Secondary battery and electronic device

Abstract

This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode and a negative electrode. The positive electrode includes a positive electrode material layer, and the negative electrode includes a negative electrode material layer. The negative electrode material layer consists only of a first material layer and a second material layer connected in sequence. The width of the first material layer is W mm, and the width of the negative electrode material layer is greater than the width of the positive electrode material layer. Half of the width difference between the negative and positive electrode material layers is L mm, and 0.2 ≤ W / L ≤ 4. The first material layer includes a first negative electrode active material, which includes a first graphite material. The second material layer includes a second negative electrode active material, which includes a second graphite material and a silicon-based material. Multiple grooves are provided on the first material layer, extending along the width direction of the unfolded negative electrode and spaced apart along the length direction of the unfolded negative electrode. This arrangement helps improve the cycle performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, have the characteristics of high specific energy, high operating voltage, low self-discharge rate, small size, and light weight, and are widely used in the consumer electronics field.

[0003] Currently, some lithium-ion battery designs employ a full-tab design, where the positive and negative tabs extend from opposite directions and are fabricated using full-tab stacking or flattening techniques. However, the full-tab flattening structure introduces electrode wetting problems, especially at the electrode edges. Electrolyte accumulates on both sides of the material layer, continuously causing side reactions and leading to capacity loss. Furthermore, when the negative electrode active material includes silicon-based materials, the crosstalk between Si and graphite further exacerbates the side reactions at the negative electrode edges, resulting in a decline in the cycle performance of the lithium-ion battery. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device to reduce side reactions in the electrode edge region, especially to reduce side reactions caused by crosstalk between two different materials, Si and graphite, thereby improving the cycle performance of the secondary battery.

[0005] It should be noted that while this application uses lithium-ion batteries as an example of secondary batteries to explain the invention, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0006] The first aspect of this application provides a secondary battery, comprising a positive electrode and a negative electrode. The positive electrode includes a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector. The negative electrode includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. Along the width direction of the unfolded negative electrode, the negative electrode material layer comprises only a first material layer and a second material layer connected in sequence. The width of the first material layer is W mm, and the width of the negative electrode material layer is greater than the width of the positive electrode material layer. Half of the width difference between the negative and positive electrode material layers is L mm, and 0.2 ≤ W / L ≤ 4; optionally, 1 ≤ W / L ≤ 3. The first material layer includes a first negative electrode active material, which includes a first graphite material. The second material layer includes a second negative electrode active material, which includes a second graphite material and a silicon-based material. A plurality of grooves are provided on the first material layer, extending along the width direction of the unfolded negative electrode and spaced apart along the length direction of the unfolded negative electrode. By setting different material layers on different regions of the negative electrode and controlling the W / L value within the range of this application, it is beneficial to reduce the risk of lithium ions diffusing to the edge of the material layer and thus continuously causing side reactions. In particular, it reduces the side reactions caused by crosstalk between the two different materials, Si and graphite. Furthermore, by setting grooves on the first material layer, the electrolyte flow channels are increased, thereby improving the cycle performance of the secondary battery.

[0007] In one or more embodiments of this application, 0.2 ≤ L ≤ 5. By adjusting the value of L within the above range, the direct contact between the edge of the positive electrode material layer and the electrolyte is reduced, thereby lowering the risk of side reactions occurring at the edge of the positive electrode during charging and discharging. The secondary battery of this application exhibits good cycle performance.

[0008] In one or more embodiments of this application, along the length direction of the unfolded negative electrode sheet, the width of a single groove is W' μm, where 20 ≤ W' ≤ 400; optionally, 90 ≤ W' ≤ 150. By adjusting the value of W within the above range, the wider first material layer plays a certain role in physical blocking and buffering, which helps to reduce the occurrence of side reactions, especially reducing the side reactions caused by crosstalk between the two different materials, Si and graphite, thereby improving the cycle performance of the secondary battery.

[0009] In one or more embodiments of this application, along the width direction after the negative electrode sheet is unfolded, the ratio of the length of a single groove to the width of the first material layer is P, where 0.2 ≤ P ≤ 1; optionally, 0.4 ≤ P ≤ 1. By adjusting the value of P within the above range, it is beneficial to accelerate the conduction of the electrolyte, further reduce the risk of side reactions caused by electrolyte accumulation at the edge of the negative electrode sheet, and improve the cycle performance of the secondary battery while taking into account processing performance.

[0010] In one or more embodiments of this application, along the thickness direction of the negative electrode sheet, the thickness of the first material layer is H μm, and the average depth of the plurality of grooves is H' μm, where 2 ≤ H' ≤ 0.8H; optionally, 2 ≤ H' ≤ 0.6H. By adjusting the value of H' within the above range, it is beneficial to accelerate the conduction of the electrolyte, reduce the risk of side reactions caused by electrolyte accumulation at the edge of the negative electrode sheet, and improve the cycle performance of the secondary battery while taking into account processing performance.

[0011] In one or more embodiments of this application, 25 ≤ H ≤ 70. By adjusting the value of H within the above range, the secondary battery exhibits good cycle performance and high energy density while also possessing good structural stability.

[0012] In one or more embodiments of this application, the distance between two adjacent grooves along the length direction of the unfolded negative electrode sheet is N mm, where 1 ≤ N ≤ 5. By adjusting the value of N within the above range, it is beneficial to accelerate the conduction of electrolyte, further reduce the risk of side reactions caused by electrolyte accumulation at the edge of the negative electrode sheet, and improve the cycle performance of the secondary battery while taking into account processing performance.

[0013] In one or more embodiments of this application, the average particle size of the second graphite material is D1 μm, and the average particle size of the silicon-based material is D2 μm, where 9 ≤ D1 ≤ 15 and 7.3 ≤ D2 ≤ 10.3. By adjusting the values ​​of D1 and D2 within the above ranges, the structural stability of the negative electrode sheet is improved, and it is beneficial to reduce the side reactions that occur when the silicon-based material comes into direct contact with the electrolyte, thereby improving the cycle performance of the secondary battery.

[0014] In one or more embodiments of this application, the secondary battery satisfies one of the following conditions: (1) 1.08 ≤ D1 / D2 ≤ 1.8; (2) 1.3 ≤ D1 / D2 ≤ 1.5. By adjusting the value of D1 / D2 within the above range, the direct contact between the silicon-based material and the second graphite material, as well as the direct contact between the silicon-based material and the electrolyte, is reduced, thereby reducing the occurrence of side reactions, especially the side reactions caused by crosstalk between the two different materials, Si and graphite, and thus improving the cycle performance of the secondary battery.

[0015] In one or more embodiments of this application, the compaction density of the first material layer is PD1 g / cm³. 3 The compaction density of the second material layer is PD2 g / cm³. 3 The values ​​of PD1 and PD2 are 0.8 ≤ PD1 ≤ 1.18 and 0.8 ≤ PD2 ≤ 1.9. By adjusting the values ​​of PD1 and PD2 within the above range, the occurrence of side reactions is reduced, especially the side reactions caused by crosstalk between the two different materials, Si and graphite, thereby improving the cycle performance of the secondary battery while maintaining energy density.

[0016] In one or more embodiments of this application, the secondary battery satisfies one of the following conditions: (1) 1.0 ≤ PD2 / PD1 ≤ 1.6; (2) 1.1 ≤ PD2 / PD1 ≤ 1.2. By adjusting the value of PD2 / PD1 within the above range, the occurrence of side reactions is reduced, especially the side reactions caused by crosstalk between the two different materials, Si and graphite, thereby improving the cycle performance of the secondary battery while taking into account energy density.

[0017] In one or more embodiments of this application, based on the mass of the first material layer, the mass percentage of the first graphite material is 95% to 99%; based on the mass of the second material layer, the mass percentage of the second graphite material is 28% to 95%, and the mass percentage of the silicon-based material is 1% to 70%. By adjusting the values ​​of w1, w2, and w3 within the above ranges, it is beneficial to improve the structural stability of the negative electrode edge. Combined with the high-capacity silicon-based material and the relatively stable second graphite material in the second negative electrode material layer, while taking into account the kinetic performance and improving the structural stability and cycle performance of the secondary battery, the secondary battery has a high energy density.

[0018] A second aspect of this application provides an electronic device that includes the secondary battery found in any of the above embodiments. Therefore, the electronic device provided by this application has good performance.

[0019] The beneficial effects of the embodiments of this application are as follows:

[0020] This application provides a secondary battery and an electronic device. By setting different material layers on different regions of the negative electrode and controlling the W / L value within the range of this application, it is beneficial to reduce the risk of lithium ions diffusing to the edge of the material layer and thus continuously causing side reactions. In particular, it reduces the side reactions caused by crosstalk between the two different materials, Si and graphite. Furthermore, grooves are set on the first material layer to increase the electrolyte flow channels, thereby improving the cycle performance of the secondary battery.

[0021] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0023] Figure 1 This is a schematic diagram showing the unfolded electrode assembly in one embodiment of this application;

[0024] Figure 2This is a schematic diagram of the negative electrode sheet unfolded along its width direction, representing one embodiment of this application.

[0025] Figure 3 for Figure 2 A cross-sectional view along the AA direction;

[0026] Reference numerals: negative electrode 10; positive electrode 20; separator 30; negative current collector 11; negative electrode material layer 12; first material layer 121; second material layer 122; positive current collector 21; positive electrode material layer 22; groove 13. Detailed Implementation

[0027] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0028] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0029] The first aspect of this application provides a secondary battery, comprising a positive electrode and a negative electrode. The positive electrode includes a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector. The negative electrode includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. Along the width direction of the unfolded negative electrode, the negative electrode material layer comprises only a first material layer and a second material layer connected in sequence. The width of the first material layer is W mm, and the width of the negative electrode material layer is greater than the width of the positive electrode material layer. Half of the width difference between the negative electrode material layer and the positive electrode material layer is L mm, and 0.2 ≤ W / L ≤ 4; optionally, 1 ≤ W / L ≤ 3. For example, the value of W / L can be 0.2, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, or a range consisting of any two of these values. The first material layer includes a first negative electrode active material, which includes a first graphite material. The second material layer includes a second negative electrode active material, which includes a second graphite material and a silicon-based material. Multiple grooves are provided on the first material layer, extending along the width direction of the unfolded negative electrode sheet and spaced apart along the length direction of the unfolded negative electrode sheet.

[0030] In this application, the length direction of the negative electrode is defined as the X direction, the width direction as the Y direction, and the thickness direction as the Z direction. It can be understood that the length, width, and thickness directions of the positive electrode and the separator are the same as those of the negative electrode in the unfolded state.

[0031] For example, such as Figure 1 As shown, the negative electrode 10 includes a negative current collector 11 and a negative electrode material layer 12 on the surface of the negative current collector 11. The positive electrode 20 includes a positive current collector 21 and a positive electrode material layer 22 on the surface of the positive current collector 21. Along the width direction Y of the unfolded negative electrode 10, the negative electrode material layer 12 includes only a first material layer 121 and a second material layer 122 connected in sequence. The width of the first material layer 121 is W mm, and the width of the negative electrode material layer 12 is greater than the width of the positive electrode material layer 22. Half of the width difference between the negative electrode material layer 12 and the positive electrode material layer 22 is L mm. Figure 2 and Figure 3 As shown, a plurality of grooves 13 are provided on the first material layer 121. The plurality of grooves 13 extend along the width direction Y after the negative electrode sheet 10 is unfolded and are spaced apart along the length direction X after the negative electrode sheet 10 is unfolded.

[0032] The inventors discovered that at the wide edge of the negative electrode, especially in the region where the edge of the negative electrode material layer extends beyond the positive electrode material layer, electrolyte tends to accumulate on both sides of the material layer, leading to continuous side reactions and capacity loss. Furthermore, during charging and discharging, silicon-based materials, in particular, experience greater stress at the edge due to their volume expansion, damaging the solid electrolyte interphase (SEI) film. The expansion of silicon particles also compresses adjacent graphite particles, and the crosstalk between Si and graphite further exacerbates the side reactions. Therefore, this application addresses this issue by setting different material layers in different regions of the negative electrode and controlling the W / L ratio within the specified range. The wider first material layer acts as a physical barrier and buffer, and the first material layer at the edge of the negative electrode only includes relatively stable graphite. This approach balances energy density while reducing lithium-ion diffusion to the edge region of the negative electrode, reducing the expansion stress generated by the second material layer, and minimizing the impact of crosstalk between Si and graphite on the edge of the negative electrode. This effectively reduces the occurrence of side reactions, especially those caused by crosstalk between Si and graphite, thus improving the cycle performance of the secondary battery. Meanwhile, the grooves on the first material layer facilitate faster electrolyte flow, further reducing the risk of side reactions caused by electrolyte buildup at the negative electrode edge. They also provide stress buffer space, thereby further improving the cycle performance of the secondary battery. However, when the W / L ratio is too low (below the lower limit of this application), it fails to provide effective physical blocking; when the W / L ratio is too high (above the upper limit of this application), it affects the energy density of the secondary battery. Through these design features, side reactions at the electrode edge are reduced, especially those caused by crosstalk between Si and graphite, achieving good cycle performance while maintaining high energy density.

[0033] The aforementioned "positive electrode material layer located on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector along its thickness direction, or on two surfaces of the positive electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface of the positive electrode current collector, or only a portion thereof; this application has no particular limitation, as long as the purpose of this application is achieved. Similarly, the aforementioned "negative electrode material layer located on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be located on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface of the negative electrode current collector, or only a portion thereof; this application has no particular limitation, as long as the purpose of this application is achieved.

[0034] In one or more embodiments of this application, 0.2 ≤ L ≤ 5. For example, the value of L can be 0.2, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, or a range of any two of these values. By adjusting the value of L within the aforementioned range, direct contact between the edge of the positive electrode material layer and the electrolyte is reduced, lowering the risk of side reactions occurring at the edge of the positive electrode during charging and discharging. Furthermore, the wider first material layer provides a certain degree of physical barrier and buffering, and the first material layer at the edge of the negative electrode only comprises relatively stable graphite material. This reduces lithium-ion diffusion to the edge region of the negative electrode while maintaining energy density, thus reducing the expansion stress generated by the second material layer and the impact of crosstalk between Si and graphite on the edge of the negative electrode. This helps reduce the occurrence of side reactions, especially those caused by crosstalk between Si and graphite. The grooves on the first material layer facilitate faster electrolyte flow, further reducing the risk of electrolyte accumulation and side reactions at the edge of the negative electrode, while also providing stress buffer space, thereby improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application exhibits excellent cycle performance.

[0035] In one or more embodiments of this application, 0.04 ≤ W ≤ 20. For example, the value of W can be 0.04, 0.05, 0.08, 0.1, 0.2, 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range of any two of these values. By adjusting the value of W within the above range, the wider first material layer plays a certain role in physical blocking and buffering. Furthermore, the first material layer at the edge of the negative electrode only includes relatively stable graphite material. While maintaining energy density, this reduces the diffusion of lithium ions to the edge region of the negative electrode, reduces the expansion stress generated by the second material layer, and decreases the impact of crosstalk between Si and graphite on the edge of the negative electrode. This helps reduce the occurrence of side reactions, especially those caused by crosstalk between Si and graphite, thereby improving the cycle performance of the secondary battery.

[0036] In one or more embodiments of this application, along the length direction of the unfolded negative electrode sheet, the width of a single groove is W' μm, where 20 ≤ W' ≤ 400; optionally, 90 ≤ W' ≤ 150. For example, the value of W' can be 20, 30, 40, 50, 70, 90, 100, 120, 150, 180, 200, 220, 250, 280, 300, 320, 350, 380, 400, or a range consisting of any two of these values. For example, as... Figure 3As shown, along the X direction of the unfolded negative electrode sheet 10, the width of a single groove 13 is W' μm. By adjusting the value of W' within the above range, it is beneficial to accelerate the conduction of electrolyte, further reduce the risk of side reactions caused by electrolyte accumulation at the edge of the negative electrode sheet, provide stress buffer space, and reduce the risk of lithium plating and cycle performance degradation due to insufficient CB at the edge of the negative electrode sheet caused by the groove. While taking into account processing performance, it improves the cycle performance of the secondary battery.

[0037] In this application, the CB value refers to the ratio between the capacity of a negative electrode sheet per unit area and the capacity of a positive electrode sheet per unit area under the same conditions, such as an ambient temperature of 25°C and a discharge rate of 0.1C. CB = (Specific capacity of negative electrode active material × Mass of negative electrode active material per unit area of ​​negative electrode sheet) / (Specific capacity of positive electrode active material × Mass of positive electrode active material per unit area of ​​positive electrode sheet). The aforementioned unit area refers to 1 mm². 2 .

[0038] In one or more embodiments of this application, along the width direction after the negative electrode sheet is unfolded, the ratio of the length of a single groove to the width of the first material layer is P, where 0.2 ≤ P ≤ 1; optionally, 0.4 ≤ P ≤ 1. For example, the value of P can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or a range consisting of any two of these values. By adjusting the value of P within the above range, it is beneficial to accelerate the conduction of the electrolyte, further reduce the risk of side reactions caused by electrolyte accumulation at the edge of the negative electrode sheet, provide stress buffer space, and reduce the risk of lithium plating and cycle performance degradation due to insufficient CB at the edge of the negative electrode sheet caused by the groove. While taking into account processing performance, it improves the cycle performance of the secondary battery.

[0039] In one or more embodiments of this application, along the thickness direction of the negative electrode sheet, the thickness of the first material layer is H μm, and the average depth of the plurality of grooves is H' μm, where 2 ≤ H' ≤ 0.8H; optionally, 2 ≤ H' ≤ 0.6H. For example, the value of H' can be 2, 0.1H, 0.2H, 0.3H, 0.4H, 0.5H, 0.6H, 0.7H, 0.8H, or a range of any two of these values. For example, as... Figure 3As shown, along the thickness direction Z of the negative electrode 10, the thickness of the first material layer 121 is H μm, and the average depth of the multiple grooves 13 is H' μm. By adjusting the value of H' within the above range, it is beneficial to accelerate the conduction of the electrolyte, reduce the risk of side reactions caused by electrolyte accumulation at the edge of the negative electrode, and also reduce the risk of the first material layer being punctured by the grooves when they are set. In addition, the loss of the first graphite material is less, and the risk of lithium plating and cycle performance degradation caused by insufficient CB at the edge of the negative electrode due to the grooves is reduced. While taking into account the processing performance, the cycle performance of the secondary battery is improved.

[0040] In this application, the thickness H' of the first material layer can be controlled by means known to those skilled in the art. For example, when the first slurry is coated on the surface of the negative electrode current collector, the thickness of the first material layer can be increased by increasing the coating weight, and vice versa, based on a certain solid content of the first slurry. Alternatively, when the negative electrode sheet is cold-pressed, the thickness of the first material layer can be decreased by increasing the cold-pressing pressure, and vice versa.

[0041] In one or more embodiments of this application, 25 ≤ H ≤ 70. For example, the value of H can be 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or a range of any two of these values. By adjusting the value of H within the above range, the secondary battery exhibits good cycle performance and high energy density while also possessing good structural stability.

[0042] In one or more embodiments of this application, the distance between two adjacent grooves along the length of the unfolded negative electrode sheet is N mm, where 1 ≤ N ≤ 5. For example, the value of N can be 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, or a range of any two values ​​therein. For example, as shown... Figure 3 As shown, along the X direction of the unfolded negative electrode sheet 10, the spacing between two adjacent grooves 13 is N mm. By adjusting the value of N within the above range, it is beneficial to accelerate the conduction of electrolyte, further reduce the risk of side reactions caused by electrolyte accumulation at the edge of the negative electrode sheet, and provide stress buffer space. It also helps to reduce the processing difficulty of the groove process and the risk of local collapse of the first material layer, and reduces the risk of lithium plating and cycle performance degradation due to insufficient CB at the edge of the negative electrode sheet caused by the groove. While taking into account processing performance, it improves the cycle performance of the secondary battery.

[0043] In this application, the spacing between two adjacent grooves refers to the distance between the centers of the widths of two adjacent grooves along the length direction after the negative electrode sheet is unfolded.

[0044] In this application, the cross-section of a single groove refers to the plane formed by the groove along its length and thickness direction after unfolding the negative electrode sheet (or the cross-section obtained by viewing the groove along its length and thickness direction after unfolding the negative electrode sheet). This application does not impose any particular limitation on the cross-sectional shape of a single groove, as long as it achieves the purpose of this application. For example, the cross-section of a single groove can be independently selected from at least one of the following: triangular, arc-shaped (with an area smaller than a semicircle with the same radius), semicircular, rectangular, trapezoidal, or square.

[0045] In one or more embodiments of this application, the average particle size of the second graphite material is D1 μm, and the average particle size of the silicon-based material is D2 μm, where 9 ≤ D1 ≤ 15 and 7.3 ≤ D2 ≤ 10.3. For example, the value of D1 can be 9, 10, 11, 12, 13, 14, 15, or a range of any two of these values, and the value of D2 can be 7.3, 7.5, 7.8, 8, 8.2, 8.5, 8.8, 9, 9.2, 9.5, 9.8, 10, 10.2, 10.3, or a range of any two of these values. By adjusting the values ​​of D1 and D2 within the above ranges, the average particle size of the second graphite material is larger. During charging and discharging, this helps to reduce the impact of the volume expansion of the silicon-based material on the negative electrode material layer, improves the structural stability of the negative electrode sheet, and helps to reduce the side reactions that occur when the silicon-based material directly contacts the electrolyte, thereby improving the cycle performance of the secondary battery.

[0046] In one or more embodiments of this application, the secondary battery satisfies one of the following conditions: (1) 1.08 ≤ D1 / D2 ≤ 1.8; (2) 1.3 ≤ D1 / D2 ≤ 1.5. For example, the value of D1 / D2 can be 1.08, 1.1, 1.12, 1.15, 1.18, 1.2, 1.22, 1.25, 1.28, 1.3, 1.32, 1.35, 1.38, 1.4, 1.42, 1.45, 1.48, 1.5, 1.52, 1.55, 1.58, 1.6, 1.62, 1.65, 1.68, 1.7, 1.72, 1.75, 1.78, 1.8, or a range consisting of any two of these values. By adjusting the value of D1 / D2 within the above range, the ratio of silicon-based material to second graphite material in the second material layer is appropriate, making the dispersion of silicon-based material and second graphite material in the second material layer more uniform. This reduces the direct contact between silicon-based material and second graphite material, as well as the direct contact between silicon-based material and electrolyte, thereby reducing the occurrence of side reactions, especially the side reactions caused by crosstalk between the two different materials, Si and graphite, and thus improving the cycle performance of the secondary battery.

[0047] This application does not impose any particular restrictions on the method of controlling the average particle size of the second graphite material and the average particle size of the silicon-based material, as long as the purpose of this application can be achieved. For example, different second graphite materials or silicon-based materials can be purchased according to the required average particle size of the second graphite material or silicon-based material. Alternatively, the average particle size of the second graphite material or silicon-based material can also be controlled by grinding or other methods.

[0048] In one or more embodiments of this application, the compaction density of the first material layer is PD1 g / cm³. 3 The compaction density of the second material layer is PD2 g / cm³. 3 0.8≤PD1≤1.18, 0.8≤PD2≤1.9. For example, the value of PD1 can be 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1, 1.02, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18, or any range of any two of these values; the value of PD2 can be 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, or any range of any two of these values. By adjusting the values ​​of PD1 and PD2 within the aforementioned range, the compaction density of the edge region of the negative electrode sheet is moderate. This helps reduce the risk of electrolyte accumulation on both sides of the negative electrode sheet and subsequent side reactions, while also enabling the secondary battery to have a higher energy density and reducing the risk of cycle performance degradation due to crosstalk reactions within the negative electrode material layer. Therefore, the occurrence of side reactions is reduced, especially those caused by crosstalk between the two different materials, Si and graphite, thus improving the cycle performance of the secondary battery while maintaining energy density.

[0049] In one or more embodiments of this application, the secondary battery satisfies one of the following conditions: (1) 1.0 ≤ PD2 / PD1 ≤ 1.6; (2) 1.1 ≤ PD2 / PD1 ≤ 1.2. For example, the value of PD2 / PD1 can be 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, or a range consisting of any two of these values. By adjusting the value of PD2 / PD1 within the above range, the compaction density of the edge region of the negative electrode sheet is moderate, which helps to reduce the risk of electrolyte enrichment on both sides of the negative electrode sheet and the occurrence of side reactions. At the same time, it enables the secondary battery to have a higher energy density and reduces the risk of cycle performance degradation caused by crosstalk reactions inside the negative electrode material layer. Thus, the occurrence of side reactions is reduced, especially the side reactions caused by crosstalk between the two different materials, Si and graphite, improving the cycle performance of the secondary battery while taking into account energy density.

[0050] This application does not impose any particular restrictions on the method of controlling the compaction density of the first material layer and the second material layer, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the cold pressing pressure, the coating amount of the first slurry, and the coating amount of the second slurry. For example, when the first slurry is coated on the surface of the negative electrode current collector, with other conditions remaining constant, increasing the coating amount of the first slurry increases the compaction density of the first material layer; when the second slurry is coated on the surface of the negative electrode current collector, with other conditions remaining constant, increasing the coating amount of the second slurry increases the compaction density of the second material layer, and vice versa; or, with other conditions remaining constant, increasing the cold pressing pressure increases the compaction density of the first material layer and the compaction density of the second material layer, and vice versa.

[0051] In one or more embodiments of this application, based on the mass of the first material layer, the mass percentage content w1 of the first graphite material is 95% to 99%. For example, the value of the mass percentage content w1 of the first graphite material can be 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, or a range consisting of any two of these values. Based on the mass of the second material layer, the mass percentage content w2 of the second graphite material is 28% to 95%, and the mass percentage content w3 of the silicon-based material is 1% to 70%. For example... The mass percentage of the second graphite material, w2, can be 28%, 29%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 92%, 95%, or any range of two of these values. The mass percentage of the silicon-based material, w3, can be 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any range of two of these values. By adjusting the values ​​of w1, w2, and w3 within the aforementioned range, the volume change of the first graphite material during the charging and discharging process of the secondary battery is relatively small, which is beneficial to improving the structural stability of the negative electrode edge. Combined with the high-capacity silicon-based material in the second negative electrode material layer and the relatively stable second graphite material, the secondary battery has a high energy density while taking into account the dynamic performance, improving the structural stability and cycle performance of the secondary battery.

[0052] This application does not impose any particular limitation on the types of the first and second graphite materials, as long as they can achieve the purpose of this application. For example, the first and second graphite materials can each be selected from at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon. This application does not impose any particular limitation on the types of silicon-based materials, as long as they can achieve the purpose of this application. For example, silicon-based materials can include at least one of elemental silicon, silicon oxide compounds, silicon carbide compounds, or silicon alloys. In this application, the silicon carbide compound is a silicon-carbon composite material. Based on the mass of the silicon-carbon composite material, the mass percentage content of silicon can be 30% to 70%, and the mass percentage content of carbon can be 30% to 70%. This application does not impose any particular limitation on the silicon-carbon composite material, as long as it can achieve the purpose of this application. For example, the silicon-carbon composite material can be a composite material obtained by deposition. Exemplarily, the silicon-carbon composite material can be silicon material deposited on a carbon skeleton, or carbon material deposited on a silicon skeleton. The silicon oxide compound includes SiOx, where 0 < x < 2. For example, the silicon oxide compound may include silicon suboxide (SiO, with a silicon to oxygen molar ratio of 1:1). This application does not particularly limit the negative electrode current collector, as long as it achieves the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. For example, the composite current collector may be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc. In this application, there is no particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application. In one or more embodiments of this application, the first negative electrode material layer includes a first conductive agent and a first binder, and the second negative electrode material layer includes a second conductive agent and a second binder. This application does not particularly limit the types of the first and second conductive agents, as long as they achieve the purpose of this application. For example, the first conductive agent and the second conductive agent can each be independently selected from at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black can include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes can include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers can include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials can include, but are not limited to, metal powders and / or metal fibers; specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer can include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole.This application does not impose any particular restrictions on the types of the first adhesive and the second adhesive, as long as they can achieve the purpose of this application. For example, the first adhesive and the second adhesive can each be independently selected from at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0053] This application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) mixing the first graphite material, the first binder and the first conductive agent evenly to obtain the first slurry; mixing the silicon-based material, the second graphite material, the second binder and the second conductive agent evenly to obtain the second slurry; (2) determining the area on the negative electrode current collector where the first material layer and the second material layer need to be set along the width direction after the negative electrode current collector is spread out, and uniformly coating the first slurry and the second slurry on the area where the first material layer and the second material layer need to be set on one surface of the negative electrode current collector, and drying to obtain a negative electrode sheet with the first material layer and the second material layer set on one side; (3) repeating the above steps on the other surface of the negative electrode current collector to obtain a negative electrode sheet with the first material layer and the second material layer set on both sides; (4) after cold pressing and cutting into strips, setting grooves on the first material layer to obtain the negative electrode sheet.

[0054] In this application, the mass percentage content of the first graphite material in the first material layer can be controlled by adjusting the mass ratio of the first graphite material, the first binder, and the first conductive agent added to the first slurry; the mass percentage content of the second graphite material and the mass percentage content of the silicon-based material in the second material layer can be controlled by adjusting the mass ratio of the silicon-based material, the second graphite material, the second binder, and the second conductive agent added to the second slurry; and the value of W can be controlled by adjusting the coating width of the first slurry in the width direction after the negative electrode sheet is unfolded.

[0055] This application does not impose any particular limitation on the solid content of the slurry, as long as the purpose of this application can be achieved. This application does not impose any particular limitation on the drying temperature and time, as long as the purpose of this application can be achieved. This application does not impose any particular limitation on the process parameters for cold pressing and slitting, as long as the purpose of this application can be achieved. This application does not impose any particular limitation on the method of setting the grooves, as long as the purpose of this application can be achieved; for example, grooves can be set using pulsed laser etching. The average depth H' and width W' of multiple grooves can be controlled by the power and defocusing amount of the pulsed laser emitter; the ratio P of the length of a single groove to the width of the first material layer can be controlled by adjusting the width of the first material layer, the power of the pulsed laser emitter, and the defocusing amount; the distance N between two adjacent grooves can be controlled by adjusting the distance between the pulsed laser emitters or the laser emission frequency.

[0056] In this application, the different features of the grooves included in the first material layer can be combined, and the implementation methods or embodiments covered by the above combinations are all within the protection scope of this application.

[0057] This application does not impose any particular limitation on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil, or composite current collectors (such as aluminum-carbon composite current collectors). The positive electrode material layer of this application includes a positive electrode active material. This application does not impose any particular limitation on the type of positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include lithium nickel cobalt manganese oxide (LiNi). 0.90 Co 0.05 Mn 0.05At least one of the following: O2 (NCM955), NCM811, NCM622, NCM523, NCM111, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, or lithium manganese iron phosphate. In this application, the positive electrode active material may also contain non-metallic elements, such as at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In this application, there are no particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application is achieved. In this application, the positive electrode material layer may also include a binder and a conductive agent. In this application, there are no particular limitations on the type of binder in the positive electrode material layer, as long as the purpose of this application is achieved. For example, the binder may include, but is not limited to, at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. This application does not impose any particular limitation on the type of conductive agent in the positive electrode material layer, as long as it can achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metallic materials, or conductive polymers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer; those skilled in the art can select according to actual needs, as long as it can achieve the purpose of this application.

[0058] In this application, the secondary battery includes a separator. For example, as shown... Figure 1As shown, a separator 30 is disposed between the positive electrode 20 and the negative electrode 10. This application does not impose any particular limitation on the separator, as long as it can achieve the purpose of this application. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The separator type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. In one or more embodiments of this application, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven membrane or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic material. In one or more embodiments of this application, the inorganic layer includes inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the binder; for example, the binder may be at least one of the binders described above. In some embodiments of this application, the polymer layer includes a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene). In this application, there is no particular limitation on the thickness of the diaphragm, as long as it can achieve the purpose of this application. For example, the thickness of the diaphragm can be from 3 μm to 30 μm.

[0059] In this application, the secondary battery includes an electrolyte, which comprises lithium salts and non-aqueous solvents. This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of lithium salts in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0060] The secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0061] The preparation process of the secondary battery in this application is well known to those skilled in the art, and this application has no particular limitations. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode sheet, separator, and negative electrode sheet in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery. Alternatively, stacking the positive electrode sheet, separator, and negative electrode sheet in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent the internal pressure of the secondary battery from rising and overcharging / discharging. In this application, the value of L can be adjusted by controlling the coating width of the positive electrode slurry, the first slurry (i.e., the slurry of the first material layer), and the second slurry (i.e., the slurry of the second material layer) in the width direction after the negative electrode sheet is unfolded.

[0062] A second aspect of this application provides an electronic device that includes the secondary battery found in any of the above embodiments. Therefore, the electronic device provided by this application has good performance.

[0063] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0064] Example

[0065] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0066] Test methods and equipment:

[0067] Tests for W and L:

[0068] At 25°C, the lithium-ion battery was discharged to the discharge cutoff voltage at a constant current of 0.5C. After removing the lithium-ion battery casing, it was soaked in diethyl carbonate for 20 minutes to remove the electrolyte, thus obtaining the electrode assembly. The electrode assembly was then placed parallel to the length of the charge-coupled device (CCD) platform, and the positive electrode, negative electrode, and separator were obtained. Keeping the positions of each component unchanged, the two edges of the entire negative electrode material layer of the negative electrode and the two edges of the positive electrode material layer of the positive electrode were determined along the width direction after the negative electrode was unfolded. Then, L was measured using a CCD.

[0069] The negative electrode sheet was removed separately and unfolded. The surface of the unfolded negative electrode sheet was observed using a scanning electron microscope (SEM). Elemental scans were performed along the width of the unfolded negative electrode sheet, targeting regions of the negative electrode material layer 30 mm from each of the two edges. Due to the different types of negative electrode active materials contained in the first and second material layers, a relatively clear boundary for Si element distribution exists between the two layers along the width of the unfolded negative electrode sheet, thus distinguishing them. W was measured based on the Si element distribution boundary obtained from the scan and the edge of the entire negative electrode material layer closest to this boundary.

[0070] Each length was measured 5 times, and the average value was taken as the final result.

[0071] In this application, the discharge cutoff voltage of the lithium-ion battery in the embodiments and comparative examples is 2.5V. It can be understood that when the voltage range marked on the outer packaging of the battery is 2.5V to 4.2V, the charging cutoff voltage is 4.2V and the discharging cutoff voltage is 2.5V.

[0072] Negative electrode sampling:

[0073] The lithium-ion battery was discharged at a constant current of 0.5C to the discharge cutoff voltage at 25°C. The battery was then disassembled under an argon atmosphere, and the negative electrode was removed. The negative electrode was then soaked in dimethyl carbonate solvent for 2 hours and dried at 60°C for 1 hour to obtain the negative electrode. The discharge cutoff voltage of the lithium-ion battery in the embodiments and comparative examples of this application is 2.5V. It is understood that when the voltage range marked on the battery packaging is 2.5V to 4.2V, the charging cutoff voltage is 4.2V, and the discharging cutoff voltage is 2.5V.

[0074] Unless otherwise specified, the following test methods shall be performed using the negative electrode sheet obtained in the above manner. The distinction between the first and second material layers can be found in "Testing of W and L".

[0075] Test of average particle size:

[0076] The average particle size D1 of the second graphite material and the average particle size D2 of the silicon-based material were measured using scanning electron microscopy (SEM). A cross-section along the thickness direction of the negative electrode was prepared and ion-polished. The cross-section of the first material layer was observed using SEM, and the silicon-based material and the second graphite material in the cross-section of the first material layer were distinguished using backscattering mode. The equivalent diameter of 32 randomly selected second graphite material particles was measured, and the average of these 32 equivalent diameters was taken as the average particle size D1 of the second graphite material. Similarly, the equivalent diameter of 32 randomly selected silicon-based material particles was measured, and the average of these 32 equivalent diameters was taken as the average particle size D2 of the silicon-based material.

[0077] In this application, the average particle size can be understood as the equivalent diameter. The equivalent diameter usually refers to the diameter of a sphere with the same volume as an irregularly shaped object. By measuring the area of ​​the particles of the silicon-based material or the second graphite material to be tested in the cross-section of the negative electrode sheet, the diameter of a circle with the same area is used as the equivalent diameter of the particles of the silicon-based material or the second graphite material to be tested.

[0078] Test of compaction density:

[0079] Using a punching machine, a small circular piece with an area of ​​S is punched from the first material layer of the negative electrode sheet. Its mass is measured as M1, and its thickness is measured as t1 using a micrometer. Along the width and thickness directions of the unfolded negative electrode sheet, the boundary lines between the first and second material layers and between the first material layer and the negative current collector are determined. The first material layer is scraped off the negative electrode sheet and its mass is measured as M2. Its thickness is measured as t2 using a micrometer. The compaction density of the first material layer is PD1 = (M1 - M2) / (t1 - t2) / S.

[0080] The second material layer was tested according to the above steps to obtain the compaction density PD2 of the second material layer.

[0081] Testing of the mass percentage of silicon-based materials and graphite materials:

[0082] Along the width and thickness directions of the unfolded negative electrode sheet, the boundary lines between the first and second material layers, and between the negative electrode material layer and the negative electrode current collector, are determined. The powder scraped off from the second material layer is weighed to obtain mass m1. Then, thermogravimetric analysis is performed at a temperature range of 25℃ to 500℃ to remove the binder. The residual material after heating is weighed to obtain mass m2. m2 / m1 gives the mass percentage of the second negative electrode active material in the second material layer.

[0083] X-ray diffraction (XRD) tests on the residual material after heating can distinguish and determine the type of silicon-based material in the second material layer.

[0084] The mass percentage of Si in the residual material after heating is determined by inductively coupled plasma (ICP) testing, which is the proportion of Si in the second negative electrode active material. Based on the specific chemical formula of the silicon-based material obtained by XRD and the mass percentage of the second negative electrode active material in the second material layer, the mass percentage of silicon-based material in the second material layer, w3, can be obtained. Subtracting the mass percentage of silicon-based material in the second material layer from the mass percentage of the second negative electrode active material in the second material layer gives the mass percentage of graphite material in the second material layer, w2.

[0085] The first material layer is scraped off from the negative electrode current collector. The powder of the scraped first material layer is weighed to obtain mass m3. Then, thermogravimetric analysis is performed at a temperature range of 25℃ to 500℃ to remove the binder. The residual material after heating is weighed to obtain mass m4. m4 / m3 gives the mass percentage of the first negative electrode active material in the first material layer, that is, the mass percentage content w1 of the first graphite material in the first material layer.

[0086] Understandably, because the content of conductive agent in the negative electrode material layer is too low, the influence of conductive graphite can be ignored when conducting mass percentage content tests.

[0087] Tests for H, H', W', P, and N:

[0088] Along the width direction after the negative electrode sheet is unfolded, find the boundary line between the first material layer and the second material layer in the negative electrode material layer, and distinguish and determine the first material layer and the second material layer of the negative electrode material layer.

[0089] The negative electrode sheet is cut along its thickness direction to obtain a longitudinal section of the first material layer. The longitudinal section of the first material layer is then ion-polished and observed using an electron scanning microscope. A clear boundary line between the first material layer and the negative current collector can be observed. The thickness H of the first material layer is measured along the thickness direction of the negative electrode sheet.

[0090] Choose any 5 grooves in the first material layer. Along the width direction after the negative electrode sheet is unfolded, determine the positions of the two ends and the midpoint of each groove in the first material layer. Then, along the thickness direction of the negative electrode sheet, measure the distance between the surface of the first material layer and the bottom surface at the two ends and the midpoint of each groove in the first material layer. Take the average value, which is the depth of each groove in the first material layer. Take the average value of the depths of the 5 grooves in the first material layer, which is the average depth H' of the multiple grooves in the first material layer.

[0091] Choose any single groove in the first material layer. Along the width direction of the unfolded negative electrode sheet, select any 5 positions on the groove. Then, along the length direction of the unfolded negative electrode sheet, measure the width of the 5 selected positions on the groove and take the average value to obtain the width W' of a single groove.

[0092] Along the length of the unfolded negative electrode sheet, measure the distance between the center of width of a single groove and the center of width of an adjacent groove. Select 5 locations and measure once each, and take the average value, which is the distance N between two adjacent grooves.

[0093] Measure the width of the first material layer along the width direction of the negative electrode sheet, and randomly select a single groove in the first single-layer region and measure the length of the groove, which is the length of the single groove. Divide the length of the single groove by the width of the first material layer to obtain the ratio P of the length of the single groove to the width of the first material layer.

[0094] Cyclic performance test:

[0095] The lithium-ion batteries in each embodiment and comparative example were subjected to charge-discharge cycle tests in a 25°C constant temperature chamber. The charge-discharge voltage range was 2.5V to 4.2V. The batteries were charged at a constant current of 2C to 4.2V, then charged at a constant voltage of 4.2V to 0.05C and allowed to rest for 5 minutes. Finally, they were discharged at a constant current of 6C to 2.5V. This charge-discharge cycle was repeated 600 times. The initial capacity C1 and the capacity C after the 600th cycle were recorded. 600 This allows for the calculation of the capacity retention rate of lithium-ion batteries.

[0096] Capacity retention rate (%) = C 600 / C1×100%.

[0097] The capacity retention rate is used to characterize the cycle performance of lithium-ion batteries. The higher the capacity retention rate, the better the cycle performance of the lithium-ion battery.

[0098] Electrode edge side reaction width test:

[0099] The lithium-ion batteries in the above embodiments and comparative examples, after being cycled 600 times, were disassembled under an argon atmosphere. The negative electrode sheet was removed, and the negative electrode sheet was soaked in dimethyl carbonate solvent for 2 hours and dried at 60°C for 1 hour to obtain the negative electrode sheet.

[0100] Along the width direction after the negative electrode sheet is unfolded, measure the distance between the starting region (grayish-black) at the edge of the negative electrode material layer and the boundary (golden yellow) of the conventional lithium intercalation region. Take the average value of five tests, which is the side reaction width at the edge of the electrode sheet of the lithium-ion battery, in mm.

[0101] Example 1

[0102] <Preparation of Negative Electrode Sheets>

[0103] The first graphite material (artificial graphite), the first conductive agent (acetylene black), the first binder (sodium carboxymethyl cellulose), and the first binder (styrene-butadiene rubber) in the first negative electrode active material are mixed in a mass ratio of 96:1.3:1.2:1.5. Deionized water is added as a solvent, and the mixture is stirred until homogeneous to obtain a first slurry with a solid content of 45 wt%. The second graphite material (SiC), the second graphite material (artificial graphite), the second conductive agent (acetylene black), the second binder (sodium carboxymethyl cellulose), and the second binder (styrene-butadiene rubber) in the second negative electrode active material are mixed in a mass ratio of 14.4:81.6:1.3:1.2:1.5 to obtain a second slurry with a solid content of 45 wt%.

[0104] A 10μm thick copper foil was used as the negative electrode current collector. Along the width of the unfolded negative electrode current collector, the areas where the first and second material layers needed to be deposited were determined. On one surface of the negative electrode current collector, the first and second slurries were uniformly coated onto the areas where the first and second material layers needed to be deposited, respectively. The foil was dried at 120℃ to obtain a negative electrode sheet with the first and second material layers on one side. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with the first and second material layers on both sides. After drying under vacuum at 120℃ for 1 hour, the foil was cold-pressed, cut, and slit, and grooves were then created on the first material layer.

[0105] Along the width direction of the unfolded negative electrode sheet, the ratio P of the length of a single groove to the width of the first material layer is set to 0.6, the groove width W' is 120 μm, and the spacing N between two adjacent grooves along the length direction of the unfolded negative electrode sheet is 3 mm. The average depth H' of multiple grooves is 20.4 μm, and H' / H is 0.4. Grooves are laser-etched on the first material layer according to the above parameters. The final negative electrode sheet with dimensions of 67.45 mm × 1436 mm is obtained.

[0106] The coating weight of the first material layer is 5.2 mg / cm³. 2 The coating weight of the second material layer is 6.14 mg / cm³. 2 The compaction density PD1 of the first material layer is 1 g / cm³. 3 The compaction density PD2 of the second material layer is 1.18 g / cm³. 3 The ratio of PD2 to PD1 is 1.18. The thickness of the first material layer is 51 μm, the width W of the first material layer is 6 mm, and the width of the second material layer is 56 mm. The average particle size D1 of the second graphite material is 14 μm, the average particle size D2 of the silicon-based material is 10 μm, and the ratio of D1 to D2 is 1.4. Based on the mass of the first anode material layer, the mass percentage w1 of the first graphite material is 96%. Based on the mass of the second anode material layer, the mass percentage w2 of the second graphite material is 81.6%, and the mass percentage w3 of the silicon-based material is 14.4%.

[0107] <Preparation of the positive electrode>

[0108] Lithium nickel cobalt manganese oxide (LiNi) is used as the positive electrode active material. 0.6 Co 0.2 Mn 0.2 O2, polyvinylidene fluoride (PVDF) binder, and conductive carbon black conductive agent were mixed in a mass ratio of 94.8:2.8:2.4. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%. After vacuum stirring, a positive electrode slurry was obtained. The positive electrode slurry was uniformly coated onto one surface of a 13 μm thick aluminum foil used as a positive electrode current collector and dried at 120°C to obtain a single-sided coated positive electrode sheet. The above steps were repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode sheet. After drying at 120°C, the sheet was cold-pressed, cut, and had tabs welded to obtain a positive electrode sheet with dimensions of 64.5 mm × 1422 mm for later use. The coating weight of the positive electrode material layer was 15 mg / cm³. 2 The compaction density of the positive electrode material layer is 3.4 g / cm³. 3 The width of the positive electrode material layer is 56mm.

[0109] <Preparation of Electrolyte>

[0110] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of 20:30:40:10 to obtain an organic solvent. Lithium hexafluorophosphate (LiPF6) was then added to the organic solvent and mixed thoroughly to obtain the electrolyte. The concentration of the lithium salt was 1 mol / L, and the remainder was the organic solvent.

[0111] <Septum>

[0112] A porous polyethylene film with a thickness of 9μm was used as the diaphragm.

[0113] <Preparation of Lithium-ion Batteries>

[0114] The prepared separator, negative electrode, and positive electrode are stacked in sequence, with the separator positioned between the positive and negative electrode to act as a separator. Along the width of the unfolded negative electrode, the two edges of the negative electrode material layer extend 3mm beyond the two edges of the positive electrode material layer. The electrode assembly is then wound to obtain the final product. After processes including flattening, current collector welding, casing, inkjet printing, vacuum drying, electrolyte injection, sealing, high-temperature settling, and capacity testing, a lithium-ion battery is obtained. The upper limit of the formation voltage is 4.15V, the formation temperature is 45℃, and the formation settling time is 12 hours.

[0115] Examples 2 to 33

[0116] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1. Specifically, when the value of W changes, the coating width of the first slurry along the width direction after the negative electrode sheet is spread is adjusted so that the value of W is as shown in Table 1, while the total width of the negative electrode material layer remains unchanged; when the value of L changes, the coating width of the positive electrode slurry along the width direction after the positive electrode sheet is spread, the coating width of the first slurry along the width direction after the negative electrode sheet is spread, and the coating width of the second slurry along the width direction after the negative electrode sheet is spread are adjusted so that the value of L is as shown in Table 1; when the values ​​of D1 and / or D2 change, the grinding time is adjusted so that the values ​​of D1 and / or D2 are as shown in Table 1; when the values ​​of H, PD1, and / or PD2 change, the coating weight is adjusted so that H, The values ​​of PD1 and / or PD2 are shown in Table 1; when the value of w1 changes, the mass percentage content of the first conductive agent and the first binder changes accordingly, while the mass ratio of the first conductive agent to the first binder remains unchanged; when the values ​​of w2 and w3 change, the mass percentage content of the second conductive agent and the second binder changes accordingly, while the mass ratio of the second conductive agent to the second binder remains unchanged; in Example 32, the silicon-based material in the second material layer is Si; in Example 33, the silicon-based material in the second material layer is SiO and Si (SiO:Si = 11:3), that is, based on the mass of the second material layer, the mass percentage content of SiO to the mass percentage content of Si is 11:3.

[0117] Comparative Example 1

[0118] Except for the preparation of the negative electrode sheet according to the following steps, the rest is the same as in Example 2.

[0119] <Preparation of Negative Electrode Sheets>

[0120] Silicon-based material SiC, second graphite material artificial graphite, second conductive agent acetylene black, second binder sodium carboxymethyl cellulose, and second binder styrene-butadiene rubber are mixed evenly in a mass ratio of 14.4:81.6:1.3:1.2:1.5 to obtain a second slurry with a solid content of 45wt%.

[0121] A 10μm thick copper foil was used as the negative electrode current collector. A second slurry was uniformly coated onto one surface of the current collector along its width. The coating was then dried at 120℃ to obtain a negative electrode sheet with a second material layer on one side. This process was repeated on the other surface of the copper foil to obtain a negative electrode sheet with a second material layer on both sides. After drying under vacuum at 120℃ for 1 hour, the electrode sheet was cold-pressed, cut, and slit to obtain a final negative electrode sheet with dimensions of 67.45mm × 1436mm. The coating weight of the second material layer was 6.14 mg / cm³. 2 The compaction density PD2 of the second material layer is 1.18 g / cm³. 3The width of the second material layer is 62mm.

[0122] Comparative Example 2

[0123] Except for the absence of grooves on the first material layer in the <Preparation of the Negative Electrode> and the adjustment of the relevant preparation parameters according to Table 1, the rest is the same as in Example 2.

[0124] Comparative Example 3

[0125] Except for the fact that the formulation of the second slurry in <Preparation of the negative electrode sheet> is the same as that in Example 33, and the relevant preparation parameters are adjusted according to Table 1, the rest is the same as that in Comparative Example 1.

[0126] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1.

[0127]

[0128]

[0129]

[0130] As can be seen from Examples 1 to 33 and Comparative Examples 1 to 3, by setting different material layers on different regions of the negative electrode sheet and setting grooves on the first material layer located at the edge of the negative electrode sheet, and controlling the W / L value within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is small, and the cycle capacity retention rate is high. This indicates that the lithium-ion battery of this application can reduce the side reactions in the electrode edge region and has good cycle performance. Comparative Examples 1 and 3 do not have a first material layer or grooves; Comparative Example 2 does not have grooves on the first material layer; the side reaction width at the electrode edge of the lithium-ion batteries in Comparative Examples 1 to 3 is large, and the cycle capacity retention rate is low, indicating that the lithium-ion batteries in the comparative examples have more side reactions in the electrode edge region and poorer cycle performance. However, the lithium-ion batteries in Examples 1 to 33 have smaller side reaction widths at the electrode edge and higher cycle capacity retention rates, indicating that the lithium-ion battery of this application can reduce the side reactions in the electrode edge region and has good cycle performance.

[0131] The value of L typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 6 and 7, when the value of L is within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is small, and the cycle capacity retention rate is high, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0132] The value of W' typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 8 to 11, when the value of L is within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is small, and the cycle capacity retention rate is high, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0133] The value of P typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 12 to 14, when the value of P is within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is small, and the cycle capacity retention rate is high, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0134] The value of H' typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 15 to 19, and 26 to 29, when the value of H' is within the range specified in this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher. This indicates that the lithium-ion battery of this application can reduce side reactions in the electrode edge region and has good cycle performance. In Example 17, due to the larger average depth of the multiple grooves, there is more local loss of the negative electrode active material, which easily leads to insufficient CB at the negative electrode edge, thereby increasing the risk of lithium plating in the secondary battery.

[0135] The value of H typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 18 to 19, and 26 to 29, when the value of H is within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0136] The value of N typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 20 to 21, when the value of N is within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is small, and the cycle capacity retention rate is high, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0137] The values ​​of D1 and D2 typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 22 to 25, when the values ​​of D1 and D2 are within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0138] The value of D1 / D2 typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 22 to 25, when the value of D1 / D2 is within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0139] The values ​​of PD1 and PD2 typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 26 to 29, when the values ​​of PD1 and PD2 are within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0140] The PD2 / PD1 ratio typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 26 to 29, when the PD2 / PD1 ratio is within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher, indicating that the lithium-ion battery of this application can reduce the side reaction in the electrode edge region and has good cycle performance.

[0141] The mass percentage of the first graphite material typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 30, and 31, when the mass percentage of the first graphite material is within the range specified in this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher. This indicates that the lithium-ion battery of this application can reduce side reactions in the electrode edge region and has good cycle performance.

[0142] The mass percentage of the second graphite material and the mass percentage of the silicon-based material typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 32 to 33, and Comparative Example 4, when the mass percentages of the second graphite material and the silicon-based material are within the range of this application, the side reaction width at the electrode edge of the lithium-ion battery is smaller, and the cycle capacity retention rate is higher. This indicates that the lithium-ion battery of this application can reduce side reactions in the electrode edge region and has good cycle performance. In Example 32, the mass percentage of the silicon-based material is smaller, meaning the mass percentage of silicon element is smaller, resulting in a lower energy density for the lithium-ion battery.

[0143] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0144] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0145] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising a positive electrode and a negative electrode, wherein the positive electrode comprises a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector, and the negative electrode comprises a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector; Along the width direction after the negative electrode sheet is unfolded, the negative electrode material layer only includes a first material layer and a second material layer connected in sequence. The width of the first material layer is W mm, and the width of the negative electrode material layer is greater than the width of the positive electrode material layer. Half of the difference between the widths of the negative electrode material layer and the positive electrode material layer is L mm, and 0.2≤W / L≤4. The first material layer includes a first negative electrode active material, which includes a first graphite material; the second material layer includes a second negative electrode active material, which includes a second graphite material and a silicon-based material. The first material layer is provided with a plurality of grooves, which extend along the width direction of the unfolded negative electrode sheet and are spaced apart along the length direction of the unfolded negative electrode sheet.

2. The secondary battery according to claim 1, wherein 0.2≤L≤5; and / or, 1≤W / L≤3.

3. The secondary battery according to claim 1, wherein Along the length direction of the unfolded negative electrode sheet, the width of a single groove is W'μm, and 20≤W'≤400.

4. The secondary battery according to claim 3, wherein, 90≤W'≤150。 5. The secondary battery according to claim 1, wherein Along the width direction after the negative electrode sheet is unfolded, the ratio of the length of a single groove to the width of the first material layer is P, where 0.2≤P≤1.

6. The secondary battery according to claim 5, wherein 0.4≤P≤1。 7. The secondary battery according to claim 1, wherein Along the thickness direction of the negative electrode sheet, the thickness of the first material layer is H μm, the average depth of the plurality of grooves is H' μm, and 2≤H'≤0.8H.

8. The secondary battery according to claim 7, wherein 2≤H'≤0.6H.

9. The secondary battery according to claim 7, wherein, 25≤H≤70。 10. The secondary battery according to claim 1, wherein Along the length of the unfolded negative electrode sheet, the distance between two adjacent grooves is N mm, where 1 ≤ N ≤ 5.

11. The secondary battery according to claim 1, wherein The average particle size of the second graphite material is D1μm, and the average particle size of the silicon-based material is D2μm, where 9≤D1≤15 and 7.3≤D2≤10.

3.

12. The secondary battery according to claim 11, wherein it satisfies one of the following conditions: (1) 1.08 ≤ D1 / D2 ≤ 1.8; (2) 1.3≤D1 / D2≤1.

5.

13. The secondary battery according to claim 1, wherein The compaction density of the first material layer is PD1 g / cm³ 3 The compaction density of the second material layer is PD2 g / cm³. 3 , 0.8≤PD1≤1.18, 0.8≤PD2≤1.

9.

14. The secondary battery according to claim 13, wherein it satisfies one of the following conditions: (1) 1.0 ≤ PD2 / PD1 ≤ 1.6; (2) 1.1≤PD2 / PD1≤1.

2.

15. The secondary battery according to any one of claims 1 to 14, wherein Based on the mass of the first material layer, the mass percentage of the first graphite material is 95% to 99%; based on the mass of the second material layer, the mass percentage of the second graphite material is 28% to 95%, and the mass percentage of the silicon-based material is 1% to 70%.

16. An electronic device comprising a secondary battery as claimed in any one of claims 1 to 15.

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