Secondary battery and electronic apparatus

Abstract

Provided in the present application are a secondary battery and an electronic apparatus. The secondary battery comprises a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, wherein 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. In an X-ray diffraction pattern of the second graphite material, a first characteristic peak is present when 2θ is between 41.5° and 43.0°, and a second characteristic peak is present when 2θ is between 43.5° and 45.5°, wherein the peak intensity of the first characteristic peak is I1, the peak intensity of the second characteristic peak is I2, and 1.5≤I2 / I1≤2.5. The above arrangements are conducive to improving the interfacial stability of the negative electrode sheet and improving the energy density and fast-charging performance of the secondary battery.

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, such as lithium-ion batteries, are characterized by high theoretical specific capacity and high safety performance, and have gradually become the main power source in the fields of computers, communications, consumer electronics (3C field) and power vehicles (EV field). In recent years, with the rapid development of electric vehicles and mobile electronic devices, the market has placed higher demands on lithium-ion batteries, but the energy density and fast charging capability of lithium-ion batteries still need to be improved.

[0003] Among them, positive electrode active materials, negative electrode active materials, and electrolytes are important components of lithium-ion batteries and have a significant impact on their performance. Therefore, continuous optimization and improvement of positive electrode active materials, negative electrode active materials, and electrolytes are particularly important. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device to improve the interface stability of the negative electrode and increase the energy density and fast charging performance of the secondary battery.

[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] Currently, improving the charging capacity of rechargeable batteries typically involves using fast-charging graphite with higher carbon coating and electrolytes with higher conductivity to improve kinetics. However, this usually reduces the high-temperature capacity retention and storage stability of the rechargeable battery, making it more prone to capacity decay and abnormal thickness expansion at high temperatures. To improve the energy density of rechargeable batteries, using silicon-based negative electrode sheets is a common and effective method. However, due to the poor electronic and ionic conductivity of silicon-based materials, the impedance of the negative electrode sheet increases, reducing the fast-charging capability of the rechargeable battery. Furthermore, because silicon-based materials undergo approximately 300% volume expansion during charging and discharging, repeated expansion and contraction can lead to particle breakage, exposing more fresh interfaces. This results in the formation of more solid electrolyte interphase (SEI) films, further reducing the capacity retention and fast-charging capability of the rechargeable battery, thus limiting the application of silicon-based materials. Based on this, this application provides a rechargeable battery and electronic device, where the rechargeable battery exhibits high energy density and good fast-charging performance. The specific technical solution is as follows:

[0007] The first aspect of this application provides a secondary battery, which includes a negative electrode sheet, 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 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 electrode 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. In the X-ray diffraction pattern of the second graphite material, a first characteristic peak exists between 41.5° and 43.0° for 2θ, and a second characteristic peak exists between 43.5° and 45.5° for 2θ. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, with 1.5 ≤ I2 / I1 ≤ 2.5, preferably 1.8 ≤ I2 / I1 ≤ 2.3. By combining the first negative electrode material layer with the second negative electrode material layer, the interface stability of the negative electrode sheet can be improved. The silicon-based material is enriched in the second negative electrode material layer, combined with the second graphite material, and the I2 / I1 ratio of the second graphite material is controlled within the above range. The conductive network formed by the graphite particles improves the conductivity of the silicon-based material, thereby increasing the charging speed of the secondary battery. This improves the fast charging performance of the secondary battery while increasing its energy density.

[0008] In some embodiments of this application, the average particle size of the second graphite material is D1 μm, and the specific surface area of ​​the second graphite material is S m. 2 / g, 5≤D1≤20, preferably 6.5≤D1≤15; 0.5≤S≤4.0, preferably 1.5≤S≤3.5. By controlling the average particle size and specific surface area of ​​the second graphite material within the above ranges, the particle size of the second graphite material is moderate, resulting in a shorter diffusion path for lithium ions. Simultaneously, the moderate specific surface area of ​​the second graphite material allows for a larger contact area between the second graphite material and the silicon-based material particles. Therefore, the secondary battery of this application has high energy density and good fast-charging performance.

[0009] In some embodiments of this application, the silicon-based material includes at least one of elemental silicon, silicon oxide, silicon carbide, or silicon alloy, and the average particle size of the silicon-based material is D2μm, where 8≤D2≤20. By selecting the above-mentioned types of silicon-based materials and controlling the average particle size of the silicon-based material within the above range, the particle size of the silicon-based material is moderate, resulting in relatively small stress during volume expansion. Furthermore, the silicon-based material and the second graphite material work synergistically, and the conductive network formed by the particles of the second graphite material further enhances the conductivity of the silicon-based material. Therefore, the secondary battery of this application has high energy density and good fast-charging performance.

[0010] In some embodiments of this application, the mass percentage of silicon-based material is W1, 10% ≤ W1 ≤ 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 risk of delamination or peeling of the negative electrode material layer due to excessive volume expansion of silicon-based material is reduced. Furthermore, in synergy with the second graphite material, the conductive network formed by the particles of the second graphite material enhances the conductivity of the silicon-based material. Therefore, the secondary battery of this application has high energy density and good fast-charging performance.

[0011] In some embodiments of this application, based on the mass of the second negative electrode material layer, the mass percentage of the second graphite material is W2, where 40% ≤ W2 ≤ 88%. By controlling the mass percentage of the second graphite material within the above range, the conductive network formed by the particles of the second graphite material, combined with the silicon-based material, further enhances the conductivity of the silicon-based material. Therefore, the secondary battery of this application, while maintaining energy density, further improves structural stability and fast-charging performance.

[0012] In some embodiments of this application, based on the mass of the first negative electrode material layer, the mass percentage of the first graphite material is W3, where 90% ≤ W3 ≤ 99%. By controlling the mass percentage of the first graphite material within the above range, it is beneficial to improve the structural stability of the negative electrode sheet. Furthermore, the first graphite material has good conductivity. Combined with the silicon-based materials in the second negative electrode material layer, the secondary battery of this application has high energy density and good fast-charging performance.

[0013] In some embodiments of this application, the thickness of the first negative electrode material layer is H1 μm, the thickness of the second negative electrode material layer is H2 μm, 0.5 ≤ H1 / H2 ≤ 8, and 20 ≤ H1 + H2 ≤ 240. By adjusting the values ​​of H1 / H2 and H1 + H2 within the above ranges, it is beneficial to form a uniform layered structure. Furthermore, while improving the energy density of the secondary battery, the expansion of the negative electrode sheet is moderate. Therefore, the interface stability of the negative electrode sheet is improved, and the energy density and fast-charging performance of the secondary battery are enhanced.

[0014] In some embodiments of this application, the coating mass per unit area of ​​the first negative electrode material layer is m1 mg / cm³. 2 The coating mass per unit area of ​​the second negative electrode material layer is m2 mg / cm³. 2 The ratio of m2 to m1 is 1:8 to 2:1. By adjusting the mass ratio of the second negative electrode material layer to the first negative electrode material layer within the above range, it is beneficial to ion transport and makes the expansion of the negative electrode sheet moderate. Therefore, it improves the interfacial stability of the negative electrode sheet and increases the energy density and fast charging performance of the secondary battery.

[0015] In some embodiments of this application, the second negative electrode material layer includes a binder, which includes at least one selected from polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the second negative electrode material layer, the mass percentage content of the binder is W4, where 0.8% ≤ W4 ≤ 10%. By selecting the above-mentioned types of binders in the second negative electrode material layer and controlling the mass percentage content of the binder within the above range, the stability of the silicon-based material is improved while the impedance of the negative electrode sheet is reduced, thereby increasing the charging speed of the secondary battery. Therefore, the secondary battery of this application further improves structural stability and fast-charging performance while maintaining energy density.

[0016] In some embodiments of this application, in the X-ray diffraction pattern of the first graphite material, a third characteristic peak exists between 40.5° and 42.0° for 2θ, and a fourth characteristic peak exists between 42.5° and 43.5° for 2θ. The peak intensity of the third characteristic peak is I1', and the peak intensity of the fourth characteristic peak is I2', with 1.2 ≤ I2' / I1' ≤ 2.5. By adjusting the value of I2' / I1' within the above range, while taking into account the structural stability and fast-charging performance of the secondary battery, it is beneficial to further improve the overall energy density of the secondary battery.

[0017] 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.

[0018] The beneficial effects of this application are:

[0019] 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. In the X-ray diffraction pattern of the second graphite material, a first characteristic peak exists between 41.5° and 43.0° for 2θ, and a second characteristic peak exists between 43.5° and 45.5° for 2θ. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, where 1.5 ≤ I2 / I1 ≤ 2.5. By combining the first negative electrode material layer with the second negative electrode material layer, the interface stability of the negative electrode sheet can be improved. The silicon-based material is enriched in the second negative electrode material layer, combined with the second graphite material, and the I2 / I1 ratio of the second graphite material is controlled within the above range. The conductive network formed by the graphite particles improves the conductivity of the silicon-based material, thereby increasing the charging speed of the secondary battery. This improves the fast charging performance of the secondary battery while increasing its energy density.

[0020] 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

[0021] 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.

[0022] 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.

[0023] 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

[0024] 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.

[0025] 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:

[0026] The first aspect of this application provides a secondary battery, which includes a negative electrode sheet, 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 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 electrode 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. In the X-ray diffraction pattern of the second graphite material, a first characteristic peak exists between 41.5° and 43.0° for 2θ, and a second characteristic peak exists between 43.5° and 45.5° for 2θ. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, with 1.5 ≤ I2 / I1 ≤ 2.5, preferably 1.8 ≤ I2 / I1 ≤ 2.3. For example, the value of I2 / I1 can be 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5 or any two of these values. The range of I2 / I1 can be 1.5 to 2.5, 1.6 to 2.4, 1.8 to 2.3, 1.9 to 2.2, 1.9 to 2.1, and all of these ranges, as well as subranges.

[0027] 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.

[0028] The inventors discovered that by dividing the negative electrode material layer into layers, the first negative electrode material layer, made of graphite, exhibits good compatibility and adhesion with the negative electrode current collector. This allows the first negative electrode material layer to act as a stable base layer, mitigating stress concentration on the current collector during the expansion of the silicon-based material in the second negative electrode material layer. The second negative electrode material layer comprises both silicon-based and graphite materials. The synergistic effect of the graphite and silicon-based materials further disperses the stress caused by volume expansion, thereby improving the interfacial stability of the negative electrode sheet. Simultaneously, by enriching the silicon-based material in the second negative electrode material layer, and utilizing the characteristic of silicon particles expanding during charging, the porosity of the second negative electrode material layer is increased. This increases the channels for electrolyte to enter the negative electrode sheet, providing more ion transport paths and effectively reducing ion transport impedance, thus increasing both charging speed and energy density. Furthermore, by adjusting the ratio of the peak intensity of the second characteristic peak to the first characteristic peak of the second graphite material within the range specified in this application, the diffusion barrier and charge transfer resistance of the second graphite material are lower, which is beneficial for the rapid insertion and extraction of lithium ions. The second graphite material in the second negative electrode material layer works synergistically with the silicon-based material. The conductive network formed by the particles of the second graphite material improves the conductivity of the silicon-based material, reduces the electron transport resistance and solid-phase ion transport resistance of the negative electrode sheet, thereby improving the charging speed of the secondary battery. When the value of I2 / I1 is outside the range specified in this application, the conductivity of the silicon-based material is poor, resulting in a slower lithium ion transport speed, making it impossible to simultaneously improve the energy density and achieve fast charging performance of the secondary battery.

[0029] Therefore, this application improves the interface stability of the negative electrode sheet and enhances the energy density and fast charging performance of the secondary battery by layering the negative electrode material layer, enriching the silicon-based material in the second negative electrode material layer, combining it with the second graphite material, and regulating the I2 / I1 ratio of the second graphite material within the above range.

[0030] This application does not impose any particular restrictions on the method of controlling I2 / I1, as long as the purpose of this application can be achieved. For example, commercially available graphite materials with different I2 / I1 ratios can be selected, and the I2 / I1 ratio of the graphite material can be determined by combining the test method of "I1, I2 testing" in this application, and the desired second graphite material can be selected. This application does not impose any particular restrictions on the type of second graphite material, as long as the purpose of this application can be achieved. For example, the second graphite material includes, but is not limited to, at least one of artificial graphite, natural graphite, or hard carbon.

[0031] In some embodiments of this application, the average particle size of the second graphite material is D1 μm, and the specific surface area of ​​the second graphite material is S m. 2 / g, 5≤D1≤20, preferably 6.5≤D1≤15; 0.5≤S≤4.0, preferably 1.5≤S≤3.5. For example, the value of D1 can be 5, 5.5, 6, 6.5, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range of any two of these values. The value range of D1 can be 5 to 20, 6 to 18, 6.5 to 16, 6.5 to 15, 8 to 15, 8 to 12, 10 to 12, and all such ranges and subranges; the value of S can be 0.5, 0 The range of values ​​for S can be 0.5 to 4.0, 0.8 to 3.8, 1 to 3.5, 1.2 to 3.5, 1.5 to 3.5, 1.8 to 3.2, 2 to 3, 2.2 to 2.8, 2.2 to 2.5, and all of these ranges, as well as subranges. By controlling the average particle size and specific surface area of ​​the second graphite material within the aforementioned range, the particle size of the second graphite material is moderate, resulting in a shorter lithium-ion diffusion path. This is beneficial for improving the charging speed of the secondary battery. During the slurry preparation and coating process of the second negative electrode material layer, a uniform coating is more easily formed, and the voids in the silicon-based material are better filled. This improves the interfacial stability and compaction density of the negative electrode sheet. Simultaneously, the moderate specific surface area of ​​the second graphite material provides a larger contact area between the particles of the second graphite material and the silicon-based material. The synergistic effect of the second graphite material and the silicon-based material in the second negative electrode material layer, with the conductive network formed by the second graphite particles, further enhances the conductivity of the silicon-based material, reducing the electron transport resistance and solid-phase ion transport resistance of the negative electrode sheet, thereby further improving the charging speed of the secondary battery. Therefore, the secondary battery of this application has high energy density and good fast-charging performance.

[0032] 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, with a silicon to oxygen molar ratio of 1:1). The average particle size of the silicon-based material particles is D²μm, where 8 ≤ D² ≤ 20. For example, the value of D2 can be 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or any range of two values ​​within these ranges. The value range of D2 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 selecting the aforementioned types of silicon-based materials and controlling the average particle size of the silicon-based materials within the above ranges, the particle size of the silicon-based materials is moderate, resulting in relatively low stress during volume expansion. Combined with the second graphite material, this allows for better filling of the pores in the second negative electrode material layer, thereby improving the interfacial stability and compaction density of the negative electrode sheet, and ultimately increasing the energy density of the secondary battery. Furthermore, the synergistic effect of the silicon-based materials and the second graphite material, with the conductive network formed by the particles of the second graphite material further enhancing the conductivity of the silicon-based materials, reduces the electron transport resistance and solid-phase ion transport resistance of the negative electrode sheet, thereby further improving the charging speed of the secondary battery. Therefore, the secondary battery of this application has high energy density and good fast charging performance.

[0033] This application does not impose any particular restrictions on the method of controlling the average particle size of the second graphite material and the average particle size of the silicon-based material, as long as the purpose of this application can be achieved. For example, 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 decreases 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. This application also does not impose any particular restrictions on the method of controlling the specific surface area of ​​the second graphite material, as long as the purpose of this application can be achieved. For example, the specific surface area of ​​the second graphite material can be controlled by classifying or grinding the particles.

[0034] In some embodiments of this application, the mass percentage of silicon-based material is W1, 10% ≤ W1 ≤ 50%, based on the mass of the second negative electrode material layer. For example, the value of W1 can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range of any two of these values. The range of W1 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. Furthermore, in synergy with the second graphite material, the conductive network formed by the particles of the second graphite material enhances the conductivity of the silicon-based material, reduces the electron transport resistance and solid-phase ion transport resistance of the negative electrode, thereby improving the charging speed of the secondary battery. Therefore, the secondary battery of this application has high energy density and good fast charging performance.

[0035] 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 W2, where 40% ≤ W2 ≤ 88%. For example, the value of W2 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 W2 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 second graphite material, combined with the silicon-based material, further enhances the conductivity of the silicon-based material through the conductive network formed by the particles of the second graphite material, reducing the electron transport resistance and solid-phase ion transport resistance of the negative electrode sheet, thereby further improving the charging speed 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, thereby improving the interface stability of the negative electrode. Therefore, the secondary battery of this application, while maintaining energy density, further improves structural stability and fast-charging performance.

[0036] 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 W3, where 90% ≤ W3 ≤ 99%. For example, the value of W3 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 W3 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 ranges, 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. A higher content of the first graphite material can form a more compact stacked structure, thereby increasing the compaction density of the negative electrode sheet. Furthermore, the first graphite material has good conductivity, which, combined with the silicon-based materials in the second negative electrode material layer, improves the fast-charging performance of the secondary battery. Therefore, the secondary battery of this application has high energy density and good fast charging performance.

[0037] In some embodiments of this application, the thickness of the first negative electrode material layer is H1 μm, the thickness of the second negative electrode material layer is H2 μm, 0.5 ≤ H1 / H2 ≤ 8, and 20 ≤ H1 + H2 ≤ 240. For example, the value of H1 / H2 can be 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, or a range of any two values ​​therein. The range of H1 / H2 can be 0.5 to 8, 1 to 7, 1.5 to 6, 2 to 5.5, 2.5 to 5, 3 to 4.5, 3.5 to 4, and any values ​​therein. There are ranges and subranges; the value of H1+H2 can be any two values ​​from 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, and 240. The value range of H1+H2 can be 20 to 240, 50 to 220, 80 to 200, 100 to 180, 120 to 160, and all ranges therein, as well as subranges. By adjusting the values ​​of H1 / H2 and H1+H2 within the aforementioned ranges, it is beneficial to form a uniform layered structure, reduce the risk of stress concentration due to excessive thickness differences, and improve the energy density of the secondary battery. The pores created by the expansion of the silicon-based material in the second negative electrode layer allow the electrolyte to better enter the first negative electrode layer, facilitating ion transport and thus increasing the charging speed of the secondary battery. Furthermore, while improving the energy density of the secondary battery, it reduces the risk of excessive graphite particles being squeezed by the silicon-based material in the second negative electrode layer after expansion, resulting in moderate expansion of the negative electrode sheet. Therefore, it improves the interface stability of the negative electrode sheet and enhances the energy density and fast-charging performance of the secondary battery. In this application, the thickness of the first negative electrode layer refers to the thickness of a single side of the first negative electrode layer, and the thickness of the second negative electrode layer refers to the thickness of a single side of the second negative electrode layer.

[0038] In some embodiments of this application, 6.67 ≤ H1 ≤ 213.33. For example, the value of H1 can be 6.67, 7, 8, 10, 30, 50, 80, 100, 120, 150, 180, 200, 205, 210, 213, 213.33, or a range consisting of any two of these values. The value range of H1 can be 6.67 to 213.33, 10 to 210, 50 to 200, 80 to 180, 100 to 160, 120 to 150, and all of these ranges, as well as sub-ranges. By adjusting the value of H1 within the above range, it is beneficial to buffer the volume expansion of silicon-based materials during charging and discharging, thereby improving the interface stability of the negative electrode sheet. Furthermore, the diffusion path of lithium ions in the first negative electrode material layer is shorter, which is beneficial to improving fast charging performance. Therefore, the secondary battery of this application has high energy density and good fast charging performance.

[0039] In some embodiments of this application, 13.33 ≤ H2 ≤ 160. For example, the value of H2 can be 13.33, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, or a range consisting of any two of these values. The range of H2 can be 13.33 to 160, 10 to 150, 30 to 120, 50 to 100, and all of these ranges, as well as sub-ranges. By controlling the value of H2 within the above range, it is beneficial to reduce the risk of interfacial stress concentration and delamination of the negative electrode material layer caused by the expansion of the silicon-based material, thereby improving the stability of the negative electrode interface. In addition, while increasing the energy density of the secondary battery, the conductive network formed by the particles of the second graphite material in the second negative electrode material layer improves the conductivity of the silicon-based material, reduces the electron transport resistance and solid-phase ion transport resistance of the negative electrode, thereby improving the charging speed of the secondary battery. Therefore, the secondary battery of this application has high energy density and good fast charging performance.

[0040] In this application, the thickness of the first negative electrode material layer and the thickness of the second negative electrode material layer can be controlled by means known to those skilled in the art. For example, when the first negative electrode slurry is coated on the surface of the negative electrode current collector, the coating amount of the first negative electrode slurry can be increased to increase the thickness of the first negative electrode material layer, based on a certain solid content of the first negative electrode slurry; when the second negative electrode slurry is coated on the second surface of the base film, the coating amount of the second negative electrode slurry can be increased to increase the thickness of the second negative electrode material layer, based on a certain solid content of the second negative electrode slurry. This application does not impose any particular restrictions, as long as the purpose of this application can be achieved.

[0041] In some embodiments of this application, the coating mass per unit area of ​​the first negative electrode material layer is m1 mg / cm³. 2The coating mass per unit area of ​​the second negative electrode material layer is m2 mg / cm³. 2 The ratio of m2:m1 is 1:8 to 2:1. For example, the value of m2:m1 can be 1:8, 1:4, 3:8, 1:2, 5:8, 3:4, 7:8, 1:1, 8:7, 4:3, 8:5, 2:1, or any two of these values. The range of values ​​for m2:m1 can be 1:8 to 2:1, 1:4 to 8:5, 3:8 to 4:3, 1:2 to 8:7, 5:8 to 1:1, 3:4 to 7:8, and all of these ranges, as well as subranges. By adjusting the mass ratio of the second negative electrode material layer to the first negative electrode material layer within the aforementioned range, the pores generated when the silicon-based material in the second negative electrode material layer expands allow the electrolyte to better enter the first negative electrode material layer, which is beneficial for ion transport and thus improves the charging speed of the secondary battery. Furthermore, while increasing the energy density of the secondary battery, it reduces the risk of the silicon-based material in the second negative electrode material layer compressing excessive graphite particles after expansion, resulting in moderate expansion of the negative electrode sheet. Therefore, it improves the interface stability of the negative electrode sheet and enhances the energy density and fast-charging performance of the secondary battery. In this application, the mass of the first negative electrode material layer refers to the mass of a single side of the first negative electrode material layer, and the mass of the second negative electrode material layer refers to the mass of a single side of the second negative electrode material layer.

[0042] This application does not impose any particular restrictions on the method of controlling the quality of the first negative electrode material layer and the second negative electrode material layer, as long as the purpose of this application can be achieved. For example, when coating the first negative electrode slurry onto the surface of the negative electrode current collector, the coating amount of the first negative electrode slurry can be increased to increase the quality of the first negative electrode material layer, while maintaining a certain solid content; similarly, when coating the second negative electrode slurry onto the second surface of the base film, the coating amount of the second negative electrode slurry can be increased to increase the quality of the second negative electrode material layer, while maintaining a certain solid content. This application does not impose any particular restrictions, as long as the purpose of this application can be achieved.

[0043] In some embodiments of this application, the second negative electrode material layer includes a binder, which includes at least one selected from polyacrylic acid (PAA), styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), or polyvinyl alcohol (PVA). Based on the mass of the second negative electrode material layer, the mass percentage of the binder is W4, where 0.8% ≤ W4 ≤ 10%. For example, the value of W4 can be 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or a range consisting of any two of these values. The range of W4 can be 0.8% to 10%, 1% to 9%, 2% to 8%, 3% to 7%, 4% to 6%, and all of these ranges, as well as sub-ranges. By selecting the aforementioned type of binder in the second negative electrode material layer and controlling the mass percentage of the binder within the aforementioned range, the binder, such as polyacrylic acid, has a high carboxyl group content in its structure. This allows it to form strong hydrogen bonds with active materials containing hydroxyl groups on their surface, enabling it to uniformly coat the particle surface of the active material. Furthermore, it exhibits less swelling in the electrolyte, especially carbonate solvents, resulting in a stable interfacial structure of the negative electrode sheet during charging and discharging. This reduces the risk of excessive expansion and fragmentation of the silicon-based material leading to repeated SEI film formation. While improving the stability of the silicon-based material, it also reduces the impedance of the negative electrode sheet, thereby increasing the charging speed of the secondary battery. Therefore, the secondary battery of this application, while maintaining energy density, further improves structural stability and fast-charging performance.

[0044] The first negative electrode material layer also includes a binder. This application does not impose any particular limitation on the type of binder in the first negative electrode material layer, as long as it achieves the purpose of this application. For example, the binder in the first negative electrode material layer includes, 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 mass percentage of the binder in the first negative electrode material layer, as long as it achieves the purpose of this application.

[0045] In some embodiments of this application, in the X-ray diffraction pattern of the first graphite material, a third characteristic peak exists between 40.5° and 42.0° for 2θ, and a fourth characteristic peak exists between 42.5° and 43.5° for 2θ. The peak intensity of the third characteristic peak is I1', and the peak intensity of the fourth characteristic peak is I2', with 1.2 ≤ I2' / I1' ≤ 2.5. For example, the value of I2' / I1' can be 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or a range consisting of any two of these values; the range of I2' / I1' can be 1.2 to 2.5, 1.3 to 2.4, 1.5 to 2.3, 1.6 to 2.2, 1.7 to 2.1, 1.8 to 2.0, and all such ranges and sub-ranges. By adjusting the values ​​of I2' / I1' within the above range, the first negative electrode material layer combined with the second negative electrode material layer can improve the overall energy density of the secondary battery while taking into account both the structural stability and fast charging performance of the secondary battery.

[0046] This application does not impose any particular restrictions on the method of controlling I2' / I1', as long as the purpose of this application can be achieved. For example, commercially available graphite materials with different I2' / I1' ratios can be selected, and the I2' / I1' ratio of the graphite material can be determined by combining the test methods of "I1', I2' testing" in this application, and the desired first graphite material can be selected. This application does not impose any particular restrictions on the type of first graphite material, as long as the purpose of this application can be achieved. For example, the first graphite material includes, but is not limited to, at least one of artificial graphite, natural graphite, or hard carbon.

[0047] This application does not impose any particular limitation on the negative electrode current collector, as long as it can achieve 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, nickel foam, copper foam, or composite current collectors (e.g., lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.). In this application, the first negative electrode material layer and the second negative electrode material layer each independently include a conductive agent. This application does not impose any particular limitation on the type of conductive agent in the first negative electrode material layer and the second negative 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 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.

[0048] 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.

[0049] 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 to the first negative electrode slurry; the mass percentage content of the silicon-based material, the mass percentage content of the second graphite material, and the mass percentage content of the binder in the second negative electrode material layer can be controlled by adjusting the mass ratio of the silicon-based material, the second graphite material, the binder, and the conductive agent added to the second negative electrode slurry.

[0050] 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.

[0051] 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.

[0052] 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 oxides, 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 polymers, 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] Example

[0058] 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.

[0059] Test methods and equipment:

[0060] Tests of I1 and I2:

[0061] 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.

[0062] 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).

[0063] 2) Use a steel scraper to scrape off the second negative electrode material layer on the surface of the negative electrode sheet, 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 scraped negative electrode material layer is considered to be the second negative electrode material layer.

[0064] 3) The powder of the second negative electrode material layer obtained by the above method is immersed in tetrahydrofuran solvent, heated to 60°C, and ultrasonically treated for 2 hours to dissolve the binder into the solvent to obtain a mixture.

[0065] 4) Centrifuge the mixture (5000 rpm, 20 minutes) to separate the supernatant and precipitate (active material + conductive agent).

[0066] 5) Utilizing the density difference between silicon-based material, active material graphite, and conductive agent (carbon nanotubes in this embodiment), the precipitate is separated by gravity separation to obtain a second graphite material. Among them, the silicon-based material has the highest density value.

[0067] 6) X-ray diffraction (XRD) was performed on the second graphite material. The second graphite material was placed in the sample stage of an XRD instrument (model Bruker, D8). The X-ray excitation target was 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 was obtained. The diffraction peak of 2θ between 41.5° and 43.0° was the first characteristic peak, and the peak intensity I1 of the first characteristic peak was recorded. The diffraction peak of 2θ between 43.5° and 45.5° was the second characteristic peak, and the peak intensity I2 of the second characteristic peak was recorded.

[0068] Tests of I1' and I2':

[0069] 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.

[0070] 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).

[0071] 2) Use a steel scraper to scrape off the second negative electrode material layer on the surface of the negative electrode sheet, 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 scraped negative electrode material layer is considered to be the second negative electrode material layer; then scrape off the remaining electrode sheet to obtain the first negative electrode material layer.

[0072] 3) The powder of the first negative electrode material layer obtained by the above method is immersed in tetrahydrofuran solvent, heated to 60°C, and ultrasonically treated for 2 hours to dissolve the binder into the solvent to obtain a mixture.

[0073] 4) Centrifuge the mixture (5000 rpm, 20 minutes) to separate the supernatant from the precipitate (active material + conductive agent).

[0074] 5) The first graphite material is obtained by separating the precipitate using the density difference between silicon-based material, active material graphite, and conductive agent (carbon nanotubes in this application embodiment) through gravity separation.

[0075] 6) X-ray diffraction (XRD) was performed on the first graphite material. The first graphite material was placed in the sample stage of an XRD instrument (model Bruker, D8). The X-ray excitation target was 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 was obtained. The diffraction peak of 2θ between 40.5° and 42.0° was the third characteristic peak, and the peak intensity I1' of the third characteristic peak was recorded. The diffraction peak of 2θ between 42.5° and 43.5° was the fourth characteristic peak, and the peak intensity I2' of the fourth characteristic peak was recorded.

[0076] Average particle size test of the second graphite material:

[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] The average particle size D1 of the second graphite material and the average particle size D2 of the silicon-based material were measured using scanning electron microscopy (SEM). A cross-section of the negative electrode sheet along its thickness direction was prepared and ion-polished to obtain the cross-section. SEM observation of the cross-section revealed a clear boundary between the two coating layers, with the first negative electrode material layer closest to the negative current collector. Backscattering mode was used to distinguish the silicon-based material and the second graphite material particles in the cross-section of the negative electrode sheet. Due to its higher atomic number and electron density, and the stronger interaction between the silicon-based material and the test electron beam compared to the graphite particles, the silicon-based material region was brighter, while the graphite particle region was darker. Twenty particles of second graphite material and twenty particles of silicon-based material were randomly selected from the cross-section of the second negative electrode material layer. The equivalent diameter of the 20 particles of second graphite material (that is, the diameter of the circle that is converted into an irregular particle in the cross-section into a circle of equal area) and the equivalent diameter of the 20 particles of silicon-based material were measured respectively. The average value of each was calculated, which is the average particle size D1 of the second graphite material and the average particle size D2 of the silicon-based material.

[0079] Specific surface area test of the second graphite material:

[0080] 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.

[0081] Referring to steps 1) to 5) of the "Tests for I1 and I2" above, the second graphite material was obtained. After drying the second graphite material in a vacuum drying oven, it was placed in a sample tube, and the specific surface area S of the second graphite material was measured using a specific surface area analyzer (Trista II 3020M) via nitrogen adsorption / desorption. The specific testing was conducted according to GB / T19587-2017 "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0082] Tests for H1 and H2:

[0083] 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.

[0084] A cross-section of the negative electrode sheet along its thickness direction was prepared, and then ion-polished to obtain the cross-section of the negative electrode sheet. Scanning electron microscopy was used to observe the cross-section of the negative electrode sheet, revealing a relatively clear boundary between the two coating layers. The layer closest to the negative current collector is the first negative electrode material layer. The distances from the surface of the negative electrode sheet to the boundary between the two coating layers, and from the boundary between the two coating layers to the coating layer and the negative current collector, were measured along the thickness direction of the negative electrode sheet to obtain the thickness H2 of the second negative electrode material layer and the thickness H1 of the first negative electrode material layer.

[0085] Tests of m1 and m2:

[0086] A 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. A 1cm × 1cm negative electrode sheet was taken as a sample.

[0087] Referring to steps 1) to 2) of the above "Test of I1' and I2'", the powder of the first negative electrode material layer and the powder of the second negative electrode material layer in the sample are obtained respectively. The mass of the first negative electrode material layer and the second negative electrode material layer are weighed using a balance and recorded as M1 mg and M2 mg respectively. The m1 = M1 and m2 = M2 are calculated.

[0088] Charging speed test:

[0089] The test temperature was maintained at 25℃. The lithium-ion batteries in the examples and comparative examples were charged at a constant current of 2C to 4.52V (the charging cutoff voltage corresponding to the product), then charged at a constant voltage of 4.52V to a current of 0.025C, and left to rest for 30 minutes. They were then discharged at a constant current of 0.2C to 3V and left to rest for 30 minutes. This process was recorded as one cycle, and each group of lithium-ion batteries was cycled three times. The charging time for each group of lithium-ion batteries was the average of the three charging times from 0% SOC to 80% SOC. Shorter charging times indicate faster charging speeds. The unit of charging time is minutes.

[0090] Energy density test:

[0091] Five lithium-ion batteries from each group were selected and subjected to their first charge and discharge cycles at 25°C. Constant current and constant voltage charging was performed at a charging current of 0.5C until the upper limit voltage was reached. Then, constant current discharge was performed at a discharging current of 0.2C until 3.0V was achieved. The discharge capacity and average discharge voltage of the lithium-ion batteries were obtained. The length, width, and thickness of each lithium-ion battery at 50% SOC were measured to calculate the battery volume. The volumetric energy density of the lithium-ion battery was calculated as: discharge capacity × average discharge voltage / battery volume. The upper limit voltage for charging the lithium-ion battery was 4.52V, and the discharge cutoff voltage was 3.0V.

[0092] Cyclic stability testing of lithium-ion batteries:

[0093] At a test temperature of 25°C, 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, and left to stand for 5 minutes. After 400 cycles of the above 3C charge / 0.5C discharge process, the thickness MMC1 of the lithium-ion battery was recorded.

[0094] Expansion rate (%) after 400 cycles = (MMC1 - MMC0) / MMC0 × 100%.

[0095] Among them, the lower the expansion rate after 400 cycles, the better the structural stability of the lithium-ion battery.

[0096] Example 1

[0097] <Preparation of Negative Electrode Sheets>

[0098] A first graphite material (I2' / I1' = 1.6), a binder of polyacrylic acid, and a conductive agent of carbon nanotubes were mixed at a mass ratio of 95:3:2. Deionized water was added as a solvent, and the mixture was stirred evenly in a vacuum mixer to prepare a first negative electrode slurry with a solid content of 45 wt%. A second graphite material (I2 / I1 = 2), a silicon-based material of silicon carbide, a binder of polyacrylic acid, and a conductive agent of carbon nanotubes were mixed at a mass ratio of 65:30:3:2. Deionized water was added as a solvent, and the mixture was stirred evenly in a vacuum mixer to prepare a second negative electrode slurry with a solid content of 45 wt%. 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. The coatings were then dried at 120°C to obtain a negative electrode sheet with a first and second negative electrode material layer on one side. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a first and second negative electrode material layer on both sides. 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 per unit area of ​​the first negative electrode material layer is 9mg / cm². 2 The thickness H1 of the first negative electrode material layer on one side is 60 μm; the coating mass per unit area of ​​the second negative electrode material layer is 9 mg / cm³. 2 The thickness H2 of the single-sided second negative electrode material layer is 60 μm; the average particle size D1 of the second graphite material is 10 μm; and the specific surface area S of the second graphite material is 2.5 m². 2 / g.

[0099] <Preparation of the positive electrode>

[0100] Lithium cobalt oxide (positive electrode active material), conductive carbon black (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 vacuum-stirred to obtain the positive electrode slurry. This 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 positive electrode sheet with a single-sided coating of the positive electrode material layer. The coating mass per unit area of ​​the positive electrode material layer was 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.

[0101] <Septum>

[0102] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0103] <Preparation of Electrolyte>

[0104] 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.

[0105] <Preparation of Lithium-ion Batteries>

[0106] 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.

[0107] Examples 2 to 26

[0108] 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 W3 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 W1, W2, and W4 change, the mass percentage content of the conductive agent in the second negative electrode material layer also changes accordingly. When the values ​​of D1 and S change, the grinding time is adjusted to achieve the values ​​shown in Table 1. When the values ​​of H1, H2, and the m2:m1 ratio change, the compaction density of the negative electrode sheet remains constant, and the coating quality is adjusted so that the values ​​of H1, H2, and the m2:m1 ratio are as shown in Table 1.

[0109] Comparative Example 1

[0110] Except for the fact that the first and second graphite materials used in the <Preparation of Negative Electrode Sheet> both have an I2 / I1 value of 3, the rest is the same as in Example 1.

[0111] Comparative Example 2

[0112] Except for the fact that the first graphite material in the <Preparation of the Negative Electrode Sheet> is exactly the same as the second graphite material in Example 1, and the second graphite material uses a graphite material with an I2 / I1 value of 3, everything else is the same as in Example 1.

[0113] Comparative Example 3

[0114] Except for the fact that the first and second graphite materials used in the <Preparation of Negative Electrode Sheet> both have an I2 / I1 value of 3, the rest is the same as in Example 23.

[0115] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1.

[0116] As can be seen from Examples 1 to 26 and Comparative Examples 1 to 3, by layering the negative electrode material layers, enriching the silicon-based material in the second negative electrode material layer, combining it with a second graphite material, and controlling the I2 / I1 ratio of the second graphite material within the aforementioned range, the lithium-ion battery exhibits a shorter charging time, higher volumetric energy density, and lower expansion rate after 400 cycles. This indicates that the lithium-ion battery of this application has a high energy density and good structural stability and fast-charging performance. In Comparative Examples 1 to 3, the I2 / I1 values ​​are outside the range of this application. The lithium-ion batteries in Comparative Examples 1 to 2 have longer charging times, lower volumetric energy densities, and higher expansion rates after 400 cycles; the lithium-ion battery in Comparative Example 3 has an even lower volumetric energy density and higher expansion rate after 400 cycles. However, the lithium-ion batteries in Examples 1 to 26 have shorter charging times, higher volumetric energy densities, and lower expansion rates after 400 cycles, indicating that the lithium-ion battery of this application has a high energy density and good structural stability and fast-charging performance.

[0117] The values ​​of D1 and S typically affect the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 6 to 11, when the values ​​of D1 and S are within the range of this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and lower expansion rate after 400 cycles, indicating that the lithium-ion battery of this application has a high energy density and good structural stability and fast-charging performance.

[0118] The type of silicon-based material and its average particle size typically affect the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 12 to 14, when the type of silicon-based material and its average particle size are within the range of this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and lower expansion rate after 400 cycles. This indicates that the lithium-ion battery of this application has a high energy density and good structural stability and fast-charging performance.

[0119] The value of W1 typically affects the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 15 to 16, when the value of W1 is within the range of this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and lower expansion rate after 400 cycles, indicating that the lithium-ion battery of this application has a higher energy density and good structural stability and fast-charging performance.

[0120] The value of W2 typically affects the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 15 to 17, when the value of W2 is within the range of this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and lower expansion rate after 400 cycles, indicating that the lithium-ion battery of this application has a higher energy density and good structural stability and fast-charging performance.

[0121] The type and content of binder in the second negative electrode material layer typically affect the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 15 to 18, when the type and content of binder in the second negative electrode material layer are within the scope of this application, the lithium-ion battery exhibits a shorter charging time, higher volumetric energy density, and a lower expansion rate after 400 cycles. This indicates that the lithium-ion battery of this application has a high energy density and good structural stability and fast-charging performance.

[0122] The value of W3 typically affects the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 19 to 20, when the value of W3 is within the range of this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and lower expansion rate after 400 cycles, indicating that the lithium-ion battery of this application has a higher energy density and good structural stability and fast-charging performance.

[0123] The values ​​of H1 / H2 and H1+H2 typically affect the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 21 to 24, and Comparative Example 3, when the values ​​of H1 / H2 and H1+H2 are within the range of this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and a lower expansion rate after 400 cycles. This indicates that the lithium-ion battery of this application has a high energy density and good structural stability and fast-charging performance.

[0124] The value of m2:m1 typically affects the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 21 to 24, when the value of m2:m1 is within the range of this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and a lower expansion rate after 400 cycles. This indicates that the lithium-ion battery of this application has a high energy density and good structural stability and fast-charging performance.

[0125] The value of I2' / I1' typically affects the energy density and fast-charging performance of lithium-ion batteries. As can be seen from Examples 1, 25, and 26, when the value of I2' / I1' is within the range specified in this application, the lithium-ion battery has a shorter charging time, higher volumetric energy density, and a lower expansion rate after 400 cycles. This indicates that the lithium-ion battery of this application has a high energy density and exhibits good structural stability and fast-charging performance.

[0126] 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.

[0127] 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. In the X-ray diffraction pattern of the second graphite material, there is a first characteristic peak between 41.5° and 43.0° for 2θ, and a second characteristic peak between 43.5° and 45.5° for 2θ. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, where 1.5 ≤ I2 / I1 ≤ 2.

5.

2. The secondary battery according to claim 1, wherein, 1.8≤I2 / I1≤2.

3.

3. The secondary battery according to claim 1 or 2, wherein, The average particle size of the second graphite material is D1 μm, and the specific surface area of ​​the second graphite material is Sm. 2 / g, 5≤D1≤20, 0.5≤S≤4.

0.

4. The secondary battery according to claim 3, wherein, 6.5≤D1≤15, 1.5≤S≤3.

5.

5. 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, and the average particle size of the silicon-based material is D2μm, where 8≤D2≤20.

6. The secondary battery according to claim 5, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the silicon-based material is W1, where 10% ≤ W1 ≤ 50%.

7. The secondary battery according to claim 6, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the second graphite material is W2, where 40% ≤ W2 ≤ 88%.

8. 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 W3, where 90% ≤ W3 ≤ 99%.

9. The secondary battery according to claim 1, wherein, The thickness of the first negative electrode material layer is H1μm, the thickness of the second negative electrode material layer is H2μm, 0.5≤H1 / H2≤8, and 20≤H1+H2≤240.

10. The secondary battery according to claim 1, wherein, The coating mass per unit area of ​​the first negative electrode material layer is m1 mg / cm³. 2 The coating mass per unit area of ​​the second negative electrode material layer is m2 mg / cm³. 2 The ratio of m2:m1 is 1:8 to 2:

1.

11. The secondary battery according to claim 1, wherein, The second negative electrode material layer includes an adhesive, which includes at least one of polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the second negative electrode material layer, the mass percentage of the adhesive is W4, where 0.8% ≤ W4 ≤ 10%.

12. The secondary battery according to claim 1, wherein, In the X-ray diffraction pattern of the first graphite material, there is a third characteristic peak between 40.5° and 42.0° of 2θ, and a fourth characteristic peak between 42.5° and 43.5° of 2θ. The peak intensity of the third characteristic peak is I1', the peak intensity of the fourth characteristic peak is I2', and 1.2≤I2' / I1'≤2.

5.

13. An electronic device comprising a secondary battery according to any one of claims 1 to 12.

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