Secondary battery and electronic device

Abstract

The present application provides a secondary battery and an electronic device. The secondary battery comprises a negative electrode sheet; the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector; the negative electrode material layer comprises a first negative electrode material layer and a second negative electrode material layer; and the first negative electrode material layer is located between the second negative electrode material layer and the negative electrode current collector. The first negative electrode material layer comprises a first graphite material, and the second negative electrode material layer comprises a silicon-based material and a second graphite material. The orientation index (OI) of the first negative electrode material layer is OI1, and the OI of the second negative electrode material layer is OI2, wherein OI1<OI2. By means of the described arrangements, the structural stability and cycle performance of the secondary battery are improved while the dynamic performance is taken into account.

Description

A secondary battery and electronic device 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, especially lithium-ion batteries, have been widely used in electronic products such as laptops and mobile phones in recent years as a convenient electrochemical device. With the market's increasing demand for longer battery life and extended lifespan in electronic products, the requirements for energy density and cycle capacity retention of lithium-ion batteries are becoming increasingly stringent. Silicon has a theoretical specific capacity far greater than that of graphite, reaching 4200 mAh / g, making silicon-based materials the most promising candidate to replace graphite as the next-generation anode material for lithium-ion batteries.

[0003] However, during the charging and discharging process of lithium-ion batteries, silicon particles undergo lithiation and delithiation, resulting in significant expansion and contraction. This large volume expansion and contraction creates voids between silicon particles, ultimately causing them to disconnect from adjacent active materials. This affects electron and lithium-ion transport, leading to cycle degradation. Furthermore, it destabilizes the interface between the electrode material layer and the current collector, compromising the structural integrity and stability of the negative electrode and reducing the safety of electronic products. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device that improves the structural stability and cycle performance of the secondary battery while taking into account dynamic performance.

[0005] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0006] Existing technologies primarily use a mixture of silicon-based and graphite materials as the negative electrode active material, thereby reducing the proportion of silicon-based material in the negative electrode sheet and thus reducing its expansion. However, this also reduces the energy density of the secondary battery, limiting the further application of silicon-based materials in secondary batteries. Therefore, this application provides a secondary battery and electronic device that improves the structural stability and cycle performance of the secondary battery while maintaining kinetic performance. The specific technical solution is as follows:

[0007] The first aspect of this application provides a secondary battery, comprising a negative electrode sheet, a negative current collector, and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer comprises a first negative electrode material layer and a second negative electrode material layer, with the first negative electrode material layer located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer comprises a first graphite material, and the second negative electrode material layer comprises a silicon-based material and a second graphite material. The orientation degree of the first negative electrode material layer is OI1, and the orientation degree of the second negative electrode material layer is OI2, where OI1 < OI2. By configuring the first and second negative electrode material layers in a combination, and ensuring that the orientation degree of the first negative electrode material layer is less than that of the second negative electrode material layer, the interfacial stability between the first and second negative electrode material layers can be improved while maintaining kinetic performance, thereby improving the structural stability of the negative electrode sheet and consequently enhancing the structural stability and cycle performance of the secondary battery.

[0008] In some embodiments of this application, 6≤OI1≤20, preferably 6≤OI1≤16; 8≤OI2≤25, preferably 15≤OI2≤25. By adjusting the values ​​of OI1 and OI2 within the above ranges, it is beneficial to balance the wetting of the electrolyte in the negative electrode material layer and the transport of lithium ions in the negative electrode material layer. Furthermore, the composite structure of silicon-based material and second graphite material in the second negative electrode material layer is more stable, and the SEI film formed on the surface of the negative electrode material layer is more uniform and dense. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0009] In some embodiments of this application, the porosity of the first negative electrode material layer is P1, and the porosity of the second negative electrode material layer is P2, where P1 < P2. This arrangement increases the channels for the electrolyte to enter the negative electrode sheet, providing stable support for the entire negative electrode material layer and facilitating the formation of a uniform and dense SEI film on the surface of the negative electrode material layer. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0010] In some embodiments of this application, 20≤P1≤45 and 30≤P2≤65. By adjusting the values ​​of P1 and P2 within the above ranges, the channels for the electrolyte to enter the negative electrode sheet are increased, which can provide stable support for the entire negative electrode material layer and facilitate the formation of a uniform and dense SEI film on the surface of the negative electrode material layer. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0011] In some embodiments of this application, the average particle size of the first graphite material is D1 μm, and the average particle size of the second graphite material is D2 μm, where D1 < D2. With this configuration, the graphite particles in the second negative electrode material layer are more loosely packed, which is beneficial for the rapid insertion and extraction of lithium ions. The graphite particles in the first negative electrode material layer are more densely packed, which can effectively disperse the stress generated by the expansion of the silicon-based material. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0012] In some embodiments of this application, 2≤D2-D1≤10, 11≤D1≤17. By adjusting the values ​​of D2-D1 and D1 within the above ranges, the graphite particles in the first negative electrode material layer are more densely packed, which can effectively disperse the stress generated by the expansion of the silicon-based material. Combined with the average particle size of the second graphite material, the structure of the entire negative electrode material layer is more stable. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0013] In some embodiments of this application, the average particle size of the silicon-based material is D3 μm, where 8 ≤ D3 ≤ 20. By adjusting the value of D3 within the above range, the secondary battery improves structural stability and cycle performance while maintaining kinetic performance.

[0014] In some embodiments of this application, the orientation degree of the first graphite material is OI3, and the orientation degree of the second graphite material is OI4, where OI3 < OI4. With this configuration, the silicon-based material undergoes significant volume expansion during charging and discharging. Combined with the more regularly arranged second graphite material, the layered structure of the second graphite material can reduce the impact of volume changes in the silicon-based material. Furthermore, the low orientation degree of the first graphite material helps reduce the overall expansion rate of the negative electrode sheet during repeated charging and discharging. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0015] In some embodiments of this application, 1≤OI3≤2.8 and 3≤OI4≤4.2. By adjusting the values ​​of OI3 and OI4 within the above ranges, it is beneficial to balance the wetting of the electrolyte in the negative electrode material layer and the transport of lithium ions in the negative electrode material layer. The secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0016] In some embodiments of this application, the mass percentage of the first graphite material is W1, based on the mass of the first negative electrode material layer, where 90% ≤ W1 ≤ 99%. By controlling the mass percentage of the first graphite material within the above range, the secondary battery achieves a high energy density while simultaneously improving kinetic performance, structural stability, and cycle performance.

[0017] In some embodiments of this application, the mass percentage of silicon-based material is W2, 10% ≤ W2 ≤ 50%, based on the mass of the second negative electrode material layer. By controlling the mass percentage of silicon-based material within the above range, the secondary battery achieves high energy density while balancing kinetic performance, improving structural stability and cycle performance.

[0018] In some embodiments of this application, the mass percentage of the second graphite material is W3, based on the mass of the second negative electrode material layer, where 40% ≤ W3 ≤ 88%. By controlling the mass percentage of the second graphite material within the above range, the secondary battery of this application further improves structural stability and cycle performance while taking into account kinetic performance.

[0019] In some embodiments of this application, the silicon-based material includes at least one of elemental silicon, silicon oxides, silicon-carbon compounds, or silicon alloys. By selecting the aforementioned silicon-based material and configuring the first negative electrode material layer and the second negative electrode material layer together, the secondary battery of this application improves energy density while taking into account kinetic performance, structural stability, and cycle performance.

[0020] In some embodiments of this application, in the X-ray diffraction pattern of the silicon-based material, a first characteristic peak exists between 26° and 30°, and a second characteristic peak exists between 47° and 52.5°. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, with 1.5 ≤ I1 / I2 ≤ 3.0. Through the above settings, the silicon-based material has a specific crystal structure and arrangement. The secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0021] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device of this application has good performance characteristics.

[0022] The beneficial effects of this application are:

[0023] This application provides a secondary battery and an electronic device. The secondary battery includes a negative electrode sheet, which includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer, with the first negative electrode material layer located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer includes a first graphite material, and the second negative electrode material layer includes a silicon-based material and a second graphite material. The orientation degree of the first negative electrode material layer is OI1, and the orientation degree of the second negative electrode material layer is OI2, where OI1 < OI2. By combining the first and second negative electrode material layers, the orientation degree of the first negative electrode material layer is less than that of the second negative electrode material layer. This improves the interfacial stability between the first and second negative electrode material layers while maintaining kinetic performance, thereby improving the structural stability of the negative electrode sheet and ultimately enhancing the structural stability and cycle performance of the secondary battery.

[0024] 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

[0025] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0026] Figure 1 is a partial cross-sectional view of the negative electrode sheet along its own thickness direction after it is unfolded in one embodiment of this application.

[0027] Reference numerals: negative electrode 10; negative electrode current collector 11; negative electrode material layer 12; first negative electrode material layer 121; second negative electrode material layer 122. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely 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.

[0029] 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:

[0030] A first aspect of this application provides a secondary battery, comprising a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector, the negative electrode material layer including a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer being located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer includes a first graphite material, the second negative electrode layer includes a silicon-based material and a second graphite material, the orientation degree of the first negative electrode material layer is OI1, the orientation degree of the second negative electrode layer is OI2, and OI1 < OI2.

[0031] In this application, the unfolded negative electrode sheet is defined with its length direction as the X direction, its width direction as the Y direction, and its thickness direction as the Z direction. For example, as shown in FIG1, the negative electrode sheet 10 includes a negative electrode current collector 11 and negative electrode material layers 12 located on the two surfaces of the negative electrode current collector. The negative electrode material layers 12 include a first negative electrode material layer 121 and a second negative electrode material layer 122, with the first negative electrode material layer 121 located between the second negative electrode material layer 122 and the negative electrode current collector 11.

[0032] The inventors discovered that a layered design combining silicon-based materials and graphite with an overlay of negative electrode material can effectively reduce the expansion of the negative electrode sheet. In existing technologies, silicon-based materials are generally used as the active negative electrode material in the negative electrode layer closest to the negative electrode current collector. However, since the silicon-based material is in direct contact with the negative electrode current collector, the expansion stress of the silicon-based material may directly act on the negative electrode current collector during the charging and discharging process of the secondary battery. This weakens the bonding force between the negative electrode current collector and the negative electrode material layer, leading to an increase in the internal resistance of the secondary battery. Furthermore, silicon-based materials are prone to structural changes during cycling, and their expansion can directly compress the upper negative electrode material layer, potentially causing delamination between the negative electrode material layers. This reduces the structural stability of the negative electrode sheet, thus affecting the structural stability and cycle performance of the secondary battery. In addition, silicon-based materials have relatively poor conductivity; using silicon-based materials as the active negative electrode material in the negative electrode layer closest to the negative electrode current collector may affect the transport of lithium ions within the negative electrode material layer, thereby affecting the kinetic performance of the secondary battery.

[0033] In this application, the active material of the first negative electrode material layer closest to the negative electrode current collector is graphite. Graphite has good mechanical strength and stability, and can withstand certain pressure and deformation. Using it as a base layer can provide more stable support for the entire negative electrode structure, reduce structural instability factors caused by volume changes and chemical reactions of silicon-based materials, and thus improve the cycle life of the secondary battery. In addition, graphite has better conductivity than silicon-based materials. Graphite has a higher lithium-ion diffusion coefficient and good electronic conductivity. When the first negative electrode material layer including graphite is closer to the negative electrode current collector, it is beneficial to improve the dynamic performance of the secondary battery.

[0034] Furthermore, placing the second negative electrode material layer containing silicon-based material on the surface helps reduce the pressure on the negative electrode current collector caused by its volume expansion, thereby reducing the risk of deformation or damage to the negative electrode current collector due to excessive stress and improving the cycle stability of the secondary battery. Enriching the second negative electrode material layer with silicon-based material utilizes the characteristic of silicon particles expanding during charging, increasing the porosity of the second negative electrode material layer. This increases the channels for electrolyte to enter the negative electrode sheet, adds more ion transport paths, effectively reduces ion transport impedance, and increases energy density while improving charging speed. Therefore, the secondary battery exhibits excellent kinetic performance.

[0035] Furthermore, the orientation degree of the first negative electrode material layer is less than that of the second negative electrode material layer. At this time, the orientation degree of the second negative electrode material layer is higher, and the arrangement of the second graphite material in the second negative electrode material layer is more regular. This is beneficial to take into account the wetting of the negative electrode material layer by the electrolyte, shorten the lithium ion transport path, reduce the charge transfer impedance, and place the second negative electrode material layer containing silicon-based material on the surface. Since the high capacity characteristics of silicon-based material participate in the reaction in the early stage of charging and discharging, the shortened ion transport channel due to the high orientation degree of the second negative electrode material layer can reduce the polarization risk of silicon due to its high capacity. Meanwhile, silicon-based materials undergo significant volume expansion during charge and discharge. By placing silicon in a highly oriented second anode material layer and combining it with a more regularly arranged second graphite material, the layered structure of the second graphite material can mitigate the impact of volume changes in the silicon-based material. The low-oriented first anode material layer exhibits good compatibility and bonding with the anode current collector, resulting in a lower expansion rate during charge and discharge. This helps reduce the overall expansion rate of the anode sheet during repeated charge and discharge cycles, providing better support and stability for the entire anode material layer, thereby improving the structural stability of the anode sheet. Furthermore, the highly oriented second anode material layer can form a uniform and dense solid electrolyte interphase (SEI) film on its surface, reducing side reactions between the anode active material and the electrolyte, thus improving the cycle performance of the secondary battery. If OI1 > OI2, lithium ions need to pass through the highly oriented first anode material layer to enter the surface silicon-based material, resulting in a significant increase in silicon reaction polarization. When the orientation of the second anode material layer is low, the arrangement of the second graphite material in the second anode material layer is relatively disordered, which cannot effectively disperse the expansion stress of silicon and is not conducive to the wetting of the anode material layer by the electrolyte. It is easy to form an uneven SEI film on the surface of the anode electrode, thereby reducing the kinetic performance, structural stability and cycle performance of the secondary battery.

[0036] Therefore, by combining the first negative electrode material layer and the second negative electrode material layer, and making the orientation degree of the first negative electrode material layer less than that of the second negative electrode material layer, this application can improve the interface stability between the first negative electrode material layer and the second negative electrode material layer while taking into account the dynamic performance, thereby improving the structural stability of the negative electrode sheet, and thus improving the structural stability and cycle performance of the secondary battery.

[0037] Typically, the orientation degree of the first and second negative electrode material layers is affected by various factors. In this application, it can be controlled by adjusting the particle size of the active material, the proportion of active material, and the orientation degree of the graphite material in the negative electrode material layer. For example, when other factors remain constant, an increase in the average particle size of the first graphite material in the first negative electrode material layer increases the orientation degree of the first negative electrode material layer, and vice versa; when other factors remain constant, an increase in the orientation degree of the first graphite material in the first negative electrode material layer increases the orientation degree of the first negative electrode material layer, and vice versa; when the mass percentage of the first graphite material in the first negative electrode material layer increases, the orientation degree of the first negative electrode material layer increases, and vice versa. The change law of the orientation degree of the second negative electrode material layer is the same as that of the first negative electrode material layer. In particular, when other factors remain constant, an increase in the average particle size of the silicon-based material increases the orientation degree of the second negative electrode material layer, and vice versa; when the mass percentage of the second graphite material in the second negative electrode material layer increases and the mass percentage of the silicon-based material decreases accordingly, the orientation degree of the second negative electrode material layer increases, and vice versa.

[0038] In some embodiments of this application, 6≤OI1≤20, preferably 6≤OI1≤16; 8≤OI2≤25, preferably 15≤OI2≤25. For example, the value of OI1 can be 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or any two of these values. The value range of OI1 can be 6 to 20, 6 to 18, 6 to 16, 8 to 15, 10 to 15, 12 to 15, and all of these ranges, as well as sub-ranges. The value of OI2 can be 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, or any two of these values. The value range of OI2 can be 8 to 25, 10 to 25, 12 to 25, 15 to 25, 18 to 22, 18 to 20, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of OI1 and OI2 within the aforementioned range, it is beneficial to balance the wetting of the negative electrode material layer by the electrolyte and the transport of lithium ions within the negative electrode material layer. Furthermore, the composite structure of the silicon-based material and the second graphite material in the second negative electrode material layer is more stable, and the first negative electrode material layer and the negative electrode current collector exhibit better compatibility and bonding force, forming a stable interface structure and improving the interfacial stability between the first and second negative electrode material layers, thereby enhancing the structural stability of the negative electrode sheet. In addition, the SEI film formed on the surface of the negative electrode material layer is more uniform and dense, thus improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application improves both structural stability and cycle performance while maintaining good kinetic performance.

[0039] In some embodiments of this application, the porosity of the first negative electrode material layer is P1, and the porosity of the second negative electrode material layer is P2, where P1 < P2. This arrangement results in a higher porosity in the second negative electrode material layer, increasing the channels for the electrolyte to enter the negative electrode sheet and providing more ion transport paths. This facilitates the rapid insertion and extraction of lithium ions, effectively reducing ion transport impedance and thus improving the kinetic performance of the secondary battery. The lower porosity of the first negative electrode material layer ensures a tighter bond with the negative electrode current collector, providing stable support for the entire negative electrode material layer and improving the structural stability of the negative electrode sheet. It also facilitates the formation of a uniform and dense SEI film on the surface of the negative electrode material layer, reducing the risk of side reaction gas generation. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining good kinetic performance.

[0040] In some embodiments of this application, 20 ≤ P1 ≤ 45, 30 ≤ P2 ≤ 65. For example, the value of P1 can be 20, 22, 25, 28, 30, 32, 35, 38, 40, 42, 45, or a range consisting of any two of these values. The range of P1 can be 20 to 45, 22 to 42, 25 to 40, 28 to 38, 30 to 35, 30 to 32, and all of these ranges, as well as sub-ranges. The value of P2 can be 30, 32, 35, 38, 40, 42, 45, 48, 50, 52, 55, 58, 60, 62, 65, or a range consisting of any two of these values. The range of P2 can be 30 to 65, 32 to 62, 35 to 60, 38 to 58, 40 to 55, 42 to 52, 45 to 50, 48 to 50, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of P1 and P2 within the aforementioned range, the second negative electrode material layer exhibits higher porosity, increasing the channels for electrolyte to enter the negative electrode sheet and providing more ion transport paths. This facilitates the rapid insertion and extraction of lithium ions, effectively reducing ion transport impedance and thus improving the kinetic performance of the secondary battery. The first negative electrode material layer has lower porosity and bonds tightly with the negative electrode current collector, providing stable support for the entire negative electrode material layer. This improves the structural stability of the negative electrode sheet and promotes the formation of a uniform and dense SEI film on the surface of the negative electrode material layer, reducing the risk of side reaction gas generation. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining good kinetic performance.

[0041] Typically, the porosity of the first and second negative electrode material layers is affected by various factors. In this application, the porosity can be controlled by adjusting the particle size of the active material, the proportion of the active material, and the orientation degree of the graphite material in the negative electrode material layer. For example, when other factors remain constant, an increase in the average particle size of the first graphite material in the first negative electrode material layer increases the porosity of the first negative electrode material layer, and vice versa; when other factors remain constant, an increase in the orientation degree of the first graphite material in the first negative electrode material layer increases the porosity of the first negative electrode material layer, and vice versa; an increase in the mass percentage of the first graphite material in the first negative electrode material layer increases the porosity of the first negative electrode material layer, and vice versa. The porosity variation of the second anode material layer is the same as that of the first anode material layer. In particular, when other factors remain constant, the porosity of the second anode material layer increases when the average particle size of the silicon-based material increases, and vice versa. When the mass percentage of the second graphite material in the second anode material layer increases and the mass percentage of the silicon-based material decreases accordingly, the porosity of the second anode material layer increases, and vice versa.

[0042] In some embodiments of this application, the average particle size of the first graphite material is D1 μm, and the average particle size of the second graphite material is D2 μm, where D1 < D2. With this configuration, the second graphite material has a larger average particle size, resulting in a looser packing of graphite particles in the second negative electrode material layer. This leads to a relatively higher porosity in the second negative electrode material layer, increasing the channels for electrolyte to enter the negative electrode sheet and providing more ion transport paths. This facilitates the rapid insertion and extraction of lithium ions, effectively reducing ion transport impedance. Furthermore, the larger particle size of the second graphite material, when combined with silicon-based materials, can provide a buffer for the volume expansion of the silicon-based materials, thereby improving the structural stability of the negative electrode sheet. Conversely, the first graphite material has a smaller average particle size, resulting in a more compact packing of graphite particles in the first negative electrode material layer. This leads to a relatively lower porosity in the first negative electrode material layer. The smaller particle size of the first graphite material provides better contact and conductivity, forming a more continuous electron conduction network, reducing the internal resistance of the secondary battery, and thus enhancing the kinetic performance of the secondary battery. Meanwhile, the first negative electrode material layer, acting as a support layer, can effectively disperse the stress generated by the expansion of the silicon-based material, reducing the risk of cracking or delamination of the negative electrode material layer due to stress concentration, and further improving the structural stability of the secondary battery. Furthermore, the composite structure of the larger-particle-size second graphite material and the silicon-based material is more stable during cycling, which is beneficial for improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application improves both structural stability and cycle performance while maintaining good kinetic performance.

[0043] In some embodiments of this application, 2 ≤ D2 - D1 ≤ 10, 11 ≤ D1 ≤ 17. For example, the value of D2 - D1 can be 2, 3, 4, 5, 6, 7, 8, 9, 10, or a range consisting of any two of these values. The value range of D2 - D1 can be 2 to 10, 3 to 9, 4 to 8, 5 to 7, 5 to 6, and all of these ranges, as well as sub-ranges. The value of D1 can be 11, 12, 13, 14, 15, 16, 17, or a range consisting of any two of these values. The value range of D1 can be 11 to 17, 12 to 16, 13 to 15, 14 to 15, and all of these ranges, as well as sub-ranges. By controlling the values ​​of D2-D1 and D1 within the aforementioned range, it is beneficial to reduce the risks of agglomeration due to excessively small particle size of the first graphite material and excessively low porosity of the first negative electrode material layer. At this point, the graphite particles in the first negative electrode material layer are more densely packed, resulting in relatively low porosity. The smaller particle size of the first graphite material particles provides better contact and conductivity, forming a more continuous electron conduction network, reducing the internal resistance of the secondary battery, and thus enhancing its kinetic performance. Simultaneously, the first negative electrode material layer, as a support layer, can effectively disperse the stress generated by the expansion of the silicon-based material, reducing the risk of cracking or delamination of the negative electrode material layer due to stress concentration, further improving the structural stability of the secondary battery. Furthermore, the average particle size of the second graphite material, combined with the second negative electrode material layer, ensures good compatibility and bonding between the two layers, resulting in a more stable overall structure and improved cycle performance of the secondary battery. Therefore, the secondary battery of this application improves both structural stability and cycle performance while maintaining good kinetic performance.

[0044] In some embodiments of this application, 13 ≤ D2 ≤ 27. For example, the value of D2 can be 13, 15, 18, 20, 22, 25, 27, or a range consisting of any two of these values. The range of D2 can be 13 to 27, 15 to 25, 18 to 23, 20 to 22, and all of these ranges, as well as sub-ranges. By adjusting the value of D2 within the above ranges, the average particle size of the second graphite material is larger. At this time, the graphite particles in the second negative electrode material layer are more loosely packed, and the porosity of the second negative electrode material layer is relatively large, which increases the channels for the electrolyte to enter the negative electrode sheet and increases the number of ion transport paths. This is beneficial for the rapid insertion and extraction of lithium ions and effectively reduces the ion transport impedance. When the large-particle-size second graphite material is combined with silicon-based materials, it can provide a certain buffer space for the volume expansion of silicon-based materials, thereby improving the structural stability of the negative electrode sheet. Moreover, the composite structure of the larger-particle-size second graphite material and silicon-based materials is also more stable during cycling, which is beneficial for improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0045] In some embodiments of this application, the average particle size of the silicon-based material is D3 μm, where 8 ≤ D3 ≤ 20. For example, the value of D3 can be 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range consisting of any two of these values. The range of D3 can be 8 to 20, 9 to 18, 10 to 16, 12 to 16, 13 to 15, and all such ranges and sub-ranges. By adjusting the value of D3 within the above range, the particle size of the silicon-based material is moderate, which is beneficial for combining with the second graphite material to form a relatively dense and continuous electron conduction network, reducing the internal resistance of the secondary battery and improving the electron conduction efficiency. Simultaneously, it reduces the stress caused by its own volume expansion during charging and discharging, reducing the risk of cracking or delamination of the negative electrode material layer, thereby improving the structural stability of the negative electrode sheet. Furthermore, the moderate particle size is conducive to forming a more uniform and dense SEI film on the surface of the negative electrode material layer, thereby improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0046] This application does not impose any particular restrictions on the method of controlling the average particle size of the first graphite material, the second graphite material, and the silicon-based material, as long as the purpose of this application can be achieved. For example, the average particle size of the graphite material or silicon-based material can be controlled by classifying or grinding the particles. For example, when other conditions remain unchanged, extending the grinding time reduces the average particle size of the graphite material or silicon-based material; shortening the grinding time increases the average particle size of the graphite material or silicon-based material.

[0047] In some embodiments of this application, the orientation degree of the first graphite material is OI3, and the orientation degree of the second graphite material is OI4, where OI3 < OI4. With this arrangement, the second graphite material has a higher orientation degree and a more regular arrangement, which is beneficial for balancing the wetting of the negative electrode material layer by the electrolyte, shortening the lithium-ion transport path, reducing charge transfer impedance, and placing the silicon-based second negative electrode material layer on the surface. Since the high capacity of the silicon-based material allows it to participate in the reaction first during the initial charging and discharging stages, the shortened ion transport channels due to the high orientation degree of the second graphite material can reduce the polarization risk caused by the high capacity of silicon. Meanwhile, silicon-based materials undergo significant volume expansion during charge and discharge. Combining them with a more regularly arranged second graphite material allows the layered structure of the second graphite to mitigate the impact of volume changes in the silicon-based material. The low-orientation first graphite material exhibits good compatibility and bonding with the negative electrode current collector, resulting in a lower expansion rate during charge and discharge. This helps reduce the overall expansion rate of the negative electrode sheet during repeated charge and discharge cycles, providing better support and stability for the entire negative electrode material layer, thereby improving the structural stability of the negative electrode sheet. Furthermore, it facilitates the formation of a uniform and dense solid electrolyte interphase (SEI) film on the surface of the negative electrode material layer, reducing side reactions between the negative electrode active material and the electrolyte, thus improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application improves both structural stability and cycle performance while maintaining good kinetic performance.

[0048] In some embodiments of this application, 1≤OI3≤2.8, 3≤OI4≤4.2. For example, the value of OI3 can be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, or any range of any two values. The value range of OI3 can be 1 to 2.8, 1.2 to 2.5, 1.5 to 2.2, 1.8 to 2, and all of these ranges, as well as sub-ranges. The value of OI4 can be 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, or any range of any two values. The value range of OI4 can be 3 to 4.2, 3.2 to 4, 3.2 to 3.8, 3.5 to 3.8, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of OI3 and OI4 within the aforementioned range, it is beneficial to balance the wetting of the negative electrode material layer by the electrolyte and the transport of lithium ions within the negative electrode material layer. Furthermore, the composite structure of the silicon-based material and the second graphite material in the second negative electrode material layer is more stable, and the first negative electrode material layer and the negative electrode current collector exhibit better compatibility and bonding force, forming a stable interface structure and improving the interfacial stability between the first and second negative electrode material layers, thereby enhancing the structural stability of the negative electrode sheet. In addition, the SEI film formed on the surface of the negative electrode material layer is more uniform and dense, thus improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application improves both structural stability and cycle performance while maintaining good kinetic performance.

[0049] This application does not impose any particular restrictions on the preparation methods of the first graphite material and the second graphite material, as long as the purpose of this application can be achieved. For example, the preparation methods of the first graphite material and the second graphite material include, but are not limited to, the following steps: pulverizing raw materials into micron-sized particles, mixing a binder on the particle surface, further pulverizing the particles to reduce the particle size to the micron level, applying pressure to make the graphite particles more uniformly distributed, immersing the graphite particles, calcining in a high-temperature furnace, then placing the graphite particles in a high-temperature furnace for graphitization to obtain a precursor, and performing a second sintering at a controlled temperature in a high-temperature furnace to obtain the first graphite material or the second graphite material.

[0050] This application does not impose any particular restrictions on raw materials, as long as they can achieve the purpose of this application. For example, raw materials include, but are not limited to, natural graphite and / or artificial graphite. This application does not impose any particular restrictions on binders, as long as they can achieve the purpose of this application. For example, binders include, but are not limited to, asphalt. This application does not impose any particular restrictions on the materials used to impregnate graphite particles, as long as they can achieve the purpose of this application. For example, materials used to impregnate graphite particles include, but are not limited to, asphalt. This application does not impose any particular restrictions on the high-temperature furnace calcination process, as long as it can achieve the purpose of this application. This application does not impose any particular restrictions on the high-temperature furnace baking process for graphitization, as long as it can achieve the purpose of this application. For example, the high-temperature baking temperature can be from 2600℃ to 3400℃. This application does not impose any particular restrictions on the secondary sintering process, as long as it can achieve the purpose of this application. In this application, the degree of graphitization of the graphite precursor can be controlled by adjusting the high-temperature baking temperature, thereby controlling the orientation degree of the graphite material.

[0051] In some embodiments of this application, based on the mass of the first negative electrode material layer, the mass percentage content of the first graphite material is W1, where 90% ≤ W1 ≤ 99%. For example, the value of W1 can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or a range consisting of any two of these values. The range of W1 can be 90% to 99%, 91% to 98%, 92% to 97%, 93% to 96%, 94% to 95%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage content of the first graphite material within the above range, the volume change of the first graphite material during the charging and discharging process of the secondary battery is small, which is beneficial to improving the structural stability of the negative electrode sheet. Furthermore, the first graphite material has good conductivity, which is beneficial to improving the kinetic performance of the secondary battery. Combined with the high-capacity silicon-based material in the second negative electrode material layer, while taking into account kinetic performance and improving the structural stability and cycle performance of the secondary battery, the secondary battery also has a high energy density.

[0052] In some embodiments of this application, the mass percentage of silicon-based material is W2, 10% ≤ W2 ≤ 50%, based on the mass of the second negative electrode material layer. For example, the value of W2 can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range of any two of these values. The range of W2 can be 10% to 50%, 15% to 45%, 20% to 40%, 25% to 35%, 25% to 30%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage of silicon-based material within the above ranges, the energy density of the secondary battery is higher, and the risk of delamination or peeling of the negative electrode material layer due to excessive silicon-based material volume expansion is reduced, thereby improving the stability of the negative electrode structure. Therefore, while considering kinetic performance and improving the structural stability and cycle performance of the secondary battery, the secondary battery also has a high energy density.

[0053] In some embodiments of this application, based on the mass of the second negative electrode material layer, the mass percentage content of the second graphite material is W3, where 40% ≤ W3 ≤ 88%. For example, the value of W3 can be 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 88%, or a range consisting of any two of these values. The range of W3 can be 40% to 88%, 45% to 85%, 50% to 80%, 55% to 75%, 60% to 70%, 60% to 65%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage content of the second graphite material within the above ranges, the combination of the second graphite material and the silicon-based material facilitates the formation of a more uniform conductive network, improves the conductivity of the second negative electrode material layer, and thus improves the kinetic performance of the secondary battery. Furthermore, during the charging and discharging process of the secondary battery, the second graphite material can provide support for the silicon-based material, reducing the impact of its volume changes on the negative electrode structure. Therefore, the secondary battery of this application not only takes into account the dynamic performance, but also further improves the structural stability and cycle performance.

[0054] In some embodiments of this application, the silicon-based material includes at least one of elemental silicon, silicon oxide compounds, silicon carbide compounds, or silicon alloys. The silicon carbide compound is a silicon-carbon composite material, wherein the mass percentage of silicon is 30% to 70%, and the mass percentage of carbon is 30% to 70%, based on the mass of the silicon-carbon composite material. This application does not impose any particular limitation on the silicon-carbon composite material, as long as it achieves 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. Exemplarily, the silicon oxide compound can include silicon suboxide (SiO, where the molar ratio of silicon to oxygen is 1:1). By selecting the above-mentioned silicon-based material, the first negative electrode material layer and the second negative electrode material layer are arranged in combination, achieving both structural stability of the negative electrode sheet and a high energy density in the secondary battery. Therefore, the secondary battery of this application improves energy density while balancing kinetic performance, structural stability, and cycle performance.

[0055] In some embodiments of this application, in the X-ray diffraction pattern of the silicon-based material, a first characteristic peak exists between 26° and 30°, and a second characteristic peak exists between 47° and 52.5°. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, where 1.5 ≤ I1 / I2 ≤ 3.0. For example, the value of I1 / I2 can be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, or a range consisting of any two of these values. The range of I1 / I2 can be 1.5 to 3.0, 1.8 to 2.8, 2 to 2.8, 2.2 to 2.8, 2.2 to 2.5, and all such ranges and sub-ranges. Through the above configuration, the silicon-based material possesses a specific crystal structure and arrangement. This creates a relatively balanced relationship between the crystalline and amorphous phases of the silicon-based material, improving electron conduction efficiency and enhancing the kinetic performance of the secondary battery. Simultaneously, the specific crystal structure and appropriate I1 / I2 ratio help disperse the stress generated by the expansion of the silicon-based material itself. This reduces the risk of cracking or delamination of the negative electrode material layer due to stress concentration, thereby improving the structural stability of the negative electrode sheet. Furthermore, it facilitates the formation of a more uniform and dense SEI film on the surface of the negative electrode material layer, further improving the cycle performance of the secondary battery. Therefore, the secondary battery of this application improves both structural stability and cycle performance while maintaining good kinetic performance.

[0056] This application does not impose any particular restrictions on the method of controlling the value of I1 / I2, as long as the purpose of this application can be achieved. For example, the value of I1 / I2 can be controlled by adjusting the type of silicon-based material.

[0057] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collectors (such as lithium copper composite current collectors, carbon copper composite current collectors, nickel copper composite current collectors, titanium copper composite current collectors, 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 this application, the first negative electrode material layer and the second negative electrode material layer each independently include an adhesive and a conductive agent. This application does not impose any particular limitation on the type of adhesive in the first negative electrode material layer and the second negative electrode material layer, as long as it achieves the purpose of this application. For example, the adhesive may include, but is not limited to, at least one of polyacrylic acid (PAA), styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), or polyvinyl alcohol (PVA). This application does not impose any particular limitation on the types of conductive agents in the first and second negative electrode material layers, as long as they 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 materials, conductive carbon black (Super P), carbon nanotubes (CNTs), vinyl glass fiber (VGCF), metal particles, or metal fibers. 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. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver.

[0058] This application does not impose any particular limitation 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, binder and conductive agent evenly to obtain the first negative electrode slurry; (2) mixing the silicon-based material, the second graphite material, binder and conductive agent evenly to obtain the second negative electrode slurry; (3) simultaneously and evenly coating the first negative electrode slurry and the second negative electrode slurry obtained above onto one surface of the copper foil of the negative electrode current collector using a double-layer coating machine, and drying to obtain a negative electrode sheet with a first negative electrode material layer and a second negative electrode material layer on one side; (4) repeating the above steps on the other surface of the negative electrode current collector to obtain a negative electrode sheet with a first negative electrode material layer and a second negative electrode material layer on both sides.

[0059] In this application, the mass percentage content of the first graphite material in the first negative electrode material layer can be controlled by adjusting the mass ratio of the first graphite material, binder and conductive agent added in the first negative electrode slurry; the mass percentage content of the silicon-based material and the mass percentage content of the second graphite material in the second negative electrode material layer can be controlled by adjusting the mass ratio of the silicon-based material, second graphite material, binder and conductive agent added in the second negative electrode slurry.

[0060] In this application, the secondary battery further includes a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The aforementioned "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the positive current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the positive current collector, as long as the purpose of this application is achieved. For example, the positive current collector can include aluminum foil, aluminum alloy foil, or composite current collectors (e.g., aluminum-carbon composite current collectors). The positive electrode material layer of this application includes a positive electrode active material; this application has no particular limitation on the type of positive electrode active material, as long as the purpose of this application is achieved. For example, the positive electrode active material can 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.

[0061] In this application, the electrolyte includes 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.

[0062] This application does not impose any particular limitation on the diaphragm, as long as it can achieve the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) mainly composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. In some embodiments of this application, the diaphragm 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, at least one surface of the substrate layer is provided with a surface treatment layer, which may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. In some embodiments of this application, the inorganic layer comprises inorganic particles and a binder. This application does not impose any particular limitation on the inorganic particles; for example, the inorganic particles may include at least one selected from 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 impose any particular limitation on 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 comprises a polymer, the polymer material of which includes at least one selected from polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene). In this application, the thickness of the diaphragm is not particularly limited, as long as it achieves the purpose of this application; for example, the thickness of the diaphragm may be from 3 μm to 30 μm.

[0063] 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.

[0064] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode 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 the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode 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 the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0065] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device of this application has good performance characteristics.

[0066] 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.

[0067] Example

[0068] 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.

[0069] Test methods and equipment:

[0070] OI1 and OI2 tests:

[0071] The lithium-ion battery discharged at 0.5C to 3.0V was disassembled, and the negative electrode sheet was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes, followed by rinsing with DMC and acetone respectively. The negative electrode sheet was then placed in an oven and baked at 80℃ for 12 hours to obtain the negative electrode sheet.

[0072] 1) The sample is cut along the thickness direction of the negative electrode sheet by focusing ion beam technology to obtain the cross section of the negative electrode material layer along its own thickness direction. The thickness of the upper and lower layers of the negative electrode material layer, namely the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer, is measured by scanning electron microscopy (SEM).

[0073] 2) The total OI value of the first negative electrode material layer and the second negative electrode material layer is obtained by testing the negative electrode sheet using X-ray diffraction (XRD). The OI value is obtained by calculating the ratio of the intensity (or integrated area) of the diffraction peaks of the (002) or (004) crystal plane to the (110) crystal plane.

[0074] 3) The second negative electrode material layer on the surface of the negative electrode sheet is scraped off by mechanical means so that the thickness of the remaining negative electrode material layer after scraping is consistent with the thickness of the first negative electrode material layer measured by SEM. At this time, the remaining negative electrode material layer is considered to be the first negative electrode material layer.

[0075] 4) Use XRD to test the OI value of the first negative electrode material layer to obtain OI1. Then the OI value of the second negative electrode material layer = the total OI value of the first negative electrode material layer and the second negative electrode material layer - the OI value of the first negative electrode material layer to obtain OI2.

[0076] OI3 and OI4 tests:

[0077] The lithium-ion battery discharged at 0.5C to 3.0V was disassembled, and the negative electrode sheet was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes, followed by rinsing with DMC and acetone respectively. The negative electrode sheet was then placed in an oven and baked at 80℃ for 12 hours to obtain the negative electrode sheet.

[0078] 1) Following the test steps of “OI1 and OI2 Test”, the powders of the first negative electrode material layer and the second negative electrode material layer were obtained respectively.

[0079] 2) The powders of the first negative electrode material layer and the second negative electrode material layer were respectively immersed in tetrahydrofuran solvent, heated to 60°C, and ultrasonically treated for 2 hours to dissolve the binder into the solvent to obtain mixture A and mixture B respectively.

[0080] 3) Centrifuge mixture A and mixture B separately (5000 rpm, 20 minutes) to separate the supernatant (containing binder) and precipitate (active material + conductive agent) of mixture A and mixture B respectively.

[0081] 4) Utilizing the density differences between the silicon-based material, the active material graphite, and the conductive agent (carbon nanotubes in this embodiment), the two precipitates are separated by gravity to obtain the first graphite and the second graphite. Among them, the silicon-based material has the highest density value.

[0082] 5) The OI values ​​of the first and second graphite were tested by XRD. The OI value was obtained by calculating the ratio of the intensity (or integrated area) of the diffraction peaks of the (002) or (004) crystal plane to that of the (110) crystal plane.

[0083] Tests for P1 and P2:

[0084] The lithium-ion battery under test was disassembled at an ambient temperature of 25°C, and the negative electrode sheet was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes, followed by rinsing with DMC and acetone respectively. The negative electrode sheet was then placed in an oven and baked at 80°C for 12 hours to obtain a sample of the negative electrode sheet.

[0085] 1) The sample is cut along the thickness direction of the negative electrode sheet by focusing ion beam technology to obtain the cross section of the negative electrode material layer along its own thickness direction. The thickness of the upper and lower layers of the negative electrode material layer, namely the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer, is measured by scanning electron microscopy (SEM).

[0086] 2) The porosity of the total negative electrode material layer was tested using the gas adsorption method to obtain the total porosity of the first negative electrode material layer and the second negative electrode material layer.

[0087] 3) The second negative electrode material layer on the surface of the negative electrode sheet is scraped off by mechanical means so that the thickness of the remaining negative electrode material layer after scraping is consistent with the thickness of the first negative electrode material layer measured by SEM. At this time, the remaining negative electrode material layer is considered to be the first negative electrode material layer.

[0088] 4) The porosity of the remaining first negative electrode material layer is tested using the gas adsorption method to obtain the porosity P1 of the first negative electrode material layer. Then, the porosity of the second negative electrode material layer = the total porosity of the first negative electrode material layer and the second negative electrode material layer - the porosity of the first negative electrode material layer, thus obtaining P2.

[0089] Tests D1, D2, and D3:

[0090] The lithium-ion battery discharged at 0.5C to 3.0V was disassembled, and the negative electrode sheet was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes, followed by rinsing with DMC and acetone respectively. The negative electrode sheet was then placed in an oven and baked at 80℃ for 12 hours to obtain the negative electrode sheet.

[0091] The average particle size D1 of the first graphite material, the average particle size D2 of the second graphite material, and the average particle size D3 of the silicon-based material were measured using scanning electron microscopy (SEM). A cross-section of the negative electrode sheet along its thickness direction was prepared and ion-polished to obtain the cross-section of the negative electrode sheet. Observation of the cross-section of the negative electrode sheet using SEM revealed a relatively clear boundary between the two coating layers. The layer closest to the negative current collector is the first negative electrode material layer. Twenty particles of the first graphite material were randomly selected from the cross-section of the first negative electrode material layer, and the equivalent diameter of each of the 20 particles (i.e., the diameter of a circle with an area equal to the size of an irregular particle in the cross-section) was measured. The average value was calculated, which is the average particle size D1 of the first graphite material.

[0092] Then, the silicon-based material and the second graphite material particles in the cross-section of the negative electrode sheet were distinguished using backscattering mode. The silicon-based material, due to its higher atomic number and higher electron density, and the stronger interaction between the silicon-based material and the test electron beam compared to the graphite particles, resulted in a brighter silicon-based material region and a darker graphite particle region. Twenty particles of the second graphite material and twenty particles of the silicon-based material were selected from the cross-section of the second negative electrode layer, respectively. The equivalent diameters of the twenty graphite particles and the twenty silicon particles were measured, and their average values ​​were calculated. These average values ​​are the average particle size D2 of the second graphite material and the average particle size D3 of the silicon-based material.

[0093] Tests of I1 and I2:

[0094] The lithium-ion battery discharged at 0.5C to 3.0V was disassembled, and the negative electrode sheet was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes, followed by rinsing with DMC and acetone respectively. The negative electrode sheet was then placed in an oven and baked at 80℃ for 12 hours to obtain the negative electrode sheet.

[0095] Referring to steps 1) to 4) of the "Testing of OI3 and OI4", silicon-based materials were obtained through gravity separation. XRD testing was performed on the silicon-based materials using an X-ray excitation target of Cu Kα. The test voltage was 30 kV, the test current was 20 mA, the scanning angle range was 5° to 80°, and the scanning rate was 0.2° / s. The XRD diffraction pattern of the silicon-based materials was obtained. The peak with 2θ between 26° and 30° was identified as the first characteristic peak, and its peak intensity (I1) was recorded. The peak with 2θ between 47° and 52.5° was identified as the second characteristic peak, and its peak intensity (I2) was recorded.

[0096] Cycle performance and structural stability testing of lithium-ion batteries:

[0097] At a test temperature of 25℃, the lithium-ion battery under test was left to stand for 5 minutes, and the initial thickness MMC0 of the lithium-ion battery was recorded. The lithium-ion battery was charged at a constant current of 1.65C to 4.26V, then charged at 1.3C to 4.28V, then charged at 1.1C to 4.32V, then charged at 1.0C to 4.35V, then charged at 0.7C to 4.4V, then charged at 0.4C to 4.52V, and then charged at a constant voltage of 4.52V to 0.05C; left to stand for 5 minutes, and then discharged at a constant current of 0.5C to 3.0V, left to stand for 5 minutes, and the discharge capacity C1 of the lithium-ion battery was recorded. After the above 3C charge / 0.5C discharge cycle was repeated 400 times, the thickness MMC1 and the discharge capacity C2 of the lithium-ion battery were recorded.

[0098] Capacity retention rate (%) after 400 cycles = C2 / C1 × 100%.

[0099] Expansion rate (%) after 400 cycles = (MMC1 - MMC0) / MMC0 × 100%.

[0100] Among them, the higher the capacity retention rate after 400 cycles, the better the cycle performance of the lithium-ion battery; the lower the expansion rate after 400 cycles, the better the structural stability of the lithium-ion battery.

[0101] Dynamic performance testing:

[0102] At a test temperature of 25℃, the lithium-ion battery under test was left to stand for 5 minutes, then charged at a constant current of 0.7C to 4.52V, and then charged at a constant voltage to 0.025C. After standing for 5 minutes, it was discharged at a constant current of 0.2C to 3.0V, yielding a discharge capacity C0.2. The above charging process was then repeated, discharging at a constant current of 2.0C to 3.0V, yielding a discharge capacity C2.0. The formula D = C2.0 / C0.2 was set. The value of D was used to determine the quality of the kinetic performance.

[0103] The larger the D value, the better the kinetic performance of the lithium-ion battery; the smaller the D value, the worse the kinetic performance of the lithium-ion battery.

[0104] Example 1

[0105] <Preparation of the First Graphite Material>

[0106] The raw material, natural graphite, is crushed into micron-sized particles. A binder, asphalt, is mixed on the surface of the particles, and the particles are crushed a second time to reduce the particle size to the micron level. Pressure is applied to make the graphite particles more evenly distributed. The graphite particles are then impregnated with asphalt and calcined in a high-temperature furnace at 800°C for 5 hours. The graphite particles are then placed in a high-temperature furnace at 3000°C and baked for 30 days to graphitize, obtaining a precursor. The precursor is then sintered again in a high-temperature furnace at 1200°C for 2 days to obtain the first graphite material with an orientation degree of 1.5, i.e., OI3 of 1.5.

[0107] <Preparation of Second Graphite Material>

[0108] The raw material, natural graphite, is crushed into micron-sized particles. A binder, asphalt, is mixed onto the surface of the particles, and the particles are then crushed a second time to reduce the particle size to the micron level. Pressure is applied to make the graphite particles more evenly distributed. The graphite particles are then impregnated with asphalt and calcined in a high-temperature furnace at 800°C for 5 hours. The graphite particles are then placed in a high-temperature furnace and baked at 2800°C for 30 days to graphitize them, obtaining a precursor. The precursor is then sintered a second time at 1200°C in a high-temperature furnace for 2 days to obtain a second graphite material with an orientation degree of 3.6, i.e., OI4 of 3.6.

[0109] <Preparation of Negative Electrode Sheets>

[0110] The first graphite material (artificial graphite), binder (polyacrylic acid), and conductive agent (carbon nanotubes) were mixed in a mass ratio of 95:3:2, with deionized water added as a solvent. After being stirred evenly in a vacuum mixer, a first negative electrode slurry with a solid content of 45 wt% was prepared. The second graphite material (artificial graphite), silicon-based material (silicon carbide), binder (polyacrylic acid), and conductive agent (carbon nanotubes) were mixed in a mass ratio of 65:30:3:2, with deionized water added as a solvent. After being stirred evenly in a vacuum mixer, a second negative electrode slurry with a solid content of 45 wt% was prepared. The first and second negative electrode slurries were simultaneously and uniformly coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector using a double-layer coating machine. After drying at 120°C, a negative electrode sheet with a single-sided first and second negative electrode material layer was obtained. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the first and second negative electrode material layers. Then, the electrode is cold-pressed, cut, and welded with tabs to obtain a negative electrode sheet with dimensions of 78mm × 875mm for later use. The coating mass of the first negative electrode material layer is 9.3 mg / cm³. 2 The coating mass of the second negative electrode material layer is 2.5 mg / cm³. 2 The compaction density (PD) of the negative electrode material layer is 1.80 g / cm³. 2 The average particle size D1 of the first graphite material is 14 μm, the average particle size D2 of the second graphite material is 20 μm, and the average particle size D3 of the silicon-based material is 14 μm.

[0111] <Preparation of the positive electrode>

[0112] LiCoO2 (positive electrode active material), Super P (conductive agent), and polyvinylidene fluoride (PVDF) (binder) were mixed in a mass ratio of 97.9:0.9:1.2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%. The mixture was then stirred under vacuum until homogeneous to obtain the positive electrode slurry. This positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector. The foil was then dried at 120°C to obtain a single-sided coated positive electrode sheet with a coating mass of 325 mg / 1540 mm². 2 Then, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a positive electrode sheet with a size of 74mm×867mm for use. The thickness of the positive electrode material layer on one side is 70μm.

[0113] <Septum>

[0114] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0115] <Preparation of Electrolyte>

[0116] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Lithium salt LiPF6 was then added to the organic solvent and mixed thoroughly to obtain the electrolyte. The concentration of the lithium salt was 1.2 mol / L, and the remainder was the organic solvent.

[0117] <Preparation of Lithium-ion Batteries>

[0118] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The upper limit of the formation voltage is 4.2V, and the formation temperature is 70°C.

[0119] Examples 2 to 21

[0120] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1. Specifically, when the value of W1 changes, the mass percentage content of the conductive agent and binder in the first negative electrode material layer also changes accordingly, while the ratio of the conductive agent to the binder in the first negative electrode material layer remains constant. When the values ​​of W2 and W3 change, the mass percentage content of the conductive agent and binder in the second negative electrode material layer also changes accordingly, while the ratio of the conductive agent to the binder in the second negative electrode material layer remains constant. When the values ​​of D1, D2, and D3 change, the grinding time is adjusted to achieve the values ​​shown in Table 1. When the values ​​of OI3 and / or OI4 change, the baking temperature of the high-temperature furnace is adjusted to achieve the values ​​of OI3 and OI4 as shown in Table 1.

[0121] Comparative Example 1

[0122] Except for the fact that the preparation steps of the first negative electrode material layer in <Preparation of the negative electrode sheet> are the same as those of the second negative electrode material layer in Example 1, and the preparation steps of the second negative electrode material layer are the same as those of the first negative electrode material layer in Example 1, and the relevant preparation parameters are adjusted according to Table 1, the rest is the same as in Example 1.

[0123] Comparative Example 2

[0124] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0125] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1.

[0126] As can be seen from Examples 1 to 21 and Comparative Examples 1 to 21, by layering the negative electrode material layers and placing the second negative electrode material layer, which includes silicon-based material, on the surface, and making the orientation degree of the first negative electrode material layer less than that of the second negative electrode material layer, the lithium-ion battery exhibits a higher capacity retention rate after 400 cycles, a lower expansion rate after 400 cycles, and a larger D-value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining kinetic performance. In Comparative Example 1, the positions of the first and second negative electrode material layers are exactly opposite to those in Example 1; in Comparative Example 2, the orientation degree of the first negative electrode material layer is greater than that of the second negative electrode material layer. In Comparative Examples 1 to 21, the lithium-ion battery exhibits a lower capacity retention rate after 400 cycles, a higher expansion rate after 400 cycles, and a smaller D-value, indicating that the lithium-ion batteries in the comparative examples cannot simultaneously achieve kinetic performance, structural stability, and cycle performance. The lithium-ion batteries in Examples 1 to 21 exhibit high capacity retention after 400 cycles, low expansion rate after 400 cycles, and large D-value, indicating that the lithium-ion batteries of this application have good structural stability and cycle performance while taking into account dynamic performance.

[0127] The values ​​of OI1 and OI2 typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 21, when the values ​​of OI1 and OI2 are within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0128] The values ​​of P1 and P2 typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 21, when the values ​​of P1 and P2 are within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0129] The values ​​of D2-D1 and D1 typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 5 and Examples 19 to 21, when the values ​​of D2-D1 and D1 are within the range of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0130] The value of D3 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 6 to 7, 19, and 21, when the value of D3 is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0131] The values ​​of OI3 and OI4 typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 8 to 12, 19, and 21, when the values ​​of OI3 and OI4 are within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0132] The value of W1 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 13 to 14, and 19 to 21, when the value of W1 is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0133] The value of W2 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 15 to 16, 19, and 21, when the value of W2 is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0134] The value of W3 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 15 to 16, 19, and 21, when the value of W3 is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0135] The type of silicon-based material typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 17 to 18, and 20, when the type of silicon-based material falls within the scope of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0136] The value of I1 / I2 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 17 to 18, and 20, when the value of I1 / I2 is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0137] 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.

[0138] 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 negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector, the negative electrode material layer comprising a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer being located between the second negative electrode material layer and the negative current collector; The first negative electrode material layer includes a first graphite material, and the second negative electrode material layer includes a silicon-based material and a second graphite material. The orientation degree of the first negative electrode material layer is OI1, and the orientation degree of the second negative electrode material layer is OI2, where OI1 < OI2.

2. The secondary battery according to claim 1, wherein, 6≤OI1≤20, 8≤OI2≤25.

3. The secondary battery according to claim 2, wherein, 6≤OI1≤16, 15≤OI2≤25.

4. The secondary battery according to claim 1 or 2, wherein, The porosity of the first negative electrode material layer is P1, and the porosity of the second negative electrode material layer is P2, where P1 < P2.

5. The secondary battery according to claim 4, wherein, 20≤P1≤45, 30≤P2≤65.

6. The secondary battery according to claim 1 or 2, wherein, The average particle size of the first graphite material is D1μm, and the average particle size of the second graphite material is D2μm, where D1 < D2.

7. The secondary battery according to claim 6, wherein, 2≤D2-D1≤10, 11≤D1≤17.

8. The secondary battery according to claim 1 or 2, wherein, The average particle size of the silicon-based material is D3μm, where 8≤D3≤20.

9. The secondary battery according to claim 1 or 2, wherein, The orientation degree of the first graphite material is OI3, and the orientation degree of the second graphite material is OI4, where OI3 < OI4.

10. The secondary battery according to claim 9, wherein, 1≤OI3≤2.8, 3≤OI4≤4.

2.

11. The secondary battery according to claim 1, wherein, Based on the mass of the first negative electrode material layer, the mass percentage of the first graphite material is W1, where 90% ≤ W1 ≤ 99%.

12. The secondary battery according to claim 1, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the silicon-based material is W2, where 10% ≤ W2 ≤ 50%.

13. The secondary battery according to claim 12, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the second graphite material is W3, 40% ≤ W3 ≤ 88%.

14. The secondary battery according to claim 1, wherein, The silicon-based material includes at least one of elemental silicon, silicon oxide, silicon carbide, or silicon alloy.

15. The secondary battery according to claim 1, wherein, In the X-ray diffraction pattern of the silicon-based material, there is a first characteristic peak between 26° and 30° and a second characteristic peak between 47° and 52.5°. The peak intensity of the first characteristic peak is I1 and the peak intensity of the second characteristic peak is I2, and 1.5≤I1 / I2≤3.

0.

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

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