at a constant current rate of 0.33C to 4.3V, then charged at a constant voltage rate to a current of 0.05C. After standing for 5 minutes, it was discharged at a constant current rate of 0.33C to 2.5V. The discharge capacity at this point was recorded, which is the 0.33C discharge capacity. The test results are recorded in Table 2 below.

[0172] (2) Cyclic performance test

[0173] At 25°C, the secondary battery is charged to 4.25V with a constant current of 0.5C, then charged at a constant voltage of 4.25V until the current drops to 0.05C, and then discharged to 2.5V with a constant current of 0.5C. The discharge specific capacity of the first cycle (Cd1) is obtained. This charging and discharging process is repeated until the 500th cycle, and the discharge specific capacity after 500 cycles is denoted as Cdn.

[0174] Capacity retention (%) = Discharge specific capacity after 500 cycles (Cdn) / Discharge specific capacity in the first cycle (Cd1). The test results are recorded in Table 2 below.

[0175] Examples 2 to 4

[0176] The secondary battery was prepared using the same method as in Example 1, except that the shape of the gasket was changed so that d1', d2', d3' and d4' were the values ​​shown in Table 1. The test results are shown in Tables 1 and 2 below.

[0177] Comparative Example 1

[0178] The secondary battery was prepared using the same method as in Example 1, except that a planar spacer was used. The upper and lower layers separated by the spacer were of the same thickness in the second direction, so that the thickness of the first negative electrode film and the second film were basically consistent within the error range in the second direction of the coated negative electrode sheet.

[0179] Examples 2 to 4 and Comparative Example 1 were tested using the same test method as Example 1. The test results are shown in Tables 1 and 2 below.

[0180] Table 1

[0181] Table 2

[0182] As can be seen from Tables 1 and 2, compared with Comparative Example 1, Examples 1 to 4, by controlling the secondary battery to be at 100% SOC, have a negative electrode sheet thickness that decreases from the center to both ends in the cross-section of the negative electrode sheet in the second direction, which improves the cycle performance of the secondary battery and achieves excellent energy density.

[0183] Examples 5 to 9

[0184] The secondary batteries were prepared using the same method as in Example 1, except that the shape of the gasket was changed so that d1', d2', d3', and d4' were the values ​​shown in Table 3. Examples 5 to 9 were tested using the same testing method as in Example 1. The test results are shown in Tables 3 and 4 below.

[0185] Table 3

[0186] Table 4

[0187] As can be seen from Tables 3 and 4, Examples 5 to 9, by ensuring that the secondary battery is in a state of charge of 0% (SOC), has d2 / d1 greater than or equal to 1.01 and d3 / d1 greater than or equal to 0.2 in the cross-section of the negative electrode sheet in the second direction, can improve the cycle performance of the secondary battery without affecting its energy density.

[0188] Examples 10 to 12

[0189] The secondary battery was prepared using the same method as in Example 1, except that the second negative electrode film also contained a silicon-carbon composite material (the mass percentage of silicon in the silicon-carbon composite material was 45%). The mass percentages of silicon in the first negative electrode film M1, the second negative electrode film M2, the first negative electrode film, and the second negative electrode film were adjusted as shown in Table 5 below.

[0190] Example 13

[0191] The secondary battery was prepared using the same method as in Example 1, except that the second negative electrode film also contained a silicon-carbon composite material (the mass percentage of silicon in the silicon-carbon composite material was 70%). The mass percentages of silicon in the first negative electrode film M1, the second negative electrode film M2, the first negative electrode film, and the second negative electrode film were adjusted as shown in Table 5 below.

[0192] Examples 10 to 13 were tested using the same testing method as Example 1. The test results are shown in Tables 5 and 6 below.

[0193] Table 5

[0194] Table 6

[0195] As can be seen from Tables 5 and 6, Examples 10 to 13, by controlling the mass ratio of silicon element M1 in the first negative electrode film layer and the mass ratio of silicon element M2 in the second negative electrode film layer to satisfy: 2.5% ≤ M1 - M2 ≤ 68.0%, can better balance the energy density and cycle performance of the secondary battery.

[0196] 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 including 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 comprising a silicon-carbon composite material. The negative electrode plate extends along a first direction, and the length of the negative electrode plate in the first direction is greater than its length in the second direction, wherein the second direction is perpendicular to the first direction. When the secondary battery is at 100% SOC, in the cross-section of the negative electrode sheet in the second direction, the thickness of the negative electrode sheet decreases from both ends towards the center.

2. The secondary battery according to claim 1, wherein, When the secondary battery is at 100% SOC, in the cross-section of the negative electrode sheet in the second direction, the minimum thickness X1 and maximum thickness Y1 of the negative electrode film layer satisfy the following relationship: 1.0 < Y1 / X1 ≤ 1.

2.

3. The secondary battery according to claim 2, wherein, The minimum thickness X1 and maximum thickness Y1 of the negative electrode film layer satisfy the following relationship. 1.01≤Y1 / X1≤1.

1.

4. The secondary battery according to any one of claims 1 to 3, wherein, When the secondary battery is at 0% SOC, in the cross-section of the negative electrode sheet in the second direction, the minimum thickness X2 and maximum thickness Y2 of the negative electrode film layer satisfy the following relationship: 1.0≤Y2 / X2≤1.

1.

5. The secondary battery according to claim 4, wherein, The minimum thickness X2 and maximum thickness Y2 of the negative electrode film layer satisfy the following relationship. 1.0≤Y2 / X2≤1.

05.

6. The secondary battery according to claim 4 or 5, wherein, The minimum thickness X2 and maximum thickness Y2 of the negative electrode film layer satisfy the following relationship. 0μm≤Y2-X2≤6μm.

7. The secondary battery according to any one of claims 1 to 6, wherein, In the second direction of the negative electrode sheet, the mass percentage of silicon in the negative electrode film layer decreases from both ends to the center.

8. The secondary battery according to any one of claims 1 to 7, wherein, The negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, which are stacked in a direction away from the negative electrode current collector. The first negative electrode film layer contains the silicon-carbon composite material, and the second negative electrode film layer contains graphite material.

9. The secondary battery according to claim 8, wherein, When the secondary battery is at 0% SOC, in the cross-section of the negative electrode sheet in the second direction, the minimum thickness of the first negative electrode film layer in the middle region is d1, and the maximum thickness of the first negative electrode film layer in the two side regions is d2, then d2 > d1.

10. The secondary battery according to claim 9, wherein, d2 / d1≥1.

01.

11. The secondary battery according to claim 9, wherein, d1＝0。 12. The secondary battery according to claim 9 or 10, wherein, In the cross-section of the negative electrode sheet in the second direction, the maximum thickness of the second negative electrode film layer in the middle region is d3, then d3 / d1≥0.

2.

13. The secondary battery according to claim 12, wherein, d3 / d1≥0.

5.

14. The secondary battery according to any one of claims 8 to 13, wherein, The mass percentage (M1) of silicon in the first negative electrode film layer is 2.5% to 68%.

15. The secondary battery according to any one of claims 8 to 14, wherein, The second negative electrode film also contains silicon, and the mass percentage (M2) of silicon in the second negative electrode film is 0% to 40%.

16. The secondary battery according to claim 15, wherein, The mass percentage of silicon in the first negative electrode film layer, M1, and the mass percentage of silicon in the second negative electrode film layer, M2, satisfy the following relationship: 2.5% ≤ M1 - M2 ≤ 68.0%.

17. The secondary battery according to any one of claims 8 to 16, wherein, The second negative electrode film layer is located between the first negative electrode film layer and the negative electrode current collector, and the graphite material in the second negative electrode film layer accounts for 50% to 99% of the total mass.

18. The secondary battery according to any one of claims 1 to 17, wherein, The mass percentage of carbon in the silicon-carbon composite material is 30% to 70%.

19. The secondary battery according to any one of claims 1 to 18, wherein, The silicon-carbon composite material satisfies one or more of the following characteristics: (1) The silicon-carbon composite material includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon, and optionally the porous carbon is hard carbon; (2) The silicon-carbon composite material further includes a carbon-containing coating layer, which coats the surface of the porous carbon and / or silicon-containing material; (3) The mass percentage of silicon in the silicon-carbon composite material is 30% to 70%; (4) The average particle size of the silicon-carbon composite material is 2μm to 15μm, and optionally, it is 5μm to 11μm; (5) The powder resistivity of the silicon-carbon composite material at 8 MPa is 4 Ω·cm to 17 Ω·cm; (6) The BET specific surface area of ​​the silicon-carbon composite material is 1.0 m². 2 / g~6.7m 2 / g.

20. The secondary battery according to any one of claims 1 to 19, wherein, The secondary battery has a wound structure.

21. An electrical device comprising a secondary battery as described in any one of claims 1 to 20.

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